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What is the speed of light? Here’s the history, discovery of the cosmic speed limit

Time travel is one of the most intriguing topics in science.

On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it’s one of the most important constants that appears in nature and defines the relationship of causality itself.

As far as we can measure, it is a constant. It is the same speed for every observer in the entire universe. This constancy was first established in the late 1800’s with the experiments of Albert Michelson and Edward Morley at Case Western Reserve University . They attempted to measure changes in the speed of light as the Earth orbited around the Sun. They found no such variation, and no experiment ever since then has either.

Observations of the cosmic microwave background, the light released when the universe was 380,000 years old, show that the speed of light hasn’t measurably changed in over 13.8 billion years.

In fact, we now define the speed of light to be a constant, with a precise speed of 299,792,458 meters per second. While it remains a remote possibility in deeply theoretical physics that light may not be a constant, for all known purposes it is a constant, so it’s better to just define it and move on with life.

How was the speed of light first measured?

In 1676 the Danish astronomer Ole Christensen Romer made the first quantitative measurement of how fast light travels. He carefully observed the orbit of Io, the innermost moon of Jupiter. As the Earth circles the Sun in its own orbit, sometimes it approaches Jupiter and sometimes it recedes away from it. When the Earth is approaching Jupiter, the path that light has to travel from Io is shorter than when the Earth is receding away from Jupiter. By carefully measuring the changes to Io’s orbital period, Romer calculated a speed of light of around 220,000 kilometers per second.

Observations continued to improve until by the 19 th century astronomers and physicists had developed the sophistication to get very close to the modern value. In 1865, James Clerk Maxwell made a remarkable discovery. He was investigating the properties of electricity and magnetism, which for decades had remained mysterious in unconnected laboratory experiments around the world. Maxwell found that electricity and magnetism were really two sides of the same coin, both manifestations of a single electromagnetic force.

James Clerk Maxwell contributed greatly to the discover of the speed of light.

As Maxwell explored the consequences of his new theory, he found that changing magnetic fields can lead to changing electric fields, which then lead to a new round of changing magnetic fields. The fields leapfrog over each other and can even travel through empty space. When Maxwell went to calculate the speed of these electromagnetic waves, he was surprised to see the speed of light pop out – the first theoretical calculation of this important number.

What is the most precise measurement of the speed of light?

Because it is defined to be a constant, there’s no need to measure it further. The number we’ve defined is it, with no uncertainty, no error bars. It’s done. But the speed of light is just that – a speed. The number we choose to represent it depends on the units we use: kilometers versus miles, seconds versus hours, and so on. In fact, physicists commonly just set the speed of light to be 1 to make their calculations easier. So instead of trying to measure the speed light travels, physicists turn to more precisely measuring other units, like the length of the meter or the duration of the second. In other words, the defined value of the speed of light is used to establish the length of other units like the meter.

How does light slow down?

Yes, the speed of light is always a constant. But it slows down whenever it travels through a medium like air or water. How does this work? There are a few different ways to present an answer to this question, depending on whether you prefer a particle-like picture or a wave-like picture.

In a particle-like picture, light is made of tiny little bullets called photons. All those photons always travel at the speed of light, but as light passes through a medium those photons get all tangled up, bouncing around among all the molecules of the medium. This slows down the overall propagation of light, because it takes more time for the group of photons to make it through.

In a wave-like picture, light is made of electromagnetic waves. When these waves pass through a medium, they get all the charged particles in motion, which in turn generate new electromagnetic waves of their own. These interfere with the original light, forcing it to slow down as it passes through.

Either way, light always travels at the same speed, but matter can interfere with its travel, making it slow down.

Why is the speed of light important?

The speed of light is important because it’s about way more than, well, the speed of light. In the early 1900’s Einstein realized just how special this speed is. The old physics, dominated by the work of Isaac Newton, said that the universe had a fixed reference frame from which we could measure all motion. This is why Michelson and Morley went looking for changes in the speed, because it should change depending on our point of view. But their experiments showed that the speed was always constant, so what gives?

Einstein decided to take this experiment at face value. He assumed that the speed of light is a true, fundamental constant. No matter where you are, no matter how fast you’re moving, you’ll always see the same speed.

This is wild to think about. If you’re traveling at 99% the speed of light and turn on a flashlight, the beam will race ahead of you at…exactly the speed of light, no more, no less. If you’re coming from the opposite direction, you’ll still also measure the exact same speed.

This constancy forms the basis of Einstein’s special theory of relativity, which tells us that while all motion is relative – different observers won’t always agree on the length of measurements or the duration of events – some things are truly universal, like the speed of light.

Can you go faster than light speed?

Nope. Nothing can. Any particle with zero mass must travel at light speed. But anything with mass (which is most of the universe) cannot. The problem is relativity. The faster you go, the more energy you have. But we know from Einstein’s relativity that energy and mass are the same thing. So the more energy you have, the more mass you have, which makes it harder for you to go even faster. You can get as close as you want to the speed of light, but to actually crack that barrier takes an infinite amount of energy. So don’t even try.

How is the speed at which light travels related to causality?

If you think you can find a cheat to get around the limitations of light speed, then I need to tell you about its role in special relativity. You see, it’s not just about light. It just so happens that light travels at this special speed, and it was the first thing we discovered to travel at this speed. So it could have had another name. Indeed, a better name for this speed might be “the speed of time.”

Related: Is time travel possible? An astrophysicist explains

We live in a universe of causes and effects. All effects are preceded by a cause, and all causes lead to effects. The speed of light limits how quickly causes can lead to effects. Because it’s a maximum speed limit for any motion or interaction, in a given amount of time there’s a limit to what I can influence. If I want to tap you on the shoulder and you’re right next to me, I can do it right away. But if you’re on the other side of the planet, I have to travel there first. The motion of me traveling to you is limited by the speed of light, so that sets how quickly I can tap you on the shoulder – the speed light travels dictates how quickly a single cause can create an effect.

The ability to go faster than light would allow effects to happen before their causes. In essence, time travel into the past would be possible with faster-than-light travel. Since we view time as the unbroken chain of causes and effects going from the past to the future, breaking the speed of light would break causality, which would seriously undermine our sense of the forward motion of time.

Why does light travel at this speed?

No clue. It appears to us as a fundamental constant of nature. We have no theory of physics that explains its existence or why it has the value that it does. We hope that a future understanding of nature will provide this explanation, but right now all investigations are purely theoretical. For now, we just have to take it as a given.

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Here's What Actually Happens When You Travel at the Speed of Light, According to NASA

NASA created a fun video to answer all of our burning questions about near-light-speed travel.

can we travel in light speed

Ever wish you could travel at the speed of light to your favorite destinations ? Once you see the reality of that speed, you may rethink everything.

"There are some important things you should probably know about approaching the speed of light," NASA's video, Guide to Near-light-speed Travel , explains. "First, a lot of weird things can happen, like time and space getting all bent out of shape."

According to the video, if you're traveling at nearly the speed of light, the clock inside your rocket would show it takes less time to travel to your destination than it would on Earth. But, since the clocks at home would be moving at a standard rate you'd return home to everyone else being quite a bit older.

"Also, because you're going so fast, what would otherwise be just a few hydrogen atoms that you'd run into quickly becomes a lot of dangerous particles. So you should probably have shields that keep them from frying your ship and also you."

Finally, the video tackles the fact that even if you were moving at the speed of light, the "universe is also a very big place, so you might be in for some surprises." For example, your rocket's clock will say it takes about nine months to get from Earth to the edge of the solar system. An Earth clock would say it took about a year and a half. Fortunately, NASA astronauts have a slew of tips for avoiding jet lag along the way.

"If you want to get to farther out vacation spots," the video explains, "you'll probably need more than a few extra snacks. A trip to the Andromeda Galaxy, our nearest large neighbor galaxy, can take over one million years. And a trip to the farthest known galaxy where it currently sits might take over 15 billion years, which is more vacation time than I think I'll ever have."

The video doesn't explain how your rocket will travel at the speed of light. Our technology just isn't there yet, but maybe the aliens will share that tech with us soon. Until then, you can track the first crew launch of Artemis II , a rocket that will fly around the moon in 2024 before making its first lunar landing in 2025.

NASA's Guide to Near-light-speed Travel

  • Released Friday, August 14, 2020
  • Produced by:
  • Chris Smith
  • Written by:
  • Visualizations by:
  • Krystofer Kim
  • Scientific consulting by:
  • Ryan DeRosa
  • and Scott Noble

So, you've just put the finishing touches on upgrades to your spaceship, and now it can fly at almost the speed of light. We're not quite sure how you pulled it off, but congratulations! Before you fly off on your next vacation, however, watch this handy video to learn more about near-light-speed safety considerations, travel times, and distances between some popular destinations around the universe. You can also download shorter clips from the video and printable postcards to send to your friends.

Near-light-speed Travel GuideThis handy video will help acquaint you with the quirks of near-light-speed travel, expected travel times, and the distances to some popular (at least, we think so) destinations!Credit: NASA's Goddard Space Flight CenterMusic: "The Tiptoe Strut" from Universal Production MusicComplete transcript available.

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Complete transcript available.

Near Light Speed 101: Effects of Near-light-speed TravelTravel at near the speed of light offers a few quirks you should be aware of, from time and space weirdness to protecting yourself from dangerous cosmic particles. This video covers some of the important ones!Credit: NASA's Goddard Space Flight CenterMusic: "Dinner With the Vicar" from Universal Production MusicComplete transcript available.

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Near Light Speed 101: Near-light-speed Travel TimesEven if you've figured out how to travel at almost the speed of light, the universe is still a huge place! Watch this video to learn more about how long it takes to cruise around the cosmos.Credit: NASA's Goddard Space Flight CenterMusic: "Dinner With the Vicar" from Universal Production MusicComplete transcript available.

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*compared to friends that stayed behind.

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Visit sunny Glerbax-29, home of eight unique, beautiful planets! The locals call it the solar system.

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A vacation to Andromeda, our nearest spiral neighbor galaxy, is a long, long, long, long, long trip, but it's worth it! And this postcard will prove it to your friends.

  • Astrophysics

Please give credit for this item to: NASA's Goddard Space Flight Center

  • Chris Smith  (USRA)
  • Krystofer Kim  (USRA)
  • Ryan DeRosa  (NASA/GSFC)
  • Scott Noble  (NASA/GSFC)

Release date

This page was originally published on Friday, August 14, 2020. This page was last updated on Wednesday, May 3, 2023 at 1:44 PM EDT.

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space traveler looking into portal on another planet

Scientists Believe Light Speed Travel Is Possible. Here’s How.

A functioning warp drive would allow humans to reach the far ends of the cosmos in the blink of an eye.

White and his team in LSI’s Houston laboratory were conducting research for the Defense Advanced Research Projects Agency, or DARPA, and had set up these particular experiments to study the energy densities within Casimir cavities, the mysterious spaces between microscopic metal plates in a vacuum. The data plot indicated areas of diminished energy between the plates, which caused them to push toward each other as if trying to fill the void. This is known as negative vacuum energy density, a phenomenon in quantum mechanics called, appropriately enough, the Casimir effect . It’s something that’s helping scientists understand the soupy physics of microscale structures, which some researchers hope can be applied to energy applications that are more practical, such as circuits and electromechanical systems.

But White noticed that the pattern of negative vacuum energy between the plates and around tiny cylindrical columns that they’d inserted in the space looked familiar. It precisely echoed the energy pattern generated by a type of exotic matter that some physicists believe could unlock high-speed interstellar travel. “We then looked, mathematically, at what happens if we placed a one-micron sphere inside of a four-micron cylinder under the same conditions, and found that this kind of structure could generate a little nanoscale warp bubble encapsulating that central region,” White explains.

That’s right—a warp bubble. The essential component of a heretofore fictional warp drive that has for decades been the obsession of physicists, engineers, and sci-fi fans. Warp drive, of course, is the stuff of Star Trek legend, a device enclosed within a spacecraft that gives the mortals aboard the ability to rip around the cosmos at superhuman speed. To the lay sci-fi fan, it’s a “black box”—a convenient, completely made-up workaround to avoid the harsh realities of interstellar travel. However, after decades of speculation, research, and experimentation, scientists believe a warp drive could actually work.

To emphasize: White didn’t actually make a warp bubble. But the data from his study led to an aha moment: For the first time, a buildable warp bubble showed promise of success.

diagram showing a negative vacuum energy in between two uncharged metallic plates

Warp technology’s core science is surprisingly sound. Though the specific mechanics of an actual device haven’t been fully unpacked, the math points toward feasibility. In short, a real-life warp drive would use massive amounts of energy, which can come in the form of mass, to create enough gravitational pull to distort spacetime in a controlled fashion, allowing a ship to speed along inside a self-generated bubble that itself is able to travel at essentially any speed. Warp drives popped up in fiction intermittently for several decades before Star Trek creator Gene Roddenberry plugged one into the USS Enterprise in 1966. But Miguel Alcubierre, PhD, a Mexican theoretical physicist and professed Star Trek enthusiast, gave the idea real-world legs when he released a paper in 1994 speculating that such a drive was mathematically possible. It was the first serious treatment of a warp drive’s feasibility, and it made headlines around the world. His breakthrough inspired more scientists to nudge the theoretical aspects of warp drive toward concrete, practical applications.

“I proposed a ‘geometry’ for space that would allow faster-than-light travel as seen from far away, essentially expanding space behind the object we want to move and contracting it in front,” Alcubierre says. “This forms a ‘bubble’ of distorted space, inside of which an object—a spaceship, say—could reside.”

Physicists tend to speak in relative terms. By injecting the sly qualifier “as seen from far away,” Alcubierre might sound like he’s describing the galactic equivalent of an optical illusion —an effect perhaps similar to driving past a truck going the opposite direction on the highway when you’re both going 60 miles an hour. Sure feels like a buck-twenty, doesn’t it? But the A-to-B speed is real; the warp effect simply shortens the literal distance between two points. You’re not, strictly speaking, moving faster than light. Inside the bubble, all appears relatively normal, and light moves faster than you are, as it should. Outside the bubble, however, you’re haulin’ the mail.

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Alcubierre’s proposal had solved one of the initial hurdles to achieving warp speeds: The very idea clashes with Einstein’s long-accepted theory of general relativity, which states that nothing can travel faster than the speed of light, but it doesn’t preclude space itself from traveling faster than that. In fact, scientists speculate that the same principles explain the rapid expansion of the universe after the Big Bang .

While concluding that warp speed was indeed possible, Alcubierre also found that it would require an enormous amount of energy to sustain the warp bubble. He theorized that negative energy—the stuff hinted at by White’s experimentation with Casimir cavities—could be a solution. The only problem is that no one has yet proved that negative energy is real. It’s the unobtanium of our spacefaring imaginations, something researchers only believe to exist. In theory, however, this unknown matter may be sufficiently powerful that future warp drive designers could channel it to contract spacetime around it. In conceptual drawings of warp-capable spacecraft , enormous material rings containing this energy source surround a central fuselage. When activated, it warps spacetime around the entire ship. The more intense the warping, the faster the warp travel is achieved.

Of course, it’s not that simple. Physicist José Natário, PhD, a professor at the Instituto Superior Técnico in Lisbon, wrote his own influential paper about the mathematical feasibility of warp drives in 2001. However, he is concerned about practical conundrums, like the amount of energy required. “You need to be able to curve spacetime quite a lot in order to do this,” he says. “We’re talking about something that would be much, much more powerful than the sun.”

Alcubierre is similarly skeptical that his theoretical ideas might ever be used to develop a working warp drive. “In order to have a bubble about 100 meters wide traveling at precisely the speed of light, you would need about 100 times the mass of the planet Jupiter converted into negative energy, which of course sounds absurd,” he says. By that standard, he concludes, a warp drive is very unlikely.

example of a warp bubble where a large object of mass pulls and contracts space to create faster than light speed

Physicists love a challenge, though. In the 29 years since Alcubierre published his paper, other scientists have wrestled with the implications of the work, providing alternative approaches to generating the energy using more accessible power sources, finding oblique entry points to the problem, and batting ideas back and forth in response to one another’s papers. They use analogies involving trampolines , tablecloths, bowling balls, balloons, conveyor belts, and music to explain the physics.

They even have their own vocabulary. It’s not faster-than-light travel; it’s superluminal travel, thank you. Then there’s nonphysical and physical—a.k.a. the critical distinction between theoretical speculation and something that can actually be engineered. (Pro tip: We’re aiming for physical here, folks.) They do mention Star Trek a lot, but never Star Wars . Even the scruffiest-looking nerf herder knows that the ships in Star Wars use hyperdrive, which consumes fuel, rather than warp drives, which don’t use propulsive technology but instead rely on, well, warping. They’re also vague about details like what passengers would experience, what gravity is like on board since you’re carrying around boatloads of energy, and what would happen if someone, say, jumped out of the ship while warping. (A speculative guess: Nothing good.)

Such research isn’t typically funded by academic institutions or the DARPAs and NASAs of the world, so much of this work occurs in the scientists’ spare time. One such scientist and Star Trek enthusiast is physicist Erik Lentz, PhD. Now a researcher at Pacific Northwest National Laboratory in Richland, Washington, Lentz was doing postdoctoral work at Göttingen University in Germany when, amid the early, isolated days of the pandemic, he mulled the idea of faster-than-light travel. He published a paper in 2021 arguing that warp drives could be generated using positive energy sources instead of the negative energy that Alcubierre’s warp drive seemed to require.

“There are a number of barriers to entry to actually being able to build a warp drive,” Lentz says. “The negative energy was the most obvious, so I tried to break that barrier down.”

He explored a new class of solutions in Einstein’s general relativity while focusing on something called the weak-energy condition, which, he explains, tracks the positivity of energy in spacetime. He hit upon a “soliton solution”—a wave that maintains its shape and moves at a constant velocity—that could both satisfy the energy-level challenge and travel faster than light. Such a warp bubble could travel along using known energy sources, though harnessing those at the levels needed are still far beyond our capabilities. The next step, he notes, may be bringing the energy requirements for a warp drive to within the range of a nuclear fusion reactor.

A fusion-powered device could theoretically travel to and from Proxima Centauri , Earth’s nearest star, in years instead of decades or millennia, and then go faster and faster as power sources improve. Current conventional rocket technology, on the other hand, would take 50,000 years just for a one-way trip—assuming, of course, there was an unlimited fuel supply for those engines.

“IF YOU COLLIDE WITH SOMETHING ON YOUR PATH, IT WOULD ALMOST CERTAINLY BE CATASTROPHIC.”

Like Alcubierre’s original thesis, Lentz’s paper had a seismic impact on the warp drive community, prompting yet another group of scientists to dig into the challenge. Physicist Alexey Bobrick and technology entrepreneur Gianni Martire have been particularly prolific. In 2021, they released a paper theorizing that a class of subliminal warp drives, traveling at just a fraction of light speed, could be developed from current scientific understanding. While that paper essentially argued that it’s perfectly acceptable to walk before you can run, they followed it up with another theory earlier this year that describes how a simulated black hole , created using sound waves and glycerin and tested with a laser beam, could be used to evaluate the levels of gravitational force needed to warp spacetime. The duo coded that breakthrough into a public app that they hope will help more quickly push theoretical ideas to practical ones. Though the team is waiting for the technology to clear a peer review stage before releasing details, the app is essentially a simulator that allows scientists to enter their warp-speed equations to validate whether they’re practical.

“When somebody publishes a warp metric for the first time, people say, ‘Okay, is your metric physical?’” Martire says. The answer to that question—whether the metric has practical potential or is strictly theoretical—is hard to establish given the challenges of testing these hypotheses. That determination could take six to eight months. “Now we can tell you within seconds, and it shows you visually how off you are or how close you are,” he says.

While useful, the app will speed up the preliminary math only for future researchers. Galaxy-sized challenges remain before we ever experience turbocharged interstellar travel. Alcubierre worries in particular about what may happen near the walls of the warp bubble. The distortion of space is so violent there, he notes, that it would destroy anything that gets close. “If you collide with something on your path, it would almost certainly be catastrophic,” he says.

Natário mulls even more practical issues, like steering and stopping. “It’s a bubble of space, that you’re pushing through space,” he says. “So, you’d have to tell space ... to curve in front of your spaceship.” But therein lies the problem: You can’t signal to the space in front of you to behave the way you want it to.

His opinion? Superluminal travel is impossible. “You need these huge deformations that we have no idea how to accomplish,” Natário says. “So yes, there has been a lot of effort toward this and studying these weird solutions, but this is all still completely theoretical, abstract, and very, very, very, very far from getting anywhere near a practical warp drive.” That’s “very” to the power of four, mind you—each crushing blow pushing us exponentially, excruciatingly further and further away from our yearned-for superluminal lives.

Ultimately, the pursuit of viable high-speed interstellar transportation also points to a more pressing terrestrial challenge: how the scientific community tackles ultra-long-term challenges in the first place. Most of the research so far has come from self-starters without direct funding, or by serendipitous discoveries made while exploring often unrelated research, such as Dr. White’s work on Casimir cavities.

Many scientists argue that we’re in a multi-decade period of stagnation in physics research, and warp drive—despite its epic time horizons before initial research leads to galaxy-spanning adventures—is somewhat emblematic of that stagnation. Sabine Hossenfelder, a research fellow at the Frankfurt Institute for Advanced Studies and creator of the YouTube channel Science Without the Gobbledygook , noted in a 2020 blog post that physics research has drifted away from frequent, persistent physical experimentation to exorbitant infusions of cash into relatively few devices. She writes that with fewer experiments, serendipitous discoveries become increasingly unlikely. Without those discoveries, the technological progress needed to keep experiments economically viable never materializes.

When asked whether this applied equally to warp drive, Hossenfelder sees a faint but plausible connection. “Warp drives are an idea that is not going to lead to applications in the next 1,000 years or so,” she says. “So they don’t play a big role in that one way to another. But when it comes to the funding, you see some overlap in the problems.”

So, despite all the advances, the horizon for a warp drive remains achingly remote. That hasn’t fazed the scientists involved, though. A few years ago, while teaching in France, White visited the Strasbourg Cathedral with his wife. While admiring its 466-foot-tall spire, he was struck by the fact that construction began in 1015 but didn’t wrap up until 1439—a span of 424 years. Those who built the basement had no chance of ever seeing the finished product, but they knew they had to do their part to aid future generations. “I don’t have a crystal ball,” White says. “I don’t know what the future holds. But I know what I need to be doing right now.”

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This Long-Range Howitzer Has Met Its Achilles Heel

3 Ways Fundamental Particles Travel at (Nearly) the Speed of Light

Light-speed travel is a staple of science fiction in space. No "Star Wars" movie seems complete until the Millennium Falcon (or a rival ship) uses its hyperdrive. And many "Star Trek" fans enjoy talking about the relative star-system-jumping speeds of the USS Enterprise, against the speeds of other Federation ships.

But in real life, physics gets in the way. Einstein's theory of special relativity essentially puts a speed limit on cosmic travel; as far as we can tell, nothing goes faster than the speed of light. Worse, any object that has mass tends to get more and more massive — dragging down the object's velocity — as it approaches light speed. So as far as we know, only small particles can get anywhere near the speed of light.

One hundred years ago, on May 29, 1919, scientists performed measurements of a solar eclipse that confirmed Einstein's work. To celebrate, NASA offered three ways that particles can accelerate to amazing speed in a new statement .

Related: Why Don't We Have a 'Star Wars' Hyperdrive Yet?  

Electromagnetic fields

The sun is a wacky environment to study physics, because it is so extreme compared to Earth. It's also a real-life laboratory showing how nuclear reactions happen. It also is an example of an environment with electromagnetic fields — which, as NASA points out, is the same force that stops magnets from falling off your fridge.

Magnetic fields and electric fields work together to accelerate particles with an electric charge. This charge allows electromagnetic fields to push particles along — sometimes at speeds approaching the speed of light.

We can even simulate this process on Earth. Huge particle accelerators (like at the Department of Energy's Fermi National Accelerator Laboratory, or at the European Organization for Nuclear Research's Large Hadron Collider ) create pulsed electromagnetic fields. These fields accelerate charged particles close to the speed of light. Next, scientists often crash these particles together to see what particles and energy are released. 

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In fractions of a second after these collisions, we can quickly observe elementary particles that were around in the first few seconds after the universe was formed. (That event, called the Big Bang , happened about 13.8 billion years ago.)

Magnetic explosions

The sun is also host to phenomena called solar flares . Dancing above the sun's surface is a tangle of magnetic fields. At times, these fields intersect and snap, sending plumes of solar material off the surface — and, sometimes, charged particles along with it.

"When the tension between the crossed lines becomes too great, the lines explosively snap and realign in a process known as magnetic reconnection," NASA officials said in the statement. "The rapid change in a region's magnetic field creates electric fields, which causes all the attendant charged particles to be flung away at high speeds."

Particles streaming off the sun may accelerate close to the speed of light, thrown from the sun thanks to magnetic reconnection. One example of such objects is the solar wind , the constant stream of charged particles the sun emits into the solar system. (There may be other factors speeding these particles as well, such as wave-particle interactions — which is explained in the next section of this article.) 

Magnetic reconnection also likely happens at large planets, such as Jupiter and Saturn. Closer to home, NASA studies magnetic reconnection near Earth using the Magnetospheric Multiscale mission , which measures our planet's magnetic field using four spacecraft. The results may be useful to better understand how particles accelerate all over the universe, NASA officials said.

Wave-particle interactions

Particles can also careen at high speeds when electromagnetic waves collide; that phenomenon is more technically called wave-particle interactions.

"When electromagnetic waves collide, their fields can become compressed. Charged particles bouncing back and forth between the waves can gain energy similar to a ball bouncing between two merging walls," NASA officials said.

These interactions take place all over the universe. Near Earth, NASA missions such as the Van Allen probes are watching wave-particle interactions to better predict particle movements — and protect electronics on satellites. That's because high-speed particles can damage these delicate spacecraft parts.

Supernovas, or star explosions, may also play a role in more far-away interactions. Researchers have theorized that after a star explodes, it creates a blast wave — a shell of hot, dense compressed gas — that zooms away from the stellar core at high speed. These bubbles are full of charged particles and magnetic fields, creating a likely environment for wave-particle interactions. This process may eject high-energy cosmic rays — which consist of particles —  at velocities close to the speed of light. 

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Elizabeth Howell

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, " Why Am I Taller ?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace

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Einstein's theory of special relativity

Why you can't travel at the speed of light

A lbert Einstein is famous for many things, not least his theories of relativity. The first, the special theory of relativity, was the one that began the physicist's reputation for tearing apart the classical worldview that had come before. Special relativity, a way of relating the motion of objects in the universe, led scientists to re-evaluate their assumptions about things as fundamental as time and space. And it led to important revelations about the relationship between energy and matter.

Special relativity was published by Einstein in 1905, in a paper titled "On the Electrodynamics of Moving Bodies". He came to it after picking on a conflict he noticed between the equations for electricity and magnetism, which the physicist James Clerk Maxwell had recently developed, and Isaac Newton's more established laws of motion.

Light, according to Maxwell, was a vibration in the electromagnetic field and it travelled at a constant speed in a vacuum. More than 100 years earlier, Newton had set down his laws of motion and, together with ideas from Galileo Galilei, these showed how the speed of an object would differ depend on who was measuring it and how they were moving relative to the object. A ball you are holding will seem still to you, even when you're in a moving car. But that ball will seem to be moving to anyone standing on the pavement.

But there was a problem in applying Newton's laws of motion to light. In Maxwell's equations, the speed of electromagnetic waves is a constant defined by the properties of the material through which the waves move. There is nothing in there that allows the speed of these waves to be different for different people depending on how they were moving relative to each other. Which is bizarre, if you think about it.

Imagine someone sitting in a stationary train, throwing a ball from where he's sitting to the opposite wall, a few metres further down the train from him. You, standing on the station platform, measure the speed of the ball at the same value as the person on the train.

Now the train starts to move (in the direction of the ball), and you again measure the speed of the ball. You would rightly calculate it as higher – the initial speed (ie, when the train was at rest) plus the forward speed of the train. On the train, meanwhile, the game-player will notice nothing different. Your two values for the speed of the ball will be different; both correct for your frames of reference.

Replace the ball with light and this calculation goes awry. If the person on the train were shining a light at the opposite wall and measured the speed of the particles of light (photons), you and the passenger would both find that the photons had the same speed at all times. In all cases, the speed of the photons would stay at just under 300,000 kilometres per second, as Maxwell's equations say they should.

Einstein took this idea – the invariance of the speed of light – as one of his two postulates for the special theory of relativity. The other postulate was that the laws of physics are the same wherever you are, whether on an plane or standing on a country road. But to keep the speed of light constant at all times and for all observers, in special relativity, space and time become stretchy and variable. Time is not absolute, for example. A moving clock ticks more slowly than a stationary one. Travel at the speed of light and, theoretically, the clock would stop altogether.

How much the time dilates can be calculated by the two equations above. On the right, Δt is the time interval between two events as measured by the person they affect. (In our example above, this would be the person in the train.) On the left, Δt' is the time interval between the same two events but measured by an outside observer in a separate frame of reference (the person on the platform). These two times are related by the Lorentz factor (γ), which in this example is a term that takes into account the velocity (v) of the train relative to the station platform, which is "at rest". In this expression, c is a constant equal to the speed of light in a vacuum.

The length of moving objects also shrink in the direction in which they move. Get to the speed of light (not really possible, but imagine if you could for a moment) and the object's length would shrink to zero.

The contracted length of a moving object relative to a stationary one can be calculated by dividing the proper length by the Lorentz factor – if it were possible for an object to reach the speed of light its length would shrink to zero.

It is important to note that if you were the person moving faster and faster, you would not notice anything: time would tick normally for you and you would not be squashed in length. But anyone watching you from the celestial station platform would be able to measure the differences, as calculated from the Lorentz factor. However, for everyday objects and everyday speeds, the Lorentz factor will be close to 1 – it is only at speeds close to that of light that the relativistic effects need serious attention.

Another feature that emerges from special relativity is that, as something speeds up, its mass increases compared with its mass at rest, with the mass of the moving object determined by multiplying its rest mass by the Lorentz factor. This increase in relativistic mass makes every extra unit of energy you put into speeding up the object less effective at making it actually move faster.

As the speed of the object increases and starts to reach appreciable fractions of the speed of light (c), the portion of energy going into making the object more massive gets bigger and bigger.

This explains why nothing can travel faster than light – at or near light speed, any extra energy you put into an object does not make it move faster but just increases its mass. Mass and energy are the same thing – this is a profoundly important result. But that is another story.

  • Albert Einstein
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What would you see if you could travel at the speed of light?

Einstein's Special Theory of Relativity was born from this very question, and the answer is as weird as you'd expect.

Robert Matthews

Asked by: Pete Groves, London

That's what Einstein asked himself as a schoolboy and it led him to his famous Special Theory of Relativity. Its equations show that objects look increasingly distorted as the speed of travel increases, with the view ahead becoming progressively brighter. Then truly bizarre effects start to kick in, with objects far ahead apparently moving further away, while those behind come into view.

Eventually, at lightspeed there's nothing but a dazzingly bright spot of light surrounded by complete blackness. Weird!

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Life's Little Mysteries

Can anything travel faster than the speed of light?

Does it matter if it's in a vacuum?

Artist's impression of beams of light

In 1676, by studying the motion of Jupiter's moon Io, Danish astronomer Ole Rømer calculated that light travels at a finite speed. Two years later, building on data gathered by Rømer, Dutch mathematician and scientist Christiaan Huygens became the first person to attempt to determine the actual speed of light, according to the American Museum of Natural History in New York City. Huygens came up with a figure of 131,000 miles per second (211,000 kilometers per second), a number that isn't accurate by today's standards — we now know that the speed of light in the "vacuum" of empty space is about 186,282 miles per second (299,792 km per second) — but his assessment showcased that light travels at an incredible speed.

According to Albert Einstein 's theory of special relativity , light travels so fast that, in a vacuum, nothing in the universe is capable of moving faster. 

"We cannot move through the vacuum of space faster than the speed of light," confirmed Jason Cassibry, an associate professor of aerospace engineering at the Propulsion Research Center, University of Alabama in Huntsville.

Question answered, right? Maybe not. When light is not in a vacuum, does the rule still apply?

Related: How many atoms are in the observable universe?

"Technically, the statement 'nothing can travel faster than the speed of light' isn't quite correct by itself," at least in a non-vacuum setting, Claudia de Rham, a theoretical physicist at Imperial College London, told Live Science in an email. But there are certain caveats to consider, she said. Light exhibits both particle-like and wave-like characteristics, and can therefore be regarded as both a particle (a photon ) and a wave. This is known as wave-particle duality.

If we look at light as a wave, then there are "multiple reasons" why certain waves can travel faster than white (or colorless) light in a medium, de Rham said. One such reason, she said, is that "as light travels through a medium — for instance, glass or water droplets — the different frequencies or colors of light travel at different speeds." The most obvious visual example of this occurs in rainbows, which typically have the long, faster red wavelengths at the top and the short, slower violet wavelengths at the bottom, according to a post by the University of Wisconsin-Madison . 

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When light travels through a vacuum, however, the same is not true. "All light is a type of electromagnetic wave, and they all have the same speed in a vacuum (3 x 10^8 meters per second). This means both radio waves and gamma rays have the same speed," Rhett Allain, a physics professor at Southeastern Louisiana University, told Live Science in an email.

So, according to de Rham, the only thing capable of traveling faster than the speed of light is, somewhat paradoxically, light itself, though only when not in the vacuum of space. Of note, regardless of the medium, light will never exceed its maximum speed of 186,282 miles per second.

Universal look

According to Cassibry, however, there is something else to consider when discussing things moving faster than the speed of light.

"There are parts of the universe that are expanding away from us faster than the speed of light, because space-time is expanding," he said. For example, the Hubble Space Telescope recently spotted 12.9 billion year-old light from a distant star known as Earendel. But, because the universe is expanding at every point, Earendel is moving away from Earth and has been since its formation, so the galaxy is now 28 billion light years away from Earth.

In this case, space-time is expanding, but the material in space-time is still traveling within the bounds of light speed.

Related: Why is space a vacuum?

Diagram of the visible color spectrum

So, it's clear that nothing travels faster than light that we know of, but is there any situation where it might be possible? Einstein's theory of special relativity, and his subsequent theory of general relativity, is "built under the principle that the notions of space and time are relative," de Rham said. But what does this mean? "If someone [were] able to travel faster than light and carry information with them, their notion of time would be twisted as compared to ours," de Rham said. "There could be situations where the future could affect our past, and then the whole structure of reality would stop making sense."

This would indicate that it would probably not be desirable to make a human travel faster than the speed of light. But could it ever be possible? Will there ever be a time when we are capable of creating craft that could propel materials — and ultimately humans — through space at a pace that outstrips light speed? "Theorists have proposed various types of warp bubbles that could enable faster-than-light travel," Cassibry said.

But is de Rham convinced?

"We can imagine being able to communicate at the speed of light with systems outside our solar system ," de Rham said. "But sending actual physical humans at the speed of light is simply impossible, because we cannot accelerate ourselves to such speed.

"Even in a very idealistic situation where we imagine we could keep accelerating ourselves at a constant rate — ignoring how we could even reach a technology that could keep accelerating us continuously — we would never actually reach the speed of light," she added. "We could get close, but never quite reach it."

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This is a point confirmed by Cassibry. "Neglecting relativity, if you were to accelerate with a rate of 1G [Earth gravity], it would take you a year to reach the speed of light. However, you would never really reach that velocity because as you start to approach lightspeed, your mass energy increases, approaching infinite. "One of the few known possible 'cheat codes' for this limitation is to expand and contract spacetime, thereby pulling your destination closer to you. There seems to be no fundamental limit on the rate at which spacetime can expand or contract, meaning we might be able to get around this velocity limit someday."

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Allain is similarly confident that going faster than light is far from likely, but, like Cassibry, noted that if humans want to explore distant planets, it may not actually be necessary to reach such speeds. "The only way we could understand going faster than light would be to use some type of wormhole in space," Allain said. "This wouldn't actually make us go faster than light, but instead give us a shortcut to some other location in space."

Cassibry, however, is unsure if wormholes will ever be a realistic option.

"Wormholes are theorized to be possible based on a special solution to Einstein's field equations," he said. "Basically, wormholes, if possible, would give you a shortcut from one destination to another. I have no idea if it's possible to construct one, or how we would even go about doing it." Originally published on Live Science.

Joe Phelan

Joe Phelan is a journalist based in London. His work has appeared in VICE, National Geographic, World Soccer and The Blizzard, and has been a guest on Times Radio. He is drawn to the weird, wonderful and under examined, as well as anything related to life in the Arctic Circle. He holds a bachelor's degree in journalism from the University of Chester. 

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Why does time change when traveling close to the speed of light? A physicist explains

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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

Why does time change when traveling close to the speed of light? – Timothy, age 11, Shoreview, Minnesota

Imagine you’re in a car driving across the country watching the landscape. A tree in the distance gets closer to your car, passes right by you, then moves off again in the distance behind you.

Of course, you know that tree isn’t actually getting up and walking toward or away from you. It’s you in the car who’s moving toward the tree. The tree is moving only in comparison, or relative, to you – that’s what we physicists call relativity . If you had a friend standing by the tree, they would see you moving toward them at the same speed that you see them moving toward you.

In his 1632 book “ Dialogue Concerning the Two Chief World Systems ,” the astronomer Galileo Galilei first described the principle of relativity – the idea that the universe should behave the same way at all times, even if two people experience an event differently because one is moving in respect to the other.

If you are in a car and toss a ball up in the air, the physical laws acting on it, such as the force of gravity, should be the same as the ones acting on an observer watching from the side of the road. However, while you see the ball as moving up and back down, someone on the side of the road will see it moving toward or away from them as well as up and down.

Special relativity and the speed of light

Albert Einstein much later proposed the idea of what’s now known as special relativity to explain some confusing observations that didn’t have an intuitive explanation at the time. Einstein used the work of many physicists and astronomers in the late 1800s to put together his theory in 1905, starting with two key ingredients: the principle of relativity and the strange observation that the speed of light is the same for every observer and nothing can move faster. Everyone measuring the speed of light will get the same result, no matter where they are or how fast they are moving.

Let’s say you’re in the car driving at 60 miles per hour and your friend is standing by the tree. When they throw a ball toward you at a speed of what they perceive to be 60 miles per hour, you might logically think that you would observe your friend and the tree moving toward you at 60 miles per hour and the ball moving toward you at 120 miles per hour. While that’s really close to the correct value, it’s actually slightly wrong.

This discrepancy between what you might expect by adding the two numbers and the true answer grows as one or both of you move closer to the speed of light. If you were traveling in a rocket moving at 75% of the speed of light and your friend throws the ball at the same speed, you would not see the ball moving toward you at 150% of the speed of light. This is because nothing can move faster than light – the ball would still appear to be moving toward you at less than the speed of light. While this all may seem very strange, there is lots of experimental evidence to back up these observations.

Time dilation and the twin paradox

Speed is not the only factor that changes relative to who is making the observation. Another consequence of relativity is the concept of time dilation , whereby people measure different amounts of time passing depending on how fast they move relative to one another.

Each person experiences time normally relative to themselves. But the person moving faster experiences less time passing for them than the person moving slower. It’s only when they reconnect and compare their watches that they realize that one watch says less time has passed while the other says more.

This leads to one of the strangest results of relativity – the twin paradox , which says that if one of a pair of twins makes a trip into space on a high-speed rocket, they will return to Earth to find their twin has aged faster than they have. It’s important to note that time behaves “normally” as perceived by each twin (exactly as you are experiencing time now), even if their measurements disagree.

You might be wondering: If each twin sees themselves as stationary and the other as moving toward them, wouldn’t they each measure the other as aging faster? The answer is no, because they can’t both be older relative to the other twin.

The twin on the spaceship is not only moving at a particular speed where the frame of references stay the same but also accelerating compared with the twin on Earth. Unlike speeds that are relative to the observer, accelerations are absolute. If you step on a scale, the weight you are measuring is actually your acceleration due to gravity. This measurement stays the same regardless of the speed at which the Earth is moving through the solar system, or the solar system is moving through the galaxy or the galaxy through the universe.

Neither twin experiences any strangeness with their watches as one moves closer to the speed of light – they both experience time as normally as you or I do. It’s only when they meet up and compare their observations that they will see a difference – one that is perfectly defined by the mathematics of relativity.

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Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

Image of galaxies, taken by the Hubble Space Telescope.

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

Illustration of GPS satellites orbiting around Earth

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

Illustration of a hand holding a phone with a maps application active.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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A Total Solar Eclipse Is Coming. Here’s What You Need to Know.

These are answers to common questions about the April 8 eclipse, and we’re offering you a place to pose more of them.

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The sun flares at the edge of the moon during a total eclipse.

By Katrina Miller

On April 8, North America will experience its second total solar eclipse in seven years. The moon will glide over the surface of our sun, casting a shadow over a swath of Earth below. Along this path, the world will turn dark as night.

Skywatchers in Mexico will be the first to see the eclipse on the mainland. From there, the show will slide north, entering the United States through Texas, then proceeding northeast before concluding for most people off the coast of Canada.

Why eclipses happen is simple: the moon comes between us and the sun. But they are also complicated. So if you’ve forgotten all of your eclipse facts, tips and how-to’s since 2017, we’re here to explain it for you.

But before we dive in, there is one thing to know that is more important than anything else: It is never safe to look directly at the sun during an eclipse (except for the few moments when the moon has fully obscured its surface). At all other times, watch the event through protective eye equipment . Read on to learn about how to watch an eclipse safely.

What is a total solar eclipse?

A solar eclipse occurs when the moon orients itself between Earth and the sun, shielding the solar surface from our view.

In cosmic terms, it is unusual that this happens: the moon is about 400 times smaller than the sun, but it is about 400 times closer to us. That means that when these two celestial bodies are aligned, they appear to be the same size in the sky.

What other types of eclipses are there?

Annular solar eclipses occur when the moon is farther from Earth and appears too small to completely shield the sun’s surface. Instead, the outer part of the solar disk remains uncovered — a “ring of fire” in the sky.

Partial solar eclipses happen when Earth, the moon and the sun are imperfectly aligned. The moon only obscures a chunk of the sun. There will be two in 2025.

Earth can also get between the moon and the sun, creating a lunar eclipse. This can be observed once or twice a year .

How dark will it be during the eclipse?

In any given place along the eclipse path , the event will last around two hours or more.

The event will commence with a partial solar eclipse, as the moon takes a small bite out of the sun’s edge, then consumes more and more of its surface. According to NASA , this can last anywhere from 70 to 80 minutes.

The phase of the eclipse where the moon has completely blocked the sun’s surface is called totality. This is the only time the event can be viewed with the naked eye.

The length of totality varies by location. In April, some places will experience this phase for more than four minutes; others, for only one to two minutes.

During totality, the sky will get dark as night and the temperature will drop. Wispy white strings of light from the sun’s outer atmosphere, or corona, will suddenly be visible. Lucky viewers may even spot a thin, reddish-pink circle around the edge of the moon. That’s the chromosphere, an atmospheric layer below the sun’s corona. Its color comes from the presence of hydrogen throughout the layer.

After totality, the sun will slowly peek out from behind the moon again — another partial eclipse that will last the same amount of time as the first one. The moon will recede until the sun is back to normal brightness in our sky.

How can I watch the solar eclipse safely?

In general, avoid looking directly at the sun without special equipment to protect your eyes. Inexpensive options for watching the eclipse include paper solar viewers and glasses. If you are using equipment purchased for a past solar eclipse, make sure to inspect it. Toss anything with scratches or other signs of damage.

According to NASA , it is not safe to look at the sun through any optical device while using paper glasses or viewers. To watch the eclipse through cameras, binoculars or telescopes, buy a special solar filter.

The only time you can view a solar eclipse with the naked eye is during the moments of totality. Once the moon begins to reveal the surface of the sun again, return to watching the event through protective equipment to avoid injury.

What happens if I look at the eclipse without protection?

In general, staring directly at the sun, even for a few seconds, can cause permanent damage to your eyes . This can range from blurry or distorted vision to something even more serious, like blind spots. Because there are no pain receptors in the retina, you won’t feel it while it’s happening.

The same is true during an eclipse — except during the brief moments of totality, when the moon has hidden the face of the sun. At all other times, use protective eye equipment to view the event.

What do I do if I can’t find eclipse glasses?

If it’s too late to get glasses or viewers, there’s always a do-it-yourself option: a pinhole camera to indirectly experience the eclipse. You can create one using cardstock , a cardboard box , a kitchen strainer or even your fingers . These designs project an image of the eclipse onto the ground or some other surface that is safe to look at.

Where are the best places to watch the eclipse?

The total eclipse will sweep across large portions of Mexico, the United States and eastern Canada. For the most dramatic show, it’s best to experience the eclipse along the path of totality , which is where the moon will completely blot out the sun.

The Path of the Eclipse

On April 8, a total solar eclipse will cross North America from Mazatlán, Mexico, to the Newfoundland coast near Gander, Canada. Viewers outside the path of the total eclipse will see a partial eclipse, if the sky is clear .

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Percentage of

the sun obscured

during the eclipse

Indianapolis

Little Rock

San Antonio

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Viewers near Mazatlán, a beach town on the Pacific shoreline of Mexico, will be the first place to experience totality on North America’s mainland. Various sites in Mexico along the eclipse’s path will experience the longest duration of totality — as long as four minutes and 29 seconds.

Cities across the United States, including Dallas, Indianapolis and Cleveland, will most likely be hot spots for the upcoming eclipse. Other notable locations include Carbondale, Ill., which also saw totality during the solar eclipse in 2017; small towns west of Austin, Texas, which are projected to have some of the best weather in the country along the eclipse path; and Niagara Falls, if the skies are clear. Six provinces of Canada are in the path of totality, but many of them have a very cloudy outlook.

When does the eclipse begin and end?

The show begins at dawn, thousands of miles southwest of the Pacific shore of Mexico. The moon starts to conceal the sun near Mazatlán at 9:51 a.m. local time. Viewers near Mazatlán will experience totality at 11:07 a.m. for four minutes and 20 seconds.

Then the moon’s shadow will swoop through Mexico, crossing over the Texas border at 1:10 p.m. Eastern time. Totality in the United States will start at 2:27 p.m. and end at 3:33 p.m. Eastern time.

Canadians will experience the solar eclipse in the afternoon for nearly three hours. The eclipse concludes beyond Canada’s boundaries when the sun sets over the Atlantic Ocean.

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How long will the eclipse last?

The duration of totality depends on how far a given location on Earth is from the moon. Places with the longest totality are closest to the moon and farther from the sun. The speed of the lunar shadow is slowest over spots with the longest totality.

In April, the longest period of totality will occur over Durango, a state in Mexico, for a total of four minutes and 29 seconds. Along the centerline, the location of shortest totality on land is on the eastern coast of Newfoundland and Labrador in Canada, for about two minutes and 54 seconds. But totality is even shorter along the edges of the total eclipse path; in some places, it lasts less than a minute.

How fast does the eclipse move?

Solar eclipses may seem to happen slowly, but the moon’s shadow is racing across the surface of Earth. Exact speeds vary by location. Eclipse calculators estimate the shadow will move between about 1,560 m.p.h. and 1,600 m.p.h. through Mexico, and more than 3,000 m.p.h. by the time it exits the United States. The eclipse will reach speeds exceeding 6,000 m.p.h. over the Atlantic Ocean.

When was the last total solar eclipse in the United States?

According to the American Astronomical Society , total solar eclipses happen once every year or so, but they can only be viewed along a narrow path on Earth’s surface. Many occur over water or other places that can be difficult to reach. A given location will experience totality once in about 400 years.

But some places get lucky: Carbondale, a college town in southern Illinois, saw the total solar eclipse in the United States on Aug. 21, 2017, and will experience another one this April. San Antonio experienced an annular eclipse last October, and is also in the path of totality for this year’s eclipse.

Do other planets experience solar eclipses?

Yes, any planet in our solar system with a moon can experience a solar eclipse. In February, a Martian rover captured Phobos , one of the red planet’s moons, transiting the sun.

The moons on other planets, though, appear either smaller or larger than the sun in the sky . Only Earth has a moon just the right size and at just the right distance to produce the unique effects of totality.

How will things on Earth change during the eclipse?

As the eclipse approaches its maximum phase, the air will get cooler, the sky will grow dimmer, shadows will sharpen and you might notice images of crescents — tiny projections of the eclipse — within them. Along the path of totality, the world will go dark while the moon inches toward perfect alignment with Earth and the sun.

Animals will also react to the solar eclipse. Bees stop buzzing , birds stop whistling and crickets begin chirping. Some pets may express confusion . Even plants are affected, scientists found after the solar eclipse in 2017 . They have diminished rates of photosynthesis and water loss similar to, though not as extreme as, what happens at night.

What if I can’t get to the path of totality?

Viewers in locations away from the eclipse path will see the moon partially blot out the sun, though how perceptible the effects are depends on the site’s distance from the centerline. (The closer you are, the more remarkable it will be.) Still, it won’t be quite like experiencing the eclipse during totality.

Remember that you should always wear protective eye equipment while watching a partial eclipse.

If you can’t make it to the path of totality but still want to experience it, many organizations are providing live video streams of the eclipse, including NASA and Time and Date . The Exploratorium, a museum in San Francisco, will also offer a sonification of the eclipse and a broadcast in Spanish.

What have we learned from solar eclipses?

In the 1800s, a French astronomer discovered the element helium by studying the spectrum of sunlight emitted during an eclipse. These events also allowed the first scientific observations of coronal mass ejections — violent expulsions of plasma from the sun’s corona — which can cause power outages and communication disruptions on Earth. Scientists also confirmed Einstein’s theory of general relativity, which says that massive objects bend the fabric of space-time, during a solar eclipse in 1919.

And there is more to discover. This April, NASA plans to fly instruments on planes to capture images of the solar corona, and launch rockets to study how the drop in sunlight during an eclipse affects Earth’s atmosphere. A radio telescope in California will try to use the moon as a shield to measure emissions from individual sunspots .

The public is joining the fun, too. During the eclipse, a team of ham radio operators will beam signals across the country to study how solar disturbances can affect communications. Some people along the path of totality will record sounds from wildlife . Others will use their phones to snap pictures of the eclipse to help sketch out the shape of the solar disk .

An earlier version of this article referred imprecisely to eclipse on other worlds. Some appear larger than the sun in sky, they are not all partial eclipses.

How we handle corrections

Katrina Miller is a science reporting fellow for The Times. She recently earned her Ph.D. in particle physics from the University of Chicago. More about Katrina Miller

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Where Will Lightspeed Stock Be in 5 Years?

Lightspeed stock (TSX:LSPD) continues to be touch and go, so what might happen in the next five years?

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Shares of Lightspeed Commerce ( TSX:LSPD ) popped yet again this week from news that the company was considering going private. It’s been the latest from a long list of headlines from Lightspeed stock, though apparently it hasn’t resulted in a high share increase.

With all this going on, what might the next few years look like for Lightspeed stock? Let’s get into it.

What happened

First off, to understand the future, let’s look at the recent past. The company’s share price has stagnated over the last few years, as it looked to improve its finances. Lightspeed stock recently saw impressive results during the last quarter, with revenue increasing 27% to US$239.7 million. Furthermore, while still reporting a net loss from investments, the company had substantially lower losses compared to last year. What’s more, it managed to reported positive adjusted earnings of US$11.8 million.

However, management then came out to say that now former chief executive officer JP Chauvet would be stepping down. Instead, founder Dax DaSilva would be coming back to the role. This was to help the company increase subscriptions once more, which seriously lagged during the last earnings report.

Why go private?

The news comes as fellow point-of-sale (POS) company Nuvei stated it was in talks for a private equity buyout. So now, Lightspeed founder DaSilva is wondering if the company should do the same, and stated they’re “open to it.”

“I still believe that the stock market is a good place for Lightspeed, but when you look at what’s happening, you wonder if going private would be a better option,” Dasilva said. “We are always open to these discussions.”

Shares jumped 5% this week at the news, coming back down slightly the day after. So it seems that nothing is off the table, leading to at least more growth in the near term as DaSilva looks to bring the company back to 2021 levels.

The next five years

First off, what would it mean if a company like Nuvei or Lightspeed goes private for shareholders? Several things could happen in this instance. Typically there  is a buyout offer for shareholders, buying up shares by the group or individual taking the company private. This buyout price would usually be a premium of the current market price. Shareholders can then accept the buyout, or sell shares for cash.

However, should the company continue as is, there are other factors worth noting that could happen as well. There is likely to be continued revenue growth, especially as it maintains the growth from its unified payments. Furthermore, with profitability on the table, this should allow for more growth in the areas that brought in subscribers.

Yet Lightspeed stock doesn’t have an easy path to success. There are many POS companies out there, and more competition on a large and small scale. So Lightspeed stock will need to identify why it deserves your dollar above the rest. And we’re still waiting to hear how DaSilva plans on doing just that.

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Inside the $1 billion love affair between Stability AI’s ‘complicated’ founder and tech investors Coatue and Lightspeed—and how it turned bitter within months

can we travel in light speed

In December 2022, someone was looking for Emad Mostaque. The founder and CEO of Stability AI, one of the hottest startups in the nascent generative AI field, could not be found. 

Even Mostaque’s wife, Zehra Qureshi, was not sure. She texted an employee, worried about Mostaque and noting that he’d left home in his pajamas. 

Mostaque would soon reappear, brushing it off. But the bizarre episode hinted at a deeper dynamic destabilizing the AI unicorn. When Mostaque was gone, employees had to scramble to communicate with investors, who’d recently poured more than $100 million into the company and wanted to speak to their star founder. The relationship between Mostaque and Stability AI’s top investors, formed practically overnight, was vital to the company’s early success—even as it began to crack almost immediately. On Saturday, many months after a pressure campaign by investors to oust him had begun and questions about his credibility had been raised, Mostaque resigned from the company he founded.

It’s a stunning fall from grace for a founder who made headlines in 2022 when he nabbed a $1 billion valuation for his startup while raising its “seed” funding—the stage at which the median tech startup is typically valued at a mere $12 million, according to PitchBook.

While not a well-known name within the tech industry, Mostaque had emerged as one of the early players in the generative AI boom thanks to Stability’s role as the driving force behind Stable Diffusion, a powerful open-source AI model that could create images of almost anything a user could think of typing—space unicorns, Lego burgers, or sports cars that look like Van Gogh paintings. Stability’s first product was image editing software called DreamStudio that sat on top of Stable Diffusion and allowed people to more easily use the model. 

And Mostaque’s pitch, that Stability AI provided an “open” alternative to privately held models like rival OpenAI, quickly attracted the backing of Coatue and Lightspeed Venture Partners, two of the tech industry’s most prestigious investors. “We believe in Emad Mostaque’s vision and we are thrilled to have led the funding round to support what is possible with AI,” Coatue partner and former Atlassian CTO Sri Viswanath wrote on LinkedIn in October 2022.

The story of how that relationship fell apart—to the point that in just one year, Coatue was demanding that Mostaque be fired—is a spectacular business debacle that may cost tens of millions of dollars, dozens of jobs, and the reputations of many of the principal actors involved. But the whirlwind romance that brought together top-tier investors and a startup in the tech industry’s most promising sector is also a reminder of the hype, FOMO, and dangerous delusions currently fueling Silicon Valley’s love affair with artificial intelligence.

This account is based on interviews with 20 Stability investors, former Stability and Coatue employees, and Coatue-backed founders—as well as hundreds of pages of documents, including lawsuit filings, balance sheets, pitch decks, business plans, and correspondence, reviewed by Fortune .

can we travel in light speed

‘Going hard’

Mostaque’s relationship with Coatue and Lightspeed blossomed alongside Stability’s sudden rise from obscurity. In early August 2022, more than two years after Mostaque founded Stability, the company publicly unveiled its text-to-image generator. Within three days, more than one thousand people had joined Stable Diffusion’s Discord channel. Techies were enchanted.

And investors were taking notice. Approximately 10 days after the initial beta launch, Malachi Price, a member of Coatue’s investment team, sent Mostaque an email introducing himself. Foundational AI models, such as the one powering Stability’s product, had the potential to “democratize” and reinvent the way people work and create content, Price wrote. Many of the companies developing large language models for AI, however, were not truly open-source. “Given the move towards open source in the software space more broadly, this has been somewhat surprising,” Price said.

The pitch was well-received by Mostaque, whose pitch deck for the company at the time described Stability as the “only competitor” to image-generation models like OpenAI’s DALL-E 2. “Thank you for your thoughtful email,” Mostaque wrote back. “I am very impressed relative to some of the discussions I have had recently and echo some of your thoughts.”

In the summer of 2022, interest rates were still low and venture capital investments in tech were booming. Coatue, a hedge fund based in New York, had been on a binge in 2021, inking more than 150 VC deals that year, according to PitchBook data. The firm was easing up its deal pace in 2022, but generative AI technology had the kind of buzz that meant exceptions could be made.

Many of the biggest names in venture capital, including Andreessen Horowitz and Sequoia Capital, were interested in Stability, and Mostaque was taking meetings with all of them, according to someone with direct knowledge of the matter. As the meetings continued, Mostaque said in an email that he was flying to New York for a week to woo funds and family offices, and he bragged to people in his circle about the interest he was seeing from Sequoia’s Pat Grady and Shaun Maguire—and even Microsoft CEO Satya Nadella, according to messages seen by Fortune . But Coatue, in particular, was “going hard,” Mostaque confided to someone close to him. One of the first Coatue partners to meet with Mostaque following Price’s outreach was David Cahn. Mostaque, a former hedge fund manager, and Cahn hit it off. “I quite liked their macro approach and really liked David,” Mostaque said in an email to Fortune .

Within 10 days of the introductory email, Mostaque relayed to someone close to him that he already had a term sheet from Coatue. And in September, Coatue had agreed to invest $50 million at a $500 million valuation, according to messages from Mostaque and someone familiar with the matter. Lightspeed later joined as co-lead investor, and when the deal closed on Sept. 19, Mostaque had managed to scoop up $101 million in total funding at a $1 billion valuation. The unicorn valuation for such a young company, and the speed at which the deal was struck, was eye-popping, even by the standards of 2022’s red-hot VC market, although investors still had more than a month overall to perform due diligence on the investment. Investors reviewed a Notion document with data and information, reviewed Stability’s revenue figures, talked to customers, reviewed Stability’s compute contract with Amazon , did a background check on Emad, and had their own data science teams replicate numbers, according to two people familiar with the matter.

For Stability, the investment was a game-changer. Just weeks earlier, the company had been valued at $100 million from smaller investors and was racking up AWS compute bills as it hired employees remotely or from its London office (which sits atop a fried chicken shop). “It was very hand-to-mouth,” one former employee recalled, describing Mostaque’s efforts to raise money from family, friends, and “anybody who would listen to him.” Stability had a payments backlog with AWS, and it wasn’t until Coatue and Lightspeed stepped in that “everything was in order” and it was fully paid off, the person said.

‘We came out of nowhere’

An AI-generated image of a grinning Mostaque riding a unicorn in space was projected on a giant on-stage screen as the crowd whooped and hollered.

The real Mostaque was also on stage, wearing a baseball cap and T-shirt, mic in hand, at San Francisco’s Exploratorium for Stability AI’s launch party on Oct. 17, 2022. News of Stability’s $101 million seed round, at a whopping $1 billion valuation, had been announced that same day. A who’s who of Silicon Valley’s royalty, including Google founder Sergey Brin, angel investor Ron Conway, and AngelList cofounder Naval Ravikant, were all there to celebrate.

“Is Stability hype or real?” Mostaque asked the crowd. “A lot of you in this room are thinking that because, look, we came out of nowhere. We did a few things, and I think we flipped the world of artificial intelligence. And I think it might be more than that soon.”

It was a triumphant moment for Stability—there was an afterparty at the Tequila Mockingbird bar that involved dancing, one employee told Fortune . But the party was also a triumph for Coatue, and the event marked Stability’s darling status within Coatue’s portfolio. Coatue had footed the bill for a portion of the lavish party, two former employees say, and packed the event with “a large guest list.” 

Coatue cofounder Thomas Laffont was among the revelers. His brother, Coatue cofounder Philippe Laffont, used Mostaque’s presentations as examples of excellence at least once, recalls one founder of another Coatue-backed AI company.

Sri Viswanath, a Coatue partner who was involved in the deal, joined Stability’s board after the funding round and took a hands-on role in hiring the company’s vice president of engineering and chief people officer, among others. 

But it didn’t take long for tensions to intrude into the honeymoon. The collaborative relationship between the investors and the promising startup gradually morphed into something more akin to that of a parent and an unruly child as the extent of internal turmoil and lack of clear direction at Stability became apparent, and even increased as Stability used its funding to expand its ranks.

Portrait of Sri Viswanath inside office space.

“There was literally no structure in place in the business at all,” says one former employee. In the London office, “one day [employees would] be working on a marketing strategy. Before that, they weren’t quite sure. And another day, they’d be working on something completely different.” That a sizable portion of Stability’s workforce was remote didn’t help.  

Another employee said there was no clear sense of business strategy: “For a long time, they were just giving these models away for free and looking at ways to catch their revenue elsewhere,” they said. The company experimented with freemium or paid consumer products and paid APIs.

Dan Jeffries, an early advisor and briefly an executive at the company, helped shape Stability’s early strategy, according to someone close to Mostaque (as well as Mostaque’s initial email exchange with Coatue). But Jeffries contends that his suggested strategy was never ultimately used. 

“The only strategy ever executed there was Emad’s. As he said many times, ‘The buck stops with me,’” Jeffries told Fortune . 

Several people within Stability’s then 100-person company chafed at interactions with Mostaque’s wife, who was also an executive at Stability, according to former employees. Zehra Qureshi had played an instrumental role in the early development of the company, shaping marketing and HR, those people said. According to some employees, she was outspoken and could be extraordinarily particular—there was even a bathroom in Stability’s London office marked off with a paper sign reading “Zehra’s toilet,” according to a photo viewed by Fortune .

“ She’s very strong-minded,” one employee says of Qureshi. “ At times, there were people that found that very uncomfortable… But the truth is that, without her, Stability would never have gotten off the ground.”

Qureshi was reached by Fortune, but ultimately didn’t respond to requests for comment in time for publication. Mostaque told Fortune in an email that since Stability was an “applied deep tech company,” it would have been “weird” to have defined business strategy during that early period. He also cited a “clash of cultures” and “some poor choices in leaders on my part who were fired.”

Stability AI did not provide comment on specific claims in this article, but a Stability AI spokesperson told Fortune via email:

“Stability AI has developed best in class generative AI models, which are the most liked on Hugging Face and have been downloaded over 150 million times. The company remains focused on commercialising its world leading models and providing partners across the creative industries with the freedom to create across image, video, audio, 3D, language and code through its membership programme, API platform, applications and model customisation.”

can we travel in light speed

‘A complicated person’

In late 2022, not long after the party, Coatue’s Cahn stepped in to help the company with day-to-day operations, according to three sources familiar with the matter. Cahn, who seemed to have a good working relationship with Mostaque, helped work on Stability’s business strategy and get documentation and other details sorted for what was expected to be additional fundraising, two of those sources told Fortune . But at the end of January 2023, Cahn left Coatue to take a job at VC firm Sequoia. Viswanath, the Coatue partner who was already on Stability’s board, became the main point person.

Despite having publicly praised Mostaque’s vision less than four months earlier, Viswanath now seemed less than enthused about the state of affairs at the buzzy AI startup he had helped tie his firm to. Around this time, Coatue asked the company for a document that would lay out Stability’s product plan and a timeline—a “road map,” according to a former employee who helped work on the document that was ultimately given to Coatue. That document “was never referenced again” after it was sent to Viswanath, the person said.

By the end of January, Viswanath’s doubts in Mostaque were serious enough that he told the company he thought it should hire a new CEO, according to a person familiar with the matter. Viswanath didn’t respond to Fortune ’s request for comment. 

The exact cause of Viswanath’s initial misgivings regarding Mostaque could not be confirmed, but whatever the resulting strain to the relationship, it did not lead to a complete rift between Coatue and Stability’s CEO.

can we travel in light speed

Investors asked Mostaque to fundraise, several sources said. There were a series of rolling attempts to gather together another giant round, this time at a valuation that Bloomberg reported to be about $4 billion. (Mostaque disputes that there were formal valuation discussions.) Stability met with Altimeter, Redpoint, and Maverick Ventures, according to two sources with direct knowledge of the matter. Dell was also at one point interested in investing, a source familiar with the matter said. “Everyone was seeing them,” one person said. Ultimately, however, the market wouldn’t bite. 

In addition to Stability’s rich valuation, part of the problem in the fundraising process may have been that Mostaque was avoidant—when asked about the special “data room” for prospective investors to peruse confidential business documents, Mostaque said “it’ll be ready in two weeks,” recalled a former employee with direct knowledge of the matter. Two weeks came and went, the former employee noted, but the data room was nowhere to be seen. (Mostaque told Fortune that existing investors and strategic investors who asked for data room access received it, but since no formal process was begun, other potential investors who signed nondisclosure agreements did not receive access to a data room.)

Mostaque, who was born in Jordan but raised in Bangladesh and the U.K., is a “complicated person,” one former Stability AI employee says. He could be charismatic, warm, and kind, but also forgetful and disorganized, that person says. In media interviews, he has said candidly that he has Asperger’s, and he told Fortune in an email that he will “take a few days off the grid a quarter” to reset because of it, including the time in December 2022 when people were looking for him. 

Devoted to public speaking events, Mostaque could be passionate, fast-talking, and brimming with confidence. In private, however, he often seemed fatigued, stressed, and ambivalent. Mostaque would sometimes tell investors that he would step down, or that he wanted to hire a president for Stability, one former employee said.

Nathan Lile, former chief of staff at Stability who frequently traveled with Mostaque, recalled the Stability founder having Wi-Fi issues on his phone during a flight layover. Mostaque went off into the terminal in search of better Wi-Fi and a new SIM card. He returned a half-hour later, holding a pair of new AirPods. 

It was a gift for Lile, who Mostaque said had been working very hard and would enjoy the noise cancellation. Lile was touched by the gesture, recalling how Mostaque’s eyes lit up when asking if Lile had ever tried spatial audio (he hadn’t). 

“I was in complete shock, because he’d just gone out and bought it,” Lile said. “And then he said something like ‘If things were a little more organized around here, I’d have gotten your initials engraved on it.’”

As Lile fiddled with the gift, he realized something else: It seemed like Mostaque had forgotten to get the SIM card, he remembers.

‘That level of souring is pretty uncommon’

In June 2023, Mostaque’s public image took a major hit when Forbes published a bombshell story raising questions about his credibility. The article contained numerous allegations that Mostaque had exaggerated his own credentials, Stability’s partnerships, and what the company could do. (Mostaque vehemently denied the article’s allegations in a blog post at the time.) 

The interactions between Mostaque and Stability’s investors changed almost immediately, people say. One person with knowledge of the matter said that Coatue was looking to help Stability hire and tout the company until just a few days before the Forbes story came out. An investor says Mostaque distanced himself from them shortly after the story came out: “After that, we sort of started to speak less,” they said.

Shortly before the Forbes story ran, there was a shakeup on the Stability board. Mostaque’s wife, Qureshi, who had served as head of communications, left her seat, and early employee Scott Trowbridge joined in her stead. (A former employee said the board “absolutely” played a role in her resignation. Mostaque disputes that the board had anything to do with it.) 

Adding to the woes, Stability’s cash position was starting to dwindle again, as the company’s compute expenses climbed. Stability, unlike other generative AI startups, didn’t have the multibillion-dollar investments with Big Tech cloud computing providers that some of its competitors enjoyed. OpenAI, for instance, had teamed up with Microsoft . Anthropic, another fast-growing AI startup, had linked up with both Amazon and later Google. 

But there was another problem. This time, it was Lightspeed that brought it up. The Silicon Valley VC firm, famous for investing early in Snapchat and for backing richly-valued startups like Stripe , had questions about Stability’s financials, Fortune has learned. Lightspeed specifically told Stability leadership that the firm was surprised about Stability’s cash position—and that some recent figures shared with Lightspeed were inconsistent with previous discussions, according to documents reviewed by Fortune .  

As 2023 came to a close, the relationship between Mostaque and his investors officially turned antagonistic. Both Coatue and Lightspeed were off the board in the fall. Gaurav Gupta, the investor at Lightspeed, gave up his board observer role, due to what Bloomberg reported was differences of opinion over the company’s direction. Coatue gave up its board seat on Oct. 5, 2023, according to someone familiar with the matter, who specified it was because of a conflict arising from a new investment into Stability by chipmaker Intel . 

And on Oct. 24, Coatue went nuclear. In a letter to Stability’s leadership, Coatue called for Mostaque to step down and to undergo sale negotiations. The letter, which was first reported by Bloomberg and independently confirmed by Fortune, specifically demanded information about compensation paid to Mostaque, Peter O’Donoghue, Scott Trowbridge, and Zehra Qureshi and asked that future bonuses and payments be approved by non-executive board members. (In response to this, Mostaque said that “all related party transactions, payments, and bonuses have always been board-approved.”)

The situation was stunning, even to some of the people close to it. “This—where you’ve got folks resigning from the board right after a $100 million round… That level of souring is pretty uncommon—at least that early anyway,” one of Stability’s investors says. 

For those in Mostaque’s camp, Coatue’s gambit was viewed as an egregious betrayal and a power grab. “Emad is a visionary and the boss,” Joseph Jacks, one of Stability’s investors, wrote Fortune in an email. “He had shitty investors who tried to kick him out of his own company,” he said, noting that he was specifically referring to Coatue and Lightspeed.  

Despite its significant investment, however, Coatue couldn’t unseat Mostaque. Mostaque told Fortune this was because he retained board control, with a majority of the voting power, and that there was no mechanism for him to be removed. While Viswanath and Lightspeed’s Gupta were out of Stability’s boardroom, Mostaque remained on the board alongside Stability executives O’Donoghue and Trowbridge, and early investor Jim O’Shaughnessy, according to a source directly involved. 

But while Mostaque managed to hang on, the leaked vote of no confidence in his leadership made it difficult for the company to progress, especially as competition in the text-to-image-generation sector was heating up. Its competitor Midjourney was releasing updated versions of its rival text-to-image generator at a prodigious rate. Adobe had released a similar image-generation product. Facebook parent company Meta announced in September it had an image generator in the works, too, while Google said it planned to have a text-to-image model in the market soon, and OpenAI released DALL-E 3 in September.

can we travel in light speed

Stability did get investments from strategic investors—from both Intel, and later Supermicro, around the end of 2023, according to someone with knowledge of the matter. Still, a succession of Stability executives and employees were heading for the door. There were seemingly a number of reasons for the turnover. For example, Ed Newton-Rex, head of Stability’s audio model, left last year after objecting to the startup’s approach to copyright. He told Fortune that Stability’s official position on the kind of training data that models should use doesn’t line up with his own. He declined to go into specifics.

But the startup that approximately 18 months ago was at the forefront of the generative AI revolution now looked like it was being left behind. 

‘Nobody tells you how hard it is’

After the year of tension between Mostaque and his investors, the Stability AI Saturday announcement that Mostaque was stepping down came abruptly. 

Reached by email, Mostaque would not discuss the rupture of his relationship with Coatue and Lightspeed, or the investors’ campaign to unseat him, apart from specifying he had majority control of Stability shares. He cited the personal toll of running a startup while suggesting that he was preparing to jump into the fray with a new project.

“Nobody tells you how hard it is to be a CEO and there are better CEOs than me to scale a business,” Mostaque said. “I am not sure anyone else would have been able to build and grow the research team to build the best and most widely used models out there and I’m very proud of the team there. I look forward to moving onto the next problem to handle and hopefully move the needle.”

Mostaque said he had been thinking about leaving over the last month and will be working on a “new initiative in decentralized AI” that will focus on “national and science/sectoral models and the protocols and infrastructure to pull them together.”

As part of Mostaque’s resignation, Stability COO Shan Shan Wong and CTO Christian Laforte will serve as interim co-CEOs of the company as it looks to bring on a new chief executive. But Mostaque is leaving behind a shell of what Stability AI once was, whose value and future prospects are uncertain.

Clément Delangue, the CEO and cofounder of Hugging Face, an AI company that’s also backed by Coatue, tweeted on Friday: “Should we acquire Stability and open-source SD3?” 

According to a person with direct knowledge of the matter, there were behind-closed-doors founder-to-founder acquisition talks with Coatue-backed Hugging Face, but the deal apparently went nowhere. There have been other rumors and reports of possible acquirers, including Cohere, but sources say a sale of the company now would be tricky. 

One of Stability’s investors laments that even with Mostaque out of the picture, the wave of employee departures and the breakup with key investors mean that new backers are unlikely to sign on.

The irony is that a golden-boy startup CEO with a compelling vision is not so easy to replace. The investors who initially rushed to hook up with Mostaque, and who bet that he would lead them to the AI promised land—even when Mostaque briefly went missing back in 2022—are now searching for something else: where the company’s future really lies, in a crowded AI field where Stability has surrendered its early-mover advantage.

Though Stability AI’s models can still generate images of space unicorns and Lego burgers, music, and videos, the company’s chances of long-term success are nothing like they once appeared. 

“It’s definitely not gonna make me rich,” the investor says.

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  1. Three Ways to Travel at (Nearly) the Speed of Light

    1) Electromagnetic Fields. Most of the processes that accelerate particles to relativistic speeds work with electromagnetic fields — the same force that keeps magnets on your fridge. The two components, electric and magnetic fields, like two sides of the same coin, work together to whisk particles at relativistic speeds throughout the universe.

  2. Will Light-Speed Space Travel Ever Be Possible?

    The idea of travelling at the speed of light is an attractive one for sci-fi writers. The speed of light is an incredible 299,792,458 meters per second. At that speed, you could circle Earth more than seven times in one second, and humans would finally be able to explore outside our solar system. In 1947 humans first surpassed the (much slower ...

  3. How to Travel at (Nearly) the Speed of Light

    The theory of special relativity showed that particles of light, photons, travel through a vacuum at a constant pace of 670,616,629 miles per hour — a speed that's immensely difficult to achieve and impossible to surpass in that environment. Yet all across space, from black holes to our near-Earth environment, particles are, in fact, being ...

  4. Speed of light: How fast light travels, explained simply and clearly

    In fact, we now define the speed of light to be a constant, with a precise speed of 299,792,458 meters per second. While it remains a remote possibility in deeply theoretical physics that light ...

  5. Here's What Would Happen If You Could Travel at the Speed of Light

    "There are some important things you should probably know about approaching the speed of light," NASA's video, Guide to Near-light-speed Travel, explains. "First, a lot of weird things can happen ...

  6. How fast does light travel?

    The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light.

  7. NASA SVS

    Near-light-speed Travel Guide. This handy video will help acquaint you with the quirks of near-light-speed travel, expected travel times, and the distances to some popular (at least, we think so) destinations! Credit: NASA's Goddard Space Flight Center. Music: "The Tiptoe Strut" from Universal Production Music. Complete transcript available.

  8. Scientists Believe Light Speed Travel Is Possible. Here's How

    It's not faster-than-light travel; it's superluminal travel, thank you. Then there's nonphysical and physical—a.k.a. the critical distinction between theoretical speculation and something ...

  9. What Would Happen If You Traveled At The Speed Of Light?

    Can We Travel At The Speed Of Light? No, humans cannot survive traveling at the speed of light. When an object moves at the speed of light, its mass increases exponentially. For instance, the speed of light is 299,792 kilometers per second (186,282 miles per second), and when an object travels at this speed, it behaves as if it has infinite ...

  10. 3 Ways Fundamental Particles Travel at (Nearly) the Speed of Light

    So as far as we know, only small particles can get anywhere near the speed of light. One hundred years ago, on May 29, 1919, scientists performed measurements of a solar eclipse that confirmed ...

  11. Can Humans Even Reach 1% the Speed of Light Ever?

    At 1% the speed of light, it would take a little over a second to get from Los Angeles to New York. This is more than 10,000 times faster than a commercial jet. The Parker Solar Probe, seen here ...

  12. Why you can't travel at the speed of light

    This explains why nothing can travel faster than light - at or near light speed, any extra energy you put into an object does not make it move faster but just increases its mass. Mass and energy ...

  13. Have we made an object that could travel 1% the speed of light?

    The fastest things ever made by humans are spacecraft, and the fastest spacecraft reached 330,000 mph - only 0.05% the speed of light. But there are ways to go faster.

  14. What would you see if you could travel at the speed of light?

    Einstein's Special Theory of Relativity was born from this very question, and the answer is as weird as you'd expect.

  15. NASA's Guide to Near-light-speed Travel

    Before you fly to other galaxies, watch this video to learn about near-light-speed safety considerations, travel times and distances between popular destinations around the universe.

  16. Warp drives: Physicists give chances of faster-than-light space travel

    The fastest ever spacecraft, the now- in-space Parker Solar Probe will reach a top speed of 450,000 mph. It would take just 20 seconds to go from Los Angeles to New York City at that speed, but it ...

  17. Can anything travel faster than the speed of light?

    Huygens came up with a figure of 131,000 miles per second (211,000 kilometers per second), a number that isn't accurate by today's standards — we now know that the speed of light in the "vacuum ...

  18. Why does time change when traveling close to the speed of light? A

    If you were traveling in a rocket moving at 75% of the speed of light and your friend throws the ball at the same speed, you would not see the ball moving toward you at 150% of the speed of light ...

  19. Is Time Travel Possible?

    The Short Answer: Although humans can't hop into a time machine and go back in time, we do know that clocks on airplanes and satellites travel at a different speed than those on Earth. We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per ...

  20. Why did the bridge collapse and what is the death toll?

    what do we know about the bridge that collapsed? The Francis Scott Key Bridge was one of three ways to cross the Baltimore Harbor and handled 31,000 cars per day or 11.3 million vehicles a year.

  21. A Total Solar Eclipse Is Coming. Here's What You Need to Know

    The speed of the lunar shadow is slowest over spots with the longest totality. In April, the longest period of totality will occur over Durango, a state in Mexico, for a total of four minutes and ...

  22. Where Will Lightspeed Stock Be in 5 Years?

    Shares of Lightspeed Commerce (TSX:LSPD) popped yet again this week from news that the company was considering going private. It's been the latest from a long list of headlines from Lightspeed ...

  23. Inside Stability AI's bad breakup with Coatue and Lightspeed Venture

    Inside the $1 billion love affair between Stability AI's 'complicated' founder and tech investors Coatue and Lightspeed—and how it turned bitter within months