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

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.

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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Speed of Light [perfect visual explanations]

The speed of light is the Universal speed limit – nothing can travel faster than light . In the vacuum (commonly denoted c), its exact value is 299,792,458 meters per second (around 186,000 miles per second). In other words, if you could travel at the speed of light, you could go around the Earth 7.5 times in one second.

It might seem blazing fast, but, in fact, when you think of the vast distances between the celestial objects in the Universe, the speed of light is actually torturously slow.

For example, Alpha Centauri, the nearest star system to the Sun is 4.3 light-years away from Earth – the light emitted from them takes 4.3 years to reach us.

Related: Leaving Solar System at the Speed Of Light

Our Milky Way galaxy is around 150-200 thousand light-years in diameter. That means sending messages back and forth on either side of the galaxy would take hundreds of thousands of years. This is one of the reasons that there may be no Kardashev Type III civilization in the Universe (a civilization that can control its own galaxy – you can think of it as Isaac Asimov’s galactic empire in the Foundation series).

As Douglas Adams pointed out,  “ Space is big . Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.”

Even in our own solar system , the speed of light is so slow, communicating with spacecraft takes sometimes hours because of that. For example, it takes more than 21 hours for the signal to reach Voyager 1 (so it is more than 21 light hours away from the Earth).

Speed of Light: see how torturously slow it is

To put things into perspective, NASA Goddard Planetary Scientist James O’ Donoghue created three animations to show how fast (or how slow) the speed of light is.

The first animation shows the light orbiting the Earth. The equatorial circumference of Earth is 40,075 km (24,901 miles). If our planet had no atmosphere (air refracts and slows downlight a little bit), a photon skimming along its surface could lap the equator nearly 7.5 times every second.

The second animation shows the light is traveling between the Earth and the moon. The average distance between the Earth and the moon is 384,400 km (238,855 miles). It takes a little more than a second for a photon to cover that distance.

The third animation shows the light traveling between the Earth and Mars. Now the speed of lights starts looking really slow. And this is just Mars, one of the closest planetary bodies to Earth.

Please note that In theory, the closest that Earth and Mars would approach each other would be when Mars is at its closest point to the sun (perihelion) and Earth is at its farthest (aphelion). This would put the planets only 33.9 million miles (54.6 million kilometers) apart. However, this has never happened in recorded history. The closest recorded approach of the two planets occurred in 2003 when they were only 34.8 million miles (56 million km) apart.

It would take around 140 hours to reach the edge of the solar system a photon emitted by the Sun – see the previous article titled “ Leaving the solar system at the speed of light “.

What Star Trek’s warp speeds would actually look like with real distance, in real-time

Dr. James O’Donoghue published another video showing what Star Trek’s warp speeds actually look like with real distance, in real-time.

Related: Carl Sagan explains the speed of light while riding a bicycle

• How Fast Does Light Travel? | The Speed of Light on Space.com
• “The speed of light is torturously slow, and these 3 simple animations by a scientist at NASA prove it” on Business Insider
• The speed of Light on Wikipedia
• How Long Does It Take to Get to Mars? on Space.com
• Warp Drive on Wikipedia
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Three ways to travel at (nearly) the speed of light.

Katy Mersmann

1) electromagnetic fields, 2) magnetic explosions, 3) wave-particle interactions.

One hundred years ago today, on May 29, 1919, measurements of a solar eclipse offered verification for Einstein’s theory of general relativity. Even before that, Einstein had developed the theory of special relativity, which revolutionized the way we understand light. To this day, it provides guidance on understanding how particles move through space — a key area of research to keep spacecraft and astronauts safe from radiation.

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 accelerated to incredible speeds, some even reaching 99.9% the speed of light.

One of NASA’s jobs is to better understand how these particles are accelerated. Studying these superfast, or relativistic, particles can ultimately help protect missions exploring the solar system, traveling to the Moon, and they can teach us more about our galactic neighborhood: A well-aimed near-light-speed particle can trip onboard electronics and too many at once could have negative radiation effects on space-faring astronauts as they travel to the Moon — or beyond.

Here are three ways that acceleration happens.

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.

In essence, electromagnetic fields accelerate charged particles because the particles feel a force in an electromagnetic field that pushes them along, similar to how gravity pulls at objects with mass. In the right conditions, electromagnetic fields can accelerate particles at near-light-speed.

On Earth, electric fields are often specifically harnessed on smaller scales to speed up particles in laboratories. Particle accelerators, like the Large Hadron Collider and Fermilab, use pulsed electromagnetic fields to accelerate charged particles up to 99.99999896% the speed of light. At these speeds, the particles can be smashed together to produce collisions with immense amounts of energy. This allows scientists to look for elementary particles and understand what the universe was like in the very first fractions of a second after the Big Bang. 

Download related video from NASA Goddard’s Scientific Visualization Studio

Magnetic fields are everywhere in space, encircling Earth and spanning the solar system. They even guide charged particles moving through space, which spiral around the fields.

When these magnetic fields run into each other, they can become tangled. When the tension between the crossed lines becomes too great, the lines explosively snap and realign in a process known as magnetic reconnection. 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. Scientists suspect magnetic reconnection is one way that particles — for example, the solar wind, which is the constant stream of charged particles from the Sun — is accelerated to relativistic speeds.

Those speedy particles also create a variety of side-effects near planets.  Magnetic reconnection occurs close to us at points where the Sun’s magnetic field pushes against Earth’s magnetosphere — its protective magnetic environment. When magnetic reconnection occurs on the side of Earth facing away from the Sun, the particles can be hurled into Earth’s upper atmosphere where they spark the auroras. Magnetic reconnection is also thought to be responsible around other planets like Jupiter and Saturn, though in slightly different ways.

NASA’s Magnetospheric Multiscale spacecraft were designed and built to focus on understanding all aspects of magnetic reconnection. Using four identical spacecraft, the mission flies around Earth to catch magnetic reconnection in action. The results of the analyzed data can help scientists understand particle acceleration at relativistic speeds around Earth and across the universe.

Particles can be accelerated by interactions with electromagnetic waves, 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.

These types of interactions are constantly occurring in near-Earth space and are responsible for accelerating particles to speeds that can damage electronics on spacecraft and satellites in space. NASA missions, like the Van Allen Probes , help scientists understand wave-particle interactions.

Wave-particle interactions are also thought to be responsible for accelerating some cosmic rays that originate outside our solar system. After a supernova explosion, a hot, dense shell of compressed gas called a blast wave is ejected away from the stellar core. Filled with magnetic fields and charged particles, wave-particle interactions in these bubbles can launch high-energy cosmic rays at 99.6% the speed of light. Wave-particle interactions may also be partially responsible for accelerating the solar wind and cosmic rays from the Sun.

Download this and related videos in HD formats from NASA Goddard’s Scientific Visualization Studio

By Mara Johnson-Groh NASA’s Goddard Space Flight Center , Greenbelt, Md.

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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.

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.

Can anything travel faster than the speed of light?

Does it matter if it's in a vacuum?

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?

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."

Related: How long is a galactic year?

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."

— What would happen if the speed of light were much lower?

— What if the speed of sound were as fast as the speed of light?

— How does the rubber pencil illusion work?

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

University Distinguished Professor of Astronomy, University of Arizona

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Chris Impey receives funding from the National Science Foundation and the Hearst Foundation.

University of Arizona provides funding as a member of The Conversation US.

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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] .

Have we made an object that could travel at at least 1% the speed of light? – Anadi, age 14, Jammu and Kashmir, India

Light is fast . In fact, it is the fastest thing that exists, and a law of the universe is that nothing can move faster than light. Light travels at 186,000 miles per second (300,000 kilometers per second) and can go from the Earth to the Moon in just over a second. Light can streak from Los Angeles to New York in less than the blink of an eye.

While 1% of anything doesn’t sound like much, with light, that’s still really fast – close to 7 million miles per hour! 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 fastest things ever made

Bullets can go 2,600 mph (4,200 kmh), more than three times the speed of sound. The fastest aircraft is NASA’s X3 jet plane , with a top speed of 7,000 mph (11,200 kph). That sounds impressive, but it’s still only 0.001% the speed of light.

The fastest human-made objects are spacecraft. They use rockets to break free of the Earth’s gravity, which takes a speed of 25,000 mph (40,000 kmh). The spacecraft that is traveling the fastest is NASA’s Parker Solar Probe . After it launched from Earth in 2018, it skimmed the Sun’s scorching atmosphere and used the Sun’s gravity to reach 330,000 mph (535,000 kmh). That’s blindingly fast – yet only 0.05% of the speed of light.

Why even 1% of light speed is hard

What’s holding humanity back from reaching 1% of the speed of light? In a word, energy. Any object that’s moving has energy due to its motion. Physicists call this kinetic energy. To go faster, you need to increase kinetic energy. The problem is that it takes a lot of kinetic energy to increase speed. To make something go twice as fast takes four times the energy. Making something go three times as fast requires nine times the energy, and so on.

For example, to get a teenager who weighs 110 pounds (50 kilograms) to 1% of the speed of light would cost 200 trillion Joules (a measurement of energy). That’s roughly the same amount of energy that 2 million people in the U.S. use in a day.

How fast can we go?

It’s possible to get something to 1% the speed of light, but it would just take an enormous amount of energy. Could humans make something go even faster?

Yes! But engineers need to figure out new ways to make things move in space. All rockets, even the sleek new rockets used by SpaceX and Blue Origins, burn rocket fuel that isn’t very different from gasoline in a car. The problem is that burning fuel is very inefficient.

Other methods for pushing a spacecraft involve using electric or magnetic forces . Nuclear fusion , the process that powers the Sun, is also much more efficient than chemical fuel.

Scientists are researching many other ways to go fast – even warp drives , the faster-than-light travel popularized by Star Trek.

One promising way to get something moving very fast is to use a solar sail. These are large, thin sheets of plastic attached to a spacecraft and designed so that sunlight can push on them, like wind in a normal sail. A few spacecraft have used solar sails to show that they work, and scientists think that a solar sail could propel spacecraft to 10% of the speed of light .

One day, when humanity is not limited to a tiny fraction of the speed of light, we might travel to the stars .

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to [email protected] . Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

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What If You Traveled Faster Than the Speed of Light?

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When we were kids, we were amazed that Superman could travel "faster than a speeding bullet." We could even picture him, chasing down a projectile fired from a weapon, his right arm outstretched, his cape rippling behind him. If he traveled at half the bullet 's speed, the rate at which the bullet moved away from him would halve. If he did indeed travel faster than the bullet, he would overtake it and lead the way. Go, Superman!

In other words, Superman's aerial antics obeyed Newton's views of space and time : that the positions and motions of objects in space should all be measurable relative to an absolute, nonmoving frame of reference [source: Rynasiewicz ].

In the early 1900s, scientists held firm to the Newtonian view of the world. Then a German-born mathematician and physicist by the name of Albert Einstein came along and changed everything. In 1905, Einstein published his theory of special relativity , which put forth a startling idea: There is no preferred frame of reference. Everything, even time, is relative.

Two important principles underpinned his theory. The first stated that the same laws of physics apply equally in all constantly moving frames of reference. The second said that the speed of light — about 186,000 miles per second (300,000 kilometers per second) — is constant and independent of the observer's motion or the source of light. According to Einstein, if Superman were to chase a light beam at half the speed of light, the beam would continue to move away from him at exactly the same speed [source: Stein , AMNH.org ].

These concepts seem deceptively simple, but they have some mind-bending implications. One of the biggest is represented by Einstein's famous equation, E = mc², where E is energy, m is mass and c is the speed of light.

According to this equation, mass and energy are the same physical entity and can be changed into each other. Because of this equivalence, the energy an object has due to its motion will increase its mass. In other words, the faster an object moves, the greater its mass. This only becomes noticeable when an object moves really quickly. If it moves at 10 percent the speed of light, for example, its mass will only be 0.5 percent more than normal. But if it moves at 90 percent the speed of light, its mass will double [source: LBL.gov ].

As an object approaches the speed of light, its mass rises precipitously. If an object tries to travel 186,000 miles per second, its mass becomes infinite, and so does the energy required to move it. For this reason, no normal object can travel as fast or faster than the speed of light.

That answers our question, but let's have a little fun and modify the question slightly.

Almost As Fast As the Speed of Light?

We covered the original question, but what if we tweaked it to say, "What if you traveled almost as fast as the speed of light?" In that case, you would experience some interesting effects. One famous result is something physicists call time dilation , which describes how time runs more slowly for objects moving very rapidly. If you flew on a rocket traveling 90 percent of light-speed, the passage of time for you would be halved. Your watch would advance only 10 minutes, while more than 20 minutes would pass for an Earthbound observer [source: May ]

You would also experience some strange visual consequences. One such consequence is called aberration , and it refers to how your entire field of view would shrink down to a tiny, tunnel-shaped "window" out in front of your spacecraft. This happens because photons (those exceedingly tiny packets of light) — even photons behind you — appear to come in from the forward direction.

In addition, you would notice an extreme Doppler effect , which would cause light waves from stars in front of you to crowd together, making the objects appear blue. Light waves from stars behind you would spread apart and appear red. The faster you go, the more extreme this phenomenon becomes until all visible light from stars in front of the spacecraft and stars to the rear become completely shifted out of the known visible spectrum (the colors humans can see). When these stars move out of your perceptible wavelength, they simply appear to fade to black or vanish against the background.

Of course, if you want to travel faster than a speeding photon, you'll need more than the same rocket technology we've been using for decades.

In a March 2021 paper published in the journal Classical and Quantum Gravity , astrophysicist Erik Lentz of the University of Göttingen in Germany proposed the idea of rearranging space-time to create a warp bubble, inside which a spacecraft might be able to travel at faster-than-light speeds.

Speed of Light FAQ

Is there anything faster than the speed of light, how fast is the speed of light in miles, why is "c" the speed of light, what is the speed of light on earth, lots more information, related articles.

• Data Sent via Infrared Light Could Make WiFi Hundreds of Times Faster
• How Light Propulsion Will Work
• How Light Works
• American Museum of Natural History. "A Matter of Time. " Amnh.org. (Feb. 16, 2022) https://www.amnh.org/exhibitions/einstein/time/a-matter-of-time
• Brandeker, Alexis. "What would a relativistic interstellar traveler see?" Usenet Physics FAQ. May 2002. (Feb. 16, 2022J) http://www.desy.de/user/projects/Physics/Relativity/SR/Spaceship/spaceship.html
• Carl Sagan's Cosmos. "Travels in Space and Time." YouTube. Video uploaded Nov. 27, 2006 (Feb. 16, 2022 ) https://www.youtube.com/watch?v=2t8hUaaZVJg
• Hawking, Stephen. "The Illustrated Brief History of Time. " Bantam. 1996. (Feb. 16. 2022) https://bit.ly/367UGpZ
• EurekAlert! "Breaking the warp barrier for faster-than-light travel. " Eurekalert.org. March 9, 2021. (Feb. 16, 2022) https://www.eurekalert.org/news-releases/642756
• Lawrence Berkeley National Laboratory. "Mass, Energy, the Speed of Light – It's Not Intuitive! " Lbl.gov. 1996. (Feb. 16, 2022) https://www2.lbl.gov/MicroWorlds/teachers/massenergy.pdf
• Lemonick, Michael D. "Will We Ever Travel at the Speed of Light?" Time. Apr. 10, 2000. (Feb. 16, 2022), 2011) http://content.time.com/time/subscriber/article/0,33009,996616,00.html
• May, Andrew. "What is time dilation? " LiveScience. Nov. 17, 2021. (Feb. 16, 2022) https://www.livescience.com/what-is-time-dilation
• NOVA Physics + Math. "Carl Sagan Ponders Time Travel." NOVA. Oct. 12, 1999. (Feb. 16, 2022) http://www.pbs.org/wgbh/nova/physics/Sagan-Time-Travel.html
• Ptak, Andy. "The Speed of Light in a Rocket." NASA's Imagine the Universe: Ask An Astrophysicist. Jan. 2, 1997. (Feb. 16, 2022) http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970102c.html
• Rynasiewicz, Robert, "Newton's Views on Space, Time, and Motion."Stanford Encyclopedia of Philosophy. Summer 2014. (Feb. 16, 2022) https://plato.stanford.edu/cgi-bin/encyclopedia/archinfo.cgi?entry=newton-stm
• Stein, Vicky. "Einstein's Theory of Special Relativity. " Space.com. Sept. 20, 2021. (Feb. 16, 2022) https://www.space.com/36273-theory-special-relativity.html
• Van Zyl, Miezam (project editor)."Universe: The Definitive Visual Guide." Dorling Kindersley Limited. 2020. (Feb. 16, 2022) https://bit.ly/33q5Mpm.

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Science News

Climate change is changing how we keep time.

Melting ice sheets are slowing Earth's rotation speed, complicating global timekeeping

The rapidly accelerating melting of Earth’s polar ice sheets — including ice atop Greenland (shown here) — is slowing the planet’s spin, which affects global timekeeping.

KEREM YUCEL/AFP via Getty Images

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By Carolyn Gramling

March 27, 2024 at 1:34 pm

Climate change may be making it harder to know exactly what time it is.

The rapid melting of the ice sheets atop Greenland and Antarctica, as measured by satellite-based gravitational measurements, is shifting more mass toward Earth’s waistline. And that extra bulge is slowing the planet’s rotation , geophysicist Duncan Agnew reports online March 27 in Nature . That climate change–driven mass shift is throwing a new wrench into international timekeeping standards.

The internationally agreed-upon coordinated universal time, or UTC, is set by atomic clocks, but that time is regularly adjusted to match Earth’s actual spin. Earth’s rotation isn’t always smooth sailing — the speed of the planet’s spin changes depending on a variety of factors, including gravitational drag from the sun and the moon, changes to the rotation speed of Earth’s core, friction between ocean waters and the seafloor, and shifts in the planet’s distribution of mass around its surface. Even earthquakes can affect the spin: The magnitude 9.1 earthquake in Indonesia in 2004, for example, altered the land surface in such a way that it caused Earth to rotate a tiny bit faster, says Agnew, of the Scripps Institution of Oceanography in La Jolla, Calif.

But the impact of that quake is much smaller than that of the ice sheets’ melting — a point that Agnew says he finds particularly startling. Humankind “has done something that affects, measurably, the rotation rate of the entire Earth.”

The need for occasional tweaks to the synchronization of atomic clocks and Earth’s rotation gave birth in 1972 to the “leap second ,” an extra tick that international timekeepers agreed to add to UTC as needed ( SN: 1/19/24 ). Timekeepers have added 27 leap seconds to the clock since the idea was introduced.

Still, metrologists — measurement scientists — aren’t overly fond of this system. For one thing, it doesn’t happen on a regular schedule, but only whenever it seems to be needed. And financial markets and satellite navigation systems, which rely on precise timing, each have their own methodologies for incorporating a leap second. Those inconsistencies can, counterproductively, make it more challenging to have a universal time. So in 2022, an international consortium of metrologists voted to do away with leap seconds in favor of adding larger chunks of time, perhaps a minute, less frequently. The group resolved to settle those details at its next meeting, in 2026.

That may not come a second too soon. The slightly slower rotation has actually delayed the need for timekeeping adjustments by a few years, Agnew says — in fact, as a result of this change, the last time a leap second was required to be inserted was in 2016. At the moment, in fact, Earth’s rotation and atomic clocks are nearly in sync.

But that’s just a brief respite, Agnew’s calculations show. The biggest changes to Earth’s rotation right now are coming from its heart: slowing rotation of Earth’s core is actually speeding up the spin of the outer layers ( SN: 1/23/23 ). That slowdown will ultimately mean that timekeepers, under the current system, must begin removing leap seconds from the UTC, rather than inserting them, to keep things in sync.

That shift in strategy might have begun as soon as in 2026. But the study suggests that, thanks to climate change, global timekeepers now have an extra two or three years before they need to adjust, notes geophysicist Jerry Mitrovica of Harvard University. But no realistic projections of future melting can forestall the inevitable beyond 2030, Mitrovica adds: One way or another, the world is going to have to start losing time — or international timekeeping guidelines will need to change.

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Total solar eclipse April 8, 2024 facts: Path, time and the best places to view

In the U.S., 31 million people already live inside the path of totality.

Scroll down to see the list of U.S. cities where the April 8 total solar eclipse will be visible, the duration of the eclipse in those locations and what time totality will begin, according to GreatAmericanEclipse.com .

"Eclipse Across America," will air live Monday, April 8, beginning at 2 p.m. ET on ABC, ABC News Live, National Geographic Channel, Nat Geo WILD, Disney+ and Hulu as well as network social media platforms.

On April 8, 2024, a historic total solar eclipse will cast a shadow over parts of the United States, prompting a mass travel event to the path of totality -- from Texas to Maine and several states and cities in between.

A total solar eclipse occurs when the moon passes between the sun and the Earth and, for a short time, completely blocks the face of the sun, according to NASA .

The track of the moon's shadow across Earth's surface is called the path of totality, and to witness the April 8 total solar eclipse, viewers must be within the 115-mile-wide path. To discover when to see the solar eclipse in totality or the partial eclipse in locations across the U.S. outside of the path, check out NASA's Eclipse Explorer tool .

Eclipse travel

In the U.S., 31 million people already live inside the path of totality, bringing the celestial phenomenon to their doorsteps, Michael Zeiler, expert solar eclipse cartographer at GreatAmericanEclipse.com told ABC News.

MORE: Eclipse glasses: What to know to keep your eyes safe

But for individuals outside of the path, investing time and money are needed to experience the event in totality.

Eclipse chasers, or umbraphiles, are individuals who will do almost anything, and travel almost anywhere, to see totality, according to the American Astronomical Society .

"There's a very active community of solar eclipse chasers and we will go to any reasonable lengths to see solar eclipses anywhere in the world," Zeiler said. "All of us are united in pursuing the unimaginable beauty of a total solar eclipse."

MORE: The surprising reason why a Texas county issued a disaster declaration ahead of April total solar eclipse

Bringing together both eclipse experts and novice sky watchers, the total solar eclipse on April 8 is projected to be the U.S.'s largest mass travel event in 2024, according to Zeiler, who likened it to "50 simultaneous Super Bowls across the nation."

"When you look at the number of people expected to come to the path of totality for the solar eclipse, we estimate those numbers are roughly the equivalent of 50 simultaneous Super Bowls across the nation, from Texas to Maine," he said.

Eclipse map, path of totality

In the U.S., the path of totality begins in Texas and will travel through Oklahoma, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire and Maine. Small parts of Tennessee and Michigan will also experience the total solar eclipse, according to NASA.

Best times, places to view eclipse

Below is a list of some American cities where the April 8 total solar eclipse will be most visible -- pending weather forecasts -- the duration of the eclipse in those locations and what time totality will begin, according to GreatAmericanEclipse.com.

• Eagle Pass, Texas, 1:27 p.m. CDT: 4 minutes, 23 seconds
• Uvalde, Texas, 1:29 p.m. CDT: 4 minutes, 16 seconds
• Kerrville, Texas, 1:32 p.m. CDT: 4 minutes, 23 seconds
• Austin, Texas, 1:36 p.m. CDT: 1 minute, 53 seconds
• Killeen, Texas, 1:36 p.m. CDT: 4 minutes, 17 seconds
• Fort Worth, Texas, 1:40 p.m. CDT: 2 minutes, 34 seconds
• Dallas, Texas, 1:40 p.m. CDT: 3 minutes, 47 seconds
• Little Rock, Arkansas, 1:51 p.m. CDT: 2 minutes, 33 seconds
• Jonesboro, Arkansas, 1:55 p.m. CDT: 2 minutes, 24 seconds
• Poplar Bluff, Arkansas, 1:56 p.m. CDT: 4 minutes, 8 seconds
• Cape Girardeau, Missouri, 1:58 p.m. CDT: 4 minutes, 6 seconds
• Carbondale, Illinois, 1:59 p.m. CDT: 4 minutes, 8 seconds
• Mount Vernon, Illinois, 2:00 p.m. CDT: 3 minutes, 40 seconds
• Evansville, Indiana, 2:02 p.m. CDT: 3 minutes, 2 seconds
• Terre Haute, Indiana, 3:04 p.m. EDT: 2 minutes, 57 seconds
• Indianapolis, Indiana, 3:06 p.m. EDT: 3 minutes, 46 seconds
• Dayton, Ohio, 3:09 p.m. EDT: 2 minutes, 46 seconds
• Wapakoneta, Ohio, 3:09 p.m. EDT: 3 minutes, 55 seconds
• Toledo, Ohio, 3:12 p.m. EDT: 1 minute, 54 seconds
• Cleveland, Ohio, 3:13 p.m. EDT: 3 minutes, 50 seconds

Pennsylvania

• Erie, Pennsylvania, 3:16 p.m. EDT: 3 minutes, 43 seconds
• Buffalo, New York, 3:18 p.m. EDT: 3 minutes, 45 seconds
• Rochester, New York, 3:20 p.m. EDT: 3 minutes, 40 seconds
• Syracuse, New York, 3:23 p.m. EDT: 1 minute, 26 seconds
• Burlington, Vermont, 3:26 p.m. EDT: 3 minutes, 14 seconds
• Island Falls, Maine, 3:31 p.m. EDT: 3 minutes, 20 seconds
• Presque Island, Maine, 3:32 p.m. EDT: 2 minutes, 47 seconds

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24/7 coverage of breaking news and live events

Human-driven climate change has 'slowed the Earth's rotation' and could affect how we measure time, study suggests

The melting of ice in Greenland and Antarctica is said to have slowed the rotation of the Earth because it has changed where the planet's mass is concentrated.

Wednesday 27 March 2024 21:24, UK

The melting of polar ice due to human-driven climate change has slightly slowed the Earth's rotation - and it could affect how we measure time, according to a study.

Although the disappearance of the ice has reduced the speed of the planet's rotation, the Earth is still spinning a bit faster than it used to.

The overall increase in speed means that for the first time in history, world timekeepers may have to consider subtracting a second from our clocks.

This means clocks may have to skip a second - called a "negative leap second" - around 2029 to keep universal time in sync with the Earth's rotation, according to the study published in the Nature journal.

If it wasn't for the impact of melting ice, the time change would have been needed three years earlier in 2026.

In recent decades, the Earth has rotated faster due to changes in its core but the melting ice has counteracted this burst of speed.

World rotation is like a figure skater twirling

Duncan Agnew, the author of the study and a geophysicist at the Scripps Institution of Oceanography at the University of California, says the ice melting in Greenland and Antarctica has changed where the Earth's mass is concentrated.

This has slowed down the Earth's rotation as less solid ice at the northern and southern areas of the planet means there is more mass around the equator, the study suggests.

Mr Agnew has used the example of a figure skater twirling on ice to explain this.

He told Sky News' US partner network NBC News: "If you have a skater who starts spinning, if she lowers her arms or stretches out her legs, she will slow down."

However, if a skater's arms are drawn inward this means she will twirl faster.

Read more: New experiment to search for mysterious particles Study reveals you need to exercise to avoid insomnia US state bans under-14s from social media

The melting of ice in Greenland and Antarctica is an accelerating trend which is said to be driven primarily by human-caused climate change.

This means humans may be causing the Earth to spin less quickly than it otherwise would be.

Mr Agnew continued: "It's kind of impressive, even to me, we've done something that measurably changes how fast the Earth rotates.

"Things are happening that are unprecedented."

The melting of polar ice would be a new factor that affects the Earth's spin.

The friction of ocean tides, due in part to the moon's gravitational pull, slows the Earth's rotation.

Meanwhile, the movement of fluid within the Earth's liquid inner core can either speed or slow down how fast the planet rotates, Mr Agnew said.

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How the Key Bridge Collapsed in Baltimore: Maps and Photos

By Weiyi Cai ,  Agnes Chang ,  Lauren Leatherby ,  Lazaro Gamio ,  Leanne Abraham and Scott Reinhard

On Tuesday, a major bridge in Baltimore collapsed into the water seconds after it was struck by a cargo ship, sending vehicles on the bridge into the river below. The ship lost power and issued a mayday call shortly before it hit the bridge.

The ship, a 948-foot-long cargo vessel called Dali, was about a half hour into its journey toward Colombo, Sri Lanka, when it hit a main pillar of the bridge. All crew members are safe, according to the ship’s owners.

Follow our live coverage .

A mayday call from the ship gave officials enough time to stop traffic at both ends of the bridge. The waters where the bridge collapsed are about 50 feet deep. By Tuesday morning, six construction workers who had been fixing potholes on the bridge remained missing as divers and other emergency workers on boats and helicopters continued to search for them. Two others had been rescued, and one was in the hospital.

Francis Scott

Patapsco River

The ship left the Port

of Baltimore around

1 a.m. on Tuesday.

Where impact occurred

Direction of the ship

The ship hit the

bridge at 1:28 a.m.

The ship hit the bridge at 1:28 a.m.

Where impact

Source: Spire Global

The New York Times; satellite image by Google Earth

The lights of the ship flickered on and off as it lost power in the minutes before the ship changed bearing and hit the bridge.

Ship approached from

the Port of Baltimore

Road repair crews

Ship changed heading

as it neared pillar

Ship hit pillar

Southern and central spans

of bridge began to collapse within

seconds of impact

Northern span began to

collapse seconds later

Within 30 seconds of impact,

the central part of bridge had

entirely collapsed.

Source: StreamTime Live via YouTube

Timestamps are from StreamTime Live video.

The New York Times

The Francis Scott Key Bridge was opened in 1977 and carried more than 12.4 million vehicles last year. The bridge was one of the three major ways to cross the Patapsco River and formed part of Baltimore’s beltway.

The Port of Baltimore is a major trade hub that handled a record amount of foreign cargo last year. It is an especially important destination — the nation’s largest by volume last year — for deliveries of cars and light trucks.

Ship impact

To Chesapeake Bay

Sources: Maryland Port Administration, OpenStreetMap, MarineTraffic

Note: Ship positions are as of 2:46 p.m. Eastern time.

Overall, Baltimore was the 17th biggest port in the United States in 2021, ranked by total tons, according to the Bureau of Transportation Statistics. The bridge collapse brought marine traffic there to a standstill, with seven cargo or tanker ships stranded in the harbor as of Tuesday afternoon.

Gov. Wes Moore declared a state of emergency for Maryland and said that his office was in close communication with Pete Buttigieg, the U.S. transportation secretary. The White House issued a statement saying that President Biden had been briefed on the collapse.

Erin Schaff/The New York Times

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