Can Anything Travel Faster Than Light?

As far as anyone has ever measured, no: no object and no message has been clocked moving through space faster than light. In empty space, light travels at 299,792,458 meters per second. Since 1983, that exact number has been used to define the meter itself. Einstein’s special relativity, from 1905, explains why the limit holds: the closer anything with mass gets to light speed, the more energy it takes to go faster, without end.

Still, “nothing is faster than light” hides some real surprises. A shadow can sweep across a wall faster than light. The distance to very faraway galaxies grows faster than light, because space itself is stretching. None of these carries a traveler or a message past a beam of light. That difference, between a pattern that moves and a signal that travels, is the heart of the question.

Why can’t a spaceship just keep speeding up?

Relativity says the energy of a moving object climbs faster and faster as its speed nears the speed of light. Right at light speed, the energy needed would be infinite, so nothing with mass can be pushed all the way there, as the Physics FAQ on faster-than-light travel explains. Particles with no mass, like particles of light (photons), are different, because they always travel at exactly light speed through empty space, never slower.

Think of a hill that gets steeper the higher you climb, until near the top it turns into a wall that never ends. You can keep climbing, but you never arrive. Particle accelerators show this every day: CERN’s Large Hadron Collider pushes protons around a 27-kilometer ring about 11,245 times a second, close to the speed of light. Near that limit, each extra push adds a lot of energy but almost no speed.

There is a deeper reason, too: in relativity, a signal that went faster than light would, for some observers, arrive before it was sent. That would open the door to messages into the past, and to paradoxes of cause and effect. This puzzle is one of the strongest arguments against faster-than-light travel.

So what does go “faster than light”?

Quite a few things seem to, but none of them breaks the rule:

• Shadows and light spots. Sweep a powerful laser across the Moon, and its spot can race across the surface faster than light, yet no object travels from one side to the other. It is like a stadium “wave”: the wave can move faster than any fan could run, yet each fan only stands up and sits down.
• Wave crests. Inside a group of waves, the individual ripples can have a “phase velocity” faster than light, but no information travels faster than light.
• Light slowed down. In water, light travels at only about 75 percent of its top speed. Fast charged particles from nuclear fuel can outrun it there, and they give off a blue glow called Cherenkov radiation, a kind of “sonic boom” made of light. They still never beat light’s speed in empty space.
• Stretching space. The distance to galaxies far enough away grows faster than light. Cosmologists have shown that we can even see some of these galaxies. This does not break relativity, because the galaxies are not racing through space; the space between us is growing. Our guide to how big the universe is explores what this means.
• Entangled particles. Measurements on two linked particles can match up instantly, even far apart, but this effect cannot be used to send a message. See what quantum entanglement is.

What happened when neutrinos seemed to beat light?

In September 2011, the OPERA experiment in Italy reported a startling result. Neutrinos (ghostly particles that pass through almost anything) had been sent 730 kilometers from CERN, near Geneva, to Italy’s Gran Sasso laboratory. Based on more than 15,000 events, the neutrinos seemed to travel about 20 parts per million faster than light. The team did not claim a revolution; instead, it asked other scientists to check its work.

They did, and in February 2012, OPERA reported two possible faults in its timing system. One was a fiber-optic connector that carried the GPS time signal to the experiment’s master clock and may not have been working properly. In March, a neighboring experiment, ICARUS, timed seven neutrinos and found their arrival consistent with light speed. By June 2012, four experiments at Gran Sasso agreed: the neutrinos kept to the cosmic speed limit. CERN said the original result could be traced to a faulty part of OPERA’s fiber-optic timing system.

That story is science working well: a surprising claim was shared openly, tested by rivals and corrected. It is a useful model for checking any viral science claim.

What happens to travelers who get close to light speed?

Nature offers a different deal: you cannot beat light, but if you travel close to its speed, less time passes for you. Relativity says that at 80 percent of light speed, a ship’s clocks run at three-fifths the rate of clocks back home, as measured from Earth. Leaving out the time spent speeding up and slowing down, a trip that takes 10 years by Earth calendars would take only 6 years on board. The crew would see it differently: to them, the distance between the stars would shrink.

This is not just math. In 2010, scientists at NIST compared two extremely precise atomic clocks. In one clock, they made the single charged atom that keeps time jiggle back and forth, at speeds like a car driving about 20 miles per hour. That clock ticked slightly slower, just as relativity predicts. The effect is tiny at everyday speeds, but it is real.

What don’t we know yet?

The biggest open door is a warp drive, which on paper might get around the rule. In 1994, physicist Miguel Alcubierre used Einstein’s theory of gravity to describe a “warp” bubble. Space would shrink in front of a ship and expand behind it. Inside the bubble, the ship would never outrun light locally, but it could cross great distances quickly. The catch is large: his idea needs “exotic matter” with negative energy. No one knows whether such matter can exist in the amounts needed, or how it could be put to work.

NASA studied daring ideas like this from 1996 to 2002. A 2004 report on that work said such goals were presumably far from being reached, and perhaps impossible. Hypothetical faster-than-light particles called tachyons have never been found either, and most physicists doubt they exist. Einstein’s theory of gravity also allows other strange shapes of space and time, such as wormholes. Nobody knows whether nature permits the exotic ingredients these would need. For now, warp drives remain a fascinating idea from theory, not a technology on the way.

What would change the answer?

One clean result could change it: a signal that clearly arrives before light could, confirmed again and again by independent teams. It would have to survive every check that OPERA’s result failed. As CERN noted in 2011, many searches for small breaks in relativity have found none so far. A full theory joining gravity with quantum physics could also reshape the question, as our guide to a theory of everything explains. So the honest answer today is simple: patterns, shadows and stretching space can outpace light, but messages and travelers, as far as we know, cannot.

If this leaves you wondering what light speed means for meeting anyone else out there, you are in good company. Keep asking: what would it take for the stars to feel a little closer?