Traveling at the speed of light has long fascinated sci-fi fans, but the physics required leave little room for optimism.
Why can’t we go faster than the speed of light?
It’s easy to bristle at limits, and this one has long been a source of ire for science fiction fans, my younger self included. As a kid, I was disappointed to learn that travel to other stars would take years or centuries; today, I’m still struck by how many people are unhappy about the hard speed limit promised by physics. Where does the light-speed boundary come from, and is there any way around it?
The short answer is no. We can’t go faster than light. It’s not even clear why we’d want to. Light travels just shy of 300,000 kilometers per second in a vacuum; that’s plenty fast for almost any purpose. It’s true that the speed of light isn’t fast compared to the distance to the stars, but relativity’s warping of space and time mean that any journey taken near the speed of light – usually abbreviated as c by physicists – will take you much less time than you might expect.
Alpha Centauri, the closest star system to our own, is a little over four light-years away. Therefore, if you traveled there at 99.9% c, you might think it would take you a little over four years to get there. That’s exactly what your friends on Earth would see as they followed your progress through their telescopes. From your perspective, however, the trip would only take a little over two months; as you traveled, the distance to Alpha Centauri would shrink by more than twenty-fold thanks to special relativity.
Traveling at (or near) the speed of light
But reaching 99.9% c is difficult – there’s no existing technology to propel a spaceship that fast, simply owing to the immense energy requirements.
The Project Mercury capsule Freedom 7, one of the smallest crewed vessels in the history of spaceflight, had a mass of about 1,000 kg (including Alan Shepard, the American astronaut inside of it). Accelerating that capsule up to 99.9% c would require all of the electrical power produced globally for 25 years straight. Pushing Freedom 7 up to 99.99% c would take another 50 years of energy production on top of that. And relativity dictates that adding the final 0.01% of c on top of that is impossible – it would require infinite energy to get any object with any mass all the way up to the speed of light itself.
If you did somehow get yourself going at the speed of light, you’d find even less reason to go faster. From the perspective of anyone traveling at c, all distances shrink to zero. You would occupy every point in the universe along your path simultaneously, and travel to any location would be instantaneous. Time wouldn’t pass for you at all.
Light speed, then, seems like it should be enough for anyone. And since you can’t actually get to c, you certainly can’t go faster than c. But say you managed it anyhow. What would happen if you did somehow go faster than light?
No going back: The paradoxes of traveling at the speed of light
Before you embark on your superluminal journey, you’d have to say goodbye to your friends and family forever, because you’re never going to get back down below the speed of light again. According to relativity, just as it takes an infinite amount of energy to get something below the speed of light to c, it also takes an infinite amount of energy to get an object going faster than light to slow down to c. If you’re traveling faster than light, you’re stuck there. If you lose energy, that will just make you speed up, rather than slowing you down.
This leads straight to another problem. An object going faster than light will emit radiation – like a luminous equivalent of a sonic boom – causing it to lose energy. That energy loss will then make it go faster, making it give off even more radiation, which speeds it up further, creating a feedback loop that drives it up to infinite speed, radiating an infinite amount of energy. This paradoxical situation would, presumably, be extraordinarily unpleasant for you and anyone else in your cosmic vicinity.
If you somehow manage to avoid that problem, another form of paradox awaits you. Relativity dictates that going faster than light is tantamount to going back in time. This holds for anything at all going faster than light, even massless information. A device allowing for superluminal signaling – which has the gorgeously science-fictional name of a “tachyonic antitelephone” – would immediately allow you to send messages to your past self, including messages telling you not to send them in the first place.
But it is possible for some things to travel faster than light, depending on your definition of “thing.” Imagine pointing an incredibly powerful laser pointer at Jupiter from the Earth, one that’s somehow powerful enough to produce a visible dot on Jupiter’s surface when you look through a telescope. If you twitch your fingers, that dot would move back and forth across the face of Jupiter faster than the speed of light.
That dot isn’t a real physical object, though; it’s just a location that happens to be reflecting light from your laser pointer. And that dot can’t carry information from one side of Jupiter to the other, because the light is coming from right here on Earth. So having it “travel” from one edge of Jupiter’s surface to the other faster than light is perfectly allowed by the laws of physics, and doesn’t present any possibility of paradox.
The speed of light, special relativity, and general relativity
This is just about all that special relativity has to offer the prospective superluminal traveler. But special relativity isn’t really the right theory for describing spacetime in most realistic situations – it only holds perfectly true in special circumstances, hence the name.
General relativity, Einstein’s theory of gravitation, is the more complicated and comprehensive sibling of special relativity, and while most of the relevant results about light-speed travel hold in general relativity too, the theory does present a few alternative potential methods for traveling faster than the speed of light.
In general relativity, spacetime warps and bends and expands, all under the influence of matter and energy. General relativity also explains the expansion of our universe. That expansion seems to provide a way for things to travel superluminally: most galaxies in the universe today are being carried away from each other faster than the speed of light by the expansion of the universe.
But that doesn’t mean they’re actually traveling faster than light. It just means that the amount of space between them is increasing faster than light can travel. In general relativity, spacetime can expand arbitrarily fast, but objects still can’t move within spacetime faster than light relative to their local environment.
What about warp drive and wormholes?
General relativity does have two other tricks which have become reliable science-fiction tropes: warp drive and wormholes. But these don’t function like their fictional analogues, and don’t really allow for travel faster than light.
In general relativity, both warp drives and wormholes require the existence of “exotic” matter that has negative mass, something that has never been observed in nature. There are also other theoretical arguments that strongly suggest that these phenomena can’t actually be used to travel faster than light, independently of the requirement for exotic matter.
The speed of light and a theory of quantum gravity
Yet general relativity may not be the final word in physics. Physicists have long suspected that general relativity will eventually have to be replaced by a theory of quantum gravity. Would such a theory provide a new loophole, one that allows us to go faster than the speed of light?
Maybe. But it still seems implausible, simply because of how well-tested these particular results preventing faster-than-light travel have been. Subatomic particles in accelerators like the Large Hadron Collider are pushed very close to c on a regular basis, and the energy needed to do so increases in perfect accord with relativity. Nobody has ever found an experimental result that violates either the special or general theory of relativity. Therefore, it seems unlikely that a theory of quantum gravity would allow for a massive violation like faster-than-light travel.
In fact, it’s entirely possible that a theory of quantum gravity would provide stricter speed limits in some situations. There’s precedent for this: before relativity, there was little reason to suspect that going faster than light was a problem at all. Revolutions in science can sometimes present new limitations that were previously unknown. And given how many tests relativity has been subjected to, the smart money would be very much against humanity ever finding a way to go faster than light.
Adam Becker is a science journalist with a PhD in astrophysics. He has written for The New York Times, the BBC, NPR, Scientific American, New Scientist, Quanta, and other publications. He is the author of two books, What Is Real? and the forthcoming More Everything Forever. He lives in California.
