Can anything travel faster than light?

The full story

The episode's narration, word for word. Numbers link to the sources below; timestamps play that moment.

Punch it 0:00

In Star Wars, ships don't cruise between the stars; they jump to hyperspace ¹. The franchise's own databank, describing its fictional galaxy, says hyperdrives let starships travel faster than the speed of light, through an alternate dimension called hyperspace ¹. Nearly every space story needs some version of that trick. So here's the season's last question: can anything really travel faster than light?

Space is too big 0:30

The nearest star after the Sun, Proxima Centauri, is four and a quarter light years away ². Light makes that trip in four and a quarter years ². The Voyager 1 spacecraft is travelling away from the Sun at about seventeen kilometres a second ². At that speed, by our calculation, it would need more than seventy-three thousand years ². No wonder screenwriters reach for a shortcut.

Two weeks of no 0:58

One director did ask for that shortcut ³. When Christopher Nolan was making Interstellar, he wanted a character to travel faster than light ³. His scientific consultant was Kip Thorne, a physicist at Caltech who had helped conceive the film ^4,5. Nolan has said that Thorne spent about two weeks beating him down, explaining it was absolutely not possible, until he conceded ³. Why was Thorne so sure?

The energy wall 1:28

The first reason is energy ⁶. In Newton's physics, doubling your speed quadruples your energy of motion, and nothing stops you going faster ⁶. But Einstein's relativity adds a factor, called gamma, that grows without limit as you approach light speed ^6,7. At ninety-nine per cent of light speed, by our calculation, a thing's energy of motion is about six times the energy locked in its own mass ⁶. At ninety-nine point nine per cent, it's twenty-one times ⁶. Reaching light speed itself would take infinite energy, so nothing with mass ever gets there ⁶.

Speeds don't add 2:06

Then why not stack speeds? Fly at half light speed and fire a probe forward at three quarters of light speed ⁸. Common sense says one and a quarter times light speed ⁸. Relativity says ninety-one per cent, because speeds combine by a formula that never passes the speed of light ⁸. Add light speed to anything and you still get light speed ⁸.

How we know 2:32

This isn't just theory ⁷. Particles called muons, made high in the atmosphere by cosmic rays, live longer when they move fast, by exactly the amount relativity predicts ⁷. And in 1971, the physicists Hafele and Keating flew atomic clocks around the world on airliners, and the clocks came back out of step, as relativity said they would ⁷.

Effect before cause 2:58

The deeper reason is cause and effect ⁹. Draw every place a light flash from here and now could reach, and you get a cone in space and time ⁹. Anything inside that cone, you can influence ⁹. Events outside it are the strange ones: different observers, moving differently, disagree on which came first ⁹. So a signal that jumped outside the cone would, for some observers, arrive before it was sent ⁹. That breaks cause and effect, and that's why physicists treat light speed as a true limit, not just a hard target ⁹.

Sixty nanoseconds 3:33

In 2011, it looked like something had broken the rule ¹⁰. The OPERA experiment fired neutrinos from CERN to an underground lab at Gran Sasso, about seven hundred and thirty kilometres away ¹⁰. They seemed to arrive about sixty billionths of a second early ¹⁰. By early 2012, OPERA had found the main culprit: a badly connected optical fibre in its timing system, adding about seventy-four billionths of a second ¹⁰. A neighbouring detector on the same beam, ICARUS, found the neutrinos arriving just as light would ¹¹. Corrected, OPERA's neutrinos matched light too ¹⁰.

Faster, but not really 4:19

There are things that look faster than light, but they never carry a message through space faster than light ^12,13. Distant galaxies are moving away from us faster than light, because the space between us is expanding ¹⁴. Nothing is outrunning a light beam where it is; space itself is stretching ^12,14. And entangled particles share strange correlations, but you can't use them to send a message ¹³. Hold on to that loophole about space, because it's the one a warp drive uses ¹².

Cardiff, 1994 4:51

In 1994, a physicist at the University of Wales in Cardiff, Miguel Alcubierre, published a five-page paper in the journal Classical and Quantum Gravity ¹². He'd found a shape of space and time, allowed by Einstein's equations, that could carry a ship between stars as fast as you like ¹². He wrote that it begs to be given the familiar name of the warp drive of science fiction ¹².

Riding a wave of space 5:20

Here's the trick ¹². Shrink space in front of the ship and stretch it out behind ¹². The ship sits in a calm bubble in between and is carried along, like someone standing still on a moving walkway ¹². Inside the bubble it never passes a light beam, so locally the rule holds ¹². The crew would feel no acceleration and suffer no time dilation ¹². The walkway analogy breaks in one place: nothing is moving through space here; space itself is doing the moving ¹².

The bill 5:53

But Alcubierre spotted the catch himself: the bubble needs negative energy, less than empty space ¹². Quantum physics does allow negative energy in some special circumstances, so he didn't rule it out ¹². In 1997, Michael Pfenning and Larry Ford worked out the minimum bill for a bubble a hundred metres in radius, moving at light speed ¹⁵. From their figures, it comes to at least three hundred billion billion galaxies' worth of negative energy ¹⁵. A later redesign by Chris Van Den Broeck cut that to about the mass of the Sun ^16,17: still negative, still far beyond anything anyone knows how to make.

Still arguing 6:37

Physicists are still arguing ^18,19. In 2021, a few papers claimed warp bubbles made from ordinary, positive energy, and one claimed it could even go faster than light ^18,20. In 2022, Jessica Santiago, Sebastian Schuster and Matt Visser replied that every physically reasonable warp drive still breaks the energy conditions ¹⁹. And Alcubierre noted that a similar trick could probably be bent into a loop through time ^12,21. On today's physics, the warp drive is a beautiful equation with no fuel.

Miller's planet 7:14

So what did Thorne give Nolan instead ³? In Interstellar, astronauts land on Miller's planet, a world of shallow ocean swept by enormous waves, orbiting a giant black hole called Gargantua ⁴. Every hour on that planet costs seven years back home ⁴. According to Thorne's own book, as reported by the astrophysicist Jean-Pierre Luminet, Nolan's demand for that time loss was non-negotiable ²². Thorne did a few hours of calculations and found it was just possible, though very unlikely ²².

One hour, seven years 7:51

Let's take the film at its word ⁴. Seven years is about sixty-one thousand hours, so the planet's clocks run about sixty-one thousand times slower ⁴. By our calculation, a single second there is seventeen hours back home ⁴. Stay for three hours, and twenty-one years pass for everyone else ⁴. That's not faster-than-light travel; it's time running slow, deep in gravity, which relativity really does allow ²³.

Why it must spin 8:21

But a planet can only orbit so close ^24,25. Around a black hole that doesn't spin, the closest stable orbit slows a clock by a factor of about one point four, not sixty-one thousand ^24,25. A spinning black hole lets orbits creep much closer to its edge ^25,26. Using the equations for a spinning black hole, we calculated how fast Gargantua must spin: within about one part in a hundred trillion of the fastest spin a black hole can have ²⁴. That's for a planet on the closest stable orbit ²⁴. Jean-Pierre Luminet, using a simpler formula for a clock hovering in place, got about one part in ten billion ^4,22. Either way, Gargantua has to spin almost impossibly close to the limit ^4,24. Even at ninety-nine point nine per cent of the maximum, about where nature is thought to put on the brakes, the slowdown is only about fourteen times ^27,24.

The cheat they chose 9:22

And the filmmakers knew ²⁷. In a 2015 paper, Thorne and the film's visual effects team wrote that Gargantua had to spin very fast to explain the time losses ²⁷. But Nolan and the effects supervisor Paul Franklin drew it spinning more slowly on screen, so its shape wouldn't confuse audiences ²⁷. That's an honest cheat: the story's physics, and a picture people could read. Two years later, Thorne shared a Nobel Prize for his part in detecting gravitational waves ²⁸.

So, can anything? 9:56

So, can anything travel faster than light? Through space, where you are, no: it would take infinite energy, and it would break cause and effect ^6,9. Space itself can stretch faster, but riding that stretch needs a kind of matter no one has ever made in bulk ^12,15,19. What relativity does allow is Interstellar's trick: you can't outrun light, but you can make time run slow ^7,23.

Looking back 10:22

This season we've asked ten questions about the movies: silent explosions, banking spaceships, airlocks, giant monsters, fireballs, asteroids, a fridge, the enhance button, laser bolts, and now light speed. Every time, the real physics was stranger and often better than the shortcut. And the films that got it right, like Interstellar, made the real thing part of the story ²⁷.

Next season 10:50

That's the end of our season of Hollywood physics, and thank you for watching. Next season, we'll take on a fresh set of questions, the kind you've always wondered about, and keep asking why until the answers make sense.

Sources

Every factual claim in the episode is tied to one of these. Spotted an error? Tell us.

1. StarWars.com Databank, "Hyperdrive" — starwars.com
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19. Jessica Santiago, Sebastian Schuster and Matt Visser, "Generic warp drives violate the… — arxiv.org
20. Alexey Bobrick and Gianni Martire, "Introducing physical warp drives", Classical and… — arxiv.org
21. Christopher Smeenk et al., "Time Travel and Modern Physics", Stanford Encyclopedia of… — plato.stanford.edu
22. Jean-Pierre Luminet, "The Warped Science of Interstellar (4/6): Time dilation and… — web.archive.org
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25. Christopher M. Hirata, "Lecture XXVII: Kerr black holes: II. Precession, circular… — tapir.caltech.edu
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27. Oliver James, Eugénie von Tunzelmann, Paul Franklin and Kip S. Thorne, "Gravitational… — arxiv.org
28. The Nobel Prize in Physics 2017, summary (Rainer Weiss, Barry C. Barish, Kip S.… — nobelprize.org

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