It's a common trope in science-fiction novels: Astronauts travel back in time by zooming through space at speeds faster than light (usually getting into trouble in the process).
Most physicists think that scenario is impossible.
But let's suspend disbelief for a second. If it time travel like this were possible, how exactly would it work?
It turns out that objects traveling faster than the speed of light could go back in time — but in the process, a pair of phantom doubles of the speedy object would pop out of thin air, and one would then go backwards and be annihilated with another, according to one hypothesis, which Robert Nemiroff, a physicist at Michigan Technological University in Houghton, Michigan, described in a paper published in May in the preprint journal arXiv.
But don't stock up on plutonium for your DeLorean just yet. The new thought experiment is probably impossible, Nemiroff said.
"I don't believe you can create a spaceship that can go faster than light," Nemiroff told Live Science. [8 Ways You Can See Einstein's Theory of Special Relativity in Real-Life]
Faster than light?
Everyone has heard it: Albert Einstein's theory of special relativity means that nothing can travel faster than light in a vacuum. That's not quite true, though: Such speeds are technically possible, but relativity dictates that anything with mass becomes heavier as it zips faster and faster, so reaching and surpassing the speed of light would take infinite energy. (Weirdly, the math would also dictate that objects could be traveling faster than the speed of light, but could not slow down to below the speed of light, Nemiroff said.)
"It is just generally believed — and I mean generally to mean almost all physicists — that there is nothing that can travel faster than light," said Sabine Hossenfelder, a theoretical physicist at Nordita in Stockholm, Sweden, who blogs at BackReaction but was not involved in the current study.
And while physicists can send subatomic particles called muons forward through time, the issue with backward time travel is causality.
Time has an arrow, and that arrow points forward. Without this safeguard, all sorts of absurd situations can occur, such as the so-called grandfather paradox, the plot device in "Back to the Future" and several other sci-fi films. If you go back in time and kill your grandfather before he has your dad, how would you exist to go back in time in the first place? [Science Fiction or Fact? The Plausibility of 10 Sci-Fi Concepts]
But oddly, neither special relativity nor particle physics has a time orientation. In fact, antiparticles, the antimatter partners of regular particles, can be interpreted as either antimatter particles going forward in time or real particles traveling back in time, Hossenfelder said. And the equations of special relativity mean that an object going faster than the speed of light would travel backward in time, she added.
Spaceship doppelganger
To understand the implications of relativistic backward time travel, Nemiroff ran the numbers for a very simple case. In his thought experiment, a spaceship would start on a launching pad on Earth, travel at five times the speed of light to a planet about 10 light-years away, then turn around to return home to a landing pad not far from the liftoff site. (Other proposed methods of time travel, such as traveling through a wormhole in curved space-time, were not addressed in the study.)
It turned out that a pair of ghost-ships, one with negative mass and one with positive mass, the researchers speculate, must appear out of thin air.
Five years after embarking, Earthlings would see a very strange apparition: Because the light from the spaceship travels slower than the spaceship itself, after the vessel returned and sat on the landing pad, Earthlings would see images of the spaceship on its way out and another doppelganger spaceship on its way back.
Eight years later, things would look even odder: An image of the spaceship sitting on the landing pad would still be visible, as would two images (perhaps like holograms) of the spaceship on its outbound and return flights. Only this time, both of those images would look farther away, as if the spaceship was traveling backward in time.
Finally, after a bit more than 10 years, the phantom spaceship pairs would annihilate each other and you'd be left with the spaceship sitting on the landing pad.
The thought experiment prompts a lot of questions. How would it all work? What would the twin spaceships be made of? Which spaceship would be the "real one? Would the phenomenon work through the quantum behavior of entangled particles? And what would the people on the spaceships be doing? Nemiroff said he can't answer those questions, and he doubts it's possible in any case.
"It doesn't make a lot of sense, and I doubt if you would look at it microscopically it would actually be possible," Hossenfelder told Live Science.
Still, the study is extremely valuable as a teaching tool, Hossenfelder said.
Real-life pair production
The time travel, pair production would be similar to an established phenomenon that occurs in illumination fronts, Nemiroff said.
"There are things that go faster than light, like shadows on the wall," Nemiroff told Live Science.
To understand illumination fronts, consider this thought experiment: If you were to aim a laser pointer at the moon (and assuming that atmospheric effects, clouds, buildings, etc. did not block the light), you would only have to flick your wrist from one side of the moon to the other faster than about 4 seconds to have the dot of light travel faster than light, Nemiroff said. If you had a powerful enough laser, an ability to take fast, time lapse-photography and an awesome telescope, you would see a pair of spots, slightly separated in distance, on the moon's surface, he said.
The trick is that in this scenario, what's traveling faster than the speed of light is not information, because there's no way for a person on one side of the moon to transmit information superluminally to the other spot via the illumination front, Nemiroff added.
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