So last week a friend pointed out a news headline that read “Time travel is impossible” and he wanted my opinions on it.
So first let’s explain what the team of scientists in Hong Kong demonstrated. Their experiment showed that individual photons, in a vacuum, always move at c, the speed of light. This was mostly aways assumed to be true of course but it is always reassuring when experiment confirms theory, even theory that is a century old. Just over a century ago Einstein conclusively showed that the speed of light was a fundamental constant in the laws of physics. As one of the consequences of doing this, he also showed that space and time are deeply linked in a very powerful way. In fact, by his Special Theory of Relativity, traveling faster than light automatically implies time travel into the past. You cannot have one without the other because space and time are deeply linked.
About 10 years ago there was experimental evidence that seemed to indicate pulses of light traveling faster than light in media like air or water or solid matter. (By the way this should not be confused with Cherenkov Light, which is a well-known, and rather beautiful, phenomena that doesn’t break any rules.) This was later shown to be an optical illusion but there was loophole left open that allowed for possibility of a single photon, a particle of light, to travel faster than light under the right circumstances. And this was what the scientists in Hong Kong were checking.
They found, reassuringly, that it was impossible for single photons in this circumstance to go faster then light. This apparently lead one of the scientists, perhaps in overenthusiastic victory, or perhaps lead some journalists to oversimplify and give us this headline.
But if we examine the technical details, the real truth is much more complicated. For example time travel into the future is happening all the time. In fact we can even time travel into the future rapidly by going very close to the speed of light or standing near a powerful gravitational source, like a neutron star or black hole. Time travel into the future isn’t troublesome because it doesn’t generate any logical paradoxes. This might seem obvious because we traveling forward in time constantly, right? But it’s kind of important to keep in mind as I explain more.
Relativity doesn’t forbid time travel into the past but it strongly limits it in an interesting way. You can travel into the past but only if your activities in the past generate causal chains the result in the present that you already knew before you left. In this form of travel to the past, you can’t change the past in any way, in fact, your arrival there and all your doings there only work to ensure the present you knew before you left. Your journey is in fact a required part of your past. This doesn’t generate any logical paradoxes. Some good examples of this form of limited time travel into the past can be seen in the first Terminator movie and 12 Monkeys or in the classic science fiction story, “–All You Zombies–.”
But this starts to take us off the subject a bit. I only point this all out to show that there are technical details the headline oversimplifies.
I’d say that time travel to the past is troublesome to physics for another reason: lack of any evidence of it. If it were possible, wouldn’t we be seeing tourists from the future all the time? Wouldn’t our present be full of ontological paradoxes were things are the sources of their own generation? Wouldn’t there be present moments constantly where, for example, a daughter is in fact her own grandmother or mother? Such a thing is allowable by relativity so why don’t we see it happening all the time? This bothered Stephen Hawking so much he proposed that there must be some set of physics we don’t understand yet that forbids even this limited form of travel into the past.
As yet, the questions in the above paragraph are still mostly unanswered. Observational evidence seems to indicate that even this limited form of time travel is not happen but theory and experiment can’t definitively rule it out. All we can safely say at this point is that we have lot to learn about how time actually works.