Sunday, December 7, 2014

Physical time travel

There is good evidence that human time travel has occurred. To explain, let’s first define the term. We mean physical time travel, not travel by wishing or dreaming or sitting still and letting time march on. In any case of physical time travel the traveler’s journey as judged by a correct clock attached to the traveler takes a different amount of time than the journey does as judged by a correct clock of someone who does not take the journey.

The physical possibility of human travel to the future is well accepted, but travel to the past is more controversial, and time travel that changes either the future or the past is generally considered to be impossible.

Our understanding of time travel comes mostly from the implications of Einstein’s general theory of relativity. This theory has never failed any of its many experimental tests, so we trust its implications for human time travel.

Einstein’s theory permits two kinds of future time travel—either by moving at high speed or by taking advantage of the presence of an intense gravitational field. Actually any motion produces time travel (relative to the clocks of those who do not travel), but if you move at extremely high speed, the time travel is more noticeable; you can travel into the future to the year 2,300 on Earth (as measured by clocks fixed to the Earth) while your personal clock measures that merely, let’s say, ten years have elapsed. You can participate in that future, not just view it; you can meet your twin sister’s descendants. But you cannot get back to the twenty-first century on Earth by reversing your velocity. If you get back, it will be via some other way.

It's not that you suddenly jump into the Earth's future of the year 2,300. Instead you have continually been traveling forward in both your personal time and the Earth’s external time, and you could have been continuously observed from Earth’s telescopes during your voyage.

How about travel to the past, the more interesting kind of time travel? This is not allowed by either Newton's physics or Einstein's special relativity, but is allowed by general relativity. In 1949, Kurt Gödel surprised Albert Einstein by discovering that in some unusual worlds that obey the equations of general relativity—but not in our world—you can continually travel forward in your personal time but eventually arrive into your own past.

Unfortunately, even if you can travel to the past in the actual world you cannot do anything that has not already been done, or else there would be a contradiction. In fact, if you do go back, you would already have been back there. For this reason, if you go back in time and try to kill your childhood self, you will fail no matter how hard you try.

While attempting this assassination, you will be in two different bodies at the same time.

Here are some philosophical arguments against past-directed time travel. I suggest that none of these are convincing. The last one is subtle.
1. If past time travel were possible, then you could be in two different bodies at the same time, which is ridiculous. 
2. If you were presently to go back in time, then your present events would cause past events, which violates our concept of causality. 
3. Time travel is impossible because, if it were possible, we should have seen many time travelers by now, but nobody has encountered any time travelers. 
4. If there were time travel, then when time travelers go back and attempt to change history they must always botch their attempts to change anything, and it will appear to anyone watching them at the time as if Nature is conspiring against them. Since observers have never witnessed this apparent conspiracy of Nature, there is no time travel. 
5. Travel to the past is impossible because it allows the gaining of information for free. Here is a possible scenario. Buy a copy of Darwin's book The Origin of Species, which was published in 1859. In the 21st century, enter a time machine with it, go back to 1855 and give the book to Darwin himself. He could have used your copy in order to write his manuscript which he sent off to the publisher. If so, who first came up with the knowledge about evolution? Neither you nor Darwin. Because this scenario contradicts what we know about where knowledge comes from, past-directed time travel isn't really possible. 
6. The philosopher John Earman describes a rocket ship that carries a time machine capable of firing a probe (perhaps a smaller rocket) into its recent past. The ship is programmed to fire the probe at a certain time unless a safety switch is on at that time. Suppose the safety switch is programmed to be turned on if and only if the “return” or “impending arrival” of the probe is detected by a sensing device on the ship. Does the probe get launched? It seems to be launched if and only if it is not launched. However, the argument of Earman’s Paradox depends on the assumptions that the rocket ship does work as intended—that people are able to build the computer program, the probe, the safety switch, and an effective sensing device. Earman himself says all these premises are acceptable and so the only weak point in the reasoning to the paradoxical conclusion is the assumption that travel to the past is physically possible.
I recommend an alternative solution to Earman’s Paradox. Nature conspires to prevent the design of the rocket ship just as it conspires to prevent anyone from building a gun that shoots if and only if it does not shoot. I cannot say what part of the gun is the obstacle, and I cannot say what part of Earman’s rocket ship is the obstacle.

Brad Dowden
Department of Philosophy
Sacramento State


  1. Brad, thanks for posting more on this great topic. Two questions.

    Is it possible to say anything more, and comprehensible to a general audience, about why General Relativity permits time travel into the past in our world?

    Also, is there currently any preferred notion of personal identity that is regarded as being most compatible with theoretical physics? From what I gather, General Relativity seems most compatible with the Many Worlds interpretation, which also requires us to be multiply instantiated.

  2. Randy, you have asked why General Relativity permits time travel into the past in our world. In a way this question is like asking why relativity theory puts a top limit on speed. The answer is that the existence of speed limits and possible time travel are theorems. But we can say a bit more. If you want to travel in time relative to your twin sister, you cannot sit still. You must move in space rapidly until your light cones start tilting far enough. But now we are getting too technical, I think.

    It is much easier to carry out this travel with an A.I. device in the spaceship than with an ordinary human being because of all the dust. If your high speed travel avoids collision with all the big objects such as planets, stars, comets and asteroids, it would not be practical to avoid each bit of dust. Yet your ship’s high speed collision with dust particles will produce a shower of gamma rays and x-rays that will annihilate ordinary human bodies.

    You also asked whether there currently is any preferred notion of personal identity that is regarded as being most compatible with theoretical physics. There is none that I know of, but if you travel back to meet your earlier self, then you absolutely must exist in two bodies at once, so that upsets any classical philosophical theory of personal identity.

    This personal identity needs only to be within just one world, the possible world we call the actual world. However, as you point out, General Relativity is compatible with the Many Worlds interpretation in which at every instant each world splits into multiple possible worlds. Metaphysicians dispute whether at a branch point your identity would split into many yous, or whether there would be just one you, the one that survives into the “actual” branch, while all the other supposed yous would merely be counterparts of you, but not actually you. Physicists avoid these discussions, and metaphysicians have no consensus on the topic.

    1. I love the idea of time travel. If I can come in on this discussion here... First, I think we should view meeting ourselves during backwards time travel as meeting an earlier time slice of ourselves. It's a bit like a worm stretching out until it went right around the world and met it's own tail. Second, I think the theory of personal identity that makes the most sense of this is the "no self" theory: we are an ever-changing bundle of experiences with no unique or unchanging core. On this view, you don't meet your self when you go back in time because there are no selves. But you do meet a being that is very much like you.

    2. Just to clarify... on my first point, I mean you meet what seems like an earlier time slice of your self, but it is a bundle of experiences that is very similar to the bundle that you are currently. Yikes!

    3. Dan, you said, in past time travel you meet an earlier version of “a bundle of experiences that is very similar to the bundle that you are currently.” What do you mean by similar? Can the experiences be similar and still be in different bodies having no atoms in common?

      So, is the following a good deal for you? Suppose I were to say I’ll give you thirty thousand dollars if you’ll step into this transport box here in Sacramento. The transport box has transported past clients to the places of their dreams; my evidence for this is that the beings who step out of the box have had no objections after they have arrived. Suppose your dream vacation is to go to Paris for a while. I claim that the human being who steps out of my box in Paris in 15 seconds will look just like you and will remember having all your experiences—teaching in New Zealand, and so forth. Inside the Sacramento box the info about your experiences will be collected, then your body will be disassembled and converted to energy. This energy and information will be sent to Paris where new hydrocarbon molecules are found to construct a human being very similar to the one that stepped into the Sacramento box, remembering very similar experiences. It’s the chance of a lifetime for you. Sign up now.

    4. The teletransporter would be fine. Death without the experience of dying or being dead after doesn't sound so bad... and that is just "adding another experience" on the no self view.

      Your first challenge I find more troubling intuitively. But an older version of the same atom being close by is no stranger than an older version of yourself being nearby.

    5. It is interesting that you speak of "close by" and "nearby." When "you" step out of the teleporter machine in Paris, your Paris atoms are not nearby your Sacramento atoms, are they?

  3. Dan, I don't understand why an experiential notion of the self should makes the most sense of what you say. Why can't the worm and the time slices you speak of be understood physically? I like your fundamental idea, and if you build it into the Many Worlds interpretation it becomes more like a hydra.

    1. Perhaps my two ideas don't go together that well. I like how the no self view attacks the intuition that we can't be in two places at once by undercutting the idea that there is a unique thing that constitutes us as individuals. Perhaps my worm analogy accounts for your atoms being near your old atoms. And perhaps the no self view best explains why there is no unique self that meets herself during backwards time travel (once the uniqueness idea of personal identity is dropped, a bundle of experiences meeting another similar bundle seems easy to accept). But you are probably right that we can do away with the no self view if we have easily ignorable intuitions.

    2. If a future "Dance of Reason" post were devoted to the problem of personal identity, we'd have a lively discussion.

  4. Kyle,
    That graphic is hilarious! Made my day.