Of the four dimensions, only time is mysterious. We can play games with the other three, the spatial dimensions — imagining, for example, 2- or even 1-dimensional universes, as in Flatland (1884). But in the end, they follow the rules. Time goes in one direction only and time travel — moving time in the other direction is easy to imagine in principle but full of nearly impossible conundrums in practice. Astrophysicists struggle to figure it out.

Our brains are adapted to keeping track of the present (the experience of “now”) and the past (memory). But that only tells us how we come to be aware of time. It doesn’t tell us what time is. We get a bit closer, says NASA astronomer Sten Odenwald, if we keep in mind that our experience of time is closely linked to entropy, the gradual running down of the universe:

But why do we remember the past and not the future? The reason for this asymmetry has to do with entropy — the amount of disorder in the universe. We have memories and historical records only because entropy in the past was lower than the entropy of the present.

Sten Odenwald , “The struggle to find the origins of time” at Astronomy Magazine (May 24, 2022)

Curiously, the one-way-only view of time does not apply in quantum mechanics:

Our world features an arrow of time where entropy increases with time. This accords with our sense of time as a one-way street, from past order to future disorder. Yet, there is no basis for the arrow of time in microscopic physics — the realm of quantum mechanics. Those equations are just as valid when time runs in reverse. Therefore, some scientists think the arrow of time exists because the universe must have started out in an incredibly orderly and unlikely state. This is called the Past Hypothesis.

Sten Odenwald , “The struggle to find the origins of time” at Astronomy Magazine (May 24, 2022)

But it does apply to us, and helps explain why we are all mortal:

As systems evolve, their possible states increase as the amount of space for their states to occupy grows. Therefore, entropy increases in the universe in the same direction in time that the universe is expanding, which we experience as the arrow of time. All of our subjective experiences take place against this backdrop. In his book The Order of Time, physicist Carlo Rovelli at Aix-Marseille University in France notes, “In order to leave a trace, it is necessary for something to become arrested, to stop moving, and this can happen only in an irreversible process — that is to say, by degrading energy into heat.” This is true when meteorites leave their impact on the ground and liberate heat, and for computer hard drives, which heat up when data is written to them. It also occurs in your brain: As Rovelli notes, every memory you have was created because it takes energy to create a memory pathway, and this recording of information both heats your brain and increases its entropy.

Sten Odenwald , “The struggle to find the origins of time” at Astronomy Magazine (May 24, 2022)

Our understanding of time and its origin has some bearing, of course, on whether we think time travel is possible. Theoretical physicist Barak Shoshany argues that the fact that Einstein’s equations allow for time travel in mathematics does not mean it can be sustained in the non-quantum realm in which we live. It’s not primarily a technical matter, he says, because we don’t know that it is technically impossible. Here is the main problem:

it is the observation that time travel seems to contradict logic, in the form of time travel paradoxes. There are several types of such paradoxes, but the most problematic are consistency paradoxes.

A popular trope in science fiction, consistency paradoxes happen whenever there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.

For example, consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I get to the past. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

But if I cannot use the time machine, then I cannot go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it. In other words, the time machine is destroyed if and only if it is not destroyed. Since it cannot be both destroyed and not destroyed simultaneously, this scenario is inconsistent and paradoxical.

Barak Shoshany, “Time Travel Necessarily Implies Existence of Multiple Histories, Physicists Say” at The Conversation (April 24, 2022)

Perhaps irresolvable stuff is just what happens if we try to reverse entropy in this universe.

In any event, Shoshany adds, don’t think that the usual sci-fi warning about the butterfly effect, as in Ray Bradbury’s classic, “A Sound of Thunder” (don’t change anything in the past!) is really any use:

Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves for this exact reason. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

But in physics, a paradox is not an event that can actually happen — it is a purely theoretical concept that points towards an inconsistency in the theory itself. In other words, consistency paradoxes don’t merely imply time travel is a dangerous endeavour, they imply it simply cannot be possible.

Barak Shoshany, “Time Travel Necessarily Implies Existence of Multiple Histories, Physicists Say” at The Conversation (April 24, 2022)

Of course, some physicists argue that if we could revisit the past, we couldn’t change it. That might be a way around the problem:

“Events readjust around anything that could cause a paradox, so the paradox does not happen,” Germain Tobar, the study’s author and a student at the University of Queensland, told IFLScience.

His work, published in the journal Classical and Quantum Gravity last week, suggests that according to the rules of theoretical physics, anything you tried to change in the past would be corrected by subsequent events.

Susie Neilson, “Time travel is theoretically possible, new calculations show. But that doesn’t mean you could change the past.” at Business Insider (September 30, 2020)

Then time travel would be like having a very good, 4-D memory. Maybe worth doing but not a cure for the ills of our present day. As so often, maybe quantum particles can be in two places at once or go back and affect their own past — but it seems we can’t.

Next: Can the future reach back and affect the past?

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