Some form of time travel might be possible…

Of the four dimensions, only time is ambiguous. We can play games with the other three, spatial dimensions – imagine, for example, two-dimensional or even one-dimensional universes, as in flat ground (1884). But in the end, they follow the rules. Time only passes in one direction and time travels – it’s easy to imagine moving time in the other direction in principle but filled with nearly impossible puzzles in practice. Astrophysicists struggle to find out.

Our brains are primed to track the present (the “now” experience) and the past (memory). But this only tells us how we perceive time. Don’t tell us anytime he is. We’re a little closer, says NASA astronomer Sten Odenwald, given that our experience with time is closely related to entropy, the gradual flow of the universe:

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

Steen Edenwald “The struggle to find the origins of time” in astronomy Magazine (May 24, 2022)

Oddly enough, unidirectional time display does not apply in quantum mechanics:

Our world is marked by a time arrow where entropy increases over time. This is consistent with our sense of time as a one-way street, from past order to future chaos. However, the arrow of time has no basis in microphysics – the world of quantum mechanics. These equations are perfectly valid when time works in reverse. Therefore, some scientists believe that the arrow of time exists because the universe must have started in an incredibly orderly and improbable state. This is called the past hypothesis.

Steen Edenwald “The struggle to find the origins of time” in astronomy Magazine (May 24, 2022)

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

As systems evolve, their potential states increase as the amount of space their states occupy grows. Therefore, the entropy of the universe increases in the same direction as the universe is expanding, which we feel as an arrow of time. All our subjective experiences are done against this background. In his book The Order of Time, physicist Carlo Rovelli of the University of Aix-Marseille in France notes, “In order to leave a trace, it is necessary for something to come to a halt, to stop moving, and this can only happen in an irreversible process—that is, by transferring energy to heat.” This is true when meteors leave their impact on Earth and release heat, and on computer hard drives, which warm up when data is written to them. This also happens in your brain: As Rovelli notes, every memory you have is created because it takes energy to create a memory pathway, and this recording of information heats up your brain and increases entropy.

Steen Edenwald “The struggle to find the origins of time” in astronomy Magazine (May 24, 2022)

Barak Shoshani

Our understanding of time and its origin has some bearing, of course, on whether we believe time travel is possible. Theoretical physicist Barak Shoshani argues that the fact that Einstein’s equations allow for time travel in mathematics does not mean that they can be preserved in the non-quantum world in which we live. He says it’s not primarily a technical issue, because we don’t know it’s technically impossible. Herein lies the main problem:

It’s the observation that time travel seems to go against logic, in the form of time travel paradoxes. There are several types of such paradoxes, but the most problematic are consistency paradoxes.

A common trope in science fiction, consistency paradoxes occur whenever a particular event leads to a change in the past, but the change itself prevents that event from happening in the first place.

For example, consider a scenario where I enter my own time machine, use it to go back in time by five minutes, and destroy the machine once I reach the past. Now that I destroyed the Time Machine, it would be impossible for me to use it after five minutes.

But if I can’t use a time machine, I can’t 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, a time machine is destroyed if and only if it is not. Since it cannot be destroyed and not destroyed at the same time, this scenario is inconsistent and contradictory.

Barak Shoshani“Time travel necessarily implies the existence of multiple histories, as physicists say” in Conversation (April 24, 2022)

Perhaps the unsolvable is exactly what happens if we try to reverse the entropy in this universe.

Anyway, Shoshani adds, don’t think the usual sci-fi warning about butterfly effectas in Ray Bradbury’s classics, “The Sound of Thunder” (Don’t Change anything in the past!) is really of any use:

Time travelers are often warned not to make major changes to the past and to avoid meeting their former selves for this very reason. Examples of this can be found in many time travel films, such as the Back to the Future trilogy.

But in physics, a contradiction is not an event that can actually happen – it is a purely theoretical concept that refers to a contradiction in the theory itself. In other words, symmetry paradoxes not only mean that time travel is a dangerous endeavor, but rather that it simply cannot be possible.

Barak Shoshani“Time travel necessarily implies the existence of multiple histories, as physicists say” in Conversation (April 24, 2022)

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

“Events adapt around anything that can cause a contradiction, so the paradox does not occur,” German Topar, study author and 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 will be corrected by subsequent events.

Susie NelsonThe new accounts show, “Time travel is theoretically possible. But that doesn’t mean you can change the past.” in interested in trade (30 September 2020)

Then time travel would be like having a very good 4D memory. Maybe it’s worth doing but it’s not a cure for today’s ailments. As is often the case, perhaps quantum particles could be in two places at once or go back and affect their past – but it seems we can’t.

next one: Can The future goes back and affects the past?

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