Do you think it will ever be possible to go back in time?

[Space60]

And of so, why aren't we getting any visitors from the future? Do we just live in a boring time?

[xxx]

it would be too paradoxical to contact you...

[alice]

I have been sent from the future to explain to you how to travel back in time, but you don't have the technlology yet.

[Travis B.]

If time travel were possible without requiring causality loops, how was Hitler never assassinated by a time traveler?

[zompist]

Sure, happens all the time, if you're an elementary particle.

In quantum mechanics, a particle can go back in time... we call it an antiparticle.



E.g. in this Feynman diagram, space is the horizontal dimension, time is vertical, going upward. An electron (e-) enters stage left, emits a photon (ɣ), moves more slowly rightward, emits another photon, then goes back in time, whereupon we call it a positron (e+). (Follow the arrows.)

However, you can also interpret this as an electron and a positron both moving forward in time, meeting at the righthand black dot, and annihilating each other, leaving a photon behind.

You could theoretically build a time machine out of antimatter. Unfortunately, the moment you let it out of its containment field it will touch ordinary matter and destroy your laboratory and in fact your entire city.

[WeepingElf]

I think there is a reason to assume that backwards time travel is impossible: Where are the visitors from the future? Of course, some people opine that UFOs are just that, but one would then rather expect that UFOs appear especially frequently near important historical events, which is not what we observe. However, they may come from a future so distant that they simply no longer know our history, and therefore choose their destinations randomly.

[Znex]

Sure, happens all the time, if you're an elementary particle.

In quantum mechanics, a particle can go back in time... we call it an antiparticle.

Hang on, I thought that was exotic matter?? Tachyons (faster-than-light, but that itself still has a strange relationship with time) and whatnot.

As far as I understood it, antimatter travels forward in time and can be created by particle interactions, and is affected by the same forces that normal matter is, such as gravity, atomic forces, electromagnetism, etc., but tachyons have not yet been demonstrated to exist.

[xxx]

some conlang words can open a door,
through time, and space,
but often between different dimensions...

but I prefer the superposition of a new layer on my own,
which shifts my vision of this world...

[Torco]

ever is a long time. I'd say not a thousand years, though, which may as well be the same.

[Space60]

Even travel to the future (outside the ordinary rate) which is easier than travel to the past is likely not going to be possible for the next few centuries.

[Space60]

It is unknown whether or not travel to the past is possible. Unlike perpetual motion which is impossible according to physics as it is currently understood, the laws of physics have not been shown to absolutely forbid backwards time travel.

[zompist]

Sure, happens all the time, if you're an elementary particle.

In quantum mechanics, a particle can go back in time... we call it an antiparticle.

Hang on, I thought that was exotic matter?? Tachyons (faster-than-light, but that itself still has a strange relationship with time) and whatnot.

No, tachyons are theoretical faster-than-light particles. Antimatter particles, including positrons, travel slower than light and are quite real.

As far as I understood it, antimatter travels forward in time

That is an interpretation, not what the science says. Feynman explains the backwards-in-time interpretation in QED and elsewhere. There is no theoretical, much less experimental, evidence for preferring one interpretation over the other.

(ETA: why make untestable alternate interpretations? One, to contest unproven statements, such as "only motion forward in time is possible". Two, because science prefers simpler explanations. Feynman's interpretation halves the number of massed particles.)

[Torco]

speaking of particle physics... isn't the matter-antimatter imbalance weird? if when we look at the past and see mostly matter matter, then maybe all the anti matter is in the future?

[bradrn]

Regarding ‘antimatter is going backwards in time’: one very important thing to understand about quantum mechanics is that it doesn’t really have an arrow of time in the first place. That is to say, the laws of quantum mechanics are perfectly reversible, such that there is no difference between ‘evolving a system forwards in time’ and ‘evolving a system backwards in time’.

By contrast, general relativity does have an arrow of time. Tachyons are a relativistic phenomenon: it is the theory of relativity which predicts that ‘traveling faster than the speed of light’ allows you to go back in time. Similarly, wormholes etc. are also relativistic. If you want to explore causality and how to break it, relativity is the theory which describes that.

So, really, there are two quite different notions of ‘going back in time here’. The quantum sense is probably more important in modern physics, but the relativistic sense is what we would normally think of as ‘time travel’.

speaking of particle physics... isn't the matter-antimatter imbalance weird? if when we look at the past and see mostly matter matter, then maybe all the anti matter is in the future?

That’s not what the problem is. Essentially: we predict that the Big Bang should have produced an equal amount of matter and antimatter, in the past. The question is: given that initial state, why do we see more matter than antimatter today?

[Darren]

That’s not what the problem is. Essentially: we predict that the Big Bang should have produced an equal amount of matter and antimatter, in the past. The question is: given that initial state, why do we see more matter than antimatter today?

If there was more antimatter, wouldn't it of blown up all the matter (including us)?

[zompist]

That’s not what the problem is. Essentially: we predict that the Big Bang should have produced an equal amount of matter and antimatter, in the past. The question is: given that initial state, why do we see more matter than antimatter today?

If there was more antimatter, wouldn't it of blown up all the matter (including us)?

Yes... and then we'd call it "matter." The sign is arbitrary.

My understanding is that we don't know why the disparity is so huge; but the culprit is probably a CPT violation. The laws of physics should be the same if you flip charge (C), parity (P), and the direction of time (T). But there are minor CPT violations.

[Darren]

That’s not what the problem is. Essentially: we predict that the Big Bang should have produced an equal amount of matter and antimatter, in the past. The question is: given that initial state, why do we see more matter than antimatter today?

If there was more antimatter, wouldn't it of blown up all the matter (including us)?

Yes... and then we'd call it "matter." The sign is arbitrary.

Sorry, I meant "mutually annihilated with all the matter".

[Jonlang]

Professor Brian Cox, around 2013, did a special lecture to a celebrity audience explaining (among other things) why time travel into the past will never be possible. It was a Doctor Who themed lecture and was a part of the BBC's 50th Anniversary celebration for DW. From what I remember it requires faster-than-light speed in order to achieve and so will never be possible. That and the fact the past isn't a "place" any more than your imagination is.

[bradrn]

If there was more antimatter, wouldn't it of blown up all the matter (including us)?

Yes... and then we'd call it "matter." The sign is arbitrary.

Sorry, I meant "mutually annihilated with all the matter".

Yes, if there was an equal amount of antimatter as matter, it would all have been annihilated, and the universe would consist of photons alone. (Which, if I must state the obvious, it does not.)

Professor Brian Cox, around 2013, did a special lecture to a celebrity audience explaining (among other things) why time travel into the past will never be possible. It was a Doctor Who themed lecture and was a part of the BBC's 50th Anniversary celebration for DW. From what I remember it requires faster-than-light speed in order to achieve and so will never be possible. That and the fact the past isn't a "place" any more than your imagination is.

‘Never’ is a difficult word. The one thing we can say about our current physics is that it does not yet completely model the universe, so if we discover new physics, it is conceivable that that might tell us it is possible to go faster than light. But I consider it exceedingly unlikely.

[Raphael]

The one thing we can say about our current physics is that it does not yet completely model the universe, so if we discover new physics, it is conceivable that that might tell us it is possible to go faster than light. But I consider it exceedingly unlikely.

And I'm not sure that even that would make time travel possible. If we discover new physics that allow us to go faster than light, then that new physics might as well contain new rules which mean that the "faster than light equals time travel" rule no longer holds true.