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

[bradrn]

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.

Yes, of course, that too.

[Darren]

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.)

In that case it makes sense that there's more of one kind than the other, since otherwise we wouldn't exist to observe said universe. 'Nuff said.

[bradrn]

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.)

In that case it makes sense that there's more of one kind than the other, since otherwise we wouldn't exist to observe said universe. 'Nuff said.

Exactly: so the question is, why is there more of one kind than another? As zompist mentioned, it means that somewhere in the laws of physics there must be a massive CPT violation, but we’re not yet sure where that violation is…

[Darren]

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.)

In that case it makes sense that there's more of one kind than the other, since otherwise we wouldn't exist to observe said universe. 'Nuff said.

Exactly: so the question is, why is there more of one kind than another? As zompist mentioned, it means that somewhere in the laws of physics there must be a massive CPT violation, but we’re not yet sure where that violation is…

Can't we just chalk it up to random chance?

[bradrn]

In that case it makes sense that there's more of one kind than the other, since otherwise we wouldn't exist to observe said universe. 'Nuff said.

Exactly: so the question is, why is there more of one kind than another? As zompist mentioned, it means that somewhere in the laws of physics there must be a massive CPT violation, but we’re not yet sure where that violation is…

Can't we just chalk it up to random chance?

Well, what do you mean by ‘random’, exactly? The known laws of physics suggest that the randomness that exists in nature should be symmetric, and thus should result in equal amounts of matter and antimatter.

[Richard W]

In that case it makes sense that there's more of one kind than the other, since otherwise we wouldn't exist to observe said universe. 'Nuff said.

Can we do more than say that there's more matter than antimatter in the visible universe? The antimatter might mostly be beyond the horizon!

[Richard W]

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.

Now, this does surprise me. I thought the catch was that one could only travel back from the future - one 'can' construct a time machine, but not one that allows one to explore the past, using only non-negative mass. What's the catch - does one need scrith or the like?

[bradrn]

In that case it makes sense that there's more of one kind than the other, since otherwise we wouldn't exist to observe said universe. 'Nuff said.

Can we do more than say that there's more matter than antimatter in the visible universe? The antimatter might mostly be beyond the horizon!

This is getting beyond what I’m familiar with, but according to Wikpedia, such a region of the universe would result in a gamma-ray signal from annihilation with our region of the universe, and we’ve seen no evidence of such a signal.

[Richard W]

This is getting beyond what I’m familiar with, but according to Wikpedia, such a region of the universe would result in a gamma-ray signal from annihilation with our region of the universe, and we’ve seen no evidence of such a signal.

It only says that for regions within the observable universe.

[Torco]

Can't we just chalk it up to random chance?

Well, what do you mean by ‘random’, exactly? The known laws of physics suggest that the randomness that exists in nature should be symmetric, and thus should result in equal amounts of matter and antimatter.

sure, but at some scales that same randomness makes biases and local regions. like, if I pour black sand and white sand in a bucket with like a shovel then stirr it up some, it's likely some bits of it are going to still only contain white sand, just by chance. some bits and chunks (regions of the bucket) that don't get mixed up well for whatever reason. if these bits of mostly mtter or antimatter in an overall universe of mostly-balanced-but-chunky mix was the structure of the big bang at some really early point, in principle yeah we could be in one: and if the chunk itself was of a size X at that point then in principle it could be bigger than the observable universe... like, do we really know the matter-antimatter *texture* of when matter is formed in the early picoseconds of the big bang? possibly, what do I know: but since apparently we don't know the size of the universe beyond the observable, we probably don't know the expansion factor we'd need. still, most macro things at least, if you look close enough, are chunky.

[linguistcat]

Also couldn't we be the small percent of difference that was left over from a much larger amount of matter and antimatter? I don't think anyone has ruled that out and if they have it would be interesting to know how so.

[Ares Land]

Also couldn't we be the small percent of difference that was left over from a much larger amount of matter and antimatter? I don't think anyone has ruled that out and if they have it would be interesting to know how so.

I think the current understanding is that what you describe is exactly what happened.

I didn't know about the antimatter interpretation!

A few years back I read Kip Thorne's Black Holes and Time Warps

Faster-than-light and time travel are related; but you can think of it the other way around. Faster than light is impossible precisely because it's equivalent to time travel, which is impossible because it breaks causality.

I don't think I understand all this well enought to explain it clearly, but basically an object travelling faster than light is moving backwards through time from some relativistic point of views. Given that all relativistic point of view are equally valid, this means time travel and FTL travel are equivalent.
It's not a terribly interesting form of time travel, but it still poses problems with causality.

Another interesting bit is that there are perfectly valid solutions to general relativity that do allow for both FTL and time travel (wormholes, Alcubierre's warp thing, and so on.)
Besides useful to SF writers, it's also an intriguing puzzle... how are paradoxes prevented?

The problem of travellers from the future is easily solved though! All the solutions compatible with relativity state require that you travel to a moment in (space)time when/where your time machine exists.

[bradrn]

This is getting beyond what I’m familiar with, but according to Wikpedia, such a region of the universe would result in a gamma-ray signal from annihilation with our region of the universe, and we’ve seen no evidence of such a signal.

It only says that for regions within the observable universe.

if these bits of mostly mtter or antimatter in an overall universe of mostly-balanced-but-chunky mix was the structure of the big bang at some really early point, in principle yeah we could be in one: and if the chunk itself was of a size X at that point then in principle it could be bigger than the observable universe... like, do we really know the matter-antimatter *texture* of when matter is formed in the early picoseconds of the big bang? possibly, what do I know: but since apparently we don't know the size of the universe beyond the observable, we probably don't know the expansion factor we'd need. still, most macro things at least, if you look close enough, are chunky.

The thing is, by definition we can only talk about that portion of the universe which is observable. We could say anything whatsoever about the rest of the universe, without ever being able to prove or disprove it. So such an explanation is not really satisfying to physicists — it’s really the equivalent of throwing up your hands and giving up on an actual explanation.

Also couldn't we be the small percent of difference that was left over from a much larger amount of matter and antimatter? I don't think anyone has ruled that out and if they have it would be interesting to know how so.

This is precisely the situation I’ve been describing! The question is, why did that small percent difference even exist in the first place?

(As I recall, people have actually calculated what the difference should have been to produce the amount of matter we see today. Wikipedia quotes it as being ‘on the order of 1 in every 1,630,000,000 (≈2×10⁹) particles’.)

[Darren]

This is precisely the situation I’ve been describing! The question is, why did that small percent difference even exist in the first place?

(As I recall, people have actually calculated what the difference should have been to produce the amount of matter we see today. Wikipedia quotes it as being ‘on the order of 1 in every 1,630,000,000 (≈2×10⁹) particles’.)

Let's say there's 10⁸⁰ particles in the universe, and 10⁷⁰ antiparticles (roughly speaking). In a universe with originally 10^1000000000 total particles, the difference would only be about 0.000008%. I could posit a pre-universe with a googolplex particles, all of which annihilated each other except for the insignificant difference which ended up becoming our current universe. Surely that falls within acceptable random chance.

Alternatively, I could say that the universe started off with 5*10⁷⁹ + 100 particles and 5*10⁷⁹ antiparticles, and then all of them except the 100 normal particles destroyed each other, but the resulting energy produced new particles (through science) of almost exactly even distribution, with the initial remainder of normal particles destroying any excess antiparticles but not any excess particles, and the process repeating until we run out of enough energy to make new particles. This would require the initial state being statistically unusually high in normal particles but not by an enormous amount (someone smart would have to run the maths on it).

Neither theory seems very plausible and I imagine neither obeys the laws of physics nor accurately predicts the composition of our universe; feel free to point out the catastrophic faults in both.

[zompist]

This is precisely the situation I’ve been describing! The question is, why did that small percent difference even exist in the first place?

(As I recall, people have actually calculated what the difference should have been to produce the amount of matter we see today. Wikipedia quotes it as being ‘on the order of 1 in every 1,630,000,000 (≈2×10⁹) particles’.)

Let's say there's 10⁸⁰ particles in the universe, and 10⁷⁰ antiparticles (roughly speaking). In a universe with originally 10^1000000000 total particles, the difference would only be about 0.000008%. I could posit a pre-universe with a googolplex particles, all of which annihilated each other except for the insignificant difference which ended up becoming our current universe. Surely that falls within acceptable random chance.

There are about 10⁸⁰ protons in the observable universe. Given the number in Brad's quote, that means there were originally 10⁸⁹ particles and 10⁸⁹ antiparticles. You'd have to read the appropriate papers to know where that number comes from.

I'm not sure what you mean by "acceptable random chance". Should cosmologists just accept the imbalance and not try to figure it out? They want to know why we have the numbers we do.

(Note, there are some complications, such as protons not existing at all at first. As CERN puts it, "About one ten-thousandth of a second after the Big Bang, protons and neutrons formed, and within a few minutes these particles stuck together to form atomic nuclei, mostly hydrogen and helium. Hundreds of thousands of years later, electrons stuck to the nuclei to make complete atoms.")

[Darren]

This is precisely the situation I’ve been describing! The question is, why did that small percent difference even exist in the first place?

(As I recall, people have actually calculated what the difference should have been to produce the amount of matter we see today. Wikipedia quotes it as being ‘on the order of 1 in every 1,630,000,000 (≈2×10⁹) particles’.)

Let's say there's 10⁸⁰ particles in the universe, and 10⁷⁰ antiparticles (roughly speaking). In a universe with originally 10^1000000000 total particles, the difference would only be about 0.000008%. I could posit a pre-universe with a googolplex particles, all of which annihilated each other except for the insignificant difference which ended up becoming our current universe. Surely that falls within acceptable random chance.

There are about 10⁸⁰ protons in the observable universe. Given the number in Brad's quote, that means there were originally 10⁸⁹ particles and 10⁸⁹ antiparticles. You'd have to read the appropriate papers to know where that number comes from.

I'm not sure what you mean by "acceptable random chance". Should cosmologists just accept the imbalance and not try to figure it out? They want to know why we have the numbers we do.

I.e. 0.000008% is easier to explain than 163000000000%

[hwhatting]

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

One potential explanation is raised here - that the world as it exists actually is the best of all possible worlds... say in every timeline without Hitler we get Uber-Nazis or something unimaginably worse at some point...
Other possible explanations: The future time travellers actually don't care, or they like their past gruesome, or our TL is the result of a constant tug-of-war between pro- and anti-Hitler-killing factions in the future...

[Raphael]

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

One potential explanation is raised here - that the world as it exists actually is the best of all possible worlds... say in every timeline without Hitler we get Uber-Nazis or something unimaginably worse at some point...
Other possible explanations: The future time travellers actually don't care, or they like their past gruesome, or our TL is the result of a constant tug-of-war between pro- and anti-Hitler-killing factions in the future...

My main problem with the idea of time travel is that you wouldn't just have a few people trying to kill Hitler... ...you'd have hundreds, or thousands, or perhaps even millions of people trying to kill, or prevent the killing of, basically every major or minor historical figure. Time travel stories rarely ever seem to account for this.

[Ares Land]

I think causality is still a show-stopper... it means either no time travel or somehow the past can never be changed.
Leaving that aside and assuming it's handwaved away or something... Maybe time travel is worked out several millenia in the future, at a time when the Nazis have lost all relevance.

The Assyrians were murderous bastards, a fact that is only known to specialists and history nerds now; picturing time travellers trying to nip the Assyrian empire in the bud feels weird.

[Creyeditor]

Maybe time travel is more like going by train. You have to build tracks and stations before you can go somewhere. This could mean that you cannot travel to a point in time where time travel was not invented yet. Et voila, no time travel to our present.