Discussion about Theoretical Physics - The Multiverse, Time Travel, and Other Related Topics

Rotating universes is another concept. Is time travel is possible? Theoretically, yes. Practically? No.

I believe in Hawking's concept of cosmic censorship... although I have my own peculiar rationale for it.

Here's how I see it:

Either the structure of space and time allows time travel, or it does not.

If it does, then at some point during the history of the universe, somebody, somewhere, will invent a time machine and will use it to change the past.

This will, in effect, "rewind the tape" and unravel all of history subsequent to the change that was made... INCLUDING the invention of the time machine. So history would forge a NEW path forward from there, and again time travel would be independantly invented. And a time machine would again be built, and used... again rewinding the tape and wiping out that version of history.

The history of the universe would be erased and rewritten, erased and rewritten, again and again. Space-time would be like a bed-sheet hanging on a clothes-line, flapping in the wind, changing shape over and over again... until by sheer accident history "locked" into a stable state, after which no further change would be possible.

... and what is the simplest random change that will protect the integrity of space-time?

Right. No time machines.

The breakdown seemed to ignore the only muktiverse idea that makes sense and yet it receives support from quite a few people. Why? because it's logical and reasonable. As soon as they move into "different laws of physics", they take leave of their senses.

Muktiverse. I shall let it remain.

Hawking's a bit over the top in a lot of things. Hawkwind had a gut instinct, like I have, and others, that time travel is impossible. Then he rationalised it by writing a science fantasy story. There is, however, no reason why using the hypothetical time machine would wipe out "that reality" in which the time machine was invented, complete with the invention itself.

Of course, they could go back and kill the person who was going to invent it.But they would still have a working time machine because that time machine comes from a later reality (or earlier, depending on how you look at it) where it did exist. So it could be copied. So his story doesn't work but that doesn't matter because Hawking got the basic idea right.

If you don't buy string theory, 9 dimensions, and the folding of the 6 dimensions at a subatomic level, that's one thing, and you can argue against the idea...but saying that to conceive of different laws of physics is taking leave of one's senses...well, let's just say that relativity in general  requires literally ignoring one's senses of perception.  Velocity, passage of time, et al. 

It's exactly what I just posted about.  Argue from evidence, not incredulity.  You're introducing bias into your "experiment", i.e. life.  To posit the existence of an atom is to take leaves of one's senses.  More power to that.  Most major advances in science happen thereby.

This one is talking specifically about the Many Worlds Interpretation (MWI), and as the "I" indicates, this is considered less a theory and more a conjecture.>>

All interpretations of QM are called interpretations because they cannot be called explanations and naturally not theory, since it's accepted that we don't know what's happening. Copenhagen is called an interpretation but I don't think that is an interpretation. It's more of a description. Bohm is an interpretation, though. Many Worlds doesn't attract that level of respect. That's an hypothesis. Mad scientist sort of level. Perpetual motion machines. Time travel. Beam me down, Scottie.

I'm trying to help make this thread accessible to anyone that is interested at any level of knowledge.  That's what it's here for, right?  There's no other reason for a bunch of people who aren't theoretical physicists to bother having this discussion on a chess forum.

Since we aren't, it's fine to discuss various hypotheses.  If we were gainfully employed physicists, somebody would say "that's all fine and well, but come back when you can predict some outcomes".

OK. Good goal.

<<It's exactly what I just posted about.  Argue from evidence, not incredulity.  You're introducing bias into your "experiment", i.e. life.  To posit the existence of an atom is to take leaves of one's senses.  More power to that.>>

I don't go with that because it just implies finding someone else who doesn't like it and using that as evidence. At this level of discussion, evidence can be a misused word.

Sorry. I meant practically possible.

I knew this already from your position on chess being proven a forced draw or not .  You are very consistent in your "I know what I know, and I pursue what I think is true" stance.

I am capable of changing my mind very quickly if presented with strong evidence. I do misinterpret things occasionally. Occasionally I think something is true that's just wrong, because either I listened to someone who was wrong or because I misread something.

There are some things I believe. Time travel is impossible. Chess is a forced draw. Apples are good for the digestion and coffee wakes me up. If I concentrate in the right way, sometimes I can affect what happens somewhere else in the World. Little, simple, obvious things like that, you know.

Socrates is dead.  My cat died.  Socrates was a cat?

Time is an arrow, (indeed).  Only one direction.  But when you speed up sufficiently, time slows to a crawl.  So take a long (very fast) trip, and come back much younger than your peers, QED.

Unless you're a Socratic cat. 

The arrow of time remains mysterious even after considerable study. Hawking identified 3 arrows of time. Possibly the most important one is the arrow of the second law of thermodynamics. Time-asymmetry in some fundamental physics - specifically the weak interaction - is another that is not obviously connected (but is empirically consistent).  I think his third arrow was that of perceived time, but this is not fundamental physics.

When I have pondered on the arrow of entropy, I find it intuitively clear that in the forward direction the microstate tends to end up in larger macrostates (subsets of the set of all microstates). (That's a way of expressing the 2nd law of thermodynamics).

But can't you use a virtually identical argument to conclude that the microstate in the past that led to the present is more likely to be in a larger macrostate of all of the microstates that could do so? I suppose the thing that convinces us this is not so is we can make a record of how entropy increases over time so we know that this has happened (and the microstate in the past was in a smaller macrostate, not a larger one.

I'm on board with all except the last one.

ya grandma

Most people here would be. I could have a go at giving part of the mechanism but I doubt it would be interesting. Then again, lots of people believe in prayer or touching wood. There use to be lots of groups on Facebook, dedicated to demolishing such ideas. I don't know if they're still active.

I'd prefer it if these ideas were explained and described because, otherwise, it's meaningless to most of us who aren't up with the nuances of thermodynamics.