
Time Travel (2006) - panic
https://www.scottaaronson.com/democritus/lec19.html
======
fouc
> if closed timelike curves existed, then quantum computers would be no more
> powerful than classical ones.

If we find quantum computers are no more powerful than classical ones, well
then perhaps CTCs exist.

\--

Although what if quantum computing is actually a result of CTC under the space
time fabric? (Probably a nonsensical proposition eh?)

~~~
jessriedel
It's generally considered inelegant but not nonsensical. There exist hidden-
variable theories that reproduce all the normal predictions of quantum
mechanics (including quantum computations), but they require non-local
interactions. I think a restricted set of CTCs could mediate those
interactions.

------
philliphaydon
I love movies and TV shows about time travel. But I just don't think it's
possible at all.

If you could travel back in time, that would mean that future and the past
must co-exist with the present. If the pasts exists for us, the future exists,
which means events have already happened, which means if time travel was to be
invented then it would have already happened and someone would have traveled
back to the past.

But if traveling back to the past, and changing something, then any changes to
the past would alter the future, but changing the past could prevent time
travel being invented which would mean the person never traveled back to
change the past to alter the future to begin with...

Unless changing the past splits time into 2 with both outcomes.

If the purpose of traveling back in time is to fix a problem that occurs in
the future, but the act of changing the past splits time, then the time you
knew would not be altered, only the alternative timeline you created.

But if the past and future already exist, and time travel is possible, then
all possible futures have ready happened and all possible pasts have already
happened which would mean you have infinite timelines of every possible
outcome of time itself. Because any change to the past in a given timeline
only splits time to another outcome for the timeline, we would never know if
our future changed or not making the entire event pointless.

I only believe it's possible to travel to the future, but not 'skip' in the
same way back to the future does. Only freeze ourself and awake in the future
which is kind of just pausing and letting time pass by.

^ my opinion.

~~~
tbabb
It is already implied by relativity that time doesn't flow. You cannot choose
a definition of "now" that some other observer who is moving relatively to you
would agree with. Under relativity, observers will disagree about which events
are happening simultaneously. So if there are two observers in the universe,
the notion of a "global now" is incompatible with relativity.

If some particular observer's definition of "now" could be determined to be
the "right" one by some experiment, that would reject the equivalence
principle, which is the foundation of relativity. In other words, not likely
to happen.

Using this principle one can build a conceptual "chain of simultaneity"
between observers that extends arbitrarily far into the future or past.

It is no surprise, then, that physics doesn't have a concept of "now". In all
physical equations, time is a free parameter, which we can set to any value,
and those equations will tell us what we should expect to observe at that
time. Physical equations do not reify any particular "t".

The fact that time seems to "flow" subjectively for us is an artifact of the
fact that information can't (that we know of) flow from the future to the
past, therefore at any given slice of time for a given observer, the past
appears "set" (the observer has information/measurements about it that confine
its possible states), but the future appears undetermined (there are no
available direct measurements of it).

 _Why_ information can't flow from the future to the past is an extremely
interesting question that is not terribly well answered, especially given that
the laws of the universe are time-symmetrical. I think most physicists would
posit that it has something to do with the perplexingly low entropy of the big
bang, statistically mandating that later states must have higher entropy, thus
birthing the second law of thermodynamics and giving rise to the "arrow of
time".

Why specifically the Second Law would prevent the flow of even a single bit of
information travelling into the past is not clear to me. As the article points
out, Deutsch 1991 showed us that grandfather paradoxes don't exist once you
factor in quantum mechanics. This is a question that has captivated me for
many years.

~~~
lisper
> Why information can't flow from the future to the past is an extremely
> interesting question that is not terribly well answered.

I took a whack at it a while back:

[http://blog.rongarret.info/2014/10/parallel-universes-and-
ar...](http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-
time.html)

~~~
tbabb
Great article, thanks for sharing! I actually found your Google talk some
years ago and found it fascinating.

I agree with basically all of your article. It certainly explains the
dichotomy of physical laws allowing a spacetime to be "played backwards",
while our experience must always be that of it being "played forwards"\-- the
time reversal of learning something is forgetting it, and this would invert
the topological sort of knowledge dependency that places slices of our
experience in the "future" or the "past". As such an observer in an isolated
part of the universe which was being "played backwards" would label our future
as their past.

If we buy into the Second Law, it makes sense to me that there would be some
direction where there are more messy entanglements between all the parts of
the universe, and in the other direction there would be fewer, and we would
name the first direction "the future" and the second direction "the past",
since there would be information embedded in all those mutual entanglements
that would over-constrain states in the past and under-constrain states in the
future.

What I don't get is what prevents me from setting up a circumstance where a
measurement I make in the present is correlated with the result of (say) a
coin flip in the future. That is easy to do if the coin flip is in the past,
but impossible the other way around. I agree that by your logic if I were
getting more information _on the whole_ from the future, then I would almost
definition label that "the past". But why _not even one bit_ from the future?
I can make a meaningful mark on a particle at time t0 and read it at t1, but
not if t0 > t1. How come?

~~~
lisper
Thanks for the kind words.

> there would be some direction where there are more messy entanglements
> between all the parts of the universe

Yes, that's exactly right, except for the word "messy". It's not "more messy",
it's just "more".

> what prevents me from setting up a circumstance where a measurement I make
> in the present is correlated with the result of (say) a coin flip in the
> future

Nothing. If you flip a coin with enough precision you can make it come up
reliably on one side or the other. ("But that's cheating!" you say. "I want
the flip to be _random_." Well, you can't have it both ways: if the flip is
random, then _by definition_ it's not going to be correlated with anything in
the past!)

> why not even one bit from the future

Because then it wouldn't be the future.

Think about this: how would you distinguish "receiving a bit from the future",
which is apparently not possible, and "making a reliable prediction about the
future", which is possible in many cases?

> I can make a meaningful mark on a particle at time t0 and read it at t1, but
> not if t0 > t1. How come?

You can't actually "make a mark" on a particle the way you can on a classical
object. You can prepare a particle in a particular quantum state, but that's
not the same as putting a mark on it.

~~~
tbabb
> You can't actually "make a mark" on a particle

Yes, of course. That was loose wording for "affect a particle a way that is
meaningful", i.e. prepare it. :)

> "But that's cheating!" you say. "I want the flip to be random."

Actually, no, I'd rather the bit be useful!

> If you flip a coin with enough precision you can make it come up reliably on
> one side or the other.

Sure, if I know enough about the universe at some moment in time, by
unitarity, I know its state at all other times. Having enough information to
run the laws of physics forward to compute the outcome of the 2020 super bowl
coin flip isn't much different, then, than running them backward to get the
2019 flip (and just as impractical). But what is interesting is that I could
correlate (say) the spin of an electron with the result of the flip and read
it later to tell what the flip _was_ , but not earlier to tell what the flip
will be.

(Obviously information about the past coin flip is available in many more
places than just in my prepared particle, but I don't _need_ anything more
than that one qubit to precisely know the result. That's a big difference from
needing to know everything in the coin's past light cone! )

If we peel away the human-imposed notions of time and causality (as you do in
your article), and see spacetime as a "block" with microscopic time symmetry,
or perhaps even further dismantled into only basis states of Hilbert space,
it's still obscure to me why particles-- even individual ones-- seem to
"carry" information only from the past and not from the future.

On an intuitive level it seems perfectly natural. On the level of (time-
symmetrical) fundamental physics, I can't pin down why it would be.

~~~
lisper
> That was loose wording for "affect a particle a way that is meaningful",
> i.e. prepare it. :)

Yes, but these details matter. Preparing a particle in a quantum state is
fundamentally different from making an identifying mark on a classical object.

> > If you flip a coin with enough precision you can make it come up reliably
> on one side or the other.

> Sure, if I know enough about the universe at some moment in time, by
> unitarity, I know its state at all other times.

That's not what I meant. I'm not talking about trying to measure the state of
the coin and the flipping apparatus in order to _predict_ the outcome, I'm
talking about building a precision flipping apparatus that allows you to
_control_ the outcome.

> I could correlate (say) the spin of an electron with the result of the flip
> and read it later to tell what the flip was, but not earlier to tell what
> the flip will be.

Actually, you can do both, and the procedures are essentially identical: to do
the former, you look at the coin and manipulate the electron state to match.
To do the latter you look at the electron and manipulate the state of the coin
to match. Easy-peasy.

Is that not what you wanted? If not, why? (Remember that when I suggested you
would want the coin flip to be random, you denied it.)

------
z3t4
If we solve Cryosleep we don't need to go faster then light, just aim
carefully and hope that the computers have not died by radiation when it's
time to wake you up in a million or so years. As time seem to appear infinite.
But what if when we wake up, all the mass in the universe (except our space-
craft) have been sucked into another dimension, will time still exist !?

~~~
saagarjha
Except that this means that you are disconnecting yourselves from anyone who
is not in cryosleep, as they would have all died. Incidentally, this problem
exists even when moving at close-to-light speeds as well :(

~~~
jacobush
Or almost as if you move away from friends and family. The human condition.

~~~
TeMPOraL
If you move away, you usually can come back.

    
    
      Now it's two months out and it's two months back, when you're pushing the speed of light
      Twenty years on your homeworld's track, pushing the speed of light
      And your friends are gone and your lovers too
      And there's damn-all left that you can do
      And you try to lie, but you know it's true, pushing the speed of light
      Pushing the speed of light
    

[https://www.youtube.com/watch?v=ud6LiVJkwyA](https://www.youtube.com/watch?v=ud6LiVJkwyA)

~~~
izzydata
This reminds me of the final battle in the anime Gunbuster where they are
trying to save the human race while traveling at light speed. They know if
they engage the alien race they will never be able to see their friends or
family again.

I think they ended up returning 12000 years later to find the entire planet
waiting for them with a light display across the Earth that says "Welcome
Home".

------
guide42
> This question has a very long history of being studied by physicists on
> weekends.

How much of science is done on weekends? I feel that investigations without
capital investment is done in a hobby-type schedule. Probably these discovers
(if any) might don't change our reality immediately, but are not worth the
investment?

------
foobaw
[https://en.wikipedia.org/wiki/Timeline_of_the_far_future](https://en.wikipedia.org/wiki/Timeline_of_the_far_future)
Maybe someone from the future could update this for us.

------
madmaniak
Time doesn't exist therefore you can't travel in it.

~~~
Koshkin
Not by itself, but it's part of the space-time continuum (through which you
_can_ travel).

~~~
madmaniak
You confuse Einstein model of physics which is powerful and help us count and
build things with what actually the reality is. We use many different models
in math or programming without believing they are reality.

------
sys_64738
I would equate time travel to being a memory and thought process. We're able
to regenerate the images of events in our minds vividly when we think about
past events which make us happy (e.g. child birth) or things that make us sad
(e.g. manager screaming at us due to another bug crashing our software). This
is time travel to the past by my definition.

For time travel to the future, I think it's about predicting future events of
a few seconds before they happen, based on past knowledge we've gained or
learned. An example of this for me is seeing a situation where somebody is
about to crash their vehicle, as the driver doesn't see another vehicle then
it happens. I recall a recent fender bender like so in Manhattan during
January. I knew it was about to happen given there wasn't the space then
crash.

So those are my definitions of time travel.

~~~
toper-centage
Have you watched Stein's Gate? If we could send a little bit of data to the
past maybe we could fit our memories.

