

Stephen Hawking outlines three theoretically realistic ideas for time travel - edw519
http://www.msnbc.msn.com/id/37018725/ns/technology_and_science-science/

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dpapathanasiou
Hawking didn't address the biggest problem, which, as everyone knows, is
grammar:

" _The major problem is quite simply one of grammar, and the main work to
consult in this matter is Dr Dan Streetmentioner's Time Traveller's Handbook
of 1001 Tense Formations. It will tell you for instance how to describe
something that was about to happen to you in the past before you avoided it by
time-jumping forward two days in order to avoid it. Most readers get as far as
the Future Semi-Conditionally Modified Subinverted Plagal Past Subjunctive
Intentional before giving up: and in fact in later editions of the book all
the pages beyond this point have been left blank to save on printing costs._ "

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Jun8
Naming time the "fourth dimension" is a misnomer, it's not a dimension in the
usual sense of the. Hence the spacetime of relativity, formulated in Minkowski
space, is different than standard 4-ED Euclidean space (see
<http://en.wikipedia.org/wiki/Minkowski_space>).

As Einstein says, "Make things as simple as possible, but not any simpler."
The time as fourth dimension thing, I think, is an example of a wrong analogy
pushed to make it easy for people to understand what in effect is quite hard
to grasp.

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aero142
I always understood a dimension as "a variable that you can change to avoid
occupying the spot." In 3d space, two objects cannot be in the same spot so
either their x,y, or z coordinate must be different. If I add time as a fourth
dimension then two objects can be at the same x,y and z, but not at the same
time. You are right that time is a special dimension, but there are many
things it has in common with the 3 spacial dimensions as well, my example
being one.

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Jun8
You're right, in an intuitive sense. In math, especially dealing with weird
stuff like extra dimensions, intuition may break down, though. For example, in
the 4D Minkowski space you can have a unit sphere with a radius of -1! One has
to understand what is meant when we say that "time is a special dimension".
otherwise teh analogies we draw may fail. The Wikipedia article I linked
explains these in a simple way.

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ryanelkins
I've never really understood why we shouldn't be able to travel backwards in
time.

Most sci-fi movies and shows seem to represent time as a line. I think of our
history as a line, but every step along that history had an infinite number of
possibilities it could have gone. To travel back in time would be to move back
along the timeline that our reality exists - but once at that point, simply by
being there, you would never be able to move forward into the timeline you
came from. You're now on a new timeline, and anything you do has no effect on
the timeline you came from.

Also, I don't know about these forward "time travel" methods described in the
article. By those definitions we are all time traveling right now - we are
moving forward in time. Its just the idea that some people move at different
speeds so that when you slow catch up to the speed of everyone else, you're
now "in the future". Really you were always there, you were just moving at a
different speed.

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ars
> I've never really understood why we shouldn't be able to travel backwards in
> time.

Because time is not a place you can go to. Time is a measure how how fast the
various reactions, and forces, and processes that make up your atoms, moves.

Those reactions can move faster, or slower. But there is no place you can go
to to travel backward in time. Time is NOT a river or a line. Time is just a
measure of how fast your reactions processed relative to the reactions of
other things.

The reason time is sometimes talked about as a line is that it works
mathematically. And the mathematics works if you reverse it too. But that
doesn't mean the math has a physical reality.

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ryanelkins
Yeah I understand what you're saying from a practical approach. I'm looking at
it more from a sci-fi (emphasis on FI) angle I guess. I don't really consider
what you're talking about as "time travel" any more than how I traveled a
minute or so into the future sitting here typing this.

When I said "I've never really understood why we shouldn't be able to travel
backwards in time." I was referring to the statement from the article:
"Ultimately, scientists may find that only travel into the future is possible,
as the laws of nature may make travel to the past impossible so the
relationship between cause and effect is maintained." particularly the part
about cause and effect.

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lmkg
Traveling backwards in time is functionally equivalent to exceeding the speed
of light; in fact, an object is traveling faster than the speed of light in
one inertial frame of reference if and only if there is a second inertial
frame of reference, with relative velocity to the first less than the speed of
light, in which it is traveling backwards in time.

Technical matters of relativity aside, consider that you can't actually change
how fast you move through time. You can't make time move faster or slower, at
least within your own frame of reference. Time doesn't just move in a fixed
direction, it moves at a fixed rate. Note that this is pretty close to how
special relativity actually works, modulo semantics of "rate" when dealing
with time.

There are also thermodynamic considerations. One direction of time does seem
to be privileged in terms of entropy. This doesn't necessarily mean that time
travel is impossible, as far as I'm aware, but it does mean that "forward" in
time isn't completely arbitrary, and reversing that may be thermodynamically
unfavored.

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crazydiamond
So what's new here ?

We always knew if you travel really fast, time slows down for you, and people
on Earth have aged faster. We always knew time moves at a different rate
depending on gravity, too. Its there in SH's Brief History. Using BH's and
WH's has been talked about for years, too.

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Jun8
You're right, that classical special theory of R, but I don't think that's
what people have in mind when they think about time travel. If they're so
inclined (like me), they think of it as a way to visit a person from the past,
e.g. Cleopatra in her imperial boat. Or, going to the year 2500.

As far as I understand, the crucial physical and philosophical problem is: Why
is time's direction one way? Why can't we reverse it. Some physicists explain
this by saying time moves in the direction of increasing entropy. But WHY does
it have to be so? There's no inherent physical law, AFAIK, that says that this
must be the case.

In your example the direction is still the same for both people, it's just the
difference of inertial frames.

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Tamerlin
That's actually also true. The only thing that physicists have identified so
far as being linked to time's directionality is entropy. Pretty much every
other aspect of physics works with time's arrow pointed in either direction.

So from a physical point of view, the only thing keeping the egg you dropped
from putting itself back together is the slope of the entropy curve associated
with that event.

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lutorm
Apart from the quantum foam wormholes, it's just describing almost century-old
(wow!) results of special and general relativity. Nothing especially exciting
if you know the tiniest amount of physics.

~~~
ErrantX
Agreed - for us.

But this was printed in a national Sunday paper here in the uk. Which I think
is great - my dad was talking about it for ages.

EDIT: huh? my point being that the audience in question, for the most part,
have little active interest in physics - thus this was a great way to expose
people to some new ideas :)

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jacquesm
All of these have been exploited in science fiction that is over 3 decades
old, and most of it wasn't new then either.

That doesn't make it less interesting, but it certainly isn't new and it
hasn't got much to do with Stephen Hawking.

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hugh3
Further support for my idea that Steven Hawking is now fully paralyzed and his
speech computer is being remote-controlled by his nurses.

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jacquesm
I don't think that's in very good taste.

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hugh3
Meh, I don't find it to be in unacceptably bad taste. The comment was made
more out of sympathy at the horrors of paralysis than out of cruelty or a
desire to offend. It's "gallows humour", not "a sick joke".

~~~
jacquesm
Let's save that discussion for when you've spent a good part of your life in a
wheelchair without a means of expressing yourself directly, other than through
some electronic gizmo.

You're making a joke at the expense of a guy that has had just about every
card in the deck stacked against him and that made in spite of that more of
his life and did more for us all than you will probably ever do.

If you don't see why it is in bad taste then I'm really sorry for you, if you
think that that was 'out of sympathy' then I must have a big problem
understanding English, and I hope I'll never be on the receiving end of your
sympathy.

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crazydiamond
If I visit the future, say 25 years hence, I'd like to be able to look back
and see what i did in that time. I'd also expect to see grown up kids, maybe
grand-kids.

If i visit the future the way Hawkins suggests, by slowing down my present...
I would land up in a future where there's been no me for that period. All
there is a memory of 5 years in my mind (while I was traveling in space).
There's a missing 20 years. I can't then (hypothetically) jump back in time
and see the last 20 years, since i was not "there" during that time.

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crazydiamond
We can't revisit the past, since it's already happened. We can't travel into
the future because it's not yet happened.

Am i missing something here ?

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ars
"Ultimately, scientists may find that only travel into the future is possible"

WOW! I did it! I traveled into the future! Not by much, but I'm at least 1
minute farther into the future than when I started.

The entire article talks only about travel into the future.

Now while that's correct from a scientific point of view, it's not really time
travel any more than day to day life is time travel.

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thirdusername
.. Or how teleportation is the same as walking. :)

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shawndrost
In quantum foam, miniscule tunnels between distinct times appear. Is it
possible to put something in such a tunnel, and push to widen it? What forces
would be exerted to counter that? Or do these "tunnels" obey totally foreign
laws of physical interaction?

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vl
Original:
[http://www.dailymail.co.uk/home/moslive/article-1269288/STEP...](http://www.dailymail.co.uk/home/moslive/article-1269288/STEPHEN-
HAWKING-How-build-time-machine.html)

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expeditious
Hawking is great. Not because these types of articles are particularly
detailed or illuminating, but because this is the kind of stuff that lures
young people into becoming rocket scientists. Go Steven!

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panacea
Time travel and teleportation are _the_ hardest sci-fi dreams to achieve, in
my opinion.

We need tesseract level API access before we can hope to use either of them.

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cousin_it
If we upload our minds into computers, we will be able to code up and inhabit
a simulated universe where time travel and teleportation are possible. I
wonder if we will want that, though.

~~~
crazydiamond
Here's a quote from today to supplement top post:

Randy D. Allen, a biologist at Oklahoma State University, argued that a smart-
enough species could develop a quantum computer and eventually transfer their
consciousnesses into it.

"No more inefficient metabolism requiring huge energy input, no chemically
derived bodies to wear out, no reproduction, no death, no taxes. Just
supermassively parallel collective consciousness with unlimited capabilities,"
he wrote. "Perhaps, through super symmetry or entanglement, they can "see" or
"feel" the entire universe. Maybe, they've gained the ability to manipulate
elementary particles and can control its evolution and its fate. They would
have become, by any human definition, Gods."

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crazydiamond
A few years back there were articles that you could travel back in time, but
only to the point when the time travel machine was invented. Now you can't
travel back at all.

