
Hello! Thanks for helping me before with my
questions, I am wondering about the possibilities
of time travel, and my teachers can't help me.
So, here it is: If, theoretically, some
one was able to travel faster than the speed of
light, thus, theoretically, traveling back in
time, would it be possible to get those effects
or results gravitationally? In other words,
instead of reaching a certain speed to reverse
time, the person stays in a very large gravity
field to do that time dilation. So, if you had a
large enough gravity field could it actually
reverse time, doing the same thing as if you were
traveling faster than C? If not, why? If so,
why/how? Is there a formula to calculate that?
Could it be done using an artificial
gravitational field?

Question Date: 20080125   Answer 1:
In a pure gravitational field, the
timedilation effect also creates a practical
barrier to access to the universe at large once
the dilation is large enough. For example, you
could take a hyperbolic orbit approaching close to
the event horizon of a sufficiently large
(megasun) hole to travel far into the future in
the sense that the trip is short for you and the
ship  but arrives far into the future of the
external frame. One issue is that in the portion
of the trip close to the horizon, all the light
that falls on you for the external length of time
occurs blueshifted in the local frame you
probably are blasted by cosmicray energy photons
which were starlight in the 'slow' external
frame. To arrive at a timepoint before you
left, requires a very rapidly spinning hole and
seems to require crossing the event horizon.
Although such a trip can miss the singularity, it
is not clear where you end up in that there is no
guarantee that it is in this universe unless our
spacetime is multiply connected. There are
other approaches to time travel which might not
require and event horizon  check out 'Tipler'
machines... still not practical, but quite
interesting.
  Answer 2:
Hello! I will try to help you: In fact, you
are quite right; creating a super large
gravitational field is the only way to do this.
You can never accelerate up to the speed of light,
and you certainly can't travel faster! First of
all, Einstein's equations of special relativity
tell us that to reach the speed of light, you
would need infinite energy. Since this is
impossible to achieve, we can't do it. Photons
have zero mass. So, the other way to do it is by
folding spacetime with a large mass this is
essentially what a wormhole does. You get
two black holes to poke through spacetime to
create a worm hole, and you have a time machine.
The only problem is, since gravity warps
spacetime, if you could create a wormhole, once
you go into it, your small mass would cause it to
collapse! Now, suppose you had the
technology to create a wormhole AND keep it open;
if you go back in time, you will alter your own
future, and then what would be the guarantee that
you would even exist, so you could not have gone
back in time. This is what we call a
paradox. Stephen Hawking has come up with a
solution to this paradox: if you went back in
time, you could not return by the same path. You
will alter the future, but not your own future.
You would be the only one to remember what you
did. Problem is no one in the new future would
believe you. A good reference about time
warps and black holes is by Kip Thorne; I forgot
the exact title, but if you look for Kip on
Amazon, you'll find it.Lawrence Krauss has also
written about this in his book, The Physics of
Star Trek. GOOD QUESTION!Best,
  Answer 3:
There is currently no known way that you can go
backward in time using either high speed or
gravity. As you said, you can almost (but not
quite) stop time if you're traveling at velocities
close to C, or in the bottom of a huge gravity
well. But neither of these can actually stop
time, much less go backward in time. If time
travel is possible, it will likely require either
worm holes or some kind of futuristic "warp field"
that suspends the laws of physics as we know them.
We obviously don't know of any way to use either
one of these right now. In fact, if worm holes
exist at all, they are apparently smaller than a
single atom; we've never detected one. Most
theories of worm holes rely on the existence of
some kind of exotic matter with negative energy
density, and this has never been observed in
practice. One humorous "proof" that time
travel will never work was the MIT Time Traveler
Convention back in 2005. The idea was to hold a
big party which future time travelers would know
about, hoping they would go back in time and visit
it. But no one from the future showed up. (Or at
least they weren't seen. Maybe they had
invisibility cloaks and wanted to avoid all the
questions.)
  Answer 4:
The problem with trying to go faster than light
is that any object with any finite mass will
become infinitely massive as it approaches the
speed of light, and therefore require an infinite
amount of energy to actually get there. If you
somehow could get there, then going even faster
would result in your acquiring an imaginary mass
(because the equations of special relativity
result in taking the square root of a negative
number if your velocity is greater than
c). The formula is:
M = m/
√(1((v/c)^{2})) , where M is mass, m is
mass at rest, v is velocity, and c is the speed of
light. Being in any gravitational well will
cause time to slow down, but it won't stop time
unless the well is infinitely deep (i.e. the
singularity at the very center of a black hole),
and it certainly won't reverse it. There is a
formula for this, but it is general relativity,
and I don't understand the math well enough to
explain it to you. So far as we know, there
is also no way to create an artificial
gravitational field. Now, there was a recent
article in the Science magazine that announced
that some mathematicians have figured out that it
is possible in theory to use a spaceship moving in
a certain way to create a gravitational well that
could propel the spaceship back in time. I don't
know how this works or what the limitations on it
are (e.g. the spaceship might need to have the
mass of a galaxy or something like that). But
there is the idea, at least.
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