Time travel? (realistically)

Yinyang, I'm sorry you are completely wrong about the nature of light, it travels at a constant speed no matter what the observer's speed is. Cyberninja, the time dilation that you are referring to is part of Einstein�s theory of relativity, and according to that then nothing can be accelerated to the speed of light because as it accelerated it's apparent mass would get larger and larger and thus require a larger and larger force to accelerate. Here is the derivation

F=ma=m(dv/dt)=d(mv)/dt=dp/dt therefore: F=d([gamma]mv)/dt=d(mv/sqrt(1-u^2/c^2))/dt=
m([gamma^-1]dv/dt-v([v/c^2][dv/dt]gamma)/(1-u^2/c^2)=
(after a little cleaning) F=ma(1-u^2/c^2)^-3/2)
a=(F/m)(1-u^2/c^2)^3/2
realitivistic mass= m[gamma] -as you approach the speed of light your mass becomes infinite as a consiquence:
as u (velocity) increases without bound (1-u^2/c^2)^3/2 becomes zero and the acceleration of the particle becomes zero.

Sorry if it's not clear, that was incredibly tedious and difficult to write. Anyways it just show that under the laws now it is impossible to accelerate a particle with mass to the speed of light.

Originally posted by Mxyztos
... Anyways it just show that under the laws now it is impossible to accelerate a particle with mass to the speed of light.

Couldn't you thoretically instantly 'jump to the speed of light?

You would still have to move from your inital speed to the speed of light, there would be a change in velocity and that means there would still be an acceleration propotional to the force applied.

There are ways to accelerate and deccelerate time.

Einstein (being the genius that he is) accurately predicted that space and time are not mutually exclusive, so we should really describe things relatively through "space-time".

Here is an EXTREME simplification of how time is slowed down.

Suppose that we have 5 "units of movement" which we use to move through "space-time". We can use those units to either move through 5 units of time, or 5 units of distance.

If we move through 5 units of distance, we won't move in time at all. Essentially, our bodies won't age and we'll be frozen in time as we travel through that 5 units of distance.

If we move through 5 units of time, we won't move anywhere in space.

The actual equations are much more complex... I can't remember exactly, but time dilation is not directly proportional to velocity. I think it's

t'/t = SQRT[1 - (v/c)^2]

so it's obviously not directly proportional, but at least it's obvious that the greater the velocity the more time become dilated. If v = c (speed of light) then basically t' = 0 (time change is 0, meaning you're frozen in time).

Note that our current equations don't even allow v > c because that would create a negative under the radical. However, if we tried

(t'/t)^2 = 1 - (v/c)^2

we'd still have to account for the fact that the velocity needs greater than the speed of light for anyone to travel backwards in time.

This is not possible because classical physics breaks down near the speed of light. Photons are able to travel at or near the speed of light because they have no mass. If you took even an electron and tried to accelerate it to the speed of light, you'd notice that the amount of force required to accelerate it begins to increase once you near the that terminal velocity of c. This is because, as Einstein said, E=mc^2. The more energy you put into the electron, the greater its mass becomes (because energy IS mass), so the more energy you'll need to accelerate.

This creates a catch-22.
- You need to put more energy (mass) into a massive object to accelerate it than you would for a light object.
- The more energy you put into the object, the more massive it becomes.

No, it's not possible to travel backwards in time with our current Einsteinian understanding of space-time topology.

Now, is it possible to shift a specific object back in time (as in reverse the aging process)?
Theoretically, THAT is possible.
Entropy (chaos) is commonly described as "the arrow of time". Reversing entropy would technically have the effect of reversing time.

Reversing entropy, however, is a statistical improbability if done on a large scale.

On a small scale it's possible to reverse the entropy of a system by drastically increasing the entropy of its environment to counterbalance the decrease of entropy of the system. That is the only way. We can't reverse the entropy of the universe to "travel back in time" because the entropy of the universe always increases (2nd Law of Thermodynamics). We can only reverse the entropy of something small scale (by making sacrifices on the large scale).

Reversing entropy, however, is a statistical improbability if done on a large scale.

Can I see some more reasoning/math? Maybe a link or two? One more thing, I just noticed that in my previous post I forgot to define gamma. gamma= 1/sqrt(1-u^2/c^2) and dmv/dt = dp/dt is momentum. Sorry bout that.

[edit on 2-12-2004 by Mxyztos]

Originally posted by Mxyztos

Reversing entropy, however, is a statistical improbability if done on a large scale.

Can I see some more reasoning/math? Maybe a link or two? One more thing, I just noticed that in my previous post I forgot to define gamma. gamma= 1/sqrt(1-u^2/c^2) and dmv/dt = dp/dt is momentum. Sorry bout that.

[edit on 2-12-2004 by Mxyztos]

I'm sorry. I was under the assumption that common sense didn't need reasoning/math/links.

I read your previous posts and I'm INCREDIBLY surprised that you'd know as much as you do about Einstein's relativity and yet have such little knowledge of the 2nd Law of Thermodynamics.
Reversing entropy is as much a statistical improbability as suppose... having all of the air molecules in a room lift to your current ceiling, suffocating you before the air molecules come back down.

I don't think I honestly need to provide a link. The Second Law can easily be Googled.

I know that reversing entropy is on a large scale is impossible but I was just wondering if you had information that is not just "googleable". Sorry if you misunderstood I wasn't trying to knock your logic it was sound. Maybe I typed it poorly, sorry.

[edit on 2-12-2004 by Mxyztos]

I honestly think there are enough links to the 2nd Law that I don't need to provide them.

Since it wont take too much time to spell things out right now in this post I'll do so.

Entropy (chaos) and energy are at odds with each other.
Increasing entropy is (to information theorists at the least) the process of losing information.

WARNING, GROSS SIMPLIFICATION AHEAD (Please do not argue about very slight trivialities. The concept is correct)

For example, we have bottle of air and another bottle with no air. The bottles are attached but have a divider keeping the air in one and not the other. One side of the divider holds air, the other side doesn't.

1 = air molecule
0 = empty
| = divider

This is the system

0000|0011

The system is more orderly (not chaotic) because the total number of permutations of air molecules given the microstates available to the system is low because one of the bottles has NO air in it.

These are the possible permutations:

0000|0011
0000|0101
0000|0110
0000|1001
0000|1010
0000|1100

Now, we KNOW that these have to be the only permutations because we have a divider there. We have energy stored.

We lift the divider for a minute or two and then let the air run through. A "pop" sound should be heard as air rushes through. That is a sign that energy has been released (sound takes energy).

We put the divider back in, but do we know what we did before lifting the divider? No information was lost. Now the permutations could be ANY of these:

0000|0011
0000|0101
0000|0110
0000|1001
0000|1010
0000|1100
0011|0000
0101|0000
0110|0000
1001|0000
1010|0000
1100|0000
0001|0001
0001|0010
... etc ... (I'm too lazy to type it all out, you should get the idea by now).

Information we HAD was lost, and we got energy for it.

The reversal of entropy (gaining information) takes energy.
Suppose we wanted to limit the permutations to just one:
000000|11
by placing the divider in and keeping those two air molecules stuck in that position.
We'd have a LOT of information. In fact, we know the exact position of those air molecules (no more "guessing").

To reverse entropy, we'd have to add energy or push the air in, compress it.
something like this
(P = pusher)
00101000
P0101000
_P101000
__P11000
___P1100
____P110
_____P11

Then we add the divider
_____P|11
And move the pusher back
000000|11

The problem is this "pusher" has energy supplied from an outside source (not part of the system). So on a small scale, we "reversed" entropy. But on a large scale we've increased entropy.

The example I used sucked, but that doesn't matter unless you're a picky quantum physicist. It was enough to get the point across.

If you ARE a really picky quantum physicist just
- replace "air molecules" with "quantum mechanical particles moving close enough to the speed of light to have noticeable wave properties"
- replace "2 bottles of space" with "universe with discrete space topology"

So yeah, that's basically how you "reverse time" on a small scale.

That's very interesting. I've heard that example given before and I've heard the "entropy arrow of time" before but I never thought to put the reversal of entropy with the reversal of time. The only thing that I'm wondering about is that in your example (and if possible on a large scale) to constitute traveling back in time wouldn't all the particles have to be in the exact same configuration? Not just represent the same total amount of entropy?

Yes, exact same configuration, not just same entropy.

That was just an example of how to decrease entropy, not to reverse time.

My point simply was that reversing entropy of a small system is possible, but would take a tremendous amount of energy from outside (which means increasing entropy on the outside, because you use energy to decrease entropy, you gain energy by increasing entropy).

Obviously, this means that we'd need to know the exact previous state of the object to time shift it back.

In fact, this would be essentially the same thing is just rebuilding the original object using the current object.

So much for "reversing the age" of the object.

So we're in agreement? Going back in time on a large scale is impossible? (save for the multiple universe theory interpretations). Like what was said before all the particles would have to be moved into the exact same previous configuration, and wouldn't this be impossible because we cannot measure the exact position of particles because of the uncertainty principle. In theory we could make all the particles behave according to their previous wave function but there would still be the chance that the particle could be somewhere else, and we could never be certain that the exact same configuration was reached. Correct me if I'm mistaken.

We are in agreement. It's impossible given our current technology.

Uncertainty Principle doesn't really say much because if we didn't know BEFORE what the precise state of an object was, what's to say we should even care about the exact state of an object should be when "shift it back in time"?

I wasn't saying that it WAS possible realistically to shift something back in time for an object. I'm just saying that it's not IMPOSSIBLE.

Traveling back in time (not counting travelling to parallel universes a la Tegmark's theory) is IMPOSSIBLE because of the paradoxes that arise (killing yourself in the past).

It's not impossible to... say... move an atom or two back to their original configuration 1 second ago (ignoring uncertainty).

one can dream...

one can also read books about what other people wrote and convince themselve that way too but in the end

that's as far as it goes.

It would be quite a feat to be able to move atoms back to a past configuration. It would also be very interesting to be able to engineer molecules to have certain properites as well, albeit off topic.

I used to always wonder about this...

My idea was that IF you could accurately track and predict wormholes (as they are in essence thought to be tunnels in space/time), and you could find such a tunnel (or predict where it would appear) to the desired designation....perhaps, IF able to convert matter into energy, then stream it into the wormhole, and then reconvert it back to matter, then maybe that would then be time travel.

I don't think that the Universe bothers itself with paradox or timelines, who knows? Perhaps you could do all kinds of things in the past, then take a tunnel back to your point of origin with nothing changed?

(There are two views on this that i have observed)
1.Changing the past will alter time into a paradox (Back to the Future2)
To enter the future is possible but traveling back would create a unbalanced universe where to of the same particles of a symmetrical base would contradict the itself in the time to when it meets itself ultimately destroying itself.
2.After all your studies you really do not have to time a wormhole just make one. lets say I make connecter gate made of all the same materials symetrical to one i will make in the future i would need to create a magenetic force 30^100 of the sun to implode an electron. to create a imaginary plane of F(x). connecting the string thereory. its like a miny blackhole in your backyard, but see you can not make one on a planet the magnetic interferance of the earth would create an paradox here on earth. electrons are needed to travel trough a wormhole when a mass enters it becomes an energy form(dark matter) where electrons connect the two gates, the other gate i make lets say 2 weeks from now ofthe same materials and the connecter like an email address the two would connect turning your body to how it was 2 weeks ago and the earth would be 2 weeks ago as it was when you made it.
what iam saying is we need to make a time window to what has happened not what has not happend yet.