time travel

[wazzuuup]

Maybe everything moves in a circle through the universe, so can't we travel through time by cutting the edges of light?

[Tormod]

Kurt Gödel argued that in a rotating universe you could do exactly that.

However, nobody has found any evidence to substantiate the idea of a rotating universe.

Here is an interesting page detailing different theories about this:
http://www.ettnet.se/~egils/essay/essay.html


Tormod

[TeleMad]

wazzuuup: I was wondering which theories exist on time travelling and what factors are tied to help explain the topic.

Einstein's special theory of relativity explains time travel. I'll try to sum it up.

1) The laws of physics are the same in all reference frames in uniform motion (Einstein’s theory of special relativity).

2) Maxwell’s equations of electromagnetism are among the laws of physics.
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Therefore, Maxwell’s equations of electromagnetism are the same in all reference frames in uniform motion.



1) Maxwell’s equations of electromagnetism are the same in all reference frames in uniform motion.

2) Maxwell’s equations of electromagnetism calculate a specific value for the velocity of electromagnetic waves (such as light) of approximately 300,000 km/s (or about 186,000 mi/s).
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Therefore, the velocity of light is the same (about 300,000 km/s) in all reference frames in uniform motion.


LIGHT CLOCK
A time keeping device that uses a light beam as its source of regularly repeating events (“ticks”) is called a light clock. One “tick” consists of a flash of light emanating from a source at the bottom of the light clock, traveling to the top of the light clock where it is reflected by a mirror, and traveling back down to the bottom of the light clock and reaching the source from which it originated.

Consider the set of events that constitute one “tick” on a single light clock, but from two separate reference frames. The first reference frame is at rest relative to the light clock while the second reference frame is moving to the left relative to the light clock (or equally, the light clock is moving to the right relative to the second reference frame). Remember, both reference frames are observing the very same set of physical events.

The height of the light clock is 1 unit. Therefore, the distance that light travels in the light clock in the first reference frame - the one at rest with respect to the light clock - is simply twice that (since the light travels up and then right back down), or 2 units.

The distance that light travels in the light clock in the other reference frame is NOT just 2 units. Why? Because between the time the light pulse left the source at the bottom and reached the mirror at the top, the mirror - and the whole light clock - had moved to the right some distance. So the path the light took, from this reference frame, is not straight up but rather a sloping, diagaonal line. And by the time the light pulse makes its way from the mirror at the top back down to the light source, the light source - and the whole light clock - will have moved even farther to the right. So again, that downward path is not vertail but rather is sloped and diagonal. Thus, the light path in the second reference frame is longer than that in the first reference frame.

1) The distances for the same set of events differ for the two reference frames in relative motion to each other: the distance in reference frame at rest with respect to the clock - known as the proper distance - is shorter.

2) All inertial reference frames will measure the same speed for light: thus, the speed of light is the same in both reference frames.

3) By definition, speed = distance / time...v = s/t.

Since the two reference frames have the same speed for light while also having light travel distances that differ, the elapsed time in the two reference frames MUST be different. Why?

By definition, velocity (v) is the change in distance (s) divided by the elapsed time (t), or v = s/t. Suppose two cars are traveling along a long stretch of open interstate highway at a steady 60 mph. The first car travels for 1 hour and cover

[TeleMad]

Shoot, after writing the previous post, I found something I wrote in my personal notes that probably does a better job...

Modern Physics
Modern physics began around the year 1900 and continues to this day. It consists of two conceptual divisions: relativity (both Einstein’s special theory of relativity and his later general theory of relativity) and quantum mechanics.

Special Relativity
Einstein’s theory of special relativity produces a Universe most of us would find counterintuitive. Loosely speaking, some examples include: it is possible for one identical twin to end up being many years younger than the other; events one observer sees as being simultaneous are not simultaneous for some other observers; velocities do not add together in simple 20mph + 20mph = 40mph fashion; time is divided not only into the present, the past, and the future, but also into something called the elsewhere; observer A’s clock can be running slower than observer B’s clock and at the same time observer B’s clock can be running slower than observer A’s, without there being any contradiction; and so on.

Yet all of the above-mentioned counterintuitive conclusions follow from the simple idea of special relativity…that the laws of physics are the same for all (observers in) uniformly moving reference frames. Let’s look briefly at what this tells us about the speed of light and then about measures of time.

Maxwell’s Equations
Magnetism and electricity – which had been thought to be two completely separate phenomena –
were unified into electromagnetism by James Clerk Maxwell in the 1800s. One consequence of his unifying equations was that there should be electromagnetic waves and these waves must travel at approximately 186,000 mi/s (or about 300,000 km/s). This result – which matched the already known speed of light, c – fell naturally out of Maxwell’s equations: he did not need to manipulate his work to force this result. No counterintuitive conclusions followed yet because it was thought that while the laws of Newtonian mechanics (i.e., the laws of motion, gravity, etc.) were the same for all observers, the laws governing electromagnetism were not.

Speed of Light is a Constant (c)
But Einstein changed all of that when, in 1905, he published his theory of special relativity. Then all of the laws of physics – including the laws of electromagnetism – were the same for all (observers in) uniformly moving reference frames. Since Maxwell’s equations are among the laws of physics and they mandate that electromagnetic waves travel at approximately 186,000 mi/s (in a vacuum), then all uniformly moving reference frames must measure the same speed of light and other electromagnetic waves, even reference frames that are moving relative to each other! An example might shed some light on this.

Suppose there are two observers, A and B, in two separate uniformly moving reference frames and a pulse of light, emitted from a star, is traveling towards them. Let us suppose that observer A is at rest here on the Earth. Since scientists have already measured the speed of light from here on Earth to be approximately 186,000 mi/s, we know that A will obtain that result, regardless of how B is moving. Let’s now look at things from B’s perspective (reference frame). First, suppose that B is in his car speeding down the highway into the oncoming light pulse at 100 mph. According to Einstein’s theory of special relativity, B will not measure the speed of that light to be 186,000 mi/s + 100 mi/hr, but rather simply 186,000 mi/s (the same speed as that measured by observer A who is at rest). Furthermore, if observer B were flying in a jet airplane into the oncoming light pulse at 600 mi/hr then he/she would not measure its speed to be 186,000 mi/s + 600 mi/hr, but rather, again, simply 186,000 mi/s (the same as observer A). In fact, if B were rocketing into the oncoming light pulse at half the speed of light, he/she would still measure the speed of that

[TeleMad]

Veritas: A thereory that really grabbed my attention is that if you travel into the past, you can't change the future. This is because you were already there to cause the event to make this present. For example, if I went back to 1913 and killed an important leader, the future wouldn't change because when 1913 was the present time, I appeared and killed him.

There actually are some hypotheses that allow one to travel back in time and change history, without it causing any inconsistencies. I haven't read the material lately, but I believe it goes something like this.

The paradoxes of time travel into the past usually arise because one could change history in such a way as to make having gone back into the past impossible. For example, if I go back in time and kill my mother, then I will have never been born...so I couldn't possibly go back in time to kill my mother in the first place. A paradox to be sure.

However, traveling back in time is usually considered possible only when constrained by being consistent with history: one must move along a timeline that already existed. For example, there is no timeline in which I existed before I was born. So I could travel back in time to my 13th birthday, or to my birth, but no farther, since doing so would be inconsistent with history.

Thus, one commonly heard paradox is avoided...one can't prevent ones own birth by killing his/her ancestors. But one could still travel back in time and kill himself/herself...another paradox...possibly. This requires being sidetracked for a minute.

In quantum mechanics, things such as electrons can be in a state of superposition - in a sense, they are both here and there at the same time, but we can't tell in which place they will be found until we perform a measurement. Where the electron is most likely and least likely to be found is described by a probabilistic wave function with multiple solutions. The act of measuring the system forces the electron to appear in only one of the possible positions - the wave function collapses to having just the one observed solution.

But according to the MWI (Multiple Worlds Interpretation) of quantum mechanics, there is no collapse of the wave function. Instead, whenever a superposition choice in nature exists, all possibilities are actually realized. How can this be? The Universe splits, producing one Universe for each possibility, in which that particular possibility occurs. If there are two possibilities, then two universes exist after the event, with each universe containing different copies of the electron and the people observing the outcome - in fact, everything in the original universe is copies - all trees, houses, planets, stars, etc. - with the only difference being that in one universe the electron was found in position A and in the other the electron was found in position B. Thus, the number of universes continually increases as splits continue to occur for all events that have multiple possible outcomes. There is not just one of you, but multiple millions of instances of you, each inhabiting a different universe. As silly as this might seem to some, it is a logically consistent theory that explains what happens at the quantum level. So in the MWI, one can think of an ever-branching tree with the beginning of time at the trunk and the multiple presents at the very ends of the branches, at the leaves. Each branch represents a timeline that has occurred (in one universe of another).

So in the MWI interpretation, one could start from his/her current "leaf" and travel back along any preexisting timeline consitent with history, and then, when coming upon a branching point, switch over to a different timeline branch that had a different future. So, at any point in the past - in any universe - in which I could have died, I could travel back to there and then kill myself. Athough I would be going back in time and changing history in a sense, I would still be followin

[afrologics]

time travel is simply base on Alberteinsten theorem of relativity.i mean time dilation and length contriction that occur when a body is assume travelly at aspeed close that of the reference frame (velocity of light).

[TeleMad]

FreeTinker: OK, if for sake of argument we ignore the impossible physics of moving back in time.

Are you positive physics says it’s impossible to move back in time?

”Motion in space can proceed in any direction and back again. Motion in time only proceeds in one direction in the everyday world, whatever seems to be going on at the particle level. It’s hard to visualize the four dimensions of space-time, each at right angles to the other, but we can leave out one dimension and imagine what this strict rule would mean if it applied to one of the three dimensions we are used to. It’s as if we were allowed to move either up or down, either forward or back, but that sideways motion was restricted to shuffling to the left, say. Movement to the right is forbidden. If we made this the central rule in a children’s game, and then told a child to find a way of reaching a prize off to the right-hand side (“backward in time”) it wouldn’t take too long for the child to find a way out of the trap. Simply turn around [to the left] to face the other way, swapping left for right, and then reach the prize by moving left. Alternatively, life down on the floor so that the prize is in the “up” direction with reference to your head. Now you can move both “up” to grasp the prize and “down” to your original position, before standing up again and returning your personal space orientation to that of the bystanders. The technique for time travel allowed by relativity theory is very similar. It involves distorting the fabric of space-time so that in a local region of space-time the time axis points in a direction equivalent to one of the three space directions in the undistorted region of space-time. One of the other space directions takes on the role of time, and by swapping space for time such a device would make true time table, there and back again, possible.

American mathematician Frank Tipler has made the calculations that prove such a trick is theoretically possible.” (emphasis added, John Gribbin, In Search of Schrodingers Cat: Quantum Physics and Reality, Bantam Book, 1984, page 193)

And more than 10 years later, in his "second edition" - and at least one other book - he repeated this.

"As I spelled out in my book In Search of the Edge of Time, there is actually nothing in the laws of physics (including those of the general theory of relativity, not just the special theory) that forbids time travel. It would be extremely difficult, and it runs counter to common sense. But it is not forbidden by the laws f physics..." (John Gribbin, Schrodingers Kittens and the Search for Reality: Solving the Quantum Mysteries, BackBay Books, 1995, page 82)

[TeleMad]

To address another issue brought up in the thread.

In order to verify time dilation, and therefore the ability to "travel into the future", we don't need any kind of an absolute reference frame to measure spatial coordinates in. Time dilation involves relative motion and each observer in uniform motion has a myriad of valid coordinate systems from which to choose. For example, when scientists confirmed time dilation based on the rate of muon decay, they didn't need to know what their "absolute, universal" coordinates were.

[paultrr]

I might also suggest: http://65.108.189.168/Docs/ROTATION%...ULED%20OUT.pdf as it appeared in or own Journal.

[paultrr]

or see attached file itself.