I just can't understand Einstein...

Okay, So I was reading about the Large Hadron Collider in WTF, and I searched Time Travel on google for some more info. I came to the Nova Time Travel website: http://www.pbs.org/wgbh/nova/time/

The third link they give you is an experiment to "Think like Einstien" in which you come to the conclusion that Time Travel is possible by looking at the relativity of distance to time. For Instance, you on a train throw a ball (the train moving 50 mph, the ball thrown at 20 mph) to a person on the sidelines the ball seems to be moving 70 mph. But f you show a light on the train, no matter if you are on the train or watching the train, the light only moves at 186,000 mph.

Then they come to the conclusion that if you traveled a year going the speed of light into space, and a year comming back at the same speed, it would only seem like 2 years to you, but theoretically 200 years to those on earth.

I don't understand that conclusion. Can anyone give me some insight?

The phenomenon you are talking about is called time dilation. An observer moving at high velocity relative to his surroundings will perceive that time in the external universe (i.e. everything that isn't moving with him) is passing at a different rate. The degree of time dilation will depend on how fast you are moving, increasing geometrically as you accelerate. So the closer you get to light speed, the more your "local" time will slow down (but note that "outside" time will actually appear slower to you, not faster, because from your perspective, it's the rest of the universe that's moving at a high velocity relative to you). So yes, if you take off on a spaceship at near light speed and fly around in space for a while, you will find that more time has passed in the outside universe than has passed aboard your spaceship.

http://en.wikipedia.org/wiki/Time_dilation


At 0.1 C (10% of light speed), the time dilation factor is going to be 1.005, meaning that if one year passes aboard your spaceship, one year and about two days (366 days and roughly 19 hours) will have passed in the outside universe. At 0.5 C, the factor will be 1.15, meaning that if one year passes aboard your spaceship, roughly a year and fifty-four days will have passed in the outside universe. At 0.9 C, the factor is about 2.29, so if a year passes on your ship, two years and a bit under four months will pass elsewhere in the universe. At .99 C it's 7.08, so if a year passes on your ship, a bit over seven years will have passed in the outside universe. And so forth. Obviously at velocities very close to C, the dilation factor will be extreme. If you hopped on your spaceship and flew around for (what seemed to you to be) a year at 99.999999999% of light speed (0.99999999999 C), you'd come back to Earth to find that nearly a quarter of a million (~223,000) years had passed. Your family wouldn't be waiting for you at the spaceport, to put it lightly. If you are interesting in playing around with this and seeing what sort of time dilation factors will occur at various velocities, here is a website with a time dilation calculator: http://www.1728.com/reltivty.htm?b0=0.9

Whether or not this is "time travel" really depends on your definition of "time travel". Unscientifically speaking, it does kind of let you travel "into the future". Our hypothetical astronaut, returning from a one-year journey at 0.99999999999 C, would get to see Earth in what is, to him, the far distant future. He'd probably be an interesting scientific specimen in the eyes of whatever humanity had evolved into at that point; sort of like if a live Neanderthal showed up today.

Einstein himself said it best- to paraphrase, "one second with your hand on a hot stove feels like a minute. One minute holding the hand of a beautiul woman feels like a second. Now that's relativity"

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Originally Posted by Rick Scarf

Einstein himself said it best- to paraphrase, "one second with your hand on a hot stove feels like a minute. One minute holding the hand of a beautiul woman feels like a second. Now that's relativity"

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that's a nice glib comment, but that also ties into why he himself lived to regret calling it the "theory of relativity," because that's not physical relativity at all.

relativity is this: if I am looking at you walking, whether you're walking away from me, up, down, or toward me is due to what angle I'm looking from relative to you.

this is because we're walking on roughly a two dimensional plane. we can "transform" one direction into another by having my perspective change, i.e. "performing a rotation."

in special relativity (i.e. relativity where space doesn't curve, a simplified case of general relativity), not only can we transform directions into each other (so that all 3 directions in space are equivalent), but time can also be transformed into space. that is saying that space and time are really one in the same.

that may not make sense intuitively, but it's the best i can come up with. not only that, but it doesn't capture the fact that time doesn't exactly "rotate" into space in the same way that one direction of space rotates into another. in technical terms, the terms of the transformation tensor terms are sinh and cosh rather than cos and sin as in the case of purely euclidian space.

now with regards to the thing about the rocket ship going into space and coming back, there is a rub in this problem. in order to travel away from the earth and come back, the rocket ship must experience an acceleration somewhere in its trajectory or it would simply travel in a straight line forever. if you were to do the math, you'd actually find that it's this acceleration that's responsible for causing the difference in elapsed times. otherwise, from the earth's perspective you'd always just see stuff in the rocket being slowed, and from the rocket's perspective you'd always just see the stuff on earth being slowed.

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Whether or not this is "time travel" really depends on your definition of "time travel". Unscientifically speaking, it does kind of let you travel "into the future". Our hypothetical astronaut, returning from a one-year journey at 0.99999999999 C, would get to see Earth in what is, to him, the far distant future. He'd probably be an interesting scientific specimen in the eyes of whatever humanity had evolved into at that point; sort of like if a live Neanderthal showed up today.

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Simply because it is unidirectional (and not into the past) doesn't make it any less time traveling. You've gone through hundreds of thousands of years into the future while in your reference frame only one year has passed. If that's not time traveling, I don't know what is.

nothing? not even a "lul no your wrong relativity means everything is relative"? :(

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Originally Posted by sycld

now with regards to the thing about the rocket ship going into space and coming back, there is a rub in this problem. in order to travel away from the earth and come back, the rocket ship must experience an acceleration somewhere in its trajectory or it would simply travel in a straight line forever. if you were to do the math, you'd actually find that it's this acceleration that's responsible for causing the difference in elapsed times.

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lul no your wrong

i thought that even without acceleration, there is a difference in elapsed times, e.g. if i'm traveling at a constant velocity and i see a rocket ship travel past me at a constant velocity

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Originally Posted by sycld

otherwise, from the earth's perspective you'd always just see stuff in the rocket being slowed, and from the rocket's perspective you'd always just see the stuff on earth being slowed.

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this isn't contradictory
assuming you mean "see" as an instantaneous action

in practice we need to somehow transmit the information back and forth, and that can only happen at a certain speed up below the speed of light, so after we send that information we have to admit that our actions have slowed down beyond the rocket's slowness

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Originally Posted by mutton

lul no your wrong

i thought that even without acceleration, there is a difference in elapsed times, e.g. if i'm traveling at a constant velocity and i see a rocket ship travel past me at a constant velocity

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oh ok would you mind coming down and telling the professor i learned this from that he's wrong?

let's say that according to the perspective of earth, the rocket ship is travelling at a constant velocity in a straight line.

from the ship's inertial reference frame, the earth is traveling at the same constant velocity in the opposite direction (okay, well, the earth is actually orbiting the sun, but let's forget about that for a moment since it's not needed in the problem).

thus, from the earth's reference frame, time is dilated within the ship. BUT, from the SHIP'S reference frame, time is just as dilated on earth.

your argument requires there to be an absolute frame of reference by which we can measure speeds. you're treating the earth as somehow being "absolutely still," as though all speeds are physically measured relative to the earth. this is not so.

apparently, you haven't gotten galilean invariance drilled sufficiently into your head yet.

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this isn't contradictory
assuming you mean "see" as an instantaneous action

in practice we need to somehow transmit the information back and forth, and that can only happen at a certain speed up below the speed of light, so after we send that information we have to admit that our actions have slowed down beyond the rocket's slowness

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first of all, no, "seeing" doesn't mean that the signal has to reach the recipient instantaneously.

secondly, if people on earth send information to the rocket, we have to admit that our actions have slowed down beyond earth's slowness. the effects of time dilation in the rocket ship relative to the earth and on earth relative to the rocket ship are symmetric, unless there's something special about rocket ships that i'm not aware of.

thirdly, i really am not even sure what you're saying here.

you are right, i misunderstood you

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Originally Posted by sycld

otherwise, from the earth's perspective you'd always just see stuff in the rocket being slowed, and from the rocket's perspective you'd always just see the stuff on earth being slowed.

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i interpreted this as contradicting what you have explained here:

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Originally Posted by sycld

thus, from the earth's reference frame, time is dilated within the ship. BUT, from the SHIP'S reference frame, time is just as dilated on earth.

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