Thread: I just can't understand Einstein...

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.