Twin Paradox....What if the twin leaves earth and stays at his destination? Who is younger?

Question

OK i guess i need to clarify the question. I understand that a moving clock runs slower to a relatively stationary observer, however in the rockets reference frame the earth twin is moving away from him at the speed of the rocket. In the earth reference frame the rocket twin is moving away. Therefore these two reference frames do not agree. Does it depend on the reference frame to tell who is younger? If so, what about an observer free of both reference frames?

Answer 1

Interesting question... however.. Time is linear. The perception of the twin leaving earth at a speed faster than light is younger....is just that... a perception. Just in that we see stars that have burned out many years ago yet we still see them. We see the "relative" past of the object that far away.

If you were on that star looking at the earth... you would be seeing the earth's past. Perhaps seeing the Civil War or something. As you traveled toward earth... things would appear to move in "fast forward" to present time as you arrived.

In as how someone would appear "younger" as they move away from earth... they would in turn appear to "catch back up" as they came back because their relative age would be at an accelerated rate as they came back.

Einstein and anyone who has followed the whole relativity issue agree that the twin would indeed be the same age. How people "age" in different environments such as weightlessness might affect her appearance...but she would be the same age.

Answer 2

The traveling twin is younger.

During the journey, you can't really say who's younger (it depends whose reference frame you go by)

Once he decelerates at his destination (which is not traveling at relativistic speed), you can compare ages.

Whether or not he comes home doesn't matter. What matters is that he returns to earth's reference frame to find himself younger.

Edit response to added info: your intuition is correct. As long as the twins are in different reference frames, they can't agree on who is older. A third observer might disagree with both. The point is, though, that once the traveling twin slows down at the destination, his reference frame switches back to his twin's. It's the act of deceleration that is important--not the trip home. Now he realizes that he has aged slowly. The third observer will also agree (now that both twins are in the same reference frame) that the traveling twin is younger (although he might disagree on the exact age).

Answer 3

The twin who left the earth is younger because in his travels he left the earth traveling at 99.99999999999% the speed of light, therefore during that time which might have taken years on earth he reaches his destination and that twin has essentially aged hardly nothing, because time for him really stood still. Then once he arrives he ages at same rate as the twin on the earth.

Einsteins Therory of Special Relativity.

Answer 4

According to Einstein Special Relativity, the twin who leaves Earth and comes back will be younger than the twin who stayed because anything that is moving has their time slowed relative to observers.

Answer 5

The comments you made under your question show that you are on the right track. This question falls in the realm of special relativity.

From the perspective of the twin on Earth, the rocket twin will age more slowly than the twin on Earth because the rocket twin is traveling away at a relativistic speed.

Now, from the perspective of the rocket twin, the Earthbound twin is flying away from the rocket at a relativistic speed, and so the Earthbound twin ages much more slowly than the rocket twin.

How can they both see the other twin as younger? It's complicated, but one way of thinking about it is like this. Think about the distance that now separates the twins. Every day they grow further apart, and the light traveling from one twin to the other takes longer and longer to reach its destination. One day it might take one year for light from one twin to reach the other twin, and the next week it could take one year plus a day. What the twins will witness is the other twin living as though in slow motion. As long as the twins remain separated, they will never reconcile the dilemma of which twin is the younger twin.

Now, lets say that they are determined to find out which twin was the youngest, and so they decide to reunite. The rocket twin turns his rocket around and heads for earth.

From the perspective of the Earth twin, the rocket twin was flying away at a fast speed and is now flying toward me at a fast speed. When he arrives, he'll be several years younger because from this frame of reference, he was the one traveling at relativistic speeds.

From the perspective of the rocket twin, the earthbound twin was flying away from rocket twin, but then rocket twin turned around and headed back toward earth. When he arrives at earth, he'll slow down and join the reference frame of the Earth twin. From that frame of reference, rocket twin sees that he is the one who flew at relativistic speeds away from and then back toward the earth, so it comes as no surprise to him that he is the younger twin.

Now here's the kicker. Say that, rather than having rocket twin turn around, they decided to have Earth twin chase after him in a rocket of his own. This rocket is a newer design, so it can travel twice as fast as rocket twin's ship.

From rocket twin's perspective, Earth twin was flying away at a great speed, but now Earth twin has turned around and is flying toward rocket twin. Rocket twin hasn't changed his reference frame at all, so it is Earth twin who has been traveling relativistically, and Earth twin will now be the younger.

From Earth twin's perspective, rocket twin was flying away, but then Earth twin up and shot off toward rocket twin, then slowed down to join rocket twin's reference frame. Now that he's in rocket twin's reference frame, there's no denying that he was the one traveling relativistically and is therefore younger.

I know, I know, it sounds crazy, but that's relativity for you.

Answer 6

The moving twin ages more slowly, but he'd have to be moving pretty darn fast for this to be signficant. The rockets currently used for space travel fall far short of that.

Answer 7

With respect to the one on earth the other is also at rest at his destination. He is not moving now. Therfore his age is the same as that he was on earth.

And with respect to the one away from earth, the one on the earth is at rest. He is not having any relative motion. Therfore his age is the same as the other one.

Only if there is relative motion between the two the time will dilate.

Answer 8

according to Eisenstein's theory of relativity, the twin that left would be younger(if in space, not faster moving planet). in theory, that is. the twins would be the same age, as they would age at the same rate, but the numerical age would be less for the space twin. NASA sent a rocket/clock into a black hole and noticed that the time display slowed as it reaches the black hole.large masses can bend space/time/light so the closer to an extremely dense object the twin would get, the slower he would(theoretically) age

Answer 9

The traveling twin will be younger - how much younger will depend on the acceleration, trip duration and ultimate velocity of his trip.
It's a concept that's certainly not very intuitive.

Answer 10

How can he leave earth and also stay at his destination? Unless he's in some kind of weird quantum state and he hasn't been observed yet... but as soon as you make the observation, she will definiately be in one place or the other.