# Time Dilation?

Tags:
1. Nov 19, 2015

### DonB

I'm relatively new here, and was told that the GP group was a good place to ask some basic questions. If this isn't he right place for the question, please direct me to the proper group.

For years I have been fascinated with the implications of the time dilation mental experiment -- the difference between what an observer sees looking at a bouncing light (or time clock) inside a spaceship going at/near the speed of light, vs. what an outside observer sees (say, on the earth) as he watches that light through the spaceship window. The former sees only vertical movement, and the latter more of a diagonal movement. Because of this, we're told, the spaceship observer must age at a slower rate than the outside one. Okay. So what about if the earth observer has the same kind of bouncing-light-beam clock thing on earth with him, and from his own perspective he sees his light beam go in a solely vertical path, but the observer in the spaceship flying by sees the light beam travel the diagonal path -- implying that the earth observer is now aging at the slower rate? How is it possible in real-world reality, if both observers are seeing the same thing, that both are aging slower than the other?

I'm sure this is a very basic question, and if it's been discussed already, please just point me to that thread(s).

(Please understand, I'm genuinely not being a I-got-it-figured-out-and-everyone-is-wrong jerk. I'm just trying to wrap my mind around the implications.)

2. Nov 19, 2015

### Staff: Mentor

3. Nov 19, 2015

### phinds

Yes, this is a very basic misunderstanding and it a very simple result of Special Relativity. Nobody ages any differently just because they are in motion relative to someone else, but both SEE each other as aging more slowly. If they meet up again, one of them will have to have accelerated and that means that their world lines are now such that when the do meet back up they will observe that they have aged by different amounts. Differential aging is real and is a result of differing world lines. Time Dilation is "real" only to the observer, never to the one being observed.

For example, you, right now as you read this, are MASSIVELY time dilated according to a particle at CERN and it is massively time dilated to you. Does that make you feel as though anything has slowed down?

4. Nov 19, 2015

### DonB

Thanks for the interesting link. So, if I understand Lasky right, this Twin Paradox is not really due to how the light beam path is viewed (vertical vs. diagonal) by the two observers (since both will see the same thing as they look at the other's time clock), but is possibly sole because of the distance-compression that Traveler experiences. This seems totally different than the (overly simplified?) explanations that I've seen to date.

But this leads to another question: The (overly-simplified?) explanations of distance-compression I've seen implied that this compression was necessary because of the implications of the the bouncing lightbeam mental experiment. If that is true, then it seems that Lasky admits that the bouncing beam does not itself demand dilation, but without it there is no explanation for distance compression which he has turned to.

5. Nov 19, 2015

### sophiecentaur

This effect is not readily seen, every day and you normally need some very high quality 'clocks' to detect any difference in the relative aging of an object and an observer.
However, there is a dramatic effect which occurs with muons, which are produced in the upper atmosphere by the arrival of cosmic rays. These little devils travel at very nearly c and have a half life of about 1.5 μs. If Relativity did not operate, you would expect only about 0.3 muons per million to survive to the surface. However, because of their high speed, they are 'aging' so much slower than observers on Earth that we can measure about 50,000 per million survivors down here. This hyperphysics link explains it and does some interactive sums for you. Relativity works!

6. Nov 19, 2015

### DonB

Okay, I get that I don't loose or gain any years just because my twin flies out to the stars and back at the speed of light -- nor does he loose/gain. It's as if one of us has a 'fold' in our time line (or world line as you called it) that allows one individual to have more years of living between Points A & B. I got that. But the problem is, by the explanations that I've seen, that the difference in how the light path is observed (vertical vs. diagonal) necessitates whose time line is shorter and whose is longer -- but that rule applies both ways, necessitating that both are both longer and shorter.

7. Nov 19, 2015

### sophiecentaur

If one twin goes into orbit around the Earth, then the age difference would be 'real'. There have been experiments with supersonic aircraft, carrying clocks which give the predicted results. But your planes and orbiters are not in inertial frames.

8. Nov 19, 2015

### phinds

No, both are shorter. Just think about an object moving directly away from you. You each see the ticks from the others clocks as having slowed down because they take longer to get to you. It's symmetrical.

Also, FYI, the GPS system would not work without taking into account both kinetic time dilation and gravitational time dilation. Well, it would work, sort of, but would have you driving into the sides of buildings and off into corn fields. As I recall, the speed of the satellites (which are not in geosynchronous orbit as many people think the are) causes a time dilation compensation of about -7 microseconds per day and the fact that they are higher in the Earths gravitational field requires a +45 microsecond compensation per day. As long as they are up there (and moving in the frame of reference in which we are at rest) and we are down here, that's just natural.

9. Nov 19, 2015

### DonB

Thanks for the interaction, and I hope to soon be able to give fuller attention to learn more of the muons. (I do know of them, but really haven't studied the subject out.) I'm not so much challenging that relativity doesn't work, just trying to see how two people observing the same thing aren't forced (as the explanations I've seen elsewhere seem to necessitate) that the individuals are both aging at faster and slower rates relative to the other.

10. Nov 19, 2015

### phinds

That's odd. All of the explanations I've ever seen are clear that the observed slowing down is symmetrical as objects are moving away and speeding up as objects move towards each other, but never both at the same time.

11. Nov 19, 2015

### Staff: Mentor

In the Scientific American article, they describe what is observed. They don't correct for Doppler shifts. As the traveling twin leaves the Earth both twins see the others clock as going a half speed and on the return journey they see the clocks going a twice the speed of their own.

The difference is the Earth twin sees the star as 6ly away whereas the travelling twin sees it as 4.8ly away.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook