Special relativity question

[Ulim]

I dont feel like really explaining it anymore because you guys just seem to be out there to ask questions that dont actually really apply here in any way.

Here is a video explaining everything.
Its math but if you are willing you can understand it.
AND PLEASE STOP ADDING NEWTON PHYSICS INTO THIS DISCUSSION THEY DONT FIT HERE
[YOUTUBE]

 

[Loveall]

The traveling Twin needs to accelerate and decelerate during the journey to come back to the meeting. It is during these acceleration stages that he sees the twin back on earth age very quickly, so when they meet everything makes sense. At other times during the trip (constant speed), you are correct: both twins see the other twin aging more slowly.[/url]

OK, that makes sense - unlike his brother, he expends energy to accelerate / decelerate, and that sets him apart. In a sense he's 'rewarded' for expending all that energy: it 'buys' him a slowdown of time. But traveling per se doesn't.

I always thought the time he spent traveling at v (close to c) would affect how much younger he'd be upon reuniting with his brother. But from what you say, that time irrelevant. The only thing that affects the difference in age between them is how close to c he got. Is that right?

The interesting thing is that when you write down how much time the traveling twin is rewarded by when accelerating, it turns out to also depend on the distance between them. So since the distance appears in the formula, the amount of time spent traveling apart matters and your original intuition is correct.

There may seem to be another contradiction here: if acceleration is the key to explain why one twin really ends up being younger, and acceleration is a 'local' effect, why does the distance between twins when the traveling twin turns around matter?

This last 'contradiction' can be cleared up by accepting that time and space are not separate entities. They are related and one is clearly affecting the other in this example. This is different than our intuition and every day experience would suggest, and thought must put into this (and excellent questions are being asked here).

 

[pitubo]

Ulim, the discussion began when you made certain claims:

Also the effects are reversed. So the guy in the train will see the world outside speed up and stretch to like 2.3 its original lenght in the direction of movement.

This is in direct contradiction with the video you posted, did you even watch the video? Skip ahead to 18:00 if you don't believe me. The video points out that both observers make equal observations about the other party.

Here is another video, that actually does address the twin paradox:
[YOUTUBE]

Pay close attention to this quote from the end of the video (at 2:45):

Trying to understand relativity just by using basic equations for time dilation and length contraction, like is often done in beginning physics classes, will often lead to confusing apparent contradictions, because these equations don't take into account the full changing of simultaneity of events and so on.

As it was stated before, the crux of the issue is the acceleration, not the motion.

Perhaps the discussion turned into nitpicking, but that is what happens when you avoid the questions and instead resort to accusing others of confusing relativity with newtonian physics, while not even properly understanding orbital motion yourself.

 

[Ulim]

If you got at c speed you dont experience time. Time is frozen for you..

 

[Ulim]

Ulim, the discussion began when you made certain claims:

Also the effects are reversed. So the guy in the train will see the world outside speed up and stretch to like 2.3 its original lenght in the direction of movement.

This is in direct contradiction with the video you posted, did you even watch the video? Skip ahead to 18:00 if you don't believe me. The video points out that both observers make equal observations about the other party.

Here is another video, that actually does address the twin paradox:

Ye my bad I forgot that this happends due to doppler effect
[YOUTUBE]

 

[Loveall]

The traveling Twin needs to accelerate and decelerate during the journey to come back to the meeting. It is during these acceleration stages that he sees the twin back on earth age very quickly, so when they meet everything makes sense. At other times during the trip (constant speed), you are correct: both twins see the other twin aging more slowly.[/url]

OK, that makes sense - unlike his brother, he expends energy to accelerate / decelerate, and that sets him apart. In a sense he's 'rewarded' for expending all that energy: it 'buys' him a slowdown of time. But traveling per se doesn't.

I always thought the time he spent traveling at v (close to c) would affect how much younger he'd be upon reuniting with his brother. But from what you say, that time irrelevant. The only thing that affects the difference in age between them is how close to c he got. Is that right?

Wouldn't that be true only, in case he travels away from his twin first, and then back?
In case of the circular motion, time would go slower for the moving object. So wouldn't that meant that for the moving twin, the not moving twin ages faster? I mean, if they had Skype or something, and he's getting N frames per second sent towards him, to him that would be more frames per second, because a second for him lasts longer. If you move away from the twin, or towards him, you get a sort of doppler effect ofcourse. But not in the case of circular motion (so in that sense, acceleration wouldn't matter either, or would it, because of it's gravitational effects on time?).

I mean, normally you have two things going on here: the doppler effect, AND the slowing down of time itself. But would that still be the case with circular motion?

Yes, I think there would be a relativistic Doppler effect in the circular motion example. One disclaimer: I don't remember the spinning example from my general relativity grad courses (we jumped right into tensor analysis ) so I could be wrong in this example but here are my thoughts.

The twin moving in circles is experiencing an acceleration as Pitubo pointed out. A practical example of this is NASA training astronauts for the large take off accelerations by spinning them around in a training chamber. From the point of view of the spinning twin if feels like they are in a high gravity environment being pushed against the back wall. If they shine light "upwards" towards their twin, it will be Doppler shifted (just like any light coming out of a gravity well). From the spinning twin's point of view, time is moving slower because they are deeper in a gravity well.

So in the spinning example, both twins would measure time going more slowly for the spinning twin. This is very different from the situation when the twins are moving apart at a constant speed where they both observe the other twin's time moving more slowly than theirs.

When I get home I can do some calculations and will update this post if I can figure out that I'm wrong. I'm getting a time dilation factor of 1+gD/c^2, where g is the acceleration the spinning twin feels against the back wall and D is the distance between then (again, the distance between them comes into play when considering acceleration itself).