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geeyathink
2014-May-29, 03:03 PM
Are time dilation and lenght contraction caused by the same thing? I had kind of thought so.

In time dilation - does the object know its out of whack?

I am asking because I looked at a website about something called a Pole-Barn-paradox (sorry) and it said that the length contraction is only a fact of the observer?

If I understood it correctly, it seems to say that the object itself (from its point of view) doesn't give a darn what we think and goes about its merry way without ever having changed?

But that is not what happens with the time dilation experiments that have been done is it?

The only thing I can think of is reading about when they carried a clock around in a plane for a long time, and when they stopped, the clock was wrong* - so it (the clock) knew and agreed with us - that it was borked? That isn't what I would expect after reading the Pole-Barn thing. Or am I looking at it wrong? (*I know the clock was not really wrong, just different.)

Please don't go into technical detail, I've read probably a thousand conversations (by way of answers to questions on forums) that stressed that there is no preferred frame - I kind of get the gist of that. I guess thats involded in a proper answer here, but I won't understand technical stuff so I don't hope for and would not understand a full answer, just the basic umph of it if possible.

Thank you for any help.

One more question after this and I'll leave you alone for another four years, I promise! jk.

StupendousMan
2014-May-29, 04:25 PM
Are time dilation and lenght contraction caused by the same thing? I had kind of thought so.

Yes.



In time dilation - does the object know its out of whack?


No.



I am asking because I looked at a website about something called a Pole-Barn-paradox (sorry) and it said that the length contraction is only a fact of the observer?


It might help for you to read this:

http://spiff.rit.edu/classes/phys200/lectures/paradox/paradox.html




The only thing I can think of is reading about when they carried a clock around in a plane for a long time, and when they stopped, the clock was wrong* - so it (the clock) knew and agreed with us - that it was borked? That isn't what I would expect after reading the Pole-Barn thing. Or am I looking at it wrong? (*I know the clock was not really wrong, just different.)


Yes, you're looking at it wrong. The clocks which were carried on the airplanes had systematic differences compared to the clocks which remained on the ground (fo details, see the final section of http://spiff.rit.edu/classes/phys200/lectures/dilation/dilation.html). But none of the clocks were "right" or "wrong." They just showed different times.

Shaula
2014-May-29, 04:27 PM
The thing about relativity is that neither observer is 'right' or 'wrong'. Take the case of a rocket fired away from us at some infeasible speed. We look at it and say "Ah ha, time dilated, length contracted". However it looks at us and says "Oh look, they are time dilated and length contracted". Who is right? They both are. From their frame the other one is indeed time dilated/length contracted. But we can bring them back together to check ,can't we?

Well, time dilation and length contraction are usually described by Special Relativity. Because the maths is way easier. If you want to compare the two objects you have to bring them back together, into the same frame of reference. So someone or something has to be accelerated. Which means that you are into General Relativity here.

In GR things like the total time elapsed on a clock (compared to another one) is path dependent, that is to say that it all depends on how you accelerate them. And acceleration induces its own time dilation. And when you do the maths you find that what appears to happen is that when you accelerate one of the clocks it causes enough time dilation to account for the end difference between them. So to both of them what appears to happen is that the other persons clock was running slowly, but then the acceleration 'fixes' it so that there is a difference in elapsed time for one clock when they are brought back together thanks to it being accelerated.

Jeff Root
2014-May-30, 07:33 AM
Length contraction and time dilation don't happen to things,
they happen to the geometric relationship between things.

Here's an analogy:

You look up at the top of the Empire State building, and see
that the angle of elevation of the topmost point is 40 degrees.
I look at the same point and measure the angle as 30 degrees.
What happened? Did one of us measure incorrectly? Is my
protractor out of whack? Was the Empire State Building shorter
when I measured it than when you measured it?

No, the difference in our angle measurements is due to the
different geometric relationships we have with the top of the
ESB. You were closer to the ESB than I was, so you measured
a larger angle.

Length contraction and time dilation work the same way. Your
measurements depend on your geometric relation to the thing
you are measuring. Measurements of a thing's length and time
can depend on your accelerations, your speed relative to the
thing, and the time you have spent at that relative speed.
If your relative speed is low, your measurements of the thing
will be similar to the thing's measurements of itself. If your
relative speed is high, you will measure the thing's length as
contracted and its time as dilated. It will measure your length
as contracted and your time as dilated. But neither you nor
the thing have actually been changed by that relative speed.
It is just the geometric relationship that changed.

The angle to the top of the Empire State building really is
different for me from what it is for you, the length of a pole
really is different for a fast-moving barn from what it is for
a motionless barn, and the time elapsed on a clock tied to
the pole really is different from the time elapsed on a clock
on the barn wall. The differences are not in the the ESB,
the observers, the clocks, the pole, or the barn, but in the
geometric relationships between them.

-- Jeff, in Minneapolis