Re: Trying figure it out - time dilation

From: Eli Botkin (elibotkin_at_optonline.net)
Date: 10/08/04 

Date: Fri, 8 Oct 2004 01:04:41 -0400

ELI WROTE
>> Kenneth:
>> Forget the "time travel." As far as Special Relativity is concerned,
>> time travel is science fiction. Time dilation refers to how time
>> operates in this universe. It is NOT a propagation delay. Time has to
>> behave that way if every observer, regardless of his/her state of "rest"
>> or "motion," is to get the same result when measuring the speed of light.

KENNETH WROTE
> Sorry, I don't get it. Say that the two us meet at point A in space. We
> synchronize our clocks and I start my spaceship and fly 1 lightyear in the
> same direction (instant acceleration). Before I left, we agreed that we
> would signal each other with a flashligth exactly after 1 year. Now, would
> you see my flash 2 years after I left ?
>

ELI WROTE
Much of what you state indicates that your doing your mental calculations
using Newtonian ideas of velocity.
So let's begin at the beginning. We begin at relative rest together and
synchronize our clocks. I do nothing. You (with instant acceleration) take
off and fly a distance that I claim to be 1 lightyear (note that one must
always say whos clock and whos ruler is doing the measuring). By agreement
we each send a light signal when our own clock reads 1 year.
I note that you haven't specified what your speed is. You first said that
you were flying out a distance of 1 lightyear and then said that you would
signal after 1 year. Are you thinking that you are 1 lightyear distant from
me after flying for 1 year? If so then relative to me you are moving at the
speed of light. A nono!

So let's be more realistic and have you move at 0.5c (half the speed of
light). So we'll drop the 1 lightyear number because in 1 year (by your
clock) you would claim that 0.5 lightyear of space separates us (as I too
would claim when my own clock reads 1 year). But I also claim that the
event which contains your clock as reading 1 year, occurs according to my
clock at 1.732 years, when you are 0.866 lightyears distant from me. From
that distance the beam you send will take 0.866 years more to reach me so,
the beam arrives at my station when my clock reads 2.598 years. Of course
different relative speeds will result in different numbers.

ELI WROTE
>> Time-intervals (delta-t) provide just another coordinate-interval between
>> events, much like position-intervals (delta-x,-y,-z) does. And as such,
>> its value depends on the state of the observer relative to other
>> observers. When two stars explode, you would assign time- and spatial
>> separation-intervals between those two events that are different from
>> what would be assigned by an observer who is moving with respect to you.
>> Neither one of you experiences a "propagation delay."

KENNETH WROTE
> I'm so sorry that my mind won't let me figure it out.
> Why isn't any of us experiencing a delay.

ELI WROTE
What sort of delay? Are you presently experiencing delays except for
knowing that starlight is received by you some time after it was emitted.
If tha's the delay you mean, then I assure you that we all experience it
(except for those who are unaware of it).

KENNETH WROTE
> The star only explodes once (hopefully). Assume that we scatter a lot of
> observers round this star.
> If I'm closest to the star I will see it explode first?
> If I'm moving away at 0.5c, I will see it ½ year later?
> If I'm moving away at 1c, I will never see it?

> If I'm 2 lightyears away and moving towards at 1c, I will see it 1 year
> before I would if I didn't move?
> And hopefully this is the truth for any of the other observers too.
>
> Why is time involved at all?
> I still only see "information" propagating and "hitting" the observers at
> different times, but the event only happened once.
> If the star was moving at say 0.25c (compared to space = nothing?), I'm
> not sure how I should predict the result, but RT says that the speed of
> ligth is constant to everyone in their own frames, which means I don't
> need to care?
> (ehhm, or did it - not sure what it means that it is constant. Is light
> propagating constantly from me, or is it constant only in relation to
> another observer)
>
>> Hope this helps.
> Always for the better, and I appreciate your help - but I warn you, that
> I'm maybe a little slow on things that is not quite logical.
>
> Thanks
> Kenneth
>
ELI WROTE
Kenneth, SRT is fully logical. I'll grant you though that it isn't
intuitive.
All our experience suggests that the composition of two velocities along a
straight line is directly additive, but alas, it isn't. So if you really
want to understand how time and distance appear to observers in different
states of motion, then you'll have to bite the bullet and learn the math
that explains SRT.

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