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Time Dillation - HSN forum

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Time Dillation


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#1 679810

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Posted 05 February 2007 - 09:17 PM

Our teacher tried explaning this to us a while ago but just a couple of quick questions. There's that whole thing about if I had a clock and so did my friend, and then I travelled at a very fast velocity. I would see his clock slow down.

A)Is this slowing down of the clocks because of the speed of light (the delay before the light from the clock reaches me)?
B)If i was to travel right round the world and back again to my friend (very big velocity), would our clocks have different times?

I don't think it's part of the course but it's just one of those things I want to know about, really annoying trying to think about it. I'm sure one of you geniuses could explain it simply tongue.gif

#2 ad absurdum

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Posted 05 February 2007 - 09:54 PM

Now I'm no genious, and I'm not pretending to be sure on any of this, but this is what I think:

A) I don't think this is quite right, although it is a very good idea I think. Suppose for a moment that faster than light travel is possible, you were a stationary observer and I was running about with a clock. Now if I was running towards you, then the light would be coming at you faster than the speed of light, and if I was moving away from you then the light would be coming at you slower than the speed of light. I think that this would have opposite effects on how you saw the time coming from my clock

However, I don't think we should even bother thinking about that. Light is always moving at the speed of light, so it's always advancing towards you at the speed of light, so I don't think it would make a difference whether I was moving towards you or away to you. Thus, I would think that the time dilation phenomena is actually an intrinsic properties of moving that fast, so isn't really to do with delays in the light reaching you, but actually the notions of time as a fixed quantity breaking down at high speeds.

B) Yeah, that would happen. In fact it did, check out: http://en.wikipedia.org/wiki/Hafele-Keating_experiment/ Note that this experiment also took into account a consequence of general relativity which has the opposite effect, but the result of the experiment was consistent with the theoretical result (which took into account both the general and special relativity parts).

This is all very much open to debate.
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#3 679810

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Posted 05 February 2007 - 10:29 PM

Right I really don't understand how light is always constant (I've been told this loads by my physics teacher. I understand what happens but just not why. That doesn't matter just now anyway:D).

Anyway thanks a lot, that's really interesting. So if say a clock was taken in an aeroplane fully round the world one way and then taken back in the opposite direction until it reached the start again, would this cancel itself out. I think I basically mean, is time dilation to do with speed or velocity.

This is all very confusing, I'll never fully understand it but is it physically slowing down the clock or what.

#4 ad absurdum

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Posted 05 February 2007 - 10:50 PM

I've never understood why light is always a constant either. I think that this is really something that happens when you spend your whole life living in a Newtonian Mechanics world, it can be difficult to understand why it all breaks down at higher velocities. I guess it's just one of these odd things of the universe that I won't even understand.

It's actually the relative speed between the stationary observer and the moving person that matters. It doesn't make a difference what way you are moving round, all that matters is how fast you are moving with respect to the stationary observer.
Anyway, suppose for a minute that it was velocity, and a person was standing at a point. Another person is going to move relative to the stationary person. If we define movement to the right as positive and movement to the left as negative, we can see that the person could have either a positive or negative velocity with respect to the stationary observer, depending what way they run. However, it doesn't actually make a difference once we put it into the time dilation equation (\delta t = \frac{\delta t'}{{\sqrt1-\frac{v^2}{c^2}}}, where the t' is the time measured by the mover and t is the time measured by the stationary person), as the velocity is squared and so it will come out postive regardless.
I think this is actually a consequence of another idea Einstein put forward: there is no absolute frame of reference. All velocity is relative, and we can only define velocities how we want to. I have absolutely no right to say that movement to the right is positive, to the left is negative, as I could have equally done it the other way round. So how do we know what one is actually right? Answer: we don't, we treat some point as a zero and consider everything in relation to that zero, but really it is all arbitrary. The only thing that is actually constant is the relativity of the motion. I can't say for certain that I'm moving at 5m/s, but I could for certain say that I was moving at 5m/s relative to something on the surface of the earth.
So in answer to that question, the effects wouldn't cancel out, but would compliment each other. I think.

I don't think that tt isn't physically slowing down the clock, but that it's physically slowing down time itself. Every clock makes its measurement by a reference to some regular event. Suppose I made a clock that ticked every time an atom in it decayed, and I had somehow set it up that the atom decayed every second. If I started moving really fast, I would still see the atom decaying every second and my clock ticking away as normal. If someone watched me, then they would see my "seconds" as actually longer than seconds, and wonder why my clock was running slow. It's just what we think of as constants (length and time) readjusting themselves to keep the one try constant (speed of light) constant.

Once again, I'm not sure about alot of that and it's just what I've gathered. I agree that it is all very confusing, but also interesting.
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#5 679810

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Posted 05 February 2007 - 11:25 PM

Right so if the opposite velocities would compliment themselves, theoretically if there was some sort of machine that could vibrate you at close to the speed of light but so that you could stay in the same position, your time would slow down, mass would increase etc.?

Edit- I suppose the speed would have a kind of modulus sin graph or something so the mass, time etc. would keep changing from 0 to a number. Just ignore me now, I'm getting far too confused

Yeah I think I'm just going to go to bed, too much thinking for something that I don't need to know (but wouldn't mind knowing)

#6 ermd

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Posted 06 February 2007 - 07:28 PM

Firstly, respect to physics. I sometimes wish I had kept studying it.

Secondly, this is banter:

QUOTE(ad absurdum @ Feb 5 2007, 09:54 PM) View Post
Now I'm no genious


Hehe. Genius. =P

ps. STILL waiting for that recording of your band playing Dream Theater stuff.

#7 skint_student

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Posted 06 February 2007 - 08:03 PM

The reasons for light being a constant involve quite complex maths that you really don't want to worry about at this stage, I'm in fourth year physics at uni and only now is it starting to make sense.

I'm not sure how much sense this will make without the maths, but might give you an idea of what's going on:

The laws of physics must hold in all reference frames, i.e. be invariant under co-ordinate transformation. Changing from one reference frame to another was originally done by Galilian transforms, however Maxwell's EM equations were not Galilean invariant - the speed of light in vacuum is a universal constant, and one cannot have c + v. Einstein replaced the Galilean transformation with the Lorentz transformtion that leaves Maxwell's equations and the speed of light invariant. Einstein modified the laws of motion so as to be Lorentz invariant - giving us the Theory of Special Relativity.

If that sounded like jibberish then just forget you ever read it.

This is a good website though without any complex maths, and links to further questions

http://www.phys.unsw.edu.au/einsteinlight/



#8 ad absurdum

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Posted 06 February 2007 - 08:15 PM

QUOTE(ermdeviation @ Feb 6 2007, 07:28 PM) View Post
Firstly, respect to physics. I sometimes wish I had kept studying it.

Secondly, this is banter:

QUOTE(ad absurdum @ Feb 5 2007, 09:54 PM) View Post
Now I'm no genious


Hehe. Genius. =P
Haha tongue.gif

QUOTE
ps. STILL waiting for that recording of your band playing Dream Theater stuff.
Heh, that band died a long time ago I'm afraid. I was only really friends with one person in the band, and he moved away. I've got another band now, but we only play easier stuff like Porcupine Tree and Radiohead, and haven't started recording yet either.

skint_student, I liked that explanation, thanks for that smile.gif
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#9 ermd

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Posted 06 February 2007 - 11:03 PM

QUOTE(ad absurdum @ Feb 6 2007, 08:15 PM) View Post
Heh, that band died a long time ago I'm afraid. I was only really friends with one person in the band, and he moved away. I've got another band now, but we only play easier stuff like Porcupine Tree and Radiohead, and haven't started recording yet either.


To be honest, I think I'd rather hear stuff by those bands! Went to see PT at the Carling Academy last year, absolutely immense. The stuff they played which is set for the next album was very impressive. =D

That explanation did confuse me a bit anyway...maybe I'd just need the maths too. That link doesn't work for me...I think the uni LAN is blocking it here.

So I'll put my question pretty simply - if light is moving in one direction away from me at 3x10^8 m/s and I start moving in the other direction at 10 m/s then how come the light, relative to me, is not moving at (3.10^8)+10 m/s?

#10 ad absurdum

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Posted 07 February 2007 - 04:37 PM

QUOTE(ermdeviation @ Feb 6 2007, 11:03 PM) View Post
To be honest, I think I'd rather hear stuff by those bands! Went to see PT at the Carling Academy last year, absolutely immense. The stuff they played which is set for the next album was very impressive. =D
I was at that gig, it was tree-mendous. The long song of the new album sounds amazing. Are you going to see them there this April?
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#11 skint_student

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Posted 07 February 2007 - 05:41 PM

QUOTE(ermdeviation @ Feb 6 2007, 11:03 PM) View Post
That explanation did confuse me a bit anyway...maybe I'd just need the maths too. That link doesn't work for me...I think the uni LAN is blocking it here.

So I'll put my question pretty simply - if light is moving in one direction away from me at 3x10^8 m/s and I start moving in the other direction at 10 m/s then how come the light, relative to me, is not moving at (3.10^8)+10 m/s?


emmmm without complex maths, it just doesn't, lol - sometimes in physics you need to through common sense out the window.
ok that's a rubbish explaination...
I have no idea how to post all the mathematical symbols on here, but to be honest it may just look like a magic trick invented to make the theory fit. There are experiments to back up Special and General relativity, so until someone conclusively prooves otherwise then we believe that the theory is correct. Though I believe all good theories should continually be tested to their limit. Everyone used to believe that the world was flat, maybe one day people will look back at laugh at Relativity... who knows?

You can look up Lorentz and Galilean transforms on Wikipedia, but I think it's a bit in depth.
Maybe try this link instead for a summary of the maths, but the other one I posted has great videos etc.

http://hyperphysics.phy-astr.gsu.edu/hbase...tiv/ltrans.html


Incidently, Hyper Physics an excellent physics website for many topics, so if you want to look through other stuff this is the home page link

http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html

#12 ermd

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Posted 07 February 2007 - 11:17 PM

QUOTE(ad absurdum @ Feb 7 2007, 04:37 PM) View Post
I was at that gig, it was tree-mendous. The long song of the new album sounds amazing. Are you going to see them there this April?


So long as the student loan holds out, yes. I've got other tickets to get before then though...Rise Against, Ash, and Whole Lotta Led muaha.





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