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Author Topic:   Expanding photons.
Son Goku
Inactive Member


Message 7 of 30 (362608)
11-08-2006 9:22 AM
Reply to: Message 1 by Larni
10-27-2006 9:00 AM


What puzzles me is that if space/time is expanding, would that not include the photon and its motion too?
The photon is a point particle (at all energy scales we've ever probed at least) so there is nothing to expand.
The expansion of the universe does effect its motion of course.
Also, if the far off galaxies are moving away faster than the near ones, would not the photons and their motion 'shift back' as it approaches our telescopes?
Photons don't move like balls, so if you take the photon picture of light then terminology like that doesn't apply.
However if you take the wave picture red shift is the "shift back" in their motion.
Although I'm not sure if I'm understanding you correctly.

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 Message 1 by Larni, posted 10-27-2006 9:00 AM Larni has replied

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Son Goku
Inactive Member


Message 14 of 30 (363035)
11-10-2006 8:22 AM
Reply to: Message 10 by Silent H
11-10-2006 6:06 AM


Light and 4D spacetime.
I made a stab at explaining this in detail before.
If you want to read:
EvC Forum: Time, a brief history
Basically messages 46 and 50-54 explain it.

This message is a reply to:
 Message 10 by Silent H, posted 11-10-2006 6:06 AM Silent H has replied

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 Message 16 by Silent H, posted 11-10-2006 12:02 PM Son Goku has replied

  
Son Goku
Inactive Member


Message 20 of 30 (363580)
11-13-2006 11:54 AM
Reply to: Message 16 by Silent H
11-10-2006 12:02 PM


Re: Light and 4D spacetime.
Sorry for the late response,.....
Anyway, yes, it essentially just measuring the speed of light. However if you accept relativity, then relativity changes the interpretation of Foucault's experiment into an experiment to find how many meters are in a second rather than a test for the speed of light.
If this isn't clear enough just say so, as there are a few subtleties here I'm glossing over.
For your other question:
The effect of changing the ratio would basically give the wrong results and wouldn't match experiment, which is one of the independant means to validate the claim that one second is 300,000,000 meters in the temporal direction.

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Son Goku
Inactive Member


Message 22 of 30 (364119)
11-16-2006 3:03 PM
Reply to: Message 21 by Jon
11-16-2006 1:31 PM


Re: Only empty space?
What book was that?
It wasn't by somebody called Mark McCutcheon was it?

This message is a reply to:
 Message 21 by Jon, posted 11-16-2006 1:31 PM Jon has replied

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Son Goku
Inactive Member


Message 28 of 30 (365117)
11-21-2006 9:50 AM
Reply to: Message 27 by JustinC
11-20-2006 4:39 PM


Re: The universe is expanding, but...
Yeah, it's a very good analogy.
Now, if the observer changes his frame of reference, he gets a new perspective on this 4D structure.
Now, i'm not sure such a radical shift in perspective is possible with the 4D structure, but is this an apt analogy?
To expand on this. Getting a different perspective in a 3D space could involve a rotation which will change your perspective on the surrounding space. (Such as turning your head around.)
In a 4D space it's pretty much the same, there are more possible roations, but nothing radically different.
However, if one of the dimensions in the 4D space is a time dimension, rotations act in a completely different manner and have subtle new properties.
A change in velocity is actually a rotation in the 4D space with 1 time dimension that we live in. (Spacetime)
It's the fact that one of the dimensions is a temporal one that accounts for such a radical shift in perspective, because it wouldn't be all that different from 3D space if all the dimensions were spatial ones.
Hope that helps a bit.

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 Message 27 by JustinC, posted 11-20-2006 4:39 PM JustinC has replied

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 Message 29 by JustinC, posted 11-21-2006 6:33 PM Son Goku has replied

  
Son Goku
Inactive Member


Message 30 of 30 (365232)
11-21-2006 7:06 PM
Reply to: Message 29 by JustinC
11-21-2006 6:33 PM


Lorentz invariance
Basically, one tells me the difference is fundamental, the other tells me its not. Where am I going wrong?
No where really, there's just a detail I haven't been explicit on. I only did things in one frame.
When we Lorentz transform we go from one guys frame, let's call it F, to another guys frame, let's call it F'.
The coordinates in F are (t,x,y,z) and in F' they are (t',x',y',z').
Let's say the Lorentz transformation is just a boost in speed along the x-direction.
Then t' = at + bx and x' = ct + dx. Where a,b,c and d are just some numbers, it doesn't matter about the details.
So you can see how the t'(time in the new frame) is a mix of t(time in the old frame) and x(space in the old frame).
So space and time don't really matter, because they're frame dependant. Now what about the distance rule, ds^2=dx^2+dy^2+dz^2-dt^2? (I prefer to leave out c^2)
Well as you might imagine the rule in the F' frame is:
ds'^2 = dx'^2+dy'^2+dz'^2-dt'^2
Which we can compare with the distance rule in F:
ds^2 = dx^2+dy^2+dz^2-dt^2
And, believe it or not, ds'^2 = ds^2.
So the answer to your question is that although time is important in the distance rule, it doesn't matter whose frame we are in, everybody gets the same answer from the distance rule. Basically no particular person's time matters, it's just that their time is important in their distance rule and everybody’s distance rule gives the same answer.
The technical term for the fact that the distance rule gives the same answer in every frame is that the distance rule is Lorentz invariant.
Hopefully that might clear something up, any questions ask away.
Edited by Son Goku, : Slight modification.

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