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plant
2010-Apr-07, 02:19 PM
I read somewhere that implied that everything is always travelling at "c" (speed of light) through space-time. The faster you go through space, the slower you go through time and vice-versa. But if you're travelling through space at c - like a photon, then presumably you're 'stationary' with respect to time. So would this mean that the photon would experience the flow of time in the rest of the universe to be infinitely fast? What i'm trying to get at is: what would the experience of the universe be like if you were a photon?

Tensor
2010-Apr-07, 04:02 PM
I read somewhere that implied that everything is always travelling at "c" (speed of light) through space-time. The faster you go through space, the slower you go through time and vice-versa.

We had a thread about this a couple of months ago. With the recent transfer to the new version of vbulletin, we don't currently have the ability to link to that discussion and why that's not quite the right way to look at it.


But if you're travelling through space at c - like a photon, then presumably you're 'stationary' with respect to time. So would this mean that the photon would experience the flow of time in the rest of the universe to be infinitely fast? What i'm trying to get at is: what would the experience of the universe be like if you were a photon?

The correct answer is:We don't know. A photon's frame of reference isn't a valid frame of reference(you end up with division by zero), so we can't really say. If, however, you take the photon's experience as a limit and extrapolate, then the photon experiences zero time and the photon's path contracts to zero length. See why it kinda, sorta doesn't make any sense? And why it's considered not valid?

tdvance
2010-Apr-07, 07:04 PM
No---you don't "travel" through spacetime really. Your speed is distance through space divided by time (which of course depends on frame of reference, which determines which directions are space and which one is time) and is generally much less than c.

astromark
2010-Apr-07, 07:34 PM
It has been the popular standard that in the vacuum of space a light photon has a velocity, c.
That at c. time does not move forward from that frame of reference.. Other sub atomic particles with 0 rest mass may also attain this velocity. Transversing a opaque substance can and does slow light. As does hitting some thing... Ouch !

plant
2010-Apr-08, 07:44 AM
ok.. but i thought that space and time weren't 'real'... just 'shadows' of space-time.... if you know what i mean...

plant
2010-Apr-08, 07:46 AM
so.. it can't go anywhere because time stands still- but it doesn't have to because it's there already!

loglo
2010-Apr-09, 02:50 AM
so.. it can't go anywhere because time stands still- but it doesn't have to because it's there already!

Yes, which is why it is called a null geodesic.

Ken G
2010-Apr-09, 03:36 AM
Here's one way you can think about this "moving through spacetime at c" business. First of all, every observer thinks themself to be stationary, and moving through (proper) time at a rate of 1 second per second. When you look at clocks zooming by you, they will appear to be ticking slowly, if you reckon their rate of ticking by using Einstein coordinates (which amount to bouncing light signals off them and reckoning that the light signal hits at the midpoint of the time you sent it out and the time you got it back on your own clock). One way you can get the right answer for how slowly you reckon their ticking is by saying that a component of their 1 second per second rate is "spent" moving through your concept of space (take their v in c units to get this component), and then a perpendicular component "remains" in the time direction. What's odd about it is that their "movement through time" is dtau/dt, where t is your clock time and tau is their clock time-- so this means that the two components we are adding up to get "1 second per second" have the odd relationship that one of them is through your concept of space (per your concept of time), and the other is through their concept of time (per your concept of time). Still, it is attractive to imagine that a moving clock is moving "partly through space and partly through time". The way you get the resultant speed to be c is to use space units instead of time units, but I don't prefer that. So in summary, saying that "everything moves through spacetime at c" is somewhat true, but it has two non-obvious elements to bear in mind:
1) it is your space and their time that they are moving through
2) you imagine the motion is through space (and time in 1/c units), rather than just through time (and through space in c units).
I prefer saying that "everything moves at the rate 1 second per second, it's just that a moving clock has a component of its motion through your space, and another component through it's own time, all measured per your time (and their spatial movement is in c units)."