# The Naked Scientists Forum

### Author Topic: Light speed?  (Read 4292 times)

#### CD13

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##### Light speed?
« on: 29/07/2011 19:25:29 »
As a scientist, but not a physicist, I've always wondered why the speed of light in a vacuum should be constant. Speed is a derived variable from distance travelled and time. Yet it's time that is the variable (OK spacetime). I understand that it all depends on the frame of reference (relativity), but it makes no sort of sense. Given that electromagnetic waves are sort of spread out anyway and are massless, why have a maximum speed?

Sorry if it's more philosophical that physical.

#### Bill S

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##### Light speed?
« Reply #1 on: 29/07/2011 23:42:16 »
Hi CD13.

Have you looked at "Why does light travel at the speed of light? (in a vacuum)".?

The answers run to almost 2 pages, but seem to boil down to "because it is".

#### Geezer

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##### Light speed?
« Reply #2 on: 30/07/2011 05:18:36 »
Welcome CD13 (unusual handle that!)

The most important thing here is never to believe anything anyone tells you (except me, of course).

Electromagnetic phenomena propagate through stuff (space/time included) at a rate determined by the permittivity and permeability of whatever stuff you have in mind.

Unfortunately, that doesn't really tell you too much about space/time. It's just one of those things.

#### Soul Surfer

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##### Light speed?
« Reply #3 on: 03/08/2011 09:23:51 »
To add one important detail to geezer's reply.  It is constant because the properties of "free space", that is space without anything else in it,  are uniform throughout what we can see of the universe.

#### Mr. Data

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##### Light speed?
« Reply #4 on: 03/08/2011 10:09:16 »
Well, Geezer is correct. The permeability and permittivity of spacetime determines the value of c... The energy density of the vacuum is also somewhat related. In theory, if you increased the overall energy density of spacetime, you can alter the speed of light, which means it changes the values of the permeability and permittivity. Notable mathematician John Barrow has done extensive work on this.

#### lightarrow

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##### Light speed?
« Reply #5 on: 03/08/2011 21:09:10 »
As a scientist, but not a physicist, I've always wondered why the speed of light in a vacuum should be constant. Speed is a derived variable from distance travelled and time. Yet it's time that is the variable (OK spacetime). I understand that it all depends on the frame of reference (relativity), but it makes no sort of sense. Given that electromagnetic waves are sort of spread out anyway and are massless, why have a maximum speed?
It's not limited to electromagnetic waves. Every massless particle with non zero energy must travel at that speed.
The fact is constant (better, *invariant*. It means it doesn't depend on the frame of reference) comes from the fact that it's the maximum speed possible, and this because of spacetime properties.

You can see it in this way: because of Lorentz contraction of lenghts, a very fast moving object measures contracted distances, so if you were inside a spaceship moving at near light speed, you would travel around the entire universe in a few seconds (zero seconds, at the limit v --> c). Clearly, you can't travel faster than that. So, in a certain sense (that is, in the sense of "rapidity", but I want to make it easy for the moment) it is as if it were infinite, and if you sum infinite to a finite number you still get infinite. For this reason, if you launch a light signal from a moving object, you "sum" the finite speed of that object with the "infinite" speed of light and so you get the same result (the *speed* is not infinite, but the rapidity is and the rapidity is a better concept, at high speeds).

Light speed is a finite number just because we measure speed as space/time, but this is a good definition of speed only at low speeds, because in this case only we have that the space travelled is independent of the interval of time.
At high speeds the usual definition loses its "common sense" meaning, as I showed in the example of the spaceship.

Don't know if all this was useful.
« Last Edit: 03/08/2011 21:24:38 by lightarrow »

Post by MikeS click to view.

#### MikeS

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##### Light speed?
« Reply #6 on: 04/08/2011 17:06:55 »
Shrunk
Well, Geezer is correct. The permeability and permittivity of spacetime determines the value of c... The energy density of the vacuum is also somewhat related. In theory, if you increased the overall energy density of spacetime, you can alter the speed of light, which means it changes the values of the permeability and permittivity. Notable mathematician John Barrow has done extensive work on this.

I pretty much agree with that as I have said in previous posts, I believe the energy/mass (gravity) density is what determines the amount of contraction or dilation of time as the photon is the universes master clock.

#### MikeS

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##### Light speed?
« Reply #7 on: 05/08/2011 07:08:58 »
As a scientist, but not a physicist, I've always wondered why the speed of light in a vacuum should be constant. Speed is a derived variable from distance travelled and time. Yet it's time that is the variable (OK spacetime). I understand that it all depends on the frame of reference (relativity), but it makes no sort of sense. Given that electromagnetic waves are sort of spread out anyway and are massless, why have a maximum speed?
It's not limited to electromagnetic waves. Every massless particle with non zero energy must travel at that speed.
The fact is constant (better, *invariant*. It means it doesn't depend on the frame of reference) comes from the fact that it's the maximum speed possible, and this because of spacetime properties.

You can see it in this way: because of Lorentz contraction of lenghts, a very fast moving object measures contracted distances, so if you were inside a spaceship moving at near light speed, you would travel around the entire universe in a few seconds (zero seconds, at the limit v --> c). Clearly, you can't travel faster than that. So, in a certain sense (that is, in the sense of "rapidity", but I want to make it easy for the moment) it is as if it were infinite, and if you sum infinite to a finite number you still get infinite. For this reason, if you launch a light signal from a moving object, you "sum" the finite speed of that object with the "infinite" speed of light and so you get the same result (the *speed* is not infinite, but the rapidity is and the rapidity is a better concept, at high speeds).

Light speed is a finite number just because we measure speed as space/time, but this is a good definition of speed only at low speeds, because in this case only we have that the space travelled is independent of the interval of time.
At high speeds the usual definition loses its "common sense" meaning, as I showed in the example of the spaceship.

Don't know if all this was useful.

That's the simplest and best explanation of that phenomena that I have come across.

#### yor_on

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##### Light speed?
« Reply #8 on: 12/08/2011 21:38:47 »
I would say that we live in a universe defined by constants. Those set the scene from where we spring, and a big 'constant' is 'c'. Maybe there are universes where the 'constants' are differently defined, and where what we would call 'magic' will be true? I don't know, but the universe remind me of a finite definition of a something, where the borders (constants) defines what is most probable. We are apparently very probable in ours :)

#### lightarrow

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##### Light speed?
« Reply #9 on: 13/08/2011 15:21:43 »
Ah, yes, without constants it would be impossible even to talk; think what would happened if, while you are saying:

"ok, now I will measure the table's lenght with this ruler..."

suddenly the ruler becomes a frog and the table a pond of water

#### The Naked Scientists Forum

##### Light speed?
« Reply #9 on: 13/08/2011 15:21:43 »