# Why, why, why, Mr. Anderson, WHY?

1. Oct 8, 2008

### Bible Thumper

Why does light have to necessarily go 186K MPS? Why not 100K MPS? Or 300K MPS?
Why strictly 186K MPS even tho there are numerous, different ways to generate light?

Why, Mr. Anderson? Why?

2. Oct 8, 2008

### dx

That's because the meter is defined so that it travels at exactly that speed. If you choose your unit of distance as the lightsecond and your unit of time as second, then light travels at speed 1. So it all depends on the choice of units. Also, the speed of light doesn't depend on how it is generated. It is constant, no matter where or how it is emitted. This is an observational fact, and also the basis of special relativity. It doesn't have any deeper explanation at the moment.

3. Oct 8, 2008

### JesseM

In some ways it's meaningless to imagine a "change" in a physical constant that has definite units like the speed of light, although you can imagine changes in "dimensionless" physical constants that are pure numbers (because they are ratios of various physical constants with units)--see this article explaining why, as well as this page summarizing all the dimensionless constants.

4. Oct 8, 2008

### atyy

Who is Mr. Anderson?

5. Oct 8, 2008

### JesseM

Keanu Reeves (it's a line from 'The Matrix')

6. Oct 8, 2008

### atyy

Ah, shoulda known that - thank you!

7. Oct 8, 2008

### Bible Thumper

Last edited by a moderator: Apr 23, 2017
8. Oct 8, 2008

9. Oct 8, 2008

### Bible Thumper

Thanks for the very useful references, JesseM.
Forgive my denseness, but it appears that whenever something is massless (or becomes massless), the speed of light is always the inevitable result. And as your reference points out, the author is also bewildered (as I am) about the fine structure constant, and why the fine structure constant is the quantity it is.
I know stuff like Celsius and Fahrenheit are arbitrary and meaningless, but absolute zero is not. Neither is the phenomena of Brownian motion--the latter being an effect of the raising of these arbitrary and meaningless temperature scales.

So I guess I'm still stuck on this 186K MPS. Create any scale you wish (or go with 300 KPS if you wish), the light will still go that speed. So will anything that's massless.
It gets odder when we introduce how light is generated. For example, we can make them by running an excessive current thru wire. Or, exposing some elements to U/V radiation (spontaneous emission). Or, by running electrons close to one another (florescent tubes). Or, by using a blender (radio emission). There are too many different ways at obtaining this same velocity.

Why, why, why?! Vagaries of perception. Temporary constructs of a feeble human intellect trying desperately to justify an existence or something!!!

10. Oct 8, 2008

### atyy

The speed of light is exact. And exactly the same for all inertial observers. If this were not so, human thought as electric current would not generate a magnetic field, and the Matrix could not exist.

http://physics.nist.gov/cgi-bin/cuu/Value?c
http://en.wikipedia.org/wiki/Magnetoencephalography
http://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation

Last edited: Oct 8, 2008
11. Oct 13, 2008

### Naty1

No one knows!! We are unable to calculate all physcial constants based on fundamental principles. Hence the speed of light, the charge of the electron, the strength of the strong nuclear force, the gravitational constant G are all determined experimentally. Another way to make an equivalent statement is that the standard model of particle physics has so far been unable to determine any of the basic constants in our universe...they are based on experimental results, not theory. String theory has the abilty to produce many constituents...too many, in fact. So far in string theory you can pick almost any value you like for many of the basic forces and particles....nobody knows how to make the approximate perterbation solutions match only what we observe in this universe.

12. Oct 17, 2008

### Anthea Lawn

Bear in mind that this is the speed of light IN A VACUUM. When light travels through a medium such as air or water, it slows down to a speed of .. well, it depends on the medium.
This slowing down is basically (without going into details) what causes refraction.

13. Oct 18, 2008

### Phrak

If the speed of light is globally changed would anyone notice?

Obituary: I see BibleThumper finally thumped-out. A slow motion, inevitable tragedy--a car crash in slow motion. It seems the contra-thumper faithful got thumped out.

The universe just made itself from nothing. I know the line.

Last edited: Oct 18, 2008
14. Oct 18, 2008

### HallsofIvy

The fact that the speed of light is that particular number IS known. It is because the meter is defined in such a way as to make the speed of light exactly equal to that.

Nor is it peculiar that the speed of light is a constant in the usual sense. It is a wave and the speed of a wave, relative to its "carrier", depends on the properties of "carrier". Light is an electromagnetic wave carried by space itself and it appears that the properties of space area always the same, hence always to same speed, relative to space.

But that should mean that we, moving through space, should then be able to use light to measure our speed "relative to space" and so get an absolute speed. Ever since the Michaelson-Moreley experiment we have known we can't do that and all of relativity is to determine why that is so. "Why is the speed of light constant relative even to moving reference systems" is the question relativity was designed to answer.

15. Oct 18, 2008

### Staff: Mentor

I'd be pretty upset, since my WiFi router would crap out and I wouldn't be able to surf PF anymore!

16. Oct 18, 2008

### Phrak

how? I'd be inclined to think that the laws of physics are invariant under a rescaling of c.

17. Oct 18, 2008

### Staff: Mentor

I also assumed you meant that the laws of physics would remain unchanged, just the value of C changed. But if you change the value of C, then things (such as radio frequencies) that depend on that value also change. Since we're speaking hypothetically, it is possible that both my router and wifi card would change the same way if their antenna and electronics are similar, but that still doesn't mean the new frequency would be able to find its way through my house.

Now, my current router only has one antenna, but my last one had two and the spacing of the antennas was based on half the wavelength of the signal being transmitted. Change the wavelength and the system loses its constructive interference.

Anything that depends on the frequency of em radiation could potentially be affected. And then there's the concern about the sun no longer working or that nuclear plant 5 mile from my house exploding...

18. Oct 18, 2008

### Anthea Lawn

It would be interesting if the speed of light were slower than the speed of sound. For example, you would hear the thunder before you saw the lightning flash.
In Terry Pratchett's Discworld novels, the speed of light is very slow, making for some interesting physical effects.

19. Oct 19, 2008

### DaveC426913

Because physical distances would NOT change. Circuits would behave differently, GPS signals would behave differently, human bodies would behave differently. Heck, molecules and atoms might behave differently.

20. Oct 20, 2008

### Phrak

I'll try to simplify. Under a rescaling of c, if all velocites didn't rescale by the same factor, you would have a point. But I don't think this is physically missible.

what experiment can you preform that would yield different results under a global rescaling of c?

Film an experiment taking place. The measured values that the experimenters obtain are independent of whether the tape is payed back at normal speed or speed-up.