# Why is the speed of light the same relative to any frame of reference?

1. Feb 29, 2004

### Rationalise

Why?!?!
What makes it the only constant in the universe?

2. Feb 29, 2004

### EL

Basically no one knows why the laws of nature look like they do. Physics just describes them...
But c is not the only constant. (Others are h-bar, G...)

3. Feb 29, 2004

### pmb_phy

Actually if it was proven that Maxwell's equations were correct and that the photon's proper mass was zero then the constancy of light could be derived from the principle of relativity and Maxwell's equations

4. Feb 29, 2004

### EL

Ok, but that just leads us to the next question...why do Maxwell´s equations look like they do?
You will always end up at something unexplainable "in the bottom"...(at least no one has yet been able to start from anything else to build up a theory from)

5. Feb 29, 2004

### Staff: Mentor

You're going to end up in an infinite "why" loop with these questions. There really isn't a good absolute answer here. The three most common are: "They just are," "God made them that way," and "if they were any different, we couldn't exist to observe them."

As you can see, "why?" is more philosophical than scientific. It can be interesting to think about/discuss, but it isn't really all that scientifically relevant.

6. Feb 29, 2004

### EL

That´s my point...

7. Mar 1, 2004

### LURCH

.
I think he meant, "only constant speed...".

I tend to use hyperdimensional reasoning to make lightspeed's frame-independance more palitable. If you can accept the fact that speed and direction are two different ways of looking at a single property, then you can make the "absolute speed" of light (a speed which is the same to all observers) an absolute direction, which is at the same angle from any path. This helps me tie lightspeed to the fourth dimension; time.

8. Mar 2, 2004

### ahrkron

Staff Emeritus
I agree with EL and Russ in that, ultimately, there are things in the universe that "just are", regardless of how much they make our neural circuitry feel "comfortable".

First (probably close to what Lurch said), when you measure the magnitude of the four-dimensional "speed" of any object, it turns out to be c always; how we measure it to move with respect to us is somewhat of an accident.

Second, I just want to emphasize that, counter-intuitive as it sounds, a constant speed of light is perfectly compatible with all experimental evidence to date.

Finally, the answer to your question is somewhat buried among the generality of maxwell eqns: from them, you can derive the speed of a wave regardless of the speed of its source.

9. Mar 2, 2004

You're talking about the motion of energy through matter. In any given medium, the EM energy can move at a given speed. Why would the speed at which the energy can migrate through a given material change simply because you are running left instead of walking right?

10. Mar 2, 2004

### Staff: Mentor

To take this a step further, some people seek the answers to some unanswerable questions in religion, but unfortunately there are always unanswerable questions, even in religion. The 'why?' of many things can be answered by citing God, but what about 'why?' questions on God himself?

11. Mar 2, 2004

### EL

At least our way of logic thinking cannot get around this problem...maybe we´re just not enough smart...

12. Mar 2, 2004

### ahrkron

Staff Emeritus
My take on it is that this is a limitation of all representation systems, and that intelligence and reason are unavoidably based on representations, implying that no matter how "smart" any species becomes, it will always have unanswerable questions.

13. Mar 2, 2004

### EL

Yes, but you are saying that from our logical point of wiev. What seems reasonable to us must not be the truth...
(Of course even my own reasoning is based on logic thinking, which makes this whole philosophical discussion paradoxial.)
However I think it´s dangerous to believe that our small brains are capable of determining what is possible to know and what is not.
But I agree that in the way humans are thinking now, we will always have unanswerable questions, no matter how smart we are...
Hope you got me...

(Sorry for leaving the field of physics)

14. Mar 10, 2004

### mhernan

Speed o lite? Constant?

Most publication use the Michelson-Morley experiment as a talking point and they refer casyually to the "null" result, implying there was no data showing light affected by the so-called aether drag. However, MM experimental results were not null, just approximately 1/20 of the 'expected' drift througth the aether. In the 1920 and after, Dayton Miller performed hundreds od MM tests which resulted ibn a finding of about 1/20 iof the 'expected'. So we have to assume the calculated value by the then current prevailing views in physics were correct in their assesment of the earth's velocity through space if we are to place any faith in the experimental results of MM..
On a slightly different angle the famous 'eclipse' experiments ofd 1919measuring the pull of mass on light from distant stars in 1919 was hailed as a huge success and Eisnstein was an immediately star. The eclipse experiments were "questionabley accurate". Where the calculated focal resolution was 2-3 arc seconds, the paper on the eclipse experiments claimed resolution of hundreths of a millimeter.
Einstein lauded the resolution of .01" in his Relativity book. This number came from comparing exposures of stars on photographic paper taken months apart and then overlaid and compared!
Is the velocity of light constant in all directions?
Yes?
No?
Prove it.

15. Mar 10, 2004

### Haelfix

Fortunately, we don't have to build interforometers anymore when analyzing special relativity. Instead we can look in modern particle accelerators, where everything is relativistic and calculable to 11 significant digits.

Any deviation in exact lorentz symmetry would output huge corrections to our results. We don't see it, ergo we still think special relativity is right.

16. Mar 10, 2004

### ahrkron

Staff Emeritus
Another piece of evidence for SR comes in the fact of the electricity bill for accelerators.

The higher the speed, the higher the mass (as measured in the lab's frame), which means that you need to have a higher magnetic field to keep your protons on track. All this numbers (from the mass to the measured field to the amount in dollars needed to keep all working) agree with SR.

17. Mar 10, 2004

### Nereid

Staff Emeritus
Here's a summary of tests of SR (there may be a better, more up-to-date summary; if any reader has found one, please let us know).

The 1919 eclipse observations were indeed 'marginal'; I would hope that no reviewer today would have agreed to its publication. Fortunately, a great many other observations have been made, involving both one-way (e.g. quasars) and two-way (e.g. Voyager on Mars, Cassini) signals.

I've not found anything better than http://relativity.livingreviews.org/Articles/lrr-2001-4/ [Broken] as a summary of GR tests.

Last edited by a moderator: May 1, 2017
18. Mar 15, 2004

### Terry Giblin

Is the speed of light travelling through a pure uniform rain droplet within a cloud, the same in every direction?

Is it therefore not logical to assume that the speed of light travelling through uniform 3-Dimensional droplet within an 11 Dimensional super cloud will also be the same in every direction?

19. Mar 15, 2004

### Nereid

Staff Emeritus
I didn't know that GR had been formulated in terms of 'an 11 Dimensional super cloud', I thought that was super-gravity, or String Theory, or M-Theory. I am also unaware of any experimental results which indicate a need for any such theories; GR has passed all its tests with flying colours.

You may want to read some of the threads in Strings, Branes & LQG, in the Physics sub-forum.

20. Mar 15, 2004

### Terry Giblin

Hi Nereid,

You are correct for point out that General Relativity, did not need additional space dimensions, but it Special Relativity (SR) would work very well without considering the time dimension and GR would not work within the frame of SR until we add the dimension of mass in for form of electrons and six additional quarks.

Where would quantum mechanics be without the electron and six quarks, in a 4-D spacetime inside a 10 or 11 dimensions super cloud.

The superstring theory describes the photon and electrons in 4-D spacetime and not forgetting the 6 quarks and the M-theory describes the cloud.

Newton, General Relativity, quantum mechanics, c,..., are all simple subsets within the cloud.