# The Theory of Relativity.

1. Mar 4, 2013

### VertexOperator

Why is the theory of relativity a theory?
Haven't we proven one of its consequences (time dilation) already using atomic synchronized clocks?

2. Mar 4, 2013

### Staff: Mentor

Just as in Geometry we have axioms and theorems. In physics, we have axioms which can be tested and theories based on them which can also be tested. The axiom for Relativity is the constancy of the speed of light for all observers in inertial frames of reference. For GR we have the equivalence principle.

3. Mar 4, 2013

4. Mar 4, 2013

### VertexOperator

Aha, I should learn my definitions then :)

5. Mar 4, 2013

### brenan

No - what you have proved is that clocks go out of sync when you accelerate them
to high velocities. The experiment proves nothing more than that.

6. Mar 4, 2013

### ZapperZ

Staff Emeritus
Actually, it does. The quantitative aspect of the change (i.e. how fast did you accelerate and by how much did the clock go out of sync) is what makes it physics, rather than just handwaving philosophy.

In any case, this is irrelevant to this thread. The OP has discovered the confusion in the term "theory" being used in science, as opposed its pedestrian usage.

Zz.

7. Mar 4, 2013

### VertexOperator

So the syllabus dot-point is: Identify that if c is constant then space and time become relative
In Newtonian physics, space and time are constants and motion is defined by them. For example, consider a spacecraft moving at half the speed of light (0.5c) towards another planet. An astronaut in the spacecraft now flashes a light beam in the direction of motion of the spacecraft.
What is the speed of the light beam as viewed by someone on the planet? Prior to Einstein, we would have said 1.5c. However, according to Einstein’s Theory of Relativity, the answer is now known to be c...
But WHY is c constant?
Is there a proper scientific explanation to why the speed of light is always c?

Last edited: Mar 4, 2013
8. Mar 4, 2013

### Staff: Mentor

No. It is just an observation, and our physical theories are based on it.
It would be possible to live in a universe with Newtonian physics as well*. But this is not our universe.

* particle physics would have to look completely different without special relativity, but let's ignore that here.

9. Mar 4, 2013

### VertexOperator

Ok, makes so much sense lol :)
How did Einstein make this observation? Please don't tell me it was a thought experiment :O

10. Mar 4, 2013

### 1977ub

Does the constancy of c seem stranger to you than Newton's laws of motion? What makes a scientific explanation "proper"?

11. Mar 4, 2013

### Staff: Mentor

Science never really answers "why" questions; no matter what answer I give you it, it will lead to another deeper "why". (Hence the old joke about "turtles all the way").

So it might be easiest to treat the constant speed of light as a postulate; that's how Einstein presented special relativity. It is, however, a good postulate, in that it leads to a logical and elegant theory that agrees with observation and makes correct predictions. You might also find the recent thread on this subject interesting.

Last edited: Mar 4, 2013
12. Mar 4, 2013

### Staff: Mentor

Nature had already given us a pretty strong hint, as Maxwell's equations of electrodynamics suggest that light should travel in a vacuum at speed c. Indeed, much of the second half of the 19th century was spent trying to reconcile classical electrodynamics with our intuitive (but not empirically confirmed) notions of how space and time should work.

It's worth noting that Einstein introduced relativity in a paper entitled "On the electrodynamics of moving bodies"... That really was the problem of the century.

13. Mar 4, 2013

### 1977ub

People did a lot of struggling with the speed of light before Einstein decided to try working out the consequences of that wild idea that c was the same for all unaccelerated observers. He didn't just pull it out of his... hat.

http://en.wikipedia.org/wiki/History_of_special_relativity

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth?" - Sherlock Holmes (Doyle)

14. Mar 4, 2013

### VertexOperator

Didn't a guy called Hertz use Maxwell's equation and radio waves to find a value for the speed of light?
Thank you for the help guy, I will treat c as a postulate by Einstein.
Also, why is it that the closer to a gravitational field an object is moving the more time dilates? Does gravity affect light in anyway?

15. Mar 4, 2013

### Staff: Mentor

It is a consequence of a similar postulate in general relativity: Physics is the same for all free-falling observers.
It affects spacetime, and light is moving in this spacetime. Light gets deflected by gravity, indeed. The strongest influence of gravity on light can be seen (or, well, not seen ;)) in black holes.

16. Mar 4, 2013

### Staff: Mentor

yes, and that's the hint from mother nature that I was referring to in #12 above.

Read that thread that I linked - the history and change of perspective over the last century is interesting.

17. Mar 4, 2013

### Staff: Mentor

Maxwell himself used his equations to predict the existence of electromagnetic waves. He used results of previous experiments on electricity and magnetism to predict the speed of those waves. It turned out to equal (within experimental uncertainty) the speed of light, which other people had recently measured fairly precisely.

18. Mar 4, 2013

### VertexOperator

I really wish we study Maxwell's equations in high school :( but we don't do contour integration in high school maths...

19. Mar 4, 2013

### WannabeNewton

I don't see why you can't learn the necessary EM on your own although contour integration is a topic from complex analysis and not, I'm assuming, what you meant to say.

20. Mar 4, 2013

### VertexOperator

But Maxwell's equations have a lot of contour integrals.
I will try to learn it on my own :)