# Theory of Relativity

1. Jan 10, 2010

### pinoyplaya

Hey guys. I am a college physics student and ever since this theory was introduced to me in high school, I refuse to believe it.

What caught my attention first is that Einstein's Principle of Relativity.

Einstein suggest that the laws of physics are the same for all observers in uniform motion relative to one another. However, using Galilean Transformations, two objects travelling on opposite directions, one with a velocity v while the other has a velocity 2v. Relative to each other, object one is travelling with a velocity 3v relative to object two and vice versa. Einstein does not refute this at all.

However, when applied to light, this should remain the same. No matter what the object was, bullet, car, rocket, or light, this transformation should remain the same. But Einstein disagrees. He suggest that light remains at the speed of light and is a constant speed (which I agree). But he also says (from what I learned) that nothing can have a velocity faster than the speed of light despite their reference frames.

So in other words, if the objects I talked about above were travelling at v=0.9c where c is the speed of light, then their relative velocities will never be faster than the speed of light. In this case their relative velocity is 3v = 2.7 times the speed of light.

How can Einstein be right if Galilean transformation holds true?

2. Jan 10, 2010

### nicksauce

The Galilean transformation does not hold true. It is superseded by the Lorentz transformation. The velocity addition rules then change, and speeds never exceed the speed of light.

3. Jan 10, 2010

### espen180

As nicksauce stated, the Galilean transformation do not hold true. They are a slow-speed (v=0, to be exact) approximation of the Lorentzian transformations.

4. Jan 10, 2010

### pinoyplaya

Ok. Then my question is why does Galilean transformation work at low speeds such as comparison car speeds but when it comes to higher velocities it is no longer valid. I know that this can be used just as an estimate but it seems accurate.

Car A headed east at 60mph and Car B passes Car A moving at a direct opposite direction. Car B has a velocity of 50mph. Both velocities are relative to the earth/ground.

The person in Car A sees Car B moving at 110 mph and Car A sees the earth/ground moving at 60mph.

Is this correct? If so, then I still don't understand why this concept cannot be applied to light.

And if Einstein is right, suggesting that light does have mass, then wouldn't an object with a slightly less mass than light can have a velocity faster than the speed of light?

Newton suggest that light is massless, then obviously nothing travels faster than the speed of light. Einstein supports that nothing can travel faster than the speed of light, but he argues with Newton and says that light does have a mass.

Edit: I guess I'm just into the idea that if nothing travels faster than the speed of light, then the speed of light should be infinite. But of course that doesn't hold true because otherwise, the light rays from the sun would not take roughly 8 minutes to pass planet earth.

Another thing that confuses me is that if the sun were to explode, we don't experience its catastrophe 8 minutes prior to the explosion. I highly disagree. We may not be able to SEE the result 8 minutes prior but its affect on planet earth should be instantaneous (as Newton suggest) but Einstein suggest otherwise. He is saying that light gets there first before anything else.

And if the sun disappears, our planet does not go out of orbit until around 8 minutes has passed since the event occurred. Newton says that our planet should get out of orbit immediately.

Think of it this way. You hang a ball on the ceiling supported by a string. You cut the string off, Einstein says that the ball (which is affected by gravity) does not fall immediately but instead, the string must pass the ball first before the ball starts to fall. And of course this is completely false.

Newton suggest that this ball will fall immediately as soon as the string is cut. And as expected, it is what should and does happen.

Last edited: Jan 10, 2010
5. Jan 10, 2010

### Staff: Mentor

6. Jan 10, 2010

### pinoyplaya

Of course it will work because these are formulas derived by Einstein himself.

If I created a formula that says 1+1=3 then it will work.

I guess the only way to show that Einstein's theory of relativity is to test it on objects with a velocity that is fractional to c. At least .5c, otherwise, its still a theory.

7. Jan 10, 2010

### nicksauce

1) Why is the Galilean transformation a good approximation at low speeds? Well, it can be shown mathematically, but I don't know a reason intuitively why it is true.

2) Light doesn't have mass. I don't know what gives you that idea.

3) Gravitational influence also propagates at the speed of light. Therefore, we see the catastrophe and feel its impact at the same time.

8. Jan 10, 2010

### nicksauce

The formulas of special relativity are extremely(!!!!) well tested. Particle colliders rely on them daily.

9. Jan 10, 2010

### pinoyplaya

Extremely tested at low velocities. Not even close to the speed of light. And the fact that it is tested on earth and not on space.

Edit: On another topic, good luck collecting anti-matter when you have a particle accelerator surrounded by full of matter. And please, don't tell me they successfully collected any for more than 1 second.

10. Jan 10, 2010

### George Jones

Staff Emeritus
No; see jtbell's posts.
Light doesn't have rest mass, while massive particles do have rest mass, and Einstein did not suggest anything that contradicts this.

11. Jan 10, 2010

### nicksauce

Particle accelerators routinely bring particles to within a fraction of the speed of light. To me, that is not a low velocity. If the equations of special relativity broke down, they would notice. And why does it matter if it's in earth or in space?

12. Jan 10, 2010

### pinoyplaya

E=mc^2

That's Einstein's famous equation isn't it? Anything that has energy has mass, light has energy therefore it has mass.

13. Jan 10, 2010

### pinoyplaya

LOL, why does it matter if its on earth or space. Newton's law of gravitation. Not to mention, Einstein says light has mass, his equation does not take into account gravity and light's mass.

And as for particle accelerators, they are not used to test Einstein's Theory of Relativity. LOL.

14. Jan 10, 2010

### nicksauce

No. This equation says that anything with rest mass has rest energy. Photons do not have rest mass. The full equation is E^2 = p^2c^2 + m^2c^4 (p is momentum), so for photons, E = pc.

15. Jan 10, 2010

### pinoyplaya

If you suggest that is wrong, then you definitely have to look at special relativity one more time.

16. Jan 10, 2010

### George Jones

Staff Emeritus
The equation actually is

$$E^2 - \left( c p \right)^2 = \left( m c^2 \right)^2,$$

where $p$ is momenum. In a frame in which a massive particle is at rest, $m \neq 0$ and $p = 0$, and the above equation becomes $E = mc^2$. Photons, which are never at rest in any frame, have, in all frames, $m = 0$ and $p \ne 0$. Consequently, for photons, the above equation becomes $E = cp$.

17. Jan 10, 2010

### nicksauce

No of course they aren't use to test relativity. Because we already know from a number of other experiments that relativity is true. But if it weren't, particle accelerators would definitely notice.

And relativity (general relativity specifically) has been tested in space, and it matches experiments extremely well.

18. Jan 10, 2010

### pinoyplaya

Photons do not have rest mass.... yes in theory. I would like to see this experimentally tested... impossible btw.

19. Jan 10, 2010

### pinoyplaya

Gravitational Influence also propagates at the speed of light -> According to Einstein.

If Einstein created new numbers and this numbers suggest 1 + 1 = 3, as we know it today it is false, but of course whoever said 2 is actually two. These are just what representations of them.

Light doesn't have mass. Well.. once again, if light has mass, then it would be very much possible to overcome the speed of light.

20. Jan 10, 2010

### nicksauce

Well I'm done here. You clearly have some sort of agenda and aren't willing to learn.