# Surpassing the speed of light? Why not?

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1. Aug 28, 2015

### quincy harman

The first postulate is perfect, the laws of physics are the same for all uniform inertial frames of reference. In fact the second postulate is perfect as well the speed of light is constant in all uniform inertial frames of reference. But here is my problem with it, the speed of any wave is constant in all inertial frames of reference. By adding speed all you're doing is giving the wave energy in the direction of motion and taking energy from it the opposite direction. So why would mass not be able to exceed the speed of light? Especially when the laws of physics are the same in all inertial reference frames. This means all motion is relative.When you surpass the speed of sound you get a sonic boom because the said object is moving to fast for the wave to keep up. But for the speed of sound we don't measure v-s or v+s, where s = speed of sound, like we would in classical mechanics but we don't calculate time dilation from it. I see a problem here. Mass cannot force light to accelerate but this doesn't necessarily mean mass cannot surpass a photon. If the laws of physics are the same in all reference frames once we we reach escape velocity why wouldn't we be able to reach speeds beyond the speed of light?

2. Aug 28, 2015

### Staff: Mentor

There is no "direction-dependent energy" where you could take something away.
No matter how much energy you give the mass, its speed will be below the speed of light.
Correct.

Sound is propagating in a medium and the speed relative to observers depends on their motion, this is completely different from light.
No matter how long and how intense you accelerate, the result is always a speed below the speed of light.
You can verify this with the formula for url=https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity]relativistic velocity addition[/url]. Take some starting speed, assume your rocket accelerates by 10m/s^2 for one second, add this to the starting speed with the relativistic formula, see how much the speed changes: it will be less than 10m/s. You can keep adding those speed increments and you will never surpass the speed of light.

3. Aug 28, 2015

### quincy harman

Red shifting if it is moving away from you blue shifting it is moving toward you, +frequency=+energy, and I understand m0/sqrt 1-v2/c2 is why it is currently thought that no mass can exceed the speed of light, same for time dilation, but the math is based off the idea that you cannot measure light with v+c,v-c, light oscillates through electromagnetic fields similar to the way sound oscillates through a medium by which I mean If a moving object can give a energy too light in the same way as it can sound, how is it different? If an object is moving towards you at 99.99% the speed of light you wouldn't see anything because it would be beyond the visible wavelength spectrum. Same if it was moving away from you. If the laws of physics are universal in all inertial frames of reference then K=1/2mv^2 is all it would take to accelerate at all velocities but relative to what? Why are the clock rates different?

Last edited: Aug 28, 2015
4. Aug 28, 2015

### Staff: Mentor

And this is based on experimental results, yes. Those results are extremely precise.
This is not similar at all.
That formula is a nonrelativistic approximation, you cannot use it in relativity.
Otherwise the speed of light could not be constant. But again, it is based on measurements. All special relativistic effects have been measured extremely well.

5. Aug 28, 2015

### quincy harman

You didn't finish reading what I said about why they are similar. They are similar because wavelength changes from the object emitting them based on the direction of motion. Of course the speed of light isn't going to oscillate the same. One oscillates threw electric and magnetic fields while another oscillates through a medium. Also I'm wanna make it very clear I'm not denying the consistency of c and if you think that you're missing my point. My point is c in vacuum is just a product of waves. wavelength*frequency=speed, I also understand that sound travels at different speeds in different mediums but given a medium ^
Also you didn't answer my question can you measure the speed of the object and the speed + the speed of sound?

Last edited: Aug 28, 2015
6. Aug 28, 2015

### Staff: Mentor

The key difference is that the speed of a sound wave is not invariant. Different reference frames disagree on how fast a given sound wave is moving. That is not at all the same as with light.

Suppose there is a lightning strike near where I am on the ground and you are in a supersonic aircraft passing overhead. We will both agree on the speed of the light, but not on the speed of the thunder.

7. Aug 28, 2015

### Staff: Mentor

Wavelength in which frame?
The answers are completely different for light and for sound.
It is not.
There is a fundamental speed in special relativity - this has nothing to do with light.
Light happens to travel at this fundamental speed, for purely historical reasons we call this speed "speed of light".
You can measure every speed. I don't understand what the question is.

8. Aug 28, 2015

### quincy harman

My point is that the consistency of c is just a product of waves and not time dilation or length contraction.
Doesn't that have to do with location? and light being much faster?

9. Aug 28, 2015

### quincy harman

you cannot measure the speed of the object that the wave is coming from + the speed of the wave

10. Aug 28, 2015

### quincy harman

okay I think I get it, the speed of light appears instantaneous for all observers because it doesn't travel through time even though it has a finite speed through space?

Last edited: Aug 28, 2015
11. Aug 28, 2015

### Staff: Mentor

No, it is a fundamental difference in the behavior of light and sound.

12. Aug 28, 2015

### quincy harman

how does no time pass for the photon yet we observe a finite speed for light?

13. Aug 28, 2015

### Staff: Mentor

No, it doesn't.

?

14. Aug 28, 2015

### quincy harman

no time passes for light because it has no rest frames

15. Aug 28, 2015

### weirdoguy

There is no notion of time for photon - because there is no reference frame of photon.

"no time passes for light because it has no rest frames"

No notion of time does not mean "no time passes". It means that you can't say anything meaningful about time for photon. It is a difference. There has been a lot of threads discussing that issue, search for them.

16. Aug 28, 2015

### bcrowell

Staff Emeritus
Most of the OP has to do with why c is frame-independent, rather than why you can't go faster than that. We have a FAQ on that topic: https://www.physicsforums.com/threads/why-is-the-speed-of-light-the-same-in-all-frames-of-reference.534862/ [Broken]

The discussion of waves is based on a common misconception that relativity, and c, are fundamentally related in some way to light waves. They aren't. See http://physics.stackexchange.com/q/35404/4552 .

Re why you can't go faster than c, see http://physics.stackexchange.com/a/61129/4552 . It has nothing to do with light waves, because neither c nor relativity have anything to do with light waves.

Last edited by a moderator: May 7, 2017
17. Aug 28, 2015

### Staff: Mentor

The statement that no time passes for a photon is not correct, although it does crop up from time to time in pop-sci works from authors who should know better.

18. Aug 28, 2015

### quincy harman

we see the photon as it left 14 billion light years ago. if you were a photon you would not experience time. but we observe it as moving through space at a finite speed
well thank you for making fun of me :)

19. Aug 28, 2015

### Staff: Mentor

Hi Quincy Harman, I am not sure what I said that could have been interpreted as making fun of you. I certainly am not doing so.

20. Aug 28, 2015

### quincy harman

can you explain what a rest frame please?