# B Can light accelerate in vacuum?

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1. Jan 18, 2016

### Likith D

Light being emitted from a source in vacuum, can photons accelerate ?

2. Jan 18, 2016

### Drakkith

Staff Emeritus
No, photons move at c the moment they are created.

3. Jan 18, 2016

### sophiecentaur

Here we are again with that perennial problem that arises because the word "particle" is used in describing a Photon. What a pity they didn't invent a special, alternative word for the photon. It's what happens when very clever people try to get very clever ideas across to us mere mortals. They can't have conceived the problems they were injecting into Science education by that choice of word.

Last edited by a moderator: Jan 18, 2016
4. Jan 18, 2016

### DrStupid

Acceleration is the time derivate of velocity and not of speed.

5. Jan 18, 2016

### Drakkith

Staff Emeritus
Ok?

6. Jan 18, 2016

### DrStupid

The local speed of light in vacuum is constant but the direction can be changed.

7. Jan 18, 2016

### DrStupid

I agree that this is very likely. However, it should be clarified that the answer doesn't refer to the question as it has been asked.

8. Jan 18, 2016

### Staff: Mentor

How? Even in general relativity, where light can seem to curve, it follows geodesics - no acceleration.

No, light in vacuum cannot accelerate.

9. Jan 18, 2016

### davenn

I suspected but didn't want to say, thanks for the clarification
should remove what I said earlier

10. Jan 19, 2016

### just dani ok

but light bend near massive object, so it is accelerated.

11. Jan 19, 2016

### A.T.

It terms of coordinate acceleration in non inertial coordinates, yes. The speed in non inertial coordinates is also not limited to c, but can take any arbitrary value.

In inertial coordinates though, which exist only locally in curved space time, light doesn't change speed nor direction.

12. Jan 19, 2016

### DrStupid

That's why I limited my statement above to the local speed.

13. Jan 19, 2016

### David Lewis

Light emitted from a source in a vacuum accelerates when it enters (or leaves) a material with refractive index > 1.

14. Jan 19, 2016

### gatopardos

Light doesnt really accelerate otherwise it would go beyond c.What changes is the phase velocity vp=c/n so when n<1 , vp>c.

15. Jan 21, 2016

### David Lewis

Acceleration is the rate of change of velocity, so negative values of acceleration (slowing down) are permitted.

16. Jan 21, 2016

### A.T.

Velocity is a vector and so is acceleration. Slowing down doesn't imply negative acceleration components, as their sign depends on the coordinate system.

17. Jan 21, 2016

### David Lewis

Thank you. If an object is going 2 m/s, and then slows down to 1 m/s, what is delta v?

18. Jan 21, 2016

### Staff: Mentor

-1m/s
+1m/s if you work with rockets.
How is that question related to the speed of light in vacuum, the topic of this thread?

19. Jan 21, 2016

### David Lewis

My assumption is that when something slows down, it accelerates (because its velocity is changing). Hence my previous assertion that light accelerates when it enters a material with refractive index >1.

20. Jan 21, 2016

### Staff: Mentor

21. Jan 21, 2016

### gatopardos

Light speed is constant.

22. Jan 21, 2016

### David Lewis

When light strikes a mirror in a vacuum, it changes direction.
And, when light is emitted from a refractive material source into a vacuum, it speeds up.

23. Jan 21, 2016

### gatopardos

Velocity is the vector of speed, the absolute value of the vector is constant there is no acceleration. Speed is constant.

24. Jan 21, 2016

### Staff: Mentor

There are no mirrors or refractive materials in a vacuum.

25. Jan 21, 2016

### gatopardos

Light doesnt stop when is hits the mirror.