# Light
**URL:** https://boards.straightdope.com/t/light/132398
**Category:** Factual Questions
**Created:** [October 15, 2002, 9:05pm UTC](https://boards.straightdope.com/t/light/132398 "2002-10-15T21:05:48Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@DarkBrigade](https://boards.straightdope.com/u/DarkBrigade)
#### Post date: [October 15, 2002, 9:05pm UTC](https://boards.straightdope.com/t/light/132398/1 "2002-10-15T21:05:48Z")
Hello,
Can light accelerate light?
Benjamin
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### Author:  [@friedo](https://boards.straightdope.com/u/friedo)
#### Post date: [October 15, 2002, 9:54pm UTC](https://boards.straightdope.com/t/light/132398/2 "2002-10-15T21:54:52Z")
No. The speed of light is constant.
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### Author:  [@scotth](https://boards.straightdope.com/u/scotth)
#### Post date: [October 15, 2002, 9:55pm UTC](https://boards.straightdope.com/t/light/132398/3 "2002-10-15T21:55:14Z")
Ummmm…
The way the question is worded doesn’t make alot of sense to me. Light by definition never accelerates (or deccelerates). It always moves at the speed of light in whatever medium that it is in.
Light can interfere with itself. For an excellent book on the way light behaves pick up Q.E.D. by Richard Feynman.
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### Author:  [@DarkBrigade](https://boards.straightdope.com/u/DarkBrigade)
#### Post date: [October 18, 2002, 9:51pm UTC](https://boards.straightdope.com/t/light/132398/4 "2002-10-18T21:51:00Z")
I wondering if light accelerate?
(Sorry about the above phrasing; I don’t know what I was thinking.)
Is light moving at 3x10^8 but is it moving at that speed the moment it is generated / transfered.
Also, my physics teacher mentioned reading about an experiment about accelerating light twenty times its current speed.
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### Author:  [@Padeye](https://boards.straightdope.com/u/Padeye)
#### Post date: [October 18, 2002, 9:57pm UTC](https://boards.straightdope.com/t/light/132398/5 "2002-10-18T21:57:25Z")
Photons have momentum but not mass. Don’t try to apply classical physics to them. Light does accelerate when going from a more dense medium to a less dense and does it instantaneously. This is how optics work.
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### Author:  [@AskNott](https://boards.straightdope.com/u/AskNott)
#### Post date: [October 19, 2002, 12:46am UTC](https://boards.straightdope.com/t/light/132398/6 "2002-10-19T00:46:28Z")
As Padeye said, photons do not have mass. They are Methodist.
–Nott
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### Author:  [@g8rguy](https://boards.straightdope.com/u/g8rguy)
#### Post date: [October 19, 2002, 12:50am UTC](https://boards.straightdope.com/t/light/132398/7 "2002-10-19T00:50:18Z")
Technically speaking, light _can_ accelerate light, in that acceleration is a change not in speed but in velocity. Since there is a process in QED known as the scattering of light by light (NB: we cannot at present detect this phenomena and can only predict it), in principle you could have two photons interact in such a way that each changes direction, which would be an acceleration.
But as far as increasing speed, as other posters have noted, you’re out of luck.
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### Author:  [@DarkBrigade](https://boards.straightdope.com/u/DarkBrigade)
#### Post date: [October 19, 2002, 3:34am UTC](https://boards.straightdope.com/t/light/132398/8 "2002-10-19T03:34:00Z")
Thank you,
I will be looking up QED
Benjamin