Photons vary in speed?

New to the forum. In discussing the speed of light, will photons vary in speed when light is bent? Black holes, magnetic fields, atmospheric refraction, other phenomena capable of bending light changes photon direction, which should in turn slow photons. Educate me...
 

Vanadium 50

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Magnetic fields don't bend light. The speed of light in media is less than the speed of light in vacuum. Changing the direction of something does not necessarily change its speed.
 

ZapperZ

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New to the forum. In discussing the speed of light, will photons vary in speed when light is bent? Black holes, magnetic fields, atmospheric refraction, other phenomena capable of bending light changes photon direction, which should in turn slow photons. Educate me...
Please start by read the FAQ thread in the General Physics forum.

Zz.
 
I don't believe FAQ's are intended to dismiss any discussion of a similar subject. FAQ's can also provoke further discussion.

Various mediums do slow photons. But what part of physics deals with photons traveling faster than the speed of light in a vacuum? The speed of light should be limitless, and the vacuum of space itself is a restrictive barrier.

It would be like our perception of how fast a 69' Mustang with a 351 engine can go. Since speed is relative, it is measured against relative barriers. The speed of a Mustang on Earth will max out when energy output = resistance. But the Mustang is already moving as fast as the planet.
 

ZapperZ

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I don't believe FAQ's are intended to dismiss any discussion of a similar subject. FAQ's can also provoke further discussion.
That's why I said that you should start with the FAQ. It should at least dispel a few misunderstanding that you had.

Various mediums do slow photons. But what part of physics deals with photons traveling faster than the speed of light in a vacuum? The speed of light should be limitless, and the vacuum of space itself is a restrictive barrier.
Er.. obviously, you didn't learn anything from the FAQ. Photons are NOT slowed down by the medium. Photons still move at c in between interaction/scattering with the medium!

And what photons are traveling faster than c in vacuum? When did you encounter one? You're asking us to provide you an explanation to something that you made up?

It would be like our perception of how fast a 69' Mustang with a 351 engine can go. Since speed is relative, it is measured against relative barriers. The speed of a Mustang on Earth will max out when energy output = resistance. But the Mustang is already moving as fast as the planet.
All speeds are relative, but not the speed of light, per Special Relativity. Also, it appears that you are not aware of how velocity addition works at speeds close to c (i.e. simple addition (Galilean transformation)) no longer works!

Zz.
 
Er.. obviously, you didn't learn anything from the FAQ. Photons are NOT slowed down by the medium. Photons still move at c in between interaction/scattering with the medium!

Zz.
Actually the speed of light does slow in a medium. If not, we wouldn't be able to observe refraction. The speed of light in water for example is 1.36:1, 1 being the speed of light. In glass the index of refraction can go up to 1.8 or 1.9, and diamond is 2.5. Not only does light slow in a medium, but different wavelengths slow to different speeds when in said mediums, this is how chromatic dispersion occurs, as different wavelengths of light change to different velocities when passing between the different refractive indexes, and exit at different trajectories. A common example in nature is called a rainbow.

The refractive index of air is actually 1.0003, so some refraction does occur, but very little. This is however why the sky turns red at sunset/sunrise, as red has the longest wavelength and refracts more than the other colours in the spectrum.

Black holes on the other hand do not refract light but instead bend spacetime, which can cause gravitational lensing affects. In this case it is not actually light that is bent but the space that it travels. Nothing can travel faster than the speed of light since c is the limit velocity that any energy can traverse, which would include massless photons. However, a recent study suggests that c itself may alter in a curved spacetime, however this still would not technically exceed the speed of light as light would still be travelling at c within the curved space. There are a couple dozen observable phenomenon in the universe that seem to violate c by exceeding light speed, all of which exist under special circumstances.
 

rcgldr

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As mentioned, in the case of a medium, electrons absorb and then emit photons with a bit of a delay, which causes a reduction in the apparent speed of light, but the photons travel at light speed from electron to electon.

The speed of light is affected by gravity:

http://en.wikipedia.org/wiki/Shapiro_delay
 
Last edited:

Rap

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You need to distinguish between group velocity and phase velocity. If you shine a light on a wall , the first photons that get there have travelled at the group velocity, always less than or equal to the speed of light. Thinking of light as a wave, the peaks travel at the phase velocity, which is the velocity given by the index of refraction. When the index is above one, the phase velocity is less than the speed of light, when its less than one, its faster than the speed of light. Most materials have an index of refraction greater than one but some have an index less than one, and the phase velocity is greater than the speed of light. But the photons always travel at the speed of light, and sometimes get absorbed, wait a bit, then re-emitted. The average velocity of the photons is the group velocity.
 

Pengwuino

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Actually the speed of light does slow in a medium. If not, we wouldn't be able to observe refraction. The speed of light in water for example is 1.36:1, 1 being the speed of light. In glass the index of refraction can go up to 1.8 or 1.9, and diamond is 2.5. Not only does light slow in a medium, but different wavelengths slow to different speeds when in said mediums, this is how chromatic dispersion occurs, as different wavelengths of light change to different velocities when passing between the different refractive indexes, and exit at different trajectories. A common example in nature is called a rainbow.
No, re-read what ZZ said. The speed of light does not slow down, it just appears to slow down due to interactions with the atoms in the substance that take time to occur.
 
Nothing can travel faster than the speed of light since c is the limit velocity that any energy can traverse, which would include massless photons. However, a recent study suggests that c itself may alter in a curved spacetime, however this still would not technically exceed the speed of light as light would still be travelling at c within the curved space. There are a couple dozen observable phenomenon in the universe that seem to violate c by exceeding light speed, all of which exist under special circumstances.
Agree, it would be like a worm hole in curved space. Excluding curved space, wouldn't it also be true that if just one observable phenomena violates c, then c itself must fall within a larger set of undetermined maximums?
 

Vanadium 50

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There are a couple dozen observable phenomenon in the universe that seem to violate c by exceeding light speed, all of which exist under special circumstances.
This thread is going in a very bad direction. Please reread the PF Rules, as well as the FAQ in the SR section.
 

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