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Author Topic: How fast do photons travel?  (Read 7997 times)

Margus

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How fast do photons travel?
« on: 02/11/2011 19:30:02 »
Margus asked the Naked Scientists:
   
How photons travel with speed slower then speed of light in matter, when particles with zero rest mass should always be traveling with speed of light.

What do you think?
« Last Edit: 02/11/2011 19:30:02 by _system »


 

Offline abacus9900

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How fast do photons travel?
« Reply #1 on: 03/11/2011 12:51:52 »
Margus asked the Naked Scientists:
   
How photons travel with speed slower then speed of light in matter, when particles with zero rest mass should always be traveling with speed of light.

What do you think?


Because it interacts with matter by being scattered, absorbed, scattered, re-absorbed etc. and so gets slowed down. In free space there is nothing to interact with.
 

Offline simplified

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How fast do photons travel?
« Reply #2 on: 04/11/2011 13:51:44 »
Margus asked the Naked Scientists:
   
How photons travel with speed slower then speed of light in matter, when particles with zero rest mass should always be traveling with speed of light.

What do you think?


Because it interacts with matter by being scattered, absorbed, scattered, re-absorbed etc. and so gets slowed down.
Then why does photon have  unchanging direction in the transparent environment?
« Last Edit: 04/11/2011 13:54:31 by simplified »
 

Offline JP

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How fast do photons travel?
« Reply #3 on: 04/11/2011 14:27:02 »
Margus asked the Naked Scientists:
   
How photons travel with speed slower then speed of light in matter, when particles with zero rest mass should always be traveling with speed of light.

What do you think?


Because it interacts with matter by being scattered, absorbed, scattered, re-absorbed etc. and so gets slowed down.
Then why does photon have  unchanging direction in the transparent environment?

Because scattering doesn't have to be in a different direction than the incoming photon.  In transparent environments, it can still be slowed down, but can scatter in the same direction as the incoming photon.
 

Offline willyp00

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How fast do photons travel?
« Reply #4 on: 04/11/2011 14:56:58 »
Margus asked the Naked Scientists:
   
How photons travel with speed slower then speed of light in matter, when particles with zero rest mass should always be traveling with speed of light.

What do you think?
Photons travel at the speed of light. Their properties don't travel. They just are, everywhere. Information is thus likewise.
 

Offline willyp00

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How fast do photons travel?
« Reply #5 on: 04/11/2011 15:01:25 »
Margus asked the Naked Scientists:
   
How photons travel with speed slower then speed of light in matter, when particles with zero rest mass should always be traveling with speed of light.

What do you think?
As photons transmute through media, there is the time delay from absorption to re-emission that causes the apparent slowness.
 

Offline lightarrow

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How fast do photons travel?
« Reply #6 on: 04/11/2011 15:37:58 »
Here it talks of light in a solid medium, but with a liquid, apart from preferred directions in the lattice and phonons, there should be no big difference:

http://www.physicsforums.com/showthread.php?t=104715

quote:
<< Do Photons Move Slower in a Solid Medium?

Contributed by ZapperZ. Edited and corrected by Gokul43201 and inha

This question appears often because it has been shown that in a normal, dispersive solid such as glass, the speed of light is slower than it is in vacuum. This FAQ will strictly deal with that scenario only and will not address light transport in anomolous medium, atomic vapor, metals, etc., and will only consider light within the visible range.

The process of describing light transport via the quantum mechanical description isn't trivial. The use of photons to explain such process involves the understanding of not just the properties of photons, but also the quantum mechanical properties of the material itself (something one learns in Solid State Physics). So this explanation will attempt to only provide a very general and rough idea of the process.

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light.

Secondly, if that assertion is true, then the index of refraction would ONLY depend on the type of atom in the material, and nothing else, since the atom is responsible for the absorption of the photon. Again, if this is true, then we see a problem when we apply this to carbon, let's say. The index of refraction of graphite and diamond are different from each other. Yet, both are made up of carbon atoms. In fact, if we look at graphite alone, the index of refraction is different along different crystal directions. Obviously, materials with identical atoms can have different index of refraction. So it points to the evidence that it may have nothing to do with an "atomic transition".

When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.

A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.>>

So, it's not simply absorption/re-emission. Photon (and of course their properties, included speed) comes from the behaviour of the entire electromagnetic field in the entire system of atoms.

--
lightarrow
« Last Edit: 04/11/2011 15:45:57 by lightarrow »
 

Offline yor_on

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How fast do photons travel?
« Reply #7 on: 04/11/2011 21:05:46 »
Nice explanation LightArrow.

"On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available."

How does that work?

"This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. "

Thinking of it as springs oscillating?
How do it re-emit the photon?
 

Offline simplified

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How fast do photons travel?
« Reply #8 on: 05/11/2011 05:10:35 »

So, it's not simply absorption/re-emission. Photon (and of course their properties, included speed) comes from the behaviour of the entire electromagnetic field in the entire system of atoms.
--
lightarrow
What is cause of reduction of wave length of photon in transparent environment?
« Last Edit: 05/11/2011 08:02:37 by simplified »
 

Offline simplified

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How fast do photons travel?
« Reply #9 on: 05/11/2011 05:34:39 »
Margus asked the Naked Scientists:
   
How photons travel with speed slower then speed of light in matter, when particles with zero rest mass should always be traveling with speed of light.

What do you think?


Because it interacts with matter by being scattered, absorbed, scattered, re-absorbed etc. and so gets slowed down.
Then why does photon have  unchanging direction in the transparent environment?

Because scattering doesn't have to be in a different direction than the incoming photon.  In transparent environments, it can still be slowed down, but can scatter in the same direction as the incoming photon.
What is cause of radiation in the same direction?
 

Offline cowlinator

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Re: How fast do photons travel?
« Reply #10 on: 27/09/2014 05:38:44 »

So, it's not simply absorption/re-emission. Photon (and of course their properties, included speed) comes from the behaviour of the entire electromagnetic field in the entire system of atoms.
--
lightarrow
That's a great explanation, thank you LightArrow.

I have to wonder, is ionization caused by high frequency photons a result of interaction with atoms or interaction with phonons?
 

Offline UltimateTheory

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Re: How fast do photons travel?
« Reply #11 on: 27/09/2014 09:18:23 »
I have to wonder, is ionization caused by high frequency photons a result of interaction with atoms or interaction with phonons?

See oil drop experiment, if you're not familiar with
http://en.wikipedia.org/wiki/Oil_drop_experiment
X-rays are created by high energy electrons in vacuum tube, and then emitted to container where we are dropping oil. X-ray photons thus eject electrons from that drop.
Ionized oil drop can therefor levitate in vertical electric field caused by plates and high voltage.

Does it explain to you how ionization can be created by high energy photons?
 

Offline PmbPhy

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Re: How fast do photons travel?
« Reply #12 on: 27/09/2014 10:53:56 »
Quote from: lightarrow
Here it talks of light in a solid medium, but with a liquid, apart from preferred directions in the lattice and phonons, there should be no big difference:

http://www.physicsforums.com/showthread.php?t=104715
Quote from: Contributed by ZapperZ. Edited and corrected by Gokul43201 and inha
<snip>
When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.
<snip>
Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.

So, it's not simply absorption/re-emission. Photon (and of course their properties, included speed) comes from the behaviour of the entire electromagnetic field in the entire system of atoms.

--
lightarrow
You, and I'm afraid to admit, ZapperZ, have hit the nail right on the head. In years past I also used the absorption/re-emission description. It gives a result which is qualitatively correct. However I hated that description because a solid is not a collection of atoms in which photons are emitted by one atom and absorbed by another. At least not to my knowledge. However I never studied quantum mechanics of the solid state so I kept my mouth shut. It wasn't too long after that I stopped using it because sooner or later I knew I was going to find out it was wrong.

My deepest respect for LightArrow and ZapperZ for this insightful commentary. Thank you very much for confirming all of it for me. :)
« Last Edit: 27/09/2014 11:00:50 by PmbPhy »
 

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Re: How fast do photons travel?
« Reply #12 on: 27/09/2014 10:53:56 »

 

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