# The Naked Scientists Forum

### Author Topic: How much of the photon's energy is kinetic?  (Read 3929 times)

#### jeffreyH

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##### How much of the photon's energy is kinetic?
« on: 07/12/2014 03:44:07 »
This may seem like a strange question but the answer is at the heart of the relationship between different types of energy. What is the answer?

#### PmbPhy

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##### Re: How much of the photon's energy is kinetic?
« Reply #1 on: 07/12/2014 07:42:54 »
Quote from: jeffreyH
This may seem like a strange question but the answer is at the heart of the relationship between different types of energy. What is the answer?
Your question isn't strange at all. In fact it's a very good question. Since the proper mass of a photon is zero, i.e. m0, the proper energy E0 = m0c2 is also zero, i.e. E0 = 0. Since energy is the sum of kinetic energy plus proper energy, i.e.

E = K + E0

It follows that since E0 = 0 that

E = K + E0 = K

i.e.

E = K

Therefore the energy of a photon is 100% kinetic energy.

#### lightarrow

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##### Re: How much of the photon's energy is kinetic?
« Reply #2 on: 07/12/2014 12:01:47 »
Quote from: jeffreyH
This may seem like a strange question but the answer is at the heart of the relationship between different types of energy. What is the answer?
Your question isn't strange at all. In fact it's a very good question. Since the proper mass of a photon is zero, i.e. m0, the proper energy E0 = m0c2 is also zero, i.e. E0 = 0. Since energy is the sum of kinetic energy plus proper energy, i.e.

E = K + E0

It follows that since E0 = 0 that

E = K + E0 = K

i.e.

E = K

Therefore the energy of a photon is 100% kinetic energy.

The result is obviously correct but I don't understand your derivation, because I don't know what you mean with "proper mass of a photon".

Here is how I would have answered the OP question:

For a photon, its invariant mass m is zero, so from the equation:

E = m*c2 + Ek

we deduce E = Ek.

--
lightarrow
« Last Edit: 07/12/2014 12:08:28 by lightarrow »

#### PmbPhy

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##### Re: How much of the photon's energy is kinetic?
« Reply #3 on: 07/12/2014 19:12:06 »
Quote from: lightarrow
The result is obviously correct but I don't understand your derivation, because I don't know what you mean with "proper mass of a photon".
It's merely a different name for invariant mass.

#### jeffreyH

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##### Re: How much of the photon's energy is kinetic?
« Reply #4 on: 07/12/2014 21:26:19 »
That is exactly what I expected the answer to be but by strange I mean the answer appears obvious but I am not convinced. The mathematics say so but is it right?

#### PmbPhy

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##### Re: How much of the photon's energy is kinetic?
« Reply #5 on: 07/12/2014 22:04:31 »
That is exactly what I expected the answer to be but by strange I mean the answer appears obvious but I am not convinced. The mathematics say so but is it right?
Yes. It's right. Don't forget that that  math is merely the language that we use to describe nature and mathematical technique's are synonymous to our use of logic and critical thinking. Therefore if the math says its right then it's right as far as our critical thinking goes.

#### jeffreyH

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##### Re: How much of the photon's energy is kinetic?
« Reply #6 on: 07/12/2014 22:28:25 »
That is exactly what I expected the answer to be but by strange I mean the answer appears obvious but I am not convinced. The mathematics say so but is it right?
Yes. It's right. Don't forget that that  math is merely the language that we use to describe nature and mathematical technique's are synonymous to our use of logic and critical thinking. Therefore if the math says its right then it's right as far as our critical thinking goes.

Well then that gives me a direction to pursue. BTW apologies for the invariant mass remarks it was the only way I could think of to prompt a confirmation of something. I will elaborate later when I have completed some calculations. I read your page on relativistic mass and it was very helpful.

#### PmbPhy

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##### Re: How much of the photon's energy is kinetic?
« Reply #7 on: 07/12/2014 22:45:28 »
Quote from: jeffreyH
Thanks, Jeff. That's very helpful. It let's me know that I'm doing something useful and that's my goal for having to be a disabled gent. :)

#### yor_on

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##### Re: How much of the photon's energy is kinetic?
« Reply #8 on: 13/12/2014 20:56:56 »
It's a pretty good question Jeffrey, and it will make you think some more, and me too :)

#### JohnDuffield

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##### Re: How much of the photon's energy is kinetic?
« Reply #9 on: 14/12/2014 12:55:13 »
That is exactly what I expected the answer to be but by strange I mean the answer appears obvious but I am not convinced. The mathematics say so but is it right?
It is. You can work it out by taking a look at Compton scattering:

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/compton.html#c1

Some of the photon E=hf wave energy is converted into the kinetic energy of the electron. In theory you could do another Compton scatter on the residual photon, and another, and another ad infinitum. In the limit all of the wave energy is converted into electron kinetic energy, and there's no wave left.

#### The Naked Scientists Forum

##### Re: How much of the photon's energy is kinetic?
« Reply #9 on: 14/12/2014 12:55:13 »