# The Naked Scientists Forum

### Author Topic: What is the mass of a photon?  (Read 20054 times)

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #25 on: 14/11/2006 17:30:01 »
e=mc^2 .06471*9*10^16 = 5.82*10^15 J

#### thebrain13

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##### Re: What is the mass of a photon?
« Reply #26 on: 14/11/2006 18:07:39 »
Heliotrope brought up the stopped photons argument. I only implied that if you could/did stop a photon, and the photon still didnt use up its mass/energy on something else (it couldnt, otherwise it wouldnt still be a photon) then its mass/energy should still remain.

And I agree with soul surfer, this topic is getting boring. Although it has a point to me.

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #27 on: 14/11/2006 19:30:54 »
e=mc^2 .06471*9*10^16 = 5.82*10^15 J
Correct. Thank you syhprum. I have changed that value from 5.82*10^11 to 5.82*10^15
« Last Edit: 14/11/2006 19:32:47 by lightarrow »

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #28 on: 14/11/2006 19:47:15 »
It is interesting to contmplate whether such an energetic photon could exist I understand that some cosmic rays run up to 10^20 ev but I dont know how that translates in joules, does one run into a Planck limit or somthing like that?

#### Heliotrope

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##### Re: What is the mass of a photon?
« Reply #29 on: 14/11/2006 19:49:14 »
No, sorry, it's not irrelevant at all! Think to this: you (A) send a light pulse to your friend (B) 10 metres apart, then, after 1 hour, your friend sends another light pulse to another friend (C) 10 metres apart from him. Does it mean that light has traveled 20 metres in 1 hour?

Of course not.
That's two separate light pulses.
I don't understand what you're driving at here.

Quote
In that experiment, Lene Hau slowed light's speed to 38 miles/hour inside a sodium atoms cloud. Do you think that, in a medium made of those sodium atoms, relativistic effects should happen at less than 38 miles/hour? Not at all!

What ?
Relativistic effects ?
You brought that up. Not me.
Of course relativistic effects don't happen at 38 mph.

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #30 on: 15/11/2006 07:58:31 »
It is interesting to contmplate whether such an energetic photon could exist I understand that some cosmic rays run up to 10^20 ev but I dont know how that translates in joules, does one run into a Planck limit or somthing like that?
1 eV = 1.6*10-19Joule.

1020 eV = 1020*1.6*10-19 = 1.6*101 = 16 Joule.

5.82*1015 Joule = 5.82*1015/1.6*10-19 = 3.64*1034 eV

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #31 on: 15/11/2006 08:04:52 »
No, sorry, it's not irrelevant at all! Think to this: you (A) send a light pulse to your friend (B) 10 metres apart, then, after 1 hour, your friend sends another light pulse to another friend (C) 10 metres apart from him. Does it mean that light has traveled 20 metres in 1 hour?

Of course not.
That's two separate light pulses.
I don't understand what you're driving at here.

Because it's the same as light propagating in a medium! As I explained, when light propagates in a medium, it's actually emitted from an atom, then it propagates in the void for a small distance between two atoms, then absorbed from the second atom and, sometimes later... re-emitted and so on. Where is the difference?

Quote
What ?
Relativistic effects ?
You brought that up. Not me.
Of course relativistic effects don't happen at 38 mph

So, you understand better why, in that experiment, light wasn't actually slowed down. Because, if Really light's speed was slowed down to 38 miles/hour, then relativistic effects should happen at less than that speed, in such a medium.
« Last Edit: 15/11/2006 08:09:12 by lightarrow »

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #32 on: 15/11/2006 08:15:24 »
Heliotrope brought up the stopped photons argument. I only implied that if you could/did stop a photon, and the photon still didnt use up its mass/energy on something else (it couldnt, otherwise it wouldnt still be a photon) then its mass/energy should still remain.

And I agree with soul surfer, this topic is getting boring. Although it has a point to me.

Yes. But the problem is that what you call "Mass" it's not what him called "Mass". For this reason you couldn't agree with him!
You intended "Relativistic" = "Total" Mass, while he intended "Rest" Mass and, as I showed in that post, they are COMPLETELY different, in concept AND in value.

Indeed, not even among experts there is a good agreement on what to call "Mass"; so, waiting for their agrerement, it's better to always specify what we are referring to.

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #33 on: 15/11/2006 19:57:28 »
I know there is a de-faco limit to the energy of cosmic rays due to their interaction with the CMBR but is there any absolute limit? does the planck length (10^-43m)? limit come into play as the minimum wavelength

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #34 on: 16/11/2006 08:06:20 »
I know there is a de-faco limit to the energy of cosmic rays due to their interaction with the CMBR but is there any absolute limit? does the planck length (10^-43m)? limit come into play as the minimum wavelength
Planck lengt is ≈ 1.6*10-35 m:

http://en.wikipedia.org/wiki/Planck_length

Photon's energy: E = p*c where p is the momentum

p = h/λ (valid for every particle)

→ E = h*c/λ ≈ 6.6*10-34*3*108/1.6*10-35 = 1.2*1010 Joule = 1.2*1010/1.6*10-19 = 7.5*1028 eV.

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #35 on: 16/11/2006 16:09:48 »
I stand corrected I was confusing the Planck length with the Planck time approx 10^-43 sec which is of course the time it takes a photon to travel the Planck unit of distance.
I am still not clear what is the highest energy photon that can exist (if there is indeed a limit), can you enlighten me?

The URL to which you have directed me makes the matter fairly clear but I am puzzled that they can suggest that the new collider at CERN may be able to generate mini black holes when such large energies are involved.
« Last Edit: 16/11/2006 16:17:22 by syhprum »

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #36 on: 16/11/2006 17:01:34 »
For thirty years I resisted getting involved with computers as I found them rather boring and not relevent to what I wanted to do but by 1977 they had caught up with me and I was forced to get involved.
The same applies to quantum physics I always used to skip these strange looking equations but now it appears I have to take notice if I am to understand the modern world
« Last Edit: 16/11/2006 17:04:09 by syhprum »

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #37 on: 17/11/2006 08:34:27 »

I am still not clear what is the highest energy photon that can exist (if there is indeed a limit), can you enlighten me?
Why do you think I have coloured with blue those values in my previous post?
Quote
...but I am puzzled that they can suggest that the new collider at CERN may be able to generate mini black holes when such large energies are involved...
7.5*1028 eV = 7.5*1016 TeV!
I don't think we will be able to reach such energies in this millennium!
Quote
The same applies to quantum physics I always used to skip these strange looking equations but now it appears I have to take notice if I am to understand the modern world
I can try to give some answers to your questions, if you don't ask me what is a photon  for example! The most strange thing of QM (quantum mechanics) is, to my point of view, the fact that sometimes it refers to something we don't exactly know what it is!

If some reader is shocked from this statement, please say what is a photon.
« Last Edit: 17/11/2006 08:40:47 by lightarrow »

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #38 on: 17/11/2006 14:20:55 »
I have already apologised I was rather slow in reading the 'Wiki' article and gaining the full significance of your formula, there has certainly been talk of the production of mini black holes at CERN but of course 10^16 Tev is out of the question, I will try and find chapter and verse and see what was meant.
PS it is rather naive I know but I visualise Photons as little packets of energy zipping along, short ones from Gamma rays and long ones from Rugby (60Khz)

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #39 on: 17/11/2006 14:28:32 »
http://unisci.com/stories/20014/1001012.htm

Here is one of the sources from which I got the black hole story

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #40 on: 17/11/2006 15:31:01 »
PS it is rather naive I know but I visualise Photons as little packets of energy zipping along, short ones from Gamma rays and long ones from Rugby (60Khz)
I wish it was!
You can think of a tennis ball as something zipping along, because you can snap photos of it in various positions.

Now, let's do the same with light. How could we know where this assumed particle is? We can use various instruments, but (one of) the point is that, in the precise moment you detect this "particle", it doesn't exist anylonger!

So, the question is: where is this "particle" between the source and the detector, if, to say it, you must place another detector in the middle of the path?

What quantum mechanics can do is to establish mathematical rules to say which detectors will "click", and nothing more!

If you think of a photon as a little packet of electromagnetic waves, then, you should explain why a single photon can spread along a vast area in the experiment of light diffraction;

If you think of a photon as a normal wave, which can expand and spread out of a vast area, you should explain why it is only detected in one point and not along that vast area...
« Last Edit: 17/11/2006 15:44:04 by lightarrow »

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #41 on: 17/11/2006 17:44:18 »
I repeat I know the idea to be naive but if you are to think about something you must have some conception of it in your mind.
Let me give two other examples, on a recent trip to Budapest while driving around I found navigation difficult because I could not make any mental pronunciation of the place names due to what was to me a very unfamiliar language.
Another thing is when I have to conceptualise the square root of negative numbers,I think of them representing quantity's at right angle's to the normal world perhaps not academically correct but it enables me to visualise them.

PS what do you think about the black holes at LHC CERN?
« Last Edit: 17/11/2006 20:53:07 by syhprum »

#### Heliotrope

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##### Re: What is the mass of a photon?
« Reply #42 on: 17/11/2006 20:57:18 »
Quote from: lightarrow
Because it's the same as light propagating in a medium! As I explained, when light propagates in a medium, it's actually emitted from an atom, then it propagates in the void for a small distance between two atoms, then absorbed from the second atom and, sometimes later... re-emitted and so on. Where is the difference?

Then clearly I am missing something.
I understand that in their journey to their destination some photons are repeatedly absorbed and reemitted by atoms in their path.
This absobtion and emission changes the direction of the photon's path and is one of the causes of scattering. As I understand it.

I however do not understand how a photon in this experiment that travels directly to my eye/detector etc... is not slowed down.

Unless, of course, that in the details of the experiments I linked to they have not stated that the photons are not travelling directly from original emission to the detector without being absorbed enroute.

If they are being absorbed and reemitted then I agree that the light has not been stopped.
It's properties have merely been "imprinted" upon an atom which is then encouraged to emit an identical photon at some later time.
In this circumstance any claim that "light has been stopped completely" is frankly fraudulent.

#### Heliotrope

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##### Re: What is the mass of a photon?
« Reply #43 on: 17/11/2006 21:08:03 »
So, the question is: where is this "particle" between the source and the detector, if, to say it, you must place another detector in the middle of the path?

What quantum mechanics can do is to establish mathematical rules to say which detectors will "click", and nothing more!

If you think of a photon as a little packet of electromagnetic waves, then, you should explain why a single photon can spread along a vast area in the experiment of light diffraction;

If you think of a photon as a normal wave, which can expand and spread out of a vast area, you should explain why it is only detected in one point and not along that vast area...

You've just reminded me of a question I have wanted to ask for many, many years :

As the photon travels down the optical track it's probability wave is spread out along it's path.
Now, as I understand it, a probability wave is a three dimensional object. Not a one dimensional object along the path.
So why don't the experimenters place photon detectors in arrays around the path of the photon instead of just in the path ?
Would this not able the experimenter to map out the actual density of the probability wave after the appropriate number of runs ?
If you did a snapshot activation of all the detectors when the photon is at the mid point of the array then eventually you'd have a complete map.
The detectors closest to the path would obviously detect more photons and thos further away less.
It might shed some light on the actual shape of the probability envelope.

Just a thought...

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #44 on: 17/11/2006 21:37:49 »
We must think of photons as both particles and waves, use double think (see 1984) in this world we have to hold many contradictory ideas at the same time

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #45 on: 18/11/2006 08:48:10 »
Quote from: lightarrow
Because it's the same as light propagating in a medium! As I explained, when light propagates in a medium, it's actually emitted from an atom, then it propagates in the void for a small distance between two atoms, then absorbed from the second atom and, sometimes later... re-emitted and so on. Where is the difference?
Then clearly I am missing something.
I understand that in their journey to their destination some photons are repeatedly absorbed and reemitted by atoms in their path.
This absobtion and emission changes the direction of the photon's path and is one of the causes of scattering. As I understand it.
I however do not understand how a photon in this experiment that travels directly to my eye/detector etc... is not slowed down.
Unless, of course, that in the details of the experiments I linked to they have not stated that the photons are not travelling directly from original emission to the detector without being absorbed enroute.
If they are being absorbed and reemitted then I agree that the light has not been stopped.
It's properties have merely been "imprinted" upon an atom which is then encouraged to emit an identical photon at some later time.
In this circumstance any claim that "light has been stopped completely" is frankly fraudulent.
Just the fact in that experiment light doesn't travel in the void, but inside matter (a cloud of super-cooled sodium atoms) means that...it's not light's speed in the void, but inside matter! Then, if it's because scattering or absorption and re-emission (as in this case) or something else, it's not very different; the fact is that it's just not light's speed in the void.
About being fraudolent, maybe it's a too strong statement, because that was the first time light's speed inside a medium reached so little values, however, I agree with you that it's a misleading claim.

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #46 on: 18/11/2006 09:00:40 »
I repeat I know the idea to be naive but if you are to think about something you must have some conception of it in your mind.
Let me give two other examples, on a recent trip to Budapest while driving around I found navigation difficult because I could not make any mental pronunciation of the place names due to what was to me a very unfamiliar language.
Another thing is when I have to conceptualise the square root of negative numbers,I think of them representing quantity's at right angle's to the normal world perhaps not academically correct but it enables me to visualise them.

PS what do you think about the black holes at LHC CERN?

About black holes (it's the easiest question ) if I have understood your question correctly, you didn't want to know a photon's energy at the quantum limit described in Wikipedia: "Planck Lenght", but just if the energies reached at LHC CERN could be enough to generate black holes (which is another story)? Probably yes, but I think Soul Surfer knows more than me on this subject. However, as you know, theory says that the less mass has a black hole, the quicker it radiates away its energy and so, its mass, so micro black holes are expected to live for only a tiny fraction of second.
I sincerely hope the theory is right. Brrr!!!

From "The Quantum Theory of Light" by  Rodney Loudon:

Quote
The use of the word "photon" to describe the quantum of electromagnetic radiation can lead to confusion and misunderstanding. It is often used in the context of interference experiments, for example Young's slits, in such phrases as "which slit does the photon goes through?" and "where do the photons hit the screen when one of the slits is covered up". The impression is given of a fuzzy globule of light that travels this way or that way through pieces of optical equpment or that light beams consist of streams of the globules, like bullets from a machine gun. Lamb has even argued that there is no such thing as a photon [1] and he has proposed that the word should be used only under license by properly qualified people!
« Last Edit: 18/11/2006 09:15:29 by lightarrow »

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #47 on: 18/11/2006 12:35:18 »
I was curious as to the greatest energy that a photon could have as there was discussion as to the energy imparted to a baseball when it was accelerated to .9 c which as you can imagine was rather large!.
I did not realises that the calculation of the limiting value of photon energy was so easy, I knew all about Planck scales and guess I should really have been able to work it out my self.
I believe the LHC black holes are expected to live about 10^-24 sec but I think until they acutely make some there is a lot of guess work involved

#### lightarrow

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##### Re: What is the mass of a photon?
« Reply #48 on: 18/11/2006 14:02:35 »
So, the question is: where is this "particle" between the source and the detector, if, to say it, you must place another detector in the middle of the path?

What quantum mechanics can do is to establish mathematical rules to say which detectors will "click", and nothing more!

If you think of a photon as a little packet of electromagnetic waves, then, you should explain why a single photon can spread along a vast area in the experiment of light diffraction;

If you think of a photon as a normal wave, which can expand and spread out of a vast area, you should explain why it is only detected in one point and not along that vast area...

You've just reminded me of a question I have wanted to ask for many, many years :

As the photon travels down the optical track it's probability wave is spread out along it's path.
Now, as I understand it, a probability wave is a three dimensional object. Not a one dimensional object along the path.
So why don't the experimenters place photon detectors in arrays around the path of the photon instead of just in the path ?
Would this not able the experimenter to map out the actual density of the probability wave after the appropriate number of runs ?
If you did a snapshot activation of all the detectors when the photon is at the mid point of the array then eventually you'd have a complete map.
The detectors closest to the path would obviously detect more photons and thos further away less.
It might shed some light on the actual shape of the probability envelope.

Just a thought...

The fact we analyze the system in two dimensions only (the plane of the paper sheet in which we draw it) is because we are assuming simmetry in the other coordinate (z); that is, an electromagnetic wave is always tridimensional, but, if it has, at least, cylindrical symmetry, every slice of it made orthogonally to Z (that is, parallel to X and Y) look exactly the same, so even the physical results on the detectors are the same: once you know what happens to those detectors in one of those slices, you automatically knows what happens for all of them in the 3-D space.

The behaviour of the wave, however, can still be different for different values of Y, but, if the wave is also plane, then the symmetry is total, and you can only analyze what happens along the coordinate X.

To generate a plane EM wave: you start from a very tiny source of light, and you place it far away from a screen which has a tiny hole in it. After going out of the hole, the wave is approximately planar; better if you use a lens after the source, to make the beam as parallel as possible.
Or, you can use a good quality laser beam.

However, you have to consider that the quantum mechanical description, which gives the probability density, makes use of wavefunctions which don't "live" at all in the ordinary 3 dimensional space, but in an abstract space called "phase space", so the 3 D representation of the wavefunction in the ordinary space is totally wrong.

Quote
Would this not able the experimenter to map out the actual density of the probability wave after the appropriate number of runs ?
No, because a detector's presence modifies completely the wavefunction. Example: Young's two slit experiment.
If the slits are free from obstacles, you have an interference pattern on the screen; if you put a detector after one of the slits, the interference disappears!

« Last Edit: 18/11/2006 14:15:02 by lightarrow »

#### syhprum

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##### Re: What is the mass of a photon?
« Reply #49 on: 19/11/2006 15:55:20 »
If I enter the temperature of the limiting energy photon (8.7*10^32 K) or the mass equivelent (1.33*10^-7 Kg)or the possible life time 1.616*10^-35 Sec into the calculater
http://xaonon.dyndns.org/hawking/
nothing seems to fit , Opinions please QM experts
« Last Edit: 19/11/2006 16:31:39 by syhprum »

#### The Naked Scientists Forum

##### Re: What is the mass of a photon?
« Reply #49 on: 19/11/2006 15:55:20 »