What is the mass of a photon?

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Offline thebrain13

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What is the mass of a photon?
« on: 27/10/2006 00:54:31 »
If a photon is not moving, then it is not a photon, it is nothing. But if it is moving then it has energy, hence it has mass. so why does everyone say a photon has no rest mass, considering there is no such thing as a rest photon?
« Last Edit: 14/06/2008 22:08:49 by chris »

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Offline science_guy

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Re: What is the mass of a photon?
« Reply #1 on: 27/10/2006 01:10:18 »
the only reason that a photon's mass has not increased proportionally to infinite, is because It begins with no mass. If a photon had the slightest amount of mass, than photons would be as big as planets when moving with the velocity of light (I think, it might just be hyperbole).  does that explain it for you?
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Offline thebrain13

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Re: What is the mass of a photon?
« Reply #2 on: 27/10/2006 01:25:03 »
Saying a photon has no mass when it begins is like saying, when this pencil came into existance, it had no wood, lead, steel, or rubber. A pencil is only a pencil if it has those things. A photon is only a photon if it is moving and has mass and energy. To me saying a photon has no mass in the beginning is like saying when this solid gold necklace was made, it had no gold.

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Offline science_guy

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Re: What is the mass of a photon?
« Reply #3 on: 27/10/2006 01:28:28 »
the Only reason that a Photon CAN move at the speed of light, is BECAUSE it has no mass.  Because it is a packet of pure energy, it is forced to move at lightspeed, because it has no inertia to stop it.  What do you think happens to the mass that daily hits your pupils, and fills all of known space?  If mass were created every fraction of a second on the sun, or any other star, It would violate the laws of conservation of mass and energy.
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Offline thebrain13

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Re: What is the mass of a photon?
« Reply #4 on: 27/10/2006 02:10:04 »
when the sun emits light it loses mass. e=mc2 the nuclear reactions reduces the mass of the element hydrogen, contained in the sun, and emit light as well. So it does not violate the laws of conservation of mass and energy. And consequently, every time an atom emits light, it loses mass. Thats where a photons mass and energy comes from.
« Last Edit: 27/10/2006 05:28:01 by thebrain13 »

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Offline science_guy

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Re: What is the mass of a photon?
« Reply #5 on: 27/10/2006 06:51:55 »
how does an atom lose mass when they emit photons?  An atom is made of protons, neutrons, and electrons.  If it were to lose any of these, it would either become a different atom, isotope, or ion, and I don't think that is recorded to happen in these nuclear reactions.  Rather, in a nuclear reactions, I thought atoms gain neutrons.

The nuclear reactions in a star are specifically nuclear fusion.  When the nuclei of two atoms touch, they fuse together, releasing large amounts of energy.  The main atoms that fuse in this kind of reaction are hydrogen atoms.  a normal hydrogen atom has no neutrons, so in order to make the resulting helium atom stable, it must have some neutrons.  That could explain the loss of mass in a star, because the neutrons are being used as placeholders in the nuclei of the newly formed atoms.
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Offline lightarrow

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Re: What is the mass of a photon?
« Reply #6 on: 27/10/2006 08:22:44 »
1. The Sun DOES indeed loose mass radiating energy: nuclear reactions convert mass into energy, which excitates atoms, which then release excitation energy as light.

2. The fact a photon has relativistic mass doesn't imply it has a rest mass. The relativistic mass is given by the formula:

mr = E/c2.
Only if the rest mass m0 is ≠ 0, we can write: mr = m0/√[1-(v/c)2].

3. A photon COULD indeed have a very slight rest mass, but in that case light's speed would depend on the reference frame. Furthermore, Maxwell's equations should be modified.

At "normal" speeds, however (that is, not relativistic) the difference would be negligible.
« Last Edit: 27/10/2006 08:24:50 by lightarrow »

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Offline syhprum

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Re: What is the mass of a photon?
« Reply #7 on: 27/10/2006 08:40:14 »
When an atom emits or adsorbs a photon of relatively low energy it is the result of electrons moving from higher or lower orbits, during radio active decay which is due to realignments of the components of the nucleus either high energy Photons (gamma rays) or actual particles are emitted and chemical properties are changed
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Offline thebrain13

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Re: What is the mass of a photon?
« Reply #8 on: 27/10/2006 21:25:01 »
My point is merely there is no such thing as a non moving photon. So the statement a photon has no rest mass makes no sense.

Especially when you consider that every atom that emits a photon loses mass.

And as I pointed out in my post, the constant velocity of baseballs, anything with a very high velocity acts like light, that is untill you nearly reach its velocity.

I bet the only reason anyone says that is so they can explain lights supposed "unique behavior" which isnt so unique when you think about it.


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Offline lightarrow

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Re: What is the mass of a photon?
« Reply #9 on: 27/10/2006 23:09:54 »
My point is merely there is no such thing as a non moving photon.
And as I pointed out in my post, the constant velocity of baseballs, anything with a very high velocity acts like light, that is untill you nearly reach its velocity.


These two statements are contradicting each other, since baseballs ecc. can be not moving
.



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So the statement a photon has no rest mass makes no sense.
Especially when you consider that every atom that emits a photon loses mass
.

The fact an atom loses (relativistic = total) mass when emits energy is because energy, and not only rest mass, have (relativistic = total) mass
.

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Offline thebrain13

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Re: What is the mass of a photon?
« Reply #10 on: 28/10/2006 00:51:27 »
Disregaurd my argument a made in a different post, on the comparison between the motion of light and baseballs, since I can not adequately explain it here. Anyways using energy and using mass are the same thing. The statement the atom uses mass to create the mass of a photon is the same as, an atom uses energy to create the mass of a photon.

Still my point remains, there is no such thing as a non moving photon. So how can anyone conclude it has no mass before it was created?

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Re: What is the mass of a photon?
« Reply #11 on: 28/10/2006 01:21:05 »
Still my point remains, there is no such thing as a non moving photon. So how can anyone conclude it has no mass before it was created?

The issue is not only wether one can show it has any mass if it were ever at rest, but whether there is any mass/energy left unaccounted for by the motion of the photon.  If 100% of the energy/mass of the photon is accounted for by relativistic kinetic energy, then there is nothing left over for the rest mass.

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Offline Heliotrope

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Re: What is the mass of a photon?
« Reply #12 on: 12/11/2006 17:54:00 »
Still my point remains, there is no such thing as a non moving photon.

Incorrect.
http://en.wikipedia.org/wiki/Slow_light
Light has been completely stopped. Allbeit for only a fraction of a second.

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So how can anyone conclude it has no mass before it was created?

Photons have no mass.
This is experimental fact.

Nothing has any mass before it is created because it doesn't exist before it comes into existence. By definition.
Things can come into existence with mass and energy. Virtual, or otherwise, particles etc...
Other things can come into existence with just energy and no rest mass.
Photons for example.

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Offline lightarrow

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Re: What is the mass of a photon?
« Reply #13 on: 13/11/2006 13:24:46 »
http://en.wikipedia.org/wiki/Slow_light
Light has been completely stopped. Allbeit for only a fraction of a second.

Not exactly. That "slowing" refers to light propagation in a material medium, not to light in the void.
When light propagates inside a material medium, it's actually absorbed, then, after a certain time, re-emitted and so on. It's This total propagation that can have different speeds than c, relating to the kind of material.

For example, inside a linear glass, light's speed is c/n where n is the refractive index. For n = 1.5, v = c/1.5

Light's speed in the void, that is, between every emission and reabsorption, is always c.
« Last Edit: 13/11/2006 13:30:36 by lightarrow »

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Offline Heliotrope

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Re: What is the mass of a photon?
« Reply #14 on: 13/11/2006 21:07:49 »
Quote from: Wikipedia
Lene Vestergaard Hau led a team from Harvard University who succeeded in slowing a beam of light to about 17 metres per second in 1999, and, in 2001, was able to momentarily stop a beam.

The medium is irrelevant.
Stopped is stopped.

As long as the optical bench wasn't moving of course.
[;D]
« Last Edit: 13/11/2006 21:09:57 by Heliotrope »
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Offline thebrain13

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Re: What is the mass of a photon?
« Reply #15 on: 13/11/2006 21:35:05 »
A stopped photon has mass. I havent read that anywhere, but I can safely assume that statement is true, otherwise conservation of mass and energy is violated. And when you think about it, that experiment only strengthens my case.

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Offline syhprum

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Re: What is the mass of a photon?
« Reply #16 on: 13/11/2006 22:34:59 »
In a gas discharge laser the photons bounce back an forth between mirrors and finally burst through, in a way they could be said to have stopped momentarily while they are still in the discharge tube (I used to have the job of getting the mirrors perfectly clean, no easy job )
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Offline Heliotrope

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Re: What is the mass of a photon?
« Reply #17 on: 13/11/2006 23:08:44 »
A stopped photon has mass.

Lots of experiments demonstrate that it doesn't.

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I havent read that anywhere,

I see...

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but I can safely assume that statement is true,

You know what they say about assumption being the mother of all screwups.
Best to do some experiments instead. Gather some data. Get some facts.
That's raaaaather more reliable than assumptions based on no data whatsoever.

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otherwise conservation of mass and energy is violated.

No it isn't.
The speed of light does not change. It's just that that photon happens to be stopped dead.
They don't change the motorway speed limit just because you've got your car in the garage now do they ?

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And when you think about it, that experiment only strengthens my case.
Don't have to do much thinking to realise that performing experiments is going to show more of reality and the facts than you, me or anyone else on this forum just thinking about it.

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Offline thebrain13

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Re: What is the mass of a photon?
« Reply #18 on: 14/11/2006 02:01:22 »
A moving photon has mass.
Explain to me how this is not a violation of conservation of mass.
Atom emits photon.
Photon has mass, that mass equals the mass lost in the atom. (no violation)
Photon stops, mass disappears.
Atom has lost mass, no object with the equivalent mass remains. (obviously not counting photons emitted from other atoms)
How is that not in violation?

The answer because mass got converted to energy is not valid. Energy has mass.

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Offline Soul Surfer

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Re: What is the mass of a photon?
« Reply #19 on: 14/11/2006 12:18:12 »
You're getting a bit boring brain with your circular and pointless arguments

A photon has energy and momentum.  There are strong reasons for believing that the fundamental property of matter and energy is momentum and not mass which is a sort of second order effect.

When a piece of matter emits a photon it will lose some mass (or momentum) and when another piece of matter absorbs it it will gain this mass by the conservation of energy.  It is energy and momentum that are conserved and not mass.  OK this may evenually be re-emitted in the form of many lower energy photons in the form of heat.

This means that the momenum of a photon could be considered as an equivalent of some mass but that is not the same as having rest mass in which the item is truly stationary (insofar as anything in this dynamic universe can ever be considered to be stationary)
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Offline lightarrow

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Re: What is the mass of a photon?
« Reply #20 on: 14/11/2006 12:53:12 »
Quote from: Wikipedia
Lene Vestergaard Hau led a team from Harvard University who succeeded in slowing a beam of light to about 17 metres per second in 1999, and, in 2001, was able to momentarily stop a beam.

The medium is irrelevant.
Stopped is stopped.


As long as the optical bench wasn't moving of course.
[;D]

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? But this is indeed the definition of light's speed in a medium! So, in a medium, light can travel 20 metres in 1 hour, but this doesn't mean that (true = in the void) light's speed is less than c.

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!
« Last Edit: 14/11/2006 13:11:44 by lightarrow »

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Offline lightarrow

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Re: What is the mass of a photon?
« Reply #21 on: 14/11/2006 12:56:41 »
In a gas discharge laser the photons bounce back an forth between mirrors and finally burst through, in a way they could be said to have stopped momentarily while they are still in the discharge tube (I used to have the job of getting the mirrors perfectly clean, no easy job )

At last we know what was your job! Very interesting, syhprum. One day I will ask you a lot of questions about it!

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Offline lightarrow

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Re: What is the mass of a photon?
« Reply #22 on: 14/11/2006 13:18:40 »
A stopped photon has mass.

Lots of experiments demonstrate that it doesn't.


Which ones? I've never heard of "stopped" photons; unless you mean something strange with this term. What does "stopped" photon mean, for you? Do you refer to Lene Hau's experiment? In this case, photons are not stopped or slowered at all, just, as I explained, their resultant propagation in a medium's specific direction is less than c. There photons always travel at c, however.
« Last Edit: 14/11/2006 13:23:09 by lightarrow »

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Offline lightarrow

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Re: What is the mass of a photon?
« Reply #23 on: 14/11/2006 13:38:30 »
A moving photon has mass.
Explain to me how this is not a violation of conservation of mass.
Atom emits photon.
Photon has mass, that mass equals the mass lost in the atom. (no violation)
Photon stops, mass disappears.
Atom has lost mass, no object with the equivalent mass remains. (obviously not counting photons emitted from other atoms)
How is that not in violation?

The answer because mass got converted to energy is not valid. Energy has mass.

1. You should define  what "Photon stops" means, exactly.

2. Relativistic mass it's NOT Rest mass. NO Energy violation.

Let's make an example with a baseball: Rest mass = 50 grams. So at rest, Relativistic Mass ( = Total mass) = Rest mass = 50 grams. If you accelerate it to 0.9c, its Total Mass = Relativistic Mass is 114.71 grams. So, if you stop it, you think that you have lost mass and so, lost energy?
 
Not at all, because, to stop it, you acquire its Kinetic Energy = Total Energy - Rest Energy =
= 114.71*10-3*c2 - 50*10-3*c2 = 64.71*10-3*c2 = 64.71*10-3*9*1016 = 5.82*1015 Joule.
If you divide that equation for c2, you have: "Kinetic Mass" = Total Mass - Rest Mass = 114.71 - 50 = 64.71 grams.
With photons you have: Kinetic Energy = Total Energy - Rest Energy = Total Energy - 0 = Total Energy.

So, if you absorb a light pulse which Total Mass ( = Relativistic mass) is 64.71 grams, you acquire a more mass of 64.71 grams.

Said in Joule: if that light pulse has an energy of 5.82*1015 Joule, after having absorbed it, you acquire 5.82*1015 Joule of energy.

Is it clear now?

« Last Edit: 14/11/2006 19:31:48 by lightarrow »

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Offline syhprum

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Re: What is the mass of a photon?
« Reply #24 on: 14/11/2006 17:13:55 »
I weigh about 85Kg I calculate that when I absorb this pulse of raidiation my temperature will be raised to 1.47*10^10 K.
« Last Edit: 14/11/2006 20:01:14 by syhprum »
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Offline 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
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Offline 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.

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Offline 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 »

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Offline 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?
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Offline 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.


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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.
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Offline 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

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Offline 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?

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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 »

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Offline 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.

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Offline 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
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Offline 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.

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Offline 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 »
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Offline 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 »
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Offline 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? [;)]
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...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!
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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 »

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Offline 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)
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Offline 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
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Offline 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 »

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Offline 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 »
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Offline 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.
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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...

Reaching out to embrace the random, reaching out to embrace whatever may come.

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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
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Offline 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.

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Offline 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!!!

About photons,
From "The Quantum Theory of Light" by  Rodney Loudon:

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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 »

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Offline 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
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Offline 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.

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  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 »

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Offline 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 »
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