# The Naked Scientists Forum

### Author Topic: What is the mass of a photon?  (Read 20520 times)

#### thebrain13

• Sr. Member
• Posts: 442
##### 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 »

#### science_guy

• Hero Member
• Posts: 701
• I'm right there... inside neilep's head!
##### 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?

#### thebrain13

• Sr. Member
• Posts: 442
##### 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.

#### science_guy

• Hero Member
• Posts: 701
• I'm right there... inside neilep's head!
##### 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.

#### thebrain13

• Sr. Member
• Posts: 442
##### 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 »

#### science_guy

• Hero Member
• Posts: 701
• I'm right there... inside neilep's head!
##### 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.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### 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 »

#### syhprum

• Neilep Level Member
• Posts: 3823
• Thanked: 19 times
##### 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

#### thebrain13

• Sr. Member
• Posts: 442
##### 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.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### 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
.

Quote
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
.

#### thebrain13

• Sr. Member
• Posts: 442
##### 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?

#### another_someone

• Guest
##### 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.

#### Heliotrope

• Sr. Member
• Posts: 151
##### 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.

Quote
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.

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### 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 »

#### Heliotrope

• Sr. Member
• Posts: 151
##### 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.
« Last Edit: 13/11/2006 21:09:57 by Heliotrope »

#### thebrain13

• Sr. Member
• Posts: 442
##### 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.

#### syhprum

• Neilep Level Member
• Posts: 3823
• Thanked: 19 times
##### 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 )

#### Heliotrope

• Sr. Member
• Posts: 151
##### 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.

Quote

I see...

Quote
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.

Quote
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 ?

Quote
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.

#### thebrain13

• Sr. Member
• Posts: 442
##### 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.

#### Soul Surfer

• Neilep Level Member
• Posts: 3345
• keep banging the rocks together
##### 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)

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### 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.

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 »

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### 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!

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### 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 »

#### lightarrow

• Neilep Level Member
• Posts: 4586
• Thanked: 7 times
##### 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 »

#### syhprum

• Neilep Level Member
• Posts: 3823
• Thanked: 19 times
##### 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 »

#### The Naked Scientists Forum

##### Re: What is the mass of a photon?
« Reply #24 on: 14/11/2006 17:13:55 »