# Thread: Does light have a mass?

1. Hi Tom

"In other words, Crisp claims that as you increase the speed of the object, the object converts the energy into mass. And as the object slows down, the mass is converted back to energy."

Where did I ever say that ? Please don't deduce conclusions from the very limited things I say about special relativity. There's a whole lot more to it than some words.

But to get back to your claim: no, energy is not converted to mass or vica versa: they are equivalent, the same. I suggest you reread the discussions on relativistic mass vs. restmass held a few times already in this thread and the forum. That will explain some things I guess.

Oh BTW: perhaps I should add this aswell: energy is relative to the observer. It's not a good idea to think of energy as some kind of absolute quantity - this only leads to contradictions. Let me give you something to think about (and please someone else, don't immediatelly give the answer). We know that in a particle collision new particles can be created: the (total) energy of a particle, and mostly its kinetic energy, can convert to matter when it smashes into a stationary target. However, from the impacting particle's point of view (imagine you are moving along with it) there is no kinetic energy since in that frame of reference it is at rest, hence the only energy available is that of the restmass E = m<sub>0</sub>c<sup>2</sup>. How can you explain from that frame of reference that new matter is created when there is not enough energy to create matter from ?

The answer is not too difficult. You probably already know it, but give it some thought. Then you'll probably have a better understanding of how energy works.

Bye!

Crisp

2. Originally posted by Crisp
[B]Hi Tom

[i]"I suggest you reread the discussions on relativistic mass vs. restmass held a few times already in this thread and the forum. That will explain some things I guess.

three things appear from relativity arguments. photons have no rest mass. photons "acquire" mass during/due to acceleration. photons travel at the speed of light as soon as they are "conceived". if photos have mass as soon as they begin to "exist", then how can you argue (save for mathematics sakes) that they have rest mass; as they are never at rest they can never have rest mass.. therefore photons HAVE mass - due to acceleration or kinetic energy or a shot of red bull, as the case may be.

i understand why you would say photons dont have mass because mathematical equations prove it so, but (note above), should'nt logic persevere, afterall, science is dependant on logic, not always the other way around, if it had been, we would never have thought the world was flat.

i know the photons have/dont have mass theory has been hashed up till the sun went up, but as it did, the photons came back.

3. """The cute thing about virtual photons is that they don't exist from our point of view (hence the term... virtual ). Therefor - apart from being a theoretical construct - nothing is known of them."""

1) YOU have told me that EM fields are seen as emittions and absorbtions of VIRTUAL photons MEANING THAT for Quantum Phyiscians they are regarded as causing REAL EFFECTS IN OUR EVERY DAY LIVES

2) Particles are observed not reaching the speed of light. It remains at 99,999 of c. The traditional explanation is that that particle becomes heavier all the time as it goes faster and faster. The traditional explanation also tells us that the additional mass (energy) is coming from those same VIRTUAL PARTICLES, ie AGAIN they have real effects for physicians.

I on the other hand have pointed out that these photons can in no way be responsible for enough increase of mass as they only remain for a split second because of Heisenberg's principle of Uncertainty. At the end of his journey, that particle will not be carrying all the aquired energy from those photons from the beginning. Only, and ONLY those of the last split second before impact takes place. This also means that the excess of energy cannot be explained with the mainstream model.

1) a small amount of mass wil be aquired during the journey of the particle but this will be a CONSTANT quantity
it's like adding and removing all the time
the mass CANNOT keep on increasing as it keeps speeding up because of Heisenberg
2) it won't reach the speed of light due to the catching-up-model
3)the excess energy measured (calculated) at collision is because when a small amount of matter is converted into energy it is a HUGE amount of energy (E=mc^2), hence virtual particles of the EM-fields, and basically any virtual particle which appears from the sea of virtual energy at that moment, will receive enough energy and will be able to obtain a PERMAMENT character.

"""About the catching-up vs. virtual photon "speed of light is the limit" discussion: I really hope an expert in field theory will come in and settle the discussion, but somehow I am afraid that that glorious day is a long way ahead."""

I have the same hope
it might seem that I always want to hear that I am right and the rest wrong but that's not the case
when someone can point out clearly why I can't possibly right I will shut up and agree

4. Crisp,

But to get back to your claim: no, energy is not converted to mass or vica versa: they are equivalent, the same.
Really?? So now energy and mass are the same.

Funny, during this entire thread you and others have claimed that photons are pure energy and that they have NO mass. Now your saying that mass and energy are the same thing. Doesn't that mean that photons HAVE mass???

Oh BTW: perhaps I should add this aswell: energy is relative to the observer. It's not a good idea to think of energy as some kind of absolute quantity - this only leads to contradictions.
This one is good!!!If energy is relative to the observer, and energy and mass are the same, that would mean that mass is relative to the observer, as well. Is there anything real or absolute in your world????

We know that in a particle collision new particles can be created: the (total) energy of a particle, and mostly its kinetic energy, can convert to matter when it smashes into a stationary target. However, from the impacting particle's point of view (imagine you are moving along with it) there is no kinetic energy since in that frame of reference it is at rest, hence the only energy available is that of the restmass E = m0c2. How can you explain from that frame of reference that new matter is created when there is not enough energy to create matter from ?
Let me guess, according to the particle, the target is moving.

Unfortunately, I don't believe in "frame of reference" crap. As c'est moi pointed out, the relativity of different frames of reference is the result of lack of information, and not some new scientific concept.

Sooner or later, the scientific community will accept the fact that everything is absolute. This will include mass, time, energy, and space. The notion that the universe differs based on how we percieve it, is one of mankinds most absurd ideas.

Tom

5. ""This will include mass, time, energy, and space. The notion that the universe differs based on how we percieve it, is one of mankinds most absurd ideas. ""

indeed, the Lorentz equations are correct but need to be interpretted in a more consistant and logic way, ie it is not space which contracts but the object itself, it is not time which dilates but clock (the clock-thing is actually the same thing as lenght dilation - the two phenomena are the same thing)

6. i belive that everything has mass, including photons. unless theres another reason why light bends due to gravity?

Oggie just retract what you said and save yourself now. You're <b>never</b> going to hear the end of it if you try to back that up.

8. explain what you mean, as i knoe this to be true. well as far as i know anyway

9. i belive that everything has mass, including photons. unless theres another reason why light bends due to gravity?

In General Relativity, it is a consequence of the Equivalence Principle which states there is no physical difference between an accelerating frame of reference and one in a gravitational field.

10. Hi all,

Okay, very tired at the moment but I just had to adress some points made in the previous posts...

c'est moi,

1) YOU have told me that EM fields are seen as emittions and absorbtions of VIRTUAL photons MEANING THAT for Quantum Phyiscians they are regarded as causing REAL EFFECTS IN OUR EVERY DAY LIVES"

I would love to see the paragraph where I said that. I never talked about virtual photons. It once came up in a small spin-off discussion between James R and myself, but in the end we both agreed that electromagnetic effects are caused by photons of the electromagnetic field and not by "virtual photons".

At the moment the only "observable" effects of virtual photons I can think of are the repulsion of two closely spaced plates and Hawking radiation. Both are unrelated to the speed of light as an absolute limit (and so are virtual photons imho).

Tom,

"Really?? So now energy and mass are the same. "

Have I ever claimed otherwise ?

"Doesn't that mean that photons HAVE mass??? "

Define mass in this context please.

"This one is good!!!If energy is relative to the observer, and energy and mass are the same, that would mean that mass is relative to the observer, as well. Is there anything real or absolute in your world???? "

Congratulations. You have just discovered one of the most important discoveries in the 20th century all by yourself. Yes, mass and energy are relative to the observer. Restmass is real for example, relativistic distance (better known as Minkowski distance) is absolute in SR.

"Unfortunately, I don't believe in "frame of reference" crap. As c'est moi pointed out, the relativity of different frames of reference is the result of lack of information, and not some new scientific concept. "

I'll be glad to give up the "frame of reference" concept if you can give me a good alternative. Unfortunately, I think you will have to look pretty hard to find loads of absolute quantities (= independent of frame of reference) in our real world.

"The notion that the universe differs based on how we percieve it, is one of mankinds most absurd ideas."

The keyword in this sentence is "perception". Just totally by coincidence I had a copy of "A short course in general relativity" in my hands this morning, and the following phrase in the section on special relativity caught my attention: "... students in special relativity often convince themselves that these effects [time dilatation and Lorentz contraction] are because of the finite speed of the transmission of signals. However, they are not....". After this section the author explains how the universality of the speed of light (i.e. that it is the same for all intertial observers) gives rise to these effects as a fundamental property of nature, and not of our perception.

The reason why I mention this is twofold: first of all, it is a nice statement on how perception can be wrong. And secondly, and more importantly: if you want some absolute quantities, for example, the speed of light, then you will have to accept the fact that absoluteness is an illusion - it is the absolute speed of light that gives rise to relativity.

Now you can argue that the speed of light is not the same for all observers... Do I hear "a frame-of-reference dependent quantity" somewhere in that sentence ?

Sorry, there is no way out of it: the amount of absolute quantities in nature is very very very small.

PS: You should really search the forums for the difference between restmass and (relativistic) mass. For one moment, just believe the "frame-of-reference" idea, picture what is going on and then abstract yourself from it. I think that will help you understand why photons have mass and why they don't have restmass at the same time.

Bye!

Crisp

11. hey, i'm not a physicist but, what i still dont understand is how something can have no mass at rest and suddenly have it when you speak of it as moving at the speed of light. if a photon does not have restmass and has mass when its moving, the mass has to be coming from somewhere, that is that it (mass) is not an intrinsic value that can be prescribed to the photon. and if it can not be prescribed to it, it must be acquiring/absorbing the said mass from elsewhere. the 'stuff' c'est moi spoke of. isnt that stuff/energy/mass, that is being absorbed then the key. that is the increase in mass may be due to kinetic energy/momentum, but it (the mass) is not a characteristic of the photon itself rather is derived from something else.

a layman's example, please feel free to criticise..
if i have a certain mass when i am at rest, you would have to say, if principles of physics apply accross the board, that when i am propelled forward my mass is increased too. OR is true only for photons (if so why are they more special? is it to do w th speed of light?)

if my masss is increased when propelled forward, then that extra mass is a product of something else. in my case the energy created from the mechanical movemetn of my feet or of the vehicle propelling me. then that extra mass is not a characteristic of me, as a photon's mass is not of it, but is a characteristic of something else, in my case it is a product of hte physical mechanism that creats my propelling energy and consequently mass.

should we not try and find the force responsible for creating that 'extra' mass in a photon? that way a photon can Never have mass, except as a by product of someother energy source. i.e what propels the photon and is that source responsible for the mass the photon appears to have?

12. Mostly Harmless:

The whole debate over whether photons have mass or not comes down to a poor understanding of the concept of mass and mass-energy, particularly the distinction between rest mass and relativistic mass.

Physicists do not get confused over this. Any physicist will tell you that a photon has zero rest mass, but it has energy related to its momentum by E=pc. If you want to regard that energy as relativistic mass, you are free to do so, but it is not a concept that physicists use very often since relativistic mass is a reference frame-dependent concept, whereas rest mass is an invariant scalar.

c'est moi:

I've already discussed your catching-up model. In the reference frame of a moving atom absorbing photons, the atom sees the energy of the photons decreasing as the atom speeds up. At the speed of light, the energy of the photons, relative to the atom, would be zero, so absorption leads to no further speed increase. That's a relativistic description similar to your catching-up model. It's not exactly the same, but you are not completely wrong.

13. So what you're sayingis that photons dont have mass, but have momentum. and due to that momentum, have energy whihc people like to call reletevistic mass.

my question was: where does that energy/momentum come from, if not intrinsically from the photon.

i understand reletavistic mass is reference-based, or as the name suggests relative to something else. rest mass being its true mass.

14. """I would love to see the paragraph where I said that. I never talked about virtual photons. It once came up in a small spin-off discussion between James R and myself, but in the end we both agreed that electromagnetic effects are caused by photons of the electromagnetic field and not by "virtual photons"."""

I find it strange that James then said that Em fields are seen as exchange of the momentum of VIRTUAL photons
where did he get the idea then? I mean, you all do have seen the quantum interpretation of Em fields in your colleges?

so magnets emit real light
funny, i don't see them glowing in the dark

"""At the moment the only "observable" effects of virtual photons I can think of are the repulsion of two closely spaced plates and Hawking radiation."""

15. Hi c'est moi,

"I find it strange that James then said that Em fields are seen as exchange of the momentum of VIRTUAL photons
where did he get the idea then? I mean, you all do have seen the quantum interpretation of Em fields in your colleges?"

There seems to be a misconception amongst non-physics students that physics students nicely get everything explained in words just the way we debate here. This is generally not true I think (and hope) - mostly we get an extended overview of the basic assumptions of a theory, how to understand these assumptions, and how some basic results can be deduced. I don't think James R ever got a quantum field lecture entitled "the quantum field nature of electromagnetic fields", and neither did I.

Interpretation is often a delicate issue in physics, that's why we only get thaught the math - that is the one thing we are sure of.

"so magnets emit real light . funny, i don't see them glowing in the dark"

There is a difference between the emission of a photon (and seeing it), and the exchange of a photon in an interaction. If you could see the photon in an interaction, that would mean it did not interact but rather went to your eye. Photons are only used in the electromagnetic picture for interaction and therefor cannot be directly observed as some glow of some sort.

You are correct in the sense that it is a theoretically predicted result (and is yet to be experimentally verified, even though - if I remember correctly - we already have quite some hints towards Hawking radiation). But then again, loads of things in physics are mathematics .

Bye!

Crisp

16. """There is a difference between the emission of a photon (and seeing it), and the exchange of a photon in an interaction. If you could see the photon in an interaction, that would mean it did not interact but rather went to your eye. Photons are only used in the electromagnetic picture for interaction and therefor cannot be directly observed as some glow of some sort."""

I bet you have seen some big magnets in action. Quite a force they excert, isn't it?

Now, how many photons, do you suppose, have to carry their momentum to make sure that such magnets appear to be so strong in interaction with another magnet ..... I suppose that this must be billions of photons and not a single one of them would reach your eye if you'd put your head between the two of them? I should see some light in the dark ...

17. Hi c'est moi,

"I suppose that this must be billions of photons and not a single one of them would reach your eye if you'd put your head between the two of them? I should see some light in the dark ..."

One more reason why I believe the photon exchange between two charged particles is because of an instantaneous absorption of a quantum of field energy, and not a "photon leaping from one point to another" situation.

Bye!

Crisp

you are not going to tell me that you haven't discussed EM-fields in some way
if you haven't, then what do you learn there?

"""One more reason why I believe the photon exchange between two charged particles is because of an instantaneous absorption of a quantum of field energy, and not a "photon leaping from one point to another" situation."""

field energy --> are you still talking about a photon here??

and if the interaction is instantaneously, how do you explain absorbtion of such a quant which must obviously have covered a certain distance or are you going the non-local way? why don't photons show their magical non-local properties then when I use my flashlight?

19. Hi c'est moi,

you are not going to tell me that you haven't discussed EM-fields in some way. if you haven't, then what do you learn there?"

To be completely honest, I dont think the word EM field was ever used ... But I haven't studied the course yet, I'll get back to you in a couple of weeks after I studied it for my exam .

"field energy --> are you still talking about a photon here??"

Ehrrrr... no, an electric field has the magic property that it can add energy to charged particles and make them move. It's the source of that energy that I refer to (it is contained within the field itself). The photon only comes up in the interaction: if a particle absorbs energy from the field, then QED states that it cannot absorb just any amount of energy: the energy the particle absorbs comes in packets... It can absorb one packet (quantum) of energy, or more, but not half a packet of energy. One of those packets of energy is a photon. In other words: I think the photon, or better: the transmitted packet of energy, is created at the moment the particle "decides" to absorb energy from the field.

"and if the interaction is instantaneously, how do you explain absorbtion of such a quant which must obviously have covered a certain distance or are you going the non-local way? why don't photons show their magical non-local properties then when I use my flashlight?"

You bet I am going the non-local way . I am referring to the non-locality of the electric field here. The field (and hence the energy of the field) is available at all locations we are interested in, and hence the particle can interact with the field everywhere.

Sidenote: flashlights do exhibit the non-local properties of light. A flashlight is perhaps not a good example (because its light is called "incoherent" which refers to the correlation of the light packets/photons and makes the wave-character of light more difficult to see), but an improved, monochromatic flashlight (ie a laser) clearly exhibits wave characteristics: just think of the Young two slit experiment (and yes, I've seen the effect with my own eyes: it is real ).

Bye!

Crisp

20. <i>my question was: where does that energy/momentum [of a photon] come from, if not intrinsically from the photon.</i>

The energy to create a photon must be provided by some external source. The photon then carries the energy from one place to another at the speed of light.

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