0 Members and 1 Guest are viewing this topic.

Just asked this on radio five live but ran out of time and didn't get to the bottom of it.I keep hearing that light is massless which confuses me. So my question is:Is a photon an actual partical which travels through space or does it only exist at the point of detection. i.e. when it hits something. Further more, if light is massless then that would imply that the electromagnetic waves passing through space don't have any energy if einsteins theory e = mc^2 is applicable to them. Unless my basic understanding of physics is completely wrong then that can't be the case which is why I have to ask how can light be massless or is this statement just being used to state the case at the point of detection. i.e. the residue of the electromagnetic wave hitting something is what we call a photon and has no mass but does have energy?thanks

if light is massless then that would imply that the electromagnetic waves passing through space don't have any energy

The Planck constant was first described as the proportionality constant between the energy (E) of a photon and the frequency (v) of its associated electromagnetic wave. This relation between the energy and frequency is called the Planck relation: E = hv (see ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN)

I keep hearing that light is massless which confuses me.

So my question is:Is a photon an actual partical which travels through space or does it only exist at the point of detection. i.e. when it hits something.

Further more, if light is massless then that would imply that the electromagnetic waves passing through space don't have any energy if einsteins theory e = mc^2 is applicable to them.

Is a photon an actual partical which travels through space or does it only exist at the point of detection. i.e. when it hits something.

There has been a debate on the definition of mass for several decades. What has happened is that particle physicists, the major “users” of special relativity, who study the intrinsic properties of particles only speak of what’s called the rest mass aka proper mass aka invariant mass of the particle. This is the mass you know from Newtonian mechanics.

Quote from: Pmb on 07/03/2013 15:31:19There has been a debate on the definition of mass for several decades. What has happened is that particle physicists, the major “users” of special relativity, who study the intrinsic properties of particles only speak of what’s called the rest mass aka proper mass aka invariant mass of the particle. This is the mass you know from Newtonian mechanics. You compute that light has zero mass with newtonian mechanics? Or, maybe, with classical electrodynamics (Mxwell's theory)? And what about the *quantum* object "photon"? Still newtonian mechanics?

Matt Austern expresses this entire argument nicely in two equations in the Baez link posted above:E = m_{rel}c^{2} and E^{2} = p^{2}c^{2} + m_{rest}^{2}c^{2} . Obviously a photon has non-zero energy, and from the first equation, m_{rel} is therefore non-zero for a photon.The second equation does allow m_{rest} to be zero for a photon, and there are good reasons for it to be zero in quantum mechanics. So how can one of these masses be zero and the other non-zero for a photon? They represent two different physical quantities. The reason they're both sometimes confusingly called "mass" is that for macroscopic objects moving at non-relativistic speeds (when you can use classical mechanics), both definitions reduce to the same thing that Newton would have called simply "mass."

That equation is WRONG in your case.It is correct only for STATIONARY BODIES.Is light stationary? Of course not.

Quote from: lightarrow on 08/03/2013 14:05:45Quote from: Pmb on 07/03/2013 15:31:19There has been a debate on the definition of mass for several decades. What has happened is that particle physicists, the major “users” of special relativity, who study the intrinsic properties of particles only speak of what’s called the rest mass aka proper mass aka invariant mass of the particle. This is the mass you know from Newtonian mechanics. You compute that light has zero mass with newtonian mechanics? Or, maybe, with classical electrodynamics (Mxwell's theory)? And what about the *quantum* object "photon"? Still newtonian mechanics?I was referring to luxons i.e. particles which have a rest frame or travel at v << c.

In any case Newtonian (inertial) mass is define as the m in p = mv. Same in relativity. I gave those links above so as to explain all of this in excrutiating detail. Anybody who reads those should pretty much be an expert on the subject by the time they're finished reading all of them.

What do you mean?Luxons don't have a rest frame and travel at c:

...sorry, you cannot view external links. To see them, please REGISTER or LOGINQuoteIn any case Newtonian (inertial) mass is define as the m in p = mv. Same in relativity. I gave those links above so as to explain all of this in excrutiating detail. Anybody who reads those should pretty much be an expert on the subject by the time they're finished reading all of them.I wrote that comment on your post because I had the ... subtle feeling that you tried to suggest to people that invariant mass is a less modern concept than relativistic mass []I believe it's the opposite...

Quote from: lightarrowThat equation is WRONG in your case.It is correct only for STATIONARY BODIES.Is light stationary? Of course not.Yeah, but as you very well know, whether it's right or wrong is a matter of what the definition of m is. If m is proper mass then it's wrong. If it's inertial mass then it's right.

Quote from: percepts on 07/03/2013 04:32:05Just asked this on radio five live but ran out of time and didn't get to the bottom of it.I keep hearing that light is massless which confuses me. So my question is:Is a photon an actual partical which travels through space or does it only exist at the point of detection. i.e. when it hits something. Further more, if light is massless then that would imply that the electromagnetic waves passing through space don't have any energy if einsteins theory e = mc^2 is applicable to them. Unless my basic understanding of physics is completely wrong then that can't be the case which is why I have to ask how can light be massless or is this statement just being used to state the case at the point of detection. i.e. the residue of the electromagnetic wave hitting something is what we call a photon and has no mass but does have energy?thanksGood questions. I'll try to answer them in order:1) In quantum mechanics, "particle" means something different than a little tiny packet of something that moves through space along well-defined trajectories. A photon is a particle in the quantum sense, in that it's the smallest piece of energy you can extract from an electromagnetic wave. At a detector, it also interacts at a point. It is not particle-like in a classical sense in that when it travels from point A to point B it does so in a wavey way that's spread out over all space, not along a single trajectory. 2) The actual equation for a particle is not necessarily E=mc^{2}, but rather E^{2}=m_{0}^{2}c^{4}+(pc)^{2}, where p is momentum. There are two important issues in this equation: first, m_{0} represents what's called the invariant mass of a particle, which for photons is zero. (Defining mass usually leads to a debate on this forum, since there are several possible definitions in relativity, but invariant mass is usually what physicists mean when they say the mass of a photon is zero). In this case, E^{2}=(pc)^{2}, and a photon does have momentum, so there's no problem.

Are you sure? Which is the transverse relativistic mass of a photon?

Quote from: lightarrowAre you sure? Which is the transverse relativistic mass of a photon?Yes. I'm sure. I'm confused as to why you don't know the answer. It's pretty trivial. I think you forgot to put on your thinking cap. Tardyons, such as electrons can be defelcted so that they accceleratet transverse to their direction. Since a photon can't be deflected

it travels in a straight line so that the transverse mass is zero. The same holds for the longitudinal mass, it's zero too. What isn't zero is the relativistic mass defined as the m in p = mv by hypothesis this is a conserved quantity.

Why a photon can't be deflected?

Let's say that you are right about relativistic mass of a photon: the formula E = mc^{2}, with m as the relativistic mass, is correct for photons only, not for tardyons.

Hmm . If you fall with the gravity, the gravity disappear for you.

Quote from: yor_on on 10/03/2013 22:39:58Hmm . If you fall with the gravity, the gravity disappear for you. No,that doesn't.Gravity increases my energy.

Quote from: lightarrowWhy a photon can't be deflected?First off I’m restraining my discussion to special relativity (SR). In SR there is nothing that interacts with a photon that can deflect it. As I recall it, and I may e wrong, I don’t consider Compton scattering to be deflecting the photon since a photon is destroyed when it hits the electron and a new one produced. Otherwise the photon can’t be deflected as an electron is i.e. via the electric field. No such field for deflecting photons exit that I’m aware of,

Quote from: lightarrowLet's say that you are right about relativistic mass of a photon: the formula E = mc^{2}, with m as the relativistic mass, is correct for photons only, not for tardyons.That is incorrect. Some people even define relativistic mass as m = E/c^{2}.It can be shown that if p = mv where m = relativistic mass then it can be shown that E = mc[sup[2[/sup]. I’ve derived that relationship here - ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN

Quote from: simplified on 11/03/2013 13:44:48Quote from: yor_on on 10/03/2013 22:39:58Hmm . If you fall with the gravity, the gravity disappear for you. No,that doesn't.Gravity increases my energy.He's correct. If you're in a uniform gravitational field and you're in free fall then you're frame of reference is for all practical purposes an inertial frame of reference with no field present. That's exactly why Einstein said that you can transform away a gravitational field.

Are you thinking that if you're in a free fall under gravity, you're still in a gravitational field?Then you have another definition of it than Einstein had Simplified, and you will need to make it a proposition that covers most any situation relativity takes up. And the point there is that it need to fit relativity, at least those parts we have measured directly, or indirectly, as muons. So you need your idea to cover, for example, why muons can go further than their allowed 'time' if measured at rest. And as they are particles of mass we find them to exist measurably. Read that paper I linked and use it to test your ideas.

So you define it such as a free fall, following a geodesic slows down time locally? Ok, then do you define it such as with different uniform motions you get different 'time'? And do you define it relative the gravity?If you do, then inside a 'black box', deep space with no tidal forces, can you prove that gravitational field? By some local experiment inside that black box. And how will you prove a motion? No windows, no outside peeking. Inside, by a experiment.

Yes, I think you can, using light as your clock, maybe splitting it relative a rotating mirror, and using a ...sorry, you cannot view external links. To see them, please REGISTER or LOGIN What will that tell you inside your black box about motion? When it comes detecting that gravitational field, you either have to assume that this will create a detectable motion inside your box, or? That you're not 'moving' at all.In relativity that uniform motion or geodesic only are described as detectable relative another frame of reference though. A acceleration is something else, detectable locally as a blue or/and red shift, depending on which way you measure in relation to the light source, and your accelerations direction. Well, as I think of it.