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Photons???


qft123
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I am new at quantum physics, so dont be harsh..

my question is that "Photons"(quanta of light) they are present in the forms of electromagnetic radiations. they have no mass..

but still they are said to be a Matter.. WHY????

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I am new at quantum physics, so dont be harsh..

my question is that "Photons"(quanta of light) they are present in the forms of electromagnetic radiations. they have no mass..

but still they are said to be a Matter.. WHY????

 

Einstein defined matter as follows. From The Foundations of the General Theory of Relativity, by Albert Einstein, Annalen der Physik, 49 (1916). Reprinted in The Principle of Relativity, Dover Pub. See bottom of page 143.

We make a distinction hereafter between “gravitational field” and “matter” in this way, that we denote everything but the gravitational field as “matter.” Our use of the word therefore includes not only matter in the ordinary sense, but the electromagnetic field itself.

 

Regarding whether photon's have mass please see

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

which explains that Photons have zero proper mass and non-zero inertial mass.

 

Particle physicists have a strong tendancy to use the term "mass" to mean "proper mass". They don't have applications otherwise. It's different when working with continuous media. Cosmologists use the term differently such as when it comes to gravitational mass.

Edited by pmb
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I am new at quantum physics, so dont be harsh..

my question is that "Photons"(quanta of light) they are present in the forms of electromagnetic radiations. they have no mass..

but still they are said to be a Matter.. WHY????

 

It depends of the definition of matter you are using. Many people distinguish between matter and radiation, with photons belonging to the latter (i.e. they consider that photons are not matter).

 

See e.g. http://www.pd.astro.it/E-MOSTRA/NEW/A4000MAT.HTM

 

Personally I consider that electrons, quarks... are matter but photons, gravitons... are not.

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In modern physics one would tend to make the distinction between the fundamental particles that "build-up" matter and the fundamental particles that "transmit" forces.

 

Photons are the particles that are responsible for the electromagnetic interaction and thus are not usually considered as matter.

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In modern physics one would tend to make the distinction between the fundamental particles that "build-up" matter and the fundamental particles that "transmit" forces.

 

Photons are the particles that are responsible for the electromagnetic interaction and thus are not usually considered as matter.

The term matter is a term which I tend to stear clear of. If I use it I mean more or lass "stuff". I'd never use it in a scientific sense myself. But if people ask me for a definition in a relativity context I quote Einstein.

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Thanks for clearing the doubt...

But as we know that photons are the elementary particles of light.. they have no mass, but how come light has mass???

Einstein proved that an object travelling at the speed of light has infinite mass..

I.E. m = m0 /sqrt(1 - v2/c2)

We will get to know that the mass is infinite (of an object travelling at the speed of light)

If photons are the elementary particles of light (with no mass), then how is it possible for light to have mass?????

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The term matter is a term which I tend to stear clear of. If I use it I mean more or lass "stuff". I'd never use it in a scientific sense myself. But if people ask me for a definition in a relativity context I quote Einstein.

 

The concept of matter given by ajb and myself is the usual in the modern Standard Model. Therefore the context is relativistic.

 

Einstein quote is in the context of classical EMTs and their role in the Hilbert & Einstein equations, which only describe matter superficially, as a continuum.

Edited by juanrga
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Thanks for clearing the doubt...

But as we know that photons are the elementary particles of light.. they have no mass, but how come light has mass???

Einstein proved that an object travelling at the speed of light has infinite mass..

I.E. m = m0 /sqrt(1 - v2/c2)

We will get to know that the mass is infinite (of an object travelling at the speed of light)

If photons are the elementary particles of light (with no mass), then how is it possible for light to have mass?????

 

m_0 = 0 for a photon.

 

The way light has mass is to introduce a new definition for mass, e.g. m=E/c^2

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m_0 = 0 for a photon.

 

The way light has mass is to introduce a new definition for mass, e.g. m=E/c^2

The definition which nefver needed changing is the way mass was defined by newton and how his second law was defined. Newton defined mass so as to be consistent with the m in p = mv. Plug in c for a photon and you get p = mc. Or if we follow French we'd get

 

From Special Relativity, A. P. French, MIT Press, page 20

Let us now try to put together some of the results we have discussed. For photons we have

 

E = cp

 

and

 

m = E/c2

 

(the first experimental, the second based on Einstein's box). Combining these, we have

 

m = p/c

The nice thing is that m = p/v is a definition whereas m = E/c2 is an equality. One that doesn't hold true in all concievable cases. E.g. the inertial energy density and the inertial mass density can be different. E.g. consider a rod which is under stress. In such a case the inertial mass density is not the same as the inertial energy density.

 

If you have Schutz's GR text please see page 110

 

Thanks for clearing the doubt...

But as we know that photons are the elementary particles of light.. they have no mass, but how come light has mass???

Einstein proved that an object travelling at the speed of light has infinite mass..

I.E. m = m0 /sqrt(1 - v2/c2)

We will get to know that the mass is infinite (of an object travelling at the speed of light)

If photons are the elementary particles of light (with no mass), then how is it possible for light to have mass?????

They have zero proper mass but the dop have inertial mass m = p/c. This comes fom the definition of mass which is implicitly defined through the relation p = mv. Such a definition says "mass m is defined such, for a system of particles, mv is a conserved quantity." This might be of use - http://home.comcast.net/~peter.m.brown/sr/inertial_mass.htm

 

The expressioin m = m0 /sqrt(1 - v2/c2)[/size] is not one I choose to use to define the mass of a luxon (a particle whose speed is c)

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So light does not have mass..

But wen it travles at the speed of light ("c"), it gains mass and the time slows down.. (Relativity)

Right??

 

Precisely photons travel to the speed of light because have not mass (m=0). As you said above Einstein showed that a massive particle with nonzero mass cannot travel at the speed of light because its mass would become infinite (which means that you need an infinite energy to accelerate one electron up to c, and this is impossible).

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but still they are said to be a Matter.. WHY????

That depends on what you mean by "matter".

 

One fairly standard definition is something that occupies space and possesses rest mass. This rules out photons on both accounts. They are massless, so strike 1. They don't occupy space as they are elementary bosons, so strike 2. This second strike also applies to the W and Z bosons, which are not massless. Per this definition, the W and Z bosons are not "matter". This second strike does not apply to mesons even though they are bosonic. Mesons do occupy space because the quarks that comprise them are fermionic and are thus subject to the Pauli exclusion principle.

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That's your own personal defintion of course.

No, it's a very standard definition.

 

So what you're saying is if it doesn't occupy space it's not matter? What about an electron?

Electrons are fermions. They are subject to the Pauli exclusion principle. They occupy space. Photons and other force carriers are bosons. They are not subject to the exclusion principle. They do not occupy space.

Edited by D H
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No, it's a very standard definition.

No, it isn't. There is no such official definition. A loose definition used is "something that has mass" and that will depend on how one defines mass. I am confused though. I can't figure out why you'd think that I'd accept a definition without proof. Do my previous posts make you believe that that is something I'm in the habit of doing?

 

Take a look at Taylor & Wheeler's SR text. Turn to page 248

Question: Does mass mean "amount of matter" "

 

Answer: Nature does not offer us any such concept as "amount of matter." History has struck down every proposal to define such a term. Even if we could count the number of atoms or by any other counting method try to evaluate amount of matter, that number would not equal mass. First, mass of a specimen changes with its temperature. Second, atoms tightly bonded in solid -- are less massive -- than the same atoms free. Third, many of natures undergo radioactive decay, with still greater changes in mass. Moreover, around us occasionally, and continually in stars, the number of atoms and number of particles themselves undergo change. How the speak honestly? Mass, yes; "amount of matter," no.

Bolding, mine.

 

 

Electrons are fermions. They are subject to the Pauli exclusion principle. They occupy space. Photons and other force carriers are bosons. They are not subject to the exclusion principle. They do not occupy space.

That's wrong. The fact that a particle follows Pauli exclusion principle doesn't tell you anything about that particle alone. It only addresses what it does it other fermions.

 

It's a well-known fact that both all point particles have zero spatial dimension. positrons are the same way as are all types of neutrinos.

 

!

Moderator Note

juanrga and pmb,

 

You're both getting caught up in a bit of off-topic discussion here, so I am going to split your posts about what mass is into a new thread and ask that you please do not continue derailing this one.

 

 

God bless ya! You can toss the other part out because I have zero interest in the subject. I'm already an expert in the matter.

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Matter is stuff that has mass and occupies space.

 

That is incorrect, considering the fact that photons don't have mass, and they don't occupy space (BOSONS), yet some people consider it at as a "matter"

I recommend you do not use this definition of matter..

Edited by qft123
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That is incorrect, considering the fact that photons don't have mass, and they don't occupy space (BOSONS), yet some people consider it at as a "matter"

I recommend you not use this definition of matter..

 

But photons are not generally considered matter. D H, juanrga and ajb have all said this.

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!

Moderator Note

juanrga and pmb,

You're both getting caught up in a bit of off-topic discussion here, so I am going to split your posts about what mass is into a new thread and ask that you please do not continue derailing this one.

 

 

Sorry, but a reply is needed here.

 

In my last response to PMB which you moved to the other thread (search post #11 in the other thread that you created), I wrote explicitly that I would not reply more about the definition of mass. Therefore I was not going to answer what PMB wrote after my last post (see #12 in the other thread) about mass. Neither I am going to reply now, where PMB continues writing about the definition of mass in #18... just after your note.

 

It was unneeded to cite me in your note.

 

That is incorrect, considering the fact that photons don't have mass, and they don't occupy space (BOSONS), yet some people consider it at as a "matter"

I recommend you not use this definition of matter..

 

In #3 I explained to you that there are different definitions of matter, but that the standard definition of matter considers that photons are not matter. I gave you a link to an astrophysics page explaining the differences between matter and radiation. This division into matter plus radiation is standard in both physics and chemistry and can be found in many textbooks.

 

Another presentation of the standard definition of matter is given in the CERN website

 

http://public.web.ce...rdmodel-en.html

 

where it is again stated that photons are not matter particles. There are several good reasons for which photons are not considered matter. See the above link.

Edited by juanrga
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But photons are not generally considered matter. D H, juanrga and ajb have all said this.

Taylor and Wheeler suggest not using the term. And I agree with them.

Edited by pmb
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Taylor and Wheeler suggest not using the term. And I agree with them.

 

Their suggestion about photons is not considered by the immense majority of physicists and chemists, because their approach to understanding matter is based in a classical coarse-grained approach, instead of the modern fine-grained approach which has generated the above SM table with the three generations of matter.

Edited by juanrga
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