The Mystery of Light Particle Mass

[benzun_1999]

dear reader,
what is the mass of light particels?

 

[Doctor Luz]

deppends on their frequency m=hν/c^2

 

[jcsd]

Photons have zero mass.

 

[jcsd]

relativistic mass is not the same as mass and is hardly ever used.

 

[Doctor Luz]

Originally posted by jcsd
relativistic mass is not the same as mass and is hardly ever used.

Relativistic mass, mass at rest, mass... It's a question on nomenclature.

 

[jcsd]

Originally posted by Doctor Luz
Relativistic mass, mass at rest, mass... It's a question on nomenclature.

Not really, because mass that is invariant under a Lorentz transformation is a much more useful defintion than one that isn't. The term 'mass' means invariant mass only.

 

[Doctor Luz]

Originally posted by jcsd
Not really, because mass that is invariant under a Lorentz transformation is a much more useful defintion than one that isn't. The term 'mass' means invariant mass only.

It's worthwhile to discuss about this. However I think that a definition of mass that include the term 'mass' is not very good.

Of course, in my first post, I was talking about the 'relativistic mass'.

 

[selfAdjoint]

And the "relativistic mass" is wrong because it would be infinite. The invariant mass of a photon is zero, and its energy and momentum are what depend on its frequency.

 

[Doctor Luz]

Originally posted by selfAdjoint
And the "relativistic mass" is wrong because it would be infinite.

Can you explain it please?

 

[jcsd]

Originally posted by selfAdjoint
And the "relativistic mass" is wrong because it would be infinite. The invariant mass of a photon is zero, and its energy and momentum are what depend on its frequency.

No: the relativistic mass is a function of the kinetic energy and therefore frequency of a photon and the correct formula for this was given by Dr. Luz. This is obviously not invariant under a Lorentz transformation as different observers will observe different frequencies and therefore each give a different value for it's energy and relativistic mass.

 

[marcus]

Luz seems to be asking for a primitive definition of mass

(he says a definition of one kind of mass based on another kind of mass would not be very good----so he probably wants a definition of mass in terms of the most basic kinds of measurement)

the only one I know is this

"the mass of a body is the inertia of the body at rest"

inertia is a ratio of force to acceleration

the potential circularity of this definition is a long-recognized minor problem in the foundations of physics

it may not be a perfect definition but at least it has a clear operational meaning and removes some of the ambiguity

I agree with several of the other posters here that relativistic mass does not seem to be a very useful concept---Einstein explicitly advised against using it (there is a letter to this effect)----and it is apt to lead to confusion. Both selfadjoint and
jcsd are right although they contradict each other----defined one way the "relativistic mass" of some light would be infinite if the light had any mass to begin with and defined another way the "relativistic mass" is just a redundant jargon synonym for the energy of the light and is not infinite but simply E/c².

So one might as well follow Einstein's advice and not use the concept. Saves endless useless discussion about terminology.

Following majority usage among working physicists, since mass is the inertia of a body measured at rest, since light cannot be at rest it has no mass.

However boxes CONTAINING light can be at rest and part of their inertia can be due to the light which they contain. The sun is soaked full of light even to its very core and that light (which has zero mass) contributes mass, inertia, gravitational attractiveness, etc. to the sun.

In other words the notion of mass prevailing in modern physics is not additive-----which is tough for some people to accept. So they have this irresistible urge to try to get people to change the way they talk so that mass can be more of an alias for energy and have the additivity that we associate with energy.

I'm for the simplest least ambiguous use of words----getting mass aligned with what most physicists mean by it.

I also appreciate when at least some types of quantity can have simple operational meanings, without a lot of theory mixed up in them.

Force can be measured purely electrically by a device called the "watt balance", which is kind of interesting. Maybe it is more primitive than mass.

 

[Doctor Luz]

Originally posted by marcus
Luz seems to be asking for a primitive definition of mass

(he says a definition of one kind of mass based on another kind of mass would not be very good----so he probably wants a definition of mass in terms of the most basic kinds of measurement)

the only one I know is this

"the mass of a body is the inertia of the body at rest"

inertia is a ratio of force to acceleration

the potential circularity of this definition is a long-recognized minor problem in the foundations of physics

it may not be a perfect definition but at least it has a clear operational meaning and removes some of the ambiguity

I agree with several of the other posters here that relativistic mass does not seem to be a very useful concept---Einstein explicitly advised against using it (there is a letter to this effect)----and it is apt to lead to confusion. Both selfadjoint and
jcsd are right although they contradict each other----defined one way the "relativistic mass" of some light would be infinite if the light had any mass to begin with and defined another way the "relativistic mass" is just a redundant jargon synonym for the energy of the light and is not infinite but simply E/c².

So one might as well follow Einstein's advice and not use the concept. Saves endless useless discussion about terminology.

Following majority usage among working physicists, since mass is the inertia of a body measured at rest, since light cannot be at rest it has no mass.

However boxes CONTAINING light can be at rest and part of their inertia can be due to the light which they contain. The sun is soaked full of light even to its very core and that light (which has zero mass) contributes mass, inertia, gravitational attractiveness, etc. to the sun.

In other words the notion of mass prevailing in modern physics is not additive-----which is tough for some people to accept. So they have this irresistible urge to try to get people to change the way they talk so that mass can be more of an alias for energy and have the additivity that we associate with energy.

I'm for the simplest least ambiguous use of words----getting mass aligned with what most physicists mean by it.

I also appreciate when at least some types of quantity can have simple operational meanings, without a lot of theory mixed up in them.

Force can be measured purely electrically by a device called the "watt balance", which is kind of interesting. Maybe it is more primitive than mass.

Thank you for your efforts and this great explanation. Realy I did not wanted a primitive definition of mass, I only was remembering my old logic teacher when he told me that "when you are going to define the concept A you must not include the word A in the definition". Do you understand me?

From now when I read mass I will think in the "invariant mass-mass at rest"

Then answering the initial question, the mass of photons are 0

 

[Dissident Dan]

If mass is "the inertia of the body at rest", there are several problems. Firstly, there is no absolute "at rest". Motion is relative. Secondly, a photon at rest has never been observed, so how can you say that it has 0 mass?

How do we measure mass, and what purpose does the concept have, other than in inertia/momentum-qualities that a photon exhibits?

 

[Arc_Central]

Can it be said that a photon has mass when it is localized, or briefly localized? Perhaps all mass is localized photons.

 

[pmb]

Originally posted by jcsd
Photons have zero mass.

Wrong - It has zero *proper mass*

 

[pmb]

Originally posted by jcsd
relativistic mass is not the same as mass and is hardly ever used.

Wrong. Relativistic mass is just another name for mass. When the term "mass" is used it means one of two things - "proper mass" or "relativistic mass" and the later is closed to being mass than the former since it retains all the properties associated with mass.

And your comment about the usage is incorrect as well.

Pmb

 

[pmb]

Originally posted by jcsd
Not really, because mass that is invariant under a Lorentz transformation is a much more useful defintion than one that isn't. The term 'mass' means invariant mass only.

As far as proper mass being more useful - that's a matter of usage - i.e. for what problem is being solved. And even then it's a matter of point of view since different people sovle the same problem in different ways.

And "invariant mass" is not what people mean when they use mass. That's just plain wrong. In almost every case the author will simply say what he means by the word "mass" and then use it as such. If it's not exlained like that then one can tell by the context. And it
also depends on the journal. Mass means relativistic mass in almost all cases in the American Journal of Physics while in Physical Review D it means proper mass in almost all cases. If you go online to a place like FermiLab then it might mean relativistic mass. One only has to look to find counter examples to your claim. Examples are all over the place. Invariant mass is important as quite useful but does not have a well defined meaning in general. For example: th term "invariant mass" (aka rest mass aka proper mass) has liitle meaingin for several particles moving in an EM field. And its quite easy for people to make mistakes if they think strictly in terms of proper mass. For example: If you asked someone if a moving particle weighs more than the same particle at rest then they're likely to say no. And that's the wrong answer. Weight increases as a body moves faster since the mass increases (no. Not proper mass. Passive gravitational mass = relativistic mass = inertial mass = just plain 'mass')

Note: Weight W = mg is defined as the magnitude of the supporting force required to supoort a particle in a gravitational field where g is the local acceleration of gravity

Pmb

 

[jcsd]

Originally posted by pmb
Wrong. Relativistic mass is just another name for mass. When the term "mass" is used it means one of two things - "proper mass" or "relativistic mass" and the later is closed to being mass than the former since it retains all the properties associated with mass.

And your comment about the usage is incorrect as well.

Pmb

I'm afraid your very wrong, 'mass' always means invariant mass, the idea of relativistic mass as 'mass' went out the window a very long time ago.

There are reasons for this for example when talking about the Chanderskar limit, relativistic mass is a completely useless defintion.

Yes some pop-sci books do occasionally muddy the water in this way, but when a scientists says mass he means invariant/rest/proper mass.

 

[pmb]

Originally posted by jcsd
I'm afraid your very wrong, 'mass' always means invariant mass, the idea of relativistic mass as 'mass' went out the window a very long time ago.

Sorry to disappoint you but that's quite far from the truth. You've gotten the wrong idea somewhere along the line. There are many new relativity texts which clearly use the concept - some heavily. In fact one of the most prominent relativists, i.e. Wolfgang Rindler, use this concept in his new text, published in 2001. Many universities do as well. The Chanderskar limit, as I recall, is an inherent property and as such it would be incorrect to refer to is as anything other than rest mass. And just because you've never found relativistic mass to be useful - in NO way implies that everyone does - That's quite far from the truth.

And when I say that it's used I don't mean in pop-sci books. I mean at the undergraduate and graduate level texts in both special and general relativity. And I also mean recent texts too. And I aslo mean in physics journals.

Pmb

 

[jcsd]

PMB, you must of learned your physics about 20 years ago because 'mass' these days means exclusively invariant mass, you will simply not find a recently published paper that refers to relativistic mass as 'mass'.

 

[jcsd]

Here's an artilce discussing the use of the term 'mass':

http://www.weburbia.demon.co.uk/physics/mass.html

 

[Isotope]

Excuse my idiocy, but would photons, although they must go at the speed of light, were stopped or slowed, would they have mass? Since they go so fast, length-contraction would effect them alot. What I'm saying is, does length-contraction cause photons to lose all mass, or would they possibly have some while fixed in a position?

 

[jcsd]

A photon can't be in a rest frame or indeed any frame where it's not traveling at c, so talking about photons slowing or stopping (in case someone mentions it I'm aware of the experiments but that's not photons being slowed that's a light pulse) is pointless, that said as all their energy is kinetic they have a rest mass of zero.

Length contraction doesn't cause any lose in mass and doesn't affect photons anyway.

 

[clicky]

Mass is a resistence to acceleration. Therefore everything that creates effects due to a change of its velocity, including light photons, should have a mass.

Pusshing relativity too hard leads to paradoxes - like length shrinking creates massless photons.

 

[jcsd]

Originally posted by clicky
Mass is a resistence to acceleration. Therefore everything that creates effects due to a change of its velocity, including light photons, should have a mass.

Pusshing relativity too hard leads to paradoxes - like length shrinking creates massless photons.

Photons can't change velocity, so they can't. The big problem with relativistic mass is that it's different for different observers.

 

[BasketDaN]

Ah... is the definition of mass really that complex? How is it not simply "how much is there"? How does a change of position alone change mass? If photons are a form of energy... wouldn't that mean they would have to have some kind of mass, because theoretically, matter and energy are interchangeable, but you simply get a lot of energy from a certain amount of matter? Basically.. If some fuel source was somehow converted into 100% light, that light it gave off would have to have mass, because the fuel it started out as also had mass.

 

[pmb]

Originally posted by jcsd
PMB, you must of learned your physics about 20 years ago

What difference does it make when I learned physics? The fact is that mass is often defined in different ways

..because 'mass' these days means exclusively invariant mass, ..

This is incorrect.

...
you will simply not find a recently published paper that refers to relativistic mass as 'mass'.

This is also incorrect.

I'd like to know how you got this impression? You don't seriously think that you actually know what all physics articles in all journals by all authors use do you?


All you have to do is to look in a physics journal to see. The American Journal of Physics is a goopd example.

For example: In the paper "An elementary derivation of E = mc^2," Fritz Rhorlich, Am. J. Phys. 58(4), April 1990 uses the term "mass" to refer to what you're calling "relativistic mass" and yet there is no use of that term. Adn Rohrlich is a well known relativist.

Pete

 

[pmb]

Originally posted by jcsd
Here's an artilce discussing the use of the term 'mass':

http://www.weburbia.demon.co.uk/physics/mass.html

Yes. I'm well aware of this FAQ. But I don't know why you posted it. It clearly states

Sometimes people say "mass" when they mean "relativistic mass", ..

And you're saying that is 100% wrong - correct?


Here's an example of why it's useful to think in terms of this usage of mass: Suppose there is a uniform gravitational field, in frame S, parallel to the z-axis. The acceleration of gravity at z = 0 is g. A particle is sliding smoothly in the z = 0 plane with velocity v. What is the weight of the particle? Now supose that, instead of being in frame S, you're in frame S' moving relative to S where S' is the frame in which the particles is nov moving. What is the weight of that particle in S'?


Take a look at Fermi Lab's website
http://www.fnal.gov/pub/inquiring/questions/light_speed_add.html

Some people thought, this formula is ugly, and they decided to introduce a new mass, called the dynamic mass M, defined by

M=m/sqrt(1-v^2/c^2)*c^2

and then the Einstein's formula will look nice again,

E=Mc^2.

This trick will make it easier to use many of the fundamental formulae from classical mechanics in Einstein's theory of relativity, just by simply exchanging the rest mass m for the dynamical mass M. ( Also it is easy to show, that for very low speeds, compared to the speed of light, m=M.)

http://www.fnal.gov/pub/inquiring/questions/accel_mass.html

One effect is that particles with mass acquire a "relativistic mass" equal to their mass at zero velocity (called the rest mass) divided by the square root of ( 1 minus (particle velocity/speed of light)squared ). So effectively a particle gets more and more mass and is therefore harder and harder to speed up further. So hard that you can't ever reach the speed of light. If you look at the equation, you see that if the particle velocity were to equal the speed of light, then you would compute a "relativistic mass" of the rest mass divided by zero. Something divided by zero is infinitely large.

http://www.fnal.gov/pub/inquiring/questions/accel_obj.html

To accelerate an object so its mass is increased by 1% then gamma, the "time dilation factor" will be simply 1.01. That is equivalent to accelerating the mass to a velocity of 14% of the speed of light or 42,000 km/sec.

A 10% increase in mass corresponds to a gamma of 1.10 or a velocity of 42% of the speed of light.

Pete

 

[jcsd]

Originally posted by pmb
What difference does it make when I learned physics? The fact is that mass is often defined in different ways

It makes a difference because relativistic mass was used years ago, now it isn't, this gives me the impression that you studied physics several decades ago.

This is incorrect.

This is also incorrect.

I'm sorry these are both correct statements, relativitic mass is an almost redunant concept these days.

I'd like to know how you got this impression? You don't seriously think that you actually know what all physics articles in all journals by all authors use do you?


All you have to do is to look in a physics journal to see. The American Journal of Physics is a goopd example.

For example: In the paper "An elementary derivation of E = mc^2," Fritz Rhorlich, Am. J. Phys. 58(4), April 1990 uses the term "mass" to refer to what you're calling "relativistic mass" and yet there is no use of that term. Adn Rohrlich is a well known relativist.

Pete

No I don't, but I do know what is standard terminology and what is not.

This question comes up in the context of wondering whether photons are really "massless," since, after all, they have nonzero energy and energy is equivalent to mass according to Einstein's equation E=mc2. The problem is simply that people are using two different definitions of mass. The overwhelming consensus among physicists today is to say that photons are massless. However, it is possible to assign a "relativistic mass" to a photon which depends upon its wavelength. This is based upon an old usage of the word "mass" which, though not strictly wrong, is not used much today. See also the Faq article Does mass change with velocity?.

The old definition of mass, called "relativistic mass," assigns a mass to a particle proportional to its total energy E, and involved the speed of light, c, in the proportionality constant:

m = E / c2. (1)

This definition gives every object a velocity-dependent mass.

The modern definition assigns every object just one mass, an invariant quantity that does not depend on velocity. This is given by

m = E0 / c2, (2)

where E0 is the total energy of that object at rest.

The first definition is often used in popularizations, and in some elementary textbooks. It was once used by practicing physicists, but for the last few decades, the vast majority of physicists have instead used the second definition. Sometimes people will use the phrase "rest mass," or "invariant mass," but this is just for emphasis: mass is mass. The "relativistic mass" is never used at all. (If you see "relativistic mass" in your first-year physics textbook, complain! There is no reason for books to teach obsolete terminology.)

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

 

[jcsd]

Originally posted by pmb
Yes. I'm well aware of this FAQ. But I don't know why you posted it. It clearly states



And you're saying that is 100% wrong - correct? This is correct some basic level textbooks and popsci explanations.

Here's an example of why it's useful to think in terms of this usage of mass: Suppose there is a uniform gravitational field, in frame S, parallel to the z-axis. The acceleration of gravity at z = 0 is g. A particle is sliding smoothly in the z = 0 plane with velocity v. What is the weight of the particle? Now supose that, instead of being in frame S, you're in frame S' moving relative to S where S' is the frame in which the particles is nov moving. What is the weight of that particle in S'?

the problem is you change refernce frames the relativitic mass changes so it's not that useful, weight is diferent from mass anyhow.

Take a look at Fermi Lab's website
http://www.fnal.gov/pub/inquiring/questions/light_speed_add.html

http://www.fnal.gov/pub/inquiring/questions/accel_mass.html

http://www.fnal.gov/pub/inquiring/questions/accel_obj.html


Pete

These are very basic explantions aimed at the general public, not at scientists.

 

[pmb]

It makes a difference because relativistic mass was used years ago, now it isn't, this gives me the impression that you studied physics several decades ago.

Well you're incorrect. Although I started college in the early 80s I wasn' much interested in relativity until the mid 90s.

I'm sorry these are both correct statements, relativitic mass is an almost redunant concept these days.

Its not redundant at all. Its a pure fact depending on who one chooses to define mass. You can write p = gamma*m*v or you can write p = mv - in either case relativistic mass is there - its "gamma*m" in the first case and m in the second case. And you're claiming that relativistic mass is not used at all by anyone - that's just wrong. It is used. Why would you think otherwise? One only need look to see. You have an incorrect notion what is "standard" terminology. This is highly dependant on the particular person and what they find useful. Some physicists use it almost exclusively while others don't. But the relativity literature is full of this notion of mass and I'm not talking about older text as I've said.

So where did you get this impression from?


Pmb

 

[jcsd]

Originally posted by pmb
Well you're incorrect. Although I started college in the early 80s I wasn' much interested in relativity until the mid 90s.


Its not redundant at all. Its a pure fact depending on who one chooses to define mass. You can write p = gamma*m*v or you can write p = mv - in either case relativistic mass is there - its "gamma*m" in the first case and m in the second case. And you're claiming that relativistic mass is not used at all by anyone - that's just wrong. It is used. Why would you think otherwise? One only need look to see. You have an incorrect notion what is "standard" terminology. This is highly dependant on the particular person and what they find useful. Some physicists use it almost exclusively while others don't. But the relativity literature is full of this notion of mass and I'm not talking about older text as I've said.

So where did you get this impression from?


Pmb

So do you haven't any formal training in relativity then?

'mass' is defined as rest mass these days. I'd like to know which physicists use 'relativitic mass' as a definiton for mass, I've never met one. Even the concept of relativitic mass isn't used much these days.

 

[pmb]

Originally posted by jcsd
the problem is you change refernce frames the relativitic mass changes so it's not that useful, weight is diferent from mass anyhow.

So what? That's relativity for you. Mass changes with speed. Electic and magnetic fields change with speed. Length changes with speed. The lifetime of a neutron changes with speed. The intensity of a gravitational field changes with speed etc. etc. etc.


re - weight - Weight is intimately related to mass. In fact the passive gravitational mass M is defined according to weight as W = Mg.

BTW - Why did you ingnore my question?

These are very basic explantions aimed at the general public, not at scientists.

Wrong. Why would you think they'd do that? If you wouldn't explain it that way they why would you think others would?

 

[jcsd]

Originally posted by pmb
So what? That's relativity for you. Mass changes with speed. Electic and magnetic fields change with speed. Length changes with speed. The lifetime of a neutron changes with speed. The intensity of a gravitational field changes with speed etc. etc. etc.


re - weight - Weight is intimately related to mass. In fact the passive gravitational mass M is defined according to weight as W = Mg.

BTW - Why did you ingnore my question?



Wrong. Why would you think they'd do that? If you wouldn't explain it that way they why would you think others would?

Yes but rest mass is more useful as it tells you how to work out the relatvistic mass in a different reference frame. Weight and mass are two different things. I ignore the question, as it involves a gravitational field, which should properly be described by GR.

Imagine a simlair situation that you are traveling at a certain velocity past a neutron star, though if you measure the Chanderskar limit using relativistic mass you may find that the neutron star shoukld collapse into a balck hole, but it doesn't.

 

[pmb]

Originally posted by jcsd
So do you haven't any formal training in relativity then?

Where id you get that idea? That's not what I said. I said I wasn't much interested in it until the mid 90s. I studied relativity as an undergrad. In the late 90s I unofficially took a course in general relativity (Ed Bertchinger's course at MIT). Unofficial because it costs $5,000 to take it and I didn't want to spend that kind of money if it was just for me learning it. But if you have the idea that I don't understand relativity in a strict formal sense - math and all - then you'd be mistaken.

'mass' is defined as rest mass these days. I'd like to know which physicists use 'relativitic mass' as a definiton for mass, I've never met one. Even the concept of relativitic mass isn't used much these days.

I already told you. Rohrlich was an example.

But I don't think anyone would ever write a term without specifically defining it first. And not everyone calls it
'relativistic mass.' Some call it 'inertial mass.'

So first - almost all authors will say what they mean by mass and then go ahead and use it as such


But here are some new relativity texts which use mass to mean relativistic mass. They use the term once to say what they mean and then simply call it mass .

"Relativity: Special, General and Cosmological," Rindler, Oxford Univ., Press, (2001)

"Basic Relativity," Mould, Springer Verlag, (1994)

"Introducing Einstein’s Relativity," D’Inverno, Oxford Univ. Press, (1992)

There are others too such as "A short course in general relativity," Foster and Nightingale who write on page 135, explaining/deriving gravitational red shift

The loss in intrinsic energy h(f_E - f_R), while the gain in potential energy is

hf_E*GM/c^2(1/r_E - 1/r_R)

on assigning the mass hf_E/c^2 to the photon.

Then there's MTW who use the term on one occasion in their text "Gravitation" to mean E/c^2 to show that the energy-momentum tensor is symetric.


Take a crack at the weight question - it will illustrate why m = E/c^2 is meaningful as well as important.


Pmb

 

[jcsd]

I'm not denying that kinetic energy doesn't add to the gravitational attraction between objects, for example even photons are graviatationally attracted to and attract masses.

It's just the case nowdays 'rest mass' is much preferred by physicists as a defintion for mass and the mass of a photon is always quoted as zero.

Yes the particle will appear to be mopre graviationally attracted in S'.

 

[pmb]

Originally posted by jcsd
Yes but rest mass is more useful as it tells you how to work out the relatvistic mass in a different reference frame.

Rest mass is useful - Sure. I agree. But that doesn't mean that relativistic mass is not useful.

Weight and mass are two different things. I ignore the question, as it involves a gravitational field, which should properly be described by GR.

You did read what I wrote right? I said that they are different but are intimately related. In fact weight is defined in terms of mass.

Its extremely simple to solve this problem - trivial even. If you think in terms of relativistic mass then the answer will jump out at you - give it a try.

Imagine a simlair situation that you are traveling at a certain velocity past a neutron star, though if you measure the Chanderskar limit using relativistic mass you may find that the neutron star shoukld collapse into a balck hole, but it doesn't.

That's not correct. The Chandrasekhar limit is an invariant quantity. It's not something that depends on the frame of referance to measure. You must be thinking about the mass of a black hole and if mass increases then it must form a black hole - That's a poor way of looking at it - the Chandrasekhar limit is a scalar - it refers to rest mass - not relativistic mass.

However the intensity of a gravitational field increases with speed - not something one would guess if youy just say that mass does not increase with speed.

Pmb

 

[pmb]

It's just the case nowdays 'rest mass' is much preferred by physicists as a defintion for mass and the mass of a photon is always quoted as zero.

That depends on the physicist. If one is a particle physicists then I don't see why they'd want to use relativistic mass. That's not what a particle physicists studies. They study intrinsic properties of particles. For example: You'll never see the lifetime of a neutron stated as anything but 15 minutes. But that doesn't mean the lifetime increases the faster the particle moves.

However if one is into other areas of relativity then this changes. Then it finds more use. For example: if you were studying the inertial properties of a charged capacitor then you can't think in terms of rest mass or relativistic mass exclusively - both mass and energy are fully described by a second rank tensor.

Pmb

 

[marcus]

Hello Pete,
if possible I would like to contribute to making peace (as an independent agent, speaking solely for myself) and also to lend support both to minority rights and clear communication.

Ive appreciated a number of your posts---especially those which didnt depend on using the "relativistic mass" concept---and I recognize your good humor, broad knowledge, and intelligence.

The troubling issue is the DEFAULT meaning (absent some special redefintion) and the issue of proselytizing.

It is pretty clear that the majority meaning of mass, these days, is the body's "inertia at rest", and that the photon has zero mass.
It is also pretty clear that people don't want some zealous missionary coming in and trying to change the way they talk.

Tom, a working physicist I gather, has come down unequivocally on this issue. A majority of the trained people actually doing physics have dropped the "relativistic mass" idea because it is confusing or because it's awkward in a QFT context----or for the reasons Lev Okun gave in that landmark Physics Today 1989 article which you know well. Or perhaps out of respect for Einstein, who advised against using the "relativistic mass" idea.

But indeed Rindler (an eminent guy of approx. emeritus age) did
answer Okun back and give arguments favoring "relativistic mass" and moreover Rindler did come out with a 2001 edition of his book. And so on. There are respectable recent instances for you to cite. And people can't be expected to be 100% consistent.
But this doesn't change the basic linguistic picture, as I see it anyway.

I value your posts a lot and just hope that when you use the
"relativistic mass" idea you make that explicit and do not suggest that it is the predominant concept or that people should change back to using it as the default.

Quite a number of people at PF have remonstrated with you and argued with you about this, including me, and now jcsd is having the same discussion. But folks get tired of arguing, so I have been avoiding threads where you talk about mass. Jcsd SHOULD be supported in upholding the majority usage of a word, for the sake of clear communication. But if my feelings are any guide, I guess people don't want to get into it with you because it leads to just one more repetitive and useless argument.

Anyway here is the standard view of mass one more time: mass is inertia (a force per acceleration idea).
Force and acceleration are primitive measurable quantities, so inertia is right down there with the most nitty-gritty concrete measurable un-theory-dependent ideas. It is good to have some
foundations-type ideas that have clear operational meaning and don't change every time you change your theory and upgrade and so on.
People can argue about what energy is and it depends to some extent on what theory they are using and theories change, and
energy is not so directly measurable----because more abstract and general---different kinds of it.
So it is a really bad idea to confuse inertia with energy. Inertia is primitive and un-abstract. Energy is something you have to calculate based on some formulas that seem to be valid.

So a lot of people, myself included, have a strong linguistic preference---almost you could say an instinct---to keep it simple and reserve mass to mean inertia.

Not the least of them being Einstein-----BTW his 1905 paper calls it "inertia" as I recall. I think the word "inertia" even occurs in the title, instead of mass. Maybe I can get the German title of the original E=mc² 1905 paper and edit it in.

YES BY JOVE! The German title of his 1905 article Annalen der Physik volume 18 page 639 was
"Ist die Traegheit eines Koerpers von seinem Energiegehalt abhaengig?"

"Is the Inertia of a Body on it's Energy-content dependent?"

And this was where the E=mc² formula first appeared, except that, interestingly enough, he wrote it
m = L/c²

Traegheit (the word Einstein used) is a wonderful German word
meaning laziness, sluggishness, indolence, slow-to-get-started, and (in physics) resistance to acceleration.

I guess the English cognate would be "Drag-hood" or "Drag-ness" except that Traege does not mean Drag but rather
lazy, sluggish, indolent, inert, dull etc.

"Is the Sluggishness of a Body on its Energy-content dependent?"

Great language, the concretness compared with english can be stunning.

 

[pmb]

if possible I would like to contribute to making peace (as an independent agent, speaking solely for myself) and also to lend support both to minority rights and clear communication.

Peace? I didn't know it was missing? To me this is just a friendly conversation.

The troubling issue is the DEFAULT meaning (absent some special redefintion) and the issue of proselytizing.

That's because I don't believe that its true. In fact observation tells me otherwise. Just look in

assets.cambridge.org/0521422701/sample/0521422701WS.pdf

for a counter example. On page 18 Peacock uses the term "mass" and he does **not** mean rest mass. I've looked at many relativity texts, new and old, various physics journals, many online lecture notes from many universities across the world as well as spoken directly to many physics teachers. The opinions are diverse and can hardly be predicted never mind counted.

It is pretty clear that the majority meaning of mass, these days, is the body's "inertia at rest", and that the photon has zero mass.
It is also pretty clear that people don't want some zealous missionary coming in and trying to change the way they talk.

You're putting words into my mouth. I've never tried to convince anyone of any such thing. What I've done is point out facts when I've seen statements made which were not true. For example: I have one text which doesn't use the term "mass" exclusively to mean "rest mass." That's Ohanian's text. Others are not always consistent - MTW is a good example. While they use the term 'mass' to mean 'rest mass' almost exclusively they use the term 'mass' to mean what you call relativistic mass. And that's used to prove that T^0j = T^j0

Tom, a working physicist I gather, has come down unequivocally on this issue. A majority of the trained people actually doing physics have dropped the "relativistic mass" idea because it is confusing or because it's awkward in a QFT context----or for the reasons Lev Okun gave in that landmark Physics Today 1989 article which you know well. Or perhaps out of respect for Einstein, who advised against using the "relativistic mass" idea.

And I'm a physicist as well. I choose otherwise. I don't see why everyone should have the same opinion anyway. In fact I think it'd ve an extremely bad idea if I just ignored what I believe to be erroneous comments just because somone else doesn't want to to correct them. In this case the comment was made that no physicists and no journal article uses the term 'mass' to mean 'relativistic mass.' I've read many articles to the contrary. Rorhlich comes to mind off hand.

Lev Okun has made some serious errors in that paper. In fact I believe all of his arguements are flawed. And as far as Einstein goes - that famous comment that's always quoted is decieving. He did mean it for particles with non-zero rest mass. However he did not mean it to apply to light. Einstein himself used the concept of 'relativistic mass' in several of his most important and most widely known papers.

I value your posts a lot and just hope that when you use the
"relativistic mass" idea you make that explicit and do not suggest that it is the predominant concept or that people should change back to using it as the default.

What's the point of any of this? If I'm going to answer a question someone asks then I'm going to as precise as I can and use terms which I choose and not terms which Lev Okun has chosen for me. Expecially due to the fact that each of his arguements are wrong.

Quite a number of people at PF have remonstrated with you and argued with you about this, including me, and now jcsd is having the same discussion. But folks get tired of arguing, so I have been avoiding threads where you talk about mass.

You do understand that this is a discussion forum don't you? I'm not about to cease thinking the way I do because somone doesn't like a particular idea. Nobody has to discuss anything with me in fact. In fact they can just not respond.

Jcsd SHOULD be supported in upholding the majority usage of a word, for the sake of clear communication.

You've never really answered my question as to why you think that this is how the majority uses the term or why it matters what the majority does. Take a particular student - convince him that whenever he sees the term "mass" that it means "rest mass." Then how do you expect him to react when he picks upp a journal and its not used that way? We don't know who we are discussing physics with. We don't know if a particular person will want to be a particle physicists or a cosmologist. And these two groups do not use the term in the same way.

Not the least of them being Einstein-----BTW his 1905 paper calls it "inertia" as I recall. I think the word "inertia" even occurs in the title, instead of mass. Maybe I can get the German title of the original E=mc² 1905 paper and edit it in.

Speaking of which - have you read Schutz's text on general relativity? He clearly and explicitly states that "inertial mass" is frame dependant.

So long story short - I will not hesitate to correct a statement that I believe to be in error. And its not very productive to try to force others not to post ideas which they don't agree with.

By the way - Can provide proof that 99% of all physics literature (journals text etc) after 1989 use mass to mean "rest mass"? Please explain why you think most people think that way when it's quite clear that many people don't. Frankly I don't see how its possible to arrive at such a conclusion since there are millions of physicists and hundreds if not thousands of physics journals - and that's not even mentioning astronomy texts.


But here - take a browse and see
http://www.geocities.com/physics_world/relativistic_mass.htm

That's a short list. I've also seen physicists at Oxford University and Harvard University do the same thing.

There are physicists who agree with me on this point about light. E.g. From
http://www.astro.washington.edu/tmurphy/phys110/faqs/AB05.05.html

But the most honest answer to your question is yes--light has mass.

Same with mass
http://www.phys.virginia.edu/classes/109N/lectures/mass_increase.html

Mass Really Does Increase with Speed

I just happened to run across another example where the term "mass" is used and the context dictates that this mass is relativistic mass. It's in Peacock's text "Cosmological Physics." On page 18 Peacock explains the energy-momentum tensor

T^00 = c^2 x (mass density) = energy density
[...]
Both momentum density and energy flux density are the product of mass density and a net velocity,..

The terms used here mean "relativistic mass" and **not** "rest mass."

The section in which this comes from is actually online so you can check it out for yourself.
http://assets.cambridge.org/0521422701/sample/0521422701WS.pdf

This text is used in Edmund Bertschinger's cosmology course at MIT.


Pmb

 

[pmb]

Originally posted by marcus
..defined one way the "relativistic mass" of some light would be infinite if the light had any mass to begin with and defined another way the "relativistic mass" is just a redundant jargon synonym for the energy of the light and is not infinite but simply E/c².

The relation

m = m_o/sqrt[1-(v/c)^2]

Applies only to particles with non-zero rest mass. This follows from the definition of relativistic mass which is the m in p = mv. The relation p = mv holds in all cases. If the particle has a non-zero rest mass then it can be shown the m is as above. But to try to apply the relation above to light is an error. To use relativistic mass consistenty then one must rever to the definition of relativistic mass and that's the ratio of momentum to velocity (magnitudes of course).

Also - relativist mass is defined according to momentum and speed. The fact the energy is proportional to it does not mean its redunant. It's just a law of nature. In fact it would be incorrect to refer to relativist mass as energy since energy, unqualified, should only mean total energy E which is the sum of kinetic energy, rest energy and potential energy. The E in E = mc^2 is the total energy of a free-particle and not energy (as in total energy). Unfortunately the term "total energy" is an overloaded term since people use it in some cases to mean free-particle energy and in other cases to mean sum of kinetic energy, rest energy and potential energy.

Pmb

 

[theriddler876]

ummm I think that light or any energy on the electromagnetic specturm could be a sub-sub atomic particle,

say you have a radioactive substance giving off just gamma radiation, one would say that gamma has no mass and no charge, but eventually after giving enough gamma radiation it will change composition, now how can it change if it's fundamentally giving nothing, I mean I can give you a whole lot of nothing and still have some left and I wouln't change because I gave you nothing right..? so say eventually an element changed to one that was equal to one atomic number less, assuming it's neutral, then it would have lost an electron and a proton, so, perhaps radiation is made of the particles that make up a proton and electrons, which would mean the mass could be calculated by how long it takes for an amount of an element giving off gamma radiation to change, and it would be a ratio of time it takes, to amount ,or mass of the change.

 

[pmb]

Originally posted by theriddler876
ummm I think that light or any energy on the electromagnetic specturm could be a sub-sub atomic particle,..

Did someone say it wasn't? If so then its not considered a sub-atomic particle since its never actually part of an atom or nuclei. You can't crack open an atom/nucleaus and find a photon.

say you have a radioactive substance giving off just gamma radiation, one would say that gamma has no mass and no charge,..

A photon has no rest mass. However it does have mass as in mass-energy. To see what that means exactly read the following two pages
http://www.geocities.com/physics_world/sr/mass_energy_equiv.htm
http://www.geocities.com/physics_world/sr/nuclear_energy.htm

The first link has the same spirit as Einstein's first derivation except this one is more straightforward. See also
http://arxiv.org/ftp/physics/papers/0308/0308039.pdf
And note that this "mass" is the m in p = mv. So anything with momentum will have mass and light has momentum. As a quantity radiates it gives off radiation and its rest mass decreases. If the body is in motion then its momentum decreases as so does its mass-energy.

Pmb

 

[theriddler876]

well if the photon is a particle, would it still be governed by conventional laws, I.E. would it slow down and stop at absolute zero, since hypothetically a super strong grativational field affects it

 

[theriddler876]

Originally posted by pmb
You can't crack open an atom/nucleaus and find a photon.
Pmb

I was referring more like cracking open a neutron, and finding a photon, or even cracking open what we find when we crack open a neutron, and finding a photon,

According to the law of conservation of mas if you have an element to start with, if it gives off alpha, then it has lost four mass, and two atomic #, but if it gives of gamma, it has lost neither mass nor, an atomic #, like I said I could give you nothing and still have some left, however, if an element gives enough gamma, or "nothing" it will change in composition, until it becomes stable

 

[pmb]

Originally posted by theriddler876
well if the photon is a particle, would it still be governed by conventional laws, I.E. would it slow down and stop at absolute zero, since hypothetically a super strong grativational field affects it

That is untrue. There's no reason to assume that a photon behaves inertial like, say, and electron. One can't assert an electric force on a photon. However one can assert a gravitational force on a photon. And a photon in a gravitational field changes speed.

Pete

 

[pmb]

Originally posted by theriddler876
I was referring more like cracking open a neutron, and finding a photon, or even cracking open what we find when we crack open a neutron, and finding a photon,..

Actually that was how the term "photon" was first used. However it became clear that this was wrong.

According to the law of conservation of mas if you have an element to start with, if it gives off alpha, then it has lost four mass, and two atomic #, but if it gives of gamma, it has lost neither mass ..

That is not true. If a nucleus emits a gamma ray its mass **will** decrease.

What is "four mass."?

Pete

 

[theriddler876]

Well photons are not affected by an electrical forces because they Have no charge, however plasma, a fourth state of matter can be created with extreme electrical forces, such as lightning

And the electromagnetic spectrum would all be photons, just at diffrent wavelenghts

and that's was what I was saying, that it will decrease, however according to the law of conservation of mass, it shouln't, since the mass and charge of gamma are zero and atomic number and mass should be conserved

what I meant by four mass was

244Pu ----> 4Hg + 240 U
94 2 92

If you add the two, you will get that the mass and atomic number are conserved,

 

[pmb]

You're using the term "mass" in two different ways in the exact same sentance. The term "mass" has two different meanings. (If can mean m = "relativistic mass" and it can mean m_o = "proper mass" (aka "rest mass"). The proper mass, m_o, of a particle is defined in terms of the particle's free-particle energy, E, and the particle's momentum, p. They are related as

E^2 - (pc)^2 = m_o^2 c^4

m is defined as the "m" in p = mv (there is a more precise definition but this is good enough for now). It can be shown that for partilces for which m_o is not zero

m = m_o/sqrt[1-(v/c)^2]


It also follows that E = mc^2

what I meant by four mass was

244Pu ----> 4Hg + 240 U
94 2 92

If you add the two, you will get that the mass and atomic number are conserved,

In this reaction m is conservered (for the same reason energy is conserved). Not m_o. I.e. the sum of the m's are constant and the sum of the m_o's is not constant.

Not that mass is conserved when there are no external forces acting on the particle. If there is then the particle will change its velocity and the mass will change. The particle exchanges energy and momentum with the source of the force.

For a worked out example of how mass is conserved in a nuclear reaction see
http://www.geocities.com/physics_world/sr/nuclear_energy.htm

Pmb

 

[theriddler876]

well a photon would have no rest mass, well we just can measure it, since it is constantly moving however if it is a particle, then it can be stopped, probably by an extreme gravity field, and relative mass is just that, it's relative, a better way to measure it would be in quanta,

say it takes 10 seconds of light shining on a metal to give off one electron, if you know the frequency, then you can find the amount of mass, in relation to the electron, per wave, does that make sense?