- #1
benzun_1999
- 260
- 0
dear reader,
what is the mass of light particels?
what is the mass of light particels?
Originally posted by jcsd
relativistic mass is not the same as mass and is hardly ever used.
Originally posted by Doctor Luz
Relativistic mass, mass at rest, mass... It's a question on nomenclature.
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.
Originally posted by selfAdjoint
And the "relativistic mass" is wrong because it would be infinite.
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.
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/c^{2}.
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.
Originally posted by jcsd
Photons have zero mass.
Originally posted by jcsd
relativistic mass is not the same as mass and is hardly ever used.
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.
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
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.
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.
Originally posted by 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'.
Originally posted by jcsd
Here's an artilce discussing the use of the term 'mass':
http://www.weburbia.demon.co.uk/physics/mass.html
Sometimes people say "mass" when they mean "relativistic mass", ..
http://www.fnal.gov/pub/inquiring/questions/accel_mass.htmlSome 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_obj.htmlOne 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.
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.
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.Originally posted by pmb
What difference does it make when I learned physics? The fact is that mass is often defined in different ways
This is incorrect.
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
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.)
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.
the problem is you change refernce frames the relativitic mass changes so it's not that useful, weight is diferent from mass anyhow.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'?
These are very basic explantions aimed at the general public, not at scientists.
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.
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.
I'm sorry these are both correct statements, relativitic mass is an almost redunant concept these days.
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 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.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.
These are very basic explantions aimed at the general public, not at scientists.
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?
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.Originally posted by jcsd
So do you haven't any formal training in relativity then?
I already told you. Rohrlich was an example.'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.
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.