Does Light have Mass?

[appleciders]

I think the title covers it. I'm so confused!

[rfgdxm]

I think the title covers it. I'm so confused!
Yes.

[Shagnasty]

IANA Physicist but I can give you the very basic answer that yes, light does have mass. The powerful gravity of a black hole sucks light into it because it has mass.

[Exapno Mapcase]

No. Absolutely not. Photons do not have mass. That's the reason that they move at the speed of light, which no particle with mass can do.

They do have energy, however, which is why they behave like a particle with mass in certain situations.

See Does light have mass? (http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html) for a physicist's better explanation.

[Squink]

INANA Physicist either, but photons do NOT have mass.

Here's a concurring physicist:
Does light have mass? (http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html) The short answer is "no", but it is a qualified "no" because there are odd ways of interpreting the question which could justify the answer "yes".

Light is composed of photons so we could ask if the photon has mass. The answer is then definitely "no": The photon is a massless particle. According to theory it has energy and momentum but no mass and this is confirmed by experiment to within strict limits. Even before it was known that light is composed of photons it was known that light carries momentum and will exert a pressure on a surface. This is not evidence that it has mass since momentum can exist without mass. [ For details see the Physics FAQ article What is the mass of the photo (http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html) ?].
Also see wikipedia on
photons (http://en.wikipedia.org/wiki/Photon).

[Squink]

Curse you Exapno, and your little mapcase too! ;)

[Exapno Mapcase]

What, it took you more than 12 minutes to find that site?

Googleloser. :)

[Mathochist]

Photons have no rest mass. Also, it's a little disingenuous to distinguish mass and energy.

Oh, and Baez is a mathematician. He knows how to use logic properly :D

[Ring]

A single photon has no mass, but a system of two photons moving anti parallel to each other does have mass.

Mass and energy are not things they’re properties of a system. And a good definition of the mass of a system is the energy of that system which cannot be transformed away by a change of reference frame.

In the case of a single photon the energy can be transformed away to a level that is as close to zero as you would like. However a system of two photons moving anti parallel to each other has a zero center of momentum frame and therefore its energy cannot be transformed away. And it therefore has mass.

[Ring]

Also, it's a little disingenuous to distinguish mass and energy.


Not true. Mass and energy are most definitely not the same thing and one cannot be transformed into the other.

[CynicalGabe]

Yes.

Thats not a complete sentence!!! :mad:

[Hermitian]

Not true. Mass and energy are most definitely not the same thing and one cannot be transformed into the other.

Um, I think Einstein cleared this up 100 years ago. Energy and mass can be transformed from one to the other. This happens every time a nuclear bomb goes off.

[appleciders]

I'm sure if I read that enough times, it would all make sense, but I have at least a vague notion. Thanks!

[scr4]

Well, at least now you know it's not a simple qustion, which is a start...

Photons do not have rest mass, i.e. the kind of mass you can measure by bringing the particle to a stop and putting it on a scale. But it does have relativistic mass, i.e. energy is mass, and a photon definitely has energy. I don't think it even needs to be a system of photons - if you trap a photon in a 100% reflective box and put the box on a scale, the box will weigh more than it would without the photon.

I have no idea what Rign meant by his post, was that supposed to be a joke?

[PETERFI]

I think the title covers it. I'm so confused!

Perhaps the right question is whether Light bothers to go to Mass on Easter Sunday which is of course a Holy Day of Obligation, presuming that Light even observes. Of course, it would be rather ungreatful of Light not to go to Mass because after all, it is written,

"And God said Let there be Light! And there was Light!"

I don't want to string you along or be to particular about it but, the theories these days have me pretty quarky with all their muonces.

This silly string theory might mean that the whole mass, no mas problem might be interdimensional and we might not know here foam here from there anymore.

So to let the cat out of the bag, if Schrodingdong won't mind, it's mostly no mass, except that it could be a little, just as light acts as particles, except when its acting like waves.

Going both waves before it was fashionable, I guess.

I know I've probably just confused you, so that puts you just about in the same indeterminate state of kantiousness that I am. I mention Kant because of his Copernican Revolution. I leave it to you to google that.

The point being that we are at a state where it is possible that within 10 years or so everything we think we know that matter-s about the universe could be turned upside down again. Or at least a little inside out.

So, you've got good answers from those above.

For now.

Fondly,

Peter Pun



The Peter Files (http://ThePeterFiles.blogspot.com)

[Tangent]

Uh oh, someone GRAPED the thread!

[Desmostylus]

I have no idea what Rign meant by his post, was that supposed to be a joke?No joke. Ring is a Feynman QED hardliner, although I think Ring is still an undergrad. There's a reasonable case to be made for Ring's interpretation. E.g. in a bomb, the energy released came from the nuclear binding energy. It was energy before the explosion, and it was still energy afterward. Mass defect isn't really mass defect, it's improperly accounted for energy.

[Ring]

This has all been discussed before. In the below thread Chronos agrees with my position -- and that's all the verification I need.

http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=111712

[Exapno Mapcase]

Photons do not have rest mass, i.e. the kind of mass you can measure by bringing the particle to a stop and putting it on a scale. But it does have relativistic mass, i.e. energy is mass, and a photon definitely has energy.
Just a comment on "relativistic mass" from the Does Light have mass? link:
Sometimes people like to say that the photon does have mass because a photon has energy E = hf where h is Planck's constant and f is the frequency of the photon. Energy, they say, is equivalent to mass according to Einstein's famous formula E = mc2. They also say that a photon has momentum and momentum is related to mass p = mv. What they are talking about is "relativistic mass", an outdated concept which is best avoided [ See Relativity FAQ article Does mass change with velocity? ] Relativistic mass is a measure of the energy E of a particle which changes with velocity. By convention relativistic mass is not usually called the mass of a particle in contemporary physics so it is wrong to say the photon has mass in this way. But you can say that the photon has relativistic mass if you really want to. In modern terminology the mass of an object is its invariant mass which is zero for a photon.

ring, can you define your use of "anti parallel"?

[scr4]

http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=111712
OK, in that context I understand and mostly agree with what you're saying. I think the only disgreement is whether "relativistic mass" is a valid and useful concept. To me, a system or object with relativistic mass still acts like it has mass, and therefore it's a useful idea. Kind of like centrifugal force.

I still think you went too far with this following statement, to the point of being overly confusing in the context of this thread:
Mass and energy are most definitely not the same thing and one cannot be transformed into the other.

[Ring]

ring, can you define your use of "anti parallel"?

Moving in the opposite direction / Having a zero Momentum frame.

Desmostylus wrote:

although I think Ring is still an undergrad.

As a matter of fact I have multiple degrees, but none of them are in physics.
I'm a plant manager not a physicist, but that doesn't mean That I'm incapable of understanding the subject.

[Ring]

OK, in that context I understand and mostly agree with what you're saying. I think the only disgreement is whether "relativistic mass" is a valid and useful concept. To me, a system or object with relativistic mass still acts like it has mass, and therefore it's a useful idea. Kind of like centrifugal force.

I still think you went too far with this following statement, to the point of being overly confusing in the context of this thread:

The problem with using relativistic mass is that it causes a lot of confusion. For instance RM has no application whatsoever in General Relativity, but on this board and others I constantly see people who think that gravity is dependent on it,

I have to disagree on the mass and energy thing. They definitely are not the same thing (a single photon vs. a system of photons), and one cannot be transformed into the other.
For instance what would be the sub atomic description of exactly what is happening during the process of converting mass to energy?

[appleciders]

It's not directly related, but it's still about light; read this article. Apparently some scientists, I believe at CERN, put a beam of photons through cesium gas, and the photons went about 300c; effectively arriving before they were sent. Check this out; am I misreading this? http://www.pbs.org/newshour/bb/science/july-dec00/light_7-21.html

[silenus]

Does light have mass?

I didn't even know light was Catholic! :D







Oh, come on...someone had to say it!

[appleciders]

And this: http://www.sciencenews.org/articles/20000610/fob7.asp
And this: http://www.soulinvitation.com/philight/ although that one's way over my head.

[Stranger On A Train]

Not true. Mass and energy are most definitely not the same thing and one cannot be transformed into the other.
Er, no. Consider the resultant of the interation between an electron and a positron. The products are photons. If mass and energy are not two different forumlations of the same fundamental quality, then where does the mass go and where does the energy come from?

I think the argument for a fundamental distinction betwen mass and energy is as disingeneous as the "particle/wave duality". There is no duality; it's just that it is mathematically convenient sometimes to describe the particle in terms of a single, indivisible chuck and sometimes as a quantitized wave. Similarly, depending on how you are measuring or observing an interaction, it may be more appropriate to speak of the "mass" of a particle or of its "energy". If you are dealing with a mechanical interaction we'll speak of mass, or momentum, or kinetic energy. thermodynamic effects, potential or binding energy is (generally) more appropriate.

We often speak of subatomic particles has having a "mass" in eV. No problem there. However, for practical purposes, it isn't really very useful to speak of a photon's "relativistic mass", since you can't measure its mass directly and it doesn't jibe well with QM. However, we often speak of the momentum of a photon in terms of its vector and frequency in ways that are equivilent to meffv. Our daily interactions with photons lend themselves to a thermodynamic view in which the momentum of the particle is less important than the energy liberated by being absorbed and re-radiated, but there are situations, such as in the middle of a nuclear bomb, or the light pressure on celestial objects in which the momentum of a photon is a mechanical consideration.

Stranger

[Ring]

Er, no. Consider the resultant of the interation between an electron and a positron. The products are photons. If mass and energy are not two different forumlations of the same fundamental quality, then where does the mass go and where does the energy come from?

Stranger

Jeez Stranger didn't you read the thread? A system of photons with a zero momentum frame has mass.

The mass didn't go anywhere it's exactly the same as before the interaction and so is the energy. They're properties of the system, and as long as you are consistent in defining the system its mass and energy cannot change.

A hot object is more massive than the same object cold.

A perfectly reflecting box with a photon bouncing around inside has more mass and higher inertia than the same box without the photon.

If a nuclear weapon is detonated in a vault the vault weighs the same before and after the detonation.

[Chronos]

In the case of a single photon the energy can be transformed away to a level that is as close to zero as you would like. However a system of two photons moving anti parallel to each other has a zero center of momentum frame and therefore its energy cannot be transformed away. And it therefore has mass.You can make this a little stronger, even. For any two photons which are not moving parallel to each other (that is, in exactly the same direction), there is some reference frame where they're moving directly away from each other, and the net momentum is zero. So almost any system of two or more photons has nonzero mass.

As for the mass/energy distinction, I think it's best to say that mass is a subset of energy. That is, all mass is energy, but not all energy is mass. For instance, suppose we look at a pion decaying into two photons. If we look at the pion as a system, it has energy, and all of its energy is mass. If we look at the two photons afterwards as a system, then it has the same amount of energy, and that energy is still all mass. So in that sense, you can never lose mass. But if we look at one of those photons at a time as a system, it has half the total energy, but that photon by itself has no mass. How much mass you have depends on how you define your system. And I think there is a practical distinctinction between the mass you have in the pion and the mass you have in the two-photon system afterwards: You can't break up the pion into smaller subsystems each of which is individually massless, but you can do so with the two-photon system. This distinction is generally described in terms of "converting mass to energy", an admittedly sloppy piece of terminology, but it works.

[Chronos]

Oh, and appleciders, I don't think you're misreading that article, but the article is miswritten. There have been many experiments in the past few years which purport to show light travelling faster than c, but in all cases, it's not, but just gives the appearance of it. In most of these experiments, a pulse of light is sent through a chamber, and the front end of the pulse exits the chamber before the back end of the pulse enters it. This, by itself, is completely, wholly unremarkable. The only thing that makes these experiments notable at all is that something in the chamber causes the pulse of light to change shape, so that the front of the pulse coming out looks like the back of the pulse did going in. So it looks like the back end of the pulse (or for that matter, any other part of the pulse) is coming out before it went in, which (if it were true) would be quite remarkable. But it's only an illusion, caused by the way the pulse changes shape in the chamber.

[Ring]

Well Chronos I’m certainly not going to argue with you, but I still think the term is extremely misleading and I believe it’s intimating that something is occurring that that most definitely is not.

It causes people to think of mass as some sort of a thing that evaporates or something, and then energy pops out. Whereas in fission, for instance, it would seem that that it’s much clearer to envision the nucleus rearranging itself into a configuration of higher binding energy and thus releasing the decreased potential energy as electromagnetic and kinetic energy.

The local mass defect can then be considered as nothing more than the consequence of the energy release, not the cause. But nonetheless I see your point and it’s certainly a valid one.

[rwjefferson]

.05.03.26

If mass and energy are not two different forumlations of the same fundamental quality, then where does the mass go and where does the energy come from?
The mass (gravity) becomes an equi-valent amount of anti-mass (energy; anti-gravity). This transformation releases of a dimensional amount of stored energy.

IIANM; it now seems the anti-gravity released since creation is accelerating the expansion off our universe.

Peace through Liberty
r~

[Stranger On A Train]

The mass (gravity) becomes an equi-valent amount of anti-mass (energy; anti-gravity). This transformation releases of a dimensional amount of stored energy.

IIANM; it now seems the anti-gravity released since creation is accelerating the expansion off our universe.
Wha--huh?

This is an attempt at a woosh, right?

Stranger

[rwjefferson]

Wha--huh? This is an attempt at a woosh, right?I may be mistaken, but I believe the data show the universe is not only expanding, but that the rate of expansion is accelerating. It seems to me that the idea that best fits, relies the existence of a "dark energy". This dark energy must possess properties opposite gravity (anti-gravity?) to counter the Big Crunch.

Is it your understanding that visible energy does not share this property?

r~