Understanding the Mass of Light: Wave or Quanta?

[FUNKER]

Does Light Have Mass?

does light be it in wave or quanta have mass in any situation?
please any answers would be appreciated

 

[HallsofIvy]

No, light does NOT have mass. Haven't we had this question a number of times?

 

[FUNKER]

thanks bro get tht shiz a crackin

 

[Link]

My physics teacher says gravity anything that has a mass. According to the General Theory of Relativity, gravity bends light... but if light don't have a mass, how can this be possible?

 

[russ_watters]

Originally posted by Link
My physics teacher says gravity anything that has a mass. According to the General Theory of Relativity, gravity bends light... but if light don't have a mass, how can this be possible?

In high school, you generally don't go past the Newtonian theory of gravity. It works in a lot of cases, but isn't quite right for all.

In Einstein's gravity, the gravitational field around an object with mass is a curvature of space, much like the way a bowling ball curves a trampoline. Light travels in a straight path in space but to us, with space itself curved, it looks bent.

 

[Link]

so the universe is a sphere?

 

[brum]

in a bomb in which some atoms are converted to light & energy (a la E=mc^2), how does that satisfy the conservation of mass then?

 

[chroot]

There is no such thing as a conservation of mass. There is a conservation of mass-energy, the sum of the two combined.

- Warren

 

[HallsofIvy]

My physics teacher says gravity (affects? attracts? verb missing!)anything that has a mass.

Okay, and that's true (hey, I wouldn't argue with your physics teacher!) but your physics teacher did not say that that was all that was affected by gravity. In the general theory of relativity, gravity works by changing space-time itself and has nothing to do with "mass". In fact, a major impetus for the creation of general relativity was the fact that mass always cancels out of problems involving gravity.

 

[Tron3k]

Isn't it interesting that a box filled with light is literally heavier than an empty box?

 

[wasteofo2]

That could actually be true, but the difference would be negligable.

I saw a special on the discovery channel about the future of space travel, and one proposition was a very small craft with a giant sort of parachute that would be propelled away from the sun based solely on the energy from the sun bouncing off of it and pushing it forward.

So this same sort of force could push down on a box slightly. Certainly nothing that you'd even care to, or be able to measure without some insance scale.

 

[brum]

Originally posted by wasteofo2
I saw a special on the discovery channel about the future of space travel, and one proposition was a very small craft with a giant sort of parachute that would be propelled away from the sun based solely on the energy from the sun bouncing off of it and pushing it forward.

So this same sort of force could push down on a box slightly. Certainly nothing that you'd even care to, or be able to measure without some insance scale.

likewise, the light would bounce of the top of the box. and the sides. (and thus, no net force)


on the parachute, that's a different story.

ever see one of those spinning things in a glass dome, about the size of your fist, that has 4 spinning squares rotating around an axis? and the 4 squares are silver-ish on one side & black on the other side, which causes the 4 squares to move in the same direction around the axis because of the sun's light being absorbed by the silver side and not by the black side.

or something like that.

 

[neutroncount]

Not quite Brum. That trick is done in a partial vacuum and is because of a temperature differential. A solor sail deals with momentum of photons being imparted onto the sail. The force is very light but given a large and long enough source of light, the final velocity can be very quick.

 

[gmorgan]

The interesting thing about light is it has no mass but has momentum determined by De Broglies(momentum=Planck's constant/wavelength) equation from QM. De Broglie believed that light did have a tiny mass such that it would be slightly slower than what we call the speed of light, however, this mass or speed difference has never been detected.

As for the rotating thing. Generally they are carnival tricks caused by temperature differentials you can however do it with a laser if the turntable is in a vacuum.

 

[rocketcity]

Why is a box of light heavier than a box of dark?

If I'm understanding this correctly, it's *not* because of light pressure (pressure is a scalar and thus non-directional) but because the captive photons, just like photons traveling through free space, are 'pulled towards' massive objects by the curvature of space.

*Would* this increase the measured weight of the box, to someone looking at a scale outside the box (even assuming an ideal scale)? I'm trying to compare this to *gas molecules* inside a box increasing its weight just by using a kinetic theory argument, i.e., the molecules collide with the bottom of the box more frequently, or at higher speeds, than those that strike the top of the box.

Is a similar phenomenon responsible for the 'weight' of the light?

Related question : if you fire a 5E15 Hz laser beam straight up from a massive object, will the light have a lower frequency at infinity due to loss of energy in climbing out of the gravity well? (S'pose the massive object is stationary in an inertial reference frame, and the observer at infinity is also similarly stationary.) (Can you even make that assumption, since massive objects inherently make reference frames non-inertial?)

P

 

[chroot]

Originally posted by rocketcity
If I'm understanding this correctly, it's *not* because of light pressure (pressure is a scalar and thus non-directional) but because the captive photons, just like photons traveling through free space, are 'pulled towards' massive objects by the curvature of space.

Pressure counts too. It gets wrapped up in the stress-energy tensor, which determines the amount of gravitational curvature.

*Would* this increase the measured weight of the box, to someone looking at a scale outside the box (even assuming an ideal scale)? I'm trying to compare this to *gas molecules* inside a box increasing its weight just by using a kinetic theory argument, i.e., the molecules collide with the bottom of the box more frequently, or at higher speeds, than those that strike the top of the box.

You can use the same argument, I believe. And yes, an observer outside the box will see the increased weight.

Related question : if you fire a 5E15 Hz laser beam straight up from a massive object, will the light have a lower frequency at infinity due to loss of energy in climbing out of the gravity well?

Yes. See the Pound & Rebka experiment for just one example.

- Warren

 

[Tron3k]

If you guys really want to know why a box full of light has more mass than an empty one, read this paper:

Light is Heavy

It's an excellent analysis.

 

[NateTG]

Originally posted by brum

ever see one of those spinning things in a glass dome, about the size of your fist, that has 4 spinning squares rotating around an axis? and the 4 squares are silver-ish on one side & black on the other side, which causes the 4 squares to move in the same direction around the axis because of the sun's light being absorbed by the silver side and not by the black side.

A radiometer?

There's a long story about how they work. It turns out that the effect has to do with interactions that occur at the edges of the blades, and not do to pressure differences caused by heat.

 

[Arcon]

Originally posted by FUNKER
does light be it in wave or quanta have mass in any situation?
please any answers would be appreciated

That depends on how you define the term mass. Light does have a non-zero relativistic mass (aka inertial mass) and has zero proper mass. Relativistic mass is the coefficient of proportionality between relativistic momentum and 3-velocity. I.e. it is the m in p = mv. It also has a non-zero active gravitational mass (it generates a gavitational field) and a non-zero passive gravitational mass (it is deflected by a gravitational field).

link wrote

My physics teacher says gravity anything that has a mass. According to the General Theory of Relativity, gravity bends light... but if light don't have a mass, how can this be possible?

You're mixing up the different meanings of the term "mass." Light does have mass because it has energy. As Feynman said in the Feynman Lectures Vol -I page 7-11, Section entitled Gravitation and Relativity

One feature of this new law is quite easy to understand is this: In Einstein relativity theory, anything which has energy has mass -- mass in the sense that it is attracted gravitationaly. Even light, which has energy, has a "mass". When a light beam, which has energy in it, comes past the sun there is attraction on it by the sun.

russ watters wrote
In Einstein's gravity, the gravitational field around an object with mass is a curvature of space, much like the way a bowling ball curves a trampoline. Light travels in a straight path in space but to us, with space itself curved, it looks bent.

The curvature in Einstein's GR is an analogy taken from geometry. And it is not spacetime curvature that is responsible for deflection of light in a gravitational field. It is the gravitational force that is responsible. Spacetime curvature is just another name for tidal force. But there is no need to assume that tidal forces are present in order for there to be a gravitational field. E.g. there is no spacetime curvature in a uniform gravitational field and yet light is still deflected. And light is deflected because it has non-zero relativistic mass.

chroot wrote
There is no such thing as a conservation of mass. There is a conservation of mass-energy, the sum of the two combined.

That is incorrect. In fact many derivations on relativity were based on the postulate of conservation of mass. Even if one wishes to define "mass" as "invariant mass" as Taylor and Wheeler do in Spacetime Physics Second Edition there is still mass conservation. I.e.
http://www.geocities.com/physics_world/stp/pg_249.htm

"Thus part of the mass of constituents has been converted into energy; but the mass of the system has not changed/"

By the way, "mass-energy" is a synonym for "relativistic mass"

HallsofIvy

In the general theory of relativity, gravity works by changing space-time itself and has nothing to do with "mass".

That is incorrect. Mass acts as the source of gravity in the same way that charge acts as the source of the EM field. The complete description of mass is the energy-momentum tensor (or mass tensor if you prefer) just as the complete description of charge is the 4-current 4-vector.

Since the time-time component of the stress-energy-momentum (SRM) tensor is proportional to relativistic mass, i.e. T⁰⁰ = c²(mass density), and since the SEM tensor is divergenceless it follows that mass is conserved. But then again that tensor is almost designed that way so one expects that to me the case. So long as mass is defined so that p = mv (p and v are 3-vectors) is conserved then mass is conserved.

For details see Cosmological Principles, Peacock, Cambridge Univ. Press, (1999). See pages 17-18
http://assets.cambridge.org/0521422701/sample/0521422701WS.pdf

rocketcity

Why is a box of light heavier than a box of dark?

Because the light transmits forces to the walls of the box and it is that transfer of momentum that causes the box to weigh more. This should be measureable in principle. There was a paper published on this subject in the American Journal of Physics. It's called

The mass of a gas of massless photons, H. Kolbenstvedt, Am. J. Phys., 63(1), Jan 1995

chroot wrote

Pressure counts too. It gets wrapped up in the stress-energy tensor, which determines the amount of gravitational curvature.

Pressure is just one component of the SEM tensor. And you need to SEM tensor to describe the inertial mass of a box. Not just to describe the gravitational mass of a body. If the SEM tensor is divergenceless then the mass of the entire system will always transform as M = gamma*M₀.

*Would* this increase the measured weight of the box, to someone looking at a scale outside the box (even assuming an ideal scale)? I'm trying to compare this to *gas molecules* inside a box increasing its weight just by using a kinetic theory argument, i.e., the molecules collide with the bottom of the box more frequently, or at higher speeds, than those that strike the top of the box.

Yes. That is correct.

 

[Monique]

Originally posted by HallsofIvy
No, light does NOT have mass. Haven't we had this question a number of times?

doesn't energy automatically have a mass associated with it? e=mc2? Arcon made a good reply..

 

[Monique]

Ah wait, invariant mass and relativistic mass are very different things :) where invariant mass would be the inertia of the body at rest? Apparently Einstein explicitly adviced against using relativistic mass..

 

[Arcon]

Originally posted by Monique
Ah wait, invariant mass and relativistic mass are very different things :) where invariant mass would be the inertia of the body at rest? Apparently Einstein explicitly adviced against using relativistic mass..

Yes. Invariant mass, m₀ (aka "rest mass," "proper mass") is different than relativistic mass, m(v). In special relativity they are related by

$$m(v) = \frac { m_{0}}{\sqrt{1-v^{2}/c^{2}}}$$

Einstein didn't exactly advise against relativistic mass. He advised against a mass M = m_o/sqrt[1-(v/c)^2]. So in special relativity he did advise agains this usage. But relativistic mass has the value

$$m = m_{0}\frac{dt}{d\tau}$$

In the case of a slowly moving particle in a gravitational field the mass is a function of the gravitational potential and is therefore different from rest mass. Einstein discussed this in his book on relativity The Meaning of Relativity in connection with Mach's Principle.

If you take a look at Thorne and Blanchard's new book which is located online. For example see
http://www.pma.caltech.edu/Courses/ph136/yr2002/chap01/0201.2.pdf

You'll see that the authors use the term "rest mass" when they mean rest mass. They do not mean invariant mass at any place when the simply write "mass." But I do see them use the term "mass" to it mean "relativistic mass", i.e.

..in the first expressioin dE is the total energy (or equivalently mass) in the volume dx dy dz. ...

In other chapters they use the term "mass-energy" i.e. in
http://www.pma.caltech.edu/Courses/ph136/yr2002/chap23/0223.1.pdf
they write

Moreover, the total density of mass-energy rho as measured in the fluid's local rest frame can be expressed as the density of rest mass-energy rho₀ plus the density of internal energy rho₀u

$$\rho = \rho _{0}(1+u)$$

That implies relativistic mass.

 

[Nim]

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chroot wrote
There is no such thing as a conservation of mass. There is a conservation of mass-energy, the sum of the two combined.


--------------------------------------------------------------------------------


That is incorrect. In fact many derivations on relativity were based on the postulate of conservation of mass. Even if one wishes to define "mass" as "invariant mass" as Taylor and Wheeler do in Spacetime Physics Second Edition there is still mass conservation. I.e.
http://www.geocities.com/physics_world/stp/pg_249.htm

"Thus part of the mass of constituents has been converted into energy; but the mass of the system has not changed/"

By the way, "mass-energy" is a synonym for "relativistic mass"

I always figured that they said mass is conserved because even though you can decrease the amount of mass in the universe, that decrease corrisponds to an increase in energy which can be converted back to mass. Is mass conserved because "anything which has energy has mass" and so the increase in energy gives rise to an increase in mass that equals the amount of mass that was converted into energy?

 

[Arcon]

Originally posted by Nim
I always figured that they said mass is conserved because even though you can decrease the amount of mass in the universe, that decrease corrisponds to an increase in energy which can be converted back to mass. Is mass conserved because "anything which has energy has mass" and so the increase in energy gives rise to an increase in mass that equals the amount of mass that was converted into energy?

That's difficult to answer until you define what you mean by "mass."

If you simply calculate the total value of the relativistic mass then that sum is a constant. To see how this works in application to, say, nuclear energy see

http://www.geocities.com/physics_world/sr/nuclear_energy.htm

Trying to define "mass" as the total energy in rest frame divided by zero and then at the same time trying to do away with relativistic mass is quite illogical. For there to be a mass-density there must be relativistic mass since such a density implies relativistic mass since it associates a location with the mass and this implies relativistic mass.

 

[Monique]

Relativistic mass is dependent on kinetic energy, which is dependent upon your frame of reference. Why can't a photon have an invariant mass m₀?

I think the answer lies somewhere in this sentence "For there to be a mass-density there must be relativistic mass since such a density implies relativistic mass since it associates a location with the mass and this implies relativistic mass." by Arcon.. ;)

 

[Monique]

Originally posted by Monique
Relativistic mass is dependent on kinetic energy, which is dependent upon your frame of reference. Why can't a photon have an invariant mass m₀?

Oh wait, never mind: speed of light is constant I'm a little confused at the moment :)

 

[Arcon]

Originally posted by Monique
Relativistic mass is dependent on kinetic energy, which is dependent upon your frame of reference. Why can't a photon have an invariant mass m₀?

I think the answer lies somewhere in this sentence "For there to be a mass-density there must be relativistic mass since such a density implies relativistic mass since it associates a location with the mass and this implies relativistic mass." by Arcon.. ;)

The term "invariant mass" refers to the m₀ in

$$E^{2} - (pc)^{2} = m_{0}^{2}c^{4}$$

This relation can be derived from

$$E = \frac{m_{0}}{\sqrt{1-v^{2}/c^{2}}}$$

The relationship between energy and momentum for radiation is E = pc. This was demonstrated by Poincare in 1900. Substitute this into the first equation and you'll get

$$E^{2} - (pc)^{2} = 0$$

It's not a really valid thing to do since the first relation above is derived with the assumption that m₀ is not zero. But that is the terminology that we're stuck with!

 

[decibel]

SO at the end of all this...light does NOT have mass ( real kind of mass)...?

 

[Arcon]

Originally posted by decibel
SO at the end of all this...light does NOT have mass ( real kind of mass)...?

I don't know why you'd conclude that. What is it you mean by "real kind of mass"? If you mean "proper mass" when that was already stated a while back - light has zero proper mass.

 

[Integral]

Note to Arcon & DW.

DO NOT START YOUR DEBATE HERE!

Integral

 

[Monique]

Originally posted by decibel
SO at the end of all this...light does NOT have mass ( real kind of mass)...?

To end all of this? Isn't a person allowed to understand? Isn't this a discussion forum? Oh well..

 

[chroot]

Originally posted by Monique
To end all of this? Isn't a person allowed to understand? Isn't this a discussion forum? Oh well..

Monique -- what do you not understand? We'll certainly try to help.

- Warren

 

[Monique]

thanks Warren, I think I have to read up and refresh my mind before I can make a sensible discussion :)

 

[Arcon]

Originally posted by Monique
To end all of this? Isn't a person allowed to understand? Isn't this a discussion forum? Oh well..

I've never understood the desire to end a polite conversation when people still wanted to discuss it. But such is life!

This is one of those topics which has been debated in the physics literature for many decades and has been rather heated even in that literature.

However there is always PM. But I'll do whatever I can to clarify anything.

 

[Michael F. Dmitriyev]

Originally posted by HallsofIvy
No, light does NOT have mass. Haven't we had this question a number of times?

We’ll check up it now.
$$E=mc^2$$ (1)
Is it right?
$$E=h \nu\$$ (2)
Is it right?
When
$$mc^2=h \nu\$$
or
$$m=k\nu\$$ (3)

here k= h/c^2
m – the rest mass.
So?