# Does Light Have Mass?

1. Feb 3, 2004

### FUNKER

Does Light Have Mass????

does light be it in wave or quanta have mass in any situation?

2. Feb 3, 2004

### HallsofIvy

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

3. Feb 3, 2004

### FUNKER

thanks bro get tht shiz a crackin

4. Feb 3, 2004

My physics teacher says gravity anything that has a mass. According to the General Theory of Relativity, gravity bends light... but if light dont have a mass, how can this be possible?

5. Feb 3, 2004

### Staff: Mentor

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.

6. Feb 3, 2004

so the universe is a sphere?

7. Feb 3, 2004

### 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?

8. Feb 3, 2004

### chroot

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

- Warren

9. Feb 3, 2004

### HallsofIvy

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.

10. Feb 3, 2004

### Tron3k

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

11. Feb 3, 2004

### 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.

12. Feb 3, 2004

### brum

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.

13. Feb 3, 2004

### 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.

14. Feb 4, 2004

### 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.

15. Feb 4, 2004

### 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 travelling 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

16. Feb 4, 2004

### chroot

Staff Emeritus
Pressure counts too. It gets wrapped up in the stress-energy tensor, which determines the amount of gravitational curvature.
You can use the same argument, I believe. And yes, an observer outside the box will see the increased weight.
Yes. See the Pound & Rebka experiment for just one example.

- Warren

17. Feb 4, 2004

### 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.

18. Feb 5, 2004

### NateTG

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.

19. Feb 5, 2004

### Arcon

Re: Does Light Have Mass????

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).
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
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.

20. Feb 5, 2004

### Monique

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

21. Feb 5, 2004

### Monique

Staff Emeritus
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..

22. Feb 5, 2004

### Arcon

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 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
That implies relativistic mass.

23. Feb 5, 2004

### 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?

24. Feb 5, 2004

### Arcon

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.

25. Feb 5, 2004

### Monique

Staff Emeritus
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.. ;)