Does light have mass?

From what I understand, mass is how much someting weighs at rest, so one argument can be that you can never take a weight of light because it is always moving at light speed. Another argument is and object with a given weight will have more "weight " when it is set into motion, if this is true, then if light had a weight at rest then its weight while moving at the speed of light would be damn near infinite.

Im not too sure how to explain the scale experiment from page one ot this thread, I have never witnessed it. But maybe it was the heat from the emmiting light source that caused the scale to give a reading?

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Originally posted here by volcanic
From what I understand, mass is how much someting weighs at rest

Well lets clear up your missunderstanding then, weight is a mesure of the force that gravety exerts on a mass. Mass is the amount of matter in an object. Two vastly difrent things, if you went to the moon you would weigh less but your mass wouldn't change. Hope that helps.

Correct, you said it better than I did, but that is how I understand it. Weight of an object is normally considered by taking this measurement at rest.

weight is a just a mesure of force exerted on an objcet, usualy the scal reads the force of gravety on a mass. In the light/vacume experiment it mesured the force created by packets of light energy (photons) hitting the scale, difrent type of force but still a force so it registers.

Great point volcanic:

It would be hard to measure weight of light because it has no mass.... and thus cannot have weight. Here is my understanding:

Mass is measured by the amount of matter something contains and then balancing that to known matter as compared to unkown matter . Weight is measured on a scale. You can't weigh a planet, or a black hole. You can just compare it's mass to space or other objects and base that on something bballad said.

Weight a measurement of the pull of gravity on something. Mass does not change when an object is moved from point A to B, weight on the otherhand does change. Also the more energy you add to an object in order to excellerate it, the more it weighs.

Your statement... "then if light had a weight at rest then its weight while moving at the speed of light would be damn near infinite." is true. If it had weight. Since you can't have weight without mass, I think you just added one more argument to the fact that light has no physical mass.

The light on the scale experiement was a test, as everything is tested, to see what the hell light is. It was first believed that because a beam of light measured something that is had mass. The was later found to be not entirely correct and the term "relative mass" was coinded to describe was is really the effects of propulsion of the photons and as an inverse to it's actual physical mass or "Invarient Mass."

So mass is measured by balancing and unkown matter with a known matter and weight is measured on a scale. Outside of Earth, in physics terms the unit of measure for mass is the KiloGram and the unit of weight as it's core effect by gravitational pull is the Newton . The meassurement of mass get's even deeper when comparing the different equations and methods used.

So at zero velocity invarient mass is equal to relativistic mass. I agree with that but the invarient mass of light is Zero. There fore light at rest will not have mass. There are some arguable experiments that almost touch on a way for light to have mass but not quite. Here is one from a posted link earlier:

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If we now return to the question "Does light have mass?" this can be taken to mean different things if the light is moving freely or trapped in a container. The definition of the invariant mass of an object is m = sqrt{E2/c4 - p2/c2}. By this definition a beam of light, is massless like the photons it is composed of. However, if light is trapped in a box with perfect mirrors so the photons are continually reflected back and forth in the box, then the total momentum is zero in the boxes frame of reference but the energy is not. Therefore the light adds a small contribution to the mass of the box. This could be measured - in principle at least - either by an increase in inertia when the box is slowly accelerated or by an increase in its gravitational pull. You might say that the light in the box has mass but it would be more correct to say that the light contributes to the total mass of the box of light. You should not use this to justify the statement that light has mass in general.