What is the mass of light?

Answer 1

The answers that a radiometer (more correctly, a Crookes radiometer) proves that light has mass is not correct, as shown by Einstien, Reynolds and Maxwell. Although these devices, sold as toys, often come with an explanation that this shows that light has mass, this is incorrect. Don't confuse mass with radiation pressure. The reason the vanes of the device move is complicated (look up Crookes radiometer in Wikipedia for a very good explanation). The device only works in a partial vacuum (with some air present) but not in a strong vacuum (where it should work, if the light has mass explanation was correct).

There is a very good reason why it is thought light does not have mass - for the very reason that it moves at the speed of light. If photons (the carriers of the electromagnetic force) had mass, as they approached the speed of light, their mass would increase dramatically. By convention, it is assumed that light has no mass; but is pure enrgy. (Again see 'photon' in Wikipedia).
Having said that, there is still dome debate on this point in current cosmological theories.

Answer 2

Light is a weird substance. It is BOTH energy AND mass at the same time.

Light can behave as a wave of energy.
Light can behave like a very small bullet.
All at the same time.

The device described by the previous person is called a "radiometer". And yes, with a $5 device, anyone can prove light has mass. Light will move a pinwheel, just like a stream of air or water will. It is best to have the radiometer in a vacuum chamber, that way you can prove air is not pushing the pinwheel.

When light hits a black vane of a pinwheel, most of the light sticks to the pinwheel ( it is absorbed ) like a baseball hitting a catcher's mit. The vane moves backwards, just like the baseball pushed the catcher's mit backwards.

When light hits a white vane of a pinwheel, most of the light bounces off, like a rock skipping off a pond. The light keeps most of it's energy, and goes on to somewhere else in the room. The energy of the photon is not transferred to the pinwheel vane.

Light has mass, but it is small.
But a flashlight will send billions of photon towards the pinwheel. So something small times a few billion particles = something anyone can measure with a $5 tool.

If you ask as physicist, he will say light when traveling, has no mass. But once it hits an object, it's energy can be turned into mass at the moment it strikes an object.

So light "sometimes" has mass.

Answer 3

The debate whether light has mass is still ongoing because of the fact that light is bent as it passes a large object such as a star (Einstein). Remember light has the properties of a particle and of a wave but it's something that's not wholly understood.

Answer 4

light DOES in fact have mass and you can prove it in your house right now...btw this is not my proof but i duplicated an experiment in school. take a needle and point it straight up. place a 4 pronged wire atop it suspended in balance. at each of the four ends place a circle or square of aluminum foil. face all of the reflective sides in the same ( clockwise) direction. use a black marker to dull the finish of the opposite sides. place a clear cake cover over the apparatus. NOW HERE WE GO...shine a flashlight at the shiny side of the foil. it will begin to turn. if something has zero mass it CANNOT initiate motion...this is a simple proof and IS VALID, PERIOD.

Answer 5

Light has no mass.

*for all intensive purposes. It can get somewhat "iffy" when you're talking about measuring light standing still having mass, but light never really stands still. Thus, zero mass.

Answer 6

light is form of energy and any sort of energy doesn't have any weight. its true that light is composed of particles called 'Photon'.but it has no weight when its at a speed of light.
its true for every particle according to the theory of relativity that the mass of a particle isn't determinable when it gains the speed of light.