Does light have mass?

Question

The only way I can concieve of testing out a theory of light having mass is by building a very long tunnel under the Earth's crust. After the tunnel is built, many sensors would be put in the long tunnel to measure a laser from one end of the tunnel to the other end. The sensors would then be able to pick up whether or not the laser bent from the Earth's gravity. Even if it showed a .0000000001... bend, wouldn't this be theoretical proof that light does have mass?

Answer 1

Light does not have mass (its rest mass is zero...it has to be in order for it to travel at the speed it does).
No object with mass can ever travel at (or faster than) the speed of light in a vacuum since to do so it would take an infinite amount of energy....and in the entire universe there is no an infinite amount of energy.


Light does, however, have momentum since it posses a certain amount of energy.

The energy contained within a single photon of light is given by,
E = hv
Where E is the energy, h is Planck's constant, and v is the frequency of the associated light.

E^2 = (mc^2)^2 + (pc)^2
where E is energy, m is the rest mass, c is the speed of light in a vacuum, and p is the momentum.

Since light has no rest mass, the (mc^2)^2 term drops out and we get,
E = pc,
solving for momentum (p),
p = E / c
Plugging back in for E,
p = hv / c
which is the same as saving,
p = h / lambda,
where lambda is the wavelength of light.

h = 6.63 E-34 J s

http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html

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On a different note, general relativity tells use that gravity will bend light...an effect known as gravitational lensing.
But this does not mean that light has mass. This phenomenon has more do with the way mass bends space-time and thus the path light must take in order to travel from one location to another.
http://astro.berkeley.edu/~jcohn/lens.html

Answer 2

Nope. Gravity bends space, and light follows space. So that wouldn't test for light having mass. But we have other tests for gravity bending space - look up Gravity Probe A.

Classically speaking, light doesn't have mass. That's a major factor in quantum mechanics and relativity theory. However, new observations (I read about this somewhere ... maybe Physics Today last year sometime) suggest light might actually have a very miniscule amount of mass - on the order of 10^-61 grams or something per photon. We'll see if their ideas/observations pan out.

Answer 3

I am afraid it would not prove light has mass. Bending of light by gravity has already been proved. (During solar eclipses, the stars visible just outside the Sun's eclipsed disk appear cluttered than when the Sun is away. This experiment proved Einstein's theory that gravity bends light.)

But it does not prove light has mass. It only proves light follows the curvature of space and this curvature is caused by gravity - more the gravity more will be the curvature.

Answer 4

If the light did bend it would be from gravity warping space time not from gravitational "force"on the photons, according to Einstein's theory of general relativity, it's what made him famous it's call gravitational lensing. there's also something about mass increasing as it approaches the speed of light so if photons did have mass the could never reach the speed of light.

Answer 5

black holes bend light... and we've detected gravitation lensing effects throughout the galaxy... we can observe these already with no further apparatus needed.

But... supposing gravity is just the warping of space/time due to mass... does a thing need mass to be affected by gravity?

Answer 6

Photons are particles which have mass. They will be effected by gravitational attraction. Electrons, ect. also display the same properties, as they are atomic particles. Very short wave length electro-magnetic radio waves such as microwave are less succeptiple to the effects of gravity which is why they are used for line of sight applications such as satellite communications. Longer wavelengths tend to be (excuse the analogy) like holding on to the end of a long stick.

Answer 7

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Answer 8

Nope.
It is already known that gravity bends light...

Answer 9

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".

Answer 10

light have mass,. you don't need to build a big tunnel