Does sunlight has weight?

Answer 1

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".

Light is composed of photons, so we could ask if the photon has mass. The answer is then definitely "no": the photon is a massless particle. According to theory it has energy and momentum but no mass, and this is confirmed by experiment to within strict limits. Even before it was known that light is composed of photons, it was known that light carries momentum and will exert pressure on a surface. This is not evidence that it has mass since momentum can exist without mass.

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 both directions symmetrically in the box, then the total momentum is zero in the box's 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 the greater force required to accelerate the box, 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.

Answer 2

Those who invoked m = E/c^2 to claim weight for sunlight are misinformed. m in the equation is so-called rest inertial mass. Inertial mass is the kind of mass that has weight, but the energy of sunlight is E = hf; where h = Plank's constant and f = light frequency. In the case of sunlight, it's made up of lots of frequencies; so there are lots of energy levels.

From the two equations, we have mc^2 = hf; so that m = hf/c^2 and this is a virtual mass having no inertia. This can be validated by the fact that light travels at the speed of...well, light. If this virtual mass were inertial mass, the m would be infinite. Since that is not the case, the m is virtual, not inertial.

As W = mg; where weight is inertial mass times acceleration due to gravity, light has no weight because its mass is virtual, not inertial. By the way, light does have momentum since mc = hf/c = p = momentum. Force is the first derivative of momentum over time; thus, dp/dt = dmc/dt = d(hf/c)/dt; thus the force (virtual weight) of photons is due to changing frequencies as the photons strike something. This results from the fact that frequency is the only variable in (hf/c). h and c are constants.

Answer 3

Weight = Mass times acceleration. Solving for Einstein's equation:
E = mc^2
m = E/c^2
But C = speed of light
so
Weight = E*c/c^2
= E/c
So the answer is: yes, sunlight does have weight. But can you imagine the infinitesimally small amount?
Energy produced by sunlight is in the order of mJ, or millijoules. c is 7.8x10^8. So dividing a milli by an eight power gets 10^-11. So the weight of sunlight is about 10^-11 kilograms, which means about 10^-14 grams, or 0.0000000000001. Damn that's light!

Answer 4

yes.
As a metter of fact I was watching a Nova episode on Einstein last night which said that the earth is hit by 4 1/2 pounds of light every second.

Answer 5

Yes. But it is very very very small. All the sunlight reaching the earth in a day weighs about 367,000 pounds.