Does light have mass???

The answer to the raised question is obvious and we all know that light is a form of energy coming from the sun. From this blog, you will learn how one of the most famous equations in the World is somehow a lie or a misconception.


Light comes from the nuclear fusion that takes place in the sun. During the fusion, 4 hydrogen nuclei are fused together under huge pressures to form one Helium. This can be illustrated by the diagram on the next. It can be perceived that there is a loss of mass by the conversion of the 4 hydrogens to the 1 helium.
On average, the sun is losing mass at a rate of approximately 4300 000 000 kg/s. This strains credulity but it is a fact.
The question is 'Where does all this mass go?'.
Well, all this mass is converted into light and light consists of small packets of energy called photons. According to one of the most famous equations on Earth (E=mc^2), we consider mass and energy to be equivalent(mass-energy equivalence) and hence anything that has mass must have energy and vice-versa.
So, does light has mass?

CRISIS
Let us jump back to 1915 when Albert Einstein developed the theory of general relativity. It stated that the gravitational effect between masses was due to the bending of the space-time fabric. Due to general relativity, we know that gravity also has an effect on light and that light gets totally absorbed by a black hole. This may suppose that light has weight as gravity has an effect on it.
Moreover, let us consider inertia. You must know that inertia is the reluctance of a body to change its state of motion(in accordance with Newton's first law) or rest and also that inertia is directly proportional to the mass.
This implies the greater the mass, the greater the inertia.
Well, light always travels at a constant speed(speed of light) and in a straight light and we cannot alter this speed. So applying the theory of inertia to it, we would possibly say that it has infinite inertia and hence infinite mass right?
But.....
Something must be definitely wrong.
What if light has another physical property that we are not considering?


Solution
It is difficult to lend credence to, but one of the world's most famous equation(E=mc^2) is quite of a lie or a misconception. As already stated above, we know that light energy comes from nuclear fusion and by conservation of energy, this energy cannot just disappear. Moreover, according to the mass-energy equivalence (E=mc^2), the light should have a mass which is not the case and this proves the mass-energy equivalence principle to be wrong.

The real equation looks like the equation on the left, where m is the rest mass of an object and ρ is momentum.
For an object of mass m, if the mass would be converted totally to energy, an energy of mc^2 would be released. The new equation also makes it possible for mass-less particles to have energy, like photons(light is made up of small packets of energy called photons or quanta).
Light has energy without mass and as per the new equation, the other physical property that we were not considering was momentum. And yes light has momentum as photons have momentum.

How can photons have momentum???
If photons did not have momentum, electrons would not have been knocked off a metal once the latter is irradiated by photons.
This is best explained by the COMPTON EFFECT.(click for more details).
In 1923, Arthur Molly Compton(American Physicist) and Peter Debye(Dutch-American Physicist and Physical Chemist) independently carried Einstein's ideas of photon's momentum further. They then realized that the scattering of x-ray photons(as shown in the diagram on the right) from electrons could best be explained by treating photons as point-like particles having energy hf and momentum hf/c or (h/λ)

Like elementary particles, photons are currently best explained by quantum mechanics and exhibit wave-particle duality and it would be an aberration to think that the momentum of photons is to be calculated using mv(mass x velocity) as they exhibit wave-particle duality and hence we must use the De Broglie's equation:
momentum(ρ)=Planck's constant(h)/wavelength(λ in metre)

Well, one could say that if photons had momentum, we would have felt a push on us light falls on us.
The Planck's constant is of order 10^(-34) and hence it can be perceived that the momentum of photons is so small that we can't experience any kind of push when light falls on us. But there are various other experiences which show that photons have momentum.

That's the end of it.
I hope that this short explanation was helpful and that it broadened your thought about the mass-energy equivalence.
For any questions, do not hesitate to leave a comment and for more science blogs, click here.

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