As we all know, the mathematical means used in the derivation of Einstein's mass-energy formula E=mc2 are so complicated.

That most of us simply don't understand! However, we all have the experience that there are often many ways to do a math problem, some of which are very simple and usually get twice the result with half the effort. Similarly, can we take a shortcut and get an E=mc2?

In fact, there is such an "unorthodox" method. Let's see:

Imagine a small ball falling on the mirror, which will put a pressure on the mirror. Similarly, does a photon hitting a mirror have pressure? 19 physical optics knows that there should be, and this pressure is called light pressure. But the intensity of light pressure is very small, and it will not knock down the mirror at all, so it is difficult for us to feel its existence in our daily life. However, the light from the sun is so strong that we can see that it is enough to push the gas of the comet and make it produce a long dazzling tail when it approaches the sun.

1899, Russian physicist Lebedev proved the existence of light pressure through experiments and found such a relationship. If we use P for light pressure, E for light energy, and C for light speed as usual, then we can get

p = 2E c

All right. Now suppose that the photon in unit time t "hits" the mirror and bounces back. The light pressure generated in this process is P, and we take the direction in which the photon "hits" the mirror as the positive direction. According to the momentum theorem we have learned (force multiplied by time equals the change of momentum), for photons, there are so

-Pt =-MC–MC =-2mc

Get rid of that negative sign,-

Pt=2mc

We said above that t is unit time, which means t= 1, so

P=2mc

Don't forget lebedev's formula of light pressure, right.

2E/c=P=2mc

About 2, multiply both sides by C.

E=mc2

Look, this "unorthodox" method seems to have some effect!

By the way, the m used above refers to the mass of photons. Photons have mass? Yes, we are talking about the gravitational mass of photons. Light has gravitational mass but no inertial mass, which is the knowledge in relativity. It is precisely because light has no inertial mass that it can move at the speed of light. In general relativity, photons have gravitational mass.

Well, it seems that "unorthodox" things are sometimes easier to understand than "orthodox" things