Why does light show a no interference result?

I was reading the first chapter in the Feynman lectures volume 3 about quantum mechanics and I understand the concept that 'interference' means $$P_{12}=P_1+P_2$$ as expected, however when the electron gun experiment is done with a light source behind the two holes the graph shows a no interference result, isn't the light supposed to interfere with the electron?

• Doesn’t Feynman go on and explain what is going on? Anyway, the light bouncing off the electrons would allow one in principle to tell which slit the electron went through. This is the famous “wavefunction collapse”. Now the two paths cannot interfere with each other. – Andrea May 22 at 17:21

1 Answer

Interference is with a single type of wave, so sound waves interfere with other sound waves, and light waves interfere with other light waves. Generally sound waves don't interfere with light waves and vice versa. In the formula you wrote it wouldn't make sense to add $$P_1$$ and $$P_2$$ if $$P_1$$ is the pressure of a sound wave and $$P_2$$ is the strength of the E-field, it would be adding apples and oranges (the units don't even match).

The electron wave is not the same type of wave as a light wave, so you would not expect that the electron wave would interfere with the light wave or vice versa. Now, of course, electrons and light do interact with each other, but that interaction is not a simple interference.