The double slit experiment shows that photons have a wave nature and a particle nature. If the slit size and distance between slits is comparable to the frequency of the light, then with only one slit opened, the phtons will behave as a particle and the "detector" will show a pattern similar to any other particle shot through one slit such as a bullet. With the two slits open, the photon will show an interference pattern on the detector. This pattern will be similar to that obtained from a wave such as ripples of water such as the professor explained. So, in one instance the photon will act as if is a particle, and in another instance the photon will act as if it is a wave.
Now, does the one photon interfer with itself, go through both slits at the same time, go through one slit and then loop around and go through the other, half go through one slit and the other half go through the other slit, or whatever? As explained in the video it is impossible to tell.
Why, you ask. Because, if you put any kind of contraption at slit one or slit 2 to detect whether the photon has gone through slit one or 2, and when your slit detector has gone off then you have already detected the photon at the slit detector. Being already detected the photon's interference pattern is broken and will not show up on the second screen.
As for wave function collapse, that is a way of explaining what may have happened. You can also look up the many paths explanation. Mathematics can describe the situation but it does not explain what really does happen ie . we are used to the behavior of macroscopic objects, but when things get really tiny other features show up and the wave-particle duality of the tiny is one of them.
An electron will behave the same way under certain conditions. but as you move up to larger and larger objects such as molecules and baseballs, then the experiment cannot be performed. a baseball would have a hard time traveling through a narrow slit.
An intelligent observer is not a prerequesit for photon, electron behavior in thsi regard. Humans are as much a part of nature as anything else. There is nothing special about a human that dictates how a photon or electron will behave. What humans can do though is devise experiments to study in a controlled fashion repeatable events such as thousands or hundreds of thousands of photons or electrons to pass through 2 slits so that we can then study that many events and see the data, do some mathematical processing, and then state that small particles act as particles and waves.
As for the paper you cite, the author talks about energy accumulation at the detector, to the point where there is enough energy to make a "pop" which may be a little bit on, as they say here, the "cranky" side of science.
Background radiation. Interesting point. thermal energy doesn't seem to have an effect since everything above absolute zero emits thermal energy. Make one slit hot and the other cold and see what that would do to the interference pattern. Maybe it would skew the pattern to one side or no effect at all. But then, you still would not be able to tell which slit an individual electron went through.
