There is a famous double-slit experiment in which a beam of light is focused upon two narrow slits. If you recall from high school physics, an interference pattern results. There are dark and light bands of light displayed on the screen behind the slits. This demonstrates that light is a wave, resulting in constructive and destructive interference, much like simultaneously dropping two pebbles in a quiet pool of water.
It is quirky, because suppose one of the slits is covered. Then the bands disappear and a single spot of light appears on the screen, indicating that light is composed of individual photons, like bullets and is not a wave. To eliminate unknown behavior, the experiment can be performed with only a single photon emitted by the light source at a time. (In this experiment, the single photon chooses to pass through one of the slits, but we do not know which one.)
If there are two slits, there is an interference band, but if one slit is covered up, the light strikes the screen as a bullet, and not venturing out toward the region of the other slit. This is amazing, because if only one photon enters a single slit at a time, there is no possibility of photons striking or interfering with each other. Yet, an interference pattern results. But not if the other slit is blocked.
What troubled Einstein was, 'how does the light know that the other slit is open or closed?' Let's make it more spooky. Again, we have a light source that emits only one photon at a time, but we also place a detector on the slits so that we know which specific slit the photon passed through. With both slits open, the interference pattern disappears. Our observation has changed how the experiment behaves. Somehow the photon knew we were observing it.
The spookiness that troubled Einstein was that the world was changed just by being observed. Einstein exclaimed, "I like to think the moon exists even when I don't look at it!" The venerable scientist was troubled that the world was changed by mankind's observation. Further, only a single observation establishes it for all of humanity. When the moon is in the sky, who caused it to be there, you or me?
The other spookiness is that the world cannot simultaneously have reality and separability. Separability implies that two independent objects are affected only by physical forces between them such a magnetism, gravity, etc. When there is no separability, then what happens to one object can affect the other, instantaneously (faster than the speed of light). Quantum theory physicists consider this the 'hidden variables' demonstration. Einstein called this, "spukehafte Fernwirkung", or spooky actions.
Spooky, quirky? Neils Bohr, one of the architects of quantum theory, said, "... It is wrong to think that the task of physics is to find out how Nature is. Physics concerns what we say about Nature." In other words, the world as it exists, is because of mankind's observation. The world would be very different, or perhaps not exist, without us. Say what?
Yesterday I read, Spooky Physics: A Brief Introduction to the Eistein-Bohr Debate on Kindle. Andrea Diem-Lane provides some additional insight to this spookiness that so troubled Einstein that he occupied the last quarter of his life trying to fix quantum theory. She also adds that Nobel physicist Richard Feynman stated that analyzing the double slit experiment alone would expose the deep mysteries of quantum mechanics.
Mmm. I'm with Einstein - I like to believe the moon exists without me looking out the window. However this is some consolation from this spookiness. Remember It's a Wonderful Life, the Christmas standard, where an angel shows George Bailey that the world would be very different without him? Quantum mechanics confirms the world would be different without each one of us.
If you are interested, another readable discussion of this spookiness is, Quantum Engima: Physics Encounters Consciousness by Rosenblum and Kuttner.
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