Is it a bird is it a particle?

August 19, 2010

Dear Hank,

I’ve always been fascinated by an experiment with light. The one where you take a candle and cut two rectangles out of cardboard and project the shadows onto a wall and (... Okay, Dan, and you can skip to the last part as this is all ancient history to you but for the rest of us...) see where the light comes through unadulterated and then you can see where shadows are and where there is overlap in the shadows. Clearly light appears to act as a wave.
The experiment continues with a change in light source. A photon gun is used. A photon gun is like a cathode ray tube and it shoots photons, which I am told are positively charged electrons. I never really understood how you know what they are or how they are charged but okay. When you shoot a stream of photons at the cardboard cut out rectangles you get the same effect as with the candle. The light goes through the slots and you see shadows like before. However, if you shoot one photon what happens?
How you shoot one photon and know you shot only one I never understood either but I assume somehow you can. Well, in my mind there are three possibilities: it goes through slot 1, slot 2, or neither. The incredible part of this experiment is that it appears to act like a wave. You get the same effect as with the candle light. Yet, you think you are shooting a particle - a photon, only one, at an object with two holes in it. It has to go through one hole or the other or neither; yet it seems to go through both.
Richard Feynman in his famous lectures on physics which you can get on tape (recorded - okay, maybe via download, maybe digitized, maybe CD) said that one of the problem’s with the single photon was, physicists thought, the photon had too much energy (I think that’s it) ad they dialed it down and watched what happened. Now you get into Schrodinger’s Cat. Whether Schrodinger ever had a cat or not is unknown to me; this is an imaginary cat, much like the famous Cheshire Cat. Schrodinger said that the very looking at the light could change the outcome of the experiment. (“Imagine if you will a cat in a box and by looking in the box to see if the cat were dead or alive changed the cat’s fate?” asked Schrodinger. Shoot the cat in the box and then you’d know I said but no one listened to me.) Feynman did a great example of this by talking about this in his lectures where he imagined having the photon gun set up and after each shot the result was announced then the energy was reduced and another shot fired. The tape went like this:
“One!...Two!...One!...” and then in a whining tone, “I don’t know!”
And that brings us to the state of the experiment until recently. Someone has done an experiment which says the cat asks you which you want (dead or alive?) and responds accordingly. In other words, someone has come up with an experiment that says you can look at the photon after it has passed a decision point on what path to take and depending on what you are looking for and how you look depends on how the photon will respond. In other - other - words, it will decide what it did after the fact.
Ponder that for a while.

Gotta go,

The B man

ps link found here:
http://www.huffingtonpost.com/robert-lanza/does-the-past-exist-yet-e_b_683103.html