There is another famous experiment that I would like to talk you through. I will try with lots of schematic drawings. The pay off is that it is another look at how the quantum world is beyond our day-to-day experience and how our basic notions are projections. For now, that is plenty! We can go deeper later.
We are going to look at what happens when a light goes through an interferometer.
Lets look at the basic set up, a “big picture” look.It is all there, but we will have to go over it step by step. First, what is in the diagram?.
There is a light source, here the green lamp in the lower left corner of the diagram.
The yellow arrows indicate the path the light takes.
There are four mirrors, one at each corner, all indicated by diagonal lines.
Two mirrors, one at the upper left corner and the other at the lower right, are indicated by a single blue line. They are full-silvered mirrors and they reflect all the light that comes to them.
Two other mirrors, one at the lower left and the other at the upper right, are half -silvered mirrors. These reflect half the light that comes to them, and lets half the light through. A very important point is that the half-silvered mirrors have a front and a back. The back, here indicated by a red line, also reflects half the light and lets half the light through, but there is a change in the reflected light when reflected off the back ( red) side of the half-silvered mirror. The light “phase” is shifted. We will get back to that; it makes all the difference.
The black trapezoid objects in the upper right par of the diagram are light detectors. That is, they will register the light that gets to them (and their color will turn from black to yellow here in these diagrams).
This next diagram shows another overview showing what will happen. We send light through and only the top light receptor registers light. Why is that? What happened to the light going toward the lower right detector?
Lets follow the light,
Here we see the yellow light that came from our lamp at the lower left in our first “big picture”diagrams. This light then enters the lower left half-silvered mirror. Half the light is reflected, and because of the mirror’s angle the light is sent up in this diagram. The other half of the light goes straight through along the bottom left to right. This is why there is a half-silvered mirror here at the beginning of our device, to split the light into two pants, an upper and lower path.
The half of the light that was reflected straight up along the upper path at the first mirror now reaches the upper left mirror and all of that light is reflected, now going along the top of the diagram left to right.
The half of the light going left to right on the lower path that went through the first half-silvered mirror next reaches the lower right mirror and is reflected up along the right side of our interferometer.
So half the light is directed to that last upper right half-silvered mirror along the upper path, the other half of the light along the lower path. The light from the upper path is again split at the half-silvered mirror at the upper right just like it was at the first half-silvered mirror at the lower left. Half of the upper path light goes through unchanged, and half is reflected up to the top light detector.
Now here is where it gets a bit tricky. The light that hits the back of the half-silvered mirror on the upper right is also split, but with a very important difference we haven’t seen yet. The half of the lower path light that goes straight through the half-silvered mirror continues up to the upper detector unchanged. That light transmitted from the lower path gets there at the same time as the light from the upper path that was reflected up to the detector, so the light reflected from the upper path and the light that goes through form the bottom path combine and the upper detector goes off.
BUT the light that ws reflected from the BACK of the upper right half-silvered mirror is now 90 degrees out of phase. This means the peaks of this light, the “out of phase” reflected off of the back of the half-silvered mirror, now in red (but don’t get confused, that is just to make it easy to follow; it doesn’t change wavelength or color) lines up with the light that went through from the upper pathway so that the peak of one lines up with the trough of the other light wave (remember, light acts like waves!)
So the two “cancel” each other out. They completely “interfere” with each other (negative interference in the jargon). The peaks, like we have seen, we can think of as +1, the troughs as -1. So you can see how the +1 lines up with the -1, and that kind of alignment of + with – goes throughout the whole wave. So the +’s combine exactly with the -‘s and camel out (+1 and -1 =0).
So NO light gets to the lower right detector, which remains black in our diagram.
So, when both pathways are open, only the upper detector detects light, the lower right detector detects nothing.
Now, if you block a pathway, then both detectors detect light! In this diagram the upper pathway is open, the lower blocked. But the situation would also be true if the upper pathway is blocked. The light reflecting off the back of the upper right half-silvered mirror is indeed phase shifted, BUT there is no other light wave to add it to, so no cancellation, no “negative interference” as there is not light coming through from the blocked pathway to interfere by being in an unchanged phase as it was when both paths were open. The detector can not sense a phase shift! It only has that effect when a light wave in a different phase is there to interact with and cancel it!
So if both detectors light up, you know that only one pathway is open!
Now this may be kind of easy to grasp if the light “arrows” are waves of light, presumably made up as a vast number of photons that are easily divided into different pathways.
But the kicker is, this happens even when you send one photon through at a time. Like the double slit experiment, we are challenged and any picture you may have about it is WRONG. Try if you want, anything you are picturing is wrong. A photon is a discreet entity on one level, it can make a discreet “ping” in one place on a detector, but here it is on two pathways and interferes with itself.
This is called “superposition”. The particle is “in” the two (or more) paths in superposition. This is a mathematical idea, of course. Superposition is a word for a phenomenon that can be mathematically described but has no four-dimensional meaning in any sense we can picture or comprehend based on our day-to-day experience and our monkey brain.
The particle is, in effect, going through all possibilities of all of the paths, every one however unlikely, including not saying on the paths we determined. Though of course that is impossible in ordinary time and space.
The indivisible, basic particle, the photon say, the discrete energy carrier of electromagnetic waves, is in both pathways. But it can’t be, it is not divisible, of course. It is a basic particle. Right? Well, yes, but no.
Maybe seems a bit of a thin punch line after working through the diagrams, but it goes to the very nature of what a “dharma” is. There are ancient Buddhist debates about what a ‘dharma” is. Not “Dharma” as in the Teaching” or the ultimate Truth, “Buddhamind” or the Dao, the Way, with an upper case “D”. But dharma as the division, the relative. A basic Buddhist idea, related to emptiness, is that all that seems to be “rings” or “events” or “phenomena” is in constant flux, change, and is the product of the ultimate contingency (all not inconstant with certain aspects of science).
To some dharmas were a bit like really basic things, somewhat like a basic particle. To others that was too dualistic, too materialistic, no matter how subtle, While they didn’t speak of waves (or even better “fields” but save that for another day) they described an emptiness that is even empty of emptiness and as such was the basis of all dharmas. This lead to later Chan poems like The Identity of the Relative and Absolute” a thousand years ago and the Heart Sutra two millennia ago. There is more to it, but a lot of debate seems to have gone on about this documented for some 2,000 + years.
So are dharma ( lower case “d”, the apparent, the relative, the divided in the undivided, as my tee-shirt from Japan says) particles or waves. Waves of what? Fields of energy contingently defined in time and space? Are photons particles or wave? Waves of what and where ( seriously unanswered questions in science, what and where are quantum waves? Seems a wave of probability? Meaning…).
So once more I find myself hinting at, rubbing up against, really stumbling into a Zen/Buddhist/Science confluence. That kind of thing can always be criticized, and I am happy to provide the disclaimer that no, I don’t think the Buddhists working on this a couple of millennia ago, and the later Chan and Zen makers who also alluded to this, had an interferometer in mind. But still, the “plus ca change” as the French say (the more things change, the more they are the same. A bit of relative and absolute right there!).
Special thanks to Prof.Benjamin Schumacher whose Great Courses lectures on quantum mechanics are very good and who presented this version of the interferometer.