Quantum Measurement Weirdness Explored (Without Any Math)
Exploring a great scientific mystery at the heart of modern science — with no math, just some drawings and pictures.
The Measurement Problem is a vexing issue at the hart of the theory that we use to explain the world. It is a thorn in the side of Quantum Mechanics, the most successful and useful scientific theory of all time. Our electronic devices, modern chemistry and biology and our basic understanding of how the universe works, all rely on this theory — but at its heart is a question that remains out of reach.
I was given the challenge to try to explain this problem to a group of very smart people who have no previous scientific background. “Use whatever you need to make this clearer” I was told “but don’t use any equations”. Seeing as that quantum mechanics is a highly abstract field which is heavily based on advanced mathematics this is indeed a daunting task, but I accepted the challenge. So here goes.
We start with an experiment that has six simple components. A battery, two metal plates (one with a hole in it) a heating coil (like the one you would find in a household heater) and a long glass tube that’s wider on one side. We’ll also need some phosphor — a material (usually a powder) that’s made from Zinc salts.
First we coat the inside of the wide side of the tube with phosphor, creating a sort of screen. We then place the metal plates in the narrow end of the tube, connect them to the battery and place the heater next to the plate in the back. Then we seal the tube and use a pump to suck all the air out of it. Done.
What we have made is a very basic Cathode Ray Tube or CRT. Those of you over 30 may remember TVs and computer monitors that had CRTs inside them. For those under 20, well, this is the original tube from YouTube.
When we activate the heater and connect the battery a bright spot appears on the screen. Moving the plate with the hole, the anode, back and forth makes the spot on the screen become bigger and smaller, like a shadow. If we lower the heat in the heater down the spot starts to flash and we can eventually see that it is composed of little individual specs of light that trace out its shape over time.
It would seem reasonable to explain what’s going on here this way: The heater is boiling off some small particles from the plate in the back, the cathode. These small particles are being pulled by the electric voltage towards the anode and some of them pass through the hole in it to impact the screen and cause it to emit a dot of light. Let’s call these particles Electrons. They come from the cathode and have a charge, so you can think of them as though they were tiny magnets that are pulled to the anode. Shoot enough particles at the anode and you get the bright spot on the screen. Behold, we have built an electron gun.
Ok, let’s try to see what these electrons are all about. We’ll place a thin barrier between the electron gun and the screen. A barrier with a slit cut in it in a top to bottom fashion.
When we turn the electron gun on we get an image of a vertical line on the screen. Makes sense. But as we narrow the slit a strange halo appears around that line. It begins to look more and more hazy. What’s going on?
This blurring effect doesn’t sit well with the theory we had about little electron particles hitting the screen. Little particles don’t blur. This seems more like what we would expect from a different physical phenomenon — a wave.
Waves, like in the ocean or a pond, are disturbances that move over a medium, in this case, water. In the early 19th century Thomas Young showed that light seems to act like water waves. This was in contrast to the then accepted notion that light was a stream of little light particles. Young did this by doing an experiment similar to ours. He shined a light on a narrow board with a slit and showed that the pattern that emerged on an adjacent screen was diffused. This is called light diffraction. Our electrons seem to be exhibiting behavior that is supposed to only be seen when dealing with waves.
Let’s take this a step further. By cutting two slits in the barrier we can try to achieve something else Young did with light — an interference pattern. When two waves collide they join to form a new wave. If two peaks meet they make a higher peak. The same happens with troughs. And when a peak hits a trough the waves cancel each other out. This creates a unique pattern called an interference pattern. The waves interfere with each other to create it.
Our experiment will now look like this:
When we place a double slit barrier in front of our electron gun we don’t see what would be expected of a barrage of particles — two bright spots in front of the two slits. We see a clear interference pattern. Even if we lower the heater and let the electrons fly one-by-one to the screen the points of light that show up on the screen slowly show the recognizable pattern of interference lines. The evidence for waves is now clear.
This experiment was actually recreated with great accuracy by Hitachi Research some years ago. There’s a great video of it here.
But, how is this possible? The electron gun is shooting single electrons, little pieces of matter — why are we seeing waves. How can single particles create interference, and with what are they interfering? A wave is a phenomenon that’s spread out over an area. It can go though two slits and change its pattern. A particle can’t do this. It can’t interfere with itself. It can only go though one slit — the left one or the right one. How do single electrons know where to go in order to make the lighter and darker areas on the screen? This makes no sense.
How about if we try to test which slit the electron went through. Let’s modify our experiment to do that. We’ll place a detector above the barrier and register which slit each electron goes through. That will help clear out what’s going on.
Would you like to guess what the detector found… Well, it detected about half of the electrons going through the right slit and half going through the left, but something else happened. The interference pattern disappeared! We get two bright spots in front of the two slits.
What? That can’t be right. We didn’t change anything from the last experiment. We just measured where the electrons went. Why did that change the outcome of the experiment? Maybe our detector caused some unknown disturbance that ruined our wave interference pattern. Let’s do something to make sure that’s not the reason.
We’ll use a different detector, one that we can place behind the phosphor screen. This post-impact detector can tell which slit the electron passed through after it has impacted the screen and made a point of light. This way there is no way for the detector to disturb the electron on its way to the screen. We turn the experiment on and we get… No interference pattern, just two bright spots. And when we turn the detector off the interference pattern returns. So what in blazes is going on here?
Let’s consider this for a moment. First we saw little particles hitting a screen and making little points of light. Then we saw that there was a diffraction pattern when we insert a barrier with narrow slit and an interference pattern when we use two slits. So we weren’t sure we were dealing with particles any more. There seemed to be waves involved. And when we tried to solve this mystery by detecting which slit the electrons went through we saw the weirdest thing — our observation caused the experiment’s results to change.
Welcome to the world of quantum weirdness. The concepts of particles and waves are no longer what you thought they were. When you measure something — you change it. An observer conducting the experiment is a part of the experiment itself. If this leaves you a little queasy — you’re not alone.
The formal scientific description of what happened when we measure the slit through which the electron passes is called “the collapse of the wave function”. This is a change in a mathematical description called the Schrödinger Equation. It’s the accepted scientific description of what happens in quantum systems in nature. When a measurement is performed or (in some interpretations) when an observer is looking at the system, it collapses from a wavelike state called “superposition” into the solid reality we know from everyday experience. This is the Measurement Problem. It is a very counterintuitive and bizarre notion. In a way it suggests that the world only exists, as we know it, when it is measured or observed.
What this collapse means and how do measurements and observers influence physical systems is still a matter of debate amongst scientists and philosophers. Einstein was very displeased with the image that arrises from quantum mechanics and is claimed to have said that it is inconceivable that “the moon is only there when I am looking at it”.
In a poll conducted among scientists at a 2013 quantum mechanics conference the participants were asked if they believe that physical objects have well-defined properties before they are measured. What a strange sounding question. Here are the results:
Let’s embrace the spookiness. If an observer or a measurement is needed in order for objects to manifest their properties does that mean that the universe was in different state of being until some observer came along to perform a measurement? Are we, as measuring observers going around with two flashlight like light cones emanating from our eyes that change everything they touch? Can cats count as observers (or measuring devices)? Can physics explain a world that is influenced by the measurements of physicists? There are no clear answers to these questions. Quantum mechanics is taught to thousands of college students every year and is used by scientists universally and it’s not exactly clear what it says about the world. There is clearly much work to be done. What a great time to be alive.
The explanations here are based on books from very smart people (see below). Any mistakes or misunderstandings in the text are totally my own.
Sources:
Quantum Mechanics and Experience — David Z Albert
The Wave Function — Alyssa Ney and David Z Albert
Philosophy of Physics, Quantum Theory — Tim Maudlin
Veritasium — The Original Double Slit Experiment
Hitachi Research — The double slit experiment
