Quantum mechanics is probably the most
mind-blowing theory of all of modern physics. In many explanations, cats are simultaneously
alive and dead, and the outcome of experiments depends on a person looking at the equipment.
Those last two things are kind of sketchy explanations that quantum practitioners have long
since abandoned. However, there are a real series of quantum measurements that are just staggeringly
bizarre and are totally worth talking about. Probably the simplest measurement to understand
the weirdnesses of quantum mechanics is the double slit experiment. And, by “simplest,” I don’t mean
simple. I just mean that it’s at least possible to understand how the experimental outcome depends
on a few relatively simple knobs in the apparatus. So how does the double slit experiment work?
Well basically you shine light on two slits and the light passes through them and you
see what pattern appears on a distant screen. The easiest way to do it is to shine a laser
at two parallel slits and see what pattern you see on a distant wall. The laser passes through
both slits, and the light going through each slit interferes with one another and the result is a
series of bright and dark lines on the screen. Where the peak of the wave of light going
through one slit coincides with the trough of light going through the other
slit, the result is a dark spot. If peaks or troughs from both slits
line up, then you get a bright spot. You can see similar behavior if you just use water
waves. After a wave passes through the slits, there are places in the water where the waves are
big and places where there are no waves at all. The first version of this experiment
with two slits was performed back in 1801 by British polymath Thomas Young. His work
showed that light acts like a wave and countless physics students reproduce his experiment
each year in introductory laboratories. So, what happens if there is only one slit, not
two? In that case, there is no interference. This stems from the fact that that light has only one
source. And, if you have some physics training, you’ll realize that I’m ignoring diffraction
here. It just simplifies the discussion and this whole thing is complicated enough that
we need as much simplification as possible. OK…to recap…with light, we can have
one slit or two. If we have one slit, we don’t have bands of light on a
distant screen. If we have two slits, we do. This pretty much proves that light is a
wave. Now let’s start talking about tricky stuff. We know from other experiments that light is
also a particle and particles of light are called photons. That is its own mind-blowing
thing, because waves and particles seem to be so different, but let’s just accept this
fact. Proving it would require its own video. So, what would we expect if light acted
like a particle when it went through the double slits? Well a particle is like a BB or
a marble or something. The particle would go through one slit or the other, but not both. The
resulting pattern on a distant screen would be two patches where the particles hit, and
the rest of the screen would have no hits. But the actual double slit behavior doesn’t look
like that. So that’s why people say that light is a wave, although there are other measurements
that I’m not going to describe here that prove equally strongly that light is a particle.
Einstein got his Nobel Prize for that insight. OK, now we can start monkeying around with the
double slit experiment. What happens if we set up a detector around the two slits to see if
the photon goes through one slit or the other? That shouldn’t happen with a
wave, but it does with a photon. And, if you do that, the pattern on the
distant screen looks like it is a particle. So, this is definitely weird. In the past, people
have interpreted this as saying that photons act like waves when you’re not looking at them, but
particles when you are. In the 1930s, people talked about the influence of human consciousness
on quantum mechanics, which has persisted for nearly a century in some communities. Science
has long since discarded the special nature of human consciousness in quantum physics, but it has
certainly started a cottage industry of quantum woo, with such books as The Tao of Physics, The
Dancing Wu-Li Masters, and a lot of things said by modern day television personality Deepak Chopra.
None of these reflect current physics thinking. Woo aside, the mind-bending behavior
of photons gets weirder still. Let me walk you through this cranial catastrophe. It turns out to be possible to turn down the
intensity of a light source so low that only one photon is emitted at a time. What happens when
you shoot a series of those photons at a double slit experiment and don’t look to see what slit it
went through? Will it act like a particle or wave? Now I need to be honest and say that, although
this experiment has been done with photons, it was first done using electrons. Electrons, like
photons, have both a wave and a particle nature. That means that qualitatively the electron
experiments can stand in for photons. And in 1986, Philippe Grangier, Gerard Roger,
and Alain Aspect did an equivalent experiment with photons. I put a URL of their actual paper
if you want some light nighttime reading. And by light, I mean heavy. And I also added a link
to a URL of a paper that details how to do the experiment with university
undergraduate laboratory equipment. You know, just in case someone out
there wants to check on what I’m saying. So, what happens? Well, when the
photon is aimed at the two slits, and if light were acting like a wave, you’d expect
a very faint interference pattern to appear. Then, with each photon, the interference
pattern will get brighter and brighter But that’s not what we see. Instead, we see
that the photon is detected at a single spot on the screen. That’s definitely the behavior you
expect from a particle. So that’s pretty weird. We have now have two instances where light acts
like a particle in the double slit experiment. If what I said seemed weird, what I am
about to say pegs the bizarre-o-meter. What happens when we send another photon through?
Well just like the first one, it appears at a single spot. But what happens with, the third,
fourth, hundredth, millionth and zillionth photon? You see that they start building up a pattern
that you should recognize. What you see is the traditional interference pattern seen by
Thomas Young way back in the early 1800s. So, this is extraordinarily weird. It seems that
individual photons travel through both slits, yet they are detected like a particle, and also
seem to be governed by the mechanics of waves. Light truly has wave and particle properties. Now the first thing I want to say is
that what I’ve told you are experimental facts. There is no question that these
things happen. These can’t be denied. The real question is “what story is the data telling us?” That’s
where things get more complicated. There is the “shut up and calculate”
school of thought, which says that you shouldn’t ask what is going on. The theory
predicts the result and that is good enough. Then there is pilot wave theory, which has
particle photons surfing waves of probability, which guide the photons to their arrival point
on the screen. The Many Worlds interpretation essentially says that all possible outcomes are
predicted, and the photon simply follows one of the paths, meaning that there is no tension
between the wave and particle nature of light. There are many interpretations of quantum
mechanical data. You can look at the Wikipedia to see the smorgasbord of ideas available to you.
And there is no lack of ideas that can reasonably be called fringe. I put a URL to the Wikipedia
page that lays out the more plausible options. I haven’t told you which of the options
is right, because the jury is still out. Few of the options are ruled out by data, so
we’re kind of stuck in the position of saying “I dunno.” By the way, fun fact, I went into
physics, hoping to solve this quantum conundrum. But as I got into it, I saw just how hard it was.
If a century of smart people thinking about it hasn’t solved the problem, probably I wasn’t going
to either. So, I went into particle physics, which was equally interesting and was a field where I
could make a difference. And the rest is history. Even though the double slit experiment is
mind-blowing, there’s more to the story. In my next episode, I’ll tell you about how we can
pick which slit the photon went through, and then erase that information. What happens in that case?
You’ll have to watch the next video to find out. Well, that was certainly fun. Thinking
about quantum mechanics is always a good way to make your head spin. If
you liked the video, please like, subscribe and share on social media. We need more
viewers. And what kind of viewers do we want? Well that’s easy. We want viewers who know the
great truth…which is that physics is everything.
