In this weeks podcast, Enrique Blair on quantum computing, Walter Bradley Center director Robert J. Marks talks with fellow computer engineer Enrique Blair about why Quantum mechanics pioneer Niels Bohr said, If quantum mechanics hasnt profoundly shocked you, you havent understood it yet. Lets look at some of the reasons he said that:

The Show Notes and transcript follow.

Enrique Blair: Its really quite different from our daily experience. Quantum mechanics really is a description of the world at the microscopic scale. And its really weird, because there are things that initially we thought maybe were particles but then we learned that they have wave-like behaviors. And there are other things that we thought were waves and then we discovered they have particle-like behaviors.

But thats hardly the strangest part. The strangest part is that a quantum particle does not actually have a position until we measure it, according to the generally accepted Copenhagen interpretation of quantum mechanics.

Robert J. Marks: Whats the Copenhagen interpretation?

Enrique Blair (pictured): Its that the quantum mechanical wave function describes measurement outcomes in probabilities. You cant predict with certainty the outcome of a measurement. Which is really shocking, because in the classical world, if you have a particle and you know its position and its velocity, you can predict where its going to be in the next second or minute or hour. Now in quantum mechanics, the really weird thing is, we say that a particle doesnt even have a position until you measure its position.

Robert J. Marks: It doesnt exist?

Enrique Blair: Not that it doesnt exist, but its position is not defined.

Dr. Marks compared quantum mechanics (QM) to one of the characters in a 1999 film, Mystery Men, featuring inept amateur superheroes, including one who says, Im invisible as long as nobodys looking at me. With QM, thats not a joke. The quantum particle doesnt have a position until we measure it. But how did we discover this? The story goes back to the early 1800s when British physicist Thomas Young (17731829) did a famous experiment with a card held up to a small window

Enrique Blair: Youngs double-slit experiment goes all the way back to 1801, where Young shot light at a couple of slits and then the light passing through the slits would show up on a screen behind them.

So light behaves like a wave, with interference patterns. But what happens when we try doing the same thing with a single particle of lighta photon? Thats something we can do nowadays.

Enrique Blair: We can reduce a beam of light so that its single photon. One photon is emitted at a time, and were shooting it at our double slit again.

What happens when each particle of light goes through these slits? Well, each particle splats up against this screen, and so you can know where the photon hits. But if you do this over a long period of time, the interference pattern shows up again. You have particles hitting the screen, so we see the particle behavior. But we also see the interference pattern which suggests that okay, weve got some wave interference going on here.

So the only way to explain both of these at the same time is that each photon, which is an indivisible packet of light, has to go through both slits at the same time and interfere with itself, and then the buildup of many, many photons gives you that interference pattern.

Robert J. Marks: A particle was hypothesized to go through both slits?

Enrique Blair: Yes, and thats the mind-blowing ramification of this thing.

Robert J. Marks: How do we decide which slit the particles go through? Suppose we went down and we tried to measure? We put out one photon and we put it through the double slit. Weve tried to measure which slit it went through. If its a particle, it can only go through one, right?

Enrique Blair: Right. That introduces this concept of measurement. Like you said, which slit does it go through? Now the interesting thing is, if we know which slit it goes through maybe we set up a detector and we say, Hey, did it go through Slit One or Slit Two? we detect that, we measure it and the interference pattern goes away because now its gone through one slit only, not both.

Robert J. Marks: Just by the act of observation, we are restricting that photon to go through one slit or the other. Observation really kind of screws things up.

Enrique Blair: Thats right. This is one of the things that is hard to understand about quantum mechanics. In the classical world that we deal with every day, we can just observe something and we dont have to interact with it. So we can measure somethings position or its velocity without altering it. But in quantum mechanics, observation or measurement inherently includes interacting with that thing, that particle.

Again, youve got this photon that goes through both slits, but then you measure it and it actually ends up going through oneonce you measure it.

Robert J. Marks: This reminds me again of Invisible Boy in Mystery Men. The photon goes through one of the two slits while youre looking at it. Unless you look away. Then it goes through both slits.

Enrique Blair: Right. Very tricky, those photons.

Next: How scientists have learned to work with the quantum world

Note: The illustration of the double-slit experiment in physics is courtesy NekoJaNekoJa and Johannes Kalliauer (CC BY-SA 4.0).

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Here's Why the Quantum World Is Just So Strange - Walter Bradley Center for Natural and Artificial Intelligence