Einstein vs. Bohr: How Their Career-Long Debate Led to Parallel Universes

Post Author:

“Can nature possibly be so absurd as it seems to us in these atomic experiments?”
— Werner Heisenberg, theoretical physicist

By the end of the 19th century, science was leaping about in a self-congratulatory dance. Electricity and magnestism had been conquered; inventions like radio and telephone had arrived; Newton's laws, summing up the behavior of objects, were allowing mankind to accurately predict a solar eclipse 1000 years into the future. As far as Lord Kelvin was concerned, physics was done. There was nothing new to be explored.

This is the story that Brian Greene — self-appointed spokesperson for physics — began to tell at World Science Festival, which he co-founded 5 years ago.

Brian Greene presents: Spooky Action!

Here comes Quantum Mechanics
In 1897 the electron was first identified as a particle. Neils Bohr noticed that when atoms agitate under heat, the electrons jump to higher orbits, but on their way back down, there are spaces in between that they never land on. Almost everyone called bullshit on this, some in German — how can the electron know beforehand what space to skip? Albert Einstein was one of the few impressed.

The Double Slit experiment
In an experiment gone wrong, an explosion accidentally led to the Double Slit experiment: when you fire particles at a screen with a slit, they land in a kind of a diffraction pattern. But if you fire them at a screen with two identical slits (which would look like “11”), the outcome goes against expectation: instead of creating a diffraction pattern roughly matching the two slits, they form a set of patches alternating between a dark and intense light; a pattern that happens to scream physics' favorite four-letter word: wave.

When two waves overlap, at some points a peak of one wave will meet a trough of the other, canceling each other out: that would explain the alternating dark spots. At other points, a peak will meet a peak, and a trough a trough, intensifying the amount of particles at those points. But why were these particles behaving like waves?

A live demo of the Double Slit experiment, showing the wave pattern.

This kind of odd behavior, due to which laws that work for everyday objects no longer hold up to the microworld of molecules, atoms, and subatomic particles, is what defines Quantum Mechanics.

A machine that performs the Double Slit experiment, on loan from Princeton University.

Playing dice
Physicist Max Born proposed a troubling solution: the wave being detected was a probability wave. Before the act of measurement, a particle could be anywhere. But if we keep subjecting it to the same conditions, a pattern will emerge: the likelihood of where the particle will be found is dictated by a probability wave. In other words, Born was suggesting that instead of particles having fixed properties, these properties existed as probabilities.

“If that turns out to be true,” said a pissed-off Max von Laue, “I'll quit physics.”

Einstein wasn't happy either:

The theory yields a lot, but it hardly brings us any closer to the secret than the old one. In any case, I'm convinced that He doesn't throw dice.”

To which Neils Bohr said:

“Einstein, don't tell God what to do.”

Einstein vs. Bohr
Here began the 30-year-long debate between Einstein and Bohr. Einstein believed the probabilities were a result of incomplete knowledge, similar to weather reports: a “50% chance of rain” actually means “we don't have enough information to say with certainty if it's going to rain.” On the contrary Bohr believed that based on probabilities, the act of observation itself had a role in producing a single definite reality out of a fuzzy, indefinite one.

Spooky Action
The debate eventually arrived at the topic of entanglement, a key feature of Quantum Mechanics. Erwin Schrödinger explains, “When two systems enter a temporary physical interaction, and when after a time of mutual influence the two systems separate again, then the two systems can no longer be described independently of one another. By their interaction, they have become entangled.”

The mysterious spin
Experiments in 1920s established that every particle spins at an unchanging fixed rate, either counter-clockwise — “up” ; or clockwise — “down”. Quantum Mechanics has demonstrated — without controversy, says Brian — that just as a particle can be here and there at the same time, it can also have both “up” and “down” spins at the same time. It's only when we measure the spin direction that it snaps to one or the other, with a 50% chance of each.

But here's the twist: after two particles have entangled, if we measure one as spinning “up”, the other — even when very far from the first — shows up as spinning “down.” Einstein deridingly called this “spooky action at a distance.” From the Quantum Mechanics viewpoint, it's as if every time a coin turns head, another magically turns tail.

Einstein pitched a simpler explanation: the two particles always had opposite spins. In this case, a better metaphor would be gloves, not coins. Put one glove in a box each: you have a 50% probability that the left hand glove is in the first box. If you open it and find a left hand glove in it, it necessarily means the other contains the right hand one. Einstein's point was: so what?

The stalemate
Both sides agreed that at the time of measurement one entangled particle shows one definite property and the other shows the opposite. There wasn't a way of testing if this was also true for the moment just before measurement, which meant there was no longer an “experimental consequence” for this debate. It was time to move on.

Inicidentally, war followed this stalemate, relocating the focus on implementation: building a bomb.

Shortly after that, Bohr and Einstein died as friends. “Not often in life has a human being caused me such joy by his mere presence as you did,” Einstein wrote to Bohr, with whom he got into such a heated discussion on a street car once that they missed their stop, and missed it again on their way back to catch it.

A modest breakthrough
About two years after Bohr's death, obscure Irish physicist John Stewart Bell continued to think about the debate in his spare time while working at CERN. He published a paper titled “On the Einsten-Podolksy-Rosen Paradox,” in which he argued that there was, in fact, an experimental consequence that could put Einstein's theory to test.

When Brian filmed a 10-part TV series, one of the episodes was dedicated to Quantum Mechanics. Bell's insight was excluded for being too mundane a topic for TV, and so it was with the sound effect of shackles and locks that he now proceeded to explain it to the live audience.

Bell realized that with a small variation on Einstein's scheme, his theory could be experimentally tested. If the detectors measure the spins along 3 axes instead of one, and the experimenter is able to pick along which axis each detector should measure, then that creates a total of 9 possibile settings (multiply 3 possibilities in one by 3 possibilities in the other).

Let's say particle A's spin along X, Y, Z axes are: up, up, down respectively. That means B's would be down, down, up. If we add it all up, we'd see that 5 times out of 9, the two detectors would record opposite directions.

This would be true for any spin combination, except one: if the spin along all 3 axes for each particle are the same — up, up, up or down, down, down — then the recorded spins would be opposites 100% of the time.

But Bell's variation still leaves us with one reliable conclusion: the recorded spins would be opposites at least 5 times out of 9, i.e. 55%. If we can test this, we can prove that Einstein was correct about particles having a definite spin direction even at a moment before measurement.

Experiments conducted since then — substituting the specific language of each experiment with the example of the spins we've been using — consistently show that the spins turn up opposites not at least 55% of the time, but rather 50% of the time. This means Einstein was wrong, Bohr was right: particles don't have definite properties when not being measured. Even Bell was surprised: “The reasonable thing just doesn't work.”

Brian says there's enough evidence to believe that even when the distance between the entangled particles is in light years, the “spooky action” remains just the same.

If we attempt to bridge the indefinite world of Quantum Mechanics and the definite world of everyday experience, we arrive at a conclusion even spookier: that “the act of observation somehow coaxes that definite reality to the fore”, out of many possibilities.

Schrödinger's famous cat
Imagine a cat that lives if a particle is found at location A and dies if the particle is found at location B. Quantum Mechanics dictates that a particle is in a constant flux of being at A and B, which would cause the cat to be in a constant flux of being dead and alive. How's that even possible.

Yes we can, parallel
Then American physicist Hugh Everett comes along and says, Hold up! It's not that the cat is in a flux of being alive and dead, but that both outcomes actually do happen, just in parallel universes. It's only when we step in as an observer that one version of us sees the particle at location A and the cat alive; in another, the particle at location B and the cat dead. In other words, we can't cheat by saying this set of rules works for particles, and this other set of rules works for big objects made out of those very particles. Whether Everett was right is a debate that continues today. Adds Brian: “In the end, what's true is not dictated by what seems more or less strange. Our response to these ideas is irrelevant to whether or not they're right.”

Q&A with the audience

Q: Why not just say god did it? Some of this clearly comes so almost impossible for a contemporary human mind to absolve it all, where does this leave our minds, even future minds: will there be always continuous doors to open that are almost unsolvable?

BG: Well, it kind of fries my mind but I guess people react in different ways. But my view is this: if you say that god did it, if that makes you happy, I'm fine with you saying that. But the thing is it doesn't yield any new insight, it doesn't yield a new prediction, it just changes one mystery that we describe using language of physics to another mystery; it's just changing one word for another. The only thing that interests me, the only thing interests most physicists is an idea which at least in principle can make an experimentally testable prediction, so we'll know whether it's right or wrong, and as far as I know, invoking god hasn't done that, and that's why, while it's okay with me, it's just not that interesting.

Q: You said that the experiment expectation was 5/9 but it came out differently, you didn't say what it was.

BG: Yes, you're exactly right, so you were playing close attention. And I gotta say, thank you for sitting through the whole 5/9ths thing. Now it's out of my system and I'll never have to say it again. So, when they did the experiment, they found not 5/9th, which is 55%, they found 50%. And the reason why 50% is a wonderful number is that if you do the standard quantum mechanical calculation of what you would predict, it's 50%. So it's not only that you rule out Einstein's view — no spooky action, definite properties yeilds more than 5/9ths, that's ruled out by the data. The data also confirms the quantum prediction: so you rule out one, you confirm the other, the best of all worlds.

Q: I thought electrons had a spin only about one axis? So what does it mean to have a spin about many axes?

BG: Well in standard quantum mechanics, we often focus upon the spin of the particle about a vertical axis, by convention. But you can take any axis at all, and you can measure the spin of the article about that axis: the z-axis, the x-axis, the y-axis, or anything in between, and what standard quantum mechanics tells us is that the spin of the particle will always be either up or down about that axis. And what Einstein was saying was simply that he believed that particles have a definite amount of that spin about any axis, even if quantum mechanics give you this weird idea of it being a mixture of both. So it's the standard quantum mechanical idea but taken to a place which quantum mechanics would not agree with, that particles have these definite features.

Q: In terms of applied physics, and practical applications, how can you distinguish between classical physics and quantum mechanics?

BG: Anything that has an integrated circuit, where did it come from? Well, if you don't know how electrons behave, then you can't manipulate them with precision, and if you can't manipulate them with precision, you can't build these ultra microscopic circuits that are in everything in the world around us. So, to my mind, one of the triumphs of quantum mechanics is that if back in the 20s and 30s, I don't know but if you would've asked people like Bohr or Schrodinger, “What's the utility of all this stuff you're worrying about?” I think they would've said, “Eh, probably not that much.” You know, we're talking about particles and atoms and we really wanna understand but it's far away from the concerns of everyday life. And then 70 or 80 years later, these ideas that started out esoteric and divorced from anything that we experience have now changed our lives, and that is an amazing transition and I think it's one that we need to keep in mind when we're talking about other fields of science that might seem abstract or esoteric at any given moment. Like string theory.

Q: I think at last year's festival, Gerard 't Hooft mentioned he was working on a theoretical work on a reality underlying quantum mechanics, and it seemed to be of a determinstic nature.

BG: Yes, so Gerard 't Hooft is a Nobel laureate in physics, and he still believes that underneath it all, not in the way that Einstein believed it — because that's ruled out — there still is a determinstic, more conventional reality. It can't be the one Einstein envisioned, but he's working hard to figure out what that might be, which is all just to say that quantum mechanics is not done. Our understanding of it is still in the works. So maybe in World Science Festival, I don't know, the 25th, if we live that long or survive the trauma to put these festivals on, we might have a talk where the nature of quantum mechanics is now shifted and a new insight changes the way we think about things. That would be a wonderful and remarkable thing to have happen.