Quirks & Quarks

How quantum particles could spawn an infinity of new universes — and we never notice them

Physicist Sean Carroll's new book describes the 'many worlds' interpretation of quantum mechanics

Physicist Sean Carroll's new book describes the 'many worlds' interpretation of quantum mechanics

Sean Carroll's new book describes how quantum phenomenon could give rise to multiple universes (NASA Terra Satellite, Reto Stöckli, Nazmi El Saleous, and Marit Jentoft-Nilsen, NASA GSFC)

Originally published on December 21, 2019.

Theoretical physicist Sean Carroll believes that his colleagues haven't been doing their job when it comes to quantum mechanics.

Quantum mechanics is an enormously successful theory, which describes the strange behaviour of tiny particles like electrons. We've harnessed it to build everything from nuclear power stations to cell phones to telecommunications satellites. 

But Carroll thinks the physics community as a whole needs to engage more thoroughly in quantum mechanics beyond investigating how it works in ways we can use in technology. He thinks physicists need to inquire more deeply into what quantum mechanics means — what it says about the nature of the universe and how we understand it. The reluctance to do this, he thinks, has held physics back.

One of the things quantum mechanics might mean is that fundamentally the universe behaves in a way that might make the most speculative science fiction writer blink in surprise. It might mean that quantum mechanical effects are constantly spawning a vast number of new parallel universes.  It's known as the 'many worlds' hypothesis, and it might be the key to understanding the nature of the universe more deeply.

Carroll's new book about this is Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime, and he spoke about it with Quirks & Quarks host Bob McDonald.  Carroll is a theoretical physicist at the California Institute of Technology.

This interview has been edited for length and clarity.


What does it mean that nobody understands quantum mechanics? It's taught in universities. It's used all the time.

Physicists use quantum mechanics to enormous credit. I mean we're able to make predictions that are then tested to 11 decimal places and it's amazing, but it's very much like I can use my smartphone. I can use it to send e-mails or texts — to take pictures. That doesn't mean I can explain to you what's going on inside. I couldn't build the smartphone myself from scratch. 

Sean Carroll is a writer and theoretical physicist at the California Institute of Technology (Rachel Porter)

And that's the relationship that physicists have with quantum mechanics. They can use it. We can solve equations we can make predictions. But when you say 'what's really happening?' they demur. They say 'well that's not our job.' And I think that that is our job. I think that we really need to understand what's really going on in nature.

In your book you suggest that the culture of mainstream physics actually discourages people from working on understanding quantum mechanics. Why is that?

Yeah that's a fascinating question. You know back in the 20s and 30s of course all the great minds were puzzling all the time over what this was all about: Einstein, Bohr, Schrodinger. But that discussion faded away and it became sort of disreputable to worry too much about the foundations of quantum mechanics. Students who worried about it left the field. Even famous researchers who were doing very well if they had good ideas about the foundations of quantum mechanics. They kept them from their colleagues because they didn't want people to think that they had lost interest in serious work.

You say that one of the promising ways that we might get to an understanding of quantum mechanics is this 'many worlds' theory.

Let's start with what we teach our students. You can have an electron — an elementary particle — and it can be spinning. When you measure the spin you only get two possible answers. Either it's spinning clockwise or spinning counterclockwise. 

So what quantum mechanics says is that when you're not looking at it that electron is in a superposition of both. It's not that it is spinning clockwise or counterclockwise and we just don't know, it's really a little bit of both. But then when you look at it — in the textbook formulation — it collapses to be either one or the other. So this smart graduate student, Hugh Everett in the 1950s, said well that's not fair because I'm not treating the observer themselves as a quantum mechanical system.

If electrons can be in superposition then people can be in superposition as well. So let's ask the question what happens to a quantum observer when they look at the electron.And the short answer is they become entangled with that electron and there gets to be one world, one universe,  one copy of physical reality, where there's an electron spinning clockwise and that's what the observer saw, and another copy where there's an electron spinning counterclockwise and that's what the observer saw.

So you're saying that we are in multiple universes as well?

Absolutely every moment there are these quantum mechanical events where some tiny quantum system becomes entangled with the wider world around it and multiple copies of the universe are created. But what's really important to emphasize here is that Everett did not say "let's take quantum mechanics and add a bunch of worlds a bunch of extra copies of reality." Those copies were always there in the formalism of quantum mechanics. If you believe that an electron can be in a superposition then there's no trouble believing that the universe can be in a superposition all Everett does is say "and that's okay."

Something Deeply Hidden by Sean Carroll. Published by Dutton books.

This is giving a lot of power to humans here. I mean I can split the universe just by looking at an electron!

Yeah but you know what? So can another electron. There's nothing special about humans — nothing special about consciousness or perception. This happens again whenever any quantum system becomes entangled with the wider world. So that's one of the huge benefits of the many worlds approach. It clears up all of this mysterious talk of measurement and observation that made it sound like human perception was really driving reality somehow.

Now when these new universes are created — say we just make two of them — are they the same after they branched out?

They're the same except for that outcome of the thing that became entangled. So in one of them the electron has been in clockwise and the other the electron spinning counterclockwise. And of course that difference can be amplified. So if you look at the electron and it's spinning clockwise in your universe, then you say I'm putting 50 bucks on red at the roulette wheel. And if it goes counterclockwise 50 bucks on black then there can be very different outcomes in those two universes.

So what's it take to make a universe? I mean does this happen every time a scientist observes a quantum phenomena?

It happens way more often than that. It happens every time a quantum system becomes entangled with the outside world. So in a typical human body there are roughly 5000 radioactive decays per second/ So every one of those radioactive decays branches the universe into multiple copies — one where the decay happened the other one where it didn't. So that's not five thousand universes. That's two to the power of 5000 new universes that are just created every second just because of you.

That's an awful lot of universes to create in this way. It sounds kind of extravagant as microscopic phenomena spawn universes like this.

Sure it's extravagant in universes but it's very simple in ideas. It's very simple and austere in concept. Basically the underlying equations provide a way of generating all these universes. The underlying laws of physics in Everett-ian 'many worlds' quantum mechanics are as simple as they can possibly be. So the fact that it makes a lot of universes really shouldn't bother you.

If there are all these other universes out there and all these multiple copies of me, can I travel from one to another and say, meet myself?

According to the laws of physics as we understand them, no you can not travel, you cannot talk to anyone in the other branches of the universe. Now Hollywood would have you believe differently and that's OK. But the laws of physics as we know them really make these into different universes — they go their own way.  That's why it makes sense to call them separate universes. What happens in one doesn't affect what happens in any of the others.

Produced and written by Jim Lebans

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