How Decoherence Splits The Quantum Multiverse

**SPEAKER_1** (0:00)
This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

**SPEAKER_2** (0:31)
This episode is brought to you by Paypal. You had plans for the last days of summer.
And back to school shopping has its own plans.
School supplies, dorm essentials, your list doesn't stop. Neither does the spending. But when you pay in for with Paypal, you can pay over time. No interest, no fees. Check out with Paypal. And get back to summer. Subject to approval. Learn more at paypal.com/payin4. Paypal, Inc. NMLS 910457

**Matt O'Dowd** (1:00)
On the quantum scale, we can see multiple histories play out and even talk to each other. So why can't we see that happen on our familiar large-scale world? Many physicists believe that the answer lies in a process known as quantum decoherence.
The Heisenberg cut. Is it the share of proceeds you send to Walter White to avoid, well, getting cut? Sure. But it's also the elusive dividing line between the quantum and classical worlds. In last week's episode, we started down this rabbit hole exploring the measurement problem. The question of why and where the blurry quantum wave function collapses into well-defined measured results. We focused on a simple question. Does conscious observation of a quantum system cause the wave function to collapse?
If you want the full story, this would be a good time to pause and catch that episode.
Or whatever, do it later. The upshot is that more and more physicists think that consciousness and even measurement don't directly cause wave function collapse. In fact, probably there is no clear Heisenberg cut. The collapse itself may be an illusion. And the alternate histories that the wave function represents may continue forever.
The question then becomes, why is it that we can see these multiple histories play out on the quantum scale, and why do we lose sight of them on our macroscopic scale? Many physicists believe that the answer lies in quantum decoherence. Quantum decoherence is a deep and developing subject. And today we're going to dip our toes and cover one aspect of it. By thinking in terms of the wavefunction. So quantum systems are described by this wavefunction thing. It's the mathematical object that defines the distribution of possible outcomes if you were to try to make a measurement of that system.
Wavefunctions evolve over time according to the Schrödinger equation, and that evolution tracks how the system's properties might change. Another way to think about it is that the time-dependent wavefunction maps all possible histories for the object. Over time the histories of a quantum system separate to represent every possible future the laws of physics allow. But those separate histories don't just split. They also merge. The probability for a system going from one state to another can be calculated by summing all possible histories that would lead to exactly that final state. This fact is also reflected in Richard Feynman's path integral formulation of quantum mechanics. But this only works if those alternate histories, those branches of the wave function remain coherent. If decoherence-

**SPEAKER_1** (4:00)
This episode is brought to you by Accenture. When your advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales.
Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

**SPEAKER_2** (4:31)
This episode is brought to you by PayPal. You have plans for the last days of summer.
In fact, the school shopping has its own plans. School supplies, dorm essentials, your list doesn't stop, neither does the spending. But when you pay in for with PayPal, you can pay over time. No interest, no fees.
Subject to approval. Learn more at paypal.com/payinfor. PayPal, Inc. NMLS 910457

**Matt O'Dowd** (5:00)
Because then those separate branches are lost from each other forever, and we lose the ability to see branches beyond our own. Let's talk about what coherence and decoherence actually mean. In general wave mechanics, we say that a set of waves are coherent if they match in frequency, and if the shapes of the waves are the same, and if there's a constant phase difference between them. The peaks and troughs either line up exactly or have a constant offset.