Sean M. Carroll Lectures on Many Worlds

NewScientist just posted a marathon series of math lectures, but almost half of it is from a 5-year-old lecture, Sean Carroll: The many worlds of quantum mechanics, that is not even on math. He gave a similar lecture last year, The Many Worlds of Quantum Mechanics | Dr. Sean Carroll.

My disagreements start early. Carroll says:

Why does 7:14 quantum mechanics have this reputation of being so hard? Here's the answer. There's an amazing feature of quantum mechanics that was nowhere to be found in classical mechanics which says that what you observe when you look at a system is not what you see.

What you see and what is really there are two different things. There's a difference between what a thing is when you're looking at it and when you're not looking at it. What you can possibly see is much less than what really exists. This sounds weird. This sounds bizarre. like it's very very different than what we had in classical mechanics.

He is trying to say that reality is the wave function, which is not directly observable.

The word see and observe are synonyms. I do not get his explanation. Even in classical physics, there are things we cannot see. We cannot see the center of the Earth.

You might think that in that 14:03 circumstance, since quantum mechanics is the foundational theory for all of modern physics, you might think that the quest to understand quantum mechanics at a deep level would be recognized as one of the most important things we could possibly do in physics. The people who devoted their lives to these would be academic superstars. You would have different universities trying to steal them away with highpriced packages and salaries and so forth. and it would be the highest prestige occupation you could have in physics.

Sadly, no, that is not what we do. It is the opposite of that. We have adopted a strategy of denial where if you're a physicist and you think hard about answering these questions, you are labeled not a physicist or a physicist who is too old to do important work anymore and you're sent off to retirement.

That is because the important issues were settled in 1930. People like Carroll want to be paid to do research on many-worlds, and real physicists consider it a complete waste of time.

36:37 Another what I think incorrect objection is that this idea cannot be tested. Can it be tested Right? It's important in science that we not just have good ideas, but that we compare these important ideas to data. Right? that we experimentally probe our ideas and people say you've invented all these new worlds. How do you ever test that idea?

The response to that is, I didn't invent any new worlds. I just took quantum mechanics seriously. The entirety of the assumptions that go into the many worlds theory is there are wave functions and they obey the Schroinger equation. That's it. Everything else is a consequence, a prediction and implication of those assumptions.

And are those assumptions testable? Hell yes they are. Of course they are. Whenever we do a quantum mechanical experiment, we're implicitly testing the many worlds interpretation.

If you want to falsify the many worlds interpretation, remember that the prediction of many worlds is wave functions don't collapse. They never do. They appear to collapse because of decoherence.

So there is no way to test many-worlds. He says it is logically correct, so everything you see confirms it. That's all.

As he admits, we see wave functions collapses. They would contradict many-worlds, except that he does not believe that they are really collapsing. He calls that a test.

This is all nonsense. I don't know why anyone takes him seriously.

Carroll also has to answer this new survey:

World's largest ever survey of physicists, results & reaction

May 12, 2026
What do physicists really think about the biggest mysteries in the universe?

In this video, leading voices in theoretical physics come together to unpack the results of the Big Mystery Survey—the largest survey ever conducted of professional physicists, prepared in collaboration with the American Physical Society's Physics Magazine." Featuring reactions from experts like Sean Carroll, Niayesh Afshordi and ,Ghazal Geshnizjani we find out what physicists really think about topics such as: What should we think about fine tuning ? What is dark matter? What is dark energy? How should we truly understand the Big Bang?What the right approach to quantum gravity ?

Whether you’re a physics enthusiast or just curious about the universe, this conversation offers a rare glimpse into how experts think about the unknown.

It reports 11% for many-worlds, 6% for Bohm pilot wave. These are the two crackpot interpretations. It is amusing to see how the leading popularizers of theoretical physics have views that are rejected by most physicists.

Carroll also has a lecture in this newly-released video on the physics of time and the history of relativity.

[4:50] All you have to do is entirely jigger your thoughts about what space and time are. And in fact, it wasn't until two years later when Hermann Minkowsky, who was a mathematician who had been one of Einstein's professors, said, "You know, the right way to think about Einstein's theory is to say that space and time aren't separate anymore. To imagine there's one thing called spacetime, and different people, different observers moving in different ways through the universe will divide it up into space and time differently.

There's no objective true fact about when I snap my fingers now what's happening light years away. That's going to depend on who's doing the observing and who is doing the measuring. ...

It can all be explained very beautifully by imagining a single four-dimensional spacetime instead of separate space and time. Einstein himself was not impressed by this move. ...

And when Minkowsky says, "I have some new math that unifies space and time based on Einstein's theories," Einstein himself is like, "I don't need that. That's like extra mathematical nonsense."

Minkowski never referred to "Einstein's theory", but got that 4D spacetime from Poincare. That 1907 Minkowski paper cited Poincare's 1905 paper with the 4D spacetime.

Yes, people like to credit Einstein for spacetime, but that was published by Poincare and Minkowski, and Einstein rejected it.

Carroll wrote a whole book on relativity, so he surely knows the history. It is weird that he gets it so wrong.

Update: The survey has just been posted in a paper.

We present results from the Big Mysteries Survey, a large-scale survey conducted through the American Physical Society's Physics Magazine on foundational and controversial topics in contemporary physics. The survey provides a snapshot of physicists' views on issues in cosmology, black-hole physics, quantum mechanics, quantum gravity, and anthropic coincidences. A central finding is that several positions often described publicly as field-wide ``consensus'' views are, in practice, supported by much narrower majorities or by pluralities rather than majorities.

19% believe string theory is the best hope for quantum gravity. 30% believe that information dropped in a black hole is preserved in Hawking radiation. 24% believe in the dark energy cosmological constant. No consensus on dark matter. 51% believe in cosmological inflation.