What this covers
Sean Carroll, CalTech, John's Hopkins, Santa Fe Institute
One of the great intellectual achievements of the twentieth century was the theory of quantum mechanics, according to which observational results can only be predicted probabilistically rather than with certainty. Yet, after decades in which the theory has been successfully used on an everyday basis, most physicists would agree that we still don’t truly understand what it means. Sean Carroll will discuss the source of this puzzlement, and explain why an increasing number of physicists are led to an apparently astonishing conclusion: that the world we experience is constantly branching into different versions, representing the different possible outcomes of quantum measurements. This could have important consequences for quantum gravity and the emergence of spacetime.
Sean Carroll is a research professor at CalTech, Homewood Professor of Natural Philosophy at John’s Hopkins University, and Fractal Faculty at SFI. His research focuses on fundamental physics and cosmology, quantum gravity and spacetime, philosophy of science, and the evolution of entropy and complexity. He’s authored “Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime;” “The Big Picture;” “The Particle at the End of the Universe;” “From Eternity to Here;” and the textbook “Spacetime and Geometry.”
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Carroll argues that the Everett (many-worlds) interpretation is the correct understanding of quantum mechanics because it treats the wave function as fundamentally real and requires only the Schrödinger equation, with no need for ad-hoc measurement rules or wave function collapse.
- The wave function is objectively real as evidenced by interference patterns in the double-slit experiment showing waves interfere with themselves across space and time
- The Copenhagen interpretation's two-rule system (wave-like behavior when unobserved, particle-like when measured) is conceptually unacceptable and leaves measurement undefined
- Everett's approach—universe is a single wave function obeying only the Schrödinger equation—elegantly explains observations without invoking collapse or observers, with branching into separate worlds being a logical consequence rather than an added assumption
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The universe is not fundamentally made of discrete bits of particles, but rather of waves, and when these wave equations are solved they naturally take on certain discrete shapes, which is where the word 'quantum' comes from—not from quantized fundamental units but from discrete solutions of wave equations.
“notice that nowhere in here this is the beginning of quantum mechanics but nowhere in here that I say the universe is made of little discrete bits that is not what quantum mechanics says if anything is the opposite the universe is made of waves but when you solve the equation for the waves you find that the waves take certain discrete shapes and that's where the word Quantum comes from”
Physicists working on particle physics have been discouraged or reluctant to work on foundations of quantum mechanics because particle physics provides immediate testable problems, and historically memos were passed around in major physics journals discouraging papers on quantum mechanics foundations.
“there were literally memos passed around in the major physics journals saying do not consider papers on the foundations of quantum mechanics...there were people who are making major contributions to the foundations of quantum mechanics like John Bell...who wouldn't tell anybody that he was working on it he hid his work on this subject”
The Rutherford model of the atom, which depicts electrons spiraling around a nucleus like a solar system, was instantly known to be false because charged particles moving in circular orbits emit electromagnetic radiation and would lose energy, causing electrons to spiral into the nucleus in approximately 10^-11 seconds (100 billionths of a second).
“if you take an electrically charged particle like an electron and you move it like in a circle it gives off radiation...if this were what the atom was like it should be giving off radiation very very quickly...the answer is 10 to the minus 11 seconds okay 1 100 of a billionth of a second”
Between 1927 and 1957 it was reasonable to think there might be a relationship between conscious observation and quantum mechanics because there were two separate sets of rules and one set involved the word 'measurement,' but now we know better through interpretations like Everett, Bohmian mechanics, and spontaneous collapse models, all of which are fully physical and materialistic.
“there was a time...when it was perfectly reasonable to think that there might be some relationship between conscious observation and quantum mechanics because there were two separate sets of rules...but we know better now...Bohmian mechanics spontaneous collapse models...are a hundred percent physical and materialistic”
Many-worlds interpretation is testable and falsifiable: you could falsify it by showing that there is something in reality besides the wave function, or by showing that the wave function doesn't always obey the Schrödinger equation, and there are actual experiments underway attempting to do exactly that by testing whether the Schrödinger equation holds at large scales.
“yes it can it's very easy to falsify it...you could easily falsify it by showing there's something in addition to the wave function...you could also falsify it by showing that the wave function doesn't always obey the Schrodinger equation and there are literally experiments going on to do exactly that”
Schrödinger's cat thought experiment was designed to show that the Copenhagen interpretation leads to an unacceptable conclusion: that macroscopic objects like cats exist in superpositions of mutually exclusive states (both alive and dead simultaneously) until measured, which Schrödinger found philosophically unacceptable.
“Schrödinger...was not trying to say like isn't quantum mechanics awesome...he was a little chagrined at what quantum mechanics had turned into...what his thought experiment was meant to show you is surely you don't believe that...the formalism of quantum mechanics says that in the Box there is literally a superposition of an awake cat and a sleep cat until you open it and you look and Schrodinger is like come on gang we got to do better than this”
In 1927, the world's greatest physicists assembled at the Solvay conference in Brussels, Belgium, and after discussing the wave-particle paradox, they concluded and announced that the resolution was: electrons behave like waves when you're not looking at them and like particles when you are looking at them, and this became the consensus view known as the Copenhagen interpretation.
“almost 100 years ago is we assembled the world's greatest Minds the smartest physicist that we knew assembled in Brussels in Belgium for this Solvay conference in 1927...they had a few Belgian ales they thought about this problem...and at the end of the week they came out and they announced okay we've got it figured out here it is electrons behave like waves when you're not looking at them and they behave like particles when you are”
Einstein and Schrödinger objected to the Copenhagen interpretation even though it became consensus, with Einstein being characterized as a smart person who understood quantum mechanics well and was not a conservative old fuddy-duddy despite popular myths, but rather someone who correctly argued that quantum mechanics was incomplete.
“there were some objectors Einstein and Schrodinger also they both they never believed this...Einstein is underrated there's this myth that goes around that...Einstein became kind of a conservative old fuddy-duddy he couldn't keep up with the new Quantum hotness...that is entirely wrong...Einstein when this picture was taken was younger than I am now”
Because of quantum entanglement, there is only one wave function for the entire universe (or any composite system), not separate wave functions for individual particles, and this universal wave function assigns a probability number to every possible configuration of everything in the universe, with the probability of observing a particular configuration given by squaring that number.
“entanglement implies there is only one wave function even when you have two particles...it's not that you have a wave function for particle 1 and a wave function for particle two you have a single wave function...the wave function of the universe...assigns a number to every possible configuration that everything in the universe could be in and you square that number and you get the probability of actually seeing the universe in that configuration”
In the Copenhagen approach, observers are treated classically (as following the laws of classical physics) while the systems they observe are treated quantum mechanically, even though observers are themselves made of atoms that obey quantum mechanics, which is a fishy and ad-hoc distinction.
“one of the moves that is made in the Copenhagen way of talking is very fishy right from the start namely you treat your little quantum mechanical system using the rules of quantum mechanics but you treat Observers classically even though they all knew that observers like human beings were made of atoms and atoms obey the rules of quantum mechanics”
Decoherence is the process by which a quantum system becomes entangled with its environment very quickly, causing the different branches of the superposition to become uncorrelated with each other such that information cannot pass between branches and they effectively become separate realities.
“in the box where the cat is there's air molecules bumping into the cat...in quantum mechanics the state of those air molecules will depend on whether the cat is awake or asleep...this is the phenomenon called decoherence the quantum system that you care about becomes entangled with its environment...this happens very very very fast almost instantaneously”
The Everett interpretation is called the 'many-worlds interpretation' not because Everett added the idea of multiple worlds as an assumption, but because following the formalism of quantum mechanics rigorously leads naturally to the conclusion that there are many worlds as a consequence.
“at no step in the argument did we say you know what would be awesome is if there were like a million worlds...all we did was follow the formalism of quantum mechanics to see what it predicted and whatever it is saying is what it predicts is exactly what we see”
The biggest open problem in physics is reconciling gravity with quantum mechanics, and the Everett interpretation approach—which has no classical presuppositions—is ideally suited for this task because you can ask how space and spacetime emerge from a purely quantum mechanical description.
“the biggest most looming one is how to reconcile gravity with quantum mechanics...maybe the mistake that we've been making is trying to quantize gravity what we want is a quantum theory that gives us gravity...the ever-ready approach that has no classical presuppositions in it is ideally Suited so let's see if we can do that”
The Copenhagen interpretation is unacceptable as a fundamental theory of nature because it is vague about what counts as a measurement—can a video camera measure something? Can a mouse measure something? Can a person without glasses who squints measure something?—and it seems to put human agency at the center of the universe, leading to pseudoscientific interpretations like quantum healing and quantum yoga.
“what does that mean what counts as a measurement can a video camera measure something can a mouse measure something what about a person who needs glasses and they're not they don't have their glasses on what if I just squint at the system...you can see how a whole book industry about Quantum healing and Quantum yoga and Quantum leadership is going to spring up in the aftermath of this because this way of talking seems to put human agency at the center of the universe”
Quantum computers and quantum computing are driving renewed interest in foundational quantum mechanics because quantum computers require systems in highly entangled coherent states, creating experimental situations where the Copenhagen interpretation is insufficient because the dividing line between classical and quantum becomes fuzzy.
“there's other kinds of experiments where the Copenhagen interpretation is not that it's proven wrong but that it's just not up to the task because the dividing line between classical and Quantum becomes fuzzier when you make quantum computers when you put lots of things into entangled coherent States at the same time”
Physics is fundamentally based on observations and measurements of data; when scientists applied the Schrödinger equation to predict what they would see when electrons are emitted, the equation predicts a spherical cloud of electron probability in all directions, but actual observations show electrons leave discrete straight-line particle trajectories.
“when you look at electrons you don't see waves you see particles here's a real image a real little movie this is a radioactive Source in a cloud chamber...all of these little lines are individual particles being emitted by the radioactive source and they leave a straight line trajectory like they're a particle with a location moving away from the source”
The Schrödinger equation itself has no directionality in time—it works equally well forward and backward in time like Newton's laws—so the arrow of time (the asymmetry between past and future) must come from initial conditions of the universe, not from the laws of physics themselves.
“this equation doesn't tell the difference between the past and the future...it has no directionality in time it works equally well...just like Newton's Laws of mechanics do not pick out a direction forward or backward”
De Broglie suggested that electrons, like light, have wave-like properties, with the electron described not as a point particle in orbit but as a wave or field that can take various shapes, and these shapes correspond to electron orbitals in atoms where electrons do not concentrate near the nucleus because that would require too much energy.
“Louis de broy suggested that maybe particles like electrons have wave-like properties...rather than having an orbit around the nucleus the wave that is the electron has a shape...those orbitals are just ways that the electron wave can settle down and none of them are all concentrated right there in the nucleus because that would actually take a lot of energy”
Quantum mechanics is understandable and is physics and science, not magic, despite its counterintuitive nature.
“it is understandable and you know some of us think we do understand it we may or may not be right but even if we're wrong it is not magic it is physics it is science and it can be understood”
From the entanglement structure in a quantum wave function, you can derive an emergent geometry of space using mathematical concepts from differential geometry (like those studied by Euclid, Gauss, and Riemann), meaning that spacetime geometry is not fundamental but emerges from quantum entanglement.
“you can derive an emergent geometry just from entanglement within a Quantum wave function...you can derive a geometry of space...the famous mathematicians who talked about geometry...you can derive an emergent geometry just from entanglement”
Understanding quantum mechanics correctly at the fundamental level matters not just philosophically but scientifically, because good understanding of deep physics can lead to better physics and solving unanswered questions in the field.
“at heart I'm still a physicist I still want to do science with this...it's more than good I don't want to downplay it it's crucially important to understand what is really going on...but it's not just to make us feel better it's because a good understanding of what's going on at the fundamental level will lead to better physics”
The Everett interpretation does not add any equations—there is no 'Everett equation'—but rather removes the measurement and collapse rules from Copenhagen and simply applies the Schrödinger equation universally to all physical systems at all times.
“he didn't add any equations there's no Everett equation...all ever it did was say...you have wave functions you have the Schrodinger equation that's all you need”
The reality problem in quantum mechanics asks whether the wave function is a real physical thing (like electromagnetic waves that you can measure) or merely a mathematical tool for predicting measurement outcomes, and the Copenhagen interpretation doesn't provide a clear answer to this question.
“there is the reality problem...we talk about the wave the electron wave or the wave function as we call it...what is that is it a real thing like an electromagnetic wave...or is it just a way of making predictions about reality”
The objection that many-worlds interpretation creates too much energy because all those worlds should cost energy to create misunderstands energy conservation: different worlds in the superposition have different 'amplitudes' or 'weights,' with the total energy across all branches remaining conserved, while individual branches become thinner and contribute less energy over time.
“there are more and more worlds but they're not created equally...different worlds have a different oomph a different thickness okay a different weight...energy is conserved in fact arguably the average interpretation is the only one where energy is conserved...your universe is Getting Thinner and thinner it is giving less and less of a contribution to the energy of the whole”
In quantum field theory, there is a natural relationship between energy and entanglement: reducing entanglement between a region and its neighbors (breaking correlations) requires energy, and conversely, regions with less entanglement than the vacuum have energy in them.
“there is automatically in Quantum field Theory a very natural relationship between energy and entanglement...I need to break the entanglement I need to decrease the amount of entanglement between one region and its neighbors so there is automatically...a relationship between energy and entanglement”
Einstein and colleagues (Podolsky and Rosen) invented or popularized the concept of quantum entanglement in 1935, showing that when an unstable particle decays into two particles, the two particles' wave functions become entangled such that a measurement of one particle's direction instantly determines what direction the other particle (even if far away) will be measured as moving, without any signal traveling between them.
“Albert Einstein and his collaborators...invented or popularized this idea called entanglement Einstein podolsky and Rosen 1935...if you look for particle number one...you'll see that it's moving in a certain direction...but now notice that the original particle...wasn't moving...momentum is conserved therefore you know if you see one particle moving in One Direction the other particle has to be seen moving in the other direction”
The standard model of particle physics is compatible with every experiment ever done in laboratory conditions on Earth, which is both a good thing (the model works) and a bad thing (it makes it hard to discover new physics when your existing theory fits everything).
“we have a model called the standard model of particle physics which is literally compatible with every experiment ever done in a laboratory here on Earth...it's a terrible situation to be in as a physicist because it's very hard to make progress when the data agree with your existing Theory”
It seems like electrons behave as waves when you're not looking at them but behave as particles when you look at them, which is strange and counterintuitive because it suggests the electron somehow knows whether it's being observed and changes its behavior accordingly.
“what we're saying is that electrons behave like waves when you're not looking at them but when you look at them they start behaving like particles now this should drive you crazy...what is the electron care whether I am looking at it or not it's an electron it just does its electron things”
When the universe branches into different versions of you seeing different measurement outcomes, those branches create different people, not different parts of one person, analogous to identical twins who start as a single zygote but split into two separate individuals with no obligation toward each other despite sharing history.
“once the branching of the of the wave function of the universe occurs the different copies of you are different people they're not a part of you in a different Universe they are almost exactly analogous to identical twins...when the wave function of the universe branches the only difference is you can't talk to all of your almost identical twins in the other universes”
The early universe was in a very special, low-entropy quantum state, which is unusual and unexpected—it's not a random or generic state—but we don't yet have consensus on why the universe started in this special state, though some physicists like Stephen Hawking have proposed ideas.
“the early Universe was in a Quantum state which by All rights is unusual is strange it's not random it's not generic it's not what you would expect it's very very special why...Stephen Hawking thought he knew the answer uh but we don't know”
You can imagine an alternative history of physics where general relativity wasn't invented until long after quantum mechanics, and physicists, understanding quantum mechanics and geometry, would predict that geometry should be dynamical and influenced by energy—essentially re-inventing general relativity by reasoning from quantum principles.
“we can at least imagine alternative history of physics where general relativity wasn't invented until long after quantum mechanics and people said you know given what I know about quantum mechanics and given what I think is probably true about geometry I bet that geometry is actually not static I bet it's dynamical in response to the amount of matter energy in the universe and you would invent general relativity that way”
The most common objection to many-worlds is 'there are just too many universes, I don't like it'; Carroll suggests this is the only real objection and that other objections people offer are intellectually dishonest rationalizations; however, accepting that electrons are waves (necessary for chemistry) already commits you to superposition, and accepting entanglement commits you to multiple particles in superposition, so the step to many worlds is trivial once you accept these prerequisites.
“the most obvious objection is that's just too many universes I don't like it this is the most common objection and between you and me I think this is the only objection I think this is the objection everyone really has when they don't like it even if they offer you other objections... but look you can easily believe that an electron can be a wave you better have to believe that if you want to explain atoms and so forth and double slit experiment that means the electron is in a superposition of possible measurement outcomes”
If you assume the early universe was in a special, low-entropy, highly entangled quantum state (for whatever reason), then the branching and time-asymmetry structure of many-worlds fits together naturally, and all the pieces of the theory cohere.
“but what we do know is if you make that assumption if you say that for whatever reason our observable universe started in a state that is very special highly entangled low entropy or highly unentangled I should say then it all fits together it all branches like this very very naturally”
Erwin Schrödinger did not instantly draw the many-worlds conclusion from entanglement in 1935, and neither did other physicists; instead, it took Hugh Everett, a graduate student, 20-30 years later to realize and formalize this implication.
“nobody said this in 1935 it was an instant conclusion you could draw from what was said in 1935. Erwin Schrodinger sort of joked about it a little bit but he didn't quite follow it through uh it was left to a graduate student 30 years later 20 or 30 years later Hugh Everett to figure things out”
Carroll wrote an op-ed in the New York Times saying 'nobody understands quantum mechanics and that's a problem' and expected multiple colleagues to email him saying they understand it, but only one colleague did, indicating lack of consensus within the physics community about the correct interpretation of quantum mechanics.
“I wrote a op-ed piece for the New York Times where I say nobody understands quantum mechanics and that's a problem and I expected at least a few of my colleagues to email me saying what do you mean I understand quantum mechanics and I was pleasantly surprised it was only one colleague who did email me about that”
The forces of nature other than gravity (electromagnetism, nuclear forces) are described by quantum field theories, where fields are fundamental objects that have values everywhere in space, and what we call particles are excitations of these quantum fields.
“the forces of nature that we do understand is that they are described by Quantum Fields...they're not really particles they look like particles in the right circumstances but they're really fields at heart...a field is just something that has a value at everywhere in space”
The Schrödinger equation describes how the wave function (represented by the Greek letter ψ) changes over time, with the equation stating that the amount of energy in the wave determines how fast it changes with time, and this equation was immediately popular with physicists in the 1920s.
“Schrodinger came up with this equation that says how the wave function of the electron or of anything else changes over time and it basically says the amount of energy in the wave tells you how fast it's changing with time”
The Feynman quote 'If you think you understand quantum mechanics, you don't understand quantum mechanics' is fake news—Feynman never said that exact phrase; he actually said 'I think that nobody understands quantum mechanics,' which is different in meaning and attribution.
“the Feynman quote is fake news he never said that...he did say uh I think that nobody understands quantum mechanics...he didn't say If you think you understand it you don't he said no one understands it whether you think you do or not”
Einstein's theory of general relativity says that gravity is the curvature of spacetime, where the geometry of spacetime is dynamical and responds to the presence of energy and mass in the universe.
“gravity is the curvature of space-time...the geometry of space-time is influenced by the amount of energy...Planet Galaxy Dark Matter curves space time in some particular way”
Max Planck made the claim that consciousness is fundamental and matter is derivative from consciousness, which Carroll regards as mistaken, noting that Planck said many dopey things including not believing in photons.
“Max Planck said that in his view Consciousness was fundamental...I'm guessing you disagree with that...Max Planck said a lot of Dopey things he never believed in photons”
The James Webb Space Telescope can observe galaxies from 13.7 billion years ago but cannot directly see the Big Bang itself because the earliest galaxies are always later than the cosmic microwave background, which comes from 380,000 years after the Big Bang.
“the James Webb...has seen back 13.7 billion years...it cannot but that's okay because we already saw the Big Bang...the cosmic microwave background...only gives us a view of what the universe looked like 380 000 years after the big bang...the earliest galaxies will always be later than the cosmic microwave background”
The cosmic microwave background is leftover radiation from the Big Bang and shows an extremely smooth distribution of energy density (one part in a hundred thousand), with galaxies, stars, and planets believed to have grown from gravity amplifying tiny density fluctuations in this smooth early universe.
“the cosmic microwave background is the leftover radiation from The Big Bang...the early Universe...was opaque...it didn't become transparent until 380 000 years...we see the radiation that last interacted at that time...it's very very smooth...one part in a hundred thousand...there were no galaxies...all of that stuff...grew through the force of gravity taking the slightly over dense regions”
The research program Carroll and his colleagues are pursuing on emergent gravity from quantum entanglement is very tentative and speculative, might crash and burn at any time, but initial indications are extremely promising.
“this is absolutely like speculation from here on where this is research that is ongoing might crash and burn at any time but it's an interesting thing to think about...the initial indications are extremely promising”
The ethical and moral implications of many-worlds are the same as the Copenhagen interpretation: in both cases, you can make predictions about measurement outcomes using quantum mechanics; the difference is only that in Everett you know there are other branches you can't see, whereas in Copenhagen you just accept probabilistic predictions without asking what is really happening.
“there is really no ethical or moral or philosophical impact to all these extra universes that would be any different than the ordinary textbook Copenhagen interpretation of quantum mechanics would let you talk about the fact that there are many copies of you but all of them have different amplitudes gives you the same utility function or whatever you want to have as you would in a universe where you could just probabilistically predict experimental outcomes”