Insight | Is quantum mechanics missing something? Three physicists on what is real

The Short Version

Three physicists give three different answers, and the difference is about meaning, since none of them disputes that the theory's predictions work. Roger Penrose says outright that quantum mechanics is incomplete and "in a certain sense, not quite right", because its basic equation never produces the single outcomes we actually see, and he thinks gravity supplies the missing piece [1]. Lisa Randall sees no need to rewrite it: for her the wave function is real, an electron is there whether or not anyone measures it, and the argument is "a disagreement on how you interpret quantum mechanics" [2]. Michelle Thaller treats the strangest feature, entanglement, as "a real experimental fact", and draws the most open conclusion: "I don't think we understand yet what reality is" [3]. So the experiments are settled and widely confirmed, while the question of what they say about the world is still argued among people who use the theory every day [1][2][3].

What They Said

Roger Penrose: something is missing, and gravity is the place to look

Penrose's doubt began in a lecture hall. As a graduate student at Cambridge he sat in on Paul Dirac's course on quantum mechanics, and in what he thinks was the very first lecture Dirac introduced the superposition principle: a particle you would normally think of as over here or over there can, in quantum mechanics, "be over here and over there at the same time" [1]. Some people say Dirac broke his piece of chalk in two to illustrate it [1]. Penrose's mind wandered, he missed the explanation that followed, and he has come to think that was fortunate: "It's probably just as well." [1] The explanation, he suspects, was the kind of thing meant to calm a student down, whereas in his case "I've worried about it ever since" [1].

What he worries about is how a superposition ever becomes one thing or the other. "There is something in quantum mechanics where these superposition become one or the other. And that's not part of quantum mechanics," he says [1]. His verdict follows: "There's something missing in the theory. The theory is incomplete. It's not just incomplete. It's in a certain sense, not quite right." [1]

The argument turns on the Schrödinger equation, the basic equation of the theory. It is deterministic and computable; you could put it on a computer [1]. Followed faithfully, it never yields a single definite result. That step is supplied separately by what people call the reduction of the state or the collapse of the wave function, "which you have to have otherwise quantum mechanics doesn't relate to the world we see" [1]. To make the theory match experience, he says, "you've got to break the Schrodinger equation" [1].

He insists this was Schrödinger's own view. The famous cat, dead and alive at the same time, is what the equation leads to, and Penrose says people today misread the point of it. Schrödinger was not announcing that such cats are possible: "he was saying, this is an absurdity." [1] There is something missing, and the collapse "is something which has to be understood" [1].

He names his company. At least three of the greatest physicists of the 20th century who were fundamental in developing quantum mechanics thought it incomplete: Einstein, Schrödinger and Dirac [1]. Dirac was cautious about saying so aloud, but if you find the right place in his writing, "he says quantum mechanics is a provisional theory" and that we need to go beyond it [1].

Penrose is equally clear about how small his camp is. There is a group of people, "all considered to be a bit, you know, a bit mavericks, who believe that quantum mechanics needs to be modified" [1]. A small minority of that minority think the modification has to involve gravity, and a smaller one still think it works in the particular way he does [1].

His reason for pointing at gravity is a conflict between two principles. The principle of equivalence goes back to Galileo: "If you fall freely, you eliminate the gravitational field." [1] Penrose illustrates it with Galileo's big rock and little rock dropped from the leaning tower, an insect on one rock watching the other and concluding there is no gravity, and a firework whose sphere of sparks stays a sphere as it falls [1]. Einstein built general relativity on that principle. Much later, Penrose came to see "a basic conflict" between it and Dirac's principle of superposition [1].

This leads to his objection to the usual research programme. Quantum gravity, as normally meant, takes the formalism of quantum mechanics as given and makes gravity fit into it [1]. He wants the influence to run both ways: "It's not just you quantize gravity. You've got to gravitate quantum mechanics." [1] "And it's a two way thing." [1]

He does not oversell it. Asked where one even starts, he says, "We say, okay, well, what's the theory? I don't know." [1] "And then we have no experiments as yet." [1] There are experiments being thought through, including one being developed by Dirk Bouwmeester, who divides his time between Leiden and Santa Barbara, which could perhaps demonstrate that quantum mechanics as now understood has to be modified once gravitational effects are brought in [1].

Penrose connects this to his best-known speculation. With the anesthesiologist Stuart Hameroff he proposes that the quantum state reduces itself in a non-computational way not yet understood, and that each such event is an element of proto-consciousness [1]. He points out that this is almost the opposite of the older idea that a conscious observer collapses the wave function [1]. By his own three grades for physical theories, superb, useful or tentative, he says a theory of consciousness probably has not even reached tentative [1]. He also declines the label materialist, "because it suggests we know what material is" [1].

Lisa Randall: the electron is there

Randall, a Harvard theoretical physicist, enters the question through a disagreement with the physicist Carlo Rovelli. Lex Fridman reads Rovelli's claim, as Randall had quoted it, that electrons do not always exist: they exist when they interact, and materialize in a place when they collide with something else [2]. He then reads her written reply: "Stocks may not achieve a precise value until they're traded, but that doesn't mean we can't approximate their worth until they change hands." [2] In the same way, "electrons might not have definite properties, but they do exist" [2]. She grants the quantum point in full. The electron does not exist as a classical object with a definite position until its position is measured, "but something was there which physicists used a wave function to describe" [2].

In conversation she puts it more bluntly. "I like to think of the universe as being out there," she says, adding that "it would be really weird if the only time things came into existence was when I saw them or I measured them" [2]. She offers a deliberately absurd comparison: she could believe the Middle East does not exist because she is not there now, and everyone would agree that is ridiculous [2]. Her conclusion: "there's only so much that we can attribute to our own powers of seeing. And the whole system doesn't come into being because I'm measuring it." [2]

She is careful to say what kind of dispute this is. It is not about the Standard Model of particle physics. "This is a disagreement on how you interpret quantum mechanics." [2] Her own interpretation is stated without hedging: "I would say that those wave functions are real" [2]. She adds a precision from particle physics. Quantum field theory does say electrons can be created and destroyed, as happens in colliders, but an electron in an atom, with its charge, is really there [2].

Randall does not claim the theory is final. Asked whether physics will reach a bottom level, she says she is more humble than many physicists: she is not sure we will, though we will keep penetrating deeper layers, and even notions like distance might break down [2]. She distinguishes fundamental physics from the effective theories that work at larger scales, such as describing a thrown ball without tracking every atom [2]. On the limits of science her answer is, "I'm smart enough to know. I have no idea." [2] A century ago, she notes, we did not know about quantum mechanics, special relativity or the expansion of the universe, so "it would be premature to say we know limitations" [2].

Her method bears on Penrose's proposal even though she does not address it. Discussing panpsychism, she says the right approach is to exhaust the possibilities within existing frameworks before jumping to the most far-out one [2]. And she describes her own motive in terms Penrose would recognize: "When things don't make sense, it really bugs me." [2]

Michelle Thaller: entanglement is a fact, and its meaning is open

Thaller, an astrophysicist and retired NASA executive, starts from the experiments. Entanglement, she tells Joe Rogan, means things are connected regardless of the distance between them, and she stresses that "this is a real experimental fact" [3]. She compares it to time dilation, another idea that sounds idiotic on first hearing and is now so routine that it is needed to calibrate GPS satellites [3].

The history she gives begins with resistance. A hundred years ago Einstein understood that quantum mechanics pointed toward this, "but he really didn't like it. He called it spooky action at a distance." [3] The implication was that two things connected quantum mechanically could be taken any distance apart and would still respond to each other instantaneously [3]. Einstein did not think that would ever actually happen. "And then back in the mid-1990s, we started to do experiments with atoms, and we found out that it was real." [3]

Her description of the effect: two electrons sharing an orbit must spin in opposite directions; separate them, and if you use an electric field to change the spin of one, "the other one immediately changes in response" [3]. Chinese researchers, she says, have done this between the ground and a space station [3]. How she reads the result is the philosophically interesting part: "the universe is saying, these two things are the same quantum mechanical system. They're basically the same object." [3] Distance is irrelevant to it: "The universe says the space and time between them doesn't matter. They're the same system." [3]

From there she moves, openly, into questions. If the Big Bang had everything in a small volume at once, "are we entangled to everything in some way?" [3] She goes on to ask, "is a part of me quantum mechanically right now in the Andromeda Galaxy?" [3]. She wonders whether a civilization could learn to harness that connectedness, and when Rogan brings up the entangled "sophons" of the science fiction series The Three-Body Problem, she says that as far as she can tell it "could be theoretically possible" [3].

Her summary is the least settled of the three: "I don't think we understand yet what reality is" [3]. She places the gap where Penrose does. "When gravity and quantum mechanics come together, those two theories, they don't work well together," she says, which is why she takes accounts of the first instants of the Big Bang with a large grain of salt [3]. Of what lies inside black holes she says, "Quite honestly, our physics gives up." [3] None of our models of matter correctly predicts the size of neutron stars, she adds [3].

Thaller frames all of this with a rule about science itself: "You have to be honest about what you don't know." [3] Science is deliberately limited to "what is reproducible, consistently reproducible," which she says does not mean other things are not real [3]. And she marks the line when she crosses it, saying at one point that she is going from observation to conjecture [3].

Where They Agree, and Where They Don't

They agree that the theory describes real phenomena. Penrose took the superposition principle from Dirac and has never doubted that it describes something [1]. Randall grants that an electron has no definite position before measurement [2]. Thaller calls entanglement an experimental fact confirmed since the 1990s [3]. No one here is disputing a measurement.

They agree that the open question is one of meaning. Randall says so in as many words [2]. Penrose's complaint is that the theory as conventionally used "doesn't relate to the world we see" without an added step nobody has explained [1]. Thaller says we do not yet understand what reality is [3].

They agree that an observer does not conjure the world. Randall rejects the idea that things come into being because she measures them [2]. Penrose's proposal has the quantum state reduce itself, the opposite of collapse by a conscious observer [1]. Rovelli, as Randall reports him, stands on the other side of this, holding that electrons exist when they interact [2].

They disagree about whether the theory itself must change. This is the real dispute. Penrose says the equation has to be broken and the theory modified [1]. Randall treats the wave function as real and the puzzle as one of interpretation, and her stated habit is to exhaust existing frameworks before reaching for radical ones [2]. Penrose holds his view because the equation, followed honestly, gives a cat that is dead and alive, and because he sees a conflict of principle with general relativity [1]. Randall holds hers because the existing description already says something is there before measurement, and denying that leads to absurdities of its own [2]. Penrose is frank that his position is that of a minority within a minority [1].

Penrose and Thaller converge on gravity, with different degrees of commitment. Both say gravity and quantum mechanics do not fit together [1][3]. Thaller states it as a known limit that shows up in black holes and the Big Bang [3]. Penrose goes further and makes gravity the cause of collapse in ordinary situations, while admitting he has no theory [1].

They respond differently to the strangeness. Penrose calls the textbook picture an absurdity that signals an error [1]. Randall plays the strangeness down: quantum weirdness, in her account, does not mean reality vanishes when unobserved [2]. Thaller takes it as a pointer to a deeper connectedness and is willing to wonder aloud where that leads [3].

None of the three directly answers the others. These are separate conversations. Randall's remarks are aimed at Rovelli, not Penrose, and Thaller addresses neither [2][3]. The debate set out here is assembled from their positions.

Why It Works

The sources explain why a theory can be this successful and this unsettled at once [1][2][3].

The equation and the experience come apart. The Schrödinger equation evolves superpositions smoothly and deterministically, and measurements give single results [1]. In practice physicists add the collapse by hand. Penrose's whole argument is that this added step is doing essential work and is not derived from anything [1].

The wave function gives an alternative reading. Randall's stock-price analogy shows how one can accept indefinite properties without denying existence: a stock has no exact price until it trades, yet its worth can be approximated in the meantime [2]. On that reading there is no gap to fill, only a non-classical kind of object to get used to [2].

Entanglement shows the description is of whole systems. Thaller's point that two separated electrons are the same quantum mechanical system explains why the theory resists pictures built from separate objects at separate places [3]. It is also why Einstein, who wanted such a picture, disliked it [3].

The founders' doubts are part of the record. Einstein's "spooky action at a distance" and the views Penrose attributes to Schrödinger and Dirac show that questioning the theory's completeness began with the people who made it [1][3].

Our intuitions were built for another scale. Randall says physics teaches us to go beyond intuition formed at everyday scales and to develop new intuitions for very different distances [2]. Penrose's refusal of the word materialist makes a related point about how little we know of what matter is [1].

The two great theories have never been joined. Penrose's conflict between equivalence and superposition and Thaller's statement that the theories do not work well together identify the same unfinished business [1][3]. That is why proposals like his aim at the meeting point.

Where It Applies, and Where It Doesn't

The predictions are not in question. Nothing in these conversations casts doubt on the experimental side. Randall calls the Standard Model, which is built on quantum theory, spectacularly successful [2]. Thaller says entanglement has been demonstrated between the ground and orbit [3].

Penrose's proposal is a conjecture, by his own account. He has no theory and, as yet, no experiment, and he places himself in a small minority [1]. His link to consciousness is more tentative still [1].

Thaller's largest ideas are questions. Whether we are entangled with everything, and whether the connection could be harnessed, are things she asks, in a passage she distinguishes from observation [3]. Her remark about the fictional sophons is an impression offered in conversation [3].

Randall's realism is an interpretation too. She presents the reality of the wave function as her own view, in an argument with another physicist who reads the same theory differently [2].

Strangeness is not a licence. Randall's rule of exhausting existing frameworks first, and Thaller's standard of consistent reproducibility, both caution against using quantum mechanics to support whatever one would like to be true [2][3]. Penrose says he rejects anything mystical and untreatable by science [1].

The question may be decided by experiment. Penrose is waiting on tests of whether gravity alters quantum behavior [1]. Randall thinks it premature to declare limits on what science can find out [2].

What To Do With It

1. Separate what is measured from what is interpreted. When reading about quantum physics, ask whether a statement is an experimental result, such as entanglement, or a reading of what it means, such as whether electrons exist unobserved [2][3].

2. Keep Randall's stock analogy to hand. Lacking a definite value is different from not existing, and it is a clear way to hold the central idea without mystery [2].

3. Read Schrödinger's cat as its author meant it. According to Penrose it was an argument that the theory leads to an absurdity, and so a claim that something is missing [1].

4. Do not put the observer at the center. Randall and Penrose, from opposite positions, both reject the idea that looking brings the world into being [1][2].

5. Notice how an expert labels a claim. Penrose says he has no theory and no experiment, Randall calls her bolder ideas speculative, and Thaller says when she is moving from observation to conjecture [1][2][3].

6. Watch the boundary between gravity and quantum theory. Two of the three locate the trouble there, and it is where Penrose expects a test [1][3].

7. Hold the two attitudes together. Randall's advice to scientists is that "you have to believe really strongly what you do while questioning it all the time." [2]

8. Be comfortable with an open question. Thaller's rule is to be honest about what you do not know, and on what quantum mechanics says about reality, that honesty is the current state of the field [3].

Sources

1. Roger Penrose on Why Consciousness Isn't Computation and the Infinite Cycles of the Universe · Lex Fridman Podcast
2. Lisa Randall on Dark Matter, the Dinosaur Extinction, and the Edges of Physics · Lex Fridman Podcast
3. Michelle Thaller on Black Holes, Time, Quantum Weirdness and the Limits of Science · The Joe Rogan Experience