Pages The Uncertainty Principle. Spectral Theory. Scattering States.

Special Cases. Many-particle Systems. Density Matrices.

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The Feynman Path Integral. Quasi-classical Analysis. Mathematical Supplement: the Calculus of Variations. Introduction to Quantum Field Theory. Supplement: Renormalization Group. Back Matter Pages In other words, photons behave like particles with detectors present and like waves without detectors.

On the very smallest scales, reality is blurry, not sharply defined — at least when no one is looking. The short answer is no one really knows, hence the crazy cornucopia of quantum interpretations. Some of the attempts to answer that question have, if anything, only added an extra dose of weirdness to the quantum brew. Perhaps the strangest of all the interpretations is the one first proposed in by Princeton physicist Hugh Everett.

In his doctoral thesis, Everett argued that the equations of quantum mechanics should be taken at face value: Quantum waves are real, with each possible wave in effect representing a separate, independent reality. The implications are staggering. At this moment, for example, an uncountable number of yous are reading this, possibly scratching their heads.

For all its universe-begetting outlandishness, the Many Worlds view has many advocates. This eliminates the paradoxes of quantum mechanics without the necessity of conjuring innumerable universes. His Rube Goldbergian experiment has six components: a steel box, a cat, a radioactive element, a Geiger counter, a hammer and a vial of cyanide.

The cat is put in the steel box; the lid is closed. During any given interval of time, the radioactive element may or may not emit a high-energy particle. If it does, the Geiger counter detects it and triggers the hammer to smash the vial, releasing poisonous fumes that kill the cat. According to the rules of quantum mechanics, the radioactive particle exists as a wave function in all its possible states — both emitted and not emitted. A single, definite state crystallizes only upon measurement.

What does that mean for the cat?

Is it both alive and dead until someone opens the box for a look? The cat is alive or dead, not alive and dead. In our everyday world, it seems, the laws of quantum theory lead to absurd results. But what about that two-slit experiment? To get the gist of the PBR argument, consider a simple card game between you and a dealer involving two decks of cards. One deck holds only red cards, the other deck only aces. The dealer gives you a card and asks which deck it came from. In most cases the answer will be easy. The aces could come from either deck.

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If the wave function is not a real physical object and instead only measures experimental probabilities, then more than one wave function could describe a single physical state, say the position of a photon just like that red ace could come from either deck. The notion that a slew of different wave functions could describe the same underlying reality falls apart in quantum mechanics, says Pusey.

He and his colleagues showed that the probabilistic interpretation becomes a problematic one. The predictions those wave functions make are so different.

## What Is Quantum Mechanics Good for?

They contend that Bohr was on to something: Our notion of an objective reality needs modification. The physical world cannot be separated from our own efforts to probe it. They call their way of looking at quantum mechanics QBism, a modified version of a theory they developed with University of New Mexico physicist Carlton Caves called Quantum Bayesianism.

QBism combines quantum mechanics with Bayesian probability, a variation on standard probability in which the odds of any given event are revised as one gains more knowledge of the many possible conditions tied to the event. For example, if a patient complains of headaches to a doctor, the initial odds of a diagnosis of brain cancer might be low. As the doctor examines the patient, the odds of a cancer diagnosis may go up or down.

## quantum mechanics | Definition, Development, & Equations | Britannica

QBism applies similar reasoning to physics experiments: Whenever physicists perform an experiment, they are updating their own subjective knowledge. There is no fixed underlying reality that different observers can independently experience. Just as a doctor must assess each patient individually, so too must a physicist approach the fresh, ever-changing phenomena presented by the quantum world.

In QBism, the experimentalist cannot be separated from the experiment — both are immersed in the same living, unpredictable moment. And reality, if QBism is right, cannot be conceived without always including the subject.

### About the book

Quantum theory, Schack says, offers profound observations about the real world, but the theory itself is not a description of the world. He posits that the right way to think of quantum mechanics is as a set of rules about how to correctly conduct experiments. They choose experiments. You could describe any experiment as a gamble on the outcome. Quantum mechanics is a useful guide to action: It tells you how to put together your experimental apparatus so that it works in the end. What sort of universe do we inhabit? Is it like a giant machine, with the future evolving from the past according to immutable laws?

Or is it inherently interactive? What is it made of? Something beyond our ability to imagine? For theoretical physicist Valentini, the answer has been there from the earliest days of quantum theory. In , the French physicist Louis de Broglie, who first proposed that particles could behave like waves, developed an interpretation of quantum mechanics called pilot wave theory, where waves and particles are both equally real.

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Each particle rides its own wave. The pilot wave is a bizarre thing — it exists in multiple dimensions — but it is a real physical object. Pilot wave theory explains the strange two-slit experiment: A particle always goes through one slit or the other; at the same time its pilot wave travels through both slits. Valentini has devoted his career to almost single-handedly resurrecting the pilot wave idea. Now his years of work actually have a chance — a small one, he admits — of being vindicated.