particle physics

For centuries, the laws of physics seemed completely deterministic. If you knew where every particle was, how fast it was moving, and what the forces were between them at any one instant, you could know exactly where they’d be and what they’d be doing at any point in the future. From Newton to Maxwell, the rules that governed the Universe had no built-in, inherent uncertainty to them in any form. Your only limits arose from your limited knowledge, measurements, and calculational power.

All of that changed a little over 100 years ago. From radioactivity to the photoelectric effect to the behavior of light when you passed it through a double slit, we began realizing that under many circumstances, we could only predict the probability that various outcomes would arise as a consequence of the quantum nature of our Universe. But along with this new, counterintuitive picture of reality, many myths and misconceptions have arisen. Here’s the true science behind 10 of them.

particle physics

By creating a track where the outside magnetic rails point in one direction and the inside magnetic rails point in the other, a Type II superconducting object will levitate, remained pinned above-or-below the track, and will move along it. This could, in principle, be scaled up to allow resistance-free motion on large scales if room-temperature superconductors are achieved.

(Credit: Henry Mühlpfordt/TU Dresden/Wikimedia Commons)

1.) Quantum effects only happen on small scales. When we think of quantum effects, we typically think about individual particles (or waves) and the bizarre properties they display. But large-scale, macroscopic effects happen that are inherently quantum in nature.

Conducting metals cooled below a certain temperature become superconductors: where their resistance drops to zero. Building superconducting tracks where magnets levitate above them and travel around them without ever slowing down is a routine student science project these days, built on an inherently quantum effects.

Superfluids can be created on large, macroscopic scales, as can quantum drums that simultaneously do and don’t vibrate. Over the past 25 years, 6 Nobel Prizes have been awarded for various macroscopic quantum phenomena.

particle physics

The energy level differences in an atom of Lutetium-177. Note how there are only specific, discrete energy levels that are acceptable. While the energy levels are discrete, the positions of the electrons are not.

(Credit: M.S. Litz and G. Merkel Army Research Laboratory, SEDD, DEPG)

2.) Quantum always means “discrete.” The idea that you can chop up matter (or energy) into individual chunks — or quanta — is an important concept in physics, but it doesn’t fully encompass what it means for something to be “quantum” in nature. For example: consider an atom. Atoms are made of atomic nuclei with electrons bound to them.

Now, think about this question: where is the electron at any moment in time?

Even though the electron is a quantum entity, its position is uncertain until you measure it. Take many atoms and bind them together (such as in a conductor), and you’ll frequently discover that although there are discrete energy levels that the electrons occupy, their positions can literally be anywhere within the conductor. Many quantum effects are continuous in nature, and it’s eminently possible that space and time, at a fundamental, quantum level, are continuous, too.

particle physics

By creating two entangled photons from a pre-existing system and separating them by great distances, we can ‘teleport’ information about the state of one by measuring the state of the other, even from extraordinarily different locations. Interpretations of quantum physics that demand both locality and realism cannot account for a myriad of observations, but multiple interpretations all appear to be equally good.

(Credit: Melissa Meister/ThorLabs)

3.) Quantum entanglement allows information to travel faster-than-light. Here’s an experiment we can perform:

  • create two entangled particles,
  • separate them by a great distance,
  • measure certain quantum properties (like the spin) of one particle on your end,
  • and you can know some information about the quantum state of other particle instantaneously: faster than the speed of light.

But here’s the thing about this experiment: no information is being transmitted faster than the speed of light. All that’s happening is that by measuring the state of one particle, you are constraining the probable outcomes of the other particle. If someone goes and measures the other particle, they will have no way of knowing that the first particle has been measured and the entanglement has been broken. The only way to determine whether entanglement has been broken or not is to bring the results of both measurements back together again: a process that can only occur at light speed or slower. No information can be passed faster than light; this was proven in a 1993 theorem.

particle physics

In a traditional Schrodinger’s cat experiment, you do not know whether the outcome of a quantum decay has occurred, leading to the cat’s demise or not. Inside the box, the cat will be either alive or dead, depending on whether a radioactive particle decayed or not. If it were a true quantum system, the cat would be neither alive nor dead, but in a superposition of both states until observed. However, you can never observe the cat to be simultaneously both dead and alive.

(Credit: Dhatfield/Wikimedia Commons)

4.) Superposition is fundamental to quantum physics. Imagine you have multiple possible quantum states that a system can be in. Maybe it can be in state “A” with 55% probability, state “B” with 30% probability, and state “C” with 15% probability. Whenever you go to make a measurement, however, you never see a mix of these possible states; you’ll only get a single-state outcome: either it’s “A,” “B,” or “C.”

Superpositions are incredibly useful as intermediate calculational steps to determine what your possible outcomes (and their probabilities) will be, but we can never measure them directly. In addition, superpositions don’t apply to all measurables equally, as you can have a superposition of momenta but not positions or vice versa. Unlike entanglement, which is a fundamental quantum phenomenon, superposition is not quantifiably or universally measurable.

particle physics

A variety of quantum interpretations and their differing assignments of a variety of properties. Despite their differences, there are no experiments known that can tell these various interpretations apart from one another, although certain interpretations, like those with local, real, deterministic hidden variables, can be ruled out.

(Credit: English Wikipedia page on Interpretations of Quantum Mechanics)

5.) There’s nothing wrong with us all choosing our favorite quantum interpretation. Physics is all about what you can predict, observe, and measure in this Universe. Yet with quantum physics, there are multiple ways to conceive of what’s occurring at a quantum level that all agree equally with experiments. Reality can be:

  • a series of quantum wavefunctions that instantaneously “collapse” when a measurement is made,
  • an infinite ensemble of quantum waves, where a measurement selects one member of the ensemble,
  • a superposition of forwards-moving and backwards-moving potentials that meet in a “quantum handshake,”
  • an infinite number of possible worlds corresponding to the possible outcomes, where we simply occupy one path,

as well as many others. Yet choosing one interpretation over another teaches us nothing except, perhaps, our own human biases. It’s better to learn what we can observe and measure under various conditions, which is physically real, than to prefer an interpretation that has no experimental benefit over any other.

particle physics

Many entanglement-based quantum networks across the world, including networks extending into space, are being developed to leverage the spooky phenomena of quantum teleportation, quantum repeaters and networks, and other practical aspects of quantum entanglement. The quantum state is “cut-and-pasted” from one location to another, but cannot be cloned, copied, or “moved” without destroying the original state. In reality, no information is being exchanged faster than light.

(Credit: S.A. Hamilton et al., 70th International Astronautical Congress, 2019)

6.) Teleportation is possible, thanks to quantum mechanics. There actually is a real phenomenon known as quantum teleportation, but it most definitively does not mean that it’s physically possible to teleport a physical object from one location to another. If you take two entangled particles and keep one close by while sending the other one to a desired distination, you can teleport the information from the unknown quantum state on one end to the other end.

This has enormous restrictions on it, however, including that it only works for single particles and that only information about an indeterminate quantum state, not any physical matter, can be teleported. Even if you could scale this up to transmit the quantum information that encodes an entire human being, transferring information is not the same as transferring matter: you cannot teleport a human, ever, with quantum teleportation.

particle physics

This diagram illustrates the inherent uncertainty relation between position and momentum. When one is known more accurately, the other is inherently less able to be known accurately. Other pairs of conjugate variables, including energy and time, spin in two perpendicular directions, or angular position and angular momentum, also exhibit this same uncertainty relation.

(Credit: Maschen/Wikimedia Commons)

7.) Everything is uncertain in a quantum Universe. Some things are uncertain, but many things are extremely well-defined and well-known in a quantum Universe. If you take an electron, for example, you cannot know:

  • its position and its momentum,
  • or its angular momentum in multiple, mutually perpendicular directions,

exactly and simultaneously under any circumstances. But some things about the electron can be known exactly! We can know its rest mass, its electric charge, or its lifetime (which appears to be infinite) with exact certainty.

The only things that are uncertain in quantum physics are pairs of physical quantities that have a specific relationship between them: that are pairs of conjugate variables. This is why there are uncertainty relations between energy and time, voltage and free charge, or angular momentum and angular position. While many pairs of quantities have an inherent uncertainty between them, many quantities are still known exactly.

particle physics

The inherent width, or half the width of the peak in the above image when you’re halfway to the crest of the peak, is measured to be 2.5 GeV: an inherent uncertainty of about +/- 3% of the total mass. The mass of the particle in question, the Z boson, is peaked at 91.187 GeV, but that mass is inherently uncertain by a significant amount owing to its excessively short lifetime. This result it remarkably consistent with Standard Model predictions.

(Credit: J. Schieck for the ATLAS Collaboration, JINST7, 2012)

8.) Every particle of the same type has the same mass. If you could take two identical particles — like two protons or two electrons — and put them on a perfectly accurate scale, they’d always have the same exact mass as one another. But that’s only because protons and electrons are stable particles with infinite lifetimes.

If you instead took unstable particles that decayed after a short while — such as two top quarks or two Higgs bosons — and put them on a perfectly accurate scale, you wouldn’t get the same values. This is because there’s an inherent uncertainty between energy and time: if a particle only lives for a finite amount of time, then there’s an inherent uncertainty in the amount of energy (and hence, from E = mc², rest mass) that the particle has. In particle physics, we call this a particle’s “width,” and it can lead to a particle’s inherent mass being uncertain by up to a few percent.

particle physics

Niels Bohr and Albert Einstein, discussing a great many topics in the home of Paul Ehrenfest in 1925. The Bohr-Einstein debates were one of the most influential occurrences during the development of quantum mechanics. Today, Bohr is best known for his quantum contributions, but Einstein is better-known for his contributions to relativity and mass-energy equivalence. As far as heroes go, both men possessed tremendous flaws in both their professional and personal lives.

(Credit: Paul Ehrenfest)

9.) Einstein himself denied quantum mechanics. It’s true that Einstein had a famous quote about how, “God does not play dice with the Universe.” But arguing against a fundamental randomness inherent to quantum mechanics — which is what the context of that quote was about — is arguing about how to interpret quantum mechanics, not an argument against quantum mechanics itself.

In fact, the nature of Einstein’s argument was that there might be more to the Universe than we can presently observe, and if we could understand the rules we have not yet uncovered, perhaps what appears to be randomness to us here might reveal a deeper, non-random truth. Although this position has not yielded useful results, explorations of the fundamentals of quantum physics continues to be an active area of research, successfully ruling out a number of interpretations involving “hidden variables” present in the Universe.

particle physics

Today, Feynman diagrams are used in calculating every fundamental interaction spanning the strong, weak, and electromagnetic forces, including in high-energy and low-temperature/condensed conditions. But it cannot be an exact picture.

(Credit: V. S. de Carvalho and H. Freire, Nucl. Phys. B, 2013)

10.) Exchanges of particles in quantum field theory completely describe our Universe. This is the “dirty little secret” of quantum field theory that physicists learn in graduate school: the technique we most commonly use for calculating the interactions between any two quantum particles. We visualize them as particles being exchanged between those two quanta, along with all possible further exchanges that could occur as intermediate steps.

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If you could extrapolate this to all possible interactions — to what scientists call arbitrary loop-orders — you’d wind up with nonsense. This technique is only an approximation: an asymptotic, non-convergent series which breaks down past a certain number of terms. It’s an incredibly useful picture, but fundamentally incomplete. The idea of virtual particle exchanges is compelling and intuitive, but is unlikely to be the final answer.

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