3683. What Is the Universe?

 By Leonard Susskind, What Is the Universe? February 2026

3458. Quantum Physics: A Very Short History

By Richard Webb, New Scientist,December 2020

What is quantum physics? Put simply, it’s the physics that explains how everything works: the best description we have of the nature of the particles that make up matter and the forces with which they interact. Quantum physics underlies how atoms work, and so why chemistry and biology work as they do. You, me and the gatepost – at some level at least, we’re all dancing to the quantum tune. If you want to explain how electrons move through a computer chip, how photons of light get turned to electrical current in a solar panel or amplify themselves in a laser, or even just how the sun keeps burning, you’ll need to use quantum physics. 

The difficulty – and, for physicists, the fun – starts here. To begin with, there’s no single quantum theory. There’s quantum mechanics, the basic mathematical framework that underpins it all, which was first developed in the 1920s by Niels Bohr, Werner Heisenberg, Erwin Schrödinger and others. It characterises simple things such as how the position or momentum of a single particle or group of few particles changes over time. 

But to understand how things work in the real world, quantum mechanics must be combined with other elements of physics – principally, Albert Einstein’s special theory of relativity, which explains what happens when things move very fast – to create what are known as quantum field theories. 

Three different quantum field theories deal with three of the four fundamental forces by which matter interacts: electromagnetism, which explains how atoms hold together; the strong nuclear force, which explains the stability of the nucleus at the heart of the atom; and the weak nuclear force, which explains why some atoms undergo radioactive decay. 

Over the past five decades or so these three theories have been brought together in a ramshackle coalition known as the “standard model” of particle physics. For all the impression that this model is slightly held together with sticky tape, it is the most accurately tested picture of matter’s basic working that’s ever been devised. Its crowning glory came in 2012 with the discovery of the Higgs boson, the particle that gives all other fundamental particles their mass, whose existence was predicted on the basis of quantum field theories as far back as 1964. 

Conventional quantum field theories work well in describing the results of experiments at high-energy particle smashers such as CERN’s Large Hadron Collider, where the Higgs was discovered, which probe matter at its smallest scales. But if you want to understand how things work in many less esoteric situations – how electrons move or don’t move through a solid material and so make a material a metal, an insulator or a semiconductor, for example – things get even more complex. 

The billions upon billions of interactions in these crowded environments require the development of “effective field theories” that gloss over some of the gory details. The difficulty in constructing such theories is why many important questions in solid-state physics remain unresolved – for instance why at low temperatures some materials are superconductors that allow current without electrical resistance, and why we can’t get this trick to work at room temperature. 

But beneath all these practical problems lies a huge quantum mystery. At a basic level, quantum physics predicts very strange things about how matter works that are completely at odds with how things seem to work in the real world. Quantum particles can behave like particles, located in a single place; or they can act like waves, distributed all over space or in several places at once. How they appear seems to depend on how we choose to measure them, and before we measure they seem to have no definite properties at all – leading us to a fundamental conundrum about the nature of basic reality. 

This fuzziness leads to apparent paradoxes such as Schrödinger’s cat, in which thanks to an uncertain quantum process a cat is left dead and alive at the same time. But that’s not all. Quantum particles also seem to be able to affect each other instantaneously even when they are far away from each other. This truly bamboozling phenomenon is known as entanglement, or, in a phrase coined by Einstein (a great critic of quantum theory), “spooky action at a distance”. Such quantum powers are completely foreign to us, yet are the basis of emerging technologies such as ultra-secure quantum cryptography and ultra-powerful quantum computing. 

But as to what it all means, no one knows. Some people think we must just accept that quantum physics explains the material world in terms we find impossible to square with our experience in the larger, “classical” world. Others think there must be some better, more intuitive theory out there that we’ve yet to discover. 

In all this, there are several elephants in the room. For a start, there’s a fourth fundamental force of nature that so far quantum theory has been unable to explain. Gravity remains the territory of Einstein’s general theory of relativity, a firmly non-quantum theory that doesn’t even involve particles. Intensive efforts over decades to bring gravity under the quantum umbrella and so explain all of fundamental physics within one “theory of everything” have come to nothing. Meanwhile, cosmological measurements indicate that over 95 percent of the universe consists of dark matter and dark energy, stuffs for which we currently have no explanation within the standard model, and conundrums such as the extent of the role of quantum physics in the messy workings of life remain unexplained. The world is at some level quantum – but whether quantum physics is the last word about the world remains an open question.

2058. Experimental Physics Finds More Evidence on Weird Properties of Subatomic Particles

By John Markoff, The New York Times, October 21, 2015

Bas Hensen, left, and Ronald Hanson helped show that objects apart can instantly affect each other. Photo: Frank Auperle/Delft University of Technology.

In a landmark study, scientists at Delft University of Technology in the Netherlands reported that they had conducted an experiment that they say proved one of the most fundamental claims of quantum theory — that objects separated by great distance can instantaneously affect each other’s behavior.

The finding is another blow to one of the bedrock principles of standard physics known as “locality,” which states that an object is directly influenced only by its immediate surroundings. The Delft study, published Wednesday in the journal Nature, lends further credence to an idea that Einstein famously rejected. He said quantum theory necessitated “spooky action at a distance,” and he refused to accept the notion that the universe could behave in such a strange and apparently random fashion.

Einstein was deeply unhappy with the uncertainty introduced by quantum theory and described its implications as akin to God’s playing dice.

But since the 1970s, a series of precise experiments by physicists are increasingly erasing doubt — alternative explanations that are referred to as loopholes — that two previously entangled particles, even if separated by the width of the universe, could instantly interact.

The new experiment, conducted by a group led by Ronald Hanson, a physicist at the Dutch university’s Kavli Institute of Nanoscience, and joined by scientists from Spain and England, is the strongest evidence yet to support the most fundamental claims of the theory of quantum mechanics about the existence of an odd world formed by a fabric of subatomic particles, where matter does not take form until it is observed and time runs backward as well as forward.

The researchers describe their experiment as a “loophole-free Bell test” in a reference to an experiment proposed in 1964 by the physicist John Stewart Bell as a way of proving that “spooky action at a distance” is real.

“These tests have been done since the late ’70s but always in the way that additional assumptions were needed,” Dr. Hanson said. “Now we have confirmed that there is spooky action at distance.”

According to the scientists, they have now ruled out all possible so-called hidden variables that would offer explanations of long-distance entanglement based on the laws of classical physics.

The Delft researchers were able to entangle two electrons separated by a distance of 1.3 kilometers, slightly less than a mile, and then share information between them. Physicists use the term “entanglement” to refer to pairs of particles that are generated in such a way that they cannot be described independently. The scientists placed two diamonds on opposite sides of the Delft University campus, 1.3 kilometers apart.

Each diamond contained a tiny trap for single electrons, which have a magnetic property called a “spin.” Pulses of microwave and laser energy are then used to entangle and measure the “spin” of the electrons.

The distance — with detectors set on opposite sides of the campus — ensured that information could not be exchanged by conventional means within the time it takes to do the measurement.

“I think this is a beautiful and ingenious experiment and it will help to push the entire field forward,” said David Kaiser, a physicist at M.I.T., who was not involved in the study. However, Dr. Kaiser, who is with another group of physicists who are preparing to perform an even more ambitious experiment next year that will soon measure light captured at the far edges of the universe, also said he did not think every scintilla of doubt had been erased by the Dutch experiment.

The tests take place in a mind-bending and peculiar world. According to quantum mechanics, particles do not take on formal properties until they are measured or observed in some way. Until then, they can exist simultaneously in two or more places. Once measured, however, they snap into a more classical reality, existing in only one place.

Beyond the immediate result, physicists noted that the experiment represented an advance in the understanding of a Lilliputian world that was once largely the province of theory. Quantum mechanics has already had a huge impact on modern technology and industry. For example, it is the foundation for modern computers and lasers.

“What I do find interesting is that the experimenters are learning how to manipulate quantum systems, and do experiments that are far beyond what was possible when I was starting in physics,” said Leonard Susskind, a theoretical physicist at Stanford. “Things which were at best ‘thought experiments’ become possible, then routine. That is incredibly impressive.”

Indeed, the experiment is not merely a vindication for the exotic theory of quantum mechanics, it is a step toward a practical application known as a “quantum Internet.” Currently, the security of the Internet and the electronic commerce infrastructure is fraying in the face of powerful computers that pose a challenge to encryption technologies based on the ability to factor large numbers and other related strategies.

Researchers like Dr. Hanson envision a quantum communications network formed from a chain of entangled particles girdling the entire globe. Such a network would make it possible to securely share encryption keys, and know of eavesdropping attempts with absolute certainty.

For some physicists, even though the new experiment claims to be “loophole free,” the matter is not completely closed.

“The experiment has closed two of the three major loopholes beautifully, but two out of three isn’t three,” Dr. Kaiser said. “I believe in my bones that quantum mechanics is the correct description of nature. But to make the strongest statement, frankly we’re not there.”

A potential weakness of the experiment, he suggested, is that an electronic system the researchers used to add randomness to their measurement may in fact be predetermined in some subtle way that is not easily detectable, meaning that the outcome might still be predetermined as Einstein believed.

To attempt to overcome this weakness and close what they believe is a final loophole, the National Science Foundation has financed a group of physicists led by Dr. Kaiser and Alan H. Guth, also at M.I.T., to attempt an experiment that will have a better chance of ensuring the complete independence of the measurement detectors by gathering light from distant objects on different sides of the galaxy next year, and then going a step further by capturing the light from objects known as quasars near the edge of the universe in 2017 and 2018.