Thought Portal

Erwin Schrödinger is one of the "fathers of quantum mechanics". He also sexually abused children. Trinity College Dublin recently denamed a lecture theatre that had been named after him - but his name is still on an equation that won the Nobel Prize for physics. And a cat.Writer and historian Subhadra Das recounts how and why you rename a university building, and retired physicist Martin Austwick considers that renaming an eponymous equation or theory might be more difficult than unscrewing a sign from a wall.

general relativity for babies

Several years ago, a physics student started wondering what a clock is. That set him and his coauthors on a path to a new understanding of timekeeping as a thermodynamic process with fundamental limits. What might it tell us about time itself?

Rich Stone, former international news editor at Science and current senior science editor at the Howard Hughes Medical Institute’s Tangled Bank Studios, joins host Sarah Crespi to talk about concerning levels of fission reactions deep in an inaccessible area of the site of the 1986 Chernobyl nuclear disaster. Though nothing is likely to come of it anytime soon, scientists must decide what—if anything—they should do tamp down reactions in this hard-to-reach place.

Also on this week’s show, Shlomi Kotler, an assistant professor in the department of applied physics at the Hebrew University of Jerusalem, joins Sarah to discuss the quantum entanglement of macroscopic objects. This hallmark of quantum physics has been confined—up until now—to microscopic items like atoms, ions, and photons. But what does it mean that two drums, each the width of a human hair, can be entangled?

Read a related insight.

The Boltzmann brain argument suggests that it is more likely for a single brain to spontaneously and briefly form in a void (complete with a false memory of having existed in our universe) than it is for our universe to have come about in the way modern science thinks it actually did. It was first proposed as a reductio ad absurdum response to Ludwig Boltzmann's early explanation for the low-entropy state of our universe.[1]

In this physics thought experiment, a Boltzmann brain is a fully formed brain, complete with memories of a full human life in our universe, that arises due to extremely rare random fluctuations out of a state of thermodynamic equilibrium. Theoretically, over an extremely large but not infinite amount of time, by sheer chance atoms in a void could spontaneously come together in such a way as to assemble a functioning human brain. Like any brain in such circumstances, it would almost immediately stop functioning and begin to deteriorate.[2]

The idea is ironically named after the Austrian physicist Ludwig Boltzmann (1844–1906), who in 1896 published a theory that tried to account for the fact that we find ourselves in a universe that is not as chaotic as the budding field of thermodynamics seemed to predict. He offered several explanations, one of them being that the universe, even one that is fully random (or at thermal equilibrium), would spontaneously fluctuate to a more ordered (or low-entropy) state. One criticism of this "Boltzmann universe" hypothesis is that the most common thermal fluctuations are as close to equilibrium overall as possible; thus, by any reasonable criterion, actual humans in the actual universe would be vastly less likely than "Boltzmann brains" existing alone in an empty universe.

Boltzmann brains gained new relevance around 2002, when some cosmologists started to become concerned that, in many existing theories about the Universe, human brains in the current Universe appear to be vastly outnumbered by Boltzmann brains in the future Universe who, by chance, have exactly the same perceptions that we do; this leads to the conclusion that statistically we ourselves are likely to be Boltzmann brains. Such a reductio ad absurdum argument is sometimes used to argue against certain theories of the Universe. When applied to more recent theories about the multiverse, Boltzmann brain arguments are part of the unsolved measure problem of cosmology. Boltzmann brains remain a thought experiment; physicists do not believe that we are actually Boltzmann brains, but rather use the thought experiment as a tool for evaluating competing scientific theories.

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The marquise began to yearn for the intellectual excitement of her childhood studies. In the spring of 1733, she asked Pierre Louis Moreau de Maupertuis, fast becoming the country’s leading scientist, to tutor her in advanced mathematics. Around this time she also met Voltaire. Her choice to take Voltaire as a lover was unusual, since he was of lower rank. But du Châtelet found something in him that she couldn’t find in the “frivolous things” of Paris. Perhaps even more telling, the country’s most famous writer and philosopher found in her a woman who could match him, wit for wit. “There is a lady in Paris, named Emilie, who, in imagination and in reason, surpasses the men who like to think they know a lot about the one and the other,” the poet wrote to a colleague.

Since du Châtelet could not join the scientific community of Paris, she and Voltaire created their own. Both disciples of Newton, they turned their backs on society life and retreated to Cirey to pursue science. They shuttered rooms with curtains to conduct experiments with light beams, and lit massive forges in the forest to study the effects of heat on metal.

EHT trained its sights on both M87’s black hole and Sagittarius A*, the supermassive black hole at the center of the Milky Way. But, it turns out, it was easier to image M87’s monster. That black hole is 55 million light-years from Earth in the constellation Virgo, about 2,000 times as far as Sgr A*. But it’s also about 1,000 times as massive as the Milky Way’s giant, which weighs the equivalent of roughly 4 million suns. That extra heft nearly balances out M87’s distance. “The size in the sky is pretty darn similar,” says EHT team member Feryal Özel.  

The idea that observers can ultimately reconcile their measurements of some kind of fundamental reality is based on several assumptions. The first is that universal facts actually exist and that observers can agree on them.

But there are other assumptions too. One is that observers have the freedom to make whatever observations they want. And another is that the choices one observer makes do not influence the choices other observers make—an assumption that physicists call locality.

If there is an objective reality that everyone can agree on, then these assumptions all hold.

But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.

Of course, there is another way out for those hanging on to the conventional view of reality. This is that there is some other loophole that the experimenters have overlooked. Indeed, physicists have tried to close loopholes in similar experiments for years, although they concede that it may never be possible to close them all.

the universe looking back to its birth and creating itself in the process.

It is enough to irk metrologists. “If aliens ever visit Earth what else would we talk about other than physics?” said Schlamminger. “If we want to talk about physics we have to agree on a set of units, but if we say our unit of mass is based on a lump of metal we keep in Paris, we’ll be the laughing stock of the universe.”

“The greatest satisfaction for me will be completing the historic arc that started with the French revolution,” said Schlamminger. “The idea was to have a measurement system for all times and for all people. They fell short on the kilogram. It has these problems with stability, so it is not for all times, and it is locked in a safe, so it is not for all people. Planck’s constant never changes, so it is the same for all time. And its value is woven into the fabric of the universe, so it is there for everyone.”