Thought Experiments


The Mistaken Assumptions That Changed Physics History

“Don’t assume,” they always say.

Last month, Avi Loeb, an astrophysicist at Harvard, published an essay on how mistaken assumptions have delayed the progress of astronomy. In the same spirit, I wanted to find out how the course of physics has been influenced by assumptions, acknowledged or otherwise. Can lessons from the past help us be more aware of the assumptions we bring to physics today? Is it desirable—or even possible—to work without assumptions?

In the years after scientists came to accept light as a wave, brilliant researchers spent untold hours chasing after the “ether,” hypothetical stuff through which light waves were thought to propagate. Water waves are disturbances in water, sound waves are disturbances in air, and so light waves must be a disturbance in something, the reasoning went. When the sophisticated experiments built to search for the ether couldn’t find it, theorists got to work trying to explain away ether’s experimental no-show. It was only when Einstein published his theory of special relativity in 1905 that the solution became clear: Ether wasn’t just hard to find. It didn’t exist.

If Einstein resolved one roadblock, he set up another. He adamantly refused to accept the randomness built in to quantum mechanics, famously quipping that God doesn’t play dice with the universe, and holding fast even as experiment after experiment (including one he helped to design) showed those dice tumbling. “My instinct for physics bristles at this,” wrote Einstein.

Dice or no dice? Credit: Yannis/Flickr, adapted under a Creative Commons license.

It’s hard to imagine an astronomer dismissing observational data on the evidence of her “instinct for astronomy,” which highlights a key difference between the two fields. “The special aspect of astronomy is that we have limited information on things far away; it’s not like experiments in the laboratory where we can control conditions,” says Loeb. Astronomers therefore have to make more judgment calls than their colleagues in physics, and when they get it wrong, it’s usually because they don’t recognize the limits of their knowledge—their mistakes are failures of humility.

Physicists, on the other hand, have heaps of data, but they approach that data with certain assumptions that feel like common sense: Waves need something to wave through! The universe should behave predictably! But, says David Kaiser, a physicist and historian of science at MIT, “Sticking to what feels like common sense or intuition can trip us up.”

Einstein was “one of the most accomplished scientists ever—he took part in the process of discovering quantum mechanics,” says Loeb, and yet “he had a prejudice that turned out to be wrong.”

To Loeb, the ideal scientific attitude is like that of the perfect detective, who brings no assumptions about the guilt or innocence of any individual to his analysis of a case. “What you want is to start with a completely blank slate,” says Loeb.

But Kaiser takes a rosier view of scientific mistakes, one that suggests a somewhat different corrective. “I think we can find instances where mistakes led to productive outcomes,” says Kaiser. “It’s not always all bad.”

Indeed, Kaiser points out, even though physicists were wrong about the ether, their investigations led to critical insights, like Maxwell’s equations, which are now part of the essential physics toolkit. “We still use Maxwell’s equations even though for Maxwell, there was no such thing as an elementary electric charge. He thought the world was made out of ether!” The equations stayed the same; the world changed around them.

Hidden assumptions can also live at the heart of how we do science. Today, for example, most theorists expect that the deepest laws of nature will turn out to be simple and elegant. So far, that assumption has held up pretty well. But, says Kaiser, it’s possible that today’s most vexing puzzles—dark matter, dark energy, the nature of black holes—signal some breakdown of our basic assumptions about elegance and simplicity.

While hindsight might lead us to brand assumptions as bald mistakes, to Kaiser, they’re more like misplaced priorities. In his 2011 book “How the Hippies Saved Physics,” he tells the story of quantum physics’ journey from the “shut up and calculate” mode, which dominated teaching and research in the years after World War II, to a more open-minded approach that embraced “late-night speculation.” What the Cold War pragmatists never guessed was that the philosophical musings of the “hippies” would yield up a raft of cutting-edge applications based on the strange phenomenon of entanglement.

“Coming out of World War II and during the war, physicists were extremely creative under tremendous time pressure,” says Kaiser. “These were extremely practical, goal-oriented applications of the Schrödinger equation. They were absolutely brilliant with world-changing impacts,” including everything from transistors to nuclear weapons. Yet a cultural resistance to “daydreaming” kept them from thinking deeply about the meaning of the equations they employed so successfully.

“We are not going to get to a land where we have no biases and no intuition,” says Kaiser. Scientists are human beings; their neutrality comes from the rigor of their methods rather than some superhuman ability to transcend personal biases. So, he argues, we should build “safety nets” into the scientific culture to “allow little pockets of support for crazy ideas”—that is, ideas that defy common sense and conventional wisdom and yet have some kernel of plausibility. Kaiser is optimistic that today’s scientific culture is making room for “weird” ideas, with microgrants and new research centers, like the Perimeter Institute, where “not-quite-mainstream stuff” can gain some purchase.

One day, those offbeat ideas just might start to seem like common sense.

Go Deeper
Editor’s picks for further reading

FQXi: Which of our basic physical assumptions are wrong?
Read the winning entries in FQXi’s 2012 essay competition.

Hyperphysics: The Michelson-Morley Experiment
Learn about the 1887 experiment that helped put ether theory to rest.

Wikipedia: Bohr-Einstein debates
Explore the twists and turns of the ongoing dispute between Albert Einstein and Niels Bohr over the foundations of quantum mechanics.

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Kate Becker

    Kate Becker is the editor of The Nature of Reality, where it is her mission to blow your mind with physics. Kate studied physics at Oberlin College and astronomy at Cornell University, and spent seven years as senior researcher for NOVA and NOVA scienceNOW. Follow her on Twitter and Facebook.