A Crisis at the Edge of Physics • 2015 Jun 5 • Adam Frank and Marcelo Gleiser • NYT • The Opinion Pages

Do physicists need empirical evidence to confirm their theories?

You may think that the answer is an obvious yes, experimental confirmation being the very heart of science. But a growing controversy at the frontiers of physics and cosmology suggests that the situation is not so simple.

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Predicted about 50 years ago, the Higgs particle is the linchpin of what physicists call the ‘standard model’ of particle physics, a powerful mathematical theory that accounts for all the fundamental entities in the quantum world (quarks and leptons) and all the known forces acting between them (gravity, electromagnetism and the strong and weak nuclear forces).

But the standard model, despite the glory of its vindication, is also a dead end. It offers no path forward to unite its vision of nature’s tiny building blocks with the other great edifice of 20th-century physics: Einstein’s cosmic-scale description of gravity. Without a unification of these two theories — a so-called theory of quantum gravity — we have no idea why our universe is made up of just these particles, forces and properties. (We also can’t know how to truly understand the Big Bang, the cosmic event that marked the beginning of time.)

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This is where the specter of an evidence-independent science arises. For most of the last half-century, physicists have struggled to move beyond the standard model to reach the ultimate goal of uniting gravity and the quantum world. Many tantalizing possibilities (like the often-discussed string theory) have been explored, but so far with no concrete success in terms of experimental validation.

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Today, the favored theory for the next step beyond the standard model is called supersymmetry (which is also the basis for string theory).

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To date, no supersymmetric particles have been found. If the Large Hadron Collider cannot detect these particles, many physicists will declare supersymmetry — and, by extension, string theory — just another beautiful idea in physics that didn’t pan out.

But many won’t. Some may choose instead to simply retune their models to predict supersymmetric particles at masses beyond the reach of the Large Hadron Collider’s power of detection — and that of any foreseeable substitute

Implicit in such a maneuver is a philosophical question: How are we to determine whether a theory is true if it cannot be validated experimentally? Should we abandon it just because, at a given level of technological capacity, empirical support might be impossible? If not, how long should we wait for such experimental machinery before moving on: ten years? Fifty years? Centuries?

Like Professor Peter Woit I found the comparison to epicycles misleading, because epicycles were actually an (inelegant) modification of the (known to be inaccurate) geocentric model that was based on empirical evidence and made predictions that were far more accurate than the predictions made by the heliocentric/circular orbit model proposed by Copernicus, whereas string theory and supersymmetry are entirely theoretical constructs that, as Prof. Woit puts it, “predict nothing and explain nothing.”

But ultimately, like the authors of the NYT opinion, I think the idea that scientific theories don’t actually need experimental confirmation as long as they are “sufficiently elegant and explanatory” is utter madness.

Although I do believe in the multiverse, I recognize that there isn’t any evidence for it.