DO physicists need empirical evidence to confirm their theories?No, quantum gravity is a stupid pipe dream that would tell us nothing about the universe.
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. ...
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.)
SUSY is at least testable and potentially explanatory, but it may soon be dead:
Today, the favored theory for the next step beyond the standard model is called supersymmetry (which is also the basis for string theory). Supersymmetry predicts the existence of a “partner” particle for every particle that we currently know. It doubles the number of elementary particles of matter in nature. The theory is elegant mathematically, and the particles whose existence it predicts might also explain the universe’s unaccounted-for “dark matter.” As a result, many researchers were confident that supersymmetry would be experimentally validated soon after the Large Hadron Collider became operational.Now physics is being overrun by non-empirical pursuits:
That’s not how things worked out, however. 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.
Consider, likewise, the cutting-edge theory in physics that suggests that our universe is just one universe in a profusion of separate universes that make up the so-called multiverse.A current essay on Scientia Salon argues that string theory and many-worlds are legitimate science and not pseudoscience, even tho there is not likely to ever have any empirical evidence for either.
Recall the epicycles, the imaginary circles that Ptolemy used and formalized around A.D. 150 to describe the motions of planets. Although Ptolemy had no evidence for their existence, epicycles successfully explained what the ancients could see in the night sky, so they were accepted as real. But they were eventually shown to be a fiction, more than 1,500 years later. Are superstrings and the multiverse, painstakingly theorized by hundreds of brilliant scientists, anything more than modern-day epicycles?This explanation of epicycles makes no sense. It says they were "imaginary" and "had no evidence", but also that the ancients saw them in the night sky.
Ptolemy approximated the night sky view of planets as main circles plus epicycles. They are real in a sense similar to saying that the phases of the Moon are real. We see them in the sky. That is all Ptolemy meant.
Saying that Ptolemy had no evidence for epicycles is like saying that he had no evidence for phases of the Moon.
You could say that electrons are modern-day epicycles in that we have very good models for what we see in electron experiments, but not necessarily a deep understanding of what an electron really is.
Superstrings are not like that at all. There is nothing relating theoretical superstrings to anything that is observed, nor is any such relationship likely in the future.
At that Scientia Essay, Massimo Pigliucci argues that the philosopher's consensus is that there is no clearcut definition of science, but he writes books attacking pseudoscience anyway. A responder gives this definition:
Sorry, but as a physicist, I’m pretty much a Popperian. I have a very narrow definition of what constitutes “science:” it is a methodology consisting of a systematic, iterative use of observation (all sense-data) and reason. Observations are used as hypotheses, reason generates models (mathematical in the advanced form) which are tested by prediction against additional observation. And Falsification is, indeed, the criterion which must be met by a model (theory) if it is to be judged part of science.That is a decent definition.
And I’m not too sure about psychology: why is it obvious that a methodology created by the human intellect should obviously be applicable to analyzing the human mind? There’s a Doug Hofstadter strange-loop problem, potentially. And then there’s “political science,” “management science,” and other comic labels.
So astrology, creation science, and natural theology are not sciences by the Popper test. Also, I would say that string theory is only a science if it generates a prediction that is at least IN PRINCIPLE falsifiable. It is near the margin, because, while there are in-principle observations that might test its predictions, these are not even close to practical. So it is certainly questionable as science. Many-worlds quantum mechanics is not science – UNLESS someone comes up with a way to test it. As I said, I have a narrow definition of the word.
Ethan Siegel tries to answer Does Quantum Gravity Need String Theory? He starts by quoting:
“I just think too many nice things have happened in string theory for it to be all wrong. Humans do not understand it very well, but I just don’t believe there is a big cosmic conspiracy that created this incredible thing that has nothing to do with the real world.” -Edward WittenLubos Motl explains:
Witten wants to say that even without definite empirical proofs, the mathematical properties of string theory make us certain that it is an incredibly tight, rich, and unique mathematical structure that seems to contain the ideas compatible with physics as well as many new structures and relationships that came as surprises and taught us to think about physical and mathematical concepts in new ways.This is just the faith of a true believer. There are lots of nice mathematical structures, from p-adic numbers to category theory, but they do not describe the structure of electrons. A lot of nice mathematics finds unexpected applications, so some of these things might be applied to physics, but it is nutty to think that electrons are based on 6-dimensional Calabi-Yau manifolds.
Siegel explains that quantum gravity is fully understood for all observable possibilities:
“So,” you reason, “we’ll simply do our quantum field theory calculations in the background of curved space!” This is known as semi-classical gravity, and it’s this type of calculation that allows us to calculate things like Hawking radiation. But even that is only at the event horizon of the black hole itself, not at the location where gravity is truly at its strongest. As Sabine Hossenfelder elegantly explained, there are multiple physical instances where we need a quantum theory of gravity, all having to do with gravitational physics on the smallest of scales: at tiny distances.Black holes are not observable inside the event horizon. We can speculate about what is inside, but according to relativity, there is no way of every knowing. It is as impossible as going faster than the speed of light.
What happens, for example, at the central locations of black holes? You might think, “oh, there’s a singularity,” but a singularity isn’t quite so much a point of infinite density, but is more likely an instance where the mathematics of General Relativity returns nonsensical answers for things like potentials and forces.
The quantum gravity researchers are concerned with the center point of the black hole, where there is supposedly a singularity with infinite density. Or maybe the matter does not collapse all the way, because of very high energy interactions that are not understood. Or maybe God is hiding in there. Believe whatever you want, because physically it is a meaningless question.