Where is Physics Headed (and How Soon Do We Get There)?

The future belongs to those who prepare for it, as scientists who petition federal agencies like NASA and the Department of Energy for research funds know all too well. The price of big-ticket instruments like a space telescope or particle accelerator can be as high as $10 billion.

And so this past June, the physics community began to consider what they want to do next, and why.

That is the mandate of a committee appointed by the National Academy of Sciences, called Elementary Particle Physics: Progress and Promise. Sharing the chairmanship are two prominent scientists: Maria Spiropulu, Shang-Yi Ch’en Professor of Physics at the California Institute of Technology, and the cosmologist Michael Turner, an emeritus professor at the University of Chicago, the former assistant director of the National Science Foundation and former president of the American Physical Society.

In the 1980s, Dr. Turner was among the scientists who began using the tools of particle physics to study the Big Bang and the evolution of the universe, and the universe to learn about particle physics. Dr. Spiropulu, born in Greece, was on the team in 2012 that discovered the long-sought Higgs boson at the European Organization for Nuclear Research, known as CERN; she now uses quantum computers to investigate the properties of wormholes. The committee’s report is scheduled for release in June 2024.

Recently The Times met with the two scientists to discuss the group’s progress, the disappointments of the last 20 years and the challenges ahead. The conversation has been edited for clarity and brevity.

Why convene this committee now?

Turner: I feel like things have never been more exciting in particle physics, in terms of the opportunities to understand space and time, matter and energy, and the fundamental particles — if they are even particles. If you asked a particle physicist where the field is going, you’d get a lot of different answers.

But what’s the grand vision? What is so exciting about this field? I was so excited in 1980 about the idea of grand unification, and that now looks small compared to the possibilities ahead.

You’re referring to Grand Unified Theories, or GUTs, which were considered a way to achieve Einstein’s dream of a single equation that encompassed all the forces of nature. Where are we on unification?

Turner: As far as we know, the basic building blocks of matter are quarks and leptons; the rules that govern them are described by the quantum field theory called the Standard Model. In addition to the building blocks, there are force carriers — the photon, of the electromagnetic force; eight gluons, of the strong color force; the W and Z bosons, of the weak nuclear force, and the Higgs boson, which explains why some particles have mass. The discovery of the Higgs boson completed the Standard Model.

But the quest for the fundamental rules is not over. Why two different kinds of building blocks? Why so many “elementary” particles? Why four forces? How do dark matter, dark energy, gravity and space-time fit in? Answering these questions is the work of elementary particle physics.

Spiropulu: The curveball is that we don’t understand the mass of the Higgs, which is about 125 times the mass of a hydrogen atom.

When we discovered the Higgs, the first thing we expected was to find these other new supersymmetric particles, because the mass we measured was unstable without their presence, but we haven’t found them yet. (If the Higgs field collapsed, we could bubble out into a different universe — and of course that hasn’t happened yet.)

That has been a little bit crushing; for 20 years I’ve been chasing the supersymmetrical particles. So we’re like deer in the headlights: We didn’t find supersymmetry, we didn’t find dark matter as a particle.

Turner. The unification of the forces is just part of what’s going on. But it is boring in comparison to the larger questions about space and time. Discussing what space and time are and where they came from is now within the realm of particle physics.

From the perspective of cosmology, the Big Bang is the origin of space and time, at least from the point of view of Einstein’s general relativity. So the origin of the universe, space and time are all connected. And does the universe have an end? Is there a multiverse? How many spaces and times are there? Does that question even make sense?

Spiropulu: To me, by the way, unification is not boring. Just saying.

Turner: I meant boring relatively speaking. It’s still very interesting!

Spiropulu: The strongest hint we have of the unity of nature comes from particle physics. At high enough energies, the fundamental forces — gravity, electromagnetism and the strong and weak nuclear forces — seem to become equal.

But we have not reached the God scale in our particle accelerators. So possibly we have to reframe the question. In my view the ultimate law remains a persistent puzzle, and the way we solve it is going to be through new thinking.

Turner: I like what Maria is saying. It feels like we have all the pieces of the puzzle on the table; it looks like the four different forces we see are just different facets of a unified force. But that may not be the right way to phrase the question.

That is the hallmark of great science: You ask a question, and often it turns out to be the wrong question, but you have to ask a question just to find out it’s the wrong one. If it is, you ask a new one.

String theory — the vaunted “theory of everything” — describes the basic particles and forces in nature as vibrating strings of energy. Is there hope on our horizon for better understanding it? This alleged stringiness only shows up at energies millions of times higher than what could be achieved by any particle accelerator ever imagined. Some scientists criticize string theory as being outside science.

Spiropulu: It’s not testable.

Turner: But it is a powerful mathematical tool. And if you look at the progress of science over the past 2,500 years, from the Milesians, who began without mathematics, to the present, mathematics has been the pacing item. Geometry, algebra, Newton and calculus, and Einstein and non-Riemannian geometry.

Spiropulu: I would be more daring and say that string theory is a framework, like other frameworks we have discovered, within which we try to explain the physical world. The Standard Model is a framework — and in the ranges of energies that we can test it, the framework has proved to be useful.

Turner: Another way to say it is that we have new words and language to describe nature. Mathematics is the language of science, and the more our language is enriched, the more fully we can describe nature. We will have to wait and see what comes from string theory, but I think it will be big.

Among the many features of string theory is that the equations seem to have 10⁵⁰⁰ solutions — describing 10⁵⁰⁰ different possible universes or even more. Do we live in a multiverse?

Turner: I think we have to deal with it, even though it sounds crazy. And the multiverse gives me a headache; not being testable, at least not yet, it isn’t science. But it may be the most important idea of our time. It’s one of the things on the table. Headache or not, we have to deal with it. It needs to go up or out; either it’s part of science or it isn’t part of science.

Why is it considered a triumph that the standard model of cosmology doesn’t say what 95 percent of the universe is? Only 5 percent of it is atomic material like stars and people; 25 percent is some other “dark matter,” and about 70 percent is something even weirder — Mike has named it “dark energy” — that is causing the universe to expand at an accelerating rate.

Turner: That’s a big success, yeah. We’ve named all the major components.

But you don’t know what most of them are.

Spiropulu: We get stalled when we reach very deep. And at some point we need to change gear — change the question or the methodology. At the end of the day, understanding the physics of the universe is not a walk in the park. More questions go unanswered than are answered.

If unification is the wrong question, what is the right one?

Turner: I don’t think you can talk about space, time, matter, energy and elementary particles without talking about the history of the universe.

The Big Bang looks like the origin of space and time, and so we can ask, What are space and time really? Einstein showed us that they’re not just the place where things happen, as Newton said. They’re dynamical: space can bend and time can warp. But now we’re ready to answer the question: Where did they come from?

We are creatures of time, so we think the universe is all about time. And that may be the wrong way to look at the universe.

We have to keep in mind what you said earlier. Many of the tools in particle physics take a very long time to develop and are very expensive. These investments always pay off, often with big surprises that change the course of science.

And that makes progress challenging. But I am bullish on particle physics because the opportunities have never been bigger and the field has been at the bleeding edge of science for years. Particle physics invented big, global science, and national and now global facilities. If history is any guide, nothing will prevent them from answering the big questions!

It took three decades to build the James Webb Space Telescope.

Spiropulu: Space — bingo!

Turner: I mean, science is all about big dreams. Sometimes the dreams are beyond your immediate reach. But science has allowed humankind to do big things — Covid vaccines, the Large Hadron Collider, the Laser Interferometer Gravitational-Wave Observatory, the Webb telescope. — that extend our vision and our power to shape our future. When we do these big things nowadays, we do them together. If we continue to dream big and work together, even more amazing things lie ahead.