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

The future belongs to those who prepare for it, as scientists who apply to federal agencies like NASA and the Department of Energy for research funds know only too well. The price of expensive instruments, such as a space telescope or a particle accelerator, can reach 10 billion dollars.

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 chair are two leading scientists: Maria Spiropulu, Shang-Yi Ch’en Professor of Physics at the California Institute of Technology, and cosmologist Michael Turner, Professor Emeritus at the University of Chicago, former assistant director of the National Science Foundation. and past 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. Greek-born Dr. Spiropulu was part of the team in 2012 that discovered the long-sought Higgs boson at the European Organization for Nuclear Research, known as CERN; he now uses quantum computers to investigate the properties of wormholes. The committee’s report is scheduled to be released in June 2024.

The Times recently caught up with the two scientists to discuss the group’s progress, the disappointments of the past 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 opportunities to understand space and time, matter and energy, and fundamental particles, if they are particles at all. If you were to ask a particle physicist where the field goes, you would get many different answers.

But what is the big vision? What is so exciting about this field? He was so excited in 1980 by the idea of ​​a grand unification, and that now seems small in comparison to the possibilities that lie ahead.

You mean the Grand Unified Theories, or GUTs, which were seen as a way to achieve Einstein’s dream of a single equation encompassing all the forces of nature. Where are we in 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 strength of strong color; 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 search for the fundamental rules is not over. Why two different types of building blocks? Why so many “elementary” particles? Why four forces? How do dark matter, dark energy, gravity, and space-time fit together? Answering these questions is the job of elementary particle physics.

spiropulu: The curve is that we do not understand the mass of the Higgs, which is about 125 times the mass of a hydrogen atom.

When we discovered the Higgs, our first expectation 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 were to collapse, we could bubble up in a different universe, and of course, that hasn’t happened yet.)

That’s been a bit overwhelming; for 20 years I have been chasing supersymmetric particles. So we’re like deer in the headlights: we don’t find supersymmetry, we don’t find dark matter as a particle.

Turner. The unification of forces is only part of what is happening. But it’s boring compared to the larger questions about space and time. Discussing what space and time are and where they come 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 make sense?

spiropulu: For me, by the way, unification is not boring. Just say.

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

spiropulu: The strongest indication we have of the unity of nature comes from particle physics. At sufficiently high energies, the fundamental forces (gravity, electromagnetism, and the strong and weak nuclear forces) appear to equalize.

But we have not reached the scale of God in our particle accelerators. So we may have to rephrase the question. In my opinion, the fundamental law remains a persistent puzzle, and the way we will solve it will be through new thinking.

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

That’s 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 only to find out it’s the wrong question. If so, you order a new one.

String theory, the vaunted “theory of everything,” describes the basic particles and forces of nature as vibrating strings of energy. Is there hope on our horizon to understand it better? This supposed stringiness only appears at energies millions of times higher than any particle accelerator ever imagined could achieve. Some scientists criticize string theory as being outside of science.

spiropulu: Not verifiable.

Turner: But it is a powerful mathematical tool. And if you look at the progress of science in the last 2,500 years, from the Milesians, who started without mathematics, to the present, mathematics has been the main theme. Geometry, algebra, Newton and calculus, and Einsteinian 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 in which we can test it, the framework has proven useful.

Turner: Another way of saying it is that we have new words and language to describe nature. Mathematics is the language of science, and the richer our language becomes, the more completely we can describe nature. We’ll have to wait and see what comes out of string theory, but I think it’s going to 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 if it sounds crazy. And the multiverse gives me a headache; since it is not testable, at least not yet, it is not science. But it may be the most important idea of ​​our time. It is one of the things that are on the table. Headache or not, we have to deal with it. You need to go up or out; It is either part of science or it is not 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 is atomic material like stars and people; 25 percent is some other “dark matter,” and about 70 percent is something even stranger, Mike has called it “dark energy,” which is causing the universe to expand at an accelerating rate.

Turner: That’s a great success, yes. We have named all the main components.

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

spiropulu: We get stuck when we get too deep. And at some point we need to change gears: change the question or the methodology. At the end of the day, understanding the physics of the universe is no walk in the park. More questions remain 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, so we can ask: What are space and time really? Einstein showed us that they are not just the place where things happen, as Newton said. They are dynamic: space can be bent and time can be warped. But now we are 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 see the universe.

We have to take into account what you said before. Many of the tools of particle physics take a 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 a challenge. But I’m bullish on particle physics because the opportunities have never been greater and the field has been at the forefront of science for years. Particle physics invented big global science and national and now global facilities. If history is any guide, nothing will stop you 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 dreams are beyond your immediate reach. But science has allowed humanity to do great things: vaccines against covid, the Large Hadron Collider, the Gravitational Wave Observatory with Laser Interferometer, the Webb telescope. — that broaden our vision and our power to shape our future. When we do these great things today, we do them together. If we continue to dream big and work together, even more amazing things await us.