Where is physics going (and how soon will 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 large instruments such as a space telescope or particle accelerator can reach $10 billion.
And so last June, the physics community started thinking about what they want to do next, and why.
That is the mandate of a committee appointed by the National Academy of Sciences Elementary Particle Physics: Progress and Promise. The presidency is shared by two prominent scientists: Maria Spiropulu, Shang-Yi Ch’en Professor of Physics at the California Institute of Technology, and the cosmologist Michael Turner, Professor Emeritus at the University of Chicago, former deputy director of the National Science Foundation and past president of the American Physical Society.
In the 1980s, Dr. Turner one of 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. Born in Greece, Spiropulu was on the team that discovered the long-sought Higgs boson at the European Organization for Nuclear Research, known as CERN, in 2012; them now uses quantum computers to investigate the properties of wormholes. The commission’s report is scheduled for publication in June 2024.
The Times recently met 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 in particle physics have never been more exciting, in terms of the possibilities of understanding space and time, matter and energy, and the fundamental particles – if they are particles at all. If you asked a particle physicist where the field is going, you’d get a lot of different answers.
But what is the big vision? What is so exciting about this field? I was so excited about the idea of great unification in 1980, and that now seems small compared to the possibilities that lie ahead.
You refer to Grand Unified Theories, or GUTs, which were considered a way to realize Einstein’s dream of a single equation encompassing all the forces of nature. Where are we with unification?
Turner: As far as we know, quarks and leptons are the basic building blocks of matter; the rules that make it happen 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 coloring force; the W and Z bosons, from 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 yet. 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 into this? Answering these questions is the work of elementary particle physics.
Spiropoulos: 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 masses we measured were unstable without their presence, but we haven’t found them yet. (If the Higgs field collapsed, we could be bubbling into another universe — which, of course, hasn’t happened yet.)
That was a bit crushing; For 20 years I’ve been chasing the supersymmetric particles. So we’re like deer in the headlights: we haven’t found supersymmetry, we haven’t found dark matter as a particle.
Turner: The unification of the forces is only part of what is going on. But it’s boring compared to the larger questions of space and time. Discussing what space and time are and where they come from now belongs to the realm of particle physics.
From a cosmological perspective, the Big Bang is the origin of space and time, at least from the point of view of Einstein’s general theory of 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 places and times are there? Does that question make sense at all?
Spiropoulos: For me, unification is not boring, by the way. Just say it.
Turner: I meant relatively boring. It remains very interesting!
Spiropoulos: The strongest clue 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—seem to equalize.
But we haven’t reached the God scale in our particle accelerators. So we may need to reformulate the question. In my opinion, the ultimate law remains a stubborn puzzle, and the way we solve it will be through new thinking.
Turner: I like what Maria says. It feels like we have all the puzzle pieces 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 right way to phrase the question.
That’s the mark of great science: you ask a question, and often it turns out to be the wrong question, but you have to ask a question to find out it’s the wrong one. If so, request a new one.
String theory – the vaunted “theory of everything” – describes the fundamental particles and forces in nature as vibrating strings of energy. Is there hope on our horizon for a better understanding? This supposed fibrousness only manifests itself at energies millions of times higher than what could ever be achieved by a particle accelerator. Some scientists criticize string theory as being outside science.
Spiropoulos: It cannot be tested.
Turner: But it is a powerful mathematical tool. And if you look at the advancement of science over the last 2,500 years, from the Milesians, who started without math, until now, math has been the most important part. Geometry, algebra, Newton and calculus, and Einstein and non-Riemannian geometry.
Spiropoulos: I would rather 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 we can test, 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 more our language is enriched, the more fully we can describe nature. We’ll have to wait and see what comes out of string theory, but I think it will be big.
One of the many features of string theory is that the equations seem to have 10⁵⁰⁰ solutions – they describe 10⁵⁰⁰ different possible universes or even more. Do we live in a multiverse?
Turner: I think we should just deal with it, even if it sounds crazy. And the multiverse gives me a headache; because it is not testable, at least not yet, it is not science. But it is perhaps 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 must go up or down; it is 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 of it is atomic material like stars and humans; 25 percent is some other “dark matter,” and about 70 percent is even stranger — Mike has called it “dark energy” — that causes the universe to expand at a faster rate.
Turner: That is a great success, yes. We’ve listed all the major components.
But you don’t know what most are.
Spiropoulos: We get stuck when we reach very deep. And at some point, we need to change gears — change the question of methodology. At the end of the day, understanding the physics of the universe is no walk in the park. More questions remain unanswered than 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 resembles the origin of space and time, and so we might ask: what are space and time anyway? Einstein showed us that they are not just the place where things happen, as Newton said. They are dynamic: 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 revolves around time. And that might 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 have a very long development time 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’m optimistic about particle physics because the opportunities have never been greater and the field has been at the cutting edge of science for years. Particle physics has invented great, global science and national and now global facilities. If history is any guide, nothing will stop them from answering the big questions!
It took three decades to build the James Webb Space Telescope.
Spiropoulos: Space – bingo!
turner: I mean, science is all about big dreams. Sometimes the dreams are beyond your immediate reach. But science has enabled humanity to do great things – Covid vaccines, the Large Hadron Collider, the Laser Interferometer Gravitational-Wave Observatory, the Webb Telescope – that expand our vision and power to shape our future . Today, when we do these big things, we do them together. If we keep dreaming big and working together, more amazing things are ahead.