The Blog


Physics | June 7, 2012

Fundamental Science and the Big Machine

Annaka Harris interviews Lisa Randall

Lisa Randall



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My motivation in writing this book was not only to describe the importance of science but also to explain how scientific research really works in practice.  We live in an age where scientific and technical advances are more critical than ever, yet many people distrust science, scientific thinking, and scientists.  I tried to explain the elements of a scientific way of thinking that can be useful when approaching the major problems of the world, as well as in scientific progress.

Creativity, for example, is essential to particle physics, cosmology, mathematics, and to other fields of science, just as it is to its more widely acknowledged beneficiaries – the arts and humanities. Scientists, writers, artists and musicians might seem very different on the surface, but the nature of skills, talents, and temperaments is not always as distinctive as you might expect.  It is important to understand the interplay between experiments, concepts, and creative thinking when we consider scientific advances.

I thought it made sense to anchor a more abstract discussion of the role of scale, risk, uncertainty, and creativity with some more concrete physics. It doesn’t hurt that it’s an exciting time for physicists like myself, with the Large Hadron Collider running extremely well and testing some of our hypotheses, as well as inspiring my colleagues and me to come up with new ideas and new ways of analyzing the enormous amount of data the LHC will produce.

I admire the courage you display throughout your book, breaking taboos and putting to rest some pervasive misconceptions about science. One misconception I encounter often is the claim that science is “unreliable” – discoveries are made one year, only to be overturned the next. You resolve this misunderstanding using the concepts of scale and effective theory. Can you explain why these concepts are so important?

It’s funny. In some respects the concept of an “effective theory” is one of the most intuitive ones you can imagine. If something is too small to matter, you can operate quite well. You can turn on a radio without knowing all its inner workings.  The “effective theory” concentrates on the particles and forces that have “effects” at the distances in question. Rather than delineating particles and interactions that describe more fundamental behavior, we formulate our theories, equations, and observations in terms of the things that are actually relevant to the scales we might detect.

You can predict where a ball will land when you throw it based on Newton’s Laws. You don’t need to use quantum mechanics. Even if you did, the difference in prediction would be far too minuscule to matter. And as a practical matter, the calculation would be far too complex. So you can use the effective theory, because it works sufficiently well.

The reason this concept is so important is that it helps us better understand how science advances, at least once it has developed to the state of contemporary physics. Newton’s Laws aren’t wrong – they are an approximation. Similarly, if and when we find new underlying structure at the LHC, it won’t invalidate the so-called Standard Model of particle physics that describes matter’s most basic elements and their interactions as we understand it today. It will improve on what we know.

Science proceeds with uncertainty at the edges, but it is advancing methodically overall. The wisdom and methods we acquired in the past survive. But theories evolve as we better understand a larger range of distances and energies.

In your book, you compare the LHC to a microscope – a new tool for looking even closer at the fabric of the universe – which really helped me connect to what is intrinsically fascinating about it. I think there’s something about the term “particle collider” that the general public has a hard time relating to – the term points to the very complicated process by which it probes matter (which most people don’t understand), rather than to its purpose, which is to see things that were previously invisible to us. One of the joys of reading your book is experiencing your excitement about the Large Hadron Collider. Will you discuss some of its awe-inspiring qualities here?

To describe the machine and technology, I end up using far more superlatives than I conventionally do, but this exception is warranted in the case of the LHC. The LHC is not merely large: it is the biggest machine ever built. It is not merely cold: it is the coldest extended region that we know of in the universe—colder than outer space.  The vacuum inside the proton-containing tubes, a 10 trillionth of an atmosphere, is the most complete vacuum over the largest region ever produced. The energy of the collisions are the highest ever generated on Earth, allowing us to study the interactions that occurred in the early universe the furthest back in time. Each LHC dipole (the magnets responsible for keeping the protons running in their tracks) contains coils of niobium-titanium superconducting cables, each of which contains stranded filaments a mere six microns thick – much smaller than a human hair. If you unwrapped all of these filaments, they would be long enough to encircle the orbit of Mars. And the last superlative that I’ll mention: the LHC’s $9 billion price tag also makes it the most expensive machine ever built.

Experiments at the LHC are designed to study substructure and interactions with a range a hundred thousand trillion times smaller than a centimeter – about a factor of ten smaller in size than anything any experiment has ever looked at before. And they are designed to produce rates for collisions—and hence potentially interesting events—that will be fifty times greater than any collider achieved before.

I still find it difficult to visualize how events get recorded in the LHC. Is it possible to describe the recording process through an analogy to something at human scale?

The recording process is in some ways straightforward, although the actual techniques and electronics are rather more technical.  Particles emanate outward from a collision. Detectors are built concentrically around the collision region so that they can record different particle properties. Charged particles ionize material to leave tracks. Particles that interact via the strong interactions dump energy in the detector element called the hadron calorimeter. Each of these layers records what goes through and the pieces get put together to determine what passed through—that is what is its charge and mass—and what is its energy and momentum.

The collisions themselves are somewhat subtle, however, because protons are not fundamental objects. They are made up of constituents known as quarks and gluons. When protons collide at LHC energies, it is really the individual constituents that collide together. It is as if you had beanbags that you throw really hard at each other. The individual beans might collide rather than the entire floppy bag. Those individual collisions of “hard” constituents are what can give rise to the potentially interesting events that make new particles.

What about particles that can’t be detected by the LHC? Aren’t there particles that will simply pass through all the detectors, unseen?

The detectors are built to capture all the energy that they can with almost complete angular coverage. So all the particles that have charge or interactions with the material of the detectors should get recorded by these hermetic detectors. Hermetic measurements ensure that even noninteracting or very weakly interacting particles can be discovered. The reason is that if “missing” transverse momentum is observed—that is, the energy doesn’t add up to the initial energy – one or more particles carry that off energy but have no directly detectable interactions must have been produced. Such particles carry energy and momentum. Even if not detected, the momentum they take away makes experimenters aware of their existence.

One particular particle that might be discovered in this way is the lightest supersymmetric particle (LSP). Supersymmetry is an extension of the symmetries of space and time into the quantum regime that pairs all known particles with supersymmetric partners. The lightest of the supersymmetric particles is expected to be very weakly interacting, so it won’t interact with any elements of the detector. This means that whenever a supersymmetric particle is produced and decays, momentum and energy will appear to be lost. The LSP will disappear from the detector and carry away momentum and energy to where it can’t be recorded, leaving as its signature missing energy.

When particles are created or “decay into” other particles, does this mean they exist inside the initial particle(s), or do they actually get created? I’m assuming it’s the latter, but what exactly does this mean?

Generally the particles we are interested in are not already present. Collisions happen that turn matter into energy. And that energy can in turn become new elementary particles as Einstein’s relation E=mc2 tells us. The same thing happens in a sense when particles decay. The original particle is destroyed, but the energy it contained is then carried off by the decay products—the new particles that are created in the decay process.

In chapter 20 you discuss dark matter and dark energy. Are you saying it’s possible that the energy we can’t account for in the universe is coming from “something” that exists in another spatial dimension but still influences 3D space? That dark energy and dark matter are dark because they’re not actually “here”?

I’d say that dark matter and dark energy are on very different footing in the sense that we can reasonably imagine a number of different candidates for dark matter. That is, with more or less conventional particle physics in three spatial dimensions, we can devise many possible models that lead to the measured amount of dark matter with no overly peculiar assumptions. So it’s possible that the explanation of dark matter will involve an additional dimension. But it certainly doesn’t seem to be required.

Dark energy, on the other hand – energy that isn’t carried by matter but just exists and permeates the universe—is much harder to understand. We aren’t even sure there will be a conventional explanation either in three dimensions or with more. It would be exciting if we were to find an answer in any number of dimensions but so far, we just don’t understand it. That is to say, we don’t understand the actual amount of dark energy that exists. In principle, we would have expected it to have been much larger—120 orders of magnitude larger in fact. Understanding dark energy remains a formidable challenge.

How many years do you think it will be before the LHC and other technologies make discoveries that fundamentally change our understanding of the structure of the universe?

In addition to the experiments at the LHC that I describe in my book, many more cosmological investigations are in store. Gravity wave detectors will look for gravitational radiation from merging black holes and other exciting phenomena involving large amounts of mass and energy. Cosmic microwave experiments will tell us more about inflation. Cosmic ray searches will tell us new details about the content of the universe. And infrared radiation detectors could find new exotic objects in the sky. Regardless of the results, the interplay between theory and data will lead us to loftier interpretations of the universe around us and expand out knowledge into currently inaccessible domains.

At the LHC, we don’t know how long it will be before we start getting answers since we don’t know what is there or what the masses and interactions might be. Some discoveries may happen within a year or two. Others could take more than a decade. Some might even require higher energies than the LHC will ever achieve. The wait is a little anxiety provoking, but the results will be mind-blowing. That should make the nail-biting worth it. They could change our view of the underlying nature of reality, or at least the matter of which we are composed. When the results are in, whole new worlds could emerge. Within our lifetimes, we just might see the universe very differently.

Lisa Randall

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