Thoughts on the LHC and ILC

Back in late July, I got email from a writer for Physics World magazine (which is sort of the UK equivalent of Physics Today), asking my opinion on a few questions relating to particle physics funding. The basis for asking me (as opposed to, you know, a particle physicist) was presumably a post from April in which I ranted a bit about the justification of Big Science projects.

The article is now out, but not available on-line, so I haven't read it. I spent a fair amount of time typing up my response, though, so I'm going to recycle it into a blog post, because I can do that.

The original email asked three questions:

• 1. Do you think that the LHC will be worth the billions of Swiss Francs being spent on it?
• 2. Do you think that it is worth building the ILC?
• 3. Do you think that we have come to the end of the line when it comes to ever bigger fundamental physics experiments, and that scientists would be better off spending their finite resources on larger numbers of smaller experiments?

My response is below the fold.

I should preface my comments with the statement that this is really not my field of expertise. I am an atomic physicist by training, and my knowledge of the state of the particle-physics art is only a little better than that of an interested layman. I can't claim real expertise that would allow me to judge claims made regarding the potential of various proposed accelerator systems.

That said, my understanding of the current state of high-energy physics is that pretty much everyone involved really expects that the energy increase provided by the LHC will be sufficient to uncover interesting physics. As I understand it, people are mildly surprised that we haven't found the Higgs boson or any of the proposed super-symmetric particles yet. For the LHC to not discover at least some of these particles would border on catastrophic for most attempts to construct a unified theory.

Given that state of affairs, I think it's absolutely worth building the LHC. There's a very reasonable expectation that it will shed light on some of the most puzzling and fundamental questions in physics, and I think the knowledge gained would be well worth the cost.

My knowledge of the ILC proposal is more limited, but my understanding is that it and the LHC are sort of a pair-- the LHC is envisioned as a sort of brute-force machine that will allow a relatively coarse survey of a heretofore unexplored energy range, while the ILC is designed to be a more controlled system that will allow finer studies of the particles expected to be produced by the LHC. The idea is that once the LHC gets the rough picture of where the new particles are, the ILC can be used to focus in on a narrower energy range, and really nail down the masses and properties of the new particles.

As such, it wouldn't make a great deal of sense to build the LHC without also laying the groundwork for the ILC. It doesn't need to be built at the same time, but the design and prototyping work should be going on while the LHC is starting up, so that once the LHC does find something, the ILC can be brought on-line in a reasonable amount of time to do those detailed studies. Having the LHC will enable the discovery of new particles, but the ILC is needed to do real precision science with them.

What to do if the LHC is turned on and doesn't see any new particles is an interesting question, though. As I understand it, the ILC would not provide a really dramatic increase in collision energy over what's possible with the LHC, which means that if the LHC failed to find new particles, the ILC probably wouldn't turn up anything, either. At which point, the question of whether it would be worth building the ILC would become a whole lot more interesting. I don't think that decision needs to be made now, though-- the time lags involved are such that there would be plenty of time to make a decision about the ILC after the LHC is operational, which is really the right time to do that.

As for whether we've reached the end of the line with big accelerator physics, that's a tough call. What I've read on the topic suggests that there aren't a lot of solid ideas on ways to make dramatic increases beyond the energy of the LHC. If the LHC fails to turn anything up, we may really have hit a wall, but it's also possible that the experience of building and running the LHC will show some clever people a new way forward. If it doesn't find anything, though, the prospects for feasible next-generation machines don't look nearly as promising as the prospects for the LHC itself. At which point, you would have to start looking seriously at other ways to answer these questions.

And I think it's very important to note that there are other ways to get at answers to some of these questions. Some of the tightest constraints on super-symmetric unified theories come not from gigantic accelerator experiments, but from "table-top physics" experiments that look for things like an electric dipole moment (EDM) of the electron and other fundamental particles. Most theories beyond the Standard Model predict non-zero EDM's for the electron, and current experiments have put limits on the size of the electron EDM that have already ruled out a whole class of "naive" supersymmetric theories, and there are experiments under construction that should push that limit down by another couple of orders of magnitude, at least. There's a thriving community of people looking for electron EDM's, including fantastically clever experiments being done by people like Dave DeMille at Yale, Dima Budker at Berkeley, and Larry Hunter at Amherst, and a host of new experiments are in the works from people like Eric Cornell at JILA. These are ingenious experiments that use the astonishing precision possible with laser spectroscopy to put limits on particle theories using atomic and molecular systems, and they don't get enough attention.

Similarly, people like Norval Fortson at the University of Washington are doing amazing things with parity non-conservation measurements, which again put limits on the types of fundamental particle interactions you can have. And there's the possibility of finding beyond-the-Standard-Model physics in measurements like Gerald Gabrielse's recent measurement of the magnetic moment of the electron (arguably the most precisely measured quantity in science), which appeared in Phys. Rev. Letters in the last month.

On the question of extra dimensions (another issue invoked in discussions of the LHC/ILC), there are some simple but amazing experiments being done by Eric Adelberger and the Eot-Wash group at the University of Washington, looking for a change in the force of gravity at extremely short distances. They're using the idea of a torsion pendulum, first used to measure the gravitational constant by Henry Cavendish in the 1790's, to look for deviations from Newton's inverse-square law at extremely short length scales, which is one of the possible consequences of the extra dimensions predicted by various theoretical models.

These measurements are not as direct and satisfying as the large accelerator measurements, but they do have the potential to at least put limits on new physics at energy scales that can't be reached by current accelerator technology. And they don't cost billions of dollars to build. They probably represent the best way forward for understanding the universe at high energy scales, if the LHC doesn't discover anything new.

Getting a little farther afield from the questions you asked above, the comments I made in the blog post you mentioned were mostly a reaction to the strategy being used to promote the LHC and ILC. While I think the physics potential of those devices is huge, I think they should be marketed to the public, and to funding agencies, on the basis of that science alone, and not as a means of generating spin-off technologies, or a jobs program for physicists. And I find it mildly annoying to hear it presented as if the entire enterprise of physics will come to a screeching halt if these machines are not built.

While these accelerators are designed to address the most fundamental questions in physics, that does not mean that they are in any way fundamental to the practice of physics as a science. The fact is, the majority of physicists around the world will have long and productive careers in fields like atomic physics, or condensed matter/ solid state physics, or even nuclear physics without ever dealing with data from one of these gigantic accelerators. The LHC and the ILC are fundamental for high-energy particle physics, but physics as a whole would survive and even thrive if they aren't built, though the world would be a poorer place for their absence.

When it comes to funding, ultimately the decision ought to be based on the quality of the science directly done by those machines. The questions the LHC and ILC are designed to answer are deep and fundamental questions about the origin, nature, and future of the universe in which we live, and we ought to fund research into those questions simply because that's one of the things that civilized modern societies do. CERN was worth building and operating not because it gave us the World Wide Web, but because it's a tangible representation of some of the best traits our species has to offer. The World Wide Web is just a nice bonus.

Similarly, we shouldn't build the LHC and the ILC because they might lead to, say, improvements in superconducting technology that will allows us to build flying cars. If we want flying cars, we should give a few billion dollars to researchers in superconductivity, and ask them to make us flying cars, not spend several billion on a particle accelerator and hope that somewhere along the line, it will produce some spin-off technology.

But that's somewhat off the original topic, and it's getting late, so I'll stop there. To sum up: 1) The LHC is definitely worth building, because there's a reasonable expectation that it will discover interesting physics beyond the Standard Model, 2) The ILC is a necessary follow-on for the LHC, and we should go ahead with the development work for it, at least until we see what the LHC finds, and 3) If the LHC doesn't find anything new, it's not the end of the world, as there are still other non-accelerator-based methods of getting at some of these questions. It might be the end of particle physics as we now know it, but physicists are clever people, and will find new ways to attack these problems.