Physics on the Perimeter

By Mark Trodden | April 26, 2007 11:16 pm

Extremely early this morning, I returned from a visit to the Perimeter Institute, the theoretical physics center in Waterloo, Canada, founded by Mike Lazaridis, Co-CEO of Research In Motion (RIM) – makers of your Blackberry. I had spent the last day at PI, delivering the institute colloquium yesterday afternoon, with a talk titled “Challenges of Explaining Cosmic Acceleration through Modified Gravity”.

Perimeter is a wonderful place, with an increasing number of researchers devoted to physics issues of “foundational” importance. These topics include Quantum Gravity (with members working on causal sets, loop quantum gravity and string theory); Quantum Information Theory, Cosmology, Quantum Foundations and Particle Theory. These people are housed in a beautiful new building that, as well as being aesthetically pleasing (to me, at least), is also a tremendous intellectual playground, with collaborative spaces around every corner, equipped with espresso machines.

Arriving in Waterloo the night before, I graded exams over dinner and a couple of beers before getting an early night so that I could get into PI reasonably early in the morning. This gave me plenty of time to get administrative details out of the way, settle into my office, and then meet up with postdocs Claudia de Rham, Andrew Tolley and Mark Wyman for coffee and a lengthy physics chat. We talked about what each of us was up to and then spent some time discussing the ghost states and strong coupling regimes of various modified gravity models. These are interesting questions, to do with some of the pathological problems that often arise when General Relativity is altered to try to explain cosmic acceleration.

My host, Cliff Burgess, grabbed me for lunch at noon, and we spent an hour or so talking about string theory, quantum gravity, the sociology of these fields, and generally chatting about life at PI. This gave me a half hour to look over my talk before the colloquium at 2pm.

Colloquium at the Perimeter Institute is a lively event. Certainly the faculty, at least, are happy to press and probe with many questions. My talk consisted of a broad introduction to the problem of cosmic acceleration, followed by an outline of the general issues presented by modifying gravity to account for this phenomenon. I used some specific examples to show how solar system constraints constrain certain models, and the appearance of ghost states renders other ones unworkable. I also talked about how we may exploit some loopholes in these constraints to arrive at viable modified gravity models. In the last part of the colloquium, I moved on to the question of the types of observations that might help distinguish modified gravity models from dark energy, or a cosmological constant, as competing explanations for cosmic acceleration – discussing comparisons between, for example, Cosmic Microwave Background measurements and large scale structure observations.

Even with the many great questions, I managed to finish roughly on time, tired, but having had a thoroughly enjoyable time during the talk. I always find speaking to an audience exhausting, and this was no exception. But here Perimeter’s ubiquitous espresso came in particularly useful. With the drug fresh in my system, Cliff and I spend a half hour in his office talking about the approaches he and his collaborators have to dark Energy in some string inspired models, before I met up with Lee Smolin to talk about approaches to related questions arising from loop quantum gravity.

Later in the afternoon I got together with my former colleague and friend Rafael Sorkin. Rafael is a remarkable guy – an experienced relativist with unique ideas about the right way to construct a quantum theory of gravity from causal sets. Some questions about my talk, an update on his progress in teasing out the physics of causal sets, and a sketch of Rafael’s other interests in the foundations of quantum theory took me all way up to when I had to leave and meet up with my hosts in the Black Hole Bistro (yes – really).

There my visit wound up with dinner, where I got to spend time with, among others, someone I’ve known for a long time – and even wrote a paper with once – the theoretical physicist Slava Mukhanov. Slava is visiting PI for a few months – he’s currently giving a cosmology mini-course there – and as well as being an accomplished physicists, is a hilarious storyteller – which made dinner wonderful fun.

I had spent an extremely long and full day at PI, punctuated with fascinating meeting after fascinating meeting. My drive home passed quickly partly because I found myself mulling over a number of the ideas people had mentioned to me during the day. This is one of the great things about traveling as an academic – the exposure to different ideas in an informal context (not like reading papers) in which you can get lots of insight and instant feedback to your questions. Nice to be home though!

  • Angus McQuarrie

    There’s always a little spark in my head whenever I read in high-profile articles about Waterloo. As a student there, you hear a lot of hype about about the University, about the Perimeter Institute, the Accelerator Center, RIM in general. All in all, most of us aren’t drinking the kool-aid, but it’s good to hear some of the propaganda is substantiated from outside sources.

  • Neil B.

    Modified Gravity – I gather it is pressed into consideration by peculiar observations (movements of and inside galaxies that can’t even be explained by dark matter, or perhaps Pioneer 10, 11 anomalies etc.) But what is the theoretical basis? There certainly wasn’t an a priori reason in theory to change anything, true?I thought Einstein’s GR had reached a rather orthodox state, and how can it be changed to incorporate all that without undermining the particular, elegant-seeming application of the geometric basis? Why doesn’t anyone (?) worry or at least talk much about the larger implications of having to change gravity theory (like, how it affects black holes, cosmic expanision and how we have evaluated it, etc.) Don’t MOND etc. have a sort of tacky inelegance? It seems we don’t have much dialog about this, it seems under the radar most of the time.

  • Neil B.

    Sorry, I know the poster is taking this issue and the listeners etc, so I meant it doesn’t seem to get out and about in the common discussion – as kids mean by “anyone” at school etc., and that we actually hear about.

  • Mark

    A number of people have been motivated to think about modified gravity as a way to explain cosmic acceleration – a phenomenon we currently do not understand – that’s the motivation. In any modified gravity theory one most certainly does have to think about the effects on black holes, the solar system, cosmic history (in fact that’s the motivtion), gravitational waves, etc. People do do this and talk about it a lot. These provide many of the constraints that rule out lots of modifications.

    As for the geometrical basis; none of the modifications I was discussing mess that up at all. One is free to think about modifications that do – but it is a huge challenge to agree with all the data I mentioned. I don’t have any idea how to do it right now.

  • Maynard Handley

    Mark, did you point out to PI how completely retarded they are concerning the internet? They have no non-painful way to browse their talks, no downloads, no podcasts, nothing. It’s like it was put together by a bunch of not very smart refugees from 1997.

    As a result, while I follow what’s going on at KITP (who have a magnificently well organized internet+podcast story) pretty well, I basically ignore what’s happening at PI. Now PI probably don’t care what I do or don’t listen to, but there probably is at least one student somewhere whose opinion does actually matter who, for the exact same reasons as me, follows KITP and ignores PI.

  • Haludza

    haha, I was just about to post about how great PI was for providing archived videos of many of their colloquia and seminars (pdf, mp3, AND streaming video no less!).

    This is all you need Richard Hadlee! :

  • Mark

    Yes Maynard, after my brief introduction in my talk, in which I thanked them for the invitation and the wonderful hospitality while there, I launched into a 10 minute rant about how “retarded” their they are in this way. They loved it! Said it was just what they’d been hoping for when they invited me!

    Also, at the end of the talk, when I could tell people were losing interest in the whole science thing, I wound up with a series of rapid-fire “Your momma’s so fat, …” jokes.

    They’ve asked me to come back for a longer visit, and I’m pretty sure it was becasue of the abuse and tough-love.

  • B

    Hi Mark,

    sorry I didn’t get to talk to you, I enjoyed your colloq – though I was late…. I find the modified source scenarios quite interesting, I’ve been thinking in that direction for a completely different reason, maybe I should look more into it (if I find the time that is).

    @ Maynard

    Regarding Websites: ARGH. You have to know that this is the *improved* version (it’s been updated last fall). It drives me nuts! Whenever I try to link to a site it will move the next day and the link is dead. It is impossible to find a seminar, even if one knows the name of the speaker (which I hardly ever do). Not to mention that there is no sensible way to put files on the web. Whenever I complain to IT they tell me they are short on time and people and they just can’t take care of all that. But sooner or later they usually address all the issues, it just takes some time…

    It would probably make a difference if you complain not here, but to the IT guys, just send an email, it’s on the websites – if you find it 😉



  • Mark

    Hi B – thanks. It would have been nice to meet up. Hopefully next time.

  • Moshe

    Hey Mark, I feel cheated, next time you’re at UBC I want the stand up routine also :-).

  • Mark

    You got it Moshe.

  • Maynard Handley

    So, Mark, what is your actual point?
    * One should NEVER criticize some aspect of an organization? or
    * One should NEVER criticize some aspect of an organization while there in person?

    It’s not like PI seems to do anything in response to emails. I have sent them at least two (a whole lot more polite than my rant, explaining the specific problems and how they might be fixed) to no obvious effect.
    Pointing out a serious issue while speaking to the people involved strikes me as a perfectly reasonable thing to do. Certainly, when I was at trade shows on behalf of my former employer, I would expect this is what people would and should do — it was there one chance to try to influence the future directions of products they cared about and wanted improved.

  • Mark

    Maynard, my actual point is neither of the things you mention. I was just mildly amused by the idea that I would visit an institution, at their invitation, and commence criticizing them while there – that’s all.

    I’ve personally never tried to deal with the online stuff from PI, and so haven’t enountered any problems. If I had, I would have contacted them, but not saved it up for a discussion while visiting – that’s not what the visit is for. Delivering an invited presentation at an academic institution is not the same thing as manning a booth at a trade show.

  • Haludza


    While I’m still puzzled at your dissatisfaction with the site, I suspect that your e-mails had the misfortunate of being sent to the wrong person (even though their job title would indicate otherwise).

    I watched the lecture by Mark online and enjoyed it very much! As with most of their online lectures, it was there literally minutes after it had ended at PI.


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Cosmic Variance

Random samplings from a universe of ideas.

About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.


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