Category Archives: work

Cloudy with a chance of Physics

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I don’t know. That’s a bit of a desperate title. But in exchange, a rather nice cloud formation, don’t you think?

clouds_3_3_2014_1

This was from the sky over Los Angeles yesterday evening (a shot of the sky in the other direction is to the right – click for larger), and my first thought was “what’s the physics behind these beautiful structures?” There’s enough regularity here to expect there to be a mechanism, but I do not know what it is. Some combination of atmospheric conditions like wind speed, clouds_3_3_2014_2temperature, perhaps some layering of different bodies of air, and so forth, resulted in this and I’d love to know more. What factors set the roughly regular size of the structures, their pretty uniform distance apart, etc? (These are typical physicist’s questions, in case you’re […] Click to continue reading this post →

Take Part in the Festival!

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I’ve just returned from a rather wonderful two rainy days in Santa Barbara celebrating the work of Joe Polchinski. (See my previous post for more about this, including a few reflections.) It was a combination of high school reunion, group hug (with Joe in the center), and serious reflection about physics, now, back then, and to come. Now the great news is that pretty much everything was recorded on video, and so you can take part in it by settling down in front of your computer (or other device – those of you in the further (but pre-singularity) future can just instruct the appropriate plug-in from [Cyberdine systems ] [Tyrell Corporation] Google to stream directly to the vision centres of your brain) and view the various excellent talks and panel discussions here.

I had the honour of chairing (and contributing to) one of the panel discussions reflecting on D-branes (as I promised last post). The title was “D-Branes: Tools of the Revolution” and it went very well thanks to my three excellent panelists (Greg Moore, Andreas Karch and Samir Mathur) and many members of the gathered audience who contributed to the free-form discussion in the 15 minutes at the end. Have a look at that right along side the really interesting and lively discussion that Steve Shenker chaired at the end of the conference (which sadly I had to miss because I had to get back to LA through the rainstorm for another engagement). The idea there was to speculate a bit about the future of physics and thereby “Planning for Joe’s 90th Birthday“.

Enjoy.

-cvj Click to continue reading this post →

Festival of Joe

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20140226-214941.jpgEarly evening. Cocktail (made with Hendricks gin, muddled tangerine, and basil…). Roast pork on the way. Old haunt.

Where am I? At Roy’s, in Santa Barbara. I’m here for a two day celebration of the work of Joe Polchinski, one of the giants of my field. It all begins tomorrow, and I am taking the opportunity to have a quiet bit of time in an old haunt. I was a postdoc of Joe’s back in the mid 1990s, just when the world of theoretical physics was waking up to the awesome power of D-branes. D-branes are a special type of dynamical extended object in physics, and Joe had discovered their importance for string theory just around that time. Roy’s opened around that time too, if I recall, and a group of us became regulars, helping it along in those early days when it was smaller than it is now. (That small group included my friend and fellow postdoc Andrew Chamblin, who passed away some years ago.)

A D-braneSo I am here to help celebrate Joe’s work on the occasion of his 60th (hard to believe that number, frankly), and it will be good to see all the people who show up, and of course it’ll be excellent to see Joe. Part of my help in the celebrations is to organize and run a panel about D-branes, which will be on at 11:00 tomorrow. I’ll be reflecting a bit on the good old days when D-branes really broke, and turned out to be the key tool of the Revolution that took place in the field. In lectures and writings from that time and long after I used to refer to them as the Heroes of the Revolution, and in honor of that and of Joe I have named this session D-Branes, Tools of the Revolutionary, or something like that. Joe helped bring about the revolution, and his tools were D-branes, you see.

I was lucky to be here as a postdoc at that time, and happily I had the good sense to be quite sure that it was going to be important to quickly spread the […] Click to continue reading this post →

Introduce a Girl to Engineering Day

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Apparently it was Introduce a Girl to Engineering Day yesterday, so some might say I missed it. But as you know, I think that every day should be Introduce a Girl to Engineering Day, so it is not too late! Have a look a website for it I found (here). What can you do? You don’t have to organise a huge event (although feel free to). It can all start small. Maybe for that next gift, consider (if you don’t already) buying your little girl (or someone else’s) a set of LEGO or an electronics kit (or other hands on building toys) instead of the (fill-in-the-blank-standard-girl-toy) everyone else is getting her. Go ahead and make it a pink kit, if you want, since that’s what society seems to want. Who cares what colour it is? Apart from buying, even just chatting a bit with a child about engineering in our lives, and all the people who can (and do) contribute to it would be a great thing to do.

Here’s a short film from our own Viterbi school of engineering:

[…] Click to continue reading this post →

Films!

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Good News Everyone!

The deadline for the USC Science Film Competition (now in its third year) passed, over the weekend. The good news is that I now have 12 films in the vault (a 50% increase on the two previous years), waiting for the next stage of the competition. They are on a wide range of topics, in a variety of narrative styles, and I’m super-excited to find a block of time and have a look at them. The next stage is judging, and I’ll be assembling the panel over the next day or two and letting them start looking at them and casting their votes.

So keep an eye out for more news soon. The showcase and awards ceremony is on the 7th of March, and I’ll be opening envelopes and giving away prizes with delight, I’m sure. I’ll also be posting films here on the blog for you to look at, once that date has passed.

-cvj Click to continue reading this post →

Mass Matters

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I was about to do a little post about where we are in the General Relativity class. We’re about to start studying our first full non-trivial solution of Einstein’s equations. Then I had this feeling I’d done it before, so I looked, and indeed the very post I thought I’d do was first posted Feb. 13th 2008. So why not re-post it for your reading pleasure?

_________________________________________________
(First posted 13th Feb. 2008)

Well, only four weeks and change behind us in this course, and… the class (see here and here) is ready to understand this wonderful equation:

the schwarzschild solution

and all that it implies. What is it? It encodes the shape of spacetime around a spherical blob of mass of total mass M. No, don’t worry too much about the details, since this is not a lecture about General Relativity….. it is just nice (I hope) every now and again to get a look at the sorts of things we use in our day to day work. This “warped” spacetime encodes what we interpret as the gravitational field (in the old Newtonian language) due to a spherical (or, to a good approximation, almost spherical) mass. Like the sun, or the earth, or that tennis ball in the corner there*. It is an exact solution […] Click to continue reading this post →

Summing Numbers in a Small World

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So here’s a slightly weird thing. So there’s been all this excitement over the web about the old old “shocker” that the sum of the positive integers is -1/12. You know, not even an integer, and not even positive. Apparently there have been articles in the New York Times and Slate and goodness knows where else… and I’ve been ignoring it all since I’m tired of what it often leads to: People wilfully using it as a device to manipulate people’s ignorance about subtleties with infinite processes to make the tired point that string theory is somehow wrong since it is based on “funny math”. I called Lawrence Krauss (who should have known better) out about it some years ago when he did that at an event I happened to attend. It’s a bit tedious, not the least because it is actually part of a wonderful field of mathematics that can get misrepresented, and of course because it has nothing to do with string theory.

So I ignored it all. Then some students in my class asked me about it. And I explained why it is interesting and so forth… Then I carried on ignoring it all.

Then a day or two ago a mathematician colleague emailed me to ask what […] Click to continue reading this post →

Straight

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In class tomorrow I’ll introduce one of my favourite equations:

[tex]
\frac{d^2x^\sigma}{d\tau^2}+\Gamma^\sigma_{\rho\nu}\frac{dx^\rho}{d\tau}\frac{dx^\nu}{d\tau}= 0\ .
[/tex]

… Wait – Where did everyone go?!

Come back! I’m not expecting you to know what it means, I just wanted to talk a bit with it sort of … nearby. If you consider yourself a bit intimidated by mathematics, be assured that it won’t bite. (No more than a piece of sheet music lying nearby will harm someone who has not learned to read music.)

It turns out that it is pretty geometry! In the equation, we’ve the object
[tex]
\Gamma^{\sigma}_{\rho\nu}\equiv\frac12 g^{\sigma\mu}\Biggl(\frac{\partial g_{\mu\nu}}{\partial x^\rho}+\frac{\partial g_{\mu\rho}}{\partial x^\nu}-\frac{\partial g_{\rho\nu}}{\partial x^\mu}\Biggr)\ ,
[/tex]
called the “Christoffel symbols”. The set of objects [tex]g_{\mu\nu}[/tex] (the “metric”) actually encode the properties of the space you wish to study (like the plane, or the sphere), and the equation at the top tells you what are the “straight lines” in that space. Well, in the plane (like your desktop) they are straight lines, while in other spaces they are the analogue of straight lines – if you want to go from one point to another point somewhere else in the space and desire to travel along the shortest path to do so, you want to follow such a line. It is called a “geodesic”. The equation is commonly called the geodesic equation.

231px-Longitude_(PSF)You know such lines, intuitively, in a non-trivial example. Next time you look at a globe (wait, does anyone but me look at maps and globes any more? I love them!), you’ll probably see examples of those lines drawn in. They are the “great circles”, the lines of longitude, and the equator. (Image used with permission.)

I just made a class worksheet that guides one through a bit of playing with this equation to get the class excited about […] Click to continue reading this post →

Hawking an Old Idea

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On the one hand it is good to get members of the general public excited about scientific research, and so having some new excitement about something Stephen Hawking said, driven by gushingly written articles in the press and online, can be good. On the other hand, it is annoying that the thrust of the articles are largely that he’s stunned the world again with a brilliant and unlooked-for idea. People just lap this stuff up, unquestioningly. It is actually an old idea (and in fact one that is being mis-reported – see below). One’s instinct is to just say “Welcome, Stephen, we’ve been waiting for you to join us”, or “Come on in, the water’s lovely”, and just move on, but it seems so unfair. The thing that’s most puzzling in all of this is Hawking’s own paper (which is all of two pages of words – a transcript of a talk he gave in August), which makes no reference at all to (for example) Samir Mathur’s work, which has been explicitly saying essentially the same thing for well over a decade, with a very definite proposal for how it might work. That work has hardly been buried in obscurity. Samir and many other people who have liked his idea have been working out the consequences of the proposal in numerous papers for over a decade and reporting on their results at all the main conferences, and even talking to him about it (I note that Samir was in the audience during the August talk and even politely asked the speaker to compare and contrast the similar-sounding proposals). So it is puzzling that you get no hint from the paper’s citations that this is a well-considered and ongoing idea, even if (perhaps) in detail it may pan out differently from other suggestions.

light cones in spacetime with a compact mass - a black holeWhat’s the idea?, you ask. Well, it is not, as you might get from most of the articles (somewhat confusingly), that black holes do not exist. It is that the black hole’s event horizon, thought of as a sharp “point of no return” boundary, may not exist. Instead, it is approximation or shorthand for the complicated physics (of both matter and spacetime) that happens in the vicinity of the black hole. Simply put, the horizon arises in classical solutions to classical (i.e. non-quantum) equations (such as in General Relativity) of gravity. (See an earlier post I did about them here, from which came the illustration […] Click to continue reading this post →

Unfinished Meditation

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figure_27th_january_2014_1_smallAfter a long weekend of a two-day meeting of an American Physical Society committee (hosted in LA this time, so I did not have to travel far), I decided last night to go and do a drop-in-and-draw session at an art studio, to clear my mind.

The model was excellent, with a smashing hairdo and good body-awareness that made for interesting poses. I was in the mood to disappear and listen to Hendrix for three hours while focusing on the simple task of representing what was in front of me. So I did.

I’m horribly slow again. Lack of regular practice (on full figures), of course. The good news is that I have been able to hold on to some of the more important foundations that allow me to lay things out, and see forms and shapes. But it does mean that I don’t get to get to some of the finishing processes that I intuitively prepare for in the earlier parts of the drawing.

As you know from many posts of mine on the subject (see e.g. here and here) I actually like incomplete drawings where you can see a lot of the process […] Click to continue reading this post →

Collecting the Cosmos

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i_2014_01_24_CollectCosmos_150x200Don’t forget that on the USC campus on Friday at 4:00pm, we’ll be kicking off the Collecting the Cosmos event! It will be in the Doheny library, and there’ll be a presentation and discussion first, and then a special opening reception for the exhibition. Be sure to get yourself on the waiting list since there’s some chance that you’ll get in even if you have not RSVPed yet. (The image is from the Visions and Voices event site, and includes parts of the artworks – by artists Victor Raphael and Clayton Spada – to be included in the exhibition, so come along and see.) The event description says, in part: […] Click to continue reading this post →

A New Group

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The new semester is underway and it’s off to a good start. I’m teaching the upper division class that I designed some years ago on General Relativity and got put on the course catalog here at USC. My thought back then was that since we are graduating streadily increasing numbers of talented physics students, many of whom are now going to the top graduate schools around the country, we really should have an opportunity for them to learn in depth about general relativity before they graduate. My thinking was also that it affords them the opportunity to not only learn about one of the most beautiful and important discoveries (almost 100 years old now) about our Universe, but also to learn skills and ways of thinking about physics and calculational techniques that they do not necessarily see in other classes. For many students it will be one of the last classes they take in physics as an undergraduate, and so they leave on a mind-expanding high note, off into the world to do wonderful things with their knowledge. For others who may take it the year before they graduate (or possibly even a year earlier) it may inspire them in their growing love and appreciation for all of physics in their remaining undergraduate years, maybe even help them decide to go to graduate school. More about all this here.

So anyway here we are. I’m happy to say that once again the group of […] Click to continue reading this post →

Thinking…

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thinkingHoliday time is upon us, and in this part of it, it is coming up on Christmas time. Really fast, it seems. The semester sort of petered out mid-week for me with the submission of the final grades for my graduate electromagnetism class (everybody did really well… fantastic group of students this year), and one last two hour committee meeting for another of those committees I can’t tell you about. I’ve found some time to think about some physics, and work on a new page for the book, among other things. There’s a panel […] Click to continue reading this post →

Not Struck…

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…by lightning or anything. Yay.

heaven_help_us

It was fun! (See previous post for what I’m talking about.) The audience seemed to like it. I got to explain that being curious and doing experiments and forming hypotheses is somehow preferable (to some) to sitting back and saying “God did it”, and that there are a lot of nice side effects of that curiosity. (You know, increased food supply, improved medicine, better communications, travel, overall quality of life, and so forth…) We even got to talk a tiny bit about physics (somehow I got on to neutrinos…. not sure why, but then… why not?). […] Click to continue reading this post →

Heaven Help Us

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Well it is 6:30pm. It was my plan to take a nap this late afternoon (maybe early evening) but I’m not going to do that anymore. Why? Well turns out I’m appearing in a show this evening. It starts at midnight so I’m a little afraid that I might just sleep all the way through, wake up tomorrow morning and so miss my spot. So while the sleep would do me some good in order to be up so late, and functioning, I think I’ll skip it.

What’s the show? Well it’s a show on stage put on by some of the Upright Citizens Brigade. They asked me to appear as a guest – not as a character, but actually as myself, a scientist. It’ll be in front of a live audience, although they will be taping it later possible broadcast. You know how it goes with me – I especially like an opportunity to put some science out there here it is not expected so this is right up my alley. My understanding is that it’s a comedy […] Click to continue reading this post →