I was sent an interesting link a while ago* that I thought I would share with you. It is a really good discussion about Dark Energy – what do we think it is, why we think it exists, why some think it does not, and how to move forward with the discussion of what is, after all apparently *most* of our universe. It is a panel discussion that was hosted by the Institute for Arts and Ideas (which I *love* the idea of!). The discussion is described on the site as follows:
Dark energy is supposed to make up two-thirds of the universe. But troublingly CERN has yet to find any evidence. Have we got our story of the universe wrong – might dark energy be the aether of our time? Do we need a new account of the universe, or is it too soon for such radical solutions?
The BBC’s Sue Nelson asks Templeton Prize winning cosmologist George Ellis, Cambridge physicist David Tong and mathematician Peter Cameron to seek the invisible.
Ok, the “troublingly CERN has yet to find any evidence” part puzzles me a bit, since nobody’s really expecting CERN to find any evidence of it, in any large scale experiments that I’m aware of (please correct me if I am wrong)… Is the writer of the abstract confusing Dark Energy and Dark Matter? Even then I think it is an odd phrase to lead with, especially if you don’t mention the huge amount of evidence from astronomy in the same footing… but I imagine the abstract was maybe not written by a physicist?
Nevertheless, I strongly recommend it as a thought-provoking discussion, and you can find it embedded below. Do also check out their many other interesting [...] Click to continue reading this post
As you may know from three previous recent posts on research (here, here, and here), I’ve been thinking and calculating a lot in the area of dynamical cosmological constant – concerning mostly (but not entirely) thermodynamics and quantum gravity. Specifically, the cosmological constant becomes the pressure variable in the thermodynamics. I think it is important, and will teach us something about things like gauge/gravity duality, string theory, black holes, and perhaps even cosmology, but I am not sure what yet. I’ve made some suggestions in recent papers, and computed some interesting things along the way.
Anyway, the larger community has not been following this story much, since: (1) It means a break with some powerful and still very fruitful frameworks where the cosmological constant being fixed is a given – like AdS/CFT – and it is not clear what that means yet, so the motivation is not super-strong; and (2) Let’s be honest, there’s no superstar working on it, so it is not going to get anyone any points. So I’ve been trying to shout about it in my little way from the periphery, as I think it might be useful, and since several people have been doing really good and interesting work on this issue for many years and it is worth more people seeing what they’ve been up to.
So imagine my pleasant surprise when I looked on the listing of new papers on the arXiv for today and saw three (!) papers on the subject, moving things forward in various ways. (They all seem to have noticed some of what I’ve [...] Click to continue reading this post
So it happened again. I got musing to myself about something and decided to do a quick computation to check it out, and it took me down an interesting rabbit hole, which resulted in me writing a nice little paper at the end of last week that appeared today on the arxiv. I think the physics is really really nice. Let me tell you a bit about it. It is in the same area of ideas that I mentioned last time, concerning that paper I wrote last month. So let me pick up the story there, since I did not really touch on the core of the story. [Note: for non-experts, the following will get somewhat technical and full of terms and ideas that I will not explain. Sorry.]
One of the things that might have struck you (if you’re an expert in the area) from my proposal to make heat engines out of black holes that do real mechanical work like the engines you read about in physics textbooks is that there ought to be no actual mechanical work since there’s no pistons – no pistons changing volumes and so forth. That is (or rather, was) a missing ingredient in the standard thermodynamics of black holes in quantum gravity. Well, that all changed a short few years ago with the work of a number of authors, particularly with the clear suggestion of David Kastor, Sourya Ray, and Jennie Traschen, and work by Brian Dolan, with a fair bit of followup investigations by various other authors including some I’ll mention below. (Update: Two reviews, with different foci, can be found in here and here.) The general idea is that if you allow the cosmological constant to be a thermodynamical variable as well (and there is a long history of authors considering this in various contexts), where it naturally acts like a pressure , (G is Newton’s constant, and I’m setting various other constants to unity in the usual way) then you naturally include a conjugate to that variable that should be the pressure.
For a simple static black hole like Schwarzschild, the volume turns out to the the naive volume you get by taking the radius of the black hole and forming [...] Click to continue reading this post
Yes, you heard me right. Holographic Heat Engines. I was thinking recently about black holes in universes with a cosmological constant and their thermodynamics. I had an idea, it led to another, then another, then some calculations, and then a couple of days of writing, calculating, and thinking… then a day to cool off and think about other things. Then I came back to it, decided it was still exciting as an idea and so tidied it all up as a paper, made some diagrams, tidied some more, and voila! A paper submitted to the arxiv.
I’m sort of pleased with all of it since it allowed me to combine a subject I think is really fun (although often so bleakly dull when presented at undergraduate level) – heat engines – with contemporary research ideas in quantum gravity and high energy physics. So I get to draw some of the cycles in the p-V plane (graph of pressure vs volume) representing the inner workings of engines of particular designs (just like you might have seen long ago in a physics class yourself) and compute their efficiency for doing mechanical work in exchange for some heat you supply. It is fundamental that you can’t do that with 100% efficiency otherwise you’d violate the second law of thermodynamics – that’s why all engines have to have some exhaust in the form of heat, giving an efficiency represented by a quantity that is less than one, where one is 100% efficient. The diagram on the left illustrates the key pieces all engines must have, no matter what working substance you’re using. The details of the design of the engine are what kind of cycle you taking it through and what the properties (“equation of state”) your working substance has. In the case of a car, for example, the working substance is cleverly mixed up with the source of heat – the air/gasoline mix forms a “working substance” that gets expanded and compressed in various ways (in the green bit of the diagram), but the fact that it also burns releasing heat means it is also the source of the heat that comes into the engine (the flow from the red bit) to be (in part) turned to work, and the remainder flowing out to the blue (exhaust). Very clever.
The cool thing here is that I’m using black holes as the working substance for [...] Click to continue reading this post
Some of you are wondering what I’m working on while on retreat. Well, actually there’s a nice coincidence here. I’m working on the graphic book that you may have heard me talk about a bit. “The Project” as I sometimes call it. I’ve been doing things on various aspects of it, such as reworking the description of it for various people to look at, writing new bits, and spending a bit of time pulling together various bits of the prototype story I used to start all of this back in 2010. The prototype bits have all of my experimentation and development of style and technique all over them, and so there are pages that needed a bit of rework (to say the least). So, on Monday, [...] Click to continue reading this post
I’m actually in hiding and silence for a week. It is Spring Break and I have locked myself away in a seaside town to do some writing, as I did last year. But I must break my silence for a little while. Why? Well there’s been a really great announcement in physics today and while being very happy that it is getting a lot of press attention – and it should since the result is very important and exciting – I’ve been stunned by how confusingly it has been reported in several news reports. So I thought I’d say a few things that might help.
But first, let me acknowledge that there’s a ton of coverage out there and so I don’t need to point to any press articles. I will just point to the press release of the BICEP2 collaboration (yes, that’s what they’re called) here, and urge you once you’ve read that to follow the link within to the wealth of data (images, text, graphs, diagrams) that they provide. It’s fantastically comprehensive, so knock yourself out. The paper is here.
I keep hearing reports saying things like “Scientists have proved the Big Bang”. No. The Big Bang, while an exciting and important result for modern cosmology, is very old news. (You can tell since there’s even a TV comedy named after it.) This is not really about the Big Bang. This is about Inflation, the mechanism that made the universe expand rapidly from super-tiny scales to more macroscopic scales in fractions of a second. (I’ll say more about the super-tiny below).
I also hear (slightly more nuanced) reports about this being the first confirmation of Inflation. That’s a point we can argue about, but I’d say that’s not true either. We’ve had other strong clues that Inflation is correct. One of the key things that pops out of inflation is that it flattens out the curvature of universe a lot, and the various observations that have been made about the Cosmic Microwave Background over the years (the CMB is that radiation left over from when the universe was very young (about 380,000 years old – remember the universe is just under 14 million years old!)) have shown us that the universes is remarkably flat. Another previous exciting result in modern cosmology. Today’s result isn’t the first evidence.
So what is today’s exciting news about then? The clue to the correct [...] Click to continue reading this post
Don’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
You’ll recall that I was in New York a short while ago to film some promotional material for a new TV series. It is called Big History, and it will be on History Channel’s H2 channel (and eventually on various international channels, but I’ve no idea which – similar ones to where you find the other show I’ve mentioned a bit, The Universe, I expect).
Rather than be primarily about astronomical and cosmological things, the show will focus each week on one of a list specific items that have affected our history, and take the long view about that item. How long a view? The longest known possible! So take something like Salt, and examine its role in civilization and culture, bringing in historians, anthropologists, etc… and physical scientists to trace that object back to its roots in the early universe… (the big bang, the cores of stars, etc.) Update: For you Breaking Bad fans, note that it’ll be narrated by Bryan Cranston, by the way.
Here’s one of the promo videos:
[…] Click to continue reading this post
I just spotted (a bit late) that Steven Weinberg (one of the giants of my field) has written a piece in the New York Review ofBooks entitled “Physics: What We Do and Don’t Know”. I recommend it. He talks about astronomy, cosmology, particle physics, and by casting his eye over the arc of their recent (intertwined) histories of ideas, experiments and discoveries, tries to put the Standard Models of particle physics and of cosmology into perspective.
The article is […] Click to continue reading this post
You might recall that last year I gave a talk at TED Youth, in their second year of short TED talks aimed at younger audiences. You’ll recall (see e.g. here and here) I made a special set of slides for it, composed from hundreds of my drawings to make it all in graphic novel style, and somehow trying to do (in 7 minutes!!) what the TED people wanted.
They wanted an explanation of string theory, but when I learned that I was the only person in the event talking about physics, I kind of insisted that (in a year when we’d discovered the Higgs boson especially!) I talk more broadly about the broader quest to understand what the world is made of, leaving a brief mention of string theory at the end as one of the possible next steps being worked on. Well, they’ve now edited it all together and made it into one of the lessons on the TED Ed site, and so you can look at it. Show it to friends, young and old, and remember that it is ok if you don’t get everything that is said… it is meant to invite you to find out more on your own. Also, as you see fit, use the pause button, scroll back, etc… to get the most out of the narrative.
I’m reasonably pleased with the outcome, except for one thing. WHY am I rocking [...] Click to continue reading this post
Well, the day is here. The Planck collaboration has announced a huge amount of results for the consumption of the scientific community and the media today. The Planck satellite looks with unprecedented precision at the very earliest radiation (“cosmic microwave background radiation”, CMB) from the universe when it was very young (a wee, cute 380,000 years old) and helps us deduce many things about what the universe was like then, and what it is like now. Here’s one of the representations of the universe using the new sky mapping Planck did (image courtesy ESA/Planck):
There’s a ton of data, and a raft of papers with analysis and conclusions. And there’s a very nice press release. I recommend looking at it. It is here, and the papers are here. The title of the press release is “Planck reveals an almost perfect Universe”, and some of the excitement is in the “almost” part. A number of anomalies that were hinted at by the previous explorer of the CMB, WMAP, seem to have been confirmed by Planck, and so there are some important things to be understood in order to figure out the origin of the anomalies (if they ultimately turn out to be real physics and not data artefacts). [Update: Andrew Jaffe has two nice posts I recommend. One on the science, and the other on the PR. Jester also has a nice post on the science from a particle physicist's perspective.]
What is the title of my post referring to? Well, the refined measurements have allowed us to update some of the vital statistics of the universe. First, it is a bit older than previous measurements have indicated. The age is now measured as 13.82 billion years. (I’m already updating pages in the draft of my book…) Second, the proportion of ingredients [...] Click to continue reading this post
Oh… I forgot to get around to letting you know the result of designing the universe required in a previous post. The result is that it is a radiation (“light”) filled universe with positive cosmological constant (and so space wants to expand due to negative pressure – much like ours seems to be doing). The radiation density wants the thing to collapse. There’s a balance between the two, and it turns out that it is when the two densities (radiation, and vacuum energy) are equal. This is only possible when there is positive curvature for the universe (so, not like ours), as you can see from the Friedman equation if you were that way inclined. So the universe is a 3-sphere, and if you work it out, the radius of this 3-sphere turns out to be . The temperature of the radiation is then computed using the usual Stefan-Boltzmann relation.
The equality of densities turns out to result from the fact that the effective potential of the equation is at a maximum, and so this universe turns out to be unstable… It is a radiation-filled version of Einstein’s matter-filled static universe, which is also unstable. It is larger than Einstein’s by a factor of .
Einstein was said to have arrived at his static universe on the grounds of what he thought was observationally clear – the universe was unchanging (on large scales). [...]
The equality of densities turns out to result from the fact that the effective potential of the equation is at a maximum, and so this universe turns out to be unstable… It is a radiation-filled version of Einstein’s matter-filled static universe, which is also unstable. It is larger than Einstein’s by a factor of .
Einstein was said to have arrived at his static universe on the grounds of what he thought was observationally clear – the universe was unchanging (on large scales). Hubble [...] Click to continue reading this post
This is an extra homework that some students of the General Relativity class did to make up for one that did not count earlier in the semester. While writing it, I realized that this universe is in fact, Heaven! You know, we become beings of light, and live forever, etc…
I thought it would be fun to share its final form:
“You work in the design section of the company that manufactures universes. (This is [...] Click to continue reading this post
Ok. So who was surprised by this one? My hand is not up… is yours? (That’s a screen shot from the Nobel Prize site to the left. More here. Cheeky of me, but it’s early in the morning and I’ve got to pack, shower, and cycle like mad to the subway to get to my train to Santa Barbara, so time is of the essence.)
I was pretty sure that this would be the prize sometime very soon, although I’ll not say that I knew it would be this year’s for sure. It is well deserved, since this was a genuinely major change in how everyone in the field thinks about the universe, and we’re still trying to get to grips with it today. The acceleration of the universe that they [...] Click to continue reading this post
As you may know already there’ll be a new NOVA series on PBS in the Fall, based on one of Brian Greene’s books, The Fabric of the Cosmos. Last Fall I did some a shoot with them for my role in it (I’ve no idea how much they will use), and I learned a short while ago that they’ll be using some of it on the NOVA website too. They extracted some parts of the on-camera interview segments I did concerning the idea of multiple universes and transcribed them into something you can read online. Have a look here. I touch on the idea in a fragmented way, mostly being led by the questions I was asked, but it’s a fun topic to chat about, and may lead you in interesting directions should you wish to learn more, so have a look.
A word on the picture they are using (er…see above left). It seems to be one that the [...] Click to continue reading this post
We had a really interesting discussion of the quantum physics of de Sitter spacetime yesterday here in Aspen, starting with a review of the behaviour of scalar fields in such a background, led by Don Marolf, and then, after lunch, an open-ended discussion led by Steve Shenker. This is all quite difficult, and is of course quite relevant, since a piece of de Sitter is relevant to discussions of inflation, which seems (from cosmological observations) to have been a dominant phase of the very early universe. As the most symmetric space with positive cosmological constant, de Sitter may also be relevant to the universe today, since dark energy (first recognized after 1998’s observations of the universe’s accelerating expansion) may well accounted for by a positive cosmological constant.
So we need to understand this type of spacetime really well… and it seems that we don’t. Now there’ve been a lot of people looking at all this and doing really excellent work, and they understand various issues really well – I am not one of them, as I’ve not worked on this in any detail as yet. Do look at the papers of Marolf, and of Shenker, and collaborators, and references therein, and catch up with what’s been going on in your own way. For what it is worth, the sense that I get is that we’re trying to solve very difficult issues of how to interpret various quantum features of the spacetime and getting a lot of puzzles by trying to make it look a lot like things we’ve done before.
Now, we may solve all these puzzles…. but my current take on this all is that we’re [...] Click to continue reading this post
You might not know the name Maurice Murphy, but I am certain that you are likely to know – and maybe even be very familiar with – his work. His is the principal trumpet playing the lead themes in very many films with music by John Williams. I have for a long time been very impressed with how so many of those themes trip so easily off the tongue (physical or mental) and seem to fit together so well (just hum the Star Wars theme, and then follow it by the Superman theme, then the Indiana Jones theme, and so on). A lot of this is due to the fact that Williams (like most good composers) is a master at recycling and modifying, creating a cluster of much loved (deservedly) themes that accompany some of our favourite movie-going memories, but I now think that the other reason is that you’re hearing them all played by the same voice! That voice is the playing of Maurice Murphy, the truly wonderful trumpeter who Williams would specifically request to play the lead on recordings of his film music. Murphy died recently, and you can dig a bit more about him and explore what I’ve been telling you further by going to the London Symphony Orchestra’s site devoted to him [...] Click to continue reading this post
This is a quick update on the school. I’ve been trying to give the students some of the core concepts they need to help them understand what string theory is, how it works, and what you can do with it. Here’s the really odd thing about all this (and an explanation of the post title): While this is a school on Quantum Gravity, after talking with the students for a while one learns that in most cases the little they’ve heard about string theory is often essentially over 20 years out of date and almost always totally skewed to the negative, to the extent that many of them are under the impression that string theory has nothing to do with quantum gravity at all! It is totally bizarre, and I suspect it is largely a result of things that are said and passed around within their research community.. So there [...] Click to continue reading this post
You can read a bit about the work of my colleague Elena Pierpaoli and her postdocs and students in this article in one of USC’s in-house publications. It focuses on the Planck observatory (image right from NASA/ESA), which we’ve discussed here before. (Recall the launch?) There’s a lot of exciting physics about the very young universe to be discovered as more data from the mission get gathered and analyzed.
Enjoy the article!
-cvj Click to continue reading this post
Recall the excitement last week about the D0 result? I wrote a post called “An Exciting Asymmetry?”. Well, there’s a rule that says if you write a title as a yes/no question, the answer is often (usually?) “No”.
Sure enough. over at Resonances, Jester reports that the CDF experiment, also at the Tevatron, has looked for a confirmation of the CP violating result that D0 claimed to see, and did not find anything abnormal where it should have. Find further details (on the technical side for the experts) and links at that post, which, as is usual with material from that blog, is well-written and interesting.
This is one reason why we (the particle physics community) build multiple detector/experiments on the same accelerator machine, and this is a prime example [...] Click to continue reading this post
A big mystery in physics is why there is more matter than anti-matter. (Of course, which we call the matter and which we call the “anti-matter” is a… matter of convention. Take your pick.) It is hoped that there is some mechanism in the laws of physics (at a very basic level concerning particle interactions) that will become apparent that explains it. It’s also hoped that the mechanism itself might have some understandable origin too. The mechanism would operate in the first tiny fractions of a second of the universe’s life when the primordial soup of particles and antiparticles (created from, roughly speaking, the energy of the big bang) began to cool down as the universe expanded. Rather than them annihilating all back into energy again, the mechanism would create an imbalance between the two, giving rise to a matter-filled universe, from which we emerged. So what could be the mechanism, can we isolate it in our theories and in our experiments? Build a good model of it? Explain it?
This is all very first and foremost in people’s minds when there’s a new experiment switched on that is probing the unknown – the Large Hadron Collider. Well, actually, it has been recently announced that a suitable mechanism (or, more precisely, its [...] Click to continue reading this post
The next in the Categorically Not! series the series of events is tomorrow, Sunday 18th April. It is, as usual, held at the Santa Monica Art Studios. It’s a series – started and run by science writer K. C. Cole – of fun and informative conversations deliberately ignoring the traditional boundaries between art, science, humanities, and other subjects. I strongly encourage you to come to them if you’re in the area. Here is the website that describes past ones, and upcoming ones. See also the links at the end of the post for some announcements and descriptions (and even video) of previous events. (At the right is an image of a beautiful sculpture by artist Yossi Govrin, who is on this week’s program.)
The theme this month is Imagine. Here’s the description from K. C. Cole: [...] Click to continue reading this post
Here’s a fun thing to get involved with. You can ask John Mather (2006 Physics Nobel Prize) a question on YouTube! Go and submit yours!
What might you ask him? Something about physics, or something else? Religion, art, politics? His favourite colour? If you consider asking a question, and whether you go ahead and ask it or not, feel free to mention in the comments what you might ask.
This is how to proceed (from NASA education)*: [...] Click to continue reading this post
Mark your calendar for Sunday! The West Hollywood Book Fair is on from 10 am to 6:00 pm that day, and there’s so much to see and do with readings, panels, discussions, authors, special celebrity guests, food, exhibitions, writing workshops, discount book offers, signings, swag (no doubt), and so forth. I’ve not been before, but as you know from reading here I’m a big fan of cities going gaga over books for a while, being a regular visitor to the LA Times Book Festival when it comes in the Spring. The calendar of events and much more about the event can be found at the website here.
Here’s another thing. Despite the fact there was no mention of a spankingly splendid [...] Click to continue reading this post
Well, ok… Boom is not quite accurate, but the idea is that there will be ten kinds of blasts/explosions/major_energetic_events discussed tonight on the History Channel’s The Universe:
The Universe is full of explosions that both create and destroy. The Chicxulub impact on the Yucatan peninsula, which may have wiped out the dinosaurs 65 million years ago, was two million times more powerful than the largest nuclear bomb ever detonated on Earth. But guess what? That’s only good enough for the very bottom of the Biggest Blasts top ten list. This episode works its way up through supernova explosions and gamma ray bursts all the way to the blast that started it all–the Big Bang.
Sounds good doesn’t it? There’s a lot of good people contributing again, so I know it’ll have some good material and explanations.
In addition, I’m reasonably confident I play a role in this one…This one was a blast (sorry) to film. You saw some posts earlier reporting on some of the filming. Assuming they used the material I did with them, you’ll get to see why I was at the [...] Click to continue reading this post
Forgot to report on this email exchange from last semester:
From one of the staff in the physics office:
Subject: 499 Syllabus
Date: Thu, 09 Jul 2009 14:01:38 -0700
To: Clifford V. Johnson
I was reading the syllabus you sent over for the 499 class. I am not sure if this is a type-o but in the Extra Books section it reads “/Black *_hoes_* and Time Warps: Einstein’s outrageous Legacy/” should it read
“/Black *_holes_* and Time Warps: Einstein’s outrageous Legacy/”.
My response: [...] Click to continue reading this post
The fires are racing up the sides of Mount Wilson as I write (19:38). They expect the burn to get to the top sometime in the next few hours, and yes, I imagine it’ll affect the Mount Wilson Observatory in some way.
[Update (20:59): You can get regular updates from the director at this page*. Seems that fire crews will remain on site for when/if the fires sweep through the grounds. ]
All of the working scientific equipment, including the solar telescopes and of course the historic telescopes (the 60 inch is pictured to the right) with which astounding discoveries were made about our universe (such as the fact that it is expanding, and the fact that the universe is vastly more than just our Milky Way Galaxy) are presumably in some danger, as well as support buildings of various kinds. I’ve no idea how much since I do not know what fire-proofing measures are in place up there, although I am sure there are several – such as keeping brush away from the buildings themselves. Here’s a camera up there on one of the solar telescopes where you can see regularly updated pictures that it snaps. I grabbed this one just now (click for larger view):
So keep your fingers crossed for luck for the instruments, and of course for all [...] Click to continue reading this post
On Friday I was involved in an interesting conversation in an unusual format. It was a chat with cosmologist Anthony Aguirre at UCSD, and it was all about research in aspects of cosmology and of string theory, touching on issues such as the nature of quantum [...] Click to continue reading this post
Tomorrow is the Big Day. For what? The launch of Planck and Herschel – Major new windows on our universe. Keep your fingers crossed for luck!
They’re on the launch pad right now. See here.
So, what are the missions and objectives of these fine spacecraft, I hear you ask.
Well, from the Planck site: [...] Click to continue reading this post
I had a lot of fun at this year’s Jet Propulsion Laboratory (JPL) open house. I’m happy to report that there were, once again, lots of people wandering around looking at the displays and demonstrations, asking questions, hanging out, and so forth, and an impressive turnout of JPL staff answering questions and being very enthusiastic about the science (something which is easy to do because it’s such an excellent topic!). I’ve made a video for you that is coming up at the end of this post. (Click on stills for larger views.)
There was the usual huge emphasis on planetary exploration with rovers and robots and so forth – this seems to capture the imagination of everyone, so why not? – but I was more than a little surprised to find virtually no showing for the Planck mission. There was one poster somewhere, but no booth, no model, no description of the truly amazing science that it will do in unlocking more about the origins of the entire [...] Click to continue reading this post