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Flip Tanedo | USLHC | USA

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Thanksgiving Feynman Turkey

Thursday, November 26th, 2009

Yesterday I was tidying up my office and I found something cute on my chalkboard:

DSC05531

Do you see it? Amidst the remnants of a discussion with one of my colleagues, it appears that I now have a Feynman diagram of a Thanksgiving turkey:

DSC05529

Actually, I’m not sure if it’s a turkey, but it’s certainly some kind of poultry. The diagram was drawn by another theory grad student, Yuhsin Tsai. The irony is that we were discussing whether this process should be considered to be a penguin diagram. There are also diagrams called seagull diagrams. I guess physicists have a thing for birds in their diagrams. (By the way, this isn’t the first time I’ve found funny-looking diagrams.)

Hopefully in the near future I’ll be able to write up a few posts explaining how Feynman diagrams work. In the meanwhile, happy Thanksgiving everyone!

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Supporting science at home and abroad

Wednesday, November 25th, 2009

This Thanksgiving particle physicists have a lot to be thankful for, not the least of which have been the exciting progress with collisions at the LHC.

Happy ATLAS Scientists

Happy ATLAS Scientists, Image from the ATLAS press release.

While images of happy LHC-ers made a big splash in the media, somewhat understated in the news was President Obama’s reaffirmation of his commitment to science and science education through the a new “Educate to Innovate” campaign whose goal is to make American science and mathematics education second to none. Here’s the video of the announcement (and the transcript):

[youtube 33_nZaOUWYw]

If I may interject some personal opinion, a concerted effort to elevate “STEM” (“science, technology, engineering, and math”) education in the US is as important (if not more so) to the sustained well-being of American science as the LHC. The president also made the key point that this is important not just for the sake of science itself, but also for the country as a whole:

The key to meeting these challenges — to improving our health and well-being, to harnessing clean energy, to protecting our security, and succeeding in the global economy — will be reaffirming and strengthening America’s role as the world’s engine of scientific discovery and technological innovation.  And that leadership tomorrow depends on how we educate our students today, especially in those fields that hold the promise of producing future innovations and innovators.  And that’s why education in math and science is so important.

The Educate to Innovate Campaign draws from the private and public sectors to find ways to promote science to kids. As someone who grew up watching Bill Nye the Science Guy, I was very pleased to see that many of these plans involve tying in science programming on television shows. Further, it was good to hear the president reaffirm the goal that we need to transform the culture of education in this country. He remarked that during his recent trip to Asia, he was impressed by the “hunger for knowledge” and “insistence on excellence” that formed the foundation of each students’ education.

Speaking of Asia, I would be remiss if I didn’t share another understated physics news item from this past week: the Institute for Physics and Mathematics of the Universe (IPMU) is in danger of funding cuts from the newly elected Japanese government. For those that are not familiar, the IPMU was recently established to be a high-profile international center for research on the interface of physics and mathematics. It has great potential to act as a focus for theoretical physics in Japan that can connect physicists and mathematicians from all over the world. As reported by Sean at Cosmic Variance, funding cuts are looming ominously for IPMU and the Japanese Ministry of Education and Science is looking for input from scientists around the world. More information is available in an IPMU press release.

Earlier this year the Science and Technology Facilities Council of the United Kingdom provided a renewed funding grant to the Institute for Particle Physics Phenomenology (IPPP) at Durham University, where I was fortunate to have been able to spend a year as a student. Hopefully IPMU will also be able to continue onwards even during tight economic times.

I know this is the US LHC blog, but the fact of the matter is that particle physics is very much an international effort. CERN itself was, in some sense, a precursor to the European Union and today scientists from around the world contribute to the forefront of particle physics research. Researchers at American universities hail from all over the world and academia flourishes in this environment of diverse backgrounds. And you know what? That’s part of what makes this line of work so much fun. Happy Thanksgiving everyone!

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The EPR paradox and B-mesons

Wednesday, November 18th, 2009

Several weeks ago it was brought to my attention that some of our readers via Facebook wanted to hear my take on the EPR paradox, so I figured I ought to get around to saying something. It turns out, further, that this is an appropriate thing to discuss since it has some applications to particle physics in how we are able to decipher what goes on inside our particle colliders.

epr

So let’s start with the basic idea. The Einstein-Podolsky-Rosen paradox is a thought experiment that was originally proposed to highlight the inadequacies of quantum mechanics. What ended up happening was that the phenomenon of quantum entanglement became the foundation for real-life applications of quantum mechanics, e.g. quantum cryptography.

A fair warning: I’m not going to give a proper, formal treatment of the paradox nor will I further discuss the original motivation. Instead, I’ll give a heuristic description and jump into an application to B mesons.

First, let’s start off by reminding ourselves that we assume that information cannot travel faster than the speed of light. This related to the fundamental principle of causality: if things could travel faster than the speed of light, then in some reference frame, it’s moving backwards in time.

Alright, now onto the EPR paradox. The idea is this:

  • You have a particle (the blue guy in the picture above) that decays into two other particles, A and B.
  • There is a conservation law that constraints some property of A and B relative to one another. For example, conservation of electric charge says that if the original particle has no charge and A has charge +1 (e.g. positron), then B must have charge -1 (e.g. electron).
  • Quantum uncertainty tells us that until we make an observation, the state of the particle is unknown. For example, we don’t know if A is an electron or a positron until we actually check. Further, quantum mechanics tells us that the particles must actually be in some “superposition” of states.
  • If, after A and B travel a long distance in this “superposition,” someone checks particle A, then the conservation law determines the state of particle B. In our example, if someone in Fermilab observes that A is an electron, then we can instantly deduce that someone at CERN (which is where B happens to be zooming past at the moment) will observe B to be a positron.
  • But the Fermilab scientist could just as likely have observed A to be the positron and thus B would be an electron; until the person at Fermilab actually measured A, it was in some intermediate state. Thus the moment A is measured, it instantly fixes what B must be.
  • To be clear: before A is measured, B really is a mixture of states and can be observed to be anything. After A is measured, B can only be observed to be the correct state to satisfy the conservation law.
  • So here’s the paradox: how the heck did B know how to behave if it’s so far away from A? (Instead of CERN, B could have been at a distant galaxy when A was measured.) It appears that information travels from A at the point of measurement at a speed faster than light to B. (In fact, at an infinite velocity.)  Einstein called this “spooky action at a distance.”

First, let me say that the effect is real. Indeed, the particles A and B are said to be entangled. (This entangled state is actually rather fragile, since you can’t let the particles interact with any other matter that would allow them to disentangle.) Second, this is not really a paradox. The point is that there is no actual “information” being transmitted since there’s no way to impose a state on A, the initial observation is always random. You can try to think up clever ways around this, but they always fail. There is no paradox. Particles can be entangled and can have weird correlations across long distances, but that’s just a prediction of quantum mechanics that is fully consistent with causality. (more…)

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Symmetry in Physcs, Pt. 4

Friday, November 6th, 2009

Alright, it’s time to start wrapping things up a bit. I’ve been going on for some time now about how symmetries play a central role in our understanding of physics. Here’s a lightning review:

  • In part 1, we thought about how the symmetries of space(time) restrict the form of our theories.
  • In part 2, we saw how antimatter comes from a discrete symmetry of spacetime (Charge-Parity)
  • In part 3, we introduced internal symmetries that have nothing to do with spacetime, but that lead to a replication in the number of particles. This “explains,” for example, why there are three copies of the electron.

Here’s a summary in graphical form:

symmetrynotes

If you wanted a nice summary in the format of a nice TED talk, (I know Mike A. is a fan), then I recommend Marcus du Sautoy’s talk earlier this year:

[youtube 415VX3QX4cU]

Now I’d like to go over some more formal results with far-reaching effects in physics, i.e. some “advanced topics.” These are usually things which are derived rigorously in successively more advanced physics courses, but here we’ll just give heuristic explanations that highlight the physical relevance. Though the topics are somewhat high brow in their nature, they address very simple questions that I think should be very accessible.

Where do conservation laws come from?

Emmy Noether was a prominent physicist and mathematician in the early 1900s when those fields were dramatically dominated by men. Today every undergraduate physics student learns Noether’s Theorem as part of analytic mechanics. The theorem can be summarized as this:

For every continuous symmetry, there is a conserved charge.

What does this mean? The first part refers to a continuous symmetry. These are like the spacetime symmetries that we discussed in part 1: rotations, translations and their relativistic generalizations (Lorentz transformations). The word continuous means that you can perform the symmetry by any arbitrary amount, as opposed to discrete symmetries (such as those in part 2).

The second part says that if you have a continuous symmetry, then you have a conserved quantity which we call charge. This is something you’re already familiar with: we know that electrons carry electric charge and that this charge is conserved: it is neither created nor destroyed, and every interaction between particles must have the same charge going out as it did going in. For example, if ten physicists entered a bar and only nine left by closing time, then the number of physicists is not conserved. (Maybe one of them had a change of heart and became a mathematician.)

This is really neat, because now we can explain the existence of conserved charges in terms of the existence of a symmetry in nature. Here are a few well known examples from non-relativistic classical physics:

  • The laws of physics are the same over time (time translation symmetry). This implies the existence of a conserved quantity that doesn’t change with time. We call this energy. i.e. the energy of a system of constant in time.
  • The laws of physics are the same at every point in space (space translation symmetry). This implies the existence of a conserved quantity that doesn’t change with space. We call this momentum.
  • The laws of physic are the same no matter how we change the direction of or coordinates, this leads to the conservation of angular momentum.

(I once convinced myself that if you think about this for a while, it makes sense ‘intuitively’ without any mathematics. However,  this depends on what you mean by ‘intuitive.’) This is now really useful because physicists building theories can generate conserved charges just by imposing that the theory obeys some symmetry. (more…)

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LHC vs. baguette bombardier bird

Friday, November 6th, 2009

I don’t really know what to think of this, but apparently there’s a news story going around that a piece of bread found its way onto an electrical connection in one of the LHC’s above-ground buildings leading to a failure of a cooling unit. The buildings are protected by high security fences so the story is that it was probably dropped by a bird. The upside is that they don’t expect this to make a dent in the current LHC start up schedule.

[As someone who has yet to visit CERN, I have to wonder how such a think could actually occur. Is it so easy for a bird to get into one of these buildings? Should someone make silly references to Spiderman 3?]

I did my best to try to track this story down to a reputable source, hopefully the US/LHC bloggers currently at CERN might be able to comment? (Mike? Seth? See any birds lately? 🙂 )

I first saw this story reported on the social news site Reddit, where it gathered plenty of rather funny comments, e.g. “So… what IS the average flight velocity of a baguette-laden swallow?” Reddit linked to a report in Popular Science, which in turn linked to The Register which claimed as its source a press briefing by Mike Lamont, the LHC Machine Coordinator.  At the time of this writing, most of the Google News references to this incident point to the article at The Register, though some spiced it up with some of their own illustrations, e.g.  the Crunch Gear blog.

Knowing The Register‘s history of making tongue-in-cheek statements, I dug a little bit deeper and I was finally able to track down a story from United Press International under their “Odd News” section with some details. This story finally references The Times of London, which has a nice write up by Nico Hines. It sounds like the fail-safes kicked in properly and it is expected that the LHC can stick to its scheduled restart (keeping in mind that this schedule gets regular minor updates).

So, on the one hand, it sounds like the story is legit. On the other hand, I maybe I should feel guilty that by posting this since I’m playing the role of “yet another blogger posting on a breaking news item and contributing nothing new other than a few snarky comments.” (What does this say about the state of blog-based journalism these days?)

[Hey, at least I provided a paper trail of references like an honest academic.]

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The LHC on the Colbert Report

Monday, November 2nd, 2009

For those that missed it, Stephen Colbert did a small bit on the LHC on his show The Colbert Report on Comedy Central. (I’m not sure if the US/LHC bloggers at CERN can watch the clips online, but I suppose they’re all too busy getting ready for collisions …)

In the first clip Colbert pokes fun at CERN while providing a summary of it’s recent troubles. He then goes on to mention a recent paper by H. B. Nielsen and M. Ninomiya that has garnered some popular press and made some waves on the blogosphere, including a post by Regina. The paper proposes a rather crazy hypothesis that there could be some kind of “backwards causation” effect that prevents physicists from directly observing the Higgs. I don’t want to spend too much time on this, but there are a few reasons this is a somewhat silly proposal from otherwise very respectable physicist:

  1. Such a proposal has little (if any) experimental or theoretical motivation from current knowledge. The authors propose that recent problems with getting the LHC up and running might be due to such a backward causation. A more plausible answer is that the LHC will attain the highest energies ever created by humans: it’s natural to expect a few hiccups as we do something that has never been done in the history of our species.
  2. Causality is a very fundamental tenet in physics and one would need a really good reason to seriously consider why the Higgs should disobey causality on macroscopic scales while every other quantum field is well behaved.
  3. Finally, as a somewhat technical side note: the Standard Model actually has four Higgs bosons. Three of them “live” in the W and Z bosons, essentially giving those particles mass. We call these the Goldstone bosons that are “eaten” by the W and Z to give them longitudinal polarizations. We have directly observed these longitudinal polarizations and hence, in a sense, we have already discovered 3/4ths of the Higgs boson! [This is a somewhat technical point that assumes the Standard Model accurately describes electroweak symmetry breaking.]

To put this proposal into some context, the claim that nature conspires to prevent us from discovering Higgs might as well say that some sentient natural intelligence (“Skynet”) sends a robot (let’s call it a “Terminator”) back in time to futz with the LHC to prevent physicists from discovering electroweak symmetry breaking. It then goes on to do the same thing three more times in increasingly non-sensical plots that not even Batman can save. [Do I watch too many movies?]

Anyway, the best summary of how most physicists feel about this paper can be found in Alessandro Strumia’s tongue-in-cheek quote on Tommaso Dorigo’s blog,

“Brezhnev, Andropov, Chernencko: these three premiers of Soviet Union unexpectedly died around 1984, such that Gorbachev could lead the process that ended with the fall of Soviet Union, such that the US congress stopped funding the SSC, such that the Higgs was not discovered. According to the new theory of backwards causation, you should be proud of having destroyed communism.”

I should also mention that Colbert goes on to interview collider theorist Brian Cox about physics and science. I liked that they chatted a but about E=mc^2, even if it was so that Cox could promote his new popular physics book.

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Berkeley’s Nobel Prize Parking

Saturday, October 17th, 2009

There’s a great story on NPR’s Morning Edition about UC Berkeley’s “Nobel Laureate” parking spots. From the article:

Winning a Nobel Prize is difficult enough. But on the campus of the University of California, Berkeley, there is something that might be even more difficult to get: a parking space on the central campus.

That’s why Berkeley has made it a practice to offer its Nobel laureates an extra-special perk: a free lifetime permit to park in the highly coveted spaces near the central campus. The spots would normally cost about $1,500 a year.

Five of the ‘NL’ spots are by the Berkeley Physics department and the article mentions ’06 physics laureate and Cal cosmologist George Smoot. I spent a summer in Berkeley in 2006 and dug up the following photo from the time:

BerkeleyNL

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Symmetry in Physics, Pt. 3: Internal Symmetries

Monday, October 12th, 2009

Now time for another installment of “symmetry in physics.” For those of you tuning in late (or who have forgotten what we’ve been discussing), we started out in part 1 with a very general discussion of the symmetries of spacetime and how this constrains the form of our theories. Next, in part 2 we looked at discrete symmetries and how they relate the notion of antimatter to charge and parity conjugation. We’ll be using some of the jargon of part 2, so make sure you brush up and remember what “CP” means. Now we’d like to address another mystery of the Standard Model: why is there so much repetition?

Family Symmetry

Let’s review the matter content of the Standard Model:

standardmodelmatter

The top two rows are quarks, the bottom two are leptons (charged leptons and neutrinos). Each row has a different electric charge. The top row has charge +2/3, the 2nd row has charge -1/3, the third row has charge -1, and the last has charge 0. As discussed in part 2, there are also the corresponding anti-particles with opposite charges [note 1]. Just about all of the matter that you’re used to is made up of only the first column. All atoms and everything they’re made of are more-or-less completely composed of up and down quarks and electrons (the neutrinos haven’t done much since early in the universe).

The replication of the structure of the first column is known as family symmetry. For each particle in the first column, there are two other particles with nearly the same properties. In fact, they would have exactly the same properties, except that they are sensitive to the Higgs field in different ways so that the copies end up having heavier masses. Technically the Higgs discriminates between different generations and breaks this symmetry, but we are still left with the question: why are there two other families of matter?

(more…)

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Symmetry in Physics, Pt. 2: Discrete Symmetries and Antimatter

Monday, September 28th, 2009

And now another installment of “Symmetry in Physics.” Recall that in part 1 we introduced the idea of symmetry and mentioned the symmetries of spacetime, such as rotations or translations. These symmetries are all ‘continuous,’ in the sense that you can rotate/translate by any arbitrary amount. Now we’ll introduce some of the discrete symmetries of spacetime, meaning that the symmetry only acts by a certain amount. In particular, we’ll focus on symmetries where one flips the sign (‘swaps the polarity’) of an object that can take two values. It turns out that this will be intimately linked to our notion of antimatter.

The spacetime symmetries we discussed in the previous post can be expanded to include three discrete symmetries: parity, charge conjugation, and time-reversal. It turns out (rather surprisingly) that physics chooses not to obey these symmetries, and this act of rebellion allowed the universe to develop interesting things like galaxies and life.

Parity

Parity is the symmetry where we reverse all of our space directions. For example, if we draw a coordinate system (x,y,z) in space, a parity transformation gives us a new coordinate system (x’,y’,z’) drawn below.paritytransform1

What’s the difference between these two coordinates? The first coordinate system obeys the ‘right hand rule.’ If you point your right hand in the x direction and curl your fingers towards the y direction, then your thumb will point in the z direction. The parity-transformed coordinate system, on the other hand, does not obey this property. It is, in fact, a left handed coordinate system. Thus a parity transform essentially swaps left and right.

Does this remind you of anything? One of my favorite puzzles as a child was the question of why a mirror reverses left and right but not up and down. The answer is that the mirror enacts a parity transformation. It reverses the forward-backward direction while maintaining the other two axes. For homework you can convince yourself that this is equivalent to our definition of ‘parity’ above. (For more discussion see this Richard Feynman video clip.)

Parity is a useful quantity when describing spinning particles: the parity transform of a particle spinning in one direction is the particle spinning in the opposite direction.

spin1

We might believe that nature obeys parity symmetry, but we’ll see that this is actually not true. Biologists might have already guessed this since the amino acids which make up proteins in cells are all left-handed.

(In fact, when the eminent theorist Wolfgang Pauli heard that Chien-Shiung Wu constructing an experiment to test whether the weak force obeys parity symmetry he scoffed that it was obvious that the answer had to be ‘yes.’ The entire community was shocked to find out that indeed, parity is not a good symmetry of nature!)

(more…)

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Symmetry in Physics, Part 1: Spacetime Symmetry

Sunday, September 20th, 2009

One of the reasons why physicists often wax poetic about the beauty of physics is that so much of the field has based on symmetry, and humans find symmetry beautiful. But it is partly because people have an intuitive sense of what symmetry is that there aren’t many attempts to really explain the central role that it has played in theoretical physics.

Over the next few posts I’d like to go over some of the ways symmetry shapes how we think about physics. I’ll be necessarily heuristic, but I hope to give an honest sense of what physicists mean by symmetry and why it’s been the primary guiding principle in theoretical particle physics for the past 50 years.

Already at an intuitive level it’s probably not hard to believe that symmetries constrain and simplify things. If I tell you to draw a blob, you might draw any unexpected shape. On the other hand, if I told you to draw a blob with rotational symmetry, you are constrained to draw circles:

blob

Similarly, if I tell you that a theory of gravity is spherically symmetric, then you know that the gravitational potential only depends only on the radial distance from the source and is independent of angular information. If you calculate the field at some distance r, then you know that the potential is the same for all other points of the same distance r. [A more advanced discussion: because the force is proportional to the change in, or gradient of, the potential, you know that the force can only be in the radial direction.]

The mathematical language of symmetry is group theory. A discussion of group theory would take us too far astray, but I heartily recommend that young physicists make a point to learn group theory when they have a chance. (The earlier the better, especially since it doesn’t require much fancy math background.)

Actually, this is a lesson in having a broad background. Murray Gell-Mann, one of the heroes of American physics and co-developer of the quark model, actually re-invented group theory for himself when trying to describe the spectrum of hadrons discovered in the 1960s. He called it the eightfold way (he was very good at naming things) and only later realized that there already existed a mathematical language for his method.

Symmetries of Spacetime

Let’s talk about the symmetries of space and time.

  • Translational symmetry: the laws of physics are the same everywhere and at all times
  • Rotational symmetry: the laws of physics don’t depend on how we orient our coordinates
  • Boost symmetry: the laws of physics don’t depend on what inertial frame we’re in. In other words, the laws of physics for someone standing still are the same as for those who are moving at a constant velocity.

Note that I didn’t say that physical observables obey these symmetries. I can survive on the surface of the Earth but not at the sun’s core. Gravity tells me that “up” and “down” are two very different directions. And if I stand still I’m fine but if I’m moving at a constant velocity I’ll eventually run into something like a building or a tree.  Our universe clearly doesn’t obey the symmetries listed above. That’s fine.

The point is that the laws of physics do obey these symmetries. You can think of “the laws of physics” as a set of fundamental laws governing the dynamics of nature. Even though nature itself isn’t symmetric (that’s a whole different story), its rules are. Classically, the law “F=ma” holds no matter where you are, when you measured it, and how you oriented yourself. Sure, the numbers might change depending on how you’re oriented, but the relation between the numbers remains true.

What does this all mean for particle physics? A particle is a particle is a particle! Suppose I had a particle. Let me draw it as a penguin (don’t ask me why). The symmetries of spacetime tell me that if I put the penguin somewhere else, or if I rotate it, or if I give it a nudge so that it’s moving at a constant velocity… then the penguin is still the same penguin that I started with. I don’t need a new framework to describe “penguin in motion rotated by 30 degrees” than I would to describe the original penguin.

penguins

This statement is so ridiculously trivial that it probably ends up sounding more complicated than it is. Obviously we know that these are all versions of the same penguin, I’ve just moved them around in ways allowed by the symmetries of space. But ‘technically’ they are all different penguins: one is tilted one way, another one is moving… if we were very, very naive we’d think “these are different things — why are you calling them the same?” The answer is that spacetime is symmetric.

[Let me say this in a different way: the symmetries of spacetime are so deeply ingrained in us as infants that we don’t even think about a penguin as being different from a penguin tilted on its side. Technically, though, they are different: one is tilted! So without any prior input, why should physics treat the tilted penguin the same way as it treats the original penguin?]

Okay… still with me? Good. Now go back and re-read the last three paragraphs with the word “penguin” replaced by “particle.” The take-home message is this: symmetries allow us to treat [naively] different objects as the same object.

Now we’re starting to get somewhere.

Before we go any further, let me stop and drop some fancy words for more advanced readers. For others, just think of these as words that you can use to impress your friends. The “full” meaning of these words is based on group theory, but we’ll try to give a sense of what they mean. The symmetries of spacetime are collectively called the Poincare symmetry. Particles are irreducible representations of the Poincare group, meaning they change in well-defined ways under Poincare symmetry. The subset of Poincare symmetry dealing only with rotations and change of inertial frames is called Lorentz symmetry. It is formulated to automatically obey the symmetries of Einstein’s special relativity (e.g. effects like time dilation and length contraction are already “built-in”). Quantum fields, which “give rise” to particles, are irreducible representations of the Lorentz group. [If you feel a little lost, don’t worry; this paragraph is just an aside.]

Epilogue, Part 1

Unfortunately, this was probably enough of a bite-sized (blog-sized) discussion for today. In terms of learning ‘sexy new truths about the universe,’ we haven’t gotten very far. What we have done, though, is lay down some foundational principles about the role of symmetry in physics. In summary,

  • Symmetries simplify things by constraining their form (e.g. a rotationally symmetric blob must be a circle)
  • Symmetries allow us to identify “naively different” objects as really being part of the same object

In our next post we’ll follow these symmetry principles and consider the discrete symmetries of spacetime. Without spoiling too much ahead of time, we’ll see that we are naturally led to the idea of antiparticles.

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