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Sep. 9th, 2008

I recently realized that the description of my research I have on my work webpage (http://physics.ucsc.edu/~jeff) probably gives the average person zero information... and gives people in other branches of physics only a very slight idea of what I'm working on. This is unfortunate, because I'm very passionate about what I do, and would love to share more of it with people who are interested. To remedy this problem, I've started writing a more readable description of what I do:


Unfortunately, I'm realizing there's a heck of a lot of background to cover before I get to my specific area of research. But it's a good exercise for me, to try to summarize in as generally-accessible terms as possible, what it is that high energy theorists and phenomenologists actually do. The more accessible you make something, the longer it takes to explain it... so it's getting pretty long (and I've only gotten to the most general stuff so far). But I hope that it's not only valuable for me to go through this exercise, but also for other people to learn more about what goes on in my field and what it's all about.

I was going to wait to post a link to this until I was done writing it, but I figure if people start giving me feedback now, then perhaps I can improve the flow/direction/level of the whole thing, before I end up too much further along. If you do read it, and you're a non-physicist, please let me know if there is anything which is unclear or highly confusing (I'm sure there is bound to be some stuff). If you read it, and you're a physicist, let me know if there's anything you think I've said that is incorrect or misleading.


( 11 comments — Leave a comment )
Sep. 10th, 2008 07:29 am (UTC)
Even though you didn't ask for general thumbs-up gestures from non-physicists, I'm happy to report that's all I have to offer. To me, with an (old) background in mathematics and a (more recent) background in philosophy, this feels like the missing piece in my understanding of your science. Thank you, and please keep going. :-)
Sep. 10th, 2008 04:42 pm (UTC)
Thanks! I'm really glad you got something out of it. I will definitely keep going.
Sep. 10th, 2008 08:16 am (UTC)
It might be best to structure this differently, so that the page as finally written up just has your research area on it, but beforehand has hyperlinks to pages for each of these other topics. This is definitely already getting quite long for a description of your research, and as far as I can tell the only thing it's mentioned about you so far is that you work on the interface of theory and phenomenology, presumably in high energy physics.

The description of high energy physics seems confusing to me. Naively, I think that because of E=mc2, high mass should at least correlate with high energy. Therefore, proteins should have higher energy than molecules, which are higher energy than atoms. But are you really talking about the energy required to isolate the things at this phase rather than the energy of the things themselves? I guess I don't know if the reaction energies of typical chemical reactions are higher or lower than the energies of typical protein interactions. But when you boil stuff at a high enough temperature, I guess the proteins break down even though the smaller molecules generally don't - is that the sense in which the smaller molecules are "higher energy" than the proteins? Also, is high energy physics more fundamental because it deals with higher energies, or is it just because it deals with the smaller things that low energy objects are composed of?

The description of experiment, theory, and phenomenology was quite helpful - I've known of this tripartite distinction, but didn't really have any sense of the distinction between phenomenology and theory until now. Does phenomenology exist solely in high energy physics though?

If you want to shorten things, I think you might cut the paragraph about symmetry groups from the quantum field theory section - it didn't really seem to clarify anything for me. Though I imagine this might be important for things you want to talk about later, in which case you'd probably need to go into more detail about what the symmetry groups mean - though that's something that would probably only help for someone with at least some mathematical background. (I've never had much sense of what the symmetry groups are supposed to mean, but I haven't read that much physics.)
Sep. 10th, 2008 05:00 pm (UTC)
Thanks or the constructive criticism.

Regarding hyperlinks... yeah, I've considered that and may decide to do that. For now, I feel like it may be able to stand alone all as one page... but I am sensing it will end up being too long for that. I'll make a decision on that after it gets a bit longer.

Regarding the ordering of different energy scales... that's a really good point, I should clarify that more because it does look confusing. In fact, I remember being confused about this very issue for a long time as I was learning physics... I never felt like professors explained it well, they always glossed over exactly what kind of energy they were talking about. Unfortunately, now I'm realizing why they glossed over it. It's a really deep and far-reaching concept but to really understand or explain it rigorously requires a lot of background and can get quite technical.

Regarding phenomenology, I think only in high energy physics is it considered a somewhat well-defined sub field. However, you could also use the word "phenomenological" or "phenomenological model" in other branches of physics (or science)... for instance, when referring to some empirical formula someone comes up with when they don't yet have any understanding of where the formula comes from or why it holds true.

I think this sense of the word comes from Kant's distinction between noumena and phenomena... if you're only interested in describing what happens, and not why it happens or what things are ultimately made of, then you're doing phenomenology. Although this makes it sound a bit like instrumentalism, which is different... so I think the connection with that original distinction isn't exact any more.
Sep. 11th, 2008 01:35 am (UTC)
The Kant usage definitely makes sense in this context. Though it might also just be a direct interpretation of the word "phenomenon", which is just "something that appears" if you look at it literally in Greek. So phenomenology is the study of appearances, which in physics means building models to explain particular experimental results (rather than deep fundamental theories that are supposed to explain much more), and in philosophy it refers to Husserl's program of understanding the mind through studying the very notions of perception and what it means to think about something (even if that something may not really exist objectively).
Sep. 10th, 2008 05:39 pm (UTC)
To try to answer your question about different energy scales...

is high energy physics more fundamental because it deals with higher energies, or is it just because it deals with the smaller things that low energy objects are composed of?

In quantum field theory, higher energy scales (in a particular sense) and smaller distance scales are always the same, so you might say it's a meaningless question. However, in string theory that is no longer true... once you get to the Planck energy scale, you can keep going to higher energy but the distance scale starts getting *larger* rather than smaller (because you've already reached the smallest distance there is). But higher energy scales are always considered more fundamental whereas smaller distances are only more fundamental if the energies you're dealing with are less than the Planck energy. I don't fully understand this myself yet, but there are a number of cases I know of where it sort of makes sense.

Naively, I think that because of E=mc2, high mass should at least correlate with high energy.

Yeah. So roughly speaking, the energy I'm talking about here is the typical energy involved in interactions between parts of a system. Unless you are doing nuclear physics, you never actually tap the mass energy stored in something. That's why nuclear physics is considered higher energy physics than, for instance, chemistry. If all that's going on is chemical reactions, you never have to think about or even know that there is energy stored in the nucleus or the mass of the atoms. All you need to know is that there are different atoms interacting, and you can treat them as indivisible. The energy scale involved in chemistry would be the "binding energy" for different kinds of molecules.

A typical nuclear binding energy (the energy it takes to rip apart the nucleus of an atom and get it to change into something else) is on the order of 1-10MeV. If you just want to excite a nucleus into a higher energy state, then I think you just need energies around 1-100keV or so. An atomic binding energy (the energy it takes to ionize an atom) is somewhere around 13eV-100eV I think (not sure about the upper bound). To just excite an atom into a higher energy state (or to get it to fluoresce for instance) is more like around 1eV. (eV = electron volt, keV = kilo-electrovolt, MeV = Mega-electronvolt.)

I don't know what molecular binding energies are, but they should be even smaller... probably just a fraction of an eV, but I think they are usually measured in Calories/mol or something instead of eV/molecule. I don't know what the typical interaction energies are for protein folding either, but I wrote that assuming they are smaller still... I should check on that to make sure.

Your example of needing to boil proteins to a high enough temperature in order to break them down into smaller molecules is one manifestation of the difference in energy scales I'm talking about. But there are a lot of other ways of looking at it too.
Sep. 10th, 2008 05:46 pm (UTC)
Oh, yeah...

So the energy scales involved in "high energy physics" are typically all above 1MeV. The QCD scale for instance, is 100MeV, which is where the strong force gets interesting. The electroweak scale is around 100GeV (100 Giga-electronvolts) which is the focus of most current experiments... both at the Tevatron in Chicago, and at LHC which is scheduled to turn on TODAY!

LHC will produce two beams of protons circulating in opposite directions, each beam having an energy of 7TeV (7 trillion electron volts, over a trillion times what it takes to do atomic physics), for a total center-of-mass energy of 14TeV. This is why it's such a big deal... nobody has ever had that much energy to try to put into a collision before.
Sep. 10th, 2008 06:51 pm (UTC)
You know... I've been thinking about this more, and I do see a big problem with the hierarchy of energy scales as I'm describing it.

I should definitely rewrite that part, and add a lot of explanation.

The problem with the definition I gave ("interaction energy of the parts of a system") is that it does work oppositely if you're talking about truly macroscopic objects. The kinetic energy of a baseball, for instance, is larger than the kinetic energy of any of the particles their colliding at LHC. And the potential energy of the earth's gravitational field (which in a sense is a "binding energy" for humans, and the moon, and such) is a lot larger than the binding energy of an atom.

For some reason, people in my field usually tend to ignore this when explaining how energy and distance are inverses of each other. But I'd like to be able to explain it without ignoring it... which means I should think about it some more.

The reason energies in macroscopic systems seems to scale with size, is simply because you're dealing with a huge collection of things so you have to add up all of their energies. So perhaps this whole correspondence should only be applied when you're thinking about systems with a relatively small number of parts. Although that doesn't quite work either, because the whole point of my definition was to regard the low energy systems as interactions between things you're considering individual parts, even though they are composed of other structure you're not worrying about. Maybe this could be fixed by instead talking about energy densities? I'm not sure if that works fully either.

In the end, it may be that all I can say is there is a huge range of distance scales for which the typical interaction energies are inversely proportional... but that if you go greater than the Planck scale it breaks down (at which point higher energy means more fundamental) or if you go above the molecular scale in terms of distances (at which point smaller distances means more fundamental?) That would fit with your intuition that it's more the distance-scale that matters... if that's what matters in everyday life, but in most of the microscopic world it's the energy that matters more. I clearly need to think about this more before cleaning it up. Thanks a lot for the input!
Sep. 10th, 2008 09:17 pm (UTC)
Does (energy involved in interactions) / (mass-energy of the system) measure the energy levels you're talking about?
Sep. 11th, 2008 03:55 am (UTC)
Qualitatively, that works pretty well. Quantitatively, there are various problems and exceptions with that definition as well. One problem is, it doesn't work for systems composed of all massless particles.

I'm starting to think it's probably best to just say that it's something that's only true for small quantum systems... once you get into the classical, macroscopic realm it just doesn't hold any more. But now I'm curious to try and figure out where the crossover point is... what size system do you need before bigger starts meaning higher energy interactions rather than lower energy? I should be able to figure this out when I get a chance.
Sep. 15th, 2008 08:54 pm (UTC)
Thanks for posting this, looking forward to further installments.

As a non-physicist I think your definition of phenomenology could be clearer and more direct (I understood it as ad-hoc connections to directly explain or guide experiments and experimental data).
( 11 comments — Leave a comment )


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