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Science discussion, continued from other thread


Kelly

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*applies ink ribbon with the words "read, but not understood"*

Carry on. :P

If that was at me, yep. I realized. :redface: I was never the best at that sort of thing.

Nah, it was this thread in general.

Really? Whoops again. Ill still leave it as standing that I misunderstood that the people who prefer more pure stuff thought they were better. Cos more pure stuff isn't better :P At least I hope that they didn't think that more pure stuff was automatically better.

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And yet maths cant do crap all on modeling ant behavior AFAIK?

Well, there is A first mathematical model of brood sorting by ants: Functional self-organization without swarm-intelligence, and A mathematical and experimental study of ant foraging trail dynamics.

There are even entire books on the subject, including Ant colony optimization and swarm intelligence: 4th international workshop (over 400 pages). Here is an excerpt:

ant1.jpg

ant2.jpg

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Coo. I had no idea those existed.

Anyway, equation 8 from the 2nd extract isn't correct as far as I know. Probably due to a lack of context, but they wont turn at any random step, they'll only turn when they meet another ant.

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So, the science of Zombies. There is a fungus which gets inside ants and controls them. Is the Zombpocalypse just around the corner?

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Biology is applied chemistry, chemistry is applied physics, and physics is applied mathematics.

Reviewing arguments for and against these claims would be interesting topic for an undergrad philosophy class.

An applicable general area of theory has come to be called 'complex systems theory'.

http://en.wikipedia.org/wiki/Complex_systems

Melanie Mitchell wrote a nice intro to the field:

http://www.amazon.com/Complexity-Guided-Tour-Melanie-Mitchell/dp/0195124413/ref=sr_1_1?ie=UTF8&s=books&qid=1294702215&sr=1-1

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A Long Time Ago

About hard mathematics, I am going up against Jackson's Electrodynamicsthis semester, gulp.

Rivan, you have an interesting plan.

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A Long Time Ago

I am kind of feeling like a dumb physicist here. String theory makes my head hurt, though it is interesting. I guess I will go back to my Navier-Stokes equation and turbulence.

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About hard mathematics, I am going up against Jackson's Electrodynamicsthis semester, gulp.

Rivan, you have an interesting plan.

There's no hard maths in Jackson. Nothing really beyond partial differential equations and vector calculus. Oh and a bit of tensors. (Only in Minkowski spacetime though - if I remember correctly there's nothing about electrodynamics in curved spacetime.)

Not that it's an easy book. It's quite detailed, comprehensive and technical. But there are no really difficult mathematical concepts in it.

(If you have any questions about any of the topics in the book feel free to ask in the science thread btw!)

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A Long Time Ago

About hard mathematics, I am going up against Jackson's Electrodynamicsthis semester, gulp.

Rivan, you have an interesting plan.

There's no hard maths in Jackson. Nothing really beyond partial differential equations and vector calculus. Oh and a bit of tensors. (Only in Minkowski spacetime though - if I remember correctly there's nothing about electrodynamics in curved spacetime.)

Not that it's an easy book. It's quite detailed, comprehensive and technical. But there are no really difficult mathematical concepts in it.

(If you have any questions about any of the topics in the book feel free to ask in the science thread btw!)

I have just heard horror stories about the horrific geometries that Jackson likes to look at. While it is just PDE's and vector calculus, evil boundary conditions can make them very very difficult mathematically. Luckily, electrodynamics is linear which makes it a bit easier. Fluids are not so and generally have really evil boundary conditions (roughness of the surface really matters).

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About hard mathematics, I am going up against Jackson's Electrodynamicsthis semester, gulp.

That might go better in the new thread.

I took that course. Yes, scary. :ph34r:

So, if you are taking that course, you are a graduate physics student, yes? :cake:

[edit]: I see that it was easy as pi for Michael. But for mortals, eeps. I found it to be challenging.

Interestingly, my professor (Pinsky) was out for much of the semester. Some of the professors spend a bit of time in the polar regions for research.

You might have seen an episode of TBBT where a joke was pulled on Sheldon regarding a magnetic monopole while at the North Pole, but Professor Pinsky actually did think that he found one and published it in Phys Rev Letters (he later retracted it and goes on record as not believing in such entities).

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I have just heard horror stories about the horrific geometries that Jackson likes to look at. While it is just PDE's and vector calculus, evil boundary conditions can make them very very difficult mathematically. Luckily, electrodynamics is linear which makes it a bit easier. Fluids are not so and generally have really evil boundary conditions (roughness of the surface really matters).

Well I would say "messy" rather than "difficult".

Yeah, no doubt the mathematics involved in fluid dynamics is a lot deeper than anything you'll find in Jackson.

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I am kind of feeling like a dumb physicist here. String theory makes my head hurt, though it is interesting. I guess I will go back to my Navier-Stokes equation and turbulence.

I will go on record as stating that I am the dumb* physicist. Indeed, I am a moron. What you guys do makes me jealous (and, like the scarecrow, wish that I had a brain).

* And regarding the other meaning of dumb, due to an injury, I was unable to speak for a period of about three years. I can once again speak well enough to be understood if there is no ambient noise, but I am still a moron.

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I don't think anyone here's dumb. And I'm not a physicist at all.

Ooh, Kelly did a post on string theory. Maybe just one comment.

The strings are attached to the 3-D brane of the universe, and cannot leave the universe. All of your electrons and such that make up you, our earth, and galaxies are bound to this membrane. You can't leave. But there are certain particles that can, namely, gravitons. They are closed strings, and are not attached to the brane. So, they can leave. And the reason why gravity is so much weaker than the other three forces is that they are not attached, and give a greatly decreased net force.

This could mislead one to thinking that gravitation strength should be a 1/r^(D-2) law where D = 10 or 11 is the number of dimensions of spacetime. Because if there are D-1 space dimensions then the flux density of gravitons should scale inversely to the area of a sphere of radius r, which is proportional to r^(D-2).

(Incidentally D = 26 is for the bosonic string, which is not physically realistic due to the absence of fermions and the presence of a tachyon. The bosonic string is a toy model, useful as a first step towards understanding the superstring - though it re-appears in a very surprising fashion in the heterotic superstring.)

Whereas in fact we observe a 1/r^2 law, which is what you'd expect from 4-d spacetime (i.e. 3-d space).

The solution is that the extra dimensions do not look like the dimensions we see around us. They are probably what's known as compactified - finite (and very small) in extent. How to generate realistic 4-d physics from the 10-d superstring model - i.e. how to compactify down - is one of the biggest phenomenological topics in the field. This is where the Calabi-Yau manifolds come in. But.... that's getting ahead. Maybe more basic stuff tomorrow.

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A Long Time Ago

So, if you are taking that course, you are a graduate physics student, yes? :cake:

[edit]: I see that it was easy as pi for Michael. But for mortals, eeps. I found it to be challenging.

Yep, I am a first year physics graduate student having to do the balance between research and classes with my first round of quals coming up in two weeks (I think I can knock the quantum one off for sure).

Oh, I am a mere mortal, though many claim that I am actually a cyborg or a robot.

Interestingly, my professor (Pinsky) was out for much of the semester. Some of the professors spend a bit of time in the polar regions for research.

You might have seen an episode of TBBT where a joke was pulled on Sheldon regarding a magnetic monopole while at the North Pole, but Professor Pinsky actually did think that he found one and published it in Phys Rev Letters (he later retracted it and goes on record as not believing in such entities).

Neat that you actually had Pinsky for at least a little bit (while not doing research in the polar regions).

I don't know about the joke in TBBT theory since I do not watch the show and have a grudge against it. People keep saying I am like Sheldon due to my poor social skills (in addition to my extreme dedication to physics and my grey-aceness). I mean, I watch the show and I see that all the characters on it have worse social skills than me. It is because of these jokes that I don't like it, I have to admit.

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The strings are attached to the 3-D brane of the universe, and cannot leave the universe. All of your electrons and such that make up you, our earth, and galaxies are bound to this membrane. You can't leave. But there are certain particles that can, namely, gravitons. They are closed strings, and are not attached to the brane. So, they can leave. And the reason why gravity is so much weaker than the other three forces is that they are not attached, and give a greatly decreased net force.

This could mislead one to thinking that gravitation strength should be a 1/r^(D-2) law where D = 10 or 11 is the number of dimensions of spacetime. Because if there are D-1 space dimensions then the flux density of gravitons should scale inversely to the area of a sphere of radius r, which is proportional to r^(D-2).

(Incidentally D = 26 is for the bosonic string, which is not physically realistic due to the absence of fermions and the presence of a tachyon. The bosonic string is a toy model, useful as a first step towards understanding the superstring - though it re-appears in a very surprising fashion in the heterotic superstring.)

Whereas in fact we observe a 1/r^2 law, which is what you'd expect from 4-d spacetime (i.e. 3-d space).

The solution is that the extra dimensions do not look like the dimensions we see around us. They are probably what's known as compactified - finite (and very small) in extent. How to generate realistic 4-d physics from the 10-d superstring model - i.e. how to compactify down - is one of the biggest phenomenological topics in the field. This is where the Calabi-Yau manifolds come in. But.... that's getting ahead. Maybe more basic stuff tomorrow.

True. It is difficult to explain the inverse square law if there are extra, open dimensions. String Theorists generally state that they should be curled up into tiny Calabi–Yau manifolds. But I have also seen descriptions where the extra 11th dimension is part of the bulk, and branes are inside of that. And that gravitons can move between them. Here is a diagram:

brane_world.jpg

Then here is a blurb (for the other peeps following the thread) on the subject:

http://willowtrees.wordpress.com/2008/05/17/cern-extra-dimensions/

...

For physicists, an energy of 1 TeV was already a landmark. Both theory and experiment had established that a mixing of the electromagnetic and weak forces begins to take place a little below that energy level. Physicists have been troubled because unification of even three forces requires much higher energies. They refer to this puzzle as the hierarchy problem.

Scientists at Stanford University and ICTP used extra dimensions in their attempt to solve the hierarchy problem. They focused first on gravity and looked for a way to make it comparable in strength to the other forces at an energy of about 1 TeV.

They accomplished that feat by hypothesizing extra dimensions that affect only gravity and are as large as 1 mm. Only a yawning gap in the scientific record makes such extra dimensions feasible. While physicists have probed the other forces of nature down to nearly 10–19 m, they’ve made extensive measurements of gravity only down to about 1 centimeter

To describe extra dimensions that would affect gravity alone, the Stanford-Trieste researchers made use of entities known as branes. Those complex, membranous objects, which can have many spatial dimensions themselves, have become a central part of string theory. In some versions of the theory, the universe itself is a brane with three spatial dimensions—a 3-brane—moving through a higher-dimensional space-time.

String theory dictates that any extra dimensions outside a brane affect only gravity. In other words, just the force-carrying particles of gravity, called gravitons, could travel in the space-time beyond the brane, leaving the other forces confined to the brane. By contrast, extra dimensions associated with the brane influence all the forces.

Therefore, even if gravity boasts an intrinsic strength similar to that of the other three forces, because it diffuses throughout the external space-time, also called the bulk, its apparent strength in the 3-brane universe is much reduced.

Any extra dimensions affecting gravity would alter Isaac Newton’s inverse-square law, which holds that objects attract each other with a force inversely proportional to the square of the distance between them. The theorists calculated that one extra dimension in the bulk would have a scale of 100 million kilometers—about the distance from Earth to the sun. That option isn’t feasible because Earth’s orbit obeys the inverse-square law.

If there were two extra dimensions, however, each would have a scale of 0.1 to 1.0 mm—large enough to be detectable but small enough not to be ruled out by tests of the inverse-square law to date. With more extra dimensions, the length scale shrinks far below the millimeter range.

Combining both approaches, “you wind up with a very compelling picture,” says Dienes, a CERN team member, now at the University of Arizona in Tucson. “These two scenarios together lower all the fundamental high-energy scales of physics.”

Accelerator searches

Inspired by these proposals, experimenters are looking for signs of extra dimensions both at accelerators and in gravitational laboratories...

(it continues with descriptions of experiments)

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I NEED A SCIENCE OFFICER!

(*goes back to watching Star Trek*)

Theres Klingons on the starboard bow captain!

Whenever there is biology, there is chemistry. Biology is applied chemistry, chemistry is applied physics, and physics is applied mathematics.

Which is just another way of saying that Maths is used in Physics, which is used in Chemistry, which is used in Biology :P

Coo. I had no idea those existed.

Anyway, equation 8 from the 2nd extract isn't correct as far as I know. Probably due to a lack of context, but they wont turn at any random step, they'll only turn when they meet another ant.

Thinking about this, I wasn't quite accurate. If it senses a chemical that indicates something to them, such as food, then they will change direction, but this is quantifiable. Hence is it either will turn, or not. Probability suggests that it may turn for some random reason.

So, the science of Zombies. There is a fungus which gets inside ants and controls them. Is the Zombpocalypse just around the corner?

The Fish refers to this Scientific American article:

http://www.scientificamerican.com/article.cfm?id=fungus-makes-zombie-ants

Wow.

Mind controll is easy when its all based on simple chemical levels. An ant, to the best of my understanding, works on chemical levels. So if it meets another ant that is emmiting the chemical that it is hungry, the ant will think something like 'If I meet another ant that is hungry in the next 20 seconds, I am 20% more likly to go out looking for food'. What I assume that this fungus does is swamp the ant with chemical(s) that means look for these conditions, then grab on tight. Then it kills them.

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I'm not even a scientist so let's not talk about how much I understand. I am merely kind of dim.

Surely a chemist is a person who is a skilled pharmacist. I've heard both chemist and pharmacist go for the same title. I suppose that differs from where you are.

I shall accept that people who dwell in the land of chemistry are chemists. (And don't say everybody is just because we all rely on chemistry.)

Whenever there is biology, there is chemistry. Biology is applied chemistry, chemistry is applied physics, and physics is applied mathematics.

Mathematics is applied.....? Reality? Innate rationality?

You say genitals are pieces of biological tissue. This is true. Chemistry allows tissue to exist. It allows atoms to form compounds, to form molecules, to give rise to macromolecules and more specifically the biopolymers (lipids, carbohydrates, proteins, and nucleic acids) that are the building blocks of life - of biological tissue.

Okay this I understand.

You must excuse my low level of scientific knowledge but I always found the line between chemistry and biology so far apart. I never really felt they connected (except when somebody died and they said: 'THIS POISON KILLED HIM'). So let's use an average plant tissue cell. (I remember those better) You've got your cell wall. That cell wall is made up of little neutrons and the protons and the electrons and they all connect (trading elections/sharing elections, etc.) and build up a giant (or tiny) structure: that is the cell wall. What's the chemical structure of a cell wall? What's the chemical structure of an average skin cell?

Am I working too small or missing a big step?

Also - I always remember, when you draw atoms and their bonds you've got these little lines linking them together. That's not really how it is, is it? Are they all squished together or held together by invisible forces?

You also say that sex is a biological action. This is also true. Biological actions directly rely on chemistry. They rely on chemical reactions. Everything we think, everything we do, even our own LIFE, is dependent on these reactions. They are so fundamental it is impossible to summarize every which way they are involved in "biological actions". From being sentient beings, to being sexual, to performing all the physiological aspects of sex... Chemistry is behind it.

My Philosophy A Level says I don't care. :lol:

(I'm not being rude but that's how I spent my philosophy classes: rejecting everything. It worked.

And then think... Physics is behind every aspect of chemistry.

The world truly is a complex yet beautiful thing.

What is beauty?

Biology is applied chemistry, chemistry is applied physics, and physics is applied mathematics.

Reviewing arguments for and against these claims would be interesting topic for an undergrad philosophy class.

An applicable general area of theory has come to be called 'complex systems theory'.

http://en.wikipedia.org/wiki/Complex_systems

Melanie Mitchell wrote a nice intro to the field:

http://www.amazon.com/Complexity-Guided-Tour-Melanie-Mitchell/dp/0195124413/ref=sr_1_1?ie=UTF8&s=books&qid=1294702215&sr=1-1

I like, I like! I'll read up on this and pretend I understand.

I suppose I like science without being a scientist... philosophy of science, language... :P I like science fiction.

To Bobandirus: Darn Klingons. They are the Marmite of the Trek Verse I believe. Some people hate them for no reason but it is possible to learn their language completely. Whereas Vulcan has not been fully created. Just saying.

STAR TREKKIN' ACROSS THE UNIVERSE!

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You've got your cell wall. That cell wall is made up of little neutrons and the protons and the electrons and they all connect (trading elections/sharing elections, etc.) and build up a giant (or tiny) structure: that is the cell wall. What's the chemical structure of a cell wall? What's the chemical structure of an average skin cell?

Am I working too small or missing a big step?

Missing 2 steps. The electrons, protons, and neutrons make up atoms. These atoms make up compounds, which then join together in varying patterns that make up large chemicals that make up the cell wall. That was very simplified, so you want to know more detail?

To Bobandirus: Darn Klingons. They are the Marmite of the Trek Verse I believe. Some people hate them for no reason but it is possible to learn their language completely. Whereas Vulcan has not been fully created. Just saying.

STAR TREKKIN' ACROSS THE UNIVERSE!

On the star ship Enterprise, under captain Kirk!

I wish we knew if there was other life. How cool would it be if Vulcan actually existed :D

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GSW and Polchinski are the definitive string theory books but there are others that are less systematic but give a somewhat quicker guide to modern topics. Barton Zwiebach's String theory for undergraduates (or titled something very similar to this - can't be bothered to look it up) is a good one.

Materials downloadable here:

http://ocw.mit.edu/courses/physics/8-251-string-theory-for-undergraduates-spring-2007/download-course-materials/

And with my limited intellect, I might be able to slog through it. I see that it is from MIT. I live near Boston, and sometimes hang out at MIT. I should spend more time sneaking into lectures and such.

Also, regarding Polchinski, I should mention that he also has a shorter version of his book on his webpage: Joe's Little Book of String. Less comprehensive than the printed version (and actually I like the BRST stuff from the Big Book of String) but it gets to the point much faster.

*downloads* Thanks.

Another similar short work is here:

http://www.phys.uu.nl/~thooft/lectures/stringnotes.pdf

It is from Gerard 't Hooft at Utrecht University in the Netherlands.

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GSW and Polchinski are the definitive string theory books but there are others that are less systematic but give a somewhat quicker guide to modern topics. Barton Zwiebach's String theory for undergraduates (or titled something very similar to this - can't be bothered to look it up) is a good one.

Materials downloadable here:

http://ocw.mit.edu/courses/physics/8-251-string-theory-for-undergraduates-spring-2007/download-course-materials/

And with my limited intellect, I might be able to slog through it. I see that it is from MIT. I live near Boston, and sometimes hang out at MIT. I should spend more time sneaking into lectures and such.

Yup. I love the way MIT make their course materials available to the public.

Here is the book I was thinking of. It's based on the course Zwiebach gave to undergrads at MIT.

http://www.amazon.com/First-Course-String-Theory/dp/0521831431

If I remember rightly, his approach is lightcone-gauge-quantization. Which is reasonable, given the constraints of trying to teach this stuff to undergrads - though the manifestly Lorentz covariant approaches to quantization are ultimately more satisfying.

Another similar short work is here:

http://www.phys.uu.nl/~thooft/lectures/stringnotes.pdf

It is from Gerard 't Hooft at Utrecht University in the Netherlands.

Yup I think I've looked at those before too. I think it's good to try and learn things from multiple sources simultaneously, because every individual account will have their own idiosyncrasies and opaque parts.

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So, the science of Zombies. There is a fungus which gets inside ants and controls them. Is the Zombpocalypse just around the corner?

And the physicists turned zombies will be looking for branes...

I am kind of feeling like a dumb physicist here. String theory makes my head hurt, though it is interesting. I guess I will go back to my Navier-Stokes equation and turbulence.

I will go on record as stating that I am the dumb* physicist. What you guys do makes me jealous (and, like the cowardly lion, wish that I had a brain).

I think you meant the scarecrow? (and are you a physicist or a zombie? Looking for brains or branes? :lol:)

Anyway, when it comes to things like string theory, I think it's a very difficult and unintuitive idea to wrap your head around so I wouldn't call anyone "dumb". And even those who do manage to wrap their heads around these kinds of ideas are often less than brilliant at explaining them to people who understand less than they do.

Like I said in one of the other threads, we're all ignorant about something. I'm too much of an amateur and find too many things interesting to spend enough time to be well-versed in much of anything. If I could I would probably take intro courses from here til the end of forever. I guess that's why I like science writing that is accessible to the layman. Isaac Asimov in his day was a master at explaining all sort of science-y things in a way I could understand. I do read books by people like Stephen Jay Gould, Richard Dawkins, and a number of others, but their books still take me longer to read than a mystery or fantasy novel, for example. So that means I don't read as much science as I would like.

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Also, regarding Polchinski, I should mention that he also has a shorter version of his book on his webpage: Joe's Little Book of String. Less comprehensive than the printed version (and actually I like the BRST stuff from the Big Book of String) but it gets to the point much faster.

*downloads* Thanks.

Another similar short work is here:

http://www.phys.uu.nl/~thooft/lectures/stringnotes.pdf

It is from Gerard 't Hooft at Utrecht University in the Netherlands.

So speaking of online sources for learning string theory, I just took a look at the String theory folder on my hard drive. This is what I have, with links to the original source. (I'm only including the expositions I have downloaded, not the research papers. And I'm also leaving out everything already mentioned.)

Sunil Mukhi's Mini-course on string theory

http://theory.tifr.res.in/~mukhi/Physics/ministring.html

I saw him give a very similar mini-course when he was visiting the UK. It was good - quite understandable. I have a single file with the parts put together if anyone wants it - dunno where I originally got it from. These might be a nice set of notes for us to work through sometime as it's intended they can be read reasonably quickly. We could also try the exercises.

John Schwarz's Introduction to superstring theory

http://arxiv.org/abs/hep-ex/0008017

Yes, the same Schwarz as in GSW and as in Becker+Becker+Schwarz. He's considered one of the fathers of String theory, because though he didn't invent it, he and Michael Green revived the subject in the 80s with their anomaly cancellation calculation. (In more modern terminology, demonstrating the existence of an anomaly-free Type I superstring.)

Tom Waterhouse's notes taken from Michael Green's course at Cambridge in 2004.

http://homepages.nyu.edu/~dy387/string_Green.pdf

I attended this exact same course too, though in a different year.

Schellekens' Introduction to String Theory

http://idisk.mac.com/aschellekens-Public/StringLectures.ps

Then some more specialist topics.

Clifford Johnson's D-Brane Primer

http://arxiv.org/abs/hep-th/0007170

Shorter version of his well-known book, D-branes.

John Schwarz's Introduction to M Theory and AdS/CFT Duality

http://arxiv.org/abs/hep-th/9812037

The second and (arguably!) third superstring revolutions, all in one set of lectures! M-theory is the theory unifying the 5 types of Superstring - proposed by Witten in the mid 90s, building on ideas of Dine, Townsend et al. The AdS/CFT correspondence is a fascinating duality, originally proposed by Juan Maldacena in 1998, between the string theoretic quantum theory of gravitation in the bulk of Anti-de-Sitter spacetime (actually AdS_5 x S_5) and a particular quantum field theory on the boundary of this space. This has led to numerous insights, one of which is the resolution of the black hole information paradox in favour of no-information-loss. (Essentially, the theory on the boundary is a quantum field theory so is manifestly unitary and therefore info-preserving. As the quantum theory of gravitation is dual to this theory, it must also be information-preserving, even in the presence of black hole evaporation. Hence the resolution of the info paradox - at least in AdS_5 x S_5 - but very likely also in other spaces). Stephen Hawking conceded a bet about this to Preskill and Thorne a few years ago.

Paul Townsend's Four lectures on M-theory

http://arxiv.org/abs/hep-th/9612121

Klebanov's TASI Lectures: Introduction to the AdS/CFT Correspondence

http://arxiv.org/abs/hep-th/0009139

Bigatti and Susskind's TASI lectures on the Holographic Principle

http://arxiv.org/abs/hep-th/0002044

The holographic principle is an old idea due to 't Hooft, essentially conjecturing that systems might have far fewer degrees of freedom than one might expect - more precisely, perhaps the degrees of freedom of a system can be thought of as just lying on the boundary of the system. This would be true if the physical theory in the bulk is always dual to one on the boundary. AdS/CFT is the most successful implementation of this idea.

Warren Siegel's Introduction to String Field Theory

http://insti.physics.sunysb.edu/~siegel/sft.pdf

An early account (1988) of String Field Theory. Dated, as is inevitable, but still really interesting. String field theory was a very promising research direction within string theory that never really took off as it was expected to, though it's still a useful tool and people still work on it. The basic idea is that string field theory is to string theory as quantum field theory is to quantum mechanics. In other words, we want a field theory, the quantum excitations of which can be described by a superstring, in much the same way that particles are the quantum excitations of quantum fields.

I think Polchinski (either in the notes we already mentioned or maybe in his What is String theory? - another set of online notes!) discusses arguments for and against the string field approach.

Incidentally Siegel also has extremely comprehensive notes on Quantum Field Theory on his website - though his style is a little idiosyncratic, you can learn a hell of a lot from him. He was also the joint author of Superspace, an approach to creating and analysing supersymmetric field theories - which you can also get on his website.

Of course there's way too much there for one person to read in a reasonable time frame, and far far more than we're going to be able to discuss on this thread. But what the heck.

(Sorry if I didn't explain any of this well - it's very late here.)

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So speaking of online sources for learning string theory, I just took a look at the String theory folder on my hard drive. This is what I have, with links to the original source. (I'm only including the expositions I have downloaded, not the research papers. And I'm also leaving out everything already mentioned.)

Cool. Thanks.

*downloads all of the PDFed lectures*

Sunil Mukhi's Mini-course on string theory

http://theory.tifr.res.in/~mukhi/Physics/ministring.html

I saw him give a very similar mini-course when he was visiting the UK…

Tom Waterhouse's notes taken from Michael Green's course at Cambridge in 2004.

http://homepages.nyu.edu/~dy387/string_Green.pdf

I attended this exact same course too, though in a different year.

That might explain why you know so much about String Theory. And those must have been interesting experiences. I am jealous.

Thanks for the documents. I will try to slog through them. I am a bit rusty at graduate-level maths, and need to practice. I am (like many) very interested in Cosmology, and if String Theory is a possible TOE, then I should learn more.

What you guys do makes me jealous (and, like the cowardly lion, wish that I had a brain).

I think you meant the scarecrow?

:redface:

Oops. In that case, let me repeat:

I am a moron.
(and are you a physicist or a zombie? Looking for brains or branes? :lol:)

branes.jpg

Anyway, when it comes to things like string theory, I think it's a very difficult and unintuitive idea to wrap your head around so I wouldn't call anyone "dumb". And even those who do manage to wrap their heads around these kinds of ideas are often less than brilliant at explaining them to people who understand less than they do.

unscientific.png

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Sunil Mukhi's Mini-course on string theory

http://theory.tifr.res.in/~mukhi/Physics/ministring.html

I saw him give a very similar mini-course when he was visiting the UK…

Tom Waterhouse's notes taken from Michael Green's course at Cambridge in 2004.

http://homepages.nyu.edu/~dy387/string_Green.pdf

I attended this exact same course too, though in a different year.

That might explain why you know so much about String Theory. And those must have been interesting experiences. I am jealous.

Actually in the case of Green's course, what I said wasn't totally true. In the year I attended it, it was given by a different lecturer and was only 16 hours not 24. And the material was significantly different; he basically zipped through both volumes of GSW and both volumes of Polchinski at lightening pace, leaving some stuff out. I'm not sure how much I learnt. I somehow managed 96% in the exam, but I think that more reflects the fact the exam was not very difficult (probably to make up for giving us a totally over-the-top course). There was also a course that year on Branes given by Paul Townsend, in which he basically gave lectures straight out of the papers he and his collaborators (all world leaders in the field) were writing at that very moment. Nobody in the entire year elected to take the exam of this course!

I don't actually regard myself as very knowledgeable about string theory. What I do know mostly comes from the time I did research on superembeddings and the pure spinor formalism of the superstring - before leaving that all behind, wandering off into the "even more la-la" land of pure mathematics.

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Oops. In that case, let me repeat:

I am a very intelligent and articulate person who doesn't know everything.

fixed that for you ;)

And lol'ing at the comics and funny stuff. :lol:

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On the subject of String Theory (and the above Zombie Feynman advocating experimental physics), recent news from CERN is that the Large Hadron Collider did not produce evidence of micro-black hole creation. String Theory requires extra dimensions, and the experiments were done in part in hoping to find such black holes that may help verify the existence of these extra dimensions. Firstly, in layman's terms, is the theory:

http://en.wikipedia.org/wiki/Micro_black_hole

It is possible that such quantum primordial black holes were created in the high-density environment of the early Universe (or big bang), or possibly through subsequent phase transitions. They might be observed by astrophysicists in the near future, through the particles they are expected to emit by Hawking radiation.

Some theories involving additional space dimensions predict that micro black holes could be formed at an energy as low as the TeV range, which will be available in particle accelerators such as the LHC (Large Hadron Collider). Popular concerns have then been raised over end-of-the-world scenarios (see Safety of particle collisions at the Large Hadron Collider). However, such quantum black holes would instantly evaporate, either totally or leaving only a very weakly interacting residue. Beside the theoretical arguments, we can notice that the cosmic rays bombarding the Earth do not produce any damage, although they reach center of mass energies in the range of hundreds of TeV...

...Can we produce micro black holes?

In familiar three-dimensional gravity, the minimum energy of a microscopic black hole is 1019 GeV, which would have to be condensed into a region on the order of the Planck length. This is far beyond the limits of any current technology. It is estimated[citation needed] that to collide two particles to within a distance of a Planck length with currently achievable magnetic field strengths would require a ring accelerator about 1000 light years in diameter to keep the particles on track. Stephen Hawking also said in chapter 6 of his Brief History of Time that physicist John Archibald Wheeler once calculated that a very powerful hydrogen bomb using all the deuterium in all the water on Earth could also generate such a black hole, but Hawking does not provide this calculation or any reference to it to support this assertion.

However, in some scenarios involving extra dimensions of space, the Planck mass can be as low as the TeV range. The Large hadron collider (LHC) has a design energy of 14 TeV for proton-proton collisions and 1150 TeV for Pb-Pb collisions. In these circumstances, it was argued in 2001 that black hole production could be an important and observable effect at the LHC or future higher-energy colliders. Such quantum black holes should decay emitting sprays of particles that could be seen by detectors at these facilities. A recent paper by Choptuik and Pretorius, published on March 17, 2010 in Physical Review Letters presents a computer-generated proof that micro black holes must form from two colliding particles with sufficient energy, which might be allowable at the energies of the LHC if additional dimensions are present other than the customary four (three space, one time).

Collision in the range of TeV have been done at the LHC (by comparison, the particle accelerators that I use at work generally run in the range of hundreds of thousands of eV, and nearly always below 1 million eV. 1TeV is one trillion electron volts, meaning that the LHC, running at a few TeV is one million times as powerful as my particle accelerators), but micro-black holes were not detected. See:

http://www.physorg.com/news/2010-12-large-hadron-collider-signatures-microscopic.html

The CMS experiment at CERN's Large Hadron Collider (LHC) has completed a search for microscopic black holes produced in high-energy proton-proton collisions. No evidence for their production was found and their production has been excluded up to a black hole mass of 3.5-4.5 TeV (1012 electron volts) in a variety of theoretical models.

Microscopic black holes are predicted to exist in some theoretical models that attempt to unify General Relativity and Quantum Mechanics by postulating the existence of extra "curled-up" dimensions, in addition to the three familiar spatial dimensions.

At the high energies of the Large Hadron Collider, such theories predict that particles may collide "closely enough" to be sensitive to these postulated extra dimensions. In such a case, the colliding particles could interact gravitationally with strengths similar to those of the other three fundamental forces – the Electromagnetic, Weak and Strong interactions. The two colliding particles might then form a microscopic black hole.

If it were so produced, a microscopic black hole would evaporate immediately, producing a distinctive spray of sub-atomic particles of normal matter. These would then be observed in the high-precision CMS detector that surrounds the LHC collision point. CMS has searched for such events amongst all the proton-proton collisions recorded during the 2010 LHC running at 7 TeV centre-of-mass energy (3.5 TeV per proton beam).

No experimental evidence for microscopic black holes has been found. This non-observation rules out the existence of microscopic black holes up to a mass of 3.5–4.5 TeV for a range of theoretical models that postulate extra dimensions.

Perhaps these black holes might be found at higher energies?

The results of the experiments as submitted to Physics Letters B are here:

http://arxiv.org/PS_cache/arxiv/pdf/1012/1012.3375v1.pdf

Note the graphs, especially on p. 7. They show the experimental results along with the expected black hole-creation results.

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