# This Stuff for Dummies

1. Sep 10, 2004

### Sak

Hello. I am a person not nearly as smart as you. But I find some of the things here very interesting, but beyond my current understanding of physics.

I was wondering if there is some resource that can define all of these things, like Branes, Loop Quantum Gravity, etc. in layman's terms. Or perhaps, if you would spare a minute of your time, to type out a simple explanation.

I am currently a high school junior, and in conceptual physics. So I can understand some very basic physics concepts.

2. Sep 10, 2004

### chroot

Staff Emeritus
Brian Green's "The Elegant Universe" is not a bad place to start.

- Warren

3. Sep 10, 2004

### Sirus

Also, the latest issue of Scientific American magazine has some accessible articles on this subject.

4. Sep 10, 2004

### marcus

my advice would be to deepen your understanding of 3 proportions in nature

c, G, and hbar

string and loop are untested theories under construction
the popularizations are full of suggestive language, fantasy, and metaphor
time spent on other people's wishful thinking may turn out to be wasted

but the certain basic proportions of the universe will probably not mislead you

the pain in the butt is that to get familiar with c,G,hbar you need to familiarize yourself with standard metric units and you need a calculator that handles powers of ten.

If a kilogram of mass is somehow converted into a flash of light
how many joules of energy is the flash? It should be second nature for you to know and respond. Think about it as you are going to sleep at night.

If the angular frequency (learn about rating frequency angular style) of a certain photon is 1030 per second, then what fraction of a joule of energy does that photon carry?

(might have met such a photon around big bang time, very hot photon, carrying rather much energy---but for example, just to calculate)
----------------

1915 Gen Rel is a theory of gravity that puts G and c together in its main equation---relating the density of energy in a region to the curvature there

1915 Gen Rel does not have hbar in it!

when there is a quantum theory of gravity it will be a "quantized" version of Gen Rel, rather likely, and it will have all three of the big constants
G and c AND hbar.

nice thing about LQG is even its simpler results like the area spectrum (the range of discrete possible values of the area of a surface) involves all three constants in an area unit called "planck area", which is the area of a square one "planck length" on a side. the planck area is Ghbar/c3

there's a free draft copy of Rovelli's book Quantum Gravity (hardcopy coming out next month from Cambridge U. Press.) downloadable from his site:

http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html

suggest just skimming the first couple of chapters. chapter 2 is largely philosophy and science history, most of the rest too mathematical for now

Last edited: Sep 10, 2004
5. Sep 10, 2004

### Sak

Thank you for the explanation, even though I lost you at angular spectrum, but I appreciate the attempt. What technology could this theory, if true, yeild?

6. Sep 10, 2004

### marcus

Sak, I am a sci and technology watcher and I'm limited to reasoning based on historical analogy (i dont know how else to project future)

1915 Gen Rel is what they call a "classical" theory, i.e. unquantized.
classical Gen Rel has given us mostly ideas instead of technology---it gives us black holes and how quasars work and dark matter and dark energy (which are puzzles still) and accelerated expansion and gravitational lensing of distant galaxies by other galaxies and how stars collapse and how pulsars orbit each other and send out gravity waves and spiral in until they merge----and because Gen Rel is the theory of the shape of spacetime it gives us our best ideas sofar about what space and time are----something that energy can bend (but that is still a puzzle) and the bending of it causes gravity.

so that is mostly a big fat zero of technological payoff for Gen Rel. In fact it is a COST because Gen Rel gives us cosmology and that is mostly what we are SPENDING money to learn about by sending up Hubble Space Telescope and Microwave Anisotropy Probe, which are very largely devoted to understanding cosmology issues---the accelerated expansion and the bumps in the microwave backround.

So with my stupid backwards looking crystal ball I have to say that in the past quantizing some classical theory has usually just made it better. Maxwell gave us 1870 Electrodynamics and in 1950 Feynmann et all quantized it and got Quantum Electrodynamics and what did that do? Basically more of the same. You could build a radio and an Xray machine in 1890 with classical electrodynamics and you could build a transistor radio and an MRI scan in 1960 with QED. Techies and Gee Whizzers can make it sound more exciting but my take is that 1915 Gen Rel is already beautiful and we are still unwrapping its goodies. And quantizing it will only make it better and it will provide more of same.

I would like to know what dark energy is about. Classic Gen Rel says it is there and equiv to about 70 percent of overall energy density in U. I think quantum Gen Rel holds answer to why and what that is. It may not even be a kind of energy, it may be an intrinsic warping in space which when geomtry is quantized we may get a better notion of.

7. Sep 10, 2004

### marcus

BTW I have a "one-step-at-a-time" bias
I think the time is ripe to quantize Gen Rel which is just one thing
a quantum theory of gravity
(after all it has been around since 1915 almost 90 years)

and I dont think the time is ripe to "unify all the forces" and have a
"theory of everything"..I think the signs of failure in that direction are
getting more and more obvious
(check out Peter Woit's blog "Not Even Wrong"
http://www.math.columbia.edu/~woit/blog/)
so the technology daydreams attendant on cooking up a "theory of everything" are, in my humble view, a waste of time.
one thing at a time and all in due time, but you may want a second opinion on that

8. Sep 10, 2004

### Tom McCurdy

9. Sep 10, 2004

### PRodQuanta

Actually Sak, I would not recommend diving right in to "The Elegant Universe". I would try and get some background information. Definitely. I say this to a fellow High School Junior because it will make much more sense. I would first recommend some reading on the theories of relativity, and how and why it changed the world's paradigm of, well, everything. I would then recommend reading a introductory book of quantum physics. I suggest "Nature Loves to Hide: A Western Perspective" by Malin. This will relate the knowledge you know about relativity and quantum mechanics, along with the history of QM. By then you should be well read up in the theoretical/present physics. I would then dive straight into The Elegant Universe and (I suggest) Three Roads to Quantum Gravity by Smolin (also addresses Loop Quantum Gravity instead of just string theory). And, I hope, once Rovelli's book comes out, that will also be a great one to read, I hope. ( I believe it should be).

By the time you have read this, not only will you have a great (basic, implying not much math) understanding of what went/is going on in physics, but you should also expect to be called by a new nickname in your highschool. I can hear it now...."Hey loop boy, string it together!"....

My \$.02

10. Sep 10, 2004

### Tom McCurdy

I was first year physics student and I thought going right in to the elegant universe was alright (junior year high school last year) but I go through it page by page taking notes
so its not a quick read if you want to absorb it all

11. Sep 11, 2004

### PRodQuanta

Without any background info on QM or SR/GR or Classical mechanics? Didn't you find it weird why such a theory would arise, or why we need these theories in the first place?