The Physics That I Don't Understand

This thread has its origins in some concepts of Physics I don't understand. These are those that frequent discussions on the origin of the universe. The following post from the "Determinism & Free Will" thread in Chit-O-Chat will serve as the starting reference.



Hithesh,
I am still confused about why one needs to know the complete extent of the physical domain in order to have an absolute frame of reference. Also confused about the flow of time in both non-absolute and absolute scales.
Bibhas

Bibhas,

Have you started on Hawking's book?

Hits

Me did Hitesh ..but lost myself in the first few pages and then didnt have the drive to move forward. As I said, I need lots of time to digest new info and being as impatient as I am, I'm still stuck with rudimentary questions in many areas. (Shaayad umar ke saath saath patience bhi aa jaaye :sigh:)

Since Shivani mentioned a few things on 'strings", Shivani -- could you explain what are "strings" ? And how does its definition help in unifying ('tying up' !) all theories together ?

For all the questions being posed here, may I suggest the recent book by Roger Penrose "The Road to Reality". Some of you may have heard about his earlier popular book titled "The Emperor's New Mind" in which he discussed the inadequacy of the laws of physics in explaining consciousness.

In his new book, String theories and the more recent version of string theory, known as M-Theory have been augmented with his newly proposed twister theory.

In my opinion, this is a fantastic and a very gentle reading on a Sunday afternoon over oodles of coffee. I assure you that you'd be enlightened. Not too expensive either.

If I may add my less than two cents:


Like Bibhas, I am not too sure of the dependence of extent of the physical domain on the notion of an absolute frame of reference. But then what is the extent? Is it the infiniteness or the unboundedness? If it is bounded but infinite, then that would keep things simpler than they would be if the universe was infinite and unbounded as well.

However things may not have to get complicated or unsolvable with changes in the frames. There are frame-invariant properties that would answer all of our questions and the properties which would not be frame-invariant would be useless anyway. To give a simple example, when a runner runs on tracks, depending on where the viewer in the stadium is, the runner's apparent position to the viewer would change but the runner's inherent properties of motion, such as velocity and acceleration would n't change.

PS: I wish that these and a few other threads had come about a bit earlier.

Nice to see you Anurag. The books that you mention --- are they in a simpler language than Stephen Hawkin's book ? Though I understand it might be difficult to explain these concepts in non-technical terms. I'll try and look for the books in my univ library.

I think it is easier to understand the effect of frames of reference when they are applied to spatial dimensions. But it is the unidirectionality of time (with respect to our frame of reference) that bothers me. If I can udnerstand the reversibility of time in another frame of reference, then probably I'll be better able to understand a few posts in other threads. I'd love to discuss on that, but that would take us into discussions on theory of relativity (and there are already many threads at HF which require deep thinking from my side )

I wish I had a better understanding of analytical math without having to relate mathematical equations to physical reality. It does a great job at doing away with physical reality and in a way, is a great tool in analysing these 'unknown' dimensions.

Vivek

PS : Please do bring out any implict assumptions that I may be making in my comments. Hits has been doing a great job at that

sigh!
I wish there was a way to do without maths.. but seems ther eis no other option.
Visu.. am on your side.. only much behind.
lets say.. "chad ja beta sooli pe bhai karenge Ram" and try to learn.
I would try to make a post on Strings, as I have understood it.
About the book Anurag has suggested.. well.. would leave it for sunday.

My understanding of String theory :-

Through the mathematics of quantum mechanics and experimental observation, it was deduced that all known particles fell into one of two classes: Bosons and Fermions Bosons are particles that transmit forces. Many bosons can occupy the same state at the same time. This is not true for fermions, only one fermion can occupy a given state at a given time, and this is why fermions are the particles that make up matter. This is why solids can't pass through one another, why we can't walk through walls -- because of Pauli repulsion -- the inability of fermions (matter) to share the same space the way bosons (forces) can.

Why string Theory - Particle physics interactions can occur at zero distance -- but Einstein's theory of gravity makes no sense at zero distance.
Relativistic quantum field theory has worked very well to describe the observed behaviors and properties of elementary particles. But the theory itself only works well when gravity is so weak that it can be neglected. Particle theory only works when we pretend gravity doesn't exist. So to speak it is applicable only on very small particles.
General relativity only works when we pretend that the Universe is purely classical and that quantum mechanics is not needed or applicable in our description of Nature. ( eg. stars)

If there were a good quantum theory of gravity, then the particle that would carry the gravitational force would have zero mass and two units of spin. This theorized particle is called the graviton.
Now, a graviton can be added to quantum field theory by hand, but the calculations do not match. This is because particle interactions occur at a single point of spacetime, at zero distance between the interacting particles. For gravitons, the mathematics behaves so badly at zero distance that the answers just don't make sense.
String theory suggested that each particle is actually made of "strings" : a wiggling tiny thread. Think of a guitar string that has been tuned by stretching the string under tension across the guitar. Depending on how the string is plucked and how much tension is in the string, different musical notes will be created by the string. These musical notes could be said to be excitation modes of that guitar string under tension.
In string theory, as in guitar playing, the string must be stretched under tension in order to become excited. However, the strings in string theory are floating in spacetime, they aren't tied down to a guitar. Nonetheless, they have tension. The string tension in string theory is denoted by the quantity 1/(2 p a'), where a' is pronounced "alpha prime"and is equal to the square of the string length scale.
If string theory is to be a theory of quantum gravity, then the average size of a string should be somewhere near the length scale of quantum gravity, called the Planck length, which is about 10-33 centimeters, or about a millionth of a billionth of a billionth of a billionth of a centimeter. These particles cannot be observed by any experiment. Then comes the question, if the string can only constitute bosons ( particles that transmit forces) or fermions, too (particles that make up matter)? Open ended or Closed ended? Both.
Different String theories are classified according to whether or not the strings are closed loops, and whether or not the particle spectrum includes fermions. If fermions are included, there must be a special kind of symmetry called supersymmetry, which means for every boson (particle that transmits a force) there is a corresponding fermion (particle that makes up matter). So supersymmetry relates the particles that transmit forces to the particles that make up matter, and these are called superstrings.
Now how does quantum mechanics apply to strings? For bosonic strings, this question is only answered in the affirmative if the spacetime dimensions number 26. For superstrings it comes down to 10. How we get down to the four spacetime dimensions we observe in our world is another story.
If we ask how to get from ten spacetime dimensions to four spacetime dimensions, then the number of string theories grows, because there are so many possible ways to make six dimensions much much smaller than the other four in string theory. This process of compactification of unwanted spacetime dimensions yields interesting physics on its own.
This period in string history has been given the name the second string revolution.
And now the biggest rush in string research is to collapse the table above into one theory, which some people want to call M theory, for it is the Mother of all theories.

I will try to make another post soon on how the the other 6 dimensions are collapsed into known 4 dimensions. Meanwhile if someone else volunteers for it.. great :-)

Yes I have. And this time around, even Harry's "Half-Blood Prince" (which my wife finished reading just as I started Hawking) didn't sway me away, at least not yet

Bibhas
p.s. It would take me a long while to make sense of Anurag's, Vivek's and Shivani's posts, so I am going to take the rest of the day off to practise the special Desh Bandish that Guruji taught me yesterday.

Make sense of my post !! maine aisa kya keh diya ? mujhe to shivani ka post dekhkar hi darrrr lag raha hai...and she's supposed to post more !!! (And to think I had asked her to post on 'strings') Ayyoooooo !!!!! Aur hits ne to shuruaat bhi nahin ki hai

It takes one to be really poor to understand what poverty really means ! Mere physics ke level kya hai, yeh tum nahiin jaante.

Need help birthday boy?

I simply llllove physics!

Yeh bhi koi poochne waali baat hai aindrajaalak ? madad chaahiye tabhi to thread shuru kiya.

Yes Mandrake. Some help would be great on what Shivani posted. Poora bouncer gaya (hehe I can dump all my astronomy queries onto you ) Woh black hol;e waala thread bhi achha tha... kb4u is apparently a physics student... would be great to have him here in this discussion too...

Haan ye baat to sahi hai. Isliye me lagbhag absent at following threads: D&FW, Mythology, Legends, RR, Photography, Shayari, dabble in regional threads Ek biowaala thread hota to wahaan bhi gaayab rehta

Me always hang around in chit-o-chat .... Me crossed 700 posts in < 2 months

Hope to hear more form you Anurag. Your posts on Math and dimensions at another thread intrigued me. Would like to learn more on that, especially on its application to visualising other dimensions. Good to have a variety of ppl here, each comfortabloe in their own field and dabbling in all other fields ( ..main apni baat kar raha tha )

mandrake, cud u throw some light on 'your' time ka funda ? (trust us .. )

and I thought atleaast this time what I have writtenis simple and understandable..
maybe it needs to start with Relativity and Quantam mechanics as well..
or maybe I just need to sit aside and learn more. (better approach..hint taken.. onto it from nowon)