The Physics That I Don't Understand, queries seeking replies Options

[shivani]

thanks a lot Bibhas and Hits..I think even I got it. (though I might come back with same Q again sometime in future)
so next have to try to visualize the three cases hits noted above.
(wish mil was here it all would have been so simple)
also .. can there be a better examples from physics rather than maths for bounded but infinite ?? not kalptaru.. as that is a magical thing

[bibhas]

Great Shivani !

Ok, here are some questions that popped into my head while reading Hawking's book and I'd be glad if anyone can answer these for me. (for those of you who haven't read it but want to, I have a pdf file of this book).

1. Is microwave radiation (observed by Penzias & Wilson as well as COBE) the only experimental evidence for Big Bang ?

2. How can Friedmann’s third model be extrapolated to state the universe to be flat ?

3.“The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space-time, it appears to us to move along a circular orbit in three-dimensional space.”. Yeh kuch samjha nahiin. Any visual analogies ?

4. Can you give an example of a particle with a spin=1/2 (no, not an electron ), i.e., one that requires a 720 degree rotation to look identical ?. Matlab, the way Hawking uses a dot for s=0, an arrow for s=1 and double-headed arrow for spin=2.

Bibhas
p.s. How can theoretical physicists live with the fact that all they do is propose theories which may never be validated exptally ? How can someone live with that kind of uncertainty within them ?

[shivani]

Bibhas
Can you please send me the pdf ?? send on my mail id. and which one is it ??
currently am reading his Brief history of time... and paperback version is gonna take eons.

[bibhas]

Bibhas
Can you please send me the pdf ?? send on my mail id. and which one is it ??
currently am reading his Brief history of time... and paperback version is gonna take eons.

Shivani,
actually it is Brief History of Time and can be downloaded from:

www.churchofsatan.org/pdf/stephen_hawking_a_brief_history_of_time.pdf

Bibhas

[shivani]

thanks Bibhas : )

[visuja]

Thanks for the link bibhas. I had left my book back at home, so this should be useful.

As for the 3 scenarios, I essentially agree with bibhas. My analysis / observations assume that relative motion between the frame of reference and the star is generally small / negligible compared to speed of light. I presume the spectral shift / Doppler effect can be corrected for to arrive at the 'true' information for that frame of reference.

1. Earth as frame of reference: Any information that we can collect about the star in observation is limited by the 'time' factor --- and the finite speed of light. So essentially what we 'see' is the star's past (ranging from 8 minutes to millions of years). A far fetched idea, but in theory, we can at the most have all information from the star's birth to its 'apparent' age as seen from the earth. ---- all info at a phase lag of the 'time difference' in question.

2. Star as the frame of reference: Essentially u live with the star. So u should have all information right from the star's birth to its current age. This would be the most accurate and up-to-date info that one could gather about the star.

3. Edge of light beam as frame of reference: Ah ! Talk of immortality ! The star would seem 'immortal' to you because you would be seeing an unchanging, unmoving star all the time. So even after millions of years on leaving the star, even if the star has ceased to exist, one would always see the star in exactly the same state as it was when the light beam left the star millions of years ago (provided one had powerful enough telescopes to view over such large distances). This is somewhat similar to scenario 2 where one 'sees' the star even if the star does not actually exist as of today. Only difference is in this scenario, the star will continue to exist till eternity (whatever that means).

Let me conduct a similar thought experiment. Now still in scenario 3, if the light beam was travelling exactly towards the earth, then on one hand we see the star as it 'was' when we left it, while regarding our perception of the earth, we would be seeing the earth 'evolve' at 'double the speed', since we'll be moving at twice the speed of light relative to the light reflected off the earth (which carry the 'info' about the earth). If the light beam we are on happens to move in an exactly opposite direction away from the earth, then this scene is exactly like scenario 3, except that now we'll be seeing both the star and the earth in exactly the same 'state' as at the instant we left the star. Our perception of the earth will be between these two scenarios for any other direction.

I hope I made some sense here . Its only when the speed barrier is broken that my thought experiments also collapse .

How can someone live with that kind of uncertainty within them ?

u steal words of my mouth . I wonder the same about all the abstract sciences ...math, physics, thermodynamics !! (how do people even conjure up things like entropy and free energy ? ) The fact that they validated these terms later is ...but to come up with these things is even

.“The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space-time, it appears to us to move along a circular orbit in three-dimensional space.”

Cant help with your other queries bibhas, but lemme try and explain the way I understood it.
Given the masses of the earth and the sun, I think the 'straight line' in space-time implies that the distance at which the earth revolves around the sun and the time taken for one revolution (or the sped of revolution) are optimum for the sun-earth system. I'm not quite sure how this optimum is defined though. My feeling is any closer to the sun, the earth would have plunged into the sun, while any farther away, the earth might drifted away from the sun's gravitational influence. Similarly any slower revolution would have spiralled it inward, while any faster would have caused it to drift away. But going by my 'theory', the planets should have been arranged in the order of increasing mass from the sun, which isnt the case. So obviously I'm missing something here. Visual analogies would be very very helpful .

I remember a program on discovery which showed an analogy to the space-time fabric. An elastic rubber sheet was held at its corners ('reasonably tightly') and steel balls of various sizes and weights were placed at various places on the sheet. The balls caused some depression in the sheet and depending on if one ball came in the 'influence' of another depression, two balls would roll into one another and so on.

I also forget why almost all orbits are elliptical and not circular ? (Now I'll have to start reading up Kepler's works before moving onto Einstein and Stephen Hawkings )

Vivek

This post has been edited by visuja: Sep 7 2005, 11:43 AM

[shivani]

.“The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space-time, it appears to us to move along a circular orbit in three-dimensional space.”

Cant help with your other queries bibhas, but lemme try and explain the way I understood it.
Given the masses of the earth and the sun, I think the 'straight line' in space-time implies that the distance at which the earth revolves around the sun and the time taken for one revolution (or the sped of revolution) are optimum for the sun-earth system. I'm not quite sure how this optimum is defined though. My feeling is any closer to the sun, the earth would have plunged into the sun, while any farther away, the earth might drifted away from the sun's gravitational influence. Similarly any slower revolution would have spiralled it inward, while any faster would have caused it to drift away. But going by my 'theory', the planets should have been arranged in the order of increasing mass from the sun, which isnt the case. So obviously I'm missing something here.

umm.. I could not understand the answer you were trying to give here Visu, instead found another question - Is there an order to planetary arrangement, in terms of the distance between a planet and its sun ??

I also forget why almost all orbits are elliptical and not circular ? unsure.gif (Now I'll have to start reading up Kepler's works before moving onto Einstein and Stephen Hawkings

This is explained in Hawkings itself.. I think first chapter ( thats all ive read so far)

3. Edge of light beam as frame of reference: Ah ! Talk of immortality ! The star would seem 'immortal' to you because you would be seeing an unchanging, unmoving star all the time. So even after millions of years on leaving the star, even if the star has ceased to exist, one would always see the star in exactly the same state as it was when the light beam left the star millions of years ago (provided one had powerful enough telescopes to view over such large distances). This is somewhat similar to scenario 2 where one 'sees' the star even if the star does not actually exist as of today. Only difference is in this scenario, the star will continue to exist till eternity (whatever that means).

Did not understand

Let me conduct a similar thought experiment. Now still in scenario 3, if the light beam was travelling exactly towards the earth, then on one hand we see the star as it 'was' when we left it, while regarding our perception of the earth, we would be seeing the earth 'evolve' at 'double the speed', since we'll be moving at twice the speed of light relative to the light reflected off the earth (which carry the 'info' about the earth). If the light beam we are on happens to move in an exactly opposite direction away from the earth, then this scene is exactly like scenario 3, except that now we'll be seeing both the star and the earth in exactly the same 'state' as at the instant we left the star. Our perception of the earth will be between these two scenarios for any other direction.

Ditto!! Did not understand.

I wonder the same about all the abstract sciences ...math, physics, thermodynamics !! (how do people even conjure up things like entropy and free energy ?

Maybe things are clearer to them the same way as your posts are clear to you.. while atleast I am getting so lost!!

people pardon my ignorance and dumbness here . The patience you guys are showing is much appreciated.

[bibhas]

Hey Vivek, thanks for that post. I realize I goofed up scenario # 3, it makes much more sense that the star would look unchanged for whatever length of time you observe it from the edge of the light beam. So then, even though Time has moved ahead, it would look like it stood still. So essentially a single moment would stretch to infinity ?
I am still unable to see how a straight line path in 4D space-time would appear to be a circular (or elliptical) path in 3D. Looking for answers to this and the other Qs I posed.

[shivani]

3) You are seated at the edge of the light beam that the star is throwing, and that's your frame.

3) This is the tricky one. I wanna say Time keeps continuously changing (is this what you call "flowing" ?) and I see everything from the star's "today" to its past "five million years ago".
So essentially I've travelled into its past, i.e., Time is not uni-directional ?

This Q /scenario I did not understand. What am I when I am trying to observe the life of star here? where exactly am I on the light beam.
Also what I could understand is.. I am constantly moving away from the star, as the beam is travellign in some direction.. or am I stationary (x-y-z only t changing?)

[bibhas]

3) You are seated at the edge of the light beam that the star is throwing, and that's your frame.

3) This is the tricky one. I wanna say Time keeps continuously changing (is this what you call "flowing" ?) and I see everything from the star's "today" to its past "five million years ago".
So essentially I've travelled into its past, i.e., Time is not uni-directional ?

This Q /scenario I did not understand. What am I when I am trying to observe the life of star here? where exactly am I on the light beam.
Also what I could understand is.. I am constantly moving away from the star, as the beam is travellign in some direction.. or am I stationary (x-y-z only t changing?)

Shivani,
The way I see it, you are you and you are on the edge (say leading edge) of the light beam, you're sitting on the first photon to come out at the given time. You are constantly moving away from the star at the speed of light and I made the assumption that the light beam is travelling towards the earth (to tie it up to scenario #1). In such a scenario, the star would appear unchanged throughout the length of your observation, i.e. time would appear unchanged to you with respect to the star under observation.
Bibhas

[shivani]

Shivani,
The way I see it, you are you and you are on the edge (say leading edge) of the light beam, you're sitting on the first photon to come out at the given time. You are constantly moving away from the star at the speed of light and I made the assumption that the light beam is travelling towards the earth (to tie it up to scenario #1). In such a scenario, the star would appear unchanged throughout the length of your observation, i.e. time would appear unchanged to you with respect to the star under observation.
Bibhas

help me understand this Bibhas.. the star would seem unchanged to me because I would have only A set or The set of information about star, when I left it. I would not be observing it ever again?? Is that assumption correct or am I somehow able to sense the changes that might be happening to the star, as I am travelling away.
If former, than yes the state is always unchanged, if later.. then.. dont know

[visuja]

You are right Shivani. Since you are travelling alongwith the light wave, u essentially are limited to the info carried by that wave ...ie how the star looked at the instant the light beam (and you) left the star. So u'd basically be seeing the star for all eternity at the same position.

oh good.. hits is here. He'll give a few pointers in a short sentence.. and then leave us to fill another page of our rambling

[hits]

Shivani,
The way I see it, you are you and you are on the edge (say leading edge) of the light beam, you're sitting on the first photon to come out at the given time. You are constantly moving away from the star at the speed of light and I made the assumption that the light beam is travelling towards the earth (to tie it up to scenario #1). In such a scenario, the star would appear unchanged throughout the length of your observation, i.e. time would appear unchanged to you with respect to the star under observation.
Bibhas

help me understand this Bibhas.. the star would seem unchanged to me because I would have only A set or The set of information about star, when I left it. I would not be observing it ever again?? Is that assumption correct or am I somehow able to sense the changes that might be happening to the star, as I am travelling away.
If former, than yes the state is always unchanged, if later.. then.. dont know

Let me point out an interesting constraint here as we discuss this - The powers of human observation are limited, for the most part, by the speed of light. Let's remember this (if and) when we shuttle back to the thread on "Weird Dimensions" (Determinism thread by Shivani).

The star is changing constantly. If you are still with respect to it, you'll know what happened to it when the light beam hits you (which will be sooner, if you are closer to the star). Now, if you are moving away from the star at or greater than the speed of light you will soon break out of the light beam thrown by the star and then lose the ability to know what's happening to the star (assuming you start your flight from the star after it started emitting light). When you are far enough from the edge of the light beam, suppose you turn around in your little contraption and ride back. Now you are going to re-live the star's "past", and progressively move towards the "present"! Ain't that interesting?!!

[hits]

Bibhas,

I owe you responses to your questions. In time...

Hits

[bibhas]

Let me point out an interesting constraint here as we discuss this - The powers of human observation are limited, for the most part, by the speed of light. Let's remember this (if and) when we shuttle back to the thread on "Weird Dimensions" (Determinism thread by Shivani).

Why not all the time ?

When you are far enough from the edge of the light beam, suppose you turn around in your little contraption and ride back. Now you are going to re-live the star's "past", and progressively move towards the "present"! Ain't that interesting?!!

Awesome (although I had to draw out a little light cone on my pad to figure that out) ! I finally realize why Einstein chose to call time as an extension of space and merged them together into spacetime. Now, someone just tell me how I can fly at the speed of light ?