Physics Discussion Thread

brihaspati · Post by **brihaspati** » 17 Jun 2011 03:29

rsingh wrote:
Amber G. wrote:Anyway .. here is another problem --

We live in a quiet neighbor-hood but there is a highway not too far from us. In the morning (or day time) we don't hear any noise but in the evening (when we take a walk) we often hear traffic noise from the high-way?
What possible could be the reason? .. (Do others have similar experience? Sound from far away places, much clearer in the evening than in the day or morning..)
Had such experience. In day time noise is canceled by other noise much closer to you. In evening or early morning not much noise near you. Heavy overloaded truks make "puuuuuuuuuur" noise which can be overheard for kilometers......if surrounding is calm.

Interesting, and the argument of cancellation by noise is more reasonable. But could also not be somewhat related to temperature inversion near the ground surface from day to night? Sound may travel more closer to the ground through denser air near the colder ground at night compared to the day when it would be likely to be dispersed away. Dont know about the magnitude of differences required to have significant observations. Just a hypothesis.

rsingh · Post by **rsingh** » 17 Jun 2011 18:40

Bade wrote:rsingh, you ask questions which are more difficult to give straight answers than AmberG.

The above will require an experiment to see the effects of coupling of the gravitational force (mass) and electromagnetic force. The current textbook understanding of when unification of such forces happen or coupling is comparable enough to have a sizable effect (changed mass/weight as data gets stored on device) is far from achievable as the energy required is large.

Sorry Saar, SHQ is very economical about special favours these days..................so this tortured dirty mind is looking some solace in Physics and BENIS dhaga

Amber G. · Post by **Amber G.** » 17 Jun 2011 19:26

..Yes, obviously one can hear the far away traffic noise clearer if there is less background noise.. (I don't know about "cancellation", unless one has installed Bose noise-cancellation system...

).. but point was, if you take other factors into account, is there really a noticeable difference between, say evening (after sun has just gone down - or late evening), night, morning and the day (say a quiet day) ...

As I said before, we could hear waves in the evening..(and night) but not that clearly in the morning (after sun rise) .. and I was told that if you do hear it in the morning, that day is going to be unusually hot and muggy ..which happened to be the case every time.

On rare occasions normal sound from sources, is uncharacteristically absent. Has anyone noticed that? ( I have read of several historical records of battles where this phenomena played a big role. One famous case I read was battle of Gettysburg ( American Civil war) where one missed hearing cannon fire less than 10 km away (which normally could be heard much further)... (quick check in google - gives several other historical records of this type)

Added later: (From Internet - In another famous case, Johnston was replaced by Lee. Johnston in his headquarters, missed hearing the noises/cannon fires which were less than 2 miles away.. (Expertise of a a physics professor was on sought on .. ityadi...)

Bade · Post by **Bade** » 17 Jun 2011 19:46

Background noise is clearly less at night, so it perhaps enhances the perception of higher intensity of individual sources of sound. It may even be related to the detector response characteristics of the eardrum. If the nature of the response of the ears is unknown, how does one account for the difference. So one will need to actually measure the sound at different times to quantify the effect first using instruments other than our ears. Has this been done for different atmospheric conditions ? Denser air column can also attenuate sound waves faster, though we know vaccum (almost zero density) does not propagate sound waves.

1) detector response (of the ear)
2) atmospheric conditions (density of air)
3) nature of the source of sound

All need to be understood separately to completely answer the question, IMO.

Rahul M · Post by **Rahul M** » 17 Jun 2011 20:13

I guess the simple physics of it is denser medium means higher range of propagation. air density should be higher (?) during cooler hours/cloudy days ?

in my experience, noises are audible from the longest distance during the darkest hours of night i.e just before dawn, which are also the coolest. of course the lack of ambient noise plays its part.

rsingh · Post by **rsingh** » 17 Jun 2011 23:12

Perhaps. I used to help my family during wheat harvest (Haryana). There was a road 2 km from our fields and could hear any traffic passing in mid day. In those day there were no tractors or any other machine in fields. There are not many birds in wheat fields so not much noise closer to you. On other hand it is almost impossible to hear anybody shouting in rain. Although air is damp, the noise closer to your ear that you do not hear any thing.

SaiK · Post by **SaiK** » 18 Jun 2011 01:32

On the moon/sun on the horizon appearing larger, there are so many theories out there. So, which one is correct?

rsingh · Post by **rsingh** » 18 Jun 2011 02:01

^^
On horizon we have fixed reference in the shape of landscape,house or trees. But when Sun or Moon is on Zenith we do not have a physical reference in the proximity.......sky is big space where heavenly bodies look smaller....... comparatively. IMO this is quite logical explanation. When we measure angle Elevation of Sun or Moon (for navigation to compare with star charts) we take upper or lower arc of these bodies ...........so that we are not influenced by their apparent size.

Amber G. · Post by **Amber G.** » 18 Jun 2011 02:26

^^^ Yes.. It is more of an illusion, if one measures (or look at sun while ignoring the surrounding) there is not much difference. BTW it is one of interesting coincidence that size or both Sun and Moon are almost the same. (Some times moon is bigger, some times it is smaller). The size is about 1/108 (radians).. some say it's the origin of special importance given to number 108).

SaiK · Post by **SaiK** » 18 Jun 2011 02:30

On the 108 number, I read somewhere a relationship established.. like the distance between Earth & Sun is about 108 times the circumference of earth or sun?.. is this true between other planets and Sun or in some multiples of 108?

so, on the horizon, it is not refraction then.

Amber G. · Post by **Amber G.** » 18 Jun 2011 03:02

^^^ The relationship is (distance of the Sun) / (diameter of the Sun) (appox=) (distance of the Moon)/diameter of the Moon) = 108
(Or apparent size of Sun (or Moon) is about 1/2 degree or about 1/108 radians ) .. No other planet/moon system in our solar system has this coincidence.

Refraction makes the Sun (at sunset/sunrise) appear higher than it actually is. This is why times of sunrise/sunset in some panchangs are off by 4-5 minutes (or even more in cold weather). In some Hindu calendars, tithis (तिथि) are determined by the position of the moon (actually angle between sun and moon) at the sunrise time so some times tithis may show discrepancies .. if one measured the apparent positions through an observatory .. ... that may be a little OT here.

negi · Post by **negi** » 18 Jun 2011 03:56

I thought refraction makes it look reddish ; shouldn't the apparent change in height and increase in size be attributed to dispersion instead ?

Amber G. · Post by **Amber G.** » 18 Jun 2011 04:19

Scattering (Rayleigh scattering) makes it ( sky too) look red.
See: http://en.wikipedia.org/wiki/Rayleigh_scattering
Refraction makes it look higher in the sky (you can see sun minutes after it has set - geometrically speaking)

See: for example: http://apollo.lsc.vsc.edu/classes/met13 ... e_set.html

Dispersion, is mainly associated with the property that different colors (or light spectrum) have different refractive index and they "disperse" (making "colors" of rainbow). (This why diamond sparkle). So in rare conditions you see weird color phenomena at sunrise/moonrise. Also when (in extreme conditions) where lower part of the sun/moon does not have the same bending (refraction) as upper part and the moon deviates (generally flattens out) from its circular shape. (Some call that dispersion too)

Hth...

For example see: (green flashes etc..)
http://hyperphysics.phy-astr.gsu.edu/hb ... edsun.html

Added later: I just saw the second reference I gave, by clicking back/next one can know about scattering, dispersion and other phenomena relating to sunset .. light etc..

Amber G. · Post by **Amber G.** » 18 Jun 2011 04:31

^^^ Also from a tall building one can see a little farther than one calculates from geometry alone... or in extreme cases one sees mirage.. all are mainly due to refraction.

negi · Post by **negi** » 18 Jun 2011 04:52

YIkes I got them mixed up.

Amber G. · Post by **Amber G.** » 18 Jun 2011 07:26

On the 108 number, I read somewhere a relationship established.. like the distance between Earth & Sun is about 108 times the circumference of earth or sun?.. is this true ...between other planets and Sun or in some multiples of 108?

^^^ The relationship is (distance of the Sun) / (diameter of the Sun) (appox=) (distance of the Moon)/diameter of the Moon) = 108
(Or apparent size of Sun (or Moon) is about 1/2 degree or about 1/108 radians ) .. No other planet/moon system in our solar system has this coincidence.

R.

To add to above, the actual numbers come out to be (If my calculation is correct):
For Moon - Varies (104.2 to 116.8 ) Average about 110.6
For Sun - Varies ( 105.6 to 109.2) Average about 107.4

If you held 1 ungli (finger - or say 1 cm thick object) about 108 unglis (about a meter) it will about cover the sun or moon.

BTW Sun's diameter is about 108 (actually 109.3) times Earths's diameter...
(What I think about 108 is it is just 27*4 .. (27 nakshatras in zodiac.. or 108 paadas as 4 paadas = 1 nakshatras)

SaiK · Post by **SaiK** » 18 Jun 2011 08:17

awesome amber..

so your ungli is object in reference to the line of sight to sun or moon, correct?

108 unglis to sun!

SriKumar · Post by **SriKumar** » 19 Jun 2011 18:51

Well, no one bit, so here goes: Here's an essay I found sometime back that I thought was interesting and seemed to explain some questions I had about the nature of Math.

On the Nature of Mathematics (my words here- and its relation to real world):
http://santiago.mapache.org/nonfiction/essays/math.html

This could be X-posted in the Math thread, however, I am posting it here because a lot of the math done is within the context of applications (engineering, physics, weather, insurance). Hence I am thinking that in a physics-based thread, the distinctions might come out more clearly between math that explains the real world we live in vs. math that does not. Also, by Math, I am excluding arithmetic and number theory, which, IHMO, is all (and only) about a format to express numbers- willing to be corrected on this. Amber.G, please feel free to cross-post in Math thread if needed.

Another thing: The author of the article calls math a language. IMHO, it is perhaps more accurate to call it a script rather than a language, since it is not spoken and only written, and its power derives from the ability to manipulate the script (unique only to this script).

Excerpt:

Mathematics, rather than being a science in its own right, is the foundation of all other sciences. Mathematics itself does not rely on the natural world for its information but on constructs of the human mind instead. (Although much of math was inspired by nature, it is still artificial.) Unlike science, there is no true knowledge to be gained from math--when the postulates were set, that determined what theorems would be true, false, or unprovable within that system. All that has ever remained is a process of identification. ..... Mathematicians have no need of hypotheses, for the postulates tell that something is either right or wrong. This absoluteness is good for the other sciences, as they are based on mathematics. Mathematics is the language in which their scientific models are written, be it the equations of quantum physics and chemical kinetics or the statistics of population ecology.

Most of us tend to assume that the rules we apply to these symbols are some sort of natural laws, but they too have been chosen arbitrarily. We could just as easily define that "2+2=0", and that through any point outside of a line, there pass an infinite number of lines parallel to the first line, and mathematicians sometimes do. Indeed, there are an infinite number of possible mathematical systems that are completely self-consistent. The one we use all the time has the advantage of describing the way our world works, when properly related to it by means of a normal language.

SaiK · Post by **SaiK** » 19 Jun 2011 19:51

Any gurus can get into general theory of relativity, that I always get into tussle keeping it in mind, in comparison with special theory that was more easier to comprehend.

So, how are the other observations in general theory actually "general" and not special?
and .. moving objects can't accelerate wrt each other.. this one beats the hell out me to understand?
and in aam-man terms, what is Einstein mushai is saying about the space-time curvature?
..ultimately I am gone bonkers on travelling at faster than the speed of light.

GuruPrabhu · Post by **GuruPrabhu** » 20 Jun 2011 17:24

SriKumar wrote: Excerpt:
Unlike science, there is no true knowledge to be gained from math--when the postulates were set, that determined what theorems would be true, false, or unprovable within that system. All that has ever remained is a process of identification. .....

It would be easy if this were universally true. But, I had posted exceptions above. In case folks missed it, I will elaborate on one of them.

Dirac, in late 1920s, set about trying to describe an electron which displayed two newly discovered phenomenon simultaneously: 1) probabilistic behavior as described by Shroedinger (quantum mechanics) and 2) kinematics as described by Einstein (relativity).

He finally came up with a hybrid differential equation which involved matrix mechanics (Pauli's spin matrices). In other words, he realized that in addition to above two properties, it was essential to include spin in the equation.

Now, pause here. Did this insight come from physical considerations? NO. It was pure math. So, what role does math play in this example?

However, this is not the punch line. Dirac could not find a solution to his equation, because 2 out of 4 solutions he has described negative energy states. But he trusted his equation enough that he stuck to his negative energy solutions and explained them away as a "sea of electrons" with energy levels below zero.

This was preposterous from a physics point of view.

A few years later the positron was discovered in cosmic ray interactions. These particles were nothing but the "negative energy electrons" of Dirac. Later on, Feynman provided the correct explanation of the solutions to Dirac equation consisting of two electrons (spin up and down) and two positrons (spin up and down).

So, essentially fiddling with math led to one of the most important discoveries in physics -- the existence of anti-matter!

Now, what role did physics play in that discovery and what role did math play in predicting it?

Bade · Post by **Bade** » 20 Jun 2011 19:58

GP, lots of physics discoveries were really not entirely inspired by experimentation and observation. Wasn't Planck's derivation of the blackbody spectrum using the hodge-podge idea of some harmonic oscillators that lead to quantum physics long before Einstein's photo-electric theory. The whole derivation is classical to begin with but ends describing what is essentially a quantum phenomena.

There definitely is something quite fundamental about the use of mathematical tools to describe nature or reality.

Amber G. · Post by **Amber G.** » 20 Jun 2011 22:26

GP, Bade et al -

Story I heard/read about Dirac was that he had a conversation (or dreamed that he had a conversation) with an intelligent dolphin watching "bubbles" going up (while scrap from ships goes down)..now are the bubbles anti-particles or just void of water ... I don't know if the story is true, but it does make a nice story ..

Dirac did teach a first-year course in QM which me and hundreds of others (filling up a big lecture hall) took, one of the math grad student ask him if his function was really a function in mathematical sense.. the student got chewed out (Dirac was quite good in rigorous math too).. Dirac, known for his shyness and politeness, did get annoyed.

Some of his ideas did come out from watching people knit ( George Gamow tells a nice anecdote about it, how he was proud of solving a topological problem ).. and stuff like that.

ramana · Post by **ramana** » 20 Jun 2011 22:36

AmberG, What is QM? Quantum Mechanics?

Amber G. · Post by **Amber G.** » 20 Jun 2011 23:30

^^^ yes, QM= Quantum Mechanics..Dirac has written an introductory text book on it which he used for the course)..

One of the famous anecdote about Dirac written in many places (Just did a google and found at ( http://www.dirac.ch/PaulDirac.html - "Statement") actually happened in Delhi (Though the actual event, I was told, is little different than mentioned here)

Bade · Post by **Bade** » 21 Jun 2011 01:10

Amber G, please share your anecdotes of Paul Dirac here. I was aware that he did visit India, not sure if he was a regular at tifr etc. The first time I had heard of him was following his passing away and the Univ had a day long special remembrance and seminars in which we fresh UGs were encouraged to participate. The talks were interesting and curiosity led me to the library and found some pics which are shown in the link you provided above. Was into sketching figures those days and promptly did a render of Dirac and Bohr, since they both fascinated me. I was really surprised that I had not heard of him before though was aware of Einstein, Bose etc. QM was not known to me and was yet to be introduced at that time so that explained it, but the event did open my eyes to the world of physics.

Rahul M · Post by **Rahul M** » 21 Jun 2011 01:24

it is said that on the last day of a visit to TIFR, dirac was asked his impressions on the visit. the reply was supposedly, "the food was good, the weather was good".

GuruPrabhu · Post by **GuruPrabhu** » 21 Jun 2011 06:28

Bade wrote:GP, lots of physics discoveries were really not entirely inspired by experimentation and observation. Wasn't Planck's derivation of the blackbody spectrum using the hodge-podge idea of some harmonic oscillators that lead to quantum physics long before Einstein's photo-electric theory.

Yes, and Lorentz transforms existed before Einstein incorporated them into relativity. But then, relativity itself is pure insightful genius! In particle physics, Gellman predicted the Omega baryon, based purely on group theory, the SU(3) of flavor. Later on the Z boson was predicted based on neutral currents and SU(2) symmetry. There are many many examples. Lets see if the Higgs boson pans out.

There definitely is something quite fundamental about the use of mathematical tools to describe nature or reality.

Yes, and it is a confounding concept. Does the universe have laws that are above mathematics and math just lucks into discovering them? Or, are the laws of universe written using mathematics? If latter, then there is the uncomfortable question of WHO wrote the laws? Laws of physics are not like Darwin's evolution, or are they? Did the laws just evolve after the big bang? From an infinity of possible laws, nature may have just randomly picked a handful and evolved accordingly. The Anthropic principle hints at that and it has found some takers lately.

It is maddeningly beautiful and frustrating at the same time.

Bade · Post by **Bade** » 21 Jun 2011 20:55

In the end mathematics including all esoteric forms is all about counting numbers or equivalently counting units. So the fundamental question then becomes what is a unit, or what decides the granularity of a unit of anything. So for some scales a specific counting methodology works, at higher resolution scales a different counting scheme had to be applied (QM being an example) but not so radically different either except in form.

So far it looks like at scales that we can see or probe, mathematical models seem to work fine as "counting" makes sense. Will it break down elsewhere as we probe deeper if possible at all.

sanjeevpunj · Post by **sanjeevpunj** » 21 Jun 2011 21:49

Amber G. wrote:Anyway .. here is another problem --

We live in a quiet neighbor-hood but there is a highway not too far from us. In the morning (or day time) we don't hear any noise but in the evening (when we take a walk) we often hear traffic noise from the high-way?
What possible could be the reason? .. (Do others have similar experience? Sound from far away places, much clearer in the evening than in the day or morning..)

This is due to different temperatures at different times.Cold air in the morning dampens sound propogation.Warm air in the evening speeds up sound propogation, and temperature barriers can even cause refraction thus give a false notion about the location from where the sound comes.

GuruPrabhu · Post by **GuruPrabhu** » 22 Jun 2011 06:01

Bade wrote:In the end mathematics including all esoteric forms is all about counting numbers or equivalently counting units.

That is an insightful statement and it took me a while to figure out if I agree or not. At the outset, you are right that physics is a science of measurables and hence, all math that serves physics has to be a math of measurements, or counting, as you put it.

However, I would like to see how you fit critical phenomena into this envelope. Typically, math of measurements is monotonic, and that aides "counting" in the sense that it can predict counts even where no experimental measurements exist. But, what if there is a "phase change", i.e., a discontinuity? What is the math "counting" in this system?

To take it to an extreme, consider spontaneous symmetry breaking in particle physics or inflation in cosmology -- the math predicts phenomena that even it doesn't control. The evolution of these systems proceeds along paths that are inherently random.

Does it not make the math a bit more than a counting tool?

Bade · Post by **Bade** » 23 Jun 2011 22:52

Guruji, was using "counting" as the underlying concept of any mathematical equation. As far as I recall Spontaneous Symmetry Breaking is also based on run of the mill mathematics of choosing the nature or functional form of the "potential", so in essence within the realm of "counting".

Let me think more on what you are saying a bit more to see if I understood you correctly.

GuruPrabhu · Post by **GuruPrabhu** » 23 Jun 2011 23:58

The form of the Higgs potential is a simple polynomial: ax^2 + bx^4.

The problem is that the U(1) symmetry that is present for the case when x is complex is broken spontaneously. The choice of phase (for the breaking angle) is random. The math can not predict where that angle is.

rsingh · Post by **rsingh** » 24 Jun 2011 00:20

Oh hoe..........simple physics onlee please.

Earth rotates on its axis from West to East. How come we feel this only if we fly very high ie in Space shuttle or space station. We do not feel this when we are in Helicopter ?

GuruPrabhu · Post by **GuruPrabhu** » 24 Jun 2011 00:53

^^^ because the helicopter is captive in the atmosphere.

rsingh · Post by **rsingh** » 24 Jun 2011 01:20

GuruPrabhu wrote:^^^ because the helicopter is captive in the atmosphere.

Captive? How. Are you saying that atmosphere on sot "A" stays same? But Earth is rotating at a speed.......Is there a place on Earth where it is windy because of Earth' rotation and not because of temperature difference- Pressure difference.

Bade · Post by **Bade** » 24 Jun 2011 01:23

So now what is the basis of the choice of form of a polynomial for the potential. Will need to read up books in field theory again.

GuruPrabhu · Post by **GuruPrabhu** » 24 Jun 2011 02:20

rsingh wrote:
GuruPrabhu wrote:^^^ because the helicopter is captive in the atmosphere.
Captive? How. Are you saying that atmosphere on sot "A" stays same? But Earth is rotating at a speed.......Is there a place on Earth where it is windy because of Earth' rotation and not because of temperature difference- Pressure difference.

I have no idea what sot A means. Not sure what your question is.

Bade, the potential is arbitrary. Its only claim to fame is that "it works" - there is no basis for it. Same is true for the inflationary potential in big bang cosmology.

rsingh · Post by **rsingh** » 25 Jun 2011 01:58

^^^

Not sure what your question is

.
That is a statement

Rahul M · Post by **Rahul M** » 25 Jun 2011 02:09

channeling dirac are we ?

SriKumar · Post by **SriKumar** » 25 Jun 2011 18:30

GuruPrabhu wrote: Bade, the potential is arbitrary. Its only claim to fame is that "it works" - there is no basis for it. Same is true for the inflationary potential in big bang cosmology.

Exactly. Wouldn't this imply that the correlation between the potential equation and the physics it represents is arbitrary, and therefore incidental (or coincidental, if you will)? It just so happens that a particular form of an equation represents a particular physics i.e. ax^2 + bx^4 represents Higgs potential. In addition, (and please correct me if I am wrong) there are other potential fields which are represented, not by the above form, but by other forms of equations- the choice of the equation being arbitrary i.e. this form fits this potential, so let's use it. In the end, the fact that a form represents a particular physics does seem a bit arbitrary.

Nothing wrong with this though, especially if the form of the equation allows us to *mathematically* manipulate it into new forms which allow us to 'predict' physical phenomena not observed. I am not against mathematical formulations, I just want understand their limitations when applying them to nature. Like I said earlier, if I have a mathematical form to represent a particular engineering or physics problem, it is a great help, whether the solution is numerical or analytical.

By the way, thanks for elaborating the example of Dirac's approach in your previous post.