Physics Discussion Thread

SaiK · Post by **SaiK** » 28 Dec 2011 02:53

but, why does desi scientists change their names to spell like cow-sik!? are they hit by neutrinos?

Bade · Post by **Bade** » 28 Dec 2011 03:27

Aha, same question as why Iyer, Aiyer and Iyengar no ?

SaiK · Post by **SaiK** » 28 Dec 2011 03:43

sure, but there is a light year difference between Iyer<->Aiyer and Iyer<->Eye-her. To me cowsik is la eyeher. /OT. nice findings.

Amber G. · Post by **Amber G.** » 28 Dec 2011 10:29

Also - some may find here too ..some familiar names of (ex)brfites ...
Closing in on the ‘God particle’

lakshmikanth · Post by **lakshmikanth** » 03 Jan 2012 07:38

I have been doing some reading on the math behind General Relativity. I have been trained in grad school linear algebra and I find that I can understand it(which is what i am really confused about)

http://amzn.com/9810235550

I thought it would be uber complicated mumbo jumbo and never looked into it before until I found this book at a friends place, I started reading chapter 2 (chapt. 1 was already familiar), and the pages flew by. I am still a little shocked to realize that the math for GR was accessible to me all this while.

Bade · Post by **Bade** » 03 Jan 2012 21:37

Lakshmiji, you are lucky to have such intuitive understanding. I can admit that more than half the class who took GR classes in grad school remained clueless through the torture and remain so to this day, including moi. It requires a different mind to wade through all the mathematics and get a intuitive physical sense out of it. Special relativity on the other hand is a breeze to many not so gifted physicists.

gakakkad · Post by **gakakkad** » 03 Jan 2012 21:43

the topic of GR has been intuitively covered in the motion mountains text . that guy can sure teach physics .

SaiK · Post by **SaiK** » 03 Jan 2012 22:55

Nice to see many physics gurus here.. perhaps it is the right time to learn some basics from you guys. I am struggling to realize any dimension more than 3. It could be the fault of my teacher.. hence the quest. And my brain goes billions of blue blistering barnacles visualizing the 11 dimensions of string theory. Can these theories be narrated for lay man?

lakshmikanth · Post by **lakshmikanth** » 05 Jan 2012 08:40

^^gakakkad, thanks, will look into that.

SaiK saar,

Its very simble only. In three dimension a point in space (which can be a particle's position) is represented by 3 independent numbers (x,y,z). Ideally if you plotted (x,y,z) at each time instant you can quantify the motion of that particle.

Einstein figured out from Maxwells equations that time is non-local, i.e. every point in space has its own little timer and then used pythagoras theorem to prove special relativity. Then the dimensions became 4 independent numbers (x,y,z,t).

Then there was a dude called Kaluza, who figured out that if he added on more independent number, he could integrate electromagnetic theory of maxwell with that of Einstein. But this 5th number does not represent a "large" dimension. It is just an angle (theta). Its like every line you draw in space is a microscopic tube and energy can be anywhere on that tube.

So it became (x,y,z,t,theta). THis was initially rejected by Einstien but later he accepted it. And so began the march of the extra dimensions. People began to propose more and more of them to account for more and more physics.

For ex I could add one more angle beta to the numbers so it becomes (x,y,z,t,theta,beta). That is 6 dimensions right there. That would mean you would have to look at each line you draw in space to have a sphere at each point at each time.

and so on and so forth. Then they figured out exotic stuff like Calabi-Yau manifolds which might be the real geometry of every point in space. See wiki: http://en.wikipedia.org/wiki/Why_10_dim ... dimensions

In string theory , the idea is we live in a universe where only a specific value of theta is allowed, the rest can vary independently. This is called a brane, which has reduced dimentionality. String theory assumes energy is made of vibrating strings in these macro (4) and micro (6) dimensions and that for our universe many of the micro dimensions are fixed at some value, but the strings can come and go between them.

Ofcourse, two branes can collide, intersect and create complex interactions. I believe no one has a clue on how to imagine it other than write the equations and solve and then see what happens when they just consider (x,y,z,t) of this universe in the solution

Parellel universe can also exist on another brane (different fixed value of micro dimensions). Its all hifi math, it has yet to be proven. I am not sure if it will be accurate. But it is very interesting indeed

Klaus · Post by **Klaus** » 05 Jan 2012 19:12

Pentagon supported physicists state that they've devised a time cloak which manipulates light rays, so as to make an event undetectable.

The breakthrough exploits the fact that frequencies of light move at fractionally different speeds.

The so-called temporal cloak starts with a beam of green light that is passed down a fibre-optic cable.

The beam goes through a two-way lens that splits it into two frequencies - blueish light that travels relatively fast, and reddish light that is slower.

The tiny difference in speed is then accentuated by placing a transparent obstacle in front of the two beams.

Eventually a time gap opens up between the red and blue beams as they travel through the optical fibre.

The gap is tiny - just 50 picoseconds, or 50 millionths of a millionth of a second.

But it is just long enough to squeeze in a pulse of laser at a different frequency from the light passing through the system.

The red and blue light are then given the reverse treatment.

They go through another obstacle, which this time speeds up the red and slows down the blue, and come to a reverse lens that reconstitutes them as a single green light.

But the 40-picosecond burst of laser is not part of the flow of photons, and thus cannot be detected.

devesh · Post by **devesh** » 22 Jan 2012 03:24

regarding General Relativity, IMO, it is better to get a grounding in the physical concepts before delving into the Maths. one thing I've noticed with grad level GR texts is that they start out with differential geometry and then end up spending half the book on mathematical concepts without really relating it to physics. and then in the second half, they get to physics. IMO, math should be developed as required.

there are some books out there which give such a treatment. "Gravity" by James Hartle comes to mind. a good book which covers a lot of topics where GR plays a role. Physics is stressed, not the math. I liked this book as the first book for GR. once that is done, the holy grail: Wheeler & Co's, "Gravitation" can be studied or some other grad level text which is rigorous on mathematical methods.

lakshmikanth · Post by **lakshmikanth** » 22 Jan 2012 04:01

Devesh saar,

Thanks for the pointers.

I am more mathematically inclined. I get an itch sometimes to read up on some of the Grad school research that I used to do

, and one of them was on differentiable manifolds, which lead me to GR. I have very limited understanding of the physics involved in GR. I may or may not get there. But the math is not too far beyond me (from what I have seen so far).

Rahul M · Post by **Rahul M** » 22 Jan 2012 04:23

have you tried weinberg's book ? I would also suggest landau's book.

devesh · Post by **devesh** » 22 Jan 2012 09:11

lakshmikanth ji,
you are definitely unique then. one of my pet peeves has always been, spending copious amounts of time on mathematics that is seemingly "useless". I just can't get myself to show interest in any math topic if I can't see how it might be applicable or how it might aid me in investigating practical stuff. give me a book on biophysics and ask me to learn all the math needed to apply those concepts to the physics of biological organisms, and I'll gladly do it.

jmtp and all that....

negi · Post by **negi** » 22 Jan 2012 09:22

^ Same here; I could never appreciate mathematics without being first told or made to understand as to HTF the equation/expression in question relates to the real world. Maths is one subject where the subject chooses you rather than the other way round.

SaiK · Post by **SaiK** » 22 Jan 2012 09:27

are certain brains more oriented towards math than others? are all math brains automatically better brains for other field of study as well? what is the physics behind this brain affinity?

Bade · Post by **Bade** » 24 Jan 2012 01:03

Regarding GR for beginners one can try Ohanian's book on GR. It has a fairly nice introduction to GR based on physical principles. I have a copy but have not fully utilized its value. The graduate level course when I took it almost two decades back did not give me much free time, among other things that I had to do on the experimental side along with coursework. I do go back to this book often to jump-start my brain cells on the mathematical formalism, but it seems like a losing battle.

Ohananian was Wheeler's (the thick GR book author) student at Princeton. I have a copy of this one I picked up recently at a Borders which was on sale.

Amber G. · Post by **Amber G.** » 29 Feb 2012 06:45

Amber G. wrote:Nandu - Wrt to you message...
You may be interested in seeing this NYtimes article:
Particles Faster Than the Speed of Light? Not So Fast, Some Say

What you said was mentioned..I do get MIT's technical review, where it was discussed.. looks likely gut reaction was correct ... anyway

From NY times:

That paper got wide attention. ...

....The Opera collaborators and other outside physicists now say Dr. van Elburg’s analysis is wrong and reflects confusion about how GPS systems work.

Rumor (actually have talked with people in the know) has it that the error was finally traced to
loose cables! (Yeah ..loose cable (or just a few feet of extra cable) connecting the instrument and timing signal can introduce that error!)

Amber G. · Post by **Amber G.** » 29 Feb 2012 06:50

Followup on an old message..

Amber G. wrote:... <snip>

Few post above, I talked about a lecture I heard (about topological insulators) about Majorana fermions... well some say (no one knows for sure) that neutrinos may be Majorana fermions (in contrast to regular elections, protons etc which obey Fermi statistics and are fermi-fermions).

In any case, interesting times ahead .. Not too many think that neutrinos are faster than light (even the authors of the paper are skeptic) but we are sure to learn more about neutrinos (Sudarshan may get his Nobel)

Faster than light neutrino --- error it seems has been traced to loose cables ..
Also another exciting rumor..(and a lecture going to be presented in the latest APS conference).. is that some one has claimed to see Majorana fermions... if confirmed, exciting times ahead.

Bade · Post by **Bade** » 29 Feb 2012 23:08

Amber G. wrote:Rumor (actually have talked with people in the know) has it that the error was finally traced to loose cables! (Yeah ..loose cable (or just a few feet of extra cable) connecting the instrument and timing signal can introduce that error!)

That is what I suspected was always the cause. Though at ~~3ft/nsec~~ 3ns/ft travel time for signal in cables IIRC and a 60 nanosecond difference seen to account for that is ~~180ft~~ 20ft of extra cable somewhere in the signal circuit and should have caught someone's attention much early.

Edit: Getting old and cannot remember all the relevant numbers.

Bade · Post by **Bade** » 01 Mar 2012 00:21

Loose cable could explain 'faster-than-light' neutrinos

Further complicating matters, even the OPERA scientists couldn't yet explain why the neutrinos clocked in as fast as they did. Now, according to Science Insider, sources familiar with the OPERA experiment say a fiber optic cable connecting a GPS receiver and an electronic card in one of the lab computers was discovered to be loose. (The GPS was used to synchronize the start and arrival times of the neutrinos).

Tightening the connection changed the time it took for data to travel the length of the fiber by 60 nanoseconds. Because this data processing time was subtracted from the overall time-of-flight in the neutrino experiment, the correction may explain the seemingly early arrival of the neutrinos. {??}To confirm this hypothesis, the OPERA team will have to repeat their experiment with the fiber optic cable secured.

AmberG, now loose connections at I/O end implies impedance mismatch at the interface, which can cause signal echos aka reflections at the interface and should have been detected too earlier, no ? Were these echos being recorded as the true trigger with the observed/measured delays attributable from the actual length of the transiting cables ? Then the clues were there already in that the seeming speed up matches up with their signal cable lengths. Something is funny in this whole episode. Or maybe I do not understand what is going on entirely as the description available is spotty.

Bade · Post by **Bade** » 01 Mar 2012 01:56

Faster-than-light neutrino measurement has two possible errors

But according to a statement OPERA began circulating today, two possible problems have now been found with its set-up. As many physicists had speculated might be the case, both are related to the experiment’s pioneering use of Global Positioning System (GPS) signals to synchronize atomic clocks at each end of its neutrino beam. First, the passage of time on the clocks between the arrival of the synchronizing signal has to be interpolated, and OPERA now says that this may not have been done correctly. Second, there was a possible faulty connection between the GPS signal and the OPERA master clock.

An anonymously sourced account on Science Insider today broke the news that OPERA may have made a mistake. That report says that the faulty connection can account exactly for the 60-nanosecond effect. OPERA’s official statement stops short of that, saying instead that its two possible sources of error point in opposite directions and it is still working things out. Its statement reads, in full:

The OPERA Collaboration, by continuing its campaign of verifications on the neutrino velocity measurement, has identified two issues that could significantly affect the reported result. The first one is linked to the oscillator used to produce the events time-stamps in between the GPS synchronizations. The second point is related to the connection of the optical fiber bringing the external GPS signal to the OPERA master clock.

These two issues can modify the neutrino time of flight in opposite directions. While continuing our investigations, in order to unambiguously quantify the effect on the observed result, the Collaboration is looking forward to performing a new measurement of the neutrino velocity as soon as a new bunched beam will be available in 2012. An extensive report on the above mentioned verifications and results will be shortly made available to the scientific committees and agencies.

So there is more to be unearthed in the error domain. PPT giri MBA equivalent alphas (spokespeople) in research rushed to make claims.

Amber G. · Post by **Amber G.** » 01 Mar 2012 21:16

Bade wrote: So there is more to be unearthed in the error domain. PPT giri MBA equivalent alphas (spokespeople) in research rushed to make claims.

Bade, to be fair, the original CERN "claim" (my son was present at the famous presentation - and I have seen the PPT of that presentation) was quite guarded. In fact, it was repeated many times that the reason they were having large audience was to find an error quickly. They were very clear that the result is quite a suspect but did not know which part caused an error and wanted help from experts in other fields.

JMT

BTW, as you may know this is one of the hot topic (and conversation) in the present APS meeting in Boston. I have had a chance to talk with the people in the know. As you have posted here, there are still some questions but they (or at least the people I talked to) are fairly certain that loose cable was the main culprit.

BTW Interesting to note that not too long ago, the critical problem which crashed the CERN's billion dollar atom smasher (and delayed the opening), was due to poor solder joints in the electric cables of the super-conducting magnet

.

Amber G. · Post by **Amber G.** » 01 Mar 2012 21:30

Bade wrote:
AmberG, now loose connections at I/O end implies impedance mismatch at the interface, which can cause signal echos aka reflections at the interface and should have been detected too earlier, no ? Were these echos being recorded as the true trigger with the observed/measured delays attributable from the actual length of the transiting cables ? Then the clues were there already in that the seeming speed up matches up with their signal cable lengths. Something is funny in this whole episode. Or maybe I do not understand what is going on entirely as the description available is spotty.

As you say things are much more complicated... and the instruments are quite delicate..GPS clock synchronization, and distance (specially under ground without a direct light path is not available) measurement is not that easy. (One is looking at the accuracy of a few feet withing hundreds of miles). In addition to determine when the neutrino was born makes it challenging ..

Here is the link to original paper and methodology...
http://static.arxiv.org/pdf/1109.4897.pdf

Amber G. · Post by **Amber G.** » 01 Mar 2012 21:34

Another big rumor, as said before, is claim by some from Netherlands of finding Majorana Fermion (A fermion which is its own anti-particle)
According to one top US expert (with whom I talked informally) the claim seems credible..

( One interesting tidbit that such fermions in superconductors can become building blocks of topological quantum computer... For those who do not know, .. Wiki may give some more info)

^^^ Added later .. did not think it will make main-stream news but it did, per google:
Peaceable matter-antimatter pairs glimpsed in the lab

On 27 February, Leo Kouwenhoven of the Delft University of Technology in the Netherlands and colleagues produced tentative evidence for such a twosome. Their Majorana particles are not free agents of the sort that might wander into a particle detector on their own, but collective excitations of electrons and "hole" states – absences of electrons – within nanoscale wires made of the semiconductor indium antimonide.

Bade · Post by **Bade** » 01 Mar 2012 21:40

BTW, as you may know this is one of the hot topic (and conversation) in the present APS meeting in Boston.

AmberG, I have been out of physics for a long time now and regret that, but that is history. Thanks for posting that link and I am not always up to date on the happenings within even HEP. I have friends who are lucky to remain in the field, only when I converse with them do I get to know inside info at times. Some of them include people whom you might know but much younger. Alok_N was one of them but much senior to me by a decade at least.

In the not very old days any suspicious results always received much scrutiny from within the collaboration before being made public. I remember the days/months preceding even the TOP quark discovery. Rumors abounded much before the claims were made. But, it was revealed only after both DO and CDF saw the similar results.

Amber G. · Post by **Amber G.** » 01 Mar 2012 21:56

Just realized, I did talk about Majoran Fermions here in the Physics dhaga ... worth reading again..
http://forums.bharat-rakshak.com/viewto ... a#p1161303

Bade · Post by **Bade** » 02 Mar 2012 00:45

This is quite readable for the lay public at least the first few pages and relates to the topic of nature of the neutrino. The Indian angle and a parallel experimental effort at INO facility in the making, thought would be of interest to any jingo.

http://arxiv.org/pdf/0803.4387v1.pdf

harbans · Post by **harbans** » 02 Mar 2012 01:07

What sort of energy and forces are released during a rope snap? What percentage energy is kinetic, how does it dissipate..any resources on snapped rope dynamics available? Working out from Fxd=.5mv2 and change in momentum/ time gives me drastically different results. It's also hard to calculate impact time and the snap velocities on impact surfaces. Any inputs/ resources i could refer to for these?

Rahul M · Post by **Rahul M** » 02 Mar 2012 01:18

bade saar , thanks for that, very lucidly written.

harbans ji, could you be a little more particular ? is it a rope snapping under its own weight ? or something else ?
anyway, this might interest you.
http://theeyegame.com/speleo/Pubs/rlenergy/Default.htm

harbans · Post by **harbans** » 02 Mar 2012 02:19

Thanks Rahul ji, i was referring to ropes snapping under tension. There are some You Tube videos of ropes being tested under load conditions but they don't obviously detail how energy dissipation takes place. On one site i read a snapped synthetic rope can have a velocity of around 200 m /s. All parts of the rope cannot move with that velocity. I wanted to know if there are resources that provide the physics/ dynamics of what exactly happens in the process.

Bade · Post by **Bade** » 02 Mar 2012 03:33

Physics trivia ! The travails of the Katrin Spectrometer through Karlruhe.
http://youtu.be/dmmVb779NP4

Amber G. · Post by **Amber G.** » 02 Mar 2012 06:16

harbans wrote:What sort of energy and forces are released during a rope snap? What percentage energy is kinetic, how does it dissipate..any resources on snapped rope dynamics available? Working out from Fxd=.5mv2 and change in momentum/ time gives me drastically different results. It's also hard to calculate impact time and the snap velocities on impact surfaces. Any inputs/ resources i could refer to for these?

Well for a physicist, the answer is, of course, EM (electromagnetic force) between rope molecules..

In practical terms, it depends on the type of rope, and is quite complicated to do it from the first principles... Generally one makes models..

One of the best place to look for is to search google scholar under mountaineering or Spe1eology..

For example:
Physics for Ropes, Loads, and Energy
or
Snapping of ropes

harbans · Post by **harbans** » 02 Mar 2012 06:32

Well for a physicist, the answer is, of course, EM (electromagnetic force) between rope molecules..

Amber G that's interesting. What percent of total energy dissipation say for a synthetic fibre and Wire rope would comprise Electromagnetic force between molecules? Any studies that have been made so far on that would help. Thanks.

Bade · Post by **Bade** » 02 Mar 2012 10:39

Harbans, the answer was provided already by AmberG. It is 100% EM. The next available force is orders of magnitude away.

In a ironic way, it is easier to answer the fundamental questions in physics than actual applied ones. We know all about radiation that one needs to know, yet it is extremely difficult to make a first principles calculation of all atmospheric contributions to what is observed by a satellite flying high above. This gets very acute when one is dealing with signals from the ocean and the errors that result are embarrassingly large even to this day. Only recourse in such situations is dependency on some functional models validated empirically.

harbans · Post by **harbans** » 02 Mar 2012 13:20

Harbans, the answer was provided already by AmberG. It is 100% EM. The next available force is orders of magnitude away.

Bade Ji, a rope snapping under very high tension and breaking loads also recoils violently. So there is a Kinetic energy component that is imparted as well. Heat and sound would also comprise a very small fraction of the energy dissipated. How could it be 100% then..

For example, this below is a Canadian Navy tug rope breakage under tension:

http://www.youtube.com/watch?v=bVsAX_i9Lis

This is a breaking load test in a test bed:

http://www.youtube.com/watch?v=_EilnYjVEdk

This below is an Aircraft Carrier accident video as the arrester ropes part on a Hornet landing..(this has a large bow effect component)

http://www.youtube.com/watch?v=uUs0hv9a ... re=related

I understand there are various factors like rope strand, material etc. But am trying to find literature that detail studies on how the dissipation takes place. How also does EM dissipation of a broken stretched rope take place exactly. I would assume you and Amber G imply that the energy stored before breakage in the Test bed video is mostly dissipated by EM with a very small percent translating to kinetic, heat and sound?

Stretch a spring for example and release it, the energy dissipation is Kinetic mostly. Same with a stretched rope under tension, i would assume dissipation on quick release of the tension would result in massive KE component..

Rahul M · Post by **Rahul M** » 02 Mar 2012 14:14

harbans ji, there are 2 different physical concepts here, force and energy. it would help not to confuse the two.

first is force. the rope is together because its constituent atoms are joined to each other because of EM force (simply put their electron fields overlap and won't be pulled apart without application of external force.) external force may take the form of 2 teams playing tug of war or mother earth pulling a load connected to an end of the rope towards herself with grav force.
but the thing is held together by EM force and nothing but EM force. 400% !
for example the floor you walk on is mostly vacuum, with tiny nuclei in the middle of the atoms surrounded by electron clouds with nothing in between. so why don't you fall through it as you are pulled towards the earth's grav centre ? because the electron clouds in the atom of your feet repel the electron clouds on the atoms of the floor (to put it simply)

the effect of external force on the rope is same, when you do work on the rope (specifically ON the point of breaking) equal or greater than the energy required to bring the rope atoms at the junction of breakage together, the atoms come apart and the rope snaps.

so what happens to the work you did on the rope ? some of it is absorbed is separating the atoms of the rope from being bound together. but you also worked on the rest of the rope that did not break ! depending on the springiness of the rope, it either goes back to its original shape, releasing some thermal energy* in the process or stays elongated with no change in energy. in real life it will be a combination of the two.
sound energy can be created by the snapped ropes going back to their original shapes and disturbing air molecules in the process.

there is another energy we have so far ignored. the rope obviously has some gravitational pot. energy by virtue of its position. as it snaps this will be changed to KE and then thermal energy and sound energy (KE of air molecules) as it hits the ground.

* thermal energy - best to think of it as agitation of the atoms. more the agitation, more the temperature.

harbans · Post by **harbans** » 02 Mar 2012 15:19

so what happens to the work you did on the rope ? some of it is absorbed is separating the atoms of the rope from being bound together. but you also worked on the rest of the rope that did not break ! depending on the springiness of the rope, it either goes back to its original shape, releasing some thermal energy* in the process or stays elongated with no change in energy. in real life it will be a combination of the two.

Rahul Ji, the rope stretches when external work is done on it. The energy stored would be half the stretch times the tension. Now if it just snaps that energy has to be released. While going back to it's original shape there is a recoil factor which i agree will vary with the material of the rope, strand structure etc. Residual elongation after stretch also is a small component. That is why rope manufacturers i see recommend renewing ropes after a number of load cycles. I am trying to get studies/ papers on a break up of energy dissipation into different components after a snap takes place.

What i am trying to understand is once you tear away the EM forces by stretching beyond the break point, how does EM become a major component of the dissipative force in the 2 halves after snap. Even if we take the spring analogy, when we release it, the dissipation is mostly KE..how then does the EM component come in as a major factor when the rope snaps?

Bade · Post by **Bade** » 02 Mar 2012 22:12

^^^ The EM forces are at work when the rope is together till the breaking point as you increase the tension. What happens after the break to each individual part of the rope (the lashing etc) has to be modeled using wave propagation. The problem then becomes effectively just the equivalent of saying if you jerked a free rope (with no tension in it) at one end held by a fixture with the same force required to break the tense one, how would the free rope behave. Then it has nothing to do EM per se, except that individual molecules within the free rope are still held together by EM forces as always.

harbans · Post by **harbans** » 02 Mar 2012 23:05

Exactly. After the break/ snap EM forces don't matter. How does a rope react and dissipate the energy held just prior to breaking. That is what i wanted to know some resources about. Recoil velocity calculations for example, how do we do that without knowing how energy is dissipated. Say a 20 m straight rope snaps midway at 10 meters. What would be the recoil. What would be the impacting force on say an object midway a a meter away. Some analysis on this is what i was looking for.