Physics Discussion Thread

Sanjay M · Post by **Sanjay M** » 18 Jun 2010 09:10

New breakthrough allows liquid mirrors to do adaptive optics:

http://www.technologyreview.com/blog/arxiv/25342/

Bade · Post by **Bade** » 18 Jun 2010 18:32

I think they are building a telescope facility using this technique in the Himalayan foothills with a US partner.

Sanjay M · Post by **Sanjay M** » 19 Jun 2010 21:54

A neutrino detector named MINOS has revealed a discrepancy between neutrinos and anti-neutrinos, which may indicate a Charge Parity violation (violation of symmetry between matter and anti-matter):

http://physicsworld.com/cws/article/news/42957

If this is occurring, then what is the reason? Does it help to explain why our universe seems to favour matter over anti-matter? Does it allow us new opportunities to explore and exploit physics for our own ends?

Bade · Post by **Bade** » 20 Jun 2010 18:20

If it turns out to be true at greater than 3-sigma level in the near future, then it just shows that we do not really understand what we mean by "mass" or "space-time". It is the last frontier of physics fundamentals we have theorized but not properly understood perhaps.

Sanjay M · Post by **Sanjay M** » 21 Jun 2010 08:13

Furthermore, the MiniBooNE experiment has also shown a symmetry violation in the oscillation of anti-neutrinos as compared to neutrinos:

http://www.symmetrymagazine.org/breakin ... fferently/

If a chink the Standard Model's armour has been found, then I'm wondering what further measurements and observations can be done to wedge our way into it, and expand it further.
If the Standard Model doesn't apply to neutrinos, not only could this alter the fundamentals of our perception of the universe, but it may be that the Standard Model doesn't apply smoothly to other areas as well.

abhishek_sharma · Post by **abhishek_sharma** » 21 Jun 2010 11:37

This is probably old, but may be useful for younger members:

Book Review of

From Eros to Gaia
by Freeman Dyson
Pantheon/A Cornelia and Michael Bessie Book, 371 pp., $25.00

http://www.nybooks.com/articles/archive ... versifier/

Sanjay M · Post by **Sanjay M** » 23 Jun 2010 08:42

Upper Bound Posited for Neutrino Mass:

http://news.bbc.co.uk/2/hi/science_and_ ... 364160.stm

Sanjay M · Post by **Sanjay M** » 24 Jun 2010 07:14

Large 'Superstorm' Observed on Exoplanet

Sanjay M · Post by **Sanjay M** » 25 Jun 2010 06:35

Here is the clever technique used to examine this exo-planet:

http://www.nature.com/nature/journal/v4 ... 1017a.html

Sanjay M · Post by **Sanjay M** » 06 Jul 2010 10:23

Gravitomagnetic Analogs of Electrical Transformers

Amber G. · Post by **Amber G.** » 12 Jul 2010 10:12

A few stories in various papers about .... 'Scientists "totally surprised" by "significant shake-up."'
Proton Smaller Than Thought

Ramesh · Post by **Ramesh** » 12 Jul 2010 11:32

This forum is a real gem in BR.
Now, if people indulge me, I have two thoughts of which I would like your comments:
1. It should be possible to travel back in time. Lets take a candle. I see it because photons hit it & are reflected back, received by my eye. Some amount of photon will travel back and keep travelling for ever if not obstructed by anything. If i light the candle now, but travel faster than light and receive the photons from unlit time, i will see unlit candle. That means i have traveled back in time.

2. Over the years so many ships and submarines have been built in the world. Plus so much of mud has been deposited in the sea due to run off of soil in the rivers. How can we be so sure that the rise in sea level is not because of these reasons?

P.S: My grey matter is in the knees/ankles, so if you find the questions stupid, my apologies in advance.

Rahul M · Post by **Rahul M** » 12 Jul 2010 11:59

1. the photons travel back through space, not time. and it is precisely because you can't travel faster than light (blame a guy called Einstein) that time travel is considered impossible.

2. AFAIK we are not even sure that there is a monotonically increasing rise in sea level and not some long term variation. in fact sea levels have probably risen anyway since last ice age and will continue to do so till next one(assuming there is one) the factors you mention, especially the ships are however too small to change the depth appreciably. but truthfully, one should calculate before saying anything.

Ramesh · Post by **Ramesh** » 12 Jul 2010 12:32

Reg time travel sir:
That is what my humble submission is: I travel in space faster than light and catch up with the earlier photons reflected from the unlit candle, hence i see unlit candle. so i travel back in time

Hope this is not stretching it too far..

Rahul M · Post by **Rahul M** » 12 Jul 2010 12:35

that's the point, you or any massive matter cannot travel faster than light.

photons are massless and hence not governed by this restriction. for massive particle(s) to travel at light speed you need infinite energy.

why sir ?

Ramesh · Post by **Ramesh** » 12 Jul 2010 12:48

I remember reading about that problem, mass will increase as velocity approaches that of light. And you need infinite energy to propel this infinite mass..etc
But there must be some way around the problem.
Anyway sir, mine was just a hypothetical case

amit · Post by **amit** » 12 Jul 2010 12:53

If I may add my two cents on this time travel discussion.

You can't travel back or forward in time. However, you can look back in time. You do so every time you look up and see the stars.

If you are looking a star (through a telescope of course) which is, say 5 billion light years away, you are actually looking at how it was 5 billion years ago because that's the time it took for the light beam - photons - to travel from that star to Earth and hence on to your retina.

Now it's entirely possible that, say, 4 billion years ago, the star went Super Nova or got eaten up by a Black Hole (I know that's a bit too imaginative but, I hope you get the point I'm trying to make). What that means is that at this point in Time the star does not exist as the form that you are seeing through the telescope. You'll get to see the event 1 billion years later.

Or if, as we speak, the star blows up, you'll get to know of that event 5 billion years later.

So in effect when you see the star you are looking back in time.

Ramesh · Post by **Ramesh** » 12 Jul 2010 13:00

Amit sir agreed.
But i remember reading somewhere many years ago, about some particles which can supposedly travel faster light. I think tachyons or something. Let me google.

amit · Post by **amit** » 12 Jul 2010 13:03

Ashu wrote:I remember reading about that problem, mass will increase as velocity approaches that of light. And you need infinite energy to propel this infinite mass..etc
But there must be some way around the problem.
Anyway sir, mine was just a hypothetical case

Yes as you approach the speed of light your mass becomes the same as the universe, I don't think you'd fit inside it.

Regarding travelling faster than time, that's till now the releam of Science Fiction. Hyper space is the common favourite. However, I really like Carl Sagan's postulate in his book Contact. He says you don't travel anywhere but you mind does.

But also need to point out that many of the things/technology we take for granted today would have been considered in the releam of Sci-Fi less than 100 years ago.

So who knows?

amit · Post by **amit** » 12 Jul 2010 13:06

Ashu wrote:Amit sir agreed.
But i remember reading somewhere many years ago, about some particles which can supposedly travel faster light. I think tachyons or something. Let me google.

Here's an interesting write-up

chilarai · Post by **chilarai** » 12 Jul 2010 13:52

amit wrote:If I may add my two cents on this time travel discussion.

You can't travel back or forward in time. However, you can look back in time. You do so every time you look up and see the stars.

I think we can and are travelling forward in time. yesterday I was in yesterday and today i am in today. Just that we all are travelling in time at the same(more or less ) rate( speed ?? ).

or i am massively wrong.

amit · Post by **amit** » 12 Jul 2010 14:59

chilarai wrote:
amit wrote:If I may add my two cents on this time travel discussion.

You can't travel back or forward in time. However, you can look back in time. You do so every time you look up and see the stars.
I think we can and are travelling forward in time. yesterday I was in yesterday and today i am in today. Just that we all are travelling in time at the same(more or less ) rate( speed ?? ).

or i am massively wrong.

Yes we are all moving forward in time but we are doing at a fixed rate/speed (clock ticking). The question is can we do so at a faster rate than the fixed rate - only that would qualify as travelling into the future?

Only way to beat that, IMHO is time dilation.

Rahul M · Post by **Rahul M** » 12 Jul 2010 17:23

Ashu wrote:Amit sir agreed.
But i remember reading somewhere many years ago, about some particles which can supposedly travel faster light. I think tachyons or something. Let me google.

correct but their presence aren't proven yet and moreover they are always constrained to moving faster than light and can't slow down to less than light speed. they are creatures of a different world so to speak. as of now the restriction that disallows light speed travel is a fundamental one and there are no ways around it. some ways around have been proposed but all are unproven.
a simple way to understand is by a light cone, the physical massive objects are inside the cone (v<C) photons are on the cones (v=C) and tachyons are outside the cone. (v>C)

the observer in the pic is at 'here and now' and his physical surrounding is represented by the horizontal plane(2-d in stead of 3-d since we can't represent 4-d on graph).

amit saar, even in time dilation time dilates only WRT the outside observer, in the object's own frame he/she moves with same clock speed.

negi · Post by **negi** » 12 Jul 2010 19:37

Is Tachyon merely an entity arrived at by physicists based on some mathematical model ? For eg. now there is enough evidence from direct observations about existence of a black hole(or at least something like it) , question is do we have any such events to support the theory about existence of tachyons ?

Even as as a kid I used to get frustrated with rigorous mathematics involved in explaining physical phenomenon now in this case the wiki article indicates that physicists have decided to assign 'imaginary mass' (i crap

) to the tachyons in order to satisfy the 'energy momentum relation'.

From my limited understanding if an 'entity' like tachyon is required for existing mathematical models to hold good the same better be established by means of some experiment else doesn't it mean there is a possibility that our very fundamental model might be wrong/inadequate ?

Bade · Post by **Bade** » 12 Jul 2010 21:04

Existence of a mathematical model does not imply its validity in the physical world.

Amber G. · Post by **Amber G.** » 13 Jul 2010 00:55

Few comments:

WRT: particles moving faster than light (Tachyons) - Some pioneering work was done by EGC Sudarshan (he coined the term 'tachyon') - one may like see:
(Seven Science quest article)
http://www.ph.utexas.edu/fogs/sudarshan_tachyons.html
Or look up the wiki entry for Tachyon or (Sudershan)
(Theoretically in sting thoery Tachyonic fields have appeared etc)

BTW, the first Bilaniuk and Sudarshan's paper (around 1969) is fairly readable if one has taken basic physics courses.

Let me add here an old classic riddle: (If you have not heard before, try to think without looking up or asking a physicist) - Imagine a person with a laser pointer, producing a light spot on a wall (on a spherical wall, the person being at the center).. as he moves the laser pointer the light spot moves.... now as the distance between the wall and the laser pointer increases, the velocity of the light spot will increase too and one can easily make if move faster than 'c'.... how's that possible?

WRT:

Now, if people indulge me, I have two thoughts of which I would like your comments:
1. It should be possible to travel back in time. Lets take a candle. I see it because photons hit it & are reflected back, received by my eye. Some amount of photon will travel back and keep travelling for ever if not obstructed by anything. If i light the candle now, but travel faster than light and receive the photons from unlit time, i will see unlit candle. That means i have traveled back in time.

There was a famous limerick:

There was a young lady named Bright,
whose speed was much faster than light.
She set off one day
in a relative way,
and returned on the previous night.

(As far as we know, no experiment has found Ms Bright - or anyone else who could travel faster than light - anyone, that is, a physical entity with a real positive mass - or one which can carry information -)

WRT:

. Over the years so many ships and submarines have been built in the world. Plus so much of mud has been deposited in the sea due to run off of soil in the rivers. How can we be so sure that the rise in sea level is not because of these reasons?

How can you be sure, by using physics and doing some calculations .. surface area of earth's ocean (not difficult to estimate- we know how big the earth is and about 70% or its is water) is about 4*10^14 sq meter.. A ship of the size of Vikrant (displaced water volume is about 20,000 cubic meter) will raise the sea level by about 0.05 nano meter.. Super tankers could be about 10x of Vikrant but even a MILLION such tankers would raise the level .. oh about half a millimeter

With respect to mud/soil deposit - again it is not difficult to calculate .. in fact scientists have used that type of calculation/estimations for the age of earth (or how long rivers existed) by measuring the mud/soil deposits..

negi · Post by **negi** » 13 Jul 2010 08:04

Bade ji I just wanted to know the significance of Tachyons i.e. do they in anyway help us in understanding a given physical phenomenon or the space around us ? Long time back I had read somewhere about Dr. J V Narlikar's work in this area and iirc there was some reference to a black hole reducing in size after having absorbed tachyons.

AmberG as for the riddle if my understanding is correct , you hinting at the fact that spot will trace a longer path in a given time (hence the apparent increase in velocity ? ) if the person was to move farther away from the wall ? But won't the angular displacement and velocity will be constant ?

amit · Post by **amit** » 13 Jul 2010 08:33

Rahul M wrote:amit saar, even in time dilation time dilates only WRT the outside observer, in the object's own frame he/she moves with same clock speed.

Rahul ji,

True the person who is travelling moves with the same clock speed.

However, suppose if the person moves in a space ship with a constant acceleration of one "g" for 20 years and then sends a message back to earth, wouldn't that message arrive way into the Earth's future?

Here's an interesting link

Of course now we fast approaching the territory of Science (Fiction)! But it's a fascinating subject.

Bade · Post by **Bade** » 13 Jul 2010 08:53

I just wanted to know the significance of Tachyons i.e. do they in anyway help us in understanding a given physical phenomenon or the space around us ? Long time back I had read somewhere about Dr. J V Narlikar's work in this area and iirc there was some reference to a black hole reducing in size after having absorbed tachyons.

AmberG has already given a nice link earlier on Tachyon related works. Any idea even if unproved or to be seen yet with experimental proof, can help crystallize our ideas towards a better understanding of physics, even when there are open ended questions on the topic. That is why many have devoted time and have published works dating to more than 30 yrs back.

Sanjay M · Post by **Sanjay M** » 13 Jul 2010 10:31

Amber G. wrote:A few stories in various papers about .... 'Scientists "totally surprised" by "significant shake-up."'
Proton Smaller Than Thought

More on this:

http://www.nature.com/news/2010/100707/ ... 0.337.html

http://arstechnica.com/science/news/201 ... e-been.ars

http://engineering.library.cornell.edu/node/4510

So this will shake the foundations of physics - but what can we do with it?

Does this affect nuclear cross-sectional calculations for nuclear applications?

What cracks does this open up in the armor of basic physics that we can exploit? Any new opportunities from this?

Amber G. · Post by **Amber G.** » 14 Jul 2010 02:09

Also from today's NYtimes:
For a Proton, a Little Off the Top (or Side) Could Be Big Trouble
Excerpts:

For most of us, 4 percent off around the waist — a couple of belt notches — would be a great triumph.

Not so for the proton, the subatomic particle that anchors atoms and is the building block of all ordinary matter, of stars, planets and people. Physicists announced last week that a new experiment had shown that the proton is about 4 percent smaller than they thought.

Instead of celebration, however, the result has caused consternation. Such a big discrepancy, say the physicists, led by Randolf Pohl of the Max Planck Institute for Quantum Optics in Garching, Germany, could mean that the most accurate theory in the history of physics, quantum electrodynamics, which describes how light and matter interact, is in trouble.
....
Putting these techniques together gave an answer of about 0.8768 femtometer for the proton’s radius, just less than a quadrillionth of a meter. By comparison, a typical atom is about 100 trillionths of a meter.

Seeking more precision, Dr. Pohl and his colleagues created atoms in which the electron had been replaced by a muon, which is a sort of fat electron. Weighing about 200 times more than an electron, the muon circles its proton more closely and thus gives a better reading of the proton size. The surprise was an answer that was 4 percent smaller, 0.84184 femtometer.

When that new radius, which is 10 times more precise than previous values, was used to calculate the Rydberg constant, a venerable parameter in atomic theory, the answer was 4 percent away from the traditionally assumed value. This means there are now two contradicting values of the Rydberg constant, Dr. Pohl explained, which means there is either something wrong with the theory, quantum electrodynamics, or the experiment.

“They are completely stunned by this,” said Dr. Pohl of his colleagues. “They are working like mad. If there is a problem with quantum electrodynamics this will be an important step forward.”

FWIW:

So this will shake the foundations of physics -..

May be. At this stage, I believe first thing everyone is doing is double checking the calcs in these Lamb shift type calculations .. repeating the experiment and double checking if something is left out...

Does this affect nuclear cross-sectional calculations for nuclear applications?

Not really.. nuclear cross-sectional measurements are/have been obtained directly from experiments..
(Nuclear cross sections generally means different from 'physical' size, that is, 'effective area' for certain types of nuclear reactions/neutron capture etc..while 'size' of, say U nucleus could be around 15 fm, the 'scattering cross section' for thermal neutron would be around 10,000 (or more) times more than what simple size would predict.

(In any case, the 'diameter' of proton (or electron) looses the traditional meaning (aka as we talk about the case of solid structure in ordinary life)

Sanjay M · Post by **Sanjay M** » 24 Jul 2010 08:04

TED Talk on the Discovery of Hundreds of Earth-like Planets

Amber G. · Post by **Amber G.** » 24 Jul 2010 21:55

Few random articles/sites - for fun...
http://www.mtu.edu/news/files/ApplPhysL ... 233503.pdf
From Applied Physics Letters - How to make Invisibility cloak ...

Also
http://www.lindau-nobel.org/LecturesOnl ... iveID=1173

These are Lindau Noble meeting lectures (past and 2010 too). (This is a annual meeting where noble prize winners interact with best and brightest of young researchers)

Amber G. · Post by **Amber G.** » 27 Jul 2010 03:51

x post

Few Indian Newspapers have story about International Physics Olympiad held at Croatia this year: eg:
Mumbai girl wins gold at International Physics Olympiad
or
IIT topper wins Physics olympiad

Congrats to all. BTW for last few years, India has been faring well in IphO ... This year I think it stood sixth overall with One Gold, three silvers and a bronze!! (Don't know why but none of the Indian newspapers I have seen mentions this, in fact, apart from gold medal it does not even mention that India won 3 silvers!

Also Congrats to US team (1 Gold, 2 Silver, 2 Bronze).

Amber G. · Post by **Amber G.** » 27 Jul 2010 21:55

Just for fun ... - One problem from recent Olympiad - Mentioned here only because there is so much interest in BRF about nuclear fission...

(Using a simple model where nucleus is thought of rigid nucleons packed together)

Fission is a nuclear process in which a nucleus splits into smaller parts (lighter nuclei). Suppose that a nucleus with A nucleons splits into only two equal parts.

a) Calculate the total kinetic energy of the fission products kin E when the centers of two lighter nuclei are separated by the distance d = 2R( where R = (A/ 2)
is their radius. The large nucleus was initially at rest.
b) Assume that d = 2R and evaluate the expression for kin E obtained in part a) for A=
100, 150, 200 and 250 (express the results in units of MeV). Estimate the values of A for
which fission is possible in the model described above?

A here is number of nucleons - for Uranium bum - it is = 235..
((Using a simple model where nucleus is thought of rigid nucleons packed together, etc...))

ArmenT · Post by **ArmenT** » 28 Jul 2010 09:26

Posted on arXiv:
Indications of room-temperature superconductivity at a metal-PZT interface
Author: Dhruba Dasgupta, Department of Physics, University of North Bengal, Siliguri

Amber G. · Post by **Amber G.** » 29 Jul 2010 23:19

^^^ Most people I know take such claims (any claims for room-temperature super conductors) skeptically -- too many papers/claims which turned out too good to be true - but then one used to think that 30K (or say) was thought to be the the upper limit ... before copper-oxide type materials were discovered (in 1986 ?) .(Heck.. at present there is not even a perfect widely accepted theory which explains how higher temperature superconductors work..and thus designing such materials is quite difficult) .

In any case, the above paper looks interesting much more reliable ..From above ...

We report the observation of an exceptionally large room-temperature electrical conductivity in silver and aluminum layers deposited on a lead zirconate titanate (PZT) substrate. The surface resistance of the silver-coated samples also shows a sharp change near 313 K. The results are strongly suggestive of a superconductive interfacial layer, and have been interpreted in the framework of Bose-Einstein condensation of bipolarons as the suggested mechanism for high-temperature superconductivity in cuprates.
...
The samples used for current-voltage measurements were (i) thin strips 2 cm × 2 mm cut off from commercial PZT discs (supplied by Central Electronics Ltd., New Delhi, India) 0.3 mm thick and with an average grain size 1 μm which were supplied in the poled state and with 0.1 mm silver coating on both faces and (ii) the same type of strips with the original silver coating removed and 4000A aluminum deposited by vacuum evaporation. The Curie temperature of the material was 3600C as specified in the manufacturer’s data sheet.

Measurements were carried out at room temperature using a four-probe arrangement with the sample placed inside a double permalloy magnetic shield, the residual magnetic field inside the enclosure being less than 10^−5 tesla. The output voltage, which was of the order of microvolts, was measured using a home-built instrumentation amplifier based on an Analog Devices AD620 chip. Data were recorded in an Agilent 54622A digital storage oscilloscope by using a sawtooth current excitation at a frequency of 20 Hz from a function generator. It was found that scanning near this rate yielded the most consistent and reproducible data, least affected by fluctuations and noise.

The experimental results reported here strongly suggest the presence of a superconducting layer near room temperature in the interface between a metal film and a PZT substrate. The data have been interpreted in the framework of the above model in terms of the experimentally-observed inhomogeneous charge patterns in high-temperature superconductors.

The fact that the results described above have been obtained from very simply-fabricated systems, without the use of any sophisticated set-up and any special attention being given to crystal purity, atomic perfection, lattice matching, etc. suggests that the physical process is a universal one, involving only an interface between a metal and an insulator with a large low-frequency dielectric constant. We note in passing that PZT and the cuprates have similar (perovskite or perovskite-based) crystal structures. This resemblance may provide an added insight into the basic mechanism of high-temperature superconductivity.

Room temperature superconductivity has potential to be very useful - eg in magnetically levitated superfast train, much better MRI's , transmission lines, powerful supercomputers, etc etc

On another note, from todays NY times - (Story about Higgs boson not (!) being found...)
Physicists Closing in on the ‘God Particle’

Sanjay M · Post by **Sanjay M** » 05 Aug 2010 09:20

Dark matter may not be there after all - 'missing' matter may actually just be in the form of gas surrounding galaxy cores:

http://www.technologyreview.com/blog/arxiv/25527/

abhishek_sharma · Post by **abhishek_sharma** » 14 Aug 2010 05:56

abhishek_sharma wrote:E.C.G. Sudarshan shares Dirac Medal with Italian

http://www.hindu.com/2010/08/14/stories ... 032400.htm

Neshant · Post by **Neshant** » 14 Aug 2010 07:46

amit wrote: Yes we are all moving forward in time but we are doing at a fixed rate/speed (clock ticking). The question is can we do so at a faster rate than the fixed rate - only that would qualify as travelling into the future?

Only way to beat that, IMHO is time dilation.

Is this guy who's researching how to build a time machine talking any sense? Something about lasers going round and round dragging a frame of sub-atomic particles in a time warp or something. He's a prof at some university. Any physicist care to weigh in on the possibilities?

Also notable is the guy is an African American who's father died at age 33 and he had a difficult life. He rose from those humble circumstances to become a prof in physics which is quite an achievement. He has a book out about his life & research :

http://en.wikipedia.org/wiki/Ronald_Mallett