Physics Discussion Thread

member_22872 · Post by **member_22872** » 04 Oct 2013 21:39

matrimc ji, too difficult for me to drop ji knowing that you are guru...I don't mind you referring me without a ji. Lasers are nice idea but don't they suffer from diffusion effects when traveling large distances? but it seems like good idea when stationed on Moon and for short voyages.

Theo ji may be they use onboard rockets to decelerate? or even lower the sail to decelerate?

Theo_Fidel · Post by **Theo_Fidel** » 04 Oct 2013 22:01

If you had rockets that big, why the sail? Lowering sail will only lower accelaration not velocity.

Amber G. · Post by **Amber G.** » 04 Oct 2013 22:03

venug wrote:matrimc garu, actually such spacecraft are designed. The sails are very huge to get an effective surface area when the photons hit the sails. So the concept is the spacecraft is maneuvered purely due to the forces these photons exert on the sails...but the draw back to such an idea is the velocities achieved by the spacecraft are not great, so you don't want such crafts where time is a factor. But are good for long space voyages where you want the spacecraft to sustain it's own flight. A..

Hi, I am sure you already know this (or get the answer doing your own calculations).. but a few points..

First, practical point about sails is that in vacuum and zero gravity - there is little draw back of having very large sails (No aerodynamic harmful effect due to shape, and little stress - no gravity - so you do not require the type requirements when you construct a large structure on earth.

The main thing about this is it really does not matter if the acceleration is tiny, if it is sustainable over a LONG period .. fuel does not run out.

In practical terms, an advanced spacecraft like voyager (or any other imaginable with current rocket technology), without a sail, will take about 50 0000 years to travel next 100 light years...
And it will still be traveling with about 60Km/sec.

With the sails (even with 1mm/sec^2 acc as you assumed) after 50,000 years, the craft will be traveling nearly with the speed of light and the trip of 100 light years may take about 200 years! ( and even much less in the time frame of the gaganaught

)

(Fairly simple calculation)

BTW, for all practical purposes, the speed achieved by crafts like voyager has not that much to with the power of rockets (or how much acceleration it may produce).. but timing when they are fired.. to take advantage of gravitational sling-shot effect. (I can explain it more, if people are interested)

member_22872 · Post by **member_22872** » 04 Oct 2013 22:05

no retrorockets need not be big(they generally aren't)...even for conventional space flight, spacecraft dynamics and controls and attitude control are achieved with retrorockets...they are not main thrust suppliers.

member_22872 · Post by **member_22872** » 04 Oct 2013 22:14

AmberG ji, that indeed is interesting, so light speeds are possible theoretically, but human flight may not be able to take advantage of. Secondly, with these velocities, no deceleration is possible with conventional means then. And do you know anything about pressure-Gravitational field transformation which Theo ji mentioned?

And yes gravity assisted sling-shot trajectories are planned for space flights to observe Sun for example, don't remember the name of the spacecarft, but they wanted to use the gravity of Saturn(?) to 'sling' the spacecraft into Sun's orbit...interesting stuff.

Vayutuvan · Post by **Vayutuvan** » 04 Oct 2013 22:41

venug wrote:Lasers are nice idea but don't they suffer from diffusion effects when traveling large distances? but it seems like good idea when stationed on Moon and for short voyages.

Not for short voyages - the idea proposed is for generational/data gathering ships. The laser is used to accelerate to some pre-determined velocity with the craft aimed at some nearby star or other and let go. I would certainly prefer unmanned craft launched this way for exploration. But lot of other tech need to be mastered - robotics, space construction, CCC. Most times funding is what will kill projects of this sort (even if feasible). If the per unit cost is low, one can live with two things - if certain number are lost due to collisions some will survive making the whole thing is worthwhile. If humankind solves the guns vs butter argument in a short time span such that all human beings are lifted up, then only we can start thinking of such projects.

Amber G. · Post by **Amber G.** » 04 Oct 2013 22:52

Quick note about slowing down of those space ships....

Yes, one would require massive amount of rocket or other power.. virtually similar to what is needed to accelerate ..

One is likely to use gravitational braking (similar to sling-shot I mentioned above - the principal is same - you save fuel and use this to accelerate (both positively and negatively) and/or redirect the path. assuming that you will have a planet or sun when you want to do this manuever

..

(BTW, this is how virtually all crafts like voyager got the lion's share of their speed - swing by Jupiter/saturn etc)

Also some have thought (and used it for landing on Mars, or Earth

) of aerodynamic braking ... let the spaceship swing by a planet like Jupiter - only close enough to go INSIDE its atmosphere

- even open a parachute - it will slow down with much smaller fuel cost - of course design has to be robust enough that people inside are shielded with immense heat produced etc..

Theo_Fidel · Post by **Theo_Fidel** » 04 Oct 2013 23:07

Venug,

There is no pressure-gravity transformation that I'm aware of. But pressure generates its own gravitational field. Like matter and energy. One component of the Suns gravitational field is its internal pressure. Even if quite small. The same is true of the Earth as well but of course much much smaller.
--------------------

WRT the large sails, the Russians had all kinds of difficulties with a small space reflector they launched some time back. The problem is transferring the tonnes of force generated by the enormous surface back through the same fragile structure to a central point load. Point loads are the bane of structural design even on earth and remain poorly understood. A moving point load supported by a gossamer thin fabric is simply unimaginable. An engineer would look at it and say it is a big deal. IMHO impossible with present understanding, techniques and materials. Maybe far in the future with remote robotic equipment. I'm talking 1000+ years at present technology improvement.

Smaller sails are feasible. NASA has a 32m one underdevelopment with a weight of 30 kg. So roughly 1 kg per sqm including the support structure. A 1 sqkm light sail would hit 1000 tonnes!

Vayutuvan · Post by **Vayutuvan** » 04 Oct 2013 23:25

If we think "unmanned" instead of "manned" - granted the former is not as sexy as the latter - lots of things will be possible sooner than later (relatively speaking, that is). Both AmberG's and Theo's objections are to do with "manned" craft - protect the humans against Gs developed when fast accel/deccel is involved and the need to have a central point load (which I presume is living quarters of the crew).

Theo_Fidel · Post by **Theo_Fidel** » 04 Oct 2013 23:50

Matrimc,

Even an unmanned would require substantial equipment. Even the Cassini probe weighed in at 5.5 tonnes. Any interstellar probe would weigh in at many multiples of that. I think any manned probe is simply out of the question.

member_22872 · Post by **member_22872** » 05 Oct 2013 00:07

Theoji thank you. Just thinking aloud...more like internal gas dynamics of a star or gas cloud leading to contraction and thus density increase. Density increase rises temperature, rise in temperature causes ionization, ionization brings into picture different subatomic particle interactions and dynamics. In the meanwhile at some point subatomic forces would become large enough to counter the contracting forces. Now based on the initial mass of the gas involved you either have a domination of contraction forces leading to star formation or sudden expulsion if the subatomic forces can overwhelm contracting forces more like super novae.

I think I misunderstood when you said pressure...but I now understand what context your used.

Theo_Fidel · Post by **Theo_Fidel** » 05 Oct 2013 01:25

No need for the Ji.

Yes, that is the oddity of Gravity. One of my professors used to say that gravity is the ultimate free lunch.

Vayutuvan · Post by **Vayutuvan** » 05 Oct 2013 02:47

Theo, there is no need for the probe to have all its electronics (and it is going to be mostly electronics) concentrated at a central point.

With near-field communication and a some kind of LAN. In that case would the numbers work out? What I am thinking is akin to modules which are sheet-like tiles with all of the electronics distributed over the surface of the tile. Several of these tiles can be snapped together along with some light weight struts. to hold things together in a mechanically stable structure. The snapping together can connect the electronics and power from solar panels (that is how we are going to get power for electronics), microfluid chemistry labs, camera sensors are all over the surface and so on. Lego mind storm like blocks so to speak.

Would this then drastically reduce (if not completely eliminate) the need to have a central (almost) point load?

SriKumar · Post by **SriKumar** » 05 Oct 2013 04:10

Theo_Fidel wrote: Srikumar, I don't know about work but potential energy has been converted to kinetic energy. So in space the box will definitely move.

Agreed. I did miss a point in what I originally thought, and thanks for catching it.
http://www.etomica.org/app/modules/site ... ound2.html
No question the box (in the link above) will move (if not fixed to anything). It will accelerate while the air is escaping (was aware of this earlier even if I did not explicitly state it). Once the pressure equalizes in both chambers, the box should start to decelerate, for the simple reason that the velocity of the escaping air has to come to zero after it reaches equilibrium in the second chamber. So the second chamber is essentially stopping the onrushing air, as a consequence decelerating the entire box. I originally thought that this action would decelerate the box to zero velocity, but I am not 100% sure about this.

Looking at it from an energy perspective: A simple equation for energy of a can under pressure is pressure*volume (gurus can correct me if this is wrong, or something major is missing). Kinetic energy is zero initially. The potential energy value, i.e. p*v remains unchanged before and after the air is allowed to escape into the second chamber. Because after the gas escapes into the second chamber, the gas volume is doubled but the pressure is halved, so p*v is still the same. By this count, the box should finally come to a halt since the potential energy of the system is unchanged (so, the KE ought to come down to zero). Not totally convinced....might be missing something. On a related note, this behavior should happen on earth as well, as long as the box rests on a frictionless surface.

Bade · Post by **Bade** » 05 Oct 2013 08:52

Theo, are you referring to Stress-Energy Tensor, when you say pressure and gravity being related ?

SaiK · Post by **SaiK** » 05 Oct 2013 09:23

I always struggle in patterns and picuturizations.. when they have examples x,y,z axis to represent 3D cartesian coords, and derive the matrix form.. how to understand the multi-dimensional examples and relate to the 3D analysis and keep it for my brain onlee pattern? pleaj help this moorkh

Bade · Post by **Bade** » 05 Oct 2013 09:48

Schaum series has some good books that deal with the math part. Vector calculus one is quite good. You will need to sit in a class at a univ if you have not taken those classes already in the past.

Vayutuvan · Post by **Vayutuvan** » 05 Oct 2013 10:02

SaiK, the only recourse is to algebra. Spaces like Hilbert space and Sobolev space (and Banach space) are infinite dimensional functional spaces. Since we live in 3D space, all we can visualize are 3D only because that is what we can see (one of our five physical senses). Since these spaces are un-visualizable it is better not even to try in spite of all the hoo-haa generated by the supercomputing community with their ray tracing and projections. It is a lost cause and a needless waste of resources - eye candy and marketing collateral if you ask me.

Theo_Fidel · Post by **Theo_Fidel** » 05 Oct 2013 11:55

Bade wrote:Theo, are you referring to Stress-Energy Tensor, when you say pressure and gravity being related ?

Yes! I think that is correct though I'm pretty much an illiterate in these matters.
Pressure is a form of energy in the equation right?

Yayavar · Post by **Yayavar** » 05 Oct 2013 21:32

I've not read any popular science books for a long time but one, even though very old, has stayed with me all these years: Heinz Pagel's "Perfect Symmetry". First read it in high-school but it did have enough of info to lay a good basis for future science readings. Probably time to read some more. Any suggestions? (Yes, hard science would be the right thing but have grown slack and only try to keep up with children's high school physics now

)

Bade · Post by **Bade** » 05 Oct 2013 22:08

Theo, the formalism of General Relativity is not easy even for a lot of physicists. Moreover it usually is only an elective even at Grad School level. So many do not take it. Pressure like force is a measurable quantity. Mass is a more fundamental quantity.

vishvak · Post by **vishvak** » 05 Oct 2013 22:11

If sun happens to pass hydrogen cloud that's free matter and more life on and on.

SriKumar · Post by **SriKumar** » 05 Oct 2013 22:18

johneeG wrote: Second, as soon as any opening is made in the chamber, the gas in the chamber will escape into the vacuum outside. This is called 'free expansion of gas'. A gas is not defined in vacuum. So, in vacuum gas does not exist.

A quick comment on the above where you mention (in prior posts) that 'free expansion of gas does not do work' (or something similar). In the examples that are shown on the web, the container in which the pressurized gas exists, is fixed. This condition is not explicitly stated but it is implicitly assumed. Here is a website that shows an alternative where 1 wall of the container (called piston) is allowed to move. Work is done in this case. http://web.mit.edu/16.unified/www/SPRIN ... ode32.html

I guess you'll say that the gas is doing work because it has a surface to push on? Well, there is a more detailed explanation I can get into if you wish but the bottomline is that if the box is not fixed, work will be done even if the gas expands into a vacuum. (Think Pascal's law, the pressurized, sealed box is already subject to numerous forces, it is not moving only because the forces are perfectly balanced).

By the way, I agree with some other things you wrote in the post....and quoting that below.

People reach understanding of things in the following manner:
a) observation (Prathyaksha)
b) inference (anumaana)
... A theory(or opinion) that best explains the available facts(observations) is generally accepted. If a better theory, comes up, then that new theory will be accepted.....But, there are several problems in the above approach:
First and foremost, not all the facts are verifiable for everyone by observing.
In such cases, people have to depend on others' words. Words can be either heard or written.

But, after a certain level, experiments are not repeatable for lay people...... In such cases, people would depend on those who claim to have done those things. So, their words have to be taken and believed. .......

The other thing one can do is to listen to the explanation and see if the explanations make sense from your perspective. Indeed one cannot read and understand all explanation for everything and that is a problem...not enough time for it. But I agree with the thought process above.

Theo_Fidel · Post by **Theo_Fidel** » 06 Oct 2013 04:33

matrimc wrote:Theo, there is no need for the probe to have all its electronics (and it is going to be mostly electronics) concentrated at a central point.

With near-field communication and a some kind of LAN. In that case would the numbers work out? What I am thinking is akin to modules which are sheet-like tiles with all of the electronics distributed over the surface of the tile. Several of these tiles can be snapped together along with some light weight struts. to hold things together in a mechanically stable structure. The snapping together can connect the electronics and power from solar panels (that is how we are going to get power for electronics), microfluid chemistry labs, camera sensors are all over the surface and so on. Lego mind storm like blocks so to speak.

Would this then drastically reduce (if not completely eliminate) the need to have a central (almost) point load?

Matrimc,

I was pondering this a bit more and yes it would definitely be one feasible idea. Some form of self assembling and self maintaining robotic technology. It is how ever very very far into the future. Many centuries away at least.

Here is a report from MIT on self assembling robots that look amazingly similar to what you were talking about. Undoubtedly you are aware of them. It is still very crude, needs a lot more development. Like centuries of development.

http://web.mit.edu/newsoffice/2013/simp ... -1004.html

Vayutuvan · Post by **Vayutuvan** » 06 Oct 2013 05:06

Theo, thanks. Will watch the video. Marvin Minsky (of MIT) is one of the first to come up with some ideas in that direction theoretically speaking.

Vayutuvan · Post by **Vayutuvan** » 06 Oct 2013 06:23

Viv

A recent book is A Clockwork universe. It is more historical than science account of royal society. I had to return midway as it was overdue and I was busy with work. Whatever I read was quite good.

An old book - more math than science - A history of pi.

Fermi Solution is also quite nice. My bro talks highly of it and I did read some interesting parts. But I am not that much into esoteric descriptive physics so I got bored a little.

Yayavar · Post by **Yayavar** » 06 Oct 2013 06:31

Thanks Matrimc. I read 'history of pi' and it adorns my bookshelf - had forgotten about it. Time to revist. Will lookup Clockwork universe - though that sounds like classical physics with the idea of God having set things in motion to be disrupted by the worthies of late 19th and early 20th century.

btw, the slingshot effect iirc was first elucidated by Arther C Clarke (and used in his 2001 Odyssey). I enjoyed his books a lot -esp Redezvous with Rama (and a few others).

Vayutuvan · Post by **Vayutuvan** » 06 Oct 2013 06:37

Theo, self assembly is far more difficult than what I am proposing. My proposal is to churn out these kind of tiles - to start with a dozen types or some absolute minimum with which the structures can be built - and assemble them manually on say moon and send them out. While that is going on, the factories on moon can be built - from these same parts and some specially constructed lights out factory-of-factories which are shipped from earth to moon - can start building self assembling parts. Requires some really advanced Group Technology - hardware and software plus manufacturing hardware and software. Centuries is an overestimate but 50-100 years something like this will happen. Hope biologists keep some of us old foots alive.

Vayutuvan · Post by **Vayutuvan** » 06 Oct 2013 06:40

Viv if you are looking for SF but more documentary style and so well researched it almost reads like and very close to science then look no further than mars trilogy by Kim Stanley Robinson. They are hard reading.

Vayutuvan · Post by **Vayutuvan** » 06 Oct 2013 06:43

"foots" should be coots or hands.

Yayavar · Post by **Yayavar** » 06 Oct 2013 08:00

matrimc wrote:Viv if you are looking for SF but more documentary style and so well researched it almost reads like and very close to science then look no further than mars trilogy by Kim Stanley Robinson. They are hard reading.

Was not looking for SF but this is good - thanks. Just downloaded digital copy from library and will read tonight - have always preferred hard science fiction, though enjoyed a few 'adventure' ones which were nothing but roman/greek history refurbished as on some remote planet.

Had mentioned the link to SF for the slingshot effect - from what I know Clarke proposed it first.

Amber G. · Post by **Amber G.** » 06 Oct 2013 22:06

Bade wrote:amberG, can you tell us who that Indian fella (teaching assistant) is in that picture with Feynman. Since it is from 1963, thought you may know.

I don't know the person. (Don't think anyone from IIT or DU) My first visit to Caltech was in late 60's..

(One thing I regretted when I first visited the place with nicer weather and beautiful beaches was not going there for graduate program, as I had a good fellowship there (slightly unusual in those days for a person fresh out from India) there but chose one of those "other" place in the east coast..

..

(Don't know why but even my sons (and DIL's) chose east cost for UG as well as the graduate programs.. All is not lost as one of the Son/DIL chose to work in the west coast (

spelling error corrected) now

'

Vayutuvan · Post by **Vayutuvan** » 06 Oct 2013 22:14

Amber G. wrote:[(Don't know why but even my sons (and DIL's) chose east cost for UG as well as the graduate programs.. All is not lost as one of the Son/DIL chose to work in the west cost now

AmberG, spelling mistake or a Freudian slip?

Amber G. · Post by **Amber G.** » 06 Oct 2013 22:53

^^

!
Anyway, for those who are interested (seriously I might add), in Freeman Dyson's sun sails and ground (or space based) Laser (or microwave).. a very good place to read about and find further resources, is from one of his interview.

Bridging the Gap: Part I

Going to another star is a terribly powerful idea, just as going to the Moon was originally. At some point in human history, there will be a leap across the great void not just to the nearest star but to any star that might be interesting to explore.

Renowned physicist, educator, and author Freeman Dyson joined Planetary Society Chairman of the Board Bruce Murray and Executive Director Louis Friedman at Society headquarters for an informal discussion about interstellar flight.

Their discussion dovetails to a proposal for sailing on solar wind. Nearly 400 years ago astronomer Johannes Kepler observed comet tails blown by a solar breeze and suggested that vessels might likewise navigate through space using appropriately fashioned sails. It is now widely recognized that sunlight does indeed produce a force which moves comet tails and a large, reflective sail could be a practical means of propelling a spacecraft. In fact, one concept explored by NASA centers is to develop an interstellar probe pushed along by sunlight reflected from an ultra-thin sail. Nearly half a kilometer wide, the delicate solar sail would be unfurled in space. Continuous pressure from sunlight would ultimately accelerate the craft to speeds about five times higher than possible with conventional rockets -- without requiring any fuel.

Read more in the original link or references provided in that link.
.

There was a famous, and quite readable, paper by FD (1968 ?) which dealt which this, but mainly with other idea of his - using atom bombs for the ship.

Note that it was not a science fiction, but real science, using technology available at that time!
(From what I know, FD actually took a year or so off from his regular work at Princeton, at the advise of Edward Taylor, to concentrate and refine the science and make it usable)

For those interested, a nice resource is a book written by his son George. (The book also has many items which, because they were classified at the time, FD could not talk about in open)

Here is the link to the book:
George Freeman book in google

Interestingly , believe it or not, the engineering part and prototyping came to halt mainly due to irrational fear of radiation, and clout some people have, not unlike the events we see at Kudankulam, or ban/resistance of nuclear power in Germany or Japan.. In any case, NPT put a halt to any idea of "exploding atom bombs" in space.

(Disclaimer: FD, at one time was my guru (also my guru's guru) and I have learnt a lot of physics and Math (some very pure math) from him directly and indirectly)

member_22872 · Post by **member_22872** » 07 Oct 2013 01:41

AmberG ji, thank you, would get the book.

Bade · Post by **Bade** » 08 Oct 2013 19:47

I protest the Noble not being given to CERN along with the two theorists, what good is a theory without experimental confirmation.

Amber G. · Post by **Amber G.** » 08 Oct 2013 20:19

Bade wrote:I protest the Noble not being given to CERN along with the two theorists, what good is a theory without experimental confirmation.

You may like this mildly amusing piece published in scientific american blog .. some time ago..

In surprise advance announcement, 2013 Nobel Prize in physics awarded to Higgs boson

Bade · Post by **Bade** » 08 Oct 2013 20:37

thanks

Is he related to the Jogalekar in Kanpur ?

Vayutuvan · Post by **Vayutuvan** » 08 Oct 2013 21:36

That sci-am blog article is nice. CERN would get
next year - possible? But in anycase I remember
Feynman's say from his book.

Bade · Post by **Bade** » 08 Oct 2013 22:10

Unlikely, that one more Nobel for Higgs for experimenters. Even the top and bottom quark discovery did not get a Nobel for any experimentalist.
From wiki,

Its existence (and that of the bottom quark) was postulated in 1973 by Makoto Kobayashi and Toshihide Maskawa to explain the observed CP violations in kaon decay,[4] and was discovered in 1995 by the CDF[5] and DØ[6] experiments at Fermilab. Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for the prediction of the top and bottom quark, which together form the third generation of quarks.[7]