Physics Discussion Thread

Vayutuvan · Post by **Vayutuvan** » 13 Apr 2015 09:56

Simulations can be fun as well as wrong (IOW physically unrealizable)

Of course in this case there in perpetual motion notecen on the computer as the simulation itself requires moving charged particles from one place to another etc.

Neshant · Post by **Neshant** » 13 Apr 2015 13:37

Fractals - Hunting the Hidden Dimension

A Nova documentary on Fractals.

Something magical about them.

Neshant · Post by **Neshant** » 13 Apr 2015 13:44

I find it hard to understand how light can travel billions of light years without losing its energy.

Its a perpetual motion machine that violates the laws of physics.

negi · Post by **negi** » 13 Apr 2015 15:48

First for light to be emitted some energy has to be spent or matter to be consumed.

Second a machine by definition has to give some useful work as an output , a stream of photons by themselves do not yield any useful work if you make an apparatus to extract energy from light for example a solar film it siezes to be a perpetual machine because the incoming stream of photons are being emitted from a source which is either converting a different form of energy into light or consuming matter moment that source of energy is extinguished or matter is consumed the solar film will stop producing electricity.

A perpetual machine/system by definition has to be self sustaining it cannot take energy or input from outside the system.

Now I am not the right guy to articulate this appropriately but light fundamentally has dual nature to be specific it needs to be treated(treated as in for visualizing it ) as matter when it is emitted/absorbed but as a wave when it propagates through a medium .

When travelling through space the speed of light will be a constant , one could argue that it cannot be the case since light does bend near massive bodies like planets (as found during the solar eclipse) however if my understanding of fundas is correct then it is not that light bends due to gravity but it is the entire spacetime continuum which is actually distorted due to gravity and light merely follows these undulations (closest analogy I could think of is light travelling via a optical fiber cable which is bent ).

Now the stream of photons aka light will continue to travel unless obstructed by another medium it could be a cloud of intergalactic matter like Nebula or a planet/asteroid/comet , I do not understand how it is a perpetual machine.

Amber G. · Post by **Amber G.** » 13 Apr 2015 19:55

vayu tuvan wrote:
Of course in this case there in perpetual motion notecen on the computer as the simulation itself requires moving charged (eg electrons) particles from one place to another etc.

Of course. In olden days one looked for mechanical types, but now why not electric types..

One of the best I have seen... the perpetual motion 'machine' requires no more than a power strip..
Let me just present a picture..
(BTW, In my life, I have see quite a few demos in physics lectures(I have put some myself) for these kind of things.. some are real fun

and even when it is not April 1 could teach some physics. This includes a Baskara's wheel type machine (BTW extremely easy to build), wheel "actually" turning and students have to figure out where the energy was coming from.. )

Now before you reprimand me with "what kind of joke is this?"""... please do see a
simulation:

Better still the you-tube...
PLEASE DO WATCH IT...

Enjoy! (But don't try at home

)
***

Vayutuvan · Post by **Vayutuvan** » 14 Apr 2015 01:41

There is a youtube video from a Russian gentleman in which a bulb connected to a solar panel on which the light from the bulb falls generating electricity to light up the bulb.

But I suggest some body to experiment in a dark room (i.e. an enclosure which isolates the light bulb-solar panel from the rest of the environment and the experimenter)

. Of course, the experimenter cannot communicate with any thing outside the dark room.

ramana · Post by **ramana** » 15 Apr 2015 01:16

Something to learn from:

http://www.saha.ac.in/web/saha-memorial-past-lectures

member_22733 · Post by **member_22733** » 15 Apr 2015 01:52

In a entropy reversed universe, it would be impossible to make NON perpetual motion machines.

Bade · Post by **Bade** » 15 Apr 2015 01:59

He has made cool videos and is into popularization of physics...

https://www.facebook.com/Dr.Don.Lincoln

Amber G. · Post by **Amber G.** » 15 Apr 2015 02:07

^^^Speaking of Saha and other greats of that time..

Modi's gift to German Chancellor Merkel were Physics papers (reproductions of CV Raman )! Merkel is, after all, a physicist. (Her degree is in quantum chemistry). Saha's time, there was a very strong communication between German physicists and Indian Physicists..People like M. Born, Sommerfield visited/taught in India and influenced schools in India.

Merkel gave Modi, Max Muller's book about Ramakrishna..

Vayutuvan · Post by **Vayutuvan** » 15 Apr 2015 06:14

Physics popularization - lots of them have been doing it. There is a ceiling to what one can learn from "popular" science/mechanics/"scientific" American/123 .../Asimov's "science" books. I feel these are not too different from Dr. D. Chopra's Quantum states (?) etc.

Physics/Science/Biology popularization is a big fail (in my v.v.v. very humble opinion, of course) and a huge waste of valuable resources (mostly intangibles like the productive time of first class scientists who should be doing science rather than educating people about various conservation laws and futility of perpetual motion machines and such). Let people sink or swim after a good k-12 education I say. But the important point is to render a good balanced education sans biases - religious or otherwise till children grow up to be adults. After that they should be left to their own devices (physical or abstract).

vishvak · Post by **vishvak** » 15 Apr 2015 08:19

LokeshC wrote:In a entropy reversed universe, it would be impossible to make NON perpetual motion machines.

But there is no such universe!

member_22733 · Post by **member_22733** » 15 Apr 2015 09:05

Yep the direction of entropy in the universe we are in is upwards.

You can at best try to keep it from increasing (fully reversible systems), but the fact that nothing travels faster than light and that the universe is expanding makes it impossible to device any means of recovering fully any losses in a system in the form of EM radiations (thermal, visible, xray etc). So its futile to even consider building a perpetual motion machine from which no energy is extracted for useful work.

ramana · Post by **ramana** » 15 Apr 2015 09:08

VT, I am an engineer by education but have very strong physics and chemistry background which has helped me in my work.

Vayutuvan · Post by **Vayutuvan** » 15 Apr 2015 09:31

Ramana garu: Strong CPM background with good communication skills is a must - no argument there. But hyping one theory or the other on PBS/NPR (or worse on Discovery channel) is at best trivializes the hard work and at worst pushes some political point/agenda. I am sure you know from your personal experience that research is staid whichever way one looks at it. Popularizing it to make it look more exciting than it is self defeating.

Bade · Post by **Bade** » 15 Apr 2015 20:37

The actual day to day work while doing research is grinding. You are right vayu. The fun is in being able to stand away from the daily grind and looking at the big picture. Very few can indulge in that aspect. A few leaders in the field and tenured profs. Rest are working the salt mines so to speak.

Still do not underestimate the popularization aspect, as the funding for this indulgence comes out of taxpayer money and the fundamental sciences folks are well aware of this and hence the need to keep the aam junta informed, even if in diluted forms. Engineers do not have this problem, as funding source is the market for their products.

Vayutuvan · Post by **Vayutuvan** » 15 Apr 2015 21:36

Bade wrote:Still do not underestimate the popularization aspect, as the funding for this indulgence comes out of taxpayer money and the fundamental sciences folks are well aware of this and hence the need to keep the aam junta informed, even if in diluted forms. Engineers do not have this problem, as funding source is the market for their products.

Thanks for clarifying. I did miss that aspect. Ultimately though popularization needs to be done in a way which is not hype. Sometimes people cross the line. Cold fusion is a case in point. The story I heard was that they were pushed by the university admin. (was it Utah?) before confirmation. Another is the hype that was generated a couple of decades back for high TC malleable superconducting material. I read stories about a professor and his group where they were working round the clock with students sleeping on bunk beds in the lab. Or another where a researcher at a well known corporate research labs was able to get a lot of computing power online (by calling the top labs leadership in the middle of the night). Next day he dramatically whipped out a high TC super-conducting tape right at the beginning of his key-note lecture.

Vayutuvan · Post by **Vayutuvan** » 15 Apr 2015 23:50

The following is not specific to Physics. Nevertheless it is about conference paper publication and the publish-or-perish culture that is sapping the energy of researchers. A question that arises out of this is whether the world needs more and more researchers? Do we have lots of data from phenomenological and experimental observations which are gathering dust and has not been made use of in applications that would solve some of the world's and human kind's pressing problems?

Ten Reasons Why Conference Papers Should Be Abolished: Prof. Donald Geman, Johns Hopkins University

I will quote the article below (with a little bit of reformatting but no changes whatsoever to the text matter itself) in its entirety as this needs to be heard far and wide.

REASON #1: The Epidemic of Communication
Blink, and another proceedings appears. Most papers turn out to be early ”progress reports”,
quickly superseded, yet pitched as mature and significant. The right place to hear
about each others fresh ideas is in our offices, not in print.

REASON #2: Superficial Reviewing
Improvements have been made, especially in the most prestigious conferences. But it is
just unreasonable to suppose that people reviewing order ten papers for at least one conference
per year will apply the same effort as in reviewing a few journal papers. And it shows in
the difference in quality, evident to anybody who has processed many journal reviews (e.g.,
as a AE or Guest Editor of a Special Issue). Anyway, the process is doomed since there
simply aren’t enough experienced reviewers.

REASON #3: Journals Work
The tradition of peer-reviewed journal articles has served science well for hundreds of years.
Why change it? It allows for major revisions and second-round reviews, often markedly
improving papers. And single-blind reviewing helps detect incremental work. The argument
about turnaround time is naive (at least in our fields); there just aren’t that many ideas or
results that are so important as to warrant speedy publication. Besides, journals have sped
up the review process.

REASON #4: Noisy Personnel Decisions
Important career evaluations are increasingly based on the number of papers appearing
in certain conferences. Given the vagaries of the review process, and the sheer numbers of
papers individuals submit, this is adding considerable noise to the decision-making process.

REASON #5: Irrational Exuberance
Community-wide, do we really believe that every few months there are several hundred
advances worthy of our attention? How many good ideas does the typical researcher (or
anybody for that matter) have in a lifetime? A Poisson distribution with a small parameter?

REASON #6: Preferential Treatment
”Area Chairs” and other committees involved in the submission process are advantaged,
and there is a clear conflict-of-interest. Holed up in an airport hotel room to make final
decisions over a weekend, it is simply human nature to favor the ones you are face-to-face
with.

REASON #7: Limited Accountability
Another corrupting factor is excessive anonymity in the chain of communication. With
journals, there is two-way communication between reviewers and AE’s, and between AE’s
and authors. In particular, as an AE, my identity is known to the authors and I take responsibility
for my decisions. In contrast, Area Chairs are often anonymous; and Program
Chairs defer to them. In the end, nobody is explicitly accountable.

REASON #8: Poor Scholarship
Apparently, for many researchers doing a ”literature search” is now reduced to looking at the
Proceedings of a few conferences over the last few years. As a result, ideas are re-invented
and re-cycled and credit is randomly distributed. This is more likely to be corrected in
journal reviewing.

REASON #9: Diminished Real Productivity
Needless to say, ”productivity” should not be measured by the number of pages written.
But as far as I can see, our young colleagues are spending as much or more time writing up
their ”results”, and searching for ”minimum publishable units” and catchy names, as they
are thinking intensely and creatively.

REASON #10: The Fog of Progress
Are we making progress? Are we steadily (if slowly) building on solid foundations? It
is difficult to know. Given all the noise due to the sheer volume of papers, the signal, namely
the important, lasting stuff, is awfully difficult to detect.

=====================
At the FBI building in Washington D.C., there used to be a red light that blinked every
time a major crime was committed in the United States. As the story goes, a visiting child
once asked ”Why not just turn off the light?”. This works in our case. Researchers in other
disciplines assemble and hear about each other’s work, which is useful and fun, without
having refereed conference proceedings; a summary (e.g., abstracts) is enough and technical
reports provide a measure of protection.
2
My own (half-serious) suggestion is to limit everybody to twenty lifetime papers. That
way we would think twice about spending a chip. And we might actually understand what
others are doing - even be eager to hear about so-and-so’s #9 (or #19).
DISCLOSURE: Although my opinions were largely formed many years ago, I have nonetheless
submitted a few dozen papers to computer vision and other conferences over the past
twenty years, mostly at the instigation of students and collaborators. In view of current
practices, I can well understand their motivation.

Donald Geman
Johns Hopkins University
November 2007

Bade · Post by **Bade** » 16 Apr 2015 00:01

This publishing craze is due to the metric setup to measure academic research achievement. I agree with the opinion above and rarely publish. I can live with that in my current job, so it is a luxury. But there are people across the cubicles who get 2-3 papers out every year. Even peer review etc made sense when there were just a handful of active researchers in a given field and equally small number of publications for each discipline. It is an industry now, just like the JEE pass manufacturing cottage industry in Kota.

There are some interesting claims being made in cold fusion, by someone I had worked with in the past tangentially so to speak, though he had some seminal ideas for that field. I have not paid much attention to his cold fusion ideas yet. Saw a paper recently...again a conference proceeding only.

Vayutuvan · Post by **Vayutuvan** » 16 Apr 2015 00:16

A couple of decades back a joke was doing the rounds (when academic jobs in one area of research took a big hit due to breaking of the bubble) "Publish-or-perish" has now become "Publish-still-perish".

Vayutuvan · Post by **Vayutuvan** » 16 Apr 2015 00:23

But there are false negatives too. This from Nature

Coping with peer rejection

Accounts of rejected Nobel-winning discoveries highlight the conservatism in science. Despite their historical misjudgements, journal editors can help, but above all, visionaries will need sheer persistence.

Not many people spend tens of thousands of dollars to tell the world that they were robbed. But that is what Raymond Damadian and his company did last week when he discovered that he hadn't won the Nobel Prize in Physiology or Medicine, and complained in full-page advertisements in The New York Times and other prominent newspapers (see page 648). He claims in his advertisement that he should have shared the prize won by Paul Lauterbur and Peter Mansfield for their work on magnetic resonance imaging.
...
Regrets
What of the journals? Nature, while proud of its content over the years, has a confession to make about this year's medicine Nobels. Not so long ago, presciently pleased with having published Lauterbur's work, we celebrated it along with other Nature greats in a promotional campaign. Lauterbur politely wrote in to point out that we had published it only after he had appealed against a rejection.

OK back to work.

Bade · Post by **Bade** » 16 Apr 2015 06:47

http://coldfusionnow.org/current-scienc ... t-in-lenr/

http://coldfusionnow.org/special-lenr-i ... lable-now/

TSJones · Post by **TSJones** » 16 Apr 2015 08:02

Neshant wrote:I find it hard to understand how light can travel billions of light years without losing its energy.

Its a perpetual motion machine that violates the laws of physics.

Neshant, if I threw a golf club in outer space w/o any other force acting on the club other than the initial motion (energy) that I gave to to it, would that club run out of momentum (energy) and come to a complete stop? No it would not, unless another object acted upon it to block its initial path. So tell me, is that golf club a perpetual motion machine? I think some dumb Englishman wrote a law about this.....

To extrapolate, a photon is a wave that statistically acts like a particle, uh, sort of like a golf club so to speak. Will it ever stop? Just like the golf club I threw in outer space, it will continue on *forever* UNLESS SOME OTHER FORCE OR OBJECT ACTS UPON IT. Trust me and shake your head yes because that dumb Englishman has never been proven wrong on this particular subject for over 300 years now.

I wish I had taken physiks in college, but unfortunately, I wasn't smart enough. So I took business basket weaving and accounting. So all I know about physiks is what I was taught in high school.

Neshant · Post by **Neshant** » 16 Apr 2015 13:32

An external force IS acting upon light right here on the earth.

The external force is gravity. But that does not seem to slow light down at all here on earth. So the golf club analogy is meaningless because the golf club is slowed down but light is not. Light is a perpetual motion machine.

Neshant · Post by **Neshant** » 16 Apr 2015 13:34

negi wrote:Second a machine by definition has to give some useful work as an output , a stream of photons by themselves do not yield any useful work

It does useful work once it lands on a solar panel or a telescope imaging chip. The bizarre part is that it can transmit information with no errors for billions of years at an incredible speed without losing the data. Truly the ultimate hard drive.

Neshant · Post by **Neshant** » 16 Apr 2015 13:39

I don't know where to put this so I'm going to put this here.
India is collaborating with US, Canada, China and Japan in building a massive telescope in Hawaii.

-----
Massive telescope to help Canadian scientists solve mysteries of the Universe
By Andrew Fazekas

After some nerve-wracking months, many a Canadian astronomer’s dream is one step closer to reality. The Federal government has finally come through with $243.5 million in funding to ensure that our nation remains a partner in the building of the largest telescope the world has ever seen.

The Thirty Meter Telescope (TMT), truly promises to be a cosmic discovery machine. It represents an ambitious $1.4 billion project to build an optical instrument that is three times larger than the biggest observatory in existence today. It will have views that will be 10 to 100 times better than anything we have now—even outdoing the venerable Hubble Space Telescope. And this incredible power will allow us to literally peer into the most distant corners of the Universe and explore alien worlds like never before. The goal is to be able to shed light on some of the most fundamental workings of the cosmos

“TMT concentrates nine times more light into one ninth the area on a detector boosting brightness a factor of 81 compared to the best current telescope,” said Raymond Carlberg, an astronomer at the University of Toronto and the Canadian project director.

Read the rest here :
https://ca.news.yahoo.com/blogs/geekqui ... 48670.html

TSJones · Post by **TSJones** » 16 Apr 2015 15:04

Neshant wrote:An external force IS acting upon light right here on the earth.

The external force is gravity. But that does not seem to slow light down at all here on earth. So the golf club analogy is meaningless because the golf club is slowed down but light is not. Light is a perpetual motion machine.

Neshant, I can assure you that the curved space that earth inhabits (we call it gravity), does try to "grab" and affect the photon. But earth's gravity is so very weak compared to say a galaxy or a huge star, that there is very little effect upon the photon, almost to the point of nonexistence. However, the galaxy or a huge star's gravity is so great that astronomers can measure the bending of the photon. Astronomers call it "gravity lensing" and it is not only measurable, but sometimes very dramatic in effect such that astronomers must be careful in assessing "duplicate" objects (mirror image) created by the bending of the photons, In some cases, gravity (curved space) is so strong that light photons cannot escape its clutches, such as in "black holes".

Honestly, I think I cannot convince you otherwise so this will be my last message on this particular topic to you.

Never the less, Newton's First Law of Motion applies to any particle whether it is a photon (which is a statistical particle) or a golf club.

And truly, quantum mechanics gets even stranger and weirder than I can actually comprehend.

more info for your perusal and edification......

http://math.ucr.edu/home/baez/physics/P ... _mass.html

What is the mass of a photon?

This question falls into two parts:

Does the photon have mass? After all, it has energy and energy is equivalent to mass.

Photons are traditionally said to be massless. This is a figure of speech that physicists use to describe something about how a photon's particle-like properties are described by the language of special relativity.

The logic can be constructed in many ways, and the following is one such. Take an isolated system (called a "particle") and accelerate it to some velocity v (a vector). Newton defined the "momentum" p of this particle (also a vector), such that p behaves in a simple way when the particle is accelerated, or when it's involved in a collision. For this simple behaviour to hold, it turns out that p must be proportional to v. The proportionality constant is called the particle's "mass" m, so that p = mv.

In special relativity, it turns out that we are still able to define a particle's momentum p such that it behaves in well-defined ways that are an extension of the newtonian case. Although p and v still point in the same direction, it turns out that they are no longer proportional; the best we can do is relate them via the particle's "relativistic mass" mrel. Thus

p = mrelv .
When the particle is at rest, its relativistic mass has a minimum value called the "rest mass" mrest. The rest mass is always the same for the same type of particle. For example, all protons have identical rest masses, and so do all electrons, and so do all neutrons; these masses can be looked up in a table. As the particle is accelerated to ever higher speeds, its relativistic mass increases without limit.

It also turns out that in special relativity, we are able to define the concept of "energy" E, such that E has simple and well-defined properties just like those it has in newtonian mechanics. When a particle has been accelerated so that it has some momentum p (the length of the vector p) and relativistic mass mrel, then its energy E turns out to be given by

E = mrelc2 , and also E2 = p2c2 + m2restc4 . (1)
There are two interesting cases of this last equation:

If the particle is at rest, then p = 0, and E = mrestc2.
If we set the rest mass equal to zero (regardless of whether or not that's a reasonable thing to do), then E = pc.
In classical electromagnetic theory, light turns out to have energy E and momentum p, and these happen to be related by E = pc. Quantum mechanics introduces the idea that light can be viewed as a collection of "particles": photons. Even though these photons cannot be brought to rest, and so the idea of rest mass doesn't really apply to them, we can certainly bring these "particles" of light into the fold of equation (1) by just considering them to have no rest mass. That way, equation (1) gives the correct expression for light, E = pc, and no harm has been done. Equation (1) is now able to be applied to particles of matter and "particles" of light. It can now be used as a fully general equation, and that makes it very useful.

Neshant · Post by **Neshant** » 17 Apr 2015 06:42

Gravity does not affect light but rather the space-time continuum through which light travels creating a longer or shorter path. Light moves at a constant speed c and never changes. So you're wrong when you say gravity affects a photon.

If gravity did exert a force on a photon, then in theory a photon bouncing around 4 enclosed mirrors should eventually come to a stop here on earth. It does not. The photon can literally keep bouncing around those 4 mirrors forever. Doesn't that meet the definition of a perpetual motion machine.

Newtonian laws do not apply for quantum mechanics.

member_22733 · Post by **member_22733** » 17 Apr 2015 07:34

Gravity does not exert a force on the photon, you are right. However, gravity bends space time around the photon.

For ex: time passes slower near earth compared to free space, but the speed of light locally is always the same. Which means that the photon will take a "little longer" to go from point A to point B near a gravitationally massive object as compared to free space.

Now assume you are on the earth (point A), your friend is in outer space (point B ) and someone sits and turns the gravity strength knob (imaginary ofcourse) from somewhere else. You send a pulse up every 10 seconds.

As the strength increases, the person in outerspace notices that the interval between the pulses slowly increases. And also the light appears redder than it normally should be. i.e. light of a lower frequency is what he is getting, and that means light lost some energy somewhere (that is energy required to climb out of the gravitational energy well).

As the strength increases even more, your friend will soon see visible light go to IR range, then to radio, then to lower and lower frequencies. i.e. it takes more and more energy for the light to escape.

At black hole strength the light never escapes. What does that mean? It means the spacetime is so curved, that light keeps traveling upwards at a constant speed and YET never reaches your friend.

You can view this in another way (simplistic view though): Imagine there is an eddy in the fabric of space, which is an infinitely stretchable fabric. The rate at which the eddie sucks surrounding space (curvature of time) is a measure of gravity. At blackhole level gravity, the rate at which the space is sucked in by the eddy at any "point" in a "given time interval" stretches the space MORE than how far light can travel at same time interval.

Here is another way to look at it:
Light is an ant, space is a rope. Ant is traveling at constant speed (with respect to the rope) on the rope. If the rope is held stationary, ant will reach point A to point B in a constant time calculated by t = Dab/Vant (where Dab == distance between A and B and Vant is the local velocity of the ant.

If the rope is being stretched or pulled between A and B then the ant will take longer, however the velocity is still constant. If the rope is being stretched so fast that the distance between any point increase at a rate faster than Vab then the ant will never reach B. It will appear from B that the ant is falling towards A.

The other way to look at it is: Light falls in gravity

TSJones · Post by **TSJones** » 17 Apr 2015 07:42

Neshant wrote:Gravity does not affect light but rather the space-time continuum through which light travels creating a longer or shorter path. Light moves at a constant speed c and never changes. So you're wrong when you say gravity affects a photon.

If gravity did exert a force on a photon, then in theory a photon bouncing around 4 enclosed mirrors should eventually come to a stop here on earth. It does not. The photon can literally keep bouncing around those 4 mirrors forever. Doesn't that meet the definition of a perpetual motion machine.

Newtonian laws do not apply for quantum mechanics.

No, it cannot bounce forever, There is no such thing as a perfect mirror.

i will let another person explain it,

from the New Scientist blog:

If the cube was made out of perfect mirrors then yes, the light would bounce around forever. Unfortunately, mirrors are not perfect - some of the light that falls on them is absorbed. A domestic mirror reflects only about 80% of the light falling on it. If you stand between two large mirrors, set up so you can see the series of reflections, you find they soon get noticeably darker. Even a high-quality telescope mirror only reflects between 95 and 99% of the light.

The other factor to consider is the speed of light. In a 1-metre cube made of mirrors with 95% reflectivity, light would be reflected 300 times in a millionth of a second and lose 5% of its brightness each time, so reducing it to under a millionth of its original brightness.

Where does it go? As the light is absorbed it warms up the surface that absorbs it, so the cube would be ever so slightly warmer.

John Romer, Great Bookham, Surrey, UK

Yayavar · Post by **Yayavar** » 22 Apr 2015 09:10

TSJones ... A nit... It was not really the ENglishman but it was the Italian who first stated it and then the other competing Frenchman who formulated the law of inertia. Though all close enough to each other geographically foreshadowing the future EU

Amber G. · Post by **Amber G.** » 22 Apr 2015 20:49

An external force IS acting upon light right here on the earth.

The external force is gravity. But that does not seem to slow light down at all here on earth. So the golf club analogy is meaningless because the golf club is slowed down but light is not. Light is a perpetual motion machine.

Actually, if you ask any physicist to answer it carefully, it's (light/photon) velocity does change depending on how far it is from the gravitational body

But it is slightly complicated, and you NEED an exact definition (or understanding) of what you mean "slow down"..specially since "slowing down" for a golf club is "obvious" (can easily be measured even with primitive instrument like looking at it).. for a photon the change is much smaller..

Let me give an example/analogy...On earth when we say "up" we mean the DEFINITE direction..

A golf ball thrown "straight up" on the first hole, or the second hole goest in the "SAME" direction.. or so we assume in ordinary conversation, but "up" in USA is not in the same direction as "up" in Australia.. (because the earth is a sphere).

My point is, although strictly speaking, two exactly vertical walls in my room are not exactly parallel but it is difficult to measure...while a "vertical walls" 5000 miles away from each other are easy to see that they are not parallel.

In the same way, it is easy to "see/observe" change of velocity of a golf ball thrown from earth (with FAR greater mass) than a photon (much smaller mass/energy -- IMPORTANT a phone may have ZERO REST MASS but not we when we talk about effect due to gravity we are not talking about rest mass -- more of this later) ..

OTOH, if you take much more massive object than earth (say a black hole) the slowing down of "light" can be observed.

****

The things are much more complicated because one has to exactly define what you mean by "velocity" ( which is distance/time) .as both distance and time are not absolute values and depend on the frame of reference..

***

So, yes, in a far away frame of reference.. (Say you are sitting far away from earth) one would see change of velocity of light (and also a golf ball)... No, this is not just theory.. ALL observations are consistent with theory.

In case you are curious.. the theory gives.. .(Just use Schwarzschild metric, in short,

for a spherical body (using black hole, because it has large gravity, is nice one to use if you want to actually do an measurement) this results into a rather simple formula,,

Velocity of photon at the distance r from center = c (1 - r_s/r).

***

As said before, always remember:

"Bol bol bol... ..
Mister Gol Matol
Just how massive is a big black hole?

Half the radius
Divide by G
and Multiply by the
square of C ...."

Amber G. · Post by **Amber G.** » 22 Apr 2015 21:06

One has to be careful..

When one talks that photon is "mass less"... yes the rest mass of photon is zero. But momentum, energy etc are not. Energy of photon depends on the color (E= h nu).. (This is why when you ask a physicist "what's new", she might answer E/h

)

They are effected by Gravity. ( When one explains it by space-time curvature etc.. physics is NOT answering "WHY".. but just how it can be explained by a theory). By this, I mean, if one does an experiment one will see the effect (which is consistent with Einstein's theory)..

One has to be careful and precise in defining "time", "distance", "mass" etc. By this one has to understand the reference system.

Velocity of light in VACUUM is constant. In air (or water or glass) it is less than c

All things (even golf balls) follow laws of quantum physics.. it is just that golf balls are so massive (say than electron) that if you just use Newton's laws, the results will not be far off.. and in practice you need not worry about it's 'wave function" etc..

All things follow relativistic laws.. it is just that golf balls travels so slow (compared to photons) that you can use Newtons laws for all practical purposes..

HTH.

Amber G. · Post by **Amber G.** » 23 Apr 2015 03:06

TSJones wrote:
Neshant wrote:Gravity does not affect light but rather the space-time continuum through which light travels creating a longer or shorter path. Light moves at a constant speed c and never changes. So you're wrong when you say gravity affects a photon.

If gravity did exert a force on a photon, then in theory a photon bouncing around 4 enclosed mirrors should eventually come to a stop here on earth. It does not. The photon can literally keep bouncing around those 4 mirrors forever. Doesn't that meet the definition of a perpetual motion machine.

Newtonian laws do not apply for quantum mechanics.
No, it cannot bounce forever, There is no such thing as a perfect mirror.

i will let another person explain it,

...

[/quote]

Let me add ..

First, to be clear let us consider the following thought experiment.

Imagine a room with two opposite walls of perfect "mirrors" , parallel to each other. A particle (golf ball or photon) is shot exactly perpendicular from one wall to other.

In space, far away from earth etc, the particle will "bounce" between these two mirrors. This is true for both a golf ball (as long as it is perfectly elastic collision), or a photon.

On earth (or near a massive gravitational object), as every one will guess, the golf ball will not travel in straight line but in a curved one (parabola). Each point of contact will be a little "lower" than previous one, eventually it will fall on the floor, so to speak.

If it is light (or a photon), one may think that the light will travel in straight line, and will keep on bouncing between these two mirrors. But this is ONLY because, the "down-word" bending is so small that in practice you can not measure it with an ordinary room on earth.

Historically what confirmed the theory of relativity was precisely this "bending" when path of photons from a far away star was bent by the sun when the photons passed through the vicinity of the sun. This was done in 1919 by Eddington and Dyson among other scientists.

HTH..

(Unfortunately to explain it more - in more precise way, a little more math is required)

Added later: FWIW, I find TSJ's posts quite accurate/insightful... TSJ, if you have time you may enjoy the QUANTUM physics online course(S) given by IIT Chennai)

TSJones · Post by **TSJones** » 23 Apr 2015 06:34

^^^^

Thanks, Amber. Unfortunately, I am quite limited in my mathematical skills. I took college algebra and basic calculus due to graduation requirements but I did not get very good grades in them. I am afraid I was strictly a "C" student in math, unfortunately. However business statistics I did better in, because the professor was a sharp, enthusiastic guy who loved to talk about gambling and risk odds. now THAT was a subject I could get into as a student.

I do read Astronomy magazine every month and not just for the awesome Hubble pictures but also for the amazing astrophysics that are within the articles. Some of those people can really explain mysterious scientific concepts so that knuckleheads like me can understand it. They also print questions from people that write to the magazine asking about such things as photons, gravity and sub atomic particles. The people who give the answers are panelists for the magazine and have Phd's in Astronomy and Astrophysics. Really high power stuff for street mopes who ask sometimes inane questions and send them to the magazine All I can say is "Jai guru deva om".

And thanks to you Amber for taking the time to illuminate the issue leading to further and greater understanding. It makes for a much nicer experience on the forum (and illuminating) when gurus such as you take the time to explain a difficult subject matter to the relatively uneducated layman such as myself.

member_28663 · Post by **member_28663** » 23 Apr 2015 11:55

Someone on the forum (this thread?) recommended The Motion Mountain book.
Cannot seem to find the post but wanted to say thanks. Have been going through it and it has been an absolute pleasure so far.

SriKumar · Post by **SriKumar** » 24 Apr 2015 04:56

^^ that would be gakakkad.

http://forums.bharat-rakshak.com/viewto ... 5#p1759585

A great read..... though I barely started it. Maybe on a weekend or something

Amber G. · Post by **Amber G.** » 24 Apr 2015 22:07

TSJones,

Thanks.
It has been mentioned here many times, and you may already know this, but Muller's "Physics and Technology for future Presidents" is a nice book, you and other may enjoy. This was/is text-book for the most popular course in physics, given by Muller in Berkley. (Formally "Physics for poets" or such title..) and there are on-line videos/courses from Berkley. (Link: http://muller.lbl.gov/teaching/Physics10/PffP.html)
(Yes, it has chapters on Relativity, quantum physics, nuclear physics etc.. without requiring calculus etc..)

I advice all bright students, (and anyone who listens to me

), (even those who are not going to be scientists or engineers) to take as many good physics UG courses, they can take, specially if they are given by good professors. And if they can handle the math part, and the course is given by a good professor, take the variant "physics for physics majors", even if there are course for Engineers or pre-med or arts major.. because a good background in physics will be helpful in all other fields..

***

One further comment I want to make about the gravity and photons vs golf-balls. I do this, because others did not comment on it, and few may not realize this...

Also, just to bring some more clarity, to comment on comments like:

..Gravity does not exert a force on the photon, you are right. However, gravity bends space time around the photon

...any way..hope the following is helpful...

The bending of path of light due to gravity is/was predicted by Newton's laws too. In other words Newton's law (and may be just simple understanding of special relativity) will predict some bending of light from a star when the light ray passes near the sun. (In fact, Laplace, for example, just using classical physics did write about "dark star" which are mathematically similar to black holes).

(One way to look at it, is acceleration due to effect of gravity on particle near a large body, according to Newton's second law is = GM/(r^2), and it does not depend on mass of the particle -- To calculate the orbit/trajectory (once rockets stopped burning the fuel) of Mangalyaaan we, for all practical purposes, do not require the mass of Mangalyaan)

Interestingly the bending - or trajectory of a golf ball or a planet or a photon (with a very very small mass - remember "rest mass" of photon is zero, but photons are not "resting"

) near sun does NOT depend on the mass of the golf ball.. and so the path taken by a photon will not differ from a golf-ball...

What Einstein added was that the bending would be MORE (about twice in case of 1919 experiment) than predicted by Newton. Actually Einstein, in his book, Relativity (written for the general reader), does say that "half of this deflection is produced by the Newtonian field of attraction of the Sun, and the other half by the geometrical curvature of space" (Einstein predicted approximately 1.7 seconds of arc deflection (vs 0.8 sec per Newton's calculation) during that eclipse. The experiment confirmed that the results matched Einstein's prediction.

Also, now this is not just some "theory" or "things we talk about in abstract". The GPS system we use in daily life, (which depend on accurate clocks on satellites, and calculations performed on the GPS units, which depend on speed of photons near earth) have to take in account of the above phenomena.

Vayutuvan · Post by **Vayutuvan** » 25 Apr 2015 02:38

I would say in addition to physics courses, some applied mathematics will be very helpful. Then one doesn't have to depend on "descriptive" physics.

sanjaykumar · Post by **sanjaykumar** » 25 Apr 2015 03:45

Neshant, if I threw a golf club in outer space w/o any other force acting on the club other than the initial motion (energy) that I gave to to it, would that club run out of momentum (energy) and come to a complete stop? No it would not, unless another object acted upon it to block its initial path. So tell me, is that golf club a perpetual motion machine? I think some dumb Englishman wrote a law about this.....

It would seem that light travelling in the universe is not free of 'forces' acting upon it. Even observable matter at any distance will capture a photon in its space-time field. All one can say is that perhaps over light years, gravitational effects become isotropic.

Even though these photons cannot be brought to rest, and so the idea of rest mass doesn't really apply to them, we can certainly bring these "particles" of light into the fold of equation (1) by just considering them to have no rest mass.

What is the speed of light for an observer travelling in a frame of reference relative to the photon at c ? What is the mass for a photon, not at rest but at c? What is time for the photon?