1

Physics, Astronomy & Cosmology / Re: The China syndrome

« on: 30/01/2006 02:39:19 »

quote:Originally posted by DoctorBeaver

OK - I know the principle of controlled nuclear reactions & that the control rods are used to regulate the flow of particles. But why is it that in most, if not all, of the nuclear reactor crisis films I've seen, the rods have to be pushed in very slowly & carefully by hand? Why can't they just all be whacked right in to shut the reactor down? Or is this simply Hollywood licence?

I don't know what movies you've been watching, but in large, power nuclear reactors the control rods are not operated by hand. There are motors, and yes, normally the rods are moved very slowly in or out. The reason for the slowness, is that reactor critically is like balancing on a wire, so fast movements are not conducive to stable operation.

In the event of an emergency, the rods are forced in at a fast rate. This is called a SCRAM, and it's been the butt of many jokes in the nuclear industry. The origin of this word goes back to Enrico Fermi's first nuclear reactor. He stationed a man at the rope that was holding the safety rod out of the reactor. In the event that the reactor went into a runaway condition, he was to chop the rope with an axe so the safety rod could fall back into the reactor. Hence the name: Safety Control Rod Axe Man (SCRAM).

"F = ma, E = mc^2, and you can't push a string."

2

General Science / Re: Do Magnets Run Out ?

« on: 30/01/2006 02:28:40 »

The permanent magnetism is usually stable, unless disturbed. The disturbance can be through heating above the Curie temperature, mechanical shock, or becoming de-magnetized by application of an external magnetic field.

"F = ma, E = mc^2, and you can't push a string."

3

General Science / Re: Crap ice from plane can kill?

« on: 30/01/2006 02:23:51 »

The issue with water leaking from an airplane is not the same as frozen rain. Obviously, frozen rain does not kill, otherwise most of us would be dead already. It's also not hail, since that is a special case and forms only in thunderstorems with large updrafts.

The real danger is that water can leak from the dump valve in the airplane, and freeze as it is coming out of the pipe that is normally used to empty the privies on the ground. The slipstream can form this into an icicle, wit a very pointed end. Then it can break off and plumet to the ground. This would be called "blue ice" because of the blue color of the disinfectant in the water. There have been press reports of these icicles crashing to the ground, and weighing up to 70 pounds. It is not procedure to dump the blue water in flight, at least over land.

"F = ma, E = mc^2, and you can't push a string."

4

Physics, Astronomy & Cosmology / Re: What are electron shells, and why don't electrons fall into the nucleus

« on: 14/01/2006 04:25:11 »

Indeed, the classical approach cannot explain electron orbitals. However, I will try to give a very simple explanation.

The electron does not ever merge with the proton in the atom because it does not have the energy to do it. This process is called "inverse beta decay" and it takes a lot of energy, and a neutrino. The proton is composed of 2 up quarks and a down quark, and the neutron is 2 down quarks and a up. (Or vice-versa, I can never remember.) Anyway, inverse beta decay is the process of combining a proton, electron, neutrino, and some energy into a neutron. The quarks must change flavor to do this. The whole process will not occur just because electrons are attracted to protons, electrically. That fact satisfies charge conservation, but is only part of the process.

The orbital is a sort of resonance where each orbital level includes a quantum of energy. In addition, electrons are fermions, so they exclude each other in the atom, and always stay apart, in space and energy. In a broad-brush explanation this is why there are discrete orbitals.

"F = ma, E = mc^2, and you can't push a string."

5

Physics, Astronomy & Cosmology / Re: EM and gravity

« on: 14/01/2006 04:04:03 »

quote:Originally posted by Rincewind

Q1: We all know light can be affected by gravity.  Does this mean that a ray of light has a gravitational attraction on massive bodies?  

A1: Couching the answer in your original terms, the answer is "yes", but it is not measurable. Photons have mass, given by m=E/c*c, so they experience the same gravitational forces as massive objects, and also generate a gravitational field of their own, just like massive particles. However, the gravitational force is so weak, no one has ever been able to verify this attraction experimentally.

Q2: And does this in turn mean that heat (internal kinetic energy) increases a body's gravitational field, as well as relative velocity?

A2: Also, yes, but we can't measure it for the same reason as above. It would take more heat than we can produce at this time to give elementary particles enough energy to change their masses. We can measure the mass increases than come from high relative velocities in particle accelerators, however. These effects are well documented.
 
Q3: I was just wondering if the effect of various forms of energy (EM, heat, kinetic) on gravity has been tested much, and if anyone could point me in the direction of any resources, or even take a shot at explaining it in your own words.

A3: I don't have a reference available. Certainly any good atomic physics book will discuss relativistic mass change. However, as I said, the gravitational forces are too weak to measure.


"F = ma, E = mc^2, and you can't push a string."

6

Physics, Astronomy & Cosmology / Re: How big is the speed of gravity ?

« on: 14/01/2006 03:49:38 »

There is perpetual confusion on this point. It is easy to see why. The earth orbits the sun, about its actual position in the sky, which is not the position we see. The visible disk of the sun is 8 minutes behind its actual position. So it would appear that gravity "transmits" with infinite speed, while light transmits with speed of light. But gravity is not being "transmitted" from the sun, and received by the earth. Gravity is a curvature of space, and space is curved ahead of the earth in its orbit, just like the road outside you door is already there. The earth follows its path in space, curved by gravity into an orbital path.

Gravitons do not enter into the orbital motion of the earth. Gravitons are only generated when a gravitational energy change occurs, which happens when a gravitation quadrupole is generated. Photons can be generated from an electromagnetic dipole, but gravitons can not be generated from a gravitational dipole.

An example of a gravitational quadrupole would be a rotating dumbell. A cosmic example would be the collapse of a rapidly rotating neutron-star binary system into a black hole. Such an event, happening in our galaxy would be detectable by gravitational telescopes.

"F = ma, E = mc^2, and you can't push a string."

7

Physics, Astronomy & Cosmology / Re: Particle Spin

« on: 20/12/2005 12:42:39 »

Fundamental particles have duality properties. In some experiments, the spin of an electron appears to be about an axis, just like a tennis ball. In other experiments, it does not. This is similar to wave-particle duality, and electrons have that property too. Macroscopic analogies to fundamental properties are very tricky, and many phycisists don't use them because of that, so frequently good papers on QM lack this intuition.

"F = ma, E = mc^2, and you can't push a string."

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General Science / Re: Is that all?

« on: 20/12/2005 12:32:12 »

quote:Originally posted by DoctorBeaver

quote:And to add to Ian's answer, there are TWELVE notes in the musical scale, not eight

Only in Western music. Arabic, Indian etc have more

There are twelve notes in a tempered musical scale. Not all western scales are tempered, although that's the modern scale. The original western (Greek) scales were pentatonic, and if you singe acapella you are using one. But this wasn't supposed to be a music theory post. It's a good subject though.

"F = ma, E = mc^2, and you can't push a string."

9

General Science / Re: RATE OF FALL

« on: 20/12/2005 12:27:02 »

quote:Originally posted by ukmicky

OK i've decided to go back over what i learn't in physics class at school .
and in case ive forgotten something ive started back at the basic's so bare with me

Now Forgetting about air resistance. do two ojects of different weights dropped from the same height at the same time really fall at the same speed and hit the ground at the same time.

Doesn't the mass of the object that falling create it own gravitaional pull on the earth, meaning a large object will fall faster than a small object with less mass because the large object pulls the earth more with it own gravitational pull .
Obviously the difference would be tiny but is it so.



Michael                                      

The two subject objects will fall at exactly the same speed. This is true for the case where the falling object is large enough to disturb the larger object, such as the earth and the moon.

This may sound like the most amazing coincidence, and between the time of Newton and Einstein, it was just that. General Relativity combines the gravitational force of gravity with the inertial properties of matter into a unified treatment. The subject effect here is known as the equivalence principle.

"F = ma, E = mc^2, and you can't push a string."

10

General Science / Re: Lightbulb filament failure

« on: 03/12/2005 13:26:31 »

Those answers are both true. The cold resistance of a tungsten filament is about 1/10 its hot resistance, so there is a powerful surge of current at turn-on. This is called "inrush". It can also cause fuses in the branch circuit to fail. The inrush causes the filament to heat at a very high rate. The rapid change in temperature causes mechanical stresses in the filament from differential expansion. This is known as "thermal shock". Most of the time the filament breaks at one end, where the thermal shock stresses are the highest. The high current also causes mechanical stress from the Lorentz force on the conductors due to magnetic fields. In small bulbs, the former effect is most important. In large bulbs, the latter becomes important.

"F = ma, E = mc^2, and you can't push a string."

11

General Science / Re: Magnetic Water Softeners

« on: 26/11/2005 13:34:03 »

They make many claims, and magnetized water does have different properties than un-magnetized water, but, water has no remanent magnetic field, so those properties are forgotten after the water leaves the field. I don't believe that disolution of calcium or magnesium is affected by such a small magnetic field.

"F = ma, E = mc^2, and you can't push a string."

12

Physics, Astronomy & Cosmology / Re: Wavefunction...made simple please ?

« on: 10/11/2005 10:44:25 »

There have been many attempts to understand the wavefunction. Some get really weird, such as Feynemans sum-of-histories. My favorite is probably Schroedinger's, who was not comfortable with QM. This is really a classical explanation, trying to breach the gulf between our intuitive macro-world, and the pico-world of QM. This is simple: The relationship between the wave and the particle is the energy-mass relationship. The energy has a wavelength, and the particle has a mass. There is uncertainty about both, so the massive particle can't be localized exactly, and the energy wavelength can't be measured exactly either. The less massive the particle, and the less energetic the wavelength, the greater the uncertainty, and the larger the wavelength. Einstein's and Max Planck's equations fit this nicely.

Unfortunately, it isn't this simple, and you would have to try to understand more of the meaning, but this is a good point-of-departure for a classical guy.

"F = ma, E = mc^2, and you can't push a string."

13

Physics, Astronomy & Cosmology / Re: antiparticles

« on: 06/11/2005 12:35:04 »

Are you really referring to the reversibility of elmentary particle reactions? E. g., beta decay and inverse-beta decay.

"F = ma, E = mc^2, and you can't push a string."

14

Physics, Astronomy & Cosmology / Re: antiparticles

« on: 01/11/2005 00:06:29 »

quote:Originally posted by Dr. Praetoria

I believe that I read that possibly, anti-particles could be thought of has travelling "backwards in time" which would contradict Einstein's Special Relativity.

Anti-matter is only normal matter with opposite signs of charge. Time and space are the same.

"F = ma, E = mc^2, and you can't push a string."

15

Physics, Astronomy & Cosmology / Re: Quantum Entangled Photons and Encryption

« on: 30/10/2005 22:25:20 »

Wave-particle duality is the original shocking result of QM. The double-beam-splitter experiment you reference shows this shocking result. The detector at A gets all the photons because constructive interference of the wave functions gives an anti-node of amplitude there. This is analogous to the bright line in a classical interference pattern. At detector B we have a node of amplitude, or a dark space in the classical double-slit experiment.

The crazy part is that the interference is happening with a single photon. Feyneman though the photon took all possible paths, which he called a "sum of histories". So the photon splits at the mirror, and two entangled photons go on both paths. They recombine at the second mirror where their wave functions interfere. Any attempt to measure one of the entangled photons collapses the entanglement, and the photon is measured at only one place. The photon is always only measured at one place. It can only be in two places at once if we are not looking.

It is really weird.

"F = ma, E = mc^2, and you can't push a string."

16

Physics, Astronomy & Cosmology / Re: Quantum Entangled Photons and Encryption

« on: 29/10/2005 16:50:28 »

The entangled photons are oppositely polarized as they separate. Of course we are using mixed metaphors when we say that. The direction of spin is the proper term for a photon. Polarization is the matching property of the photon's wave function.

I cannot answer your question, however. As far as I understand the process, querying the spin of the photon requires its destruction. How this affects the entangled photon is unclear. It should result in a matching destruction if spin is to be conserved, but how would the matching photon be read at the other location if that happened?  This is intriguing, and I would like to study the details, assuming they are public.

"F = ma, E = mc^2, and you can't push a string."

17

Physics, Astronomy & Cosmology / Re: antiparticles

« on: 29/10/2005 16:39:53 »

quote:Originally posted by McQueen
... And since electron/positron pairs have  photons as both the causative and end factors in their existence ,  the propagation of electromagnetic radiation must be  closely involved with these “virtual” particles. But , and this is the question , how are they involved? Quantum mechanics ( or QED) gives a totally unsatisfactory answer to these questions. Am I right?

As I understand it, virtual particles are closely involved with the transmission of forces from electromagnetic radiation. The magnetic forces you can feel are transmitted by virtual photons. The electrostatic force is transmitted by virtual photons. Virtual photons cannot perform work, however. Real photons are required for that.

"F = ma, E = mc^2, and you can't push a string."

18

Physics, Astronomy & Cosmology / Re: Does ' Light ' wear out ?

« on: 29/10/2005 16:17:54 »

quote:Originally posted by itsjustme

light does not age because of time dilation, you will understand why if you understand einstein's special relativity theory. light years are just the measure of distance in space 1 light year means that light will take 1 year to travel from one point to the other. if you were traveling at the speed of light you would not age. at 99.95% the speed of light you can travel 1000 light years and come back and you would of only aged 10 years while the earth would of age a lot more. photons do not age but as light travels it loses amplitude.

"Loss of amplitude" is another way of saying that photons do interact with matter on their way across space. If a photon encounters a particle of matter equal-to-or-greater than its wavelength, it can be absorbed or scattered. Gasses also have an ability to absorb certain wavelengths of radiation, especially UV, which is how emission nebulae glow so beautifully in visible light. Without matter to affect its course, a photon of EM radiation will not age, and retains its identity throughout the age of the universe. This fact allows astronomers to look back in time, and reconstruct the story of the universe, all the way to the CBR at age ~300k years. Before that, the universe was so hot that it was ionized, and was opaque to radiation. So that is the limit for EM studies.

I wonder if there is a gravitational background that could be probed, even further.

"F = ma, E = mc^2, and you can't push a string."

19

Physics, Astronomy & Cosmology / Re: Newton

« on: 25/10/2005 23:39:46 »

Actually, the special theory of relativity was engendered by Maxwell's equations. When a moving magnetic field gives rise to a current in a conductor, the form of the equations is completely different than when a conductor moves through a stationary magnetic field. I remember encountering this, and immediately recognizing that the equations must be wrong. Of course they give the right answer, but they are not describing correctly what is happening. Einstein's contribution was in correcting the Maxwell's equations to give the right description for both cases.

Of course, Maxwell was following Newton's lead, because he used an absolute coordinate system. This gives the wrong equations, since only the relative movement needs description.

"F = ma, E = mc^2, and you can't push a string."

20

Physics, Astronomy & Cosmology / Re: Quantum Entangled Photons and Encryption

« on: 25/10/2005 17:40:25 »

quote:Originally posted by McQueen

I was browsing the web when I came across this article on “Quantum Encryption” published by New Scientist :
http://www.newscientist.com/article.ns?id=dn4914 (P.s If you see an empty space at the head of the page , scroll down. ) Can anyone tell me what is happening  ? Here is my take on things. Two quantum entangled photons are made through a process known as “parametric down conversion”. Since the photons are “entangled” they posses identical but opposite polarization. One of the photons is sent to a different location (B) , where its polarization is detected , the photon which remained at (A) must now have the opposite polarization. If the photon sent to (B) is interfered with , then the polarization of the photon at (A) will no longer correspond and it is immediately apparent that someone has tried to interfere with the transfer. But what does this achieve ? If the time and duration of the transfer is known together with the intervals at which the photons are sent , then even a child could break the encryption . So why not just use morse code instead , the only advantage is that with entangled photons , one would be aware of any attempts to interfere with the transfer process. Am I right in thinking this , or have I missed something. Further what does it prove about non-locality ?

The entangled photons are used only to transmit a cryptographic key to two separated locations. The value of the key is random, but it is known in two separated locations, and the transmission of the key was secure. Now this key is used in one location to encrypt a message. Then the message is sent conventionally to the other location, where the key is used to decrypt the message. This key is used only once. A new key is transmitted by entangled photons for the following message.

"F = ma, E = mc^2, and you can't push a string."