Superconductivity at room temperature??

[Ziggurat]

So am I. I've never heard of this solution, if indeed it exists, and as I said I'm not familiar with the details of classical radiation reaction---neither the historical development nor the present state of affairs. It's annoying that the author does not provide references, and I don't have time to go textbook-mining to figure out what he means.

If you've got Griffiths' "Intro to Electrodynamics", there's a bit about it (including references I haven't looked up) in section 11.2.2 (3rd edition) or 9.3.1 (2nd edition). Alternatively it's also mentioned in Jackson section 17.2 of the 2nd edition.

 

[Skwinty]

Here is another:

Particles being accelerated by magnetic fields.
OK, in a superconductor, this will only happen on the surface of the conductor.

This image shows three frames from a QuickTime animation (click the image to view) illustrating how magnetic waves in the Sun's outer atmosphere (corna) accelerate the solar wind. The corona is seen as a feathery yellow ring around the lunar disk during eclipse in the first image, the particles making up the solar wind (red and green) spiral around the magnetic field lines (white lines), accelerating away from the Sun. The spiraling solar wind particles take energy from the magnetic waves, canceling them out as the particles rush into space.
http://science.nasa.gov/science-news/science-at-nasa/1999/ast08jul99_2/

 

[ben m]

If you've got Griffiths' "Intro to Electrodynamics", there's a bit about it (including references I haven't looked up) in section 11.2.2 (3rd edition) or 9.3.1 (2nd edition). Alternatively it's also mentioned in Jackson section 17.2 of the 2nd edition.

Someday I will encounter an E&M problem that isn't discussed in Jackson. Maybe. Also: 2nd edition? The green one? You must be older than I am.

 

[ben m]

Here is another:

Sorry, Skwinty, the issue at hand is really just E&M, it's not something having to do with plasmas or electrophoresis. And in fact it turns out that the standard textbook (Jackson) discusses it in the same way as Mathpages.com does.

The runaway solution arises only when you overinterpret what turns out to be an approximate equation (16.9 in Jackson 3rd ed.) and does not appear in the correct perturbative treatment of the problem (eq. 16.10)

 

[Skwinty]

OK, I was concentrating on particles "surfing" on their own fields or gaining energy from a magnetic field.

Perhaps, in the case of electrophoresis and charged particles, the ratchet description, which to my mind, implies a boost of some sort.

During the day I have to deal with tangible things, but at night I like to read about things less obvious.

No need to be sorry, I can handle rejection.

ETA: If you have some spare time in the next few days, could you scan and post the relevant section of Jackson. Thanks.

 

[Ziggurat]

Someday I will encounter an E&M problem that isn't discussed in Jackson. Maybe. Also: 2nd edition? The green one? You must be older than I am.

The 2nd edition is the red one, the 1st edition is the green one. And I'm not that old, I got it not long before the 3rd edition came out, which was only in 1998.

Oh god, that was 12 years ago. I am getting old!

 

[Skwinty]

I found this article, it seems to me that the cat is loose amongst the pidgeons.

Waves up dudes, lets go surfing!



"Yet experiments led by SIMES researchers Makoto Hashimoto and Rui-Hua to suggest that the electron-pairing model does not describe what happens in the newer high-temperature superconductors.
"In 2006, our group published a paper suggesting that there are two types of distinct energy gaps," He said. "This more recent work provides a conclusive argument that there are two different mechanisms involved here."
The researchers trained the X-ray beam of SLAC's Stanford Synchrotron Radiation Lightsource on a high-temperature superconductor to reveal the material's electronic structure and explore the nature of the pseudogap. They were on a hunt for evidence of the electron pairing seen in conventional superconductors, in the form of what's called "electron-hole symmetry"; if it were present in the pseudogap, then the road to designing even higher temperature superconductors would be to make the pairs dance together instead of resting dormant.
But that's not what the researchers found; under the bright X-ray beam, the high-temperature superconductor showed a clear lack of the telltale symmetry—and thus of electron pairing. This suggests that the electron pairs were not lying dormant; they were simply not there.
The researchers posit that the electrons do not pair in this temperature range and instead travel in a wave; what they observed at SSRL were crests and troughs of electron density. The electrons' tendency to travel in a density wave may compete with their efforts to pair, suggesting that scientists will need a different approach in order to create a room-temperature superconductor.
"This is a very difficult problem, but an important one to solve," said Hashimoto. "We don't yet know the details of the density wave, but by extending our studies to different materials we are now seeking to understand it."


http://www.physorg.com/news189798601.html

ETA: link to publication http://www.nature.com/nphys/journal/v6/n6/abs/nphys1632.html

Come back Sol, all is forgiven. You did say all would be explained by symmetry breaking if I recall.

 

[johanfprins]

You have declared this to be so, but your reasoning is wrong. Charge carriers can and do accelerate without emitting radiation.

Not under the action of an electric-field.

The scattering is irrelevant to the question of whether or not accelerating charges must emit radiation.

It is not irrelevant since the charge-carriers can then dissipate more energy by scattering than by emitting radiation.

So is the superconductivity.

Not so, if the charge-carriers within a superconductor can be accelerated by an electric-field as you have all along claimed, they will and must emit EM radiation.[

I find that picked the simplest example to prove a point is a good thing. And it's not irrelevant. You made an absolute claim.

You are dishonest. We all know that we have been talking all along about acceleration by an electric-field!

Either the claim is true, or it is false. Since the claim is absolute, any counterexample will disprove it.

You know that we were arguing all along about acceleration under the action of an electric-field

I provided a counterexample, thereby disproving your claim.

A counterexample outside the topic we have been discussing.

There's nothing irrelevant or intellectually dishonest about that. That's how rigorous, logical debate proceeds.

It is irrelevant and intellectually dishonest!

Why should the superconductivity or the lack of scattering make any difference? It doesn't. In fact, an exact parallel to my example of a spinning ring exists for superconductors. Get a superconducting ring, apply a magnetic field to the ring, and you'll induce a current around the ring. The current is persistent and time-independent. The charge carriers in the superconducting ring are traveling around a circle. Circular motion 101: they're constantly accelerating.

If they did accelerate they would have emitted EM radiation. The fact that they do not emit radiation, proves that they are not generating the current by acceleration. This is in complete support with Onnes's original measurement which proves that when a current moves from one point to another within a superconductor, there is zero-voltage difference: And this is experimentally incontrovertible proof that the charge-carriers within a superconductor constitute a current without being accelerated.

Do they radiate? No, they do not. They could not, not without violating energy conservation.

Why would it violate energy conservation? They radiate since they are accelerated all the time. This work must be radiated away or else their speed will keep on increasing; it is you who are violating energy conservation.

You write as if scattering prevented charge carriers from radiating. But that's nonsense.

I did not write that at all. Obviously while they are accelerating they will radiate, but they dissipate more energy by scattering.

Furthermore, the exact same situation arises in non-scattering superconductors as well. And I've already pointed out the example to you. When a current passes through a superconducting wire with a bend in it, the charge carriers must accelerate to move around the bend. Just like with a copper wire, that acceleration is due to an electric field, which is created by a charge carrier density imbalance between the outside and inside edge of the bend.

If they can accelerate the material is not a superconductor.

I'm very much aware of the physics of rigid body motion. I'm also quite aware that rotation of rigid bodies involves acceleration of each part of that rigid body. That's why objects break apart if you spin them too fast.

Congratulations: I am impressed!

I claim that it doesn't MATTER why the charge distribution is accelerating, the fact remains that it accelerates. In fact, due to linearity, Maxwell's equations require that the cause of the acceleration be irrelevant. Whether or not a background electric field exists will make no difference to the field produced by the charge in question.

The experimental fact remains that when you accelerate charges by an electric-field they will emit EM radiation and that Onnes proved that within a superconductor charges move without being accelerated. You say they can be accelerated: I say that any material in which charge-carriers can be accelerated cannot be a superconductor.

But yes, you can construct an exactly equivalent scenario in which the acceleration of non-scattering charges is from an electric field.

I hope you are not again dishonest. I am talking about charges which are not mechanically bonded but are each accelerated by an electric-field.

 

[johanfprins]

Let me add a few clarifications to the accelerating-charges scenario. Maxwell's Equations, as we've said repeatedly, don't fundamentally care about acceleration. They care about time-varying EM fields. Nor do they care about the causes of accelerations: a charged pith ball, in (say) uniform circular motion, has certain EM fields that depend on q, r(t), and v(t)---nothing else. The fields don't care whether the pith ball is tied to a string, glued to a toy train, or hovering in a cyclotron.

Oh yes they do since this determines the boundary conditions under which you must solve Maxwell's wave equations. I know that in modern physics it has become the custom to ignore boundary conditions but should not be done if you model real physics.

Here's the scenario. Consider a cylindrical rope of uniform volumetric charge density d. The rope-material is incompressible, but deformable, so I can change its radius---envision a wire going through an extruder. I have a straight run of this rope stretched through the region from x=0 to x=L. It's moving, but at a non-constant velocity. When it enters the region, at x=0, the rope has radius r = R0 and is moving at a low velocity v = v0. It gets thinner as it goes along---and since it's incompressible that is only possible if its velocity increases. By the time it gets to x=L, the velocity has doubled to v = 2 v0 and the radius has decreased to r = R0/sqrt(2). At any point x in between, the rope's velocity is v = v0(1+ x/L) and its radius is r = R0 / sqrt(1 + x/L). We can make any assumption we like about how the rope behaves outside the box---does it loop back from x=L to x=0? Are we looking at the midpoint of an infinitely long rope? I don't care.)

I assume that this is a thought experiment? Well you have forgotten to apply the Lorentz transformations: So it is a worthless experiment which proves nothing. A pie-in-the sky experiment.

All of the charges in the rope are indeed accelerating---and accelerating in a straight line, just like (say) a particle beam in an accelerator. Let's look at the electric and magnetic fields of these accelerating charges.

So each part of the rope is an individual charge-carrier being accelerated by an electric-field?

The electric field is given by Coulomb's Law. At any point in space, the charge density is constant---in a quasi-cylindrical approximation where L >> R0, there's more charge per unit x on the left (thick rope) and less on the right (thin rope) but these densities do not vary in time.

Hu? you have a charge density moving relative to you which must according to the Lorentz transformation have a changing time coordinate along its length? And you claim that you have a constant time-independent charge? God help me!

The magnetic field is given by the Biot-Savart law. At any point in space, the current density is constant. The current densities are higher on the right (faster-moving charge) and lower on the left (slower-moving), but the net current I(x) is constant in x. There's a small radial component of the current, since charges have to migrate inward, but that component is also constant in time.

Constant E field? Check. Constant B field? Check. Solve Maxwell's Equations---what far-field EM fields match these boundary conditions? Looks like it's static in the far field also. That's a nonradiating linear accelerator, Johan. Run it backwards and it's a nonradiating decelerator.

Nonradiating. Let me say it again:

As I pointed out you are violating Maxwell's equations which are Lorentz invariant. And this means that along your rope the charges have different time coordinates: So it is impossible to observe a constant electric field and a constant magnetic field. Furthermore your example has nothing to do with charges being accelerated by an applied electric field. It is pure fictional nonsense.

There is no fundamental classical law saying that accelerating charges radiate. None whatsoever. It is common for simple accelerating charges to radiate, but according to Maxwell's Equations this common observation is not a general rule. If you learned otherwise, you learned wrongly.

So then accelerate an electron without it radiating EM. Stop talking nonsense.

 

[johanfprins]

Prins is saying everything that accelerates radiates, and the rest of us are saying, no, Maxwell's Equations tell us that some things radiate and other things do not.

That is a lie: I say that any charge-carrier which you accelerate by applying an external electric field to it will radiate EM.

I have nothing to contribute to posts #562 to #570. Greetings till next time.

 

[ben m]

Oh yes they do since this determines the boundary conditions under which you must solve Maxwell's wave equations. I know that in modern physics it has become the custom to ignore boundary conditions but should not be done if you model real physics.

I will go so far as to clarify, in case it wasn't obvious, that it does matters if there are movable charges, conductors, or dielectrics in the near zone---i.e. it matters if (say) the experiment is taking place inside a conducting sphere or next to a Jeff Koons sculpture or something. Other than that there is no boundary condition that matters.

I assume that this is a thought experiment? Well you have forgotten to apply the Lorentz transformations: So it is a worthless experiment which proves nothing. A pie-in-the sky experiment.

Hu? you have a charge density moving relative to you which must according to the Lorentz transformation have a changing time coordinate along its length? And you claim that you have a constant time-independent charge? God help me!

Johan, at this point it sounds like you're pulling details out of thin air and guessing that they're fatal to my example. You guessed wrong in this case again. Pick an inertial observer---one who agrees with my statement that v=v0(1 + x/L). You think this observer sees a time varying EM field?

Tell me---does this observer see an increasing E field, or a decreasing E field, at t=0? "Time-independent" means that dE/dt = 0 and dB/dt = 0; if you think they're nonzero, surely you can tell me at least the signs of the derivatives.

Furthermore your example has nothing to do with charges being accelerated by an applied electric field. It is pure fictional nonsense.

I could arrange it such that the acceleration was due to an electric field; it doesn't matter. Even for point-charge Larmor-formula radiation it doesn't matter.

It is pure fiction, but that's how thought experiments work. It could be built, and if it were built it would obey Maxwell's Equations.

 

[ben m]

Let me add (since I have Jackson out another reason) that you can confirm everything we've said for yourself. Do you have a copy of Jackson, "Electrodynamics", or a similar E&M textbook? Look for the Larmor formula---the justification, presumably, for the factoid you keep quoting that "accelerated charges radiate". Jackson derives this formula from Maxwell's Equations, it's not an axiom.

Nowhere in the derivation does he mention the cause of the acceleration---he doesn't need to. Jackson (3rd ed ch. 14) starts with an isolated point particle with an arbitrary velocity V(t) and position r(t). From these two quantities he can calculate the 4-vector potential A(x,t) (equivalent to calculating the E and B fields) and from the potential he calculates the radiation. Done. No approximations here, this is exact and relativistic right from step 1. None of your objections are visible.

In fact, Jackson prefaces the chapter with a few appropriate comments, emphasis mine:

"In chapter 9 we discussed examples of radiation by macroscopic time-varying charge and current densities, which are fundamentally charges in motion. But in one class of radiation phenomena the source is a moving point charge or a small number of such charges. In such problems it is useful to develop the formalism in a way that relates the radiation intensity and polarization directly to properties of the charge's trajectory and motion."

For the 9th time: "accelerated charges radiate" is a special case, encountered in the (very common) instance of a isolated accelerating point charges. It is not a general rule.

 

[johanfprins]

Yes, alpha, beta and neutrons are particles and not waves.

May I again plead with anybody on this forum to clearly define what "a particle" is? How is it possible that we have talked about "particles" for hundreds of years and even classified them as not being waves, while nobody ever defined what "a particle" is: And then we call ourselves physicists?

 

[johanfprins]

They are waves, just not electromagnetic waves; they obey a wave equation which is not Maxwell's Equations.

How do you know that the correct differential wave equation for an electron does not relate to Maxwell's equations? Does a light wave trapped within a single mass-wave (BEC) not have mass energy? After all it is stationary and not moving with a speed c. So is an electron wave not just a light wave which has inertia. Pair formation indicates that it must be so.

 

[johanfprins]

There is no physical distinction (though there are technological ones) between our ability to isolate a gamma ray and our ability to isolate a helium atom --- or a radio-frequency photon for that matter. Each of them is simultaneously a particle and a wave.

Why make a distinction between "particle" and "wave" if nobody can define what "a particle" is?

 

[ben m]

May I again plead with anybody on this forum to clearly define what "a particle" is? How is it possible that we have talked about "particles" for hundreds of years and even classified them as not being waves, while nobody ever defined what "a particle" is: And then we call ourselves physicists?

Um. There has been over 100 years of discussion of what wavefunctions mean and how they relate to particle and wave descriptions. The only thing I find unsatisfactory about it is the semantics. Have you studied that discussion in detail and found it unsatisfactory? How much detail?

(If you want to discuss this please start a new thread.)

 

[ben m]

How do you know that the correct differential wave equation for an electron does not relate to Maxwell's equations? Does a light wave trapped within a single mass-wave (BEC) not have mass energy? After all it is stationary and not moving with a speed c. So is an electron wave not just a light wave which has inertia. Pair formation indicates that it must be so.

Johan, are you just making this up as you go along? I can't count the number of crackpots who have come through here saying "electrons are just made of light". And at least one, I remember, said "pair production proves it". It's nonsense; electrons have two conserved quantities (angular momentum 1/2 and charge -e) which cannot be generated by putting photons (spin 1, charge 0) together in any combination.

If you want to reinvent quantum mechanics and particle physics, start a new thread. This thread is to discuss superconductivity, with an emergency detour to basic E&M.

 

[Skwinty]

May I again plead with anybody on this forum to clearly define what "a particle" is? How is it possible that we have talked about "particles" for hundreds of years and even classified them as not being waves, while nobody ever defined what "a particle" is: And then we call ourselves physicists?

The way I understood the difference was:

Sub atomic entities are called wave/particles with characteristics of both, depending on the experiment.

The wave component relates to probabilities whilst the particle component relates to a physical existence with a rest mass.

But then again, I am not a physicist.

 

[johanfprins]

keep saying that, but several of us have explained the relevance, as in post #557.

You might think so, but of course you admitted that you are a clown.

In Newtonian mechanics, motion around a ring always involves acceleration.

Congratulations, the first intelligent statement from you with which I can agree with. The movement of superconducting charge-carriers do not occur by means of Newtonian mechanics.

For lurkers: John Bardeen^WP is the only person to have received two Nobel Prizes in Physics. In his biography, True Genius: The Life and Science of John Bardeen, Lillian Hoddeson and Vicki Daitch wrote:

The very next paragraph of that biography alludes to the falling out between Bardeen and Shockley, and the paragraph after that begins the story of the research that won Bardeen his second Nobel Prize (for the BCS theory of superconduction).

Good: Just a pity that the BCS theory is plain wrong. I am sure that Schockley would have expected that. Not that I like the man. I would have liked Bardeen more; but that still does not mean that he did not make a massive booboo with the BCS model!

 

[ben m]

Congratulations, the first intelligent statement from you with which I can agree with. The movement of superconducting charge-carriers do not occur by means of Newtonian mechanics.

You've leapt back to superconductors without answering the simpler questions.

According to Newtonian physics (if even that), do you think that the charges on a uniformly charged spinning disk are accelerating? Yes or no.

According to Maxwell's Equations, do you think a uniformly-charged spinning disk should radiate? Yes or no.