Ummmm... the way you stated that is less than precise.
For your second question:
E = m c^2 was not confirmed by Mercury's orbit. Deviations in Mercury's orbit from Newtonian predictions provided supporting evidence for Einstein's theory of general relativity.
E = m c^2 is not even correct (E = m c^2 for you only if the particle is at rest in your frame, provided your frame is inertial). E = m c^2 came from special relativity. Special relativity has been validated in many many ways (the energy yield of nuclear weapons to name a dramatic example).
Regarding your first question, the precise answer:
The gravitational Einstein curvature tensor at a point in space-time is proportional to the stress energy tensor at that same point in space-time (with an additive term for the local metric tensor if you are into the cosmological constant).
What does that mean?
Neglecting the stuff about the cosmological constant (the differential geometry equivalent to a constant of integration), regions with a higher energy density generate a stronger gravitational field.
Contributions to the stress energy at a point in space-time include rest mass density (i.e. air makes less curvature that steel), pressure (higher temperatures --> faster random motion --> more energy), electromagnetic fields (the photon radiation spectrum contributes to the tensor),...
An issue of some contention is whether or not the gravitational curvature contributes to the stress energy tensor. Einstein thought it would be double counting but others are not so sure.
Most of the time (i.e. on earth), only the rest mass density is significant and Einstein's theory simplifies to Newtonian gravity.
However, in the core of a neutron star for instance, the contribution to the T_tt component of the stress energy from the pressure makes a significant contribution and needs to be accounted for.
Similarly, when accounting for the gravitional curvature due to say a black body radiation spectrum of photons, all the gravitional curvature comes from relativistic effects. Photons have no rest mass (more precisely, experimentally, photons have such a small rest mass as to make neutrinos look extra beefy); look up the equation of state for an extremely relativitistic ideal gas to get an idea of how photons contribute to gravitation.
A good resource for these things at a level beyond high school physics might be P. J. E. Peebles "Introduction to Modern Cosmology". It is often used as a text in advanced undergradute, intro graduate level courses in astrophysics.
But what do I know? After all, it is late into the thread.
Ugh... a couple of the posting in here are scientifically dubious at best as the moderators happily mod up anything vaguely resembling their high school physics class.
Disclaimer: While I have a Ph.D. in plasma physics and did a large amount of scientific computing in my thesis, this is not an area on which I am an expert. However I do know that a number of high quality physicists have given this a fair amount of thought (like Feynmann and Wheeler for instance) and have read some of their work.
The big limit is thermodynamic. The minimum energy it takes to flip a bit is of order k_b T_a where k_b is Boltzmann's constant and T_a is the ambient temperature (I think Wheeler was the first to show this limit through clever gedanken experiments but I could be wrong). The ambient temperature of the universe as measured to high precision by the cosmic microwave background black body radiation spectrum is T_a ~ 2.8 K (that is ~ -270 C or ~ -460 F for the unit challenged but remember Celsius and Fahrenheit are not referenced from absolute zero for the following formula).
So, suppose your calculation needs to flip N bits and you want to do it in time tau. Then the thermodynamic minimum theoretical power requirements for your computer are of order:
P ~ N k_b T_a / tau
So you want to do a complex calculation in on a Plank time scale length? I hope you have the power output of a supernova available. Of course, this is the minimum. You have to account all the inefficiences in generation, cooling... In the end, you might need a couple of simultaneous supernovas.
Also, for reference, the Plank length and Plank time are the measurement scales made by constructing quantities of the appropriate unit out of Plank's constant h, the speed of light c and the gravitational coupling constant G. Crudely speaking, it is the length scale at which conjectured quantum gravity effects dominate. Planck length considerations aren't really factored into theoretical limits of computation as other more obvious limits are reached first (like the above limit).
A more practical issue is whether or not computer miniturization can continue below the rapidly approaching atomic length scale (~1 A). For example, could you make logic gates based on complex inter-nuclear interactions or based out of non-linear vacuum dielectric polarization of hard gamma rays (i.e. compton backscattering off virtual electron-positron pairs) or other such known exotica of modern physics?
Use ATLAS (http://www.netlib.org, platform self tuning BLAS and LAPACK) and FFTW (run-time algorithm optimized Fourier transforms).
Both are portable and both approach or beat the performance of proprietary hand-tuned assembly written libraries.
But don't take my word for it. MATLAB (http://www.mathworks.com) now uses the ATLAS implementation of LAPACK / BLAS and MIT's FFTW in the their computational core.
I've used the ASCI Red BLAS and FFT stuff. I think the reason that it is not freely distributed is that it was developed in colloboration with Intel employees. However, ASCI Red libraries always had the disclaimer to the effect that if you had a compelling reason to have the source something could be worked out.
Check out how FFTW works. It is one of the few things I've seen that I would actually consider clever. Basically, FFTW designs a algorithm at run-time which is optimal for your cache size, register file depth, memory bandwidtch and transform type; powers of two sizes are not required. What really impressed me is that FFTW's codelet generator stumbled across a couple of hitherto unknown algorithms with reduced flops for computing strange sized FFTs.
ATLAS is pretty clever too. For kicks, run the installation and watch it tune the kernels. The routines for portably diagnosing FPU register size, FPU MAC performance and cache sizes are useful to have around.
Hmmm... what you are describing sounds more like a U-bomb than an H-bomb. Doing this from memory:
U-bombs were tested in the late 60s. Basically, the idea was to wrap a H-bomb with uranium. The A-bomb trigger would go off initiating an H-bomb explosion which in turn would induce fission in the outer uranium layer for an increased yield. It was a very messy weapon and didn't have a whole lot of strategic value as such. I think the one test killed a bunch of fisherman in a Japanese boat called the "Lucky Dragon" (I forget the original Japanese name) from fallout who were trolling a bit too close to the test site. However this might be more than a little bit mixed up.
It has been a while since I've thought about the history of above-grounds weapons testing and if you are relying on Slashdot for a nuclear physics education you are just plain silly. As a Ph.D. in plasma physics, it is distressing to see the number of scientific dubious assertions that get moderated up (it is as though science here is decided through committee and not experiment).
In any case, because there are many variants of atomic weapons means I don't have a great deal of faith in the previous poster. Depending on the details of the trigger and fusion fuel you can get all sorts of weapons (optimized for explosive yield, optimized for EMP, optimized for neutron yield, optimized for fallout or lack thereof,...)
A blanket statement that all fusion bombs are fission powered is a bit misleading.
Furthermore, if I recall correctly nuclear proliferation treaties were entered into which effectively arrested development of U-bomb type weapons. Your level of cynicism will dictate whether that means anything to you.
What?? Orwell was a well-known member of the U.K. socialist party if memory serves. He certainty wasn't very optimistic about socialism as demonstrated by 1984. (I tend to share his bleak outlook about utopian societies and thus I prefer my governments to have a touch of libertarian.)
Maybe you are working from a different definition of conservative (quite possible if you are not in the U.S.)
Opposite group and phase velocity is pretty easy to come by though. Think of the standard LC ladder with C as the shunt element. This turns into a standard transmission line in the limit of infinitesimal L and C.
Make the L the shunt element. Ta-da. Backwards waves (i.e. group velocity and phase velocity in opposite directions).
One minus the quantity of the plasma frequency squared divded by the wave frequency squared.
I agree that HTML should support LaTeX or some other such equation typesetting convention. I know efforts exist but nothing is widely adopted. Until then, my emails and posts will use a bastardized quasi-LaTeX-ish equation markup.
- You are right that AM is reflected by the ionosphere as a consequence of the negative dielectric constant. FM is not reflected because the carrier frequency is higher than the ionosphere's plasma frequency (giving a positive dielectric constant for FM).
- By the n = c_vac/sqrt(eps*mu) definition you mentioned, if both eps and mu are negative, the index of refraction is still positive. Perhaps the article meant to say 0 n 1 (still interesting; for example, laser-solenoid fusion is a wacky idea needing n1). Or possibly the researchers are using a more generalized definition of n. (General press is not the best place to get scientific details or scientifically accurate terms.)
- Having both eps and mu 0 has other stranger consequences (which is why such materials seem new and strange). The two interesting ones I can think of it that the material is left-handed and the material exhibits an inverted Cherenkov effect.
- Negative dielectric constants by themselves do not preclude wave propagation. Standard plane wave propagation in bulk negative dielectric media is out but many kinds of waves are possible at the material interfaces or via magnetic effects. Whistlers (the whistle sound you can hear on an AM radio) are an example of such wave in the ionosphere. (Whistlers start out as a lightning strike in the Southern hemisphere.) Schumann resonances of the ionosphere are another example.
- The article linked to did not specifically state that negative magnetic permeabilities had been created but similar research which made a couple of headlines last year did the same trick using arrays of split ring resonantors and other such microwave voodoo. As far as I can tell, this is the either same thing or a recent extension. In any case, devices like TWTs and backwards wave oscillators and what not rely heavily on creating devices which do similar strange things to wave propagation in them.
Kevin
Why does Slashdot keeps eating my less-than signs.
Having just completed a Ph.D. in this field I can say with some certainty that negative indexes of refraction are not new.
The relative dielectric constant of a plasma (cold, unmagnitized, above the ion plasma frequency) is:
1 - wp^2 / w^2
where w is the frequency and wp is the plasma frequency. Below the electron plasma frequency, the dielectric constant of a plasma is negative. (Actually, part of my thesis addes terms to handle electron pressure and density gradient effects.)
Hell, Rayleigh (think 1900s) was using such treatments to calculate resonance frequencies for things like the sun (wp/sqrt(3) by the way).
What was somewhat new about the research referred to is they simultaneously created negative dielectric constant and a negative magnetic permeability.
However, the techniques they used to do so have been around since the 1950s and form the basis of all sorts of electron devices like traveling wave tubes (a staple of satellite communication).
That is true. I quote myself: "tradeoff games between _attenuation_, dispersion, bandwidth"
1.55 um is at a minimum between where Rayleigh scattering losses dominate and losses due to a vibrational absorption line in water (which is unintentionally incorporated into the fiber during manufacturing). However, this is what I recall off the top of my head so it may not be completely accurate.
Actually, you are right. The important thing to remember is that typically different modes in an optical fiber have different optical path lengths and this can limit the speed you can transit information.
There are graded index optical fibers such that the optical path length of modes launched in an arbitrary (confined) direction is equal within in the limits of ray-tracing theory (which may or may not be appropriate for a given fiber). In fact, calculating the grading profile to equalize the optical path length is a standard textbook problem (at least in graduate courses).
As to the earlier comment stating you can not do much about other sources of dispersion in a fiber:
That is not quite correct. Carefully picking the wavelength, modulation scheme, fiber materials, fiber grading and what not can be used to play tradeoff games between attenuation, dispersion, bandwidth... The typical 1.55 um wavelength of operation for a fiber was not picked out of a hat.
Also, non-linear crystals can be used to play games with the spectral characteristics of a signal in an optical fiber to invert the dispersion. (By the way, similar spectral tricks are played in the new-fangled 10^18 W/cm^2 intensity desktop pulsed lasers.)
It is probably too late for anybody to see
this (thus, doomed to no moderation) but
electromagnetics is one of my Ph.D. specialities.
While I don't think his idea will work (well),
many people here are citing conservation of
momentum as the reason why.
However, every post I have seen has not accounted
for field momentum.
For example, it is possible to generate
thrust from an end fire phased array antenna.
Photons carry the momentum away (allowing
Newton's laws to be satified). This is not
practical though because, photons carry very
so little momentum
Even though the photon has a zero rest mass
(if you could bring it to rest), it has a
directed momentum of magnitude:
p_photon = h_planck * f_photon / c
(Consult a quantum textbook for more details.)
If the scientist proposing this idea does it
right, the RF oscillations will constructively
add to spit off photons in a opposite direction
of the device's acceleration.
However, my guess is that to get appreciable
thrust from the field momentum would require
fields and photon fluxes not easily attainable
(I am thinking GW sized power plants for micro
Netwons of force). Thus, I don't expect this
technology to compete with say high specific
impulse plasma thrusters anytime soon.
Having just interviewed at Bell Labs, Lucent is planning on spinning off microelectronics and related Bell Labs research into a separate company. Thus, I would not look for Lucent to go on an acquistion spree and buy PARC. But I could be wrong.
What is up with the moderation? The numerous factually incorrect things being moderated up is disturbing. I'll stick out my neck for being a troll but this is not insightful.
Electrons do not travel anywhere close to the speed of light in a material medium.
In electron positron colliders maybe, but not in a microchip.
It has already been posted here that the drift velocity of an electron in a solid state medium is quite slow (really really slow).
Also, the interaction of a photon with matter (and a photon traveling in a material medium) can be influenced by static electric fields (ferro-electric materials), magnetic fields (ferro-magnetic materials) and numerous other effects (non-linear photon iteractions).
A typical Compton wavelength for an electron is _much_ shorter than the comparable Compton wavelength of a photon.
This is why electron microscopes are used for incredibly resolving incredibly fine features (and not optical microscopes). Resolution liminiting wave effects creep up at much higher resolutions when using an electron microscope.
In order to get a photon whose wavelength is comparable to the typical Compton wavelengths of an electron, you are talking high energy photons (X-rays ).
In an all optical microchip, the photons would be piped around the chip in dielectric waveguides (essentially optical fibers). The cross sectional sizes of the dielectric waveguide are to be on the order of the wavelength of photon being transported (esp. if you want/need single mode transmission).
To pack these optical equivalents of wires with the same density as you would expect to find on a modern electron based processor, the wavelength of these photons would be well outside the optical spectrum.
However, if you don't believe me, calculate the energy / frequency for a photon whose wavelength is much smaller than feature sizes on modern microchips. (Hint: modern etching processes use deep ultraviolet light).
All optical microchips do have quite a few benefits. Compactness is not one of them.
Kevin
P.S. Before anyone objects on the grounds that you can just use higher energy photons, X-rays and deep ultraviolet light are very difficult to generate and control (esp. in a semiconductor medium). As such they are completely unsuitable for use in an optical microchip.
As a former employee of Los Alamos, I would like to point out the lab is LOS ALAMOS (not Las - as stated repeatedly in the story intro). I don't mind typos, especially in user comments. However, the story intros should at least be minimally proofread.
Or am I out of the loop and Las is the new hip way to talk about the lab?
I will have to diagree with you on that (at least for non-DC fields).
For transmission lines, the energy of the wave is stored in the fields (not in conduction current). Currents are induced on the surface of the conductors that guide the wave.
However, if you suspect that electrons are literally responsible for moving you information down a transmission line, consider that in an ideal transmission line (which by definition has a group velocity equal to the speed of light of the surrounding medium) electrons would have relativistic velocities in a solid state medium (which is quite ludicrous).
You might want to read up on Brilliuon precursors as well.
If other conductors are nearby, then coupling / cross-talk can occur. This is due to the fields of one wave system excited modes in another wave system.
Read Jackson's "Classical Electrodynamics", Ramo, Whinnery and Van Duzer's "Fields and Waves in Communication Electronics" or D.M. Pozar's "Microwave Engineering" for details. These are all pretty standard texts for teaching E.M. (the first is theoretical, the latter are more oriented towards engineers).
Yes you can do this with any particle with spin (electrons for example). You can also do it with optical polarization of photons. I'm sure other nonlocally entangled two state systems exist.
However, you cannot use this to transmit information. For instance, two entangled particles are emitted separating from each other at a rate of 2c. You measure the state of one of the particles (suppose it is spin up), then the distant particle will then become the appropriate state (say spin down).
Measurement doesn't force the spin state to a value of your choosing... it only forces the global system to be consistent with your local measurement. You can use this to transmit a one time pad but the protocols used also require a normal slower than light channel between the sender and reciever.
Some experiments have been conducted which claim evanescant modes can transmit information faster than the speed of light (yes, I know the difference between group velocity and phase velocity). I am remembering a claim from a group which stated they were transmitting Mozart through a crystal at 4.7X the speed of light (if you don't mind the received signal being attenuated 80 dB). However, objections have been raised to the research (most revelvant was that the audio signal + carrier was easily predictable in a mathematical sense thus it is difficult to conclusively say when in fact the signal arrived). I could dig up an abstract for the research.
The research at Los Alamos to which you are referring actually was concerned with Quantum Crytography. In Quantum Cryptography, a random pad is distributed securely. The distribution mechanism involves strange EPR-paradox type faster than light effects. The Los Alamos 95-96 Physics Division Progress Report gives 205m for the distance under which free space quantum key distribution has been performed. I imagine greater distances have been achieved. I could dig up some abstracts here too if necessary.
Slashdot Mirror
Ummmm ... the way you stated that is less than precise.
...
For your second question:
E = m c^2 was not confirmed by Mercury's orbit. Deviations in Mercury's orbit from Newtonian predictions provided supporting evidence for Einstein's theory of general relativity.
E = m c^2 is not even correct (E = m c^2 for you only if the particle is at rest in your frame, provided your frame is inertial). E = m c^2 came from special relativity. Special relativity has been validated in many many ways (the energy yield of nuclear weapons to name a dramatic example).
Regarding your first question, the precise answer:
The gravitational Einstein curvature tensor at a point in space-time is proportional to the stress energy tensor at that same point in space-time (with an additive term for the local metric tensor if you are into the cosmological constant).
What does that mean?
Neglecting the stuff about the cosmological constant (the differential geometry equivalent to a constant of integration), regions with a higher energy density generate a stronger gravitational field.
Contributions to the stress energy at a point in space-time include rest mass density (i.e. air makes less curvature that steel), pressure (higher temperatures --> faster random motion --> more energy), electromagnetic fields (the photon radiation spectrum contributes to the tensor),
An issue of some contention is whether or not the gravitational curvature contributes to the stress energy tensor. Einstein thought it would be double counting but others are not so sure.
Most of the time (i.e. on earth), only the rest mass density is significant and Einstein's theory simplifies to Newtonian gravity.
However, in the core of a neutron star for instance, the contribution to the T_tt component of the stress energy from the pressure makes a significant contribution and needs to be accounted for.
Similarly, when accounting for the gravitional curvature due to say a black body radiation spectrum of photons, all the gravitional curvature comes from relativistic effects. Photons have no rest mass (more precisely, experimentally, photons have such a small rest mass as to make neutrinos look extra beefy); look up the equation of state for an extremely relativitistic ideal gas to get an idea of how photons contribute to gravitation.
A good resource for these things at a level beyond high school physics might be P. J. E. Peebles "Introduction to Modern Cosmology". It is often used as a text in advanced undergradute, intro graduate level courses in astrophysics.
But what do I know? After all, it is late into the thread.
Kevin
Ugh ... a couple of the posting in here are scientifically dubious at best as the moderators happily mod up anything vaguely resembling their high school physics class.
... In the end, you might need a couple of simultaneous supernovas.
Disclaimer: While I have a Ph.D. in plasma physics and did a large amount of scientific computing in my thesis, this is not an area on which I am an expert. However I do know that a number of high quality physicists have given this a fair amount of thought (like Feynmann and Wheeler for instance) and have read some of their work.
The big limit is thermodynamic. The minimum energy it takes to flip a bit is of order k_b T_a where k_b is Boltzmann's constant and T_a is the ambient temperature (I think Wheeler was the first to show this limit through clever gedanken experiments but I could be wrong). The ambient temperature of the universe as measured to high precision by the cosmic microwave background black body radiation spectrum is T_a ~ 2.8 K (that is ~ -270 C or ~ -460 F for the unit challenged but remember Celsius and Fahrenheit are not referenced from absolute zero for the following formula).
So, suppose your calculation needs to flip N bits and you want to do it in time tau. Then the thermodynamic minimum theoretical power requirements for your computer are of order:
P ~ N k_b T_a / tau
So you want to do a complex calculation in on a Plank time scale length? I hope you have the power output of a supernova available. Of course, this is the minimum. You have to account all the inefficiences in generation, cooling
Also, for reference, the Plank length and Plank time are the measurement scales made by constructing quantities of the appropriate unit out of Plank's constant h, the speed of light c and the gravitational coupling constant G. Crudely speaking, it is the length scale at which conjectured quantum gravity effects dominate. Planck length considerations aren't really factored into theoretical limits of computation as other more obvious limits are reached first (like the above limit).
A more practical issue is whether or not computer miniturization can continue below the rapidly approaching atomic length scale (~1 A). For example, could you make logic gates based on complex inter-nuclear interactions or based out of non-linear vacuum dielectric polarization of hard gamma rays (i.e. compton backscattering off virtual electron-positron pairs) or other such known exotica of modern physics?
Kevin
Why bother?
Use ATLAS (http://www.netlib.org, platform self tuning BLAS and LAPACK) and FFTW (run-time algorithm optimized Fourier transforms).
Both are portable and both approach or beat the performance of proprietary hand-tuned assembly written libraries.
But don't take my word for it. MATLAB (http://www.mathworks.com) now uses the ATLAS implementation of LAPACK / BLAS and MIT's FFTW in the their computational core.
I've used the ASCI Red BLAS and FFT stuff. I think the reason that it is not freely distributed is that it was developed in colloboration with Intel employees. However, ASCI Red libraries always had the disclaimer to the effect that if you had a compelling reason to have the source something could be worked out.
Check out how FFTW works. It is one of the few things I've seen that I would actually consider clever. Basically, FFTW designs a algorithm at run-time which is optimal for your cache size, register file depth, memory bandwidtch and transform type; powers of two sizes are not required. What really impressed me is that FFTW's codelet generator stumbled across a couple of hitherto unknown algorithms with reduced flops for computing strange sized FFTs.
ATLAS is pretty clever too. For kicks, run the installation and watch it tune the kernels. The routines for portably diagnosing FPU register size, FPU MAC performance and cache sizes are useful to have around.
Kevin
Hmmm ... what you are describing sounds more like a U-bomb than an H-bomb. Doing this from memory:
...)
U-bombs were tested in the late 60s. Basically, the idea was to wrap a H-bomb with uranium. The A-bomb trigger would go off initiating an H-bomb explosion which in turn would induce fission in the outer uranium layer for an increased yield. It was a very messy weapon and didn't have a whole lot of strategic value as such. I think the one test killed a bunch of fisherman in a Japanese boat called the "Lucky Dragon" (I forget the original Japanese name) from fallout who were trolling a bit too close to the test site. However this might be more than a little bit mixed up.
It has been a while since I've thought about the history of above-grounds weapons testing and if you are relying on Slashdot for a nuclear physics education you are just plain silly. As a Ph.D. in plasma physics, it is distressing to see the number of scientific dubious assertions that get moderated up (it is as though science here is decided through committee and not experiment).
In any case, because there are many variants of atomic weapons means I don't have a great deal of faith in the previous poster. Depending on the details of the trigger and fusion fuel you can get all sorts of weapons (optimized for explosive yield, optimized for EMP, optimized for neutron yield, optimized for fallout or lack thereof,
A blanket statement that all fusion bombs are fission powered is a bit misleading.
Furthermore, if I recall correctly nuclear proliferation treaties were entered into which effectively arrested development of U-bomb type weapons. Your level of cynicism will dictate whether that means anything to you.
Kevin
"As the well-known conservative George Orwell ..."
What?? Orwell was a well-known member of the U.K. socialist party if memory serves. He certainty wasn't very optimistic about socialism as demonstrated by 1984. (I tend to share his bleak outlook about utopian societies and thus I prefer my governments to have a touch of libertarian.)
Maybe you are working from a different definition of conservative (quite possible if you are not in the U.S.)
Kevin
The stuff is interesting alright.
Opposite group and phase velocity is pretty easy to come by though. Think of the standard LC ladder with C as the shunt element. This turns into a standard transmission line in the limit of infinitesimal L and C.
Make the L the shunt element. Ta-da. Backwards waves (i.e. group velocity and phase velocity in opposite directions).
Kevin
1 - wp^2 / w^2
In English:
One minus the quantity of the plasma frequency squared divded by the wave frequency squared.
I agree that HTML should support LaTeX or some other such equation typesetting convention. I know efforts exist but nothing is widely adopted. Until then, my emails and posts will use a bastardized quasi-LaTeX-ish equation markup.
Kevin
A couple of brief comments:
- You are right that AM is reflected by the ionosphere as a consequence of the negative dielectric constant. FM is not reflected because the carrier frequency is higher than the ionosphere's plasma frequency (giving a positive dielectric constant for FM).
- By the n = c_vac/sqrt(eps*mu) definition you mentioned, if both eps and mu are negative, the index of refraction is still positive. Perhaps the article meant to say 0 n 1 (still interesting; for example, laser-solenoid fusion is a wacky idea needing n1). Or possibly the researchers are using a more generalized definition of n. (General press is not the best place to get scientific details or scientifically accurate terms.)
- Having both eps and mu 0 has other stranger consequences (which is why such materials seem new and strange). The two interesting ones I can think of it that the material is left-handed and the material exhibits an inverted Cherenkov effect.
- Negative dielectric constants by themselves do not preclude wave propagation. Standard plane wave propagation in bulk negative dielectric media is out but many kinds of waves are possible at the material interfaces or via magnetic effects. Whistlers (the whistle sound you can hear on an AM radio) are an example of such wave in the ionosphere. (Whistlers start out as a lightning strike in the Southern hemisphere.) Schumann resonances of the ionosphere are another example.
- The article linked to did not specifically state that negative magnetic permeabilities had been created but similar research which made a couple of headlines last year did the same trick using arrays of split ring resonantors and other such microwave voodoo. As far as I can tell, this is the either same thing or a recent extension. In any case, devices like TWTs and backwards wave oscillators and what not rely heavily on creating devices which do similar strange things to wave propagation in them.
Kevin
Why does Slashdot keeps eating my less-than signs.
Having just completed a Ph.D. in this field I can say with some certainty that negative indexes of refraction are not new.
The relative dielectric constant of a plasma (cold, unmagnitized, above the ion plasma frequency) is:
1 - wp^2 / w^2
where w is the frequency and wp is the plasma frequency. Below the electron plasma frequency, the dielectric constant of a plasma is negative. (Actually, part of my thesis addes terms to handle electron pressure and density gradient effects.)
Hell, Rayleigh (think 1900s) was using such treatments to calculate resonance frequencies for things like the sun (wp/sqrt(3) by the way).
What was somewhat new about the research referred to is they simultaneously created negative dielectric constant and a negative magnetic permeability.
However, the techniques they used to do so have been around since the 1950s and form the basis of all sorts of electron devices like traveling wave tubes (a staple of satellite communication).
Kevin
That is true. I quote myself: "tradeoff games between _attenuation_, dispersion, bandwidth"
1.55 um is at a minimum between where Rayleigh scattering losses dominate and losses due to a vibrational absorption line in water (which is unintentionally incorporated into the fiber during manufacturing). However, this is what I recall off the top of my head so it may not be completely accurate.
Actually, you are right. The important thing to remember is that typically different modes in an optical fiber have different optical path lengths and this can limit the speed you can transit information.
... The typical 1.55 um wavelength of operation for a fiber was not picked out of a hat.
There are graded index optical fibers such that the optical path length of modes launched in an arbitrary (confined) direction is equal within in the limits of ray-tracing theory (which may or may not be appropriate for a given fiber). In fact, calculating the grading profile to equalize the optical path length is a standard textbook problem (at least in graduate courses).
As to the earlier comment stating you can not do much about other sources of dispersion in a fiber:
That is not quite correct. Carefully picking the wavelength, modulation scheme, fiber materials, fiber grading and what not can be used to play tradeoff games between attenuation, dispersion, bandwidth
Also, non-linear crystals can be used to play games with the spectral characteristics of a signal in an optical fiber to invert the dispersion. (By the way, similar spectral tricks are played in the new-fangled 10^18 W/cm^2 intensity desktop pulsed lasers.)
It is probably too late for anybody to see
this (thus, doomed to no moderation) but
electromagnetics is one of my Ph.D. specialities.
While I don't think his idea will work (well),
many people here are citing conservation of
momentum as the reason why.
However, every post I have seen has not accounted
for field momentum.
For example, it is possible to generate
thrust from an end fire phased array antenna.
Photons carry the momentum away (allowing
Newton's laws to be satified). This is not
practical though because, photons carry very
so little momentum
Even though the photon has a zero rest mass
(if you could bring it to rest), it has a
directed momentum of magnitude:
p_photon = h_planck * f_photon / c
(Consult a quantum textbook for more details.)
If the scientist proposing this idea does it
right, the RF oscillations will constructively
add to spit off photons in a opposite direction
of the device's acceleration.
However, my guess is that to get appreciable
thrust from the field momentum would require
fields and photon fluxes not easily attainable
(I am thinking GW sized power plants for micro
Netwons of force). Thus, I don't expect this
technology to compete with say high specific
impulse plasma thrusters anytime soon.
Kevin
Having just interviewed at Bell Labs, Lucent is planning on spinning off microelectronics and related Bell Labs research into a separate company. Thus, I would not look for Lucent to go on an acquistion spree and buy PARC. But I could be wrong.
What is up with the moderation? The numerous factually incorrect things being moderated up is disturbing. I'll stick out my neck for being a troll but this is not insightful.
Electrons do not travel anywhere close to the speed of light in a material medium.
In electron positron colliders maybe, but not in a microchip.
It has already been posted here that the drift velocity of an electron in a solid state medium is quite slow (really really slow).
Also, the interaction of a photon with matter (and a photon traveling in a material medium) can be influenced by static electric fields (ferro-electric materials), magnetic fields (ferro-magnetic materials) and numerous other effects (non-linear photon iteractions).
Kevin
Eeegads ... No!
A typical Compton wavelength for an electron is _much_ shorter than the comparable Compton wavelength of a photon.
This is why electron microscopes are used for incredibly resolving incredibly fine features (and not optical microscopes). Resolution liminiting wave effects creep up at much higher resolutions when using an electron microscope.
In order to get a photon whose wavelength is comparable to the typical Compton wavelengths of an electron, you are talking high energy photons (X-rays ).
In an all optical microchip, the photons would be piped around the chip in dielectric waveguides (essentially optical fibers). The cross sectional sizes of the dielectric waveguide are to be on the order of the wavelength of photon being transported (esp. if you want/need single mode transmission).
To pack these optical equivalents of wires with the same density as you would expect to find on a modern electron based processor, the wavelength of these photons would be well outside the optical spectrum.
However, if you don't believe me, calculate the energy / frequency for a photon whose wavelength is much smaller than feature sizes on modern microchips. (Hint: modern etching processes use deep ultraviolet light).
All optical microchips do have quite a few benefits. Compactness is not one of them.
Kevin
P.S. Before anyone objects on the grounds that you can just use higher energy photons, X-rays and deep ultraviolet light are very difficult to generate and control (esp. in a semiconductor medium). As such they are completely unsuitable for use in an optical microchip.
As a former employee of Los Alamos, I would like to point out the lab is LOS ALAMOS (not Las - as stated repeatedly in the story intro). I don't mind typos, especially in user comments. However, the story intros should at least be minimally proofread.
Or am I out of the loop and Las is the new hip way to talk about the lab?
Eagerly awaiting (-1: offtopic).
I will have to diagree with you on that (at least for non-DC fields).
For transmission lines, the energy of the wave is stored in the fields (not in conduction current). Currents are induced on the surface of the conductors that guide the wave.
However, if you suspect that electrons are literally responsible for moving you information down a transmission line, consider that in an ideal transmission line (which by definition has a group velocity equal to the speed of light of the surrounding medium) electrons would have relativistic velocities in a solid state medium (which is quite ludicrous).
You might want to read up on Brilliuon precursors as well.
If other conductors are nearby, then coupling / cross-talk can occur. This is due to the fields of one wave system excited modes in another wave system.
Read Jackson's "Classical Electrodynamics", Ramo, Whinnery and Van Duzer's "Fields and Waves in Communication Electronics" or D.M. Pozar's "Microwave Engineering" for details. These are all pretty standard texts for teaching E.M. (the first is theoretical, the latter are more oriented towards engineers).
Kevin
Yes you can do this with any particle with spin (electrons for example). You can also do it with optical polarization of photons. I'm sure other nonlocally entangled two state systems exist.
... it only forces the global system to be consistent with your local measurement. You can use this to transmit a one time pad but the protocols used also require a normal slower than light channel between the sender and reciever.
However, you cannot use this to transmit information. For instance, two entangled particles are emitted separating from each other at a rate of 2c. You measure the state of one of the particles (suppose it is spin up), then the distant particle will then become the appropriate state (say spin down).
Measurement doesn't force the spin state to a value of your choosing
Kevin
Two things:
Some experiments have been conducted which claim evanescant modes can transmit information faster than the speed of light (yes, I know the difference between group velocity and phase velocity). I am remembering a claim from a group which stated they were transmitting Mozart through a crystal at 4.7X the speed of light (if you don't mind the received signal being attenuated 80 dB). However, objections have been raised to the research (most revelvant was that the audio signal + carrier was easily predictable in a mathematical sense thus it is difficult to conclusively say when in fact the signal arrived). I could dig up an abstract for the research.
The research at Los Alamos to which you are referring actually was concerned with Quantum Crytography. In Quantum Cryptography, a random pad is distributed securely. The distribution mechanism involves strange EPR-paradox type faster than light effects. The Los Alamos 95-96 Physics Division Progress Report gives 205m for the distance under which free space quantum key distribution has been performed. I imagine greater distances have been achieved. I could dig up some abstracts here too if necessary.
Kevin