Gravity, QED and the Casimir effect.
After the post on gravity for Adi’s benefit I began to ponder some of what I was offering by way of hypothesis and decided it was worth investigating further in search of a testable model. While I was doing so I encountered the basis of the now popular Zero Point Energy hypothesis and found out I was in good company.
I discovered a poorly written but very interesting paper by this physicist, Dr. H. E. Puthoff.
What I found fascinating was an observation by Andrei Sakharov that he offers in this passage:
http://www.ldolphin.org/zpe.html
With regard to the gravitational attraction that is described so well by Einstein's theory, its fundamental nature is still not well understood. Whether addressed simply in terms of Newton's Law, or with the full rigor of general relativity, gravitational theory is basically descriptive in nature, without revealing the underlying dynamics for that description. As a result, attempts to unify gravity with the other forces (electromagnetic, strong and weak nuclear forces) or to develop a quantum theory of gravity have foundered again and again on difficulties that can be traced back to a lack of understanding at a fundamental level. To rectify these difficulties, theorists by and large have resorted to ever-increasing levels of mathematical sophistication and abstraction, as in the recent development of supergravity and superstring theories. Taking a completely different tack when addressing these difficulties in the sixties, the well-known Russian physicist Andrei Sakharov put forward the somewhat radical hypothesis that gravitation might not be a fundamental interaction at all, but rather a secondary or residual effect associated with other (non- gravitational) fields. (6)
Specifically, Sakharov suggested that gravity might be an induced effect brought about by changes in the zero-point energy of the vacuum, due to the presence of matter. If correct, gravity would then be understood as a variation on the Casimir theme, in which background zero-point-energy pressures were again responsible. Although Sakharov did not develop the concept much further, he did outline certain criteria such a theory would have to meet such as predicting the value of the gravitational constant G in terms of zero-point-energy parameters. The approach to gravity outlined by Sakharov has recently been addressed in detail, and with positive results, again by the author. (7) The gravitational interaction is shown to begin with the fact that a particle situated in the sea of electromagnetic zero-point fluctuations develops a "jitter" motion, or ZITTERBEWEGUNG as it is called. When there are two or more particles they are each influenced not only by the fluctuating background field, but also by the fields generated by the other particles, all similarly undergoing ZITTERBEWEGUNG motion, and the inter-particle coupling due to these fields results in the attractive gravitational force.
I had found the article because I was searching for background information on the Casimir effect when a similar insight occurred to me during our discussion in Adi’s thread; Why is light the fastest?
It also brought me to a thread that I had missed here in our forum on the discovery of antigravity that deserves to be cited in this one if for no other reason than the numerous good links provided, not to mention the insights.
Antigravity discovered
What I am suggesting is that the Casimir effect, or Vacuum Energy as it is also called was consistent with the basic idea I was suggesting that gravity could be understood as a kind of background radiation filling empty space with a kind of *virtual particle* that is being concentrated (lensed) by the presence of matter/energy due to its attraction to matter but basically having zero mass. This also might be a testable hypothesis.
The idea of a force that comes from *nothing* and projecting towards all *something*, essentially pushing toward the center of all mass, in a kind of reverse inverse square law would thus have an observational basis. It would also be consistent with GR.
I also found this interesting post in a Physics Forum describing some background on Vacuum Energy.
Field and particle theories
Quantum field theory considers the vacuum ground state not to be completely empty, but to consist of a seething mass of virtual particles and fields. Since these fields do not have a permanent existence, they are called vacuum fluctuations. In the Casimir effect, two metal plates can cause a change in the vacuum energy density between them which generates a measurable force.
Some believe that vacuum energy might be the "dark energy" (also called quintessence) associated with the cosmological constant in General relativity, thought to be similar to a negative force of gravity. Observations that the expanding Universe appears to be accelerating seem to support the Cosmic inflation theory —first proposed by Alan Guth (1981) — in which the nascent Universe passed through a phase of exponential expansion driven by a negative vacuum energy density (positive vacuum pressure).
Implications
Vacuum energy has a number of consequences. Vacuum fluctuations are always created as particle/antiparticle pairs. The creation of these "virtual particles" near the event horizon of a black hole has been hypothesized by physicist Stephen Hawking to be a mechanism for the eventual "evaporation" of black holes. The net energy of the universe remains zero so long as the particle pairs annihilate each other within Planck time. If one of the pair is pulled into the black hole before this, then the other particle becomes "real" and energy/mass is essentially radiated into space from the black hole. This loss is cumulative and could result in the black hole's disappearance over time. The time required is dependent on the mass of the black hole, but could be on the order of 10^100 years for large solar-mass black holes.
The Grand unification theory predicts a non-zero cosmological constant from the energy of vacuum fluctuations. Examining normal physical processes with knowledge of these field phenomena can lead to an interesting insight in electrodynamics. During discussions of perpetual motion, the topic of vacuum energy usually encourages serious inquiries.
History
In 1934, Georges Lemaître used an unusual perfect-fluid equation of state to interpret the cosmological constant as due to vacuum energy. In 1973, Edward Tryon proposed that the Universe may be a large scale quantum mechanical vacuum fluctuation where positive mass-energy is balanced by negative gravitational potential energy. During the 1980s, there were many attempts to relate the fields that generate the vacuum energy to specific fields that were predicted by the Grand unification theory, and to use observations of the Universe to confirm that theory. These efforts had failed so far, and the exact nature of the particles or fields that generate vacuum energy, with a density such as that required by the Inflation theory, remains a mystery.
While looking for further explanation and description of what Casimir Force is that I found this observation, which brought me to the thread cited above about antigravity.
The Force of Empty Space
The Casimir force is so weak that it has rarely been detected at all, but now a team reports in the 23 November PRL that they have made the most precise measurement ever of the phenomenon. They claim that their technique, using an atomic force microscope, has the capacity to test the strangest aspects of the Casimir effect, ones that have never before been tested.
The simplest explanation of the Casimir effect is that the two metal plates attract because their reflective surfaces exclude virtual photons of wavelengths longer than the separation distance. This reduces the energy density between the plates compared with that outside, and--like external air pressure tending to collapse a slightly evacuated vessel--the Casimir force pulls the plates toward one another. But the most puzzling aspect of the theory is that the force depends on geometry: If the plates are replaced by hemispherical shells, the force is repulsive. Spherical surfaces somehow "enhance" the number of virtual photons. There is no simple or intuitive way to tell which way the force will go before carrying out the complicated calculations.
Since the discovery of the theory by Casimir 50 years ago, there have been only two previous documented detections of the effect. One was in 1958 and had 100% uncertainty, and the second was last year [Phys. Rev. Lett. 78, 5 (1997)], when the theory was verified to within 5%. Umar Mohideen and Anushree Roy, of the University of California at Riverside, claim their new results verify the theory to within 1%.
Mohideen and Roy exploited the exquisite sensitivity of the atomic force microscope (AFM), which can sense forces as small as 10-18 newtons. In an AFM, the force causes a slight deflection of a microscopic cantilever, which is detected with a laser system. The team affixed an aluminum-plated, 200-µm-diameter sphere to the cantilever and recorded the deflection as it approached a flat, aluminum-plated surface to within 100 nm. They corrected the raw data for several small effects, including the electrostatic force between the surfaces caused by a small excess charge on the sphere. Because the approach was so close, the team also had to correct the theoretical curve to account for the microscopic roughness of the metal surfaces, which they measured using the AFM in a more traditional mode. They applied a further correction to the ideal theory to account for the lack of perfectly reflecting surfaces.
Mohideen says the new method is superior to previous ones because the electrostatic corrections to the data amounted to only a few percent of the size of the Casimir force, whereas in the previous experiment, those corrections were 5 times the Casimir force. But the real importance of the new technique, says Mohideen, is that its precision can be dramatically improved, allowing studies of the weird geometry dependence of the Casimir effect and even its predicted dependence on temperature.
The passage in this experiment that I emphasized above caught my attention because it could be explained by the lensing effect as described in GR by the material shape of the plates. More importantly it goes toward explaining how a manipulation of the Casimir Effect in this manner – changing the shape of the plates from flat to spheroid - could resemble a model for antigravity, and even more so if in fact the two forces are really just descriptions of the same force (Gravity and Casimir) under different conditions.
In this online analysis offered by the Calphysics Institute describing Zero Point energy, a similar explanation is also offered with respect to Gravity, GR and the Casimir Effect.
Zero Point Energy and Zero Point Field
A major discovery in astrophysics in the late 1990s was the finding from type Ia supernovae redshift-luminosity observations that the expansion of the universe is accelerating. This led to the concept of dark energy, which is in effect a resurrection of Einstein's cosmological constant. (The universe now appears to consist of about 70 percent dark energy, 25 percent dark matter and five percent ordinary matter.) Zero-point energy has the desired property of driving an accelerated expansion, and thus having the requisite properties of dark energy, but to an absurdly greater degree than required, i.e. 120 orders of magnitude.
According to relativity theory, energy is equivalent to mass as a source of gravity, thus zero-point energy should gravitate, which according to general relativity means producing a positive curvature in space-time. At first glance one might assume that if there is an enormous amount of zero-point energy underlying the universe, its effect would be to dramatically curve the universe to a minute size. Indeed, if the spectrum of zero-point energy extends to the Planck scale, its energy density would be the mass equivalent of about 1093 grams per cubic centimeter which would reduce the universe to a size smaller than an atomic nucleus.
Zero-point energy behaves differently. For ordinary radiation, the ratio of pressure to energy density is w=1/3c2, which is customarily expressed in units whereby c=1, and thus the ratio is expressed as w=,1/3. But for zero-point energy the ratio is w=-1. This is owing to the circumstance that the zero-point energy density is assumed to be constant: no matter how much the universe expands it does not become diluted, but instead more zero-point energy is assumed to be created out of nothing.
A further peculiarity is that a ratio of w=-1 implies that the zero-point energy exerts a negative pressure which, counter-intuitively, leads to an expansion of space-time.
Thus zero-point energy would appear to be identical with the mysterious dark energy, but unfortunately if the energy spectrum does continue up to the Planck frequency, there may be 120 orders of magnitude more energy per cubic centimeter than the observations of cosmic acceleration permit. Indeed, this amount of zero-point energy, interpreted this way, would have accelerated the universe into oblivion in microseconds.
NASA offers this page with numerous explicative links to the subject described in this thread. http://apod.nasa.gov...d/ap020917.html
NASA may be interested because the Casimir Effect is also construed to be evidence of a Quantum Vacuum state and is suggested as a means of developing a theoretical Wormhole technology to achieve a Faster Than Light model.
The Casimir Effect
The Casimir effect is one of several phenomena that provide convincing evidence for the reality of the quantum vacuum – the equivalent in quantum mechanics of what, in classical physics, would be described as empty space. It has been linked to the possibility of faster-than-light travel.
According to modern physics, a vacuum is full of fluctuating electromagnetic waves of all possible wavelengths which imbue it with a vast amount of energy, normally invisible to us. Casimir realized that between two plates, only those unseen electromagnetic waves whose wavelengths fit a whole number of times into the gap should be counted when calculating the vacuum energy. As the gap between the plates is narrowed, fewer waves can contribute to the vacuum energy and so the energy density between the plates falls below the energy density of the surrounding space. The result is a tiny force trying to pull the plates together – a force that has been measured and thus provides proof of the existence of the quantum vacuum.
This may be relevant to space travel because the region inside a Casimir cavity has negative energy density. Zero energy density, by definition, is the energy density of normal " empty space." Since the energy density between the conductors of a Casimir cavity is less than normal, it must be negative. Regions of negative energy density are thought to be essential to a number of hypothetical faster-than-light propulsion schemes, including stable wormholes and the Alcubierre warp drive .
Please consider this somewhat lengthy introduction as an open invitation to focus the discussion begun elsewhere that relates to these exciting and important aspects of Grand Unified Theory with respect to gravity, General Relativity, Quantum Electrodynamics, its *fundamental nature* and the possible description of them as observable through the Casimir Effect.
I also found this interesting departmental discussion from the University of California that offers reasoning remarkably similar to my own in the thread about whether gravity is faster than light.
What's the Energy Density of the Vacuum?
We have two fundamental theories of physics: quantum field theory and general relativity. Quantum field theory takes quantum mechanics and special relativity into account, and it's a great theory of all the forces and particles except gravity, but it ignores gravity. General relativity is a great theory of gravity, but it ignores quantum mechanics. Nobody knows how to reconcile these theories yet. That's what people working on "quantum gravity" are trying to do.
Now, the reason I'm telling you this is that quantum field theory and general relativity have really different attitudes towards the energy density of the vacuum. The reason is that quantum field theory only cares about energy differences. If you can only measure energy differences, you can't determine the energy density of the vacuum - it's just a matter of convention. As far as we know, you can only determine the energy density of the vacuum by experiments that involve general relativity - namely, by measuring the curvature of spacetime.
So, when you ask about the energy density of the vacuum, you get different answers depending on whether the person answering you is basing their answer on general relativity or quantum field theory. Let me run through the 5 most common answers, explaining how people reach these different answers:
1. We can measure the energy density of the vacuum through astronomical observations that determine the curvature of spacetime. All the measurements that have been done agree that the energy density is VERY CLOSE TO ZERO. In terms of mass density, its absolute value is less than 10-26 kilograms per meter. In terms of energy density, this is about 10-9 joules per cubic meter.
One can know something is very close to zero without knowing whether it is positive, negative or zero. For a long time that's how it was with the cosmological constant. But, recent measurements by the Wilkinson Microwave Anisotropy Probe and many other experiments seem to be converging on a positive cosmological constant, equal to about 6 × 10-27 kilograms per cubic meter. This corresponds to a positive energy density of about 9 × 10-10 joules per cubic meter.
The reason they get a positive energy density is very interesting. Thanks to the redshifts of distant galaxies and quasars, we've known for a long time that the universe is expanding. The new data shows something surprising: this expansion is speeding up. Ordinary matter can only make the expansion slow down, since gravity attracts - at least for ordinary matter.
What can possibly make the expansion speed up, then? Well, general relativity says that if the vacuum has energy density, it must also have pressure! In fact, it must have a pressure equal to exactly -3 times its energy density, in units where the speed of light and Newton's gravitational constant equal 1. (It sounds weird, but ultimately this is because there are 3 dimensions of space.) Positive energy density makes the expansion of the universe tend to slow down... but negative pressure makes the expansion tend to speed up, and that factor of 3 means the negative pressure wins. So, if the vacuum has positive energy density, the expansion of the universe will tend to speed up. This is what people see. And, this is the most plausible explanation known for what's going on.
Of course, to believe this argument at all, one must have some confidence in general relativity. To believe scientists' attempts to determine an actual value for the energy density of spacetime, one must have more confidence in general relativity, and also other assumptions about cosmology. However, the basic fact that the energy density of spacetime is very close to zero is almost unarguable: for it to be false, general relativity would have to be very wrong. (There are more answers to the problem posed offered at the link to the paper)
Yes, I suspect the value for the energy density of spacetime must be very close to zero but perhaps equal to the Casimir Effect and similarly, gravity.
Casimir effect (Wiki)
Dark Energy Fills the Universe (Berkeley Lab)
