The Quantum Vacuum

There's much talk in the popular physics literature about the 'quantum vacuum'. Fluctuations of the vacuum purportedly give rise to observable effects such as the Lamb shift in the energy levels of a Hydrogen atom, or the Casimir effect, an attractive force between closely-separated parallel conducting plates. However, the popular literature tends to speak only of the vacuum, and neglects to explain that:

(i) Each different theory tends to have its own notion of the vacuum, and

(ii) Each different field within a fixed theory has a vacuum associated with it.

So, for example, quantum field theory and general relativity have different concepts of the vacuum, and within quantum field theory, there is the vacuum state of the electromagnetic field, there is the vacuum state of the electron field, and there is the vacuum state of the interacting electron-electromagnetic field, and they are all distinct.

Nevertheless, the various vacua of all the fields in the standard model of particle physics (an application of quantum field theory) add up to give a vacuum energy density which is many orders of magnitude greater than the upper bound placed upon the vacuum energy of empty space ('dark energy') by cosmological observations. This is the 'cosmological constant problem', superbly analysed by Rugh and Zinkernagel in 2001. The accelerating expansion of the universe can be explained by some sort of vacuum energy, but not that predicted by quantum field theory.

Rugh and Zinkernagel suggest that the existence of the quantum vacuum has not actually been demonstrated. And, indeed, the fact that the purported vacuum fluctuations do not contribute to the 'scattering amplitudes' (i.e., the probabilities of various outcomes of a collision) between, say, photons and electrons, requires an explanation from those who believe in the reality of the quantum vacuum. Rugh and Zinkernagel also propose that there may be some false beliefs about the assumed relation between quantum field theory in curved space-time and general relativity.

However, another possibility, that provided by supersymmetry, seems most intriguing. There are two types of elementary particles: bosons and fermions. Supersymmetry postulates that each type of boson with which we are currently familiar, has a supersymmetric fermionic partner, and each type of fermion with which we are currently familiar, has a supersymmetric bosonic partner. So, for example, the photon (a boson) has a supersymmetric partner called the photino (a fermion). The idea is that at some time in the universe's early history, the higher energy levels were such that each supersymmetric boson-fermion pair were merely different states of the same type of elementary particle. As the energy levels dropped, supersymmetric symmetry breaking took place, and, as a consequence, the particles with which we are currently familiar, interact very rarely with their supersymmetric partners. These weakly-interacting supersymmetric particles provide a nice candidate to explain the existence of dark matter in astronomy and cosmology. Moreover, as Rugh and Zinkernagel point out "In a supersymmetric theory the fermion and boson contributions to the vacuum energy would cancel to an exact zero (they are equally large and have opposite sign), so if we lived in a world in which each particle had a superpartner, we would understand why the vacuum energy vanishes." Sadly, supersymmetric symmetry breaking seems to produce a non-zero energy density which exceeds that required to explain dark energy. Perhaps we need to combine supersymmetry with a refined understanding of the link between quantum field theory and general relativity. Food for thought...

Kubica again

Robert Kubica has been prevented by the FIA doctors from taking part in this weekend's American Grand Prix, following his violent accident in last Sunday's Canadian Grand Prix. There is therefore no need for me to be impressed.

Blind light

Today I happened to be in London, and the opportunity arose in the afternoon to visit either the Star Wars exhibition, or Antony Gormley's exhibition, 'Blind Light', at the Hayward...

There were some very cool installations at the Gormley exhibition. The centrepiece is 'Blind Light', a room suffuse with water vapour and bright fluorescent light. Within moments of entering, you lose sight of anyone else, and everything becomes a complete white-out. Coughing in the water vapour, you stumble forward until a subtle darkening indicates that you're approaching one of the glass walls. Eventually your outstretched hand touches glass, and you begin to feel your way around.

Also very cool are the sentinels placed on various buildings around the Hayward. There are even some in the far distance, on buildings across the Thames. And in terms of a demonstration of craft, I was most impressed by 'Matrices and Expansions', a collection of 3-d polygonal wireframe meshes, with human figures embedded within.

Kubica's accident

Poland's Robert Kubica intends to race at the American Grand Prix on Sunday, after this accident yesterday. I'd be impressed.

Kinaesthetic sensations

The philosophy of perception has very little to say about kinaesthetic sensations. These sensations tell us about the inertial forces and spatial motion of our bodies. The semicircular canals within the inner ear function as a set of accelerometers in three perpendicular planes. Each canal detects angular acceleration about an axis perpendicular to the plane of the canal. Whilst the semicircular canals detect rotational acceleration, linear acceleration is detected by adjacent organs within the inner ear, called the otoliths. These are small particles of calcium carbonate, suspended in space, which move and deflect sensitive hair cells when we undergo a linear acceleration.

So we know when we're undergoing a linear or rotational acceleration, and we also know our orientation relative to the gravitational field of the Earth: we know when we're upright, or lying horizontal, or hanging upside-down. These kinaesthetic sensations surely have implications for any philosophical claim that space is a merely a geometrical mental construct, rather than something which objectively exists, and defines inertial forces.

Air Traffic - Shooting Star

This is Air Traffic, Bournemouth's most impressive musical export since Max Bygraves. I really like this song, although it's all a bit Coldplay, and the bloke even sounds like the execrable Chris Martin. Some nicely built guitar riffs though. Although I'm not totally sure what a riff is.

Millennium Simulation

Here's a nice animated 'flythrough' of the current distribution of galaxies and galaxy clusters, produced from the results of 2005's Millennium Simulation. This simulation was able to take the initial distribution of matter, inferred from observations of the cosmic microwave background radiation, and by applying a 'cold dark matter' model, was able to reproduce the current galaxy distribution some 10 billion or so years later. No doubt, there was some 'parameter-fitting' going on, but it's impressive nevertheless.

The Schwinger limit

Physicists are threatening to boil the quantum vacuum! The quantum vacuum is, purportedly, a seething foment of 'virtual' particles and anti-particles; each particle--anti-particle pair is created spontaneously from nothing, and mutually annihilates shortly thereafter. These particles are deemed virtual because they describe trajectories in space-time which violate the relativistic relationship between mass and energy-momentum; they are said to be 'off the mass-shell'.

However, if a force field can be created with a potential gradient greater than the Schwinger limit of 8 x 10¹⁸ volts per metre, then this will be sufficient to move the virtual particles onto the mass shell, and for the particle--anti-particle pairs to separate. In effect, these particles will begin to boil from the quantum vacuum. Some physicists are starting to think that high intensity laser technology will, within a few decades, be able to generate electromagnetic fields which exceed the Schwinger limit.

Rain on Neptune!

It seems there's a 40% chance of rain on Neptune in 8 billion years! That's got to be the best long-range weather forecast ever!

Sunday evening radio

Why is the radio so turgid on a Sunday evening? It's full of self-absorbed people discussing their 'relationships' and personal problems. For chrissake, ditch the yapping and play some decent music! That'll cheer people up, not this mawdling, insincere, sub-counselling rubbish.

Moonbows

The LA Times recently printed this spectacular photo of a moonbow over Yosemite falls at night, (courtesy of Dr Karl and Radio 5's 'Up all Night' programme). A moonbow is a rainbow at night. Light from the full moon, shining upon water vapour in the air, can produce a rainbow via the same mechanism by which light from the Sun produces a daytime rainbow. The full Moon is required to produce sufficient light to excite the colour receptors in the eye.