Tuesday, August 31, 2010

'Out There' Astrophysics Impacts Technology (again)

A favorite staple of high-tech science fiction, the gigantic laser weapon, may have some limitations imposed by fundamental physics.

Physics Central: Lasers reaching their limit

In 1997 at the Stanford Linear ACcelerator (SLAC), electrons with 47 GeV (giga-electron volts) of energy were collided with the beam of a green laser.   The high-energy electrons collide against the photons at an angle that transfers energy to the photons in the laboratory rest frame, a process known as the inverse Compton Effect.  This increased the energy of the laser photons from the green wavelength of light up into the gamma-ray range.  These gamma-ray photons subsequently collided with other low energy photons in the laser beam, creating electron-positron pairs.  The two photons collided with enough energy in the center-of-momentum (CM) frame that their combined energy exceeded 1.1MeV (million electron volts), the threshold for pair production.  This was the first time photon energy was directly converted into matter.  It is the inverse process of electrons and positrons colliding to form gamma-rays.  [NY Times: Scientists Use Light to Create Particles, 4]

It is now becoming clear that above some photon energy density, this pair-production process can happen spontaneously - enough photons will have energy above the threshold that they will start a cascade of pair production,  followed by pair annihilation, followed by pair production...  This would suggest there is a quantum-imposed limit to the energy density of lasers[1].

While this might not seem to be an astrophysics issue, one needs to investigate the history.  I mentioned some of this in an earlier post (see Testing Science at the Leading Edge)

When antimatter was first discovered in 1932, with the identification of the positron, we had the first experimental verification of the process of matter-antimatter annihilation, where the collision of an electron and positron would produce two photons (with no other particles around, at least two photons are required to conserve momentum).

One of the heavily tested (but by no means proven) fundamental principles of physics is that sub-atomic processes are reversible in time.  It is a principle that has been tested in many cases and found to hold, but it has not been demonstrated as an absolute.  However, it holds so well that it is generally assumed valid for interactions where it has not yet been tested.  If an opportunity arises where it is tested and fails, there will undoubtedly be a Nobel prize for that researcher. 

So if an electron and positron can collide to produce two photons, by time-reversal symmetry, it stands to reason that one can collide two photons of sufficient energy (in excess of 1.1 MeV) and create an electron-positron pair.  The probability for such a reaction was first calculated in 1934, shortly after the discovery of the positron, by Breit and Wheeler[2,3].  This reaction probability was sufficiently small that no one in the 1930s had the technology to test it, so it remained an interesting concept.

But in the 1960s, x-ray detectors (wikipedia, NASA/GSFC) launched on board rockets above the Earth's atmosphere (which was too thick for cosmic x-rays to penetrate) began detecting high-energy point sources in space.  Gamma-ray detectors would detect photons with energies in excess of the 1.1 MeV threshold and the question arose as to what could produce these high-energy photons (wikipediaNASA/GSFC).

One of the processes recognized as a possible source of these photons were extremely high temperature plasmas of electrons, positrons, and photons, also called a pair plasma.  Here are just a few of the papers published studying the environment created by such as plasma.

    •    1964, Neutrino Processes and Pair Formation in Massive Stars and Supernovae
    •    1979, Photon Pair Production in Astrophysical Transrelativistic Plasmas
    •    1981, Annihilation radiation from a hot e/+/-e/-/ plasma
    •    1982, Relativistic thermal plasmas - Pair processes and equilibria
    •    1983, Radiation spectrum of optically thin relativistic electron-positron plasma
    •    1984, Spectra from pair-equilibrium plasmas
    •    1995, Thermal Comptonization in Mildly Relativistic Pair Plasmas

Astrophysicists have been exploring this type of plasma environment for thirty years, prior to verification of the process in the laboratory, based only on the extrapolation of some very fundamental physical principles. 

There are a surprising number of phenomena where a fundamental principle has been subjected to pretty heavy testing at current laboratory scales: energy-momentum conservation, time reversibility, Lorentz invariance, the wave function properties of fermions and bosons, etc.  Astrophysicists have occasionally explored the extreme limits of these principles and obtained some unusual predictions.  For example, the fact that electrons and neutrons are fermions (no more than two can occupy the same quantum state at the same time, AKA the Pauli Principle) implies that there high density configurations where an object can be held up by the 'pressure' created by this limit.  Computations demonstrate that such objects would have sizes and masses consistent with white dwarf and neutron stars.  I'm still assembling some of the fascinating nuclear physics surrounding these ideas.

  1. Limitations on the attainable intensity of high power lasers
  2. G. Breit and J. A. Wheeler. Collision of Two Light Quanta. Physical Review, 46:1087–1091, December 1934. doi: 10.1103/PhysRev.46.1087.
  3. M. S. Plesset and J. A. Wheeler. Inelastic Scattering of Quanta with Production of Pairs. Physical Review,  48:302–306, August 1935. doi: 10.1103/PhysRev.48.302.
  4. D. L. Burke, R. C. Field, G. Horton-Smith, J. E. Spencer, D. Walz, S. C. Berridge, W. M. Bugg, K. Shmakov, A. W. Weidemann, C. Bula, K. T. McDonald, E. J. Prebys, C. Bamber, S. J. Boege, T. Koffas, T. Kotseroglou, A. C. Melissinos, D. D. Meyerhofer, D. A. Reis, and W. Ragg. Positron Production in Multiphoton Light-by-Light Scattering. Physical Review Letters, 79:1626–1629, September 1997. doi: 10.1103/Phys-RevLett.79.1626.

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