## Gamma-Ray Photon Observations Indicate Space-Time Is Smooth 81 81

eldavojohn writes

*"Seven billion light years away (seven billion years ago), a gamma-ray burst occurred. The observation of four Fermi-detected gamma-ray bursts (GRBs) has led physicists to speculate that space-time is indeed smooth (abstract and a pre-publication PDF both available). A trio of photons were observed to arrive very close together, and the observers believe that these are from the same burst, which means there was nothing diffracting their paths from the gamma-ray burst to Earth. This observation doesn't prove that space-time is infinitesimally smooth like Einstein predicted, but does indicate it's smooth for a range of parameters. Before we can totally discount the theory that space-time is comprised of Planck-scale pixels, we must now establish that the proposed pixels don't disrupt the photons in ways independent of their wavelengths. For example, this observation did not disprove the possibility that the pixels exert a subtler 'quadratic' influence over the photons, nor could it determine the presence of birefringence — an effect that depends on the polarization of the light particles."*
## Re:Size matters... (Score:5, Informative)

Probably they refer to the electron's Compton length, which in some sense can be viewed as effective size of the electron. If you try to resolve the electron beyond that size, you inevitably get particle creation.

However if I'm not mistaken, a billionth of a billionth of the electron's Compton wavelength is still about five orders of magnitude larger than the Planck length.

## Re:Size matters... (Score:3, Informative)

The electromagnetic field of a point particle is infinitely large. In the collisions, the particles don't

reallycollide; they just get so close that the force between them gets quite large.## Re:Size matters... (Score:4, Informative)

## Re:Size matters... (Score:4, Informative)

For a number of scientific considerations, one can treat elementary particles (like the electron) as point-like objects, and legitimately so.

But the Planck length is a unit that is about 18 magnitudes (i.e., 18 powers of 10) smaller than anything one might define as the "size" of an electron.

If you imagine a ruler with a dozen Planck lenghts as units printed on it instead of inches, then in comparison an electron would be an enormous object, much bigger than the size of a planet.

## Re:Space/time duration/distance (Score:5, Informative)

There are multiple distance measures in cosmology - they are all in principle exact (at least, if you know all your cosmological parameters), but they differ significantly once you start getting above about 1 billion light years. Much above that, and they can differ incredibly much. Some of these measures are based on idealized measurements, others on the physics directly.

Some measures used in cosmological work are,

- proper motion distance (the distance a parallax measurement would give you)

- luminosity distance (the distance you would infer from the apparent brightness of a standard candle)

- angular diameter distance (the distance you would infer from the apparent angular size of a standard sized object). The angular diameter distance is notorious for getting smaller if you get far enough away in many cosmologies (including, apparently, the one we live in).

- look back distance (if you imagine that everyone has a clock synchronized at the big band, the difference between your time and the time you would read on the remote clock, if you could read it). This is also called the light travel time.

- proper distance (what some long yardstick would read).

- comoving distance (the proper distance divided by the scale factor - 1 plus the redshift, z - for the remote observer, to get a distance that doesn't change with cosmological time).

And, finally, each cosmological model will have a coordinate distance (the difference between the coordinates of two different places), which need not have a simple relation to any of the above.

It is fair to say that one of the easiest ways to make a fool of yourself in cosmology is to mix up distance scales. (As an additional cause of mixups, only proper distances can be subtracted - for the rest, the distance between A and B is NOT the difference of the distance to A and the distance to B, even if A and B are on a straight line as seen from the Earth.)

In this case, the Gamma Ray Burst 090510A was at a red shift of 0.897. Go to the Cosmology Calculator [ucla.edu] and you find that that

For Ho = 71, OmegaM = 0.270, Omegavac = 0.730, z = 0.897

It is now 13.666 Gyr since the Big Bang.

The age at redshift z was 6.376 Gyr.

The light travel time was 7.290 Gyr.

The comoving radial distance, which goes into Hubble's law, is 3053.8 Mpc or 9.960 Gly.

The angular size distance DA is 1609.8 Mpc or 5.2505 Gly.

The luminosity distance DL is 5793.1 Mpc or 18.895 Gly.

The proper distance is (1+z) times the comoving distance, or 18.89 Gly.

## Re:Size matters... (Score:4, Informative)

It is not so much that elementary particles are mathematical points (zero dimensional objects) as that they have no internal "structure" like the 'non-elementary' particles (protons, for example) and no even more 'elementary' constituents. At the deepest level in the Standard Model all 'particles' are described as excitations of quantum fields and have positive probability density over a region in space which is not a point. Electrons are conventionally referred to as "point particles" but that is slang for the deeper description.