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Oct
01

There is a serious physics error in the Discover article:

Another example of the trickiness of coordinates, drawn from general relativity, is the black hole. In the canonical Schwarschild coordinates describing a black hole, it looks like terrible things (e.g., singularities) happen at the event horizon [or Schwarschild radius, which represents the 'surface'] of the black hole. But these are a problem with the coordinates. In truth, nothing particularly weird happens as you cross the surface of a black hole (besides gravitational lensing causing the sky to appear bent and warped). This can be seen by writing the exact same spacetime in different coordinates (e.g., Kruskal coordinates), where everything becomes well behaved (except for the singularity itself). No big deal crossing the event horizon (though all hell breaks loose as you approach the singularity). A similar confusion resided in the nature of gravitational waves.

Something weird does happen if you are lowered on a cable, or have rocket engines thrusting to the center of the black hole. There is the quantum Unruh effect, such static observers will see black body radiation of temperature

kBT(r) = (hcrs/r^2)(1 - rs/r)^-1/2 + hc/rs---> infinity as r ---> rs+

whilst a locally coincident geodesic observer only sees the Hawking black body radiation of temperature hc/rskB

Also, these static observers will also collide with in-falling matter that is infinitely blue shifted at the event horizon.

On Sep 30, 2010, at 4:55 PM, nick herbert wrote:*Here's a nice blog post by daniel holz (feynman professor at Los Alamos) about the history of gravitational waves:*

http://blogs.discovermagazine.com/cosmicvariance/2007/04/25/the-difficult-childhood-of-gravitational-waves/*The comments are particularly amusing and instructive.
Since gravity is so weak there are few opportunities to test Einstein's theory (and its competitors) experimentally. The newest test is the lifetime of the Hulse-Taylor binary pulsar which agrees with GR to 0.2% and for which Hulse & Taylor appropriately received the 1993 Nobel Prize. First indirect measurement of Einsteinian gravitational waves.
The intensity of such waves depends on how many polarization degrees of freedom one assigns to the graviton. For a speed of light spin-2 graviton the number of degrees is 2. The intensity of G-radiation depends directly on these degrees of freedom so if you double the degrees of freedom you double the intensity and seriously mess up the agreement with experiment. Graviton degrees of freedom is not a small effect.
Jack has proposed a clever path to quantum gravity that invokes spin-1 tetrads as the basis out of which the spin-2 graviton is composed and he has not failed to recognize that in quantum mechanics 1 + 1 can add up not only to 2 but to zero and to one also. Therefore on the face of it, Jack's suggestion predicts not only a spin-2 graviton but a spin-1 and a spin-0 graviton as well--for which I propose the names scalar and vector Sarfattions. *

The spin 0 and spin 1 gravitons may have rest mass. As you point out the Hulse-Taylor data only shows massless spin 2. Yukawa and all that.

But the Hulse-Taylor results show that GR does not need extra polarization degrees of freedom. Therefore in order for the universe to make sense:

ALL LOW-SPIN SARFATTIONS MUST BE SUPPRESSED!

Perhaps the first step in the necessary suppression of Sarfattions is to keep them from being mentioned at scientific conferences. Or even in private e-mail conversations such as this one. Since I have already let the cat out of the bag I offer a compromise--that the three Sarfattions be referred to only by their initials in a kind of secret code known only to the inner circle. Thus SS, VS, and TS for the scalar, vector and tensor "you-know-whats".

Jack's new "you-know-what" particles possess one unique property. They are one of the few new particles NOT PREDICTED TO BE SEEN at the Large Hadron Collider.

Good point, especially if they have rest mass. In any case, the Hulse-Taylor data shows that if they exist they are not massless, therefore, we can't detect them in the far-field.

Nick Herbert

http://quantumtantra.blogspot.com