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On 2017-06-15 14:27, Louis Proyect via Marxism wrote:
http://www.salon.com/2017/06/14/extragalactic-black-hole-collision-deletes-two-suns-worth-of-mass-from-the-universe/
This 3rd detection of gravitational waves with high confidence is a
significant and newsworthy event. However the article in Salon that
Louis points us too is of rather poor quality, not written by a
physicist; below are some better articles I found. [I would generally
advise against choosing an article with the most intriguing headline.
More accurate information will usually avoid sensational headlines,
better reflecting the slow, methodical, and careful pace of serious
scientific experimentation.]
In particular, the article's saying that the black hole merger 3 billion
years ago "deletes two suns’ worth of mass from the universe" is
misleading, essentially wrong. It is only true in the sense that all
energy has an equivalence in mass (Einstein's famous E=mc^2), and that
the energy created in gravitational waves by this merger of two black
holes has a mass equivalence of two suns, an immensely huge energy.
Because that energy radiated away, the resulting mass of the merged
black hole was two solar masses less than the combined masses of the
original black holes. No physicist would call that "deleting mass." It
was not even similar to the annihilation of a positron and an electron,
in which those two particles' mass is converted into energy in the form
of gamma rays. Rather, consider that you have a rock heated to a high
temperature, and you put it on a scale. As the rock cools, losing its
heat energy, the mass of the rock will actually decrease by a tiny
amount (no, you couldn't possibly detect that with any balance!) even
though every single atom in the original rock is still there. The lost
mass m would satisfy E=mc^2 where E is the thermal energy lost from the
rock as it cooled.
The source of the huge energy generated in this case was not direct
annihilation of massive particles, but conversion of gravitational
potential energy to kinetic energy, "gravitational collapse." That's
essentially no different than if you have a boulder at the top of a
building and drop it out of the window: as it hits the ground it will
have a large kinetic energy equal to the loss of potential energy (its
height above the ground times its weight). If you then put the boulder
in an elevator and bring it back upstairs, the elevator will supply the
same amount of energy back to the boulder, not changing the boulder
itself but only its position in the earth's gravitational field which we
call potential energy, gravitational energy in this case. The huge
masses of the black holes falling into each other likewise liberates a
huge kinetic energy, of which mc^2 was radiated away as gravitational
waves, with m being twice the mass of our sun. That is what the author
should have written.
For better articles on this gravitational wave detection, see:
https://phys.org/news/2017-06-gravitational-insight-black-holes.html
https://www.ligo.caltech.edu/news/ligo20170601
- Jeff
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