Saturday, March 19, 2016

Tale of the Recent Gravity Wave Discovery GW150914

[This is a guest blog by Mark G. Gray, a physicist who understands people and many other things. Reading it put a whole lot of perpective in my life.]

Thirteen hundred million years ago in a galaxy thirteen billion trillion kilometers away, a small dark sphere looms ominously near a slightly larger dark sphere.

The small sphere is one hundred thirty kilometers in diameter.  Its surface is neither solid, nor liquid, nor gas, nor plasma.  Nor is it visible except by absence; no light passes through, nor is emitted from, nor reflected by it.  But its presence is felt throughout the universe: any unfortunate structure within a few hundred kilometers of it would be torn apart by tidal forces, with the pieces glowing X-rays as they fall into the surface and disappear from the universe.  Even the light from distant stars streaming around its edges is twisted to form a distorted halo.

The large sphere is nearly identical to the small sphere, but is one hundred seventy kilometers in diameter.

Although they pass within a few hundred kilometers of each other, the spheres do not tear each other apart.  Instead they twirl around a point approximately midway between them, moving closer and faster on each orbit, past the point where their diameters overlap, finally rotating seventy-five times per second.

When the smaller's center collides with the larger's surface, there is no sound, no light, no X-rays, no ejection of debris, nothing to indicate a collision in the conventional sense, but instead just a wobble in the merged dark shapes and a ripple in space-time that alternately doubles and halves nearby lengths relative to widths as it passes.

The remnant of the encounter is a single dark sphere three hundred seventy kilometers in diameter, a spherical wave in space-time expanding at the speed of light, and perhaps the explosion of a nearby star triggered as the wave passes through it.

Meanwhile, only two hundred forty thousand trillion kilometers from the center of the Milky Way galaxy on its Orion arm, the planet Earth is in the middle of its Mesoproterozoic era.  The super-continent Rodinia has just formed from three pre-existing continents. Eukaryotes, cells with a well defined nucleus and organelles have emerged, but not yet evolved into multi-cellular life.  The Moon, which is still geologically active, orbits the Earth in a little over three weeks.

Two hundred thousand years ago the wave front reaches the Small Magellanic Cloud, a dwarf galaxy in the Milky Way's neighborhood.  The planet Earth is in the late Pleistocene epoch of its Cenozoic era. The seven contemporary continents are in place, the glaciers are in retreat, and modern humans have just emerged and invented agriculture. The Moon, geologically dead for over a billion years, orbits the Earth in a little under four weeks.

One hundred years ago, as the wave front passes through the stars in the Milky Way's Hydrus constellation, the human Albert Einstein uses his theory of General Relativity to show that accelerating masses can produce gravitational waves in space-time.  Karl Schwarzschild publishes the first solution to Einstein's General Relativity for a spherical mass.

Seventy-seven years ago J. Robert Oppenheimer uses S. Chandrasenkahr's work on stellar deaths to predict massive stars that have exhausted their nuclear fuel would collapse under their own weight to form a singularity.

Fifty-eight years ago David Finkelstein uses Schwarzschild's solution to show Oppenheimer's singularity would be surrounded by a spherical event horizon, a black hole, from which nothing, not even light, can escape.

Fifty-four years ago M. E. Gertsenshtein and V. I. Pustoviot describe how interfering perpendicular beams of correlated light can detect gravitational waves.

Thirty-two years ago Kip Thorne, Ronald Drever, and Rainier Weiss establish the Laser Interferometer Gravitational-wave Observatory (LIGO).

Twenty-eight years ago they secure funding for LIGO.

Twenty-two years ago LIGO construction begins.

Fourteen years ago LIGO becomes operational.  LIGO operates for eight years without seeing a gravitational wave.

Six years ago LIGO is shut down for improvements.  The gravitational wave moves among our sun's nearest neighbor stars.

On September 12, 2015 the Advanced LIGO starts its first operational run, with just enough sensitivity to detect the gravitational wave that is now about four times further from Earth than Voyager 1.

At 09:50:45 UTC on September 14, 2015 the Advanced LIGO at Livingston, Louisiana detects the gravitational wave when its four kilometer length oscillates relative to its four kilometer width by a fraction of the size of a subatomic particle.  Several thousandths of a second later and three thousand two kilometers away, the Advanced LIGO at Hanford, Washington detects the gravitational wave.  The signal, designated GW150914, cycles eight times, increasing in both intensity and frequency, until it reaches an intense chirp at its crescendo.

On February 11, 2016 the Advanced LIGO team announces their detection of a gravitational wave.  The coincidence of the signal at the two detectors implies a non-local source.  The similarity of the two signals implies the detection of a single, real event.  The time difference between the signals triangulates a direction, and the red-shift of the signal gives a distance to the source.  The spectrum of the signal matches general relativity's prediction for the inspiral and merger of binary black holes and lets them reconstruct what happened:

    A black hole twenty-nine times the mass of our sun encounters
    another black hole thirty-six times the mass of our sun.  As the
    black holes scatter around their center-of-mass by mutual
    gravitational attraction, they lose kinetic energy radiated away
    as gravitational waves.  The binary black holes, now trapped in
    orbit, centripetally accelerate around their center, radiating
    more gravitational waves, losing more energy, moving ever
    closer together and orbiting ever faster.  They finally merge,
    emitting a blast of gravitational waves, to form a single black
    hole about sixty-two times the mass of the sun, with three solar
    masses converted entirely to gravitational wave energy in a
    spherical front moving outward at the speed of light.  At its peak
    the merger produces several times more power than all the stars
    in the observable universe.

p.s. No, the GW doesn't stand for Gerald Weinberg, nor for anything as small as our Earth or even our little corner of the Universe.

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