Sky & Telescope - June 2022 - 4

SPECTRUM by Peter Tyson
Mind the Gap
GRAVITATIONAL-WAVE ASTRONOMY, the science behind our cover
story this month, has many amazing aspects: That we can actually
detect these Einstein-predicted ripples in spacetime. That they
arrive at our detectors from billions of light-years away. That they
open a whole new window on the universe, allowing us to " hear "
events that are unseeable, like the merging of two black holes.
To me, the most amazing thing of all is the almost unimaginable gap
between the magnitude of the events observed and the magnitude of the
gravitational-wave signal we receive at our detectors. (For details on the supersensitive
instruments, including LIGO and Virgo, that catch these waves, see
S&T: Feb. 2018, p. 32.) The phenomena that trigger gravitational waves are some
of the most energetic in the universe. Yet the notice we get of these inconceivably
violent affairs makes " brief, short, and faint " sound overstated.
That's not hyperbole. The first convergence of two black holes ever detected
- on September 14, 2015 - generated a signal at the LIGO detectors that lasted
just two-tenths of a second. And while those two black holes each contained
roughly 30 times the Sun's mass, the change in distance between the LIGO mirrors
that their union caused - and that scientists measured - was equivalent to
altering Earth's orbit around the Sun by the diameter of a hydrogen atom.
As for sound, we " hear " the echo
pSpectrogram of " chirp " showing
gravitational waves (greenish line) rising in
frequency as two neutron stars merge
of these wildly powerful astrophysical
episodes as mere chirps. That's the term
scientists use, because that's what the
signals sound like when converted to
audible frequencies we can hear. Have a
listen at https://is.gd/LIGOchirp.
It's extraordinary enough that scientists
can tease those ultra-truncated,
deeply hushed signals out of the dominant
" noise " at their detectors. But what they learn from that chirp is equally
stupendous, as Camille Carlisle outlines in her feature article on page 12. They
can gauge the event's distance and its rough location in space. They can tell
what kind of merging entities spawned the event - whether two black holes,
two neutron stars, or one of each. They can determine how massive the objects
were before they coalesced, and how massive the resulting behemoth was.
Finally, or I should say firstly, that tiny blip tells them how much the passing
gravitational waves distort spacetime. That is, how much the waves released by
these colossal collisions engender tiny stretches and compressions
in the coordinates describing the spacetime.
Mind the gap indeed!
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Sky & Telescope - June 2022

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