LIGO sensitivity compared with the human eye

[Jos Bergervoet]

Recently in another newsgroup (nl.wetenschap) the question
came up of LIGO's sensitivity compared to the human eye.

a) LIGO can (easily) see a source at one billion light years
distance which emits the energy of 3 solar masses in 1 second
(like the black hole mergers!)

b) The eye can (also quite easily) see a source at 10 lightyears
distance emitting the energy of 0.1 solar mass in 10 billion
years (like some of the sun-like nearby stars, burning 10% mass
in their entire lifetime).

Comparing the energy flux received:
- In case a) source power (energy per second) is 1e19 x higher.
- 1/r^2 attenuation with distance is 1e16 x larger for a).

So it seems that LIGO receives 1000 times stronger energy flux
and therefore the eye is 1000 times more sensitive than LIGO.

Still, since we are playing with many orders of magnitude here,
the difference is remarkably small. Also the cases compared are
not the absolute sensitivity levels of the two systems, both
LIGO and the eye can see somewhat weaker signals. So the simple
estimate here might not completely settle it. Should anything
be adjusted in the comparison above?

(And if not, when will LIGO's successors surpass the eye?)

[Michael Asherman]

[Moderator's note: Quoted text moved to top, and trimmed. -P.H.]

Post by Jos Bergervoet
a) LIGO can (easily) see a source at one billion light years
distance which emits the energy of 3 solar masses in 1 second
(like the black hole mergers!)

Does the figure of 3 solar masses include only gravitational energy, or is
it total energy including electromagnetic?

[Thomas 'PointedEars' Lahn]

Post by Michael Asherman
[Moderator's note: Quoted text moved to top, and trimmed. -P.H.]

Post by Jos Bergervoet
a) LIGO can (easily) see a source at one billion light years
distance which emits the energy of 3 solar masses in 1 second
(like the black hole mergers!)

Does the figure of 3 solar masses include only gravitational energy, or is
it total energy including electromagnetic?

By contrast to the merger of neutron stars (e.g. GW170817), there is no
electromagnetic (EM) energy coming from the merger of black holes (BHs).

*That* they do NOT emit EM radiation is why they are called *black* holes.

The result of such a merger is also a BH whose mass is necessarily
larger than each of the progenitor masses; therefore its Schwarzschild
radius is larger, too (râ=9B = 2 G M/c²), and any EM radiation produced
in the merger is then already beyond the event horizon of the resulting
BH.

The number of 3 Mâ probably refers to the first event detected by
Advanced LIGO, GW150914, where two BHs of masses 29 M_sun and 36 M_sun
merged to form a BH of 62 M_sun (instead of 65 M_sun), emitting the
*equivalent* of 3 M_sun (this is a *mass*, NOT an energy) in gravitational
energy (the energy is E = 3 M_sun c² instead). But that was NOT emitted
in 1 s, but in \_a fraction of a second_/, precisely 20 ms = 0.02 s.

<https://www.ligo.caltech.edu/page/press-release-gw150914>
<https://arxiv.org/abs/1602.03837>

<https://www.ligo.caltech.edu/page/press-release-gw170817>

[jacobnavia]

Le 12/01/2019 =C3=A0 12:01, Jos Bergervoet a =C3=A9crit=C2=A0:

The human eye can detect a SINGLE PHOTON...

See:

Direct detection of a single photon by humans
https://www.nature.com/articles/ncomms12172

That is the absolute minimum I would say.

Now, what is the quantum particle of gravity???

Well, better leave that question open isn't it?

:-)

[Steve Willner]

Post by Jos Bergervoet
Recently in another newsgroup (nl.wetenschap) the question
came up of LIGO's sensitivity compared to the human eye.

This is a hard comparison to make. Not the least of the problems is
that LIGO measures "strain," which falls off only linearly with
distance, not with distance squared. That means improving LIGO
sensitivity by a factor of 2 increases the volume surveyed by a
factor of 8.

I suppose one could pick some distance and ask what energy release is
needed to produce a detectable signal in each case. However,
efficiency of energy conversion is also relevant. When black holes
merge, all the energy that doesn't end up in the final object goes
into gravitational waves. By contrast, a supernova emits only a tiny
fraction (about 10^-4 from unreliable memory) of its energy in
visible light.
--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone [phone] ***@cfa.harvard.edu
Cambridge, MA 02138 USA

[[Mod. note -- Another very important difference in comparing LIGO/Virgo
sensitivity with that of any optical detector (e.g., the human eye) is
that the optical detector absorbs the indident photons, whereas LIGO/Virgo
is 99.9999...% transparent to gravitational waves. That is, only a very
tiny fraction of an incident gravitational wave's energy is transferred
to LIGO/Virgo -- the vast majority (much more than 99.999999%) passes
right through LIGO/Virgo (and right through the Earth).

One analogy I've read is that building a gravitational-wave detector
like LIGO/Virgo is like building a radio antenna out of wood. So it's
only by incredibly sophisticated optical and mechanical engineering that
LIGO/Virgo can detect gravitational waves at all.
-- jt]]