John
Michell (1724-1793) was Rector
of Thornhill,
England, and a friend of Henry
Cavendish. Michell
has
several claims to fame: Seismology:
Michell was one of the first to explore
the theory of earthquakes, and suggested that they might originate as
an event at a specific place (such as under a volcano), with the
shockwave then being transmitted through the Earth to affect a wider
region. A violent event ocurring in the Earth's crust out at sea might
shake
cities inland, with the 1760 Lisbon
quake being a likely example ... Lisbon's quake didn't
necessarily mean that there was anything
badly wrong actually underneath Lisbon, and didn't
mean that the city (or other cities) were
in danger of being suddenly swallowed up by the Earth (which must have
been a
reassuring idea to people who lived in them). If people
could record the precise times that they
experienced a
'quake (said Michell), we could
calculate the location where the disturbance actually originated.
Michell's
ideas were put into practice ~125 years later, by John
Milne
(1850-1913), and are the basis of modern seismology.
Light-momentum:
One of
Michell's more unsuccessful
experiments was to try to measure light-pressure. To do this, he used
a reflector to focus sunlight onto one side of a
compass needle. The needle got hot, briefly went in the wrong
direction (which Michell attributed to outgassing), and then melted.
Weighing the Earth:
Michell
developed an extremely
sensitive torsion balance, with the aim of measuring the amount of
gravity associated with a known amount of mass (the Gravitational
Constant). If one knew the ratio between gravitaitonal
and inertial
mass, and one knew the strength of the gravitational fields of
planets (by the orbits of objects circling them), then one could
calculate their masses.
Michell died
before he could carry out
the test, but Henry Cavendish
inherited his apparatus,
continued to develop it, and used it to perform the Cavendish
Experiment to
calculate the Earth's mass. Ancestor
of the Black Hole: In 1783,
Michell wrote a letter to
Cavendish (published in the 1784 volume of the prestigious Royal
Society's journal) discussing how we might measure the masses
of
distant stars. Michell's idea was that since light ought to be
weakened when it climbs out of a star's strong gravitaitonal field,
we should be able (in theory) to see a shift in the
characteristics of this starlight, that might tell us the star's
surface gravity. Michell reasoned that when weakened light from a
strong-gravity star was passed through a prism, it ought to be
deflected differently due to its reduced energy, and
this might result in the star's image being deflected differently by
the prism, with the offset between the refracted images of different
stars telling us the
difference in surface gravity between them. In practice, a shift in the
frequency of a star's visible light simply moves other frequencies
into the visible range (making Michell's suggestion impractical), but
the basic principle was sound, and modern
spectroscopy instead uses identifiable "notches" in a
star's spectrum as references for spectral shifts. While some
sources tell us that Einstein was the first person to come up
with the
idea of gravitational shifts (in
1911), Michell was already going beyond this and
exploring ways that future astronomers might be able to make practical
use of the effect, back in the Eighteenth Century.
Michell
also calculated that if a star
was big enough or dense enough, its surface escape velocity could
equal or exceed the speed of light, and we woudn't be able to see the
thing directly. The critical radius at which this happens can be
written in simplified form as "r=2M", where M
is the mass
of the star, and various natural constants (such as c
and G) are deliberately left
out. Michell's distance is what we would now refer to as the Schwarzchild
radius
associated with the event horizon of a black
hole under general relativity, and we now refer to the
sort of object
described by Michell as a "dark star",
to distinguish it
from GR1915's "black holes".
Although we
couldn't see these dark
stars directly, if a binary star system included one visible star and
one dark star, we'd be able to identify the "dark" object
by the motion of its visible companion (another idea often thought to
have originated in the Twentieth-Century). Michell suggested a
catalogue of binary
stars in order to find what proportion were "dark", in the hope
of producing some sort of statistical baseline for estimating
the amount
of other "dark", gravitationally-cloaked material there
might be in the universe.
The history
to Michell's "dark
star" piece is discussed in Thorne (1994), and possible reasons
for its obscurity until the mid-late C20th are discussed
in Baird
(2007). A transcript of the letter itself, is here.
Background references
- Isaac Newton, Opticks
(1704-)
- John
Michell
"On the
means of discovering the distance, magnitude etc. of the fixed stars ..."
Philosophical Transactions of the Royal Society
(1784)
- Albert
Einstein
"On
the Influence of Gravitation on the Propagation of Light"
(1911)
- Isaac
Asimov, Asimov's Biographical Encyclopedia of
Science
and Technology (revised 1972 edition)
–
lists most of Michell's key
achievements, but doesn't mention his "dark star" paper
- Clyde R. Hardin
"The scientific work of the Reverend John Michell",
Annals of Science 22 27-47
- Russell McCormmach
"John Michell and Henry Cavendish: Weighing the stars",
British Journal for the History of Science 4
126-155
- Simon
Schaffer "John Michell and
black holes", Journal for the History of Astronomy 10
42-43 (1979)
- Gary
Gibbons "The man who
invented black holes [his work emerges out of the dark after two
centuries]" New
Scientist 28 June pp.1101 (1979)
- Hawking
and Israel (eds), Three hundred
years of gravitation (1987), Werner
Israel, "Dark stars: The
evolution of an idea", pp. 199-276
- J.
Eisenstaedt "De L'influence
de la gravitation sur la propagation
de la lumière en théorie Newtonienne. L'archéologie des trous noirs",
Archive for the History of Exact Sciences 42
315-386 (1991)
- Kip
S. Thorne, Black Holes and Time warps
(1994), especially Chapter 3
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