- Levenson, Thomas.
The Hunt for Vulcan.
New York: Random House, 2015.
ISBN 978-0-8129-9898-6.
-
The history of science has been marked by discoveries in
which, by observing where nobody had looked before, with
new and more sensitive instruments, or at different
aspects of reality, new and often surprising phenomena
have been detected. But some of the most profound of
our discoveries about the universe we inhabit have come
from things we didn't observe, but expected to.
By the nineteenth century, one of the most solid pillars of
science was Newton's law of universal gravitation. With a
single equation a schoolchild could understand, it
explained why objects fall, why the Moon orbits the Earth and
the Earth and other planets the Sun, the tides, and the
motion of double stars. But still, one wonders: is the law
of gravitation exactly as Newton described, and does it work
everywhere? For example, Newton's gravity gets weaker as the
inverse square of the distance between two objects (for example, if
you double the distance, the gravitational force is four times
weaker [2² = 4]) but has unlimited range: every
object in the universe attracts every other object, however
weakly, regardless of distance. But might gravity not, say,
weaken faster at great distances? If this were the case,
the orbits of the outer planets would differ from the predictions
of Newton's theory. Comparing astronomical observations to
calculated positions of the planets was a way to discover
such phenomena.
In 1781 astronomer
William Herschel
discovered
Uranus, the
first planet not known since antiquity. (Uranus is dim but
visible to the unaided eye and doubtless had been seen
innumerable times, including by astronomers who included it
in star catalogues, but Herschel was the first to note its
non-stellar appearance through his telescope, originally
believing it a comet.) Herschel wasn't looking for a new
planet; he was observing stars for another project when he
happened upon Uranus. Further observations of the object
confirmed that it was moving in a slow, almost circular orbit,
around twice the distance of Saturn from the Sun.
Given knowledge of the positions, velocities, and masses of
the planets and Newton's law of gravitation, it should be possible
to predict the past and future motion of solar system bodies
for an arbitrary period of time. Working backward, comparing the
predicted influence of bodies on one another with astronomical
observations, the masses of the individual planets can be estimated
to produce a complete model of the solar system. This great work
was undertaken by
Pierre-Simon Laplace
who published his
Mécanique céleste
in five volumes between 1799 and 1825. As the middle of the 19th
century approached, ongoing precision observations of the planets
indicated that all was not proceeding as Laplace had foreseen.
Uranus, in particular, continued to diverge from where it was expected
to be after taking into account the gravitational influence upon
its motion by Saturn and Jupiter. Could Newton have been wrong,
and the influence of gravity different over the vast distance of
Uranus from the Sun?
In the 1840s two mathematical astronomers,
Urbain Le Verrier
in France and
John Couch Adams
in Britain, working independently, investigated the possibility that
Newton was right, but that an undiscovered body in the outer solar system
was responsible for perturbing the orbit of Uranus. After almost
unimaginably tedious calculations (done using
tables
of logarithms and pencil and paper arithmetic), both Le Verrier and
Adams found a solution and predicted where to observe the new planet.
Adams failed to persuade astronomers to look for the new world, but Le Verrier
prevailed upon an astronomer at the Berlin Observatory to try, and
Neptune was duly
discovered within one degree (twice the apparent size of the full Moon)
of his prediction.
This was Newton triumphant. Not only was the theory vindicated, it
had been used, for the first time in history, to predict the existence
of a previously unknown planet and tell the astronomers right where to
point their telescopes to observe it. The mystery of the outer solar
system had been solved. But problems remained much closer to the Sun.
The planet
Mercury
orbits the Sun every 88 days in an eccentric orbit which never exceeds
half the Earth's distance from the Sun. It is a small world, with
just 6% of the Earth's mass. As an inner planet, Mercury never appears more
than 28° from the Sun, and can best be observed in the morning or
evening sky when it is near its maximum elongation from the Sun.
(With a telescope, it is possible to observe Mercury in broad
daylight.) Flush with his success with Neptune, and rewarded with
the post of director of the Paris Observatory, in 1859 Le Verrier
turned his attention toward Mercury.
Again, through arduous calculations (by this time Le Verrier had a
building full of minions to assist him, but so grueling was the
work and so demanding a boss was Le Verrier that during his
tenure at the Observatory 17 astronomers and 46 assistants
quit) the influence of all of the known planets upon the motion
of Mercury was worked out. If Mercury orbited a spherical Sun
without other planets tugging on it, the point of its closest
approach to the Sun (perihelion) in its eccentric orbit would
remain fixed in space. But with the other planets exerting their
gravitational influence, Mercury's perihelion should advance around the
Sun at a rate of 526.7 arcseconds per century. But astronomers
who had been following the orbit of Mercury for decades measured the
actual advance of the perihelion as 565 arcseconds per century.
This left a discrepancy of 38.3 arcseconds, for which there was
no explanation. (The modern value, based upon more precise
observations over a longer period of time, for the
perihelion
precession of Mercury is 43 arcseconds per century.) Although
small (recall that there are 1,296,000 arcseconds in a full circle),
this anomalous precession was much larger than the margin of error
in observations and clearly indicated something was amiss.
Could Newton be wrong?
Le Verrier thought not. Just as he had done for the anomalies of
the orbit of Uranus, Le Verrier undertook to calculate the properties
of an undiscovered object which could perturb the orbit of Mercury
and explain the perihelion advance. He found that a planet closer
to the Sun (or a belt of asteroids with equivalent mass) would do
the trick. Such an object, so close to the Sun, could easily have
escaped detection, as it could only be readily observed during a
total solar eclipse or when passing in front of the Sun's disc (a
transit).
Le Verrier alerted astronomers to watch for transits
of this intra-Mercurian planet.
On March 26, 1859,
Edmond
Modeste Lescarbault, a provincial
physician in a small town and passionate amateur astronomer
turned his (solar-filtered) telescope toward the Sun. He saw
a small dark dot crossing the disc of the Sun, taking one hour
and seventeen minutes to transit, just as expected by Le
Verrier. He communicated his results to the great man, and
after a visit and detailed interrogation, the astronomer certified
the doctor's observation as genuine and computed the orbit for
the new planet. The popular press jumped upon the story. By
February 1860,
planet
Vulcan was all the rage.
Other observations began to arrive, both from credible and unknown
observers. Professional astronomers mounted worldwide campaigns to
observe the Sun around the period of predicted transits of Vulcan.
All of the planned campaigns came up empty. Searches for Vulcan
became a major focus of solar eclipse expeditions. Unless the
eclipse happened to occur when Vulcan was in
conjunction
with the Sun, it should be readily observable when the Sun was
obscured by the Moon. Eclipse expeditions prepared detailed star
charts for the vicinity of the Sun to exclude known stars for the
search during the fleeting moments of totality. In 1878, an
international party of eclipse chasers including Thomas Edison
descended on Rawlins, Wyoming to hunt Vulcan in an eclipse
crossing that frontier town. One group spotted Vulcan; others
didn't. Controversy and acrimony ensued.
After 1878, most professional astronomers lost interest in Vulcan.
The anomalous advance of Mercury's perihelion was mostly set
aside as “one of those things we don't understand”,
much as astronomers regard
dark matter
today. In 1915, Einstein published his theory of gravitation:
general relativity. It predicted that when objects moved rapidly
or gravitational fields were strong, their motion would deviate
from the predictions of Newton's theory. Einstein recalled the
moment when he performed the calculation of the motion of Mercury
in his just-completed theory. It predicted precisely the perihelion
advance observed by the astronomers. He said that his heart shuddered
in his chest and that he was “beside himself with joy.”
Newton was wrong! For the extreme conditions of Mercury's orbit,
so close to the Sun, Einstein's theory of gravitation is required to
obtain results which agree with observation. There was no need for
planet Vulcan, and now it is mostly forgotten. But the episode is
instructive as to how confidence in long-accepted theories and wishful
thinking can lead us astray when what might be needed is an overhaul of
our most fundamental theories. A century hence, which of our beliefs
will be viewed as we regard planet Vulcan today?
January 2016 