It is sometimes said that
the constancy of physical law is an assumption of science. That may have
been true once, but constancy has been the subject of a great deal of
research. Today, experimental evidence places an upper limit on how much
the "constants" could have changed. Broadly, the answer is: at most one
percent over the lifetime of the universe.
Details
One nice piece of evidence comes from Supernova 1987a, which was special
because it was not very far away. Theory predicts that such a supernova
would create about 0.1 solar masses of nickel-56, which is radioactive.
Nickel-56 decays with a half-life of 6.1 days into cobalt-56, which in
turn decays with a half-life of 77.1 days. Both kinds of decay give off
very distinctive gamma rays. Analysis of the gamma rays from SN1987a
showed mostly cobalt-56, exactly as predicted. And, the amount of those
gamma rays died away with exactly the half-life of cobalt-56. For more
details, read:
- The Compton
Gamma Ray Observatory, Neil Gehrels et al, Scientific American,
December 1993, pp.68-77
- SN1987a Light Curves, P. Whitelock et al., in Proceedings of
the Tenth Santa Cruz Workshop in Astronomy and Astrophysics,
Springer- Verlag, 1991.
Since SN1987a was 170,000
light years away we were seeing light generated 170,000 years ago. This
means that radioactive decay ran at the same speed 170,000 years ago as
it does now.
Another evidence is the natural nuclear reactor at Oklo, in Gabon. This
reactor was actually just an unusually rich body of radioactive ore. So
rich, in fact, that when it was formed, it approached critical mass.
Studies of the unusual elements found there indicate that reactors acted
the same two billion years ago as they do now. If the fine structure
constant had been different by as little as one part in a million, the
Oklo measurements should have detected that.
Another evidence is in the light from distant galaxies. When you pass
starlight through a prism, you can see spectral lines, which just means
that there is an excess (or shortage) of light at specific frequencies.
Certain atoms (or molecules or reactions) produce distinctive spectral
lines. Modern physics has a solid theory for such things, and we can
calculate the frequencies from fundamental constants. Therefore, if we
look at a distant galaxy, we can tell if certain fundamental constants
are different there. Most of the references below discuss this.
Other methods mentioned in
the references:
Searches for changes in
the radius of Mercury, the Moon, and Mars. These would change
because of changes in the strength of interactions within the
materials that they are formed from.
Searches for long term
(secular) changes in the orbits of the moon and the earth, as
measured by looking at such diverse phenomena as ancient solar
eclipses and coral growth patterns.
Ranging data for the
distance from earth to Mars, using the Viking spacecraft.
Data on the orbital
motion of a binary pulsar PSR 1913+16.
Observations of
long-lived isotopes that decay by beta decay (Re 187, K 40, Rb 87)
and comparisons to isotopes that decay by different mechanisms.
Searches for differences
in gravitational attraction between different elements.
Absorption lines of
quasars. These measure fine structure and hyperfine splittings.
Laboratory searches for
changes in the mass difference between the K0 meson and its
antiparticle.
Non-physicists may be
surprised that all of these things are interconnected. For example, the
radioactive decay of some elements is governed by the strong force. So,
a change in their decay rate implies a different binding energy. Energy
curves space, so a different binding energy implies a change in the
amount of gravity, and that implies a change in orbital motion.
If you followed that, I said that if a planet has been in the same orbit
for a long time, then Uranium-235's radioactive decay rate has been
unchanged for that same amount of time. And so on.
Physics creates a huge web
of connections between astronomy and geology. You may find something
debatable about any one of these results. However, it is very hard to
argue against a great many independent results, each of which fits into
a connected web, and each of which places strong constraints on how fast
change could be happening.
References
Clifford W. Will, Was
Einstein Right, Basic Books 1986. See Chapter 9, "Is the
Gravitational Constant Constant?"
Clifford W. Will, Theory
and Experiment in Gravitational Physics, chap. 8.4 and part of
14.3(c)
Cowie & Songaila,
"Astrophysical Limits on the Evolution of Dimensionless Physical
Constants over Cosmological Time," Astrophysical Journal v.453,
p.596 1995
The Oklo Natural Nuclear
Reactor (note: this link can be slow.)
P. Sisterna and H.
Vucetich, "Time variation of fundamental constants: Bounds from
geophysical and astronomical data," Physical Review D 41 (1990) 1034
and 44 (1991) 3096
E. Richard Cohen in
Gravitational Measurements, Fundamental Metrology and Constants, V.
De Sabbata and V.N. Melnikov, editors, NATO ASI Series C Vol. 230,
Kluwer Academic Publishers, 1988
Barrow, J.D. and Tipler,
F.J. The Anthropic Cosmological Principle, Oxford University Press,
London 1986
Philosophical
Transactions of the Royal Society, London A, 310 (1983) 209-363, was
a special issue on "The Constants of Physics."
A.D. Tubbs and A.M.
Wolfe, "Evidence for Large-Scale Uniformity of Physical Laws,"
Astrophysical Journal 236 (1980) L105
W.A. Baum and R.
Florentin-Nielsen, Astrophysical Journal 209 (1976) 319.
It should be noted that
Young-Earth Creationists Norman and Setterfield, who argued for a
decreasing speed of light, state that the dimensionless fundamental
constants have not varied. (Otherwise, life as we know it would have
been impossible in the recent historical past.) They were apparently
unaware that these two claims are inconsistent with each other.