dimensions and a detector with a good enough angular resolution.
In the early days of quantum mechanics, people like Bohr and
Heisenberg imagined that there was simply a clear division between
the macroscopic and microscopic worlds. Big things and small things
just had different rules: Newton’s laws in one case, quantum me-
chanics in the other. But this is no longer a tenable position, because
we now know that there is no limit on the distance scales over which
quantum-mechanical behavior can occur. For example, a commu-
nication satellite carried out a demonstration in 2017 in which a
coherence length of 1200 km was demonstrated using photons.^9
The insight about decoherence was the following. Consider the
most massive material object that has so far been successfully dif-
fracted through a grating, which was a molecule consisting of about
810 atoms in an experiment by Eibenbergeret al.in 2013.^10 While
this molecule was propagating through the apparatus as a wave,
the experimenters needed to keep it from simply being stopped by
a collision with an air molecule. For this reason, they had to do
the experiment inside a vacuum chamber, with an extremely good
vacuum. But even then, the molecule was being bombarded by
photons of infrared light emitted from the walls of the chamber. The
effect of this bombardment is to disrupt the molecule’s wavefunction
and reduce its coherence length (p. 824).b/A large molecule such as
the one in the Eibenberger ex-
periment is represented by its
wavepacket. As the molecule
starts out, its coherence length,
shown by the arrows, is quite
long. As it flies to the right,
it is bombarded by infrared pho-
tons, which randomize its phase,
causing its coherence length to
shorten exponentially: by a fac-
tor of two in the second panel,
and by a further factor of two in
the final one. When the packet
enters the double slit, its coher-
ence length is on the same order
of magnitude as the slits’ spacing
d, which will worsen but not en-
tirely eliminate the observability of
interference fringes. (This is only
a schematic representation, with
the wavepacket shown as being
many orders of magnitude bigger
than its actual size in relation to
the vacuum chamber. Also, the
real experiment used a reflecting
grating, not a transmitting double
slit.)
This causes an effect similar to the one in the situation illus-
trated in figure a, where we spy on one slit of a double-slit appa-
ratus. The microscope would operate by bouncing photons off of
the electron, and the result is to disrupt the coherence of the elec-
tron’s wavefunction, so that the coherence length is no longer as
large as the distance between the slits. The infrared photons in
the Eibenberger experiment were not introduced intentionally, but(^9) Yinet al.,arxiv.org/abs/1707.01339
(^10) arxiv.org/abs/1310.8343
1000 Chapter 14 Additional Topics in Quantum Physics