Provides Further Evidence That Reality Doesn't
Exist Until We Measure It
Pieter Kuiper via Wikimedia Commons. A comparison of double slit interference patterns with different widths. Similar patterns produced by atoms have confirmed the dominant model of quantum mechanics]
By Stephen Luntz
Physicists have succeeded in confirming one of the theoretical aspects of quantum physics: Subatomic objects switch between particle and wave states when observed, while remaining in a dual state beforehand. In the macroscopic world, we are used to waves being waves and solid objects being particle-like. However, quantum theory holds that for the very small this distinction breaks down. Light can behave either as a wave, or as a particle. The same goes for objects with mass like electrons.
raises the question of what determines when a
photon or electron will behave like a wave or a
particle. How, anthropomorphizing madly, do these
things “decide” which they will be at a particular
time?The dominant model of quantum mechanics holds
that it is when a measurement is taken that the
“decision” takes place. Erwin Schrodinger came up
with his famous thought experiment using a cat to
ridicule this idea. Physicists think that quantum
behavior breaks down on a large scale, so
Schrödinger's cat would not really be both alive
and dead—however, in the world of the very small,
strange theories like this seem to be the only way
to explain what we we see.
In 1978, John Wheeler proposed a series of thought
experiments to make sense of what happens when a
photon has to either behave in a wave-like or
particle-like manner. At the time, it was
considered doubtful that these could ever be
implemented in practice, but in 2007 such an
experiment was achieved. Now, Dr. Andrew Truscott
of the Australian National University has reported
the same thing in Nature Physics, but this time
using a helium atom, rather than a photon.
“A photon is in a sense quite simple,” Truscott
told IFLScience. “An atom has significant mass and
couples to magnetic and electric fields, so it is
much more in tune with its environment. It is more
of a classical particle in a sense, so this was a
test of whether a more classical particle would
behave in the same way.”
Trustcott's experiment involved creating a
Bose-Einstein Condensate of around a hundred
helium atoms. He conducted the experiment first
with this condensate, but says the possibility
that atoms were influencing each other made it
important to repeat after ejecting all but one.
was passed through a “grate” made by two laser
beams that can scatter an atom in a similar manner
to a solid grating that can scatter light. These
have been shown to cause atoms to either pass
through one arm, like a particle, or both, like a
wave. A random number
generator was then used to determine whether a
second grating would appear further along the
atom's path. Crucially, the number was only
generated after the atom had passed the first
The second grating, when applied, caused an interference pattern in the measurement of the atom further along the path. Without the second grating, the atom had no such pattern.
An optical version of Wheeler's delayed choice experiment (left) and an atomic version as used by Truscott (right). Credit: Manning et al.
Truscott says that there are two possible explanations for the behavior observed. Either, as most physicists think, the atom decided whether it was a wave or a particle when measured, or “a future event (the method of detection) causes the photon to decide its past.” In the bizarre world of quantum mechanics, events rippling back in time may not seem that much stranger than things like “spooky action at a distance” or even something being a wave and a particle at the same time. However, Truscott said, “this experiment can't prove that that is the wrong interpretation, but it seems wrong, and given what we know from elsewhere, it is much more likely that only when we measure the atoms do their observable properties come into reality.