|Can quantum erasers be used for erasing the past,
for example, could they be used in a time machine
to undo what you just did to how it was before
and "leave without a trace"?
|Question Date: 2007-06-08|
We don't have any sign of a way to erase or
rewrite the past, and Einstein's theories of
relativity seem to prohibit it. One way science
fiction writers have gotten around this problem is
to ask if we could somehow jump to a parallel
universe where the past was only slightly
different. Physicists don't agree on the
interpretation of some theories, but some
physicists interpret quantum mechanics to mean
there are an infinite number of universes
branching off from ours at any given point in
time. In this view (which not everyone agrees
with), any time a decision is made, *both* results
occur--just in different universes. There's no
way to prove that this is what happens, though, so
many other scientists don't consider it a
On the other hand, it has
been known for many years that either Einstein was
wrong, or quantum mechanics is wrong, or both.
So maybe you'll be the one to figure out how to
make the "leap"!
I am uncertain on the concept of quantum
erasers, but the undoing of large-scale
interactions over long time periods, such as the
action of any human, are highly unlikely. Time
travel is theoretically possible on time scales of
10^-40 seconds or so, and that's only in theory -
the theory is completely unsupported by any data.
Though it's a topic that receives attention in
theoretical physics, time travel is not a real
concept that science regularly tackles.Although
the theory of relativity shows that time travel to
the future is effectively possible, time travel to
the past is a not grounded in the real world,
serving only as a way to tell interesting
stories.Stephen Hawking, a renowned physicist once
jokingly also pointed out that if time travel
worked, you'd expect to see tourists from the
future hanging around.
The concept of a
quantum eraser is actually describing a subtle and
complex experiment for measuring the strange and
intriguing behavior of subatomic particles in
quantum mechanics. Quantum mechanics tells us
that these tiny particles (like photons of light)
are both waves and particles at the same time.
One interesting experiment that proved this was
called the "double slit" experiment. In it, a
stream of photons is aimed at a metal plate with
two small holes in it, with a detector behind it.
If the photons were particles, you'd expect them
to go through one hole or the other, meaning that
two clumps of photons would be detected. But
that's not what happens! You actually get a
pattern of clumps of photons on the detector, as
if the photons were behaving as waves, interfering
with each other and making a high and low rippled
pattern. A refinement of this experiment is the
quantum eraser experiment, where they use
polarized glass before the detector, allowing them
to measure the photons more closely. The nature
of quantum mechanics is such that measuring
anything very closely can change its behavior (the
Heisenberg uncertainty principle,for instance),
and indeed, this "erases" the quantum interference
pattern that the detector sees-- suddenly it only
sees two clumps of photons, as if the photons were
behaving only as particles.
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