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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
Answer 1:

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 scientific answer.

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"!

Answer 2:

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.

Answer 3:

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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