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We know that today's behavior will affect tomorrow's events, but imagine turning the scene upside down: future behavior may actually affect your situation today. Is the future likely to affect the present? The delayed selective quantum erasure experiment, an enhanced version of the famous double-slit experiment, implies such a result.

Wheeler's delayed selection experiment in 1978, physicist Wheeler proposed a thought experiment called delayed selection. Wheeler's idea is to imagine light from distant quasars billions of light-years away, bent by the gravity of nearer galaxies. As a result, quasars appear to be in two slightly different locations because of the gravitational lens effect of galaxies between Earth and quasars.

Wheeler later pointed out that this light can be observed on Earth in two different ways. The first way is to have a detector for each image. Because the exact source of this light is known, it is measured as a light particle when observed. However, if an optical interferometer is placed at the junction of the two light sources, the combined light from the two images will be measured as waves because its exact source is unknown. Because this is not measured until billions of years after the light is emitted, and the method of measurement depends on the choice of the observer, the experiment is called delayed selection.

This quasar experiment is not practical, but modern equipment allows us to do similar experiments in the laboratory. In fact, his thought experiment has been confirmed: when the path information is known, light is a particle; when using an interferometer to erase the path information, light is a wave. Light began its journey billions of years ago, long before we decided which method of measurement to choose. It seems that quasar light "knows" whether it will be seen as a particle or a wave billions of years before it is experimented on Earth.

Does this prove that the particle's measurement of its current state affects its past state in some way? The most traditional way to explain quantum theory is to assume that the quantum has a wave function, which means that it is not actually a particle or a wave. When it is observed, it collapses into a definite state. In this view, measurement behavior makes quantum particles a reality. So in the delayed selection experiment, this means that the quantum does not become "real" until you measure it. So this experiment does not prove that the present affects the past, because light may be both waves and particles and become real only when it is measured.

Delayed selective erasure experiment however, another newer experimental setting uses a more complex method to determine this idea that the future will affect the past, introducing something called a quantum eraser for delayed selection. Therefore, it is called delayed selective quantum erasure experiment. This is a complex structure in which entangled photon pairs are introduced into Wheeler's delay selection experiment. However, we will use a simpler setting that will illustrate the concept in a more understandable way.

First of all, a standard double-slit device is used to emit a single photon, but instead of letting the photon enter the screen after passing through the double slit, the photon is divided into two entangled photons using a barium borate crystal. Because these photons are entangled, we can measure one photon without touching another photon. In addition, we will place a lens so that regardless of whether the photons are emitted from the top or bottom slits, the two beams will always be combined at detector 1, so that we cannot know which slit the photons come from. So this mirror acts as an eraser because it erases the path information of photons.

Next, we will measure the entangled photon pairs coming out of each slit several times. We place a spectroscope so that the photons have a 50% chance of turning downward and a 50% chance of passing through the spectroscope. When the beam turns downward, the two beams from the top and bottom slits merge so that the path information cannot be known. In other words, if detector 4 detects a photon, it has no information about which path the photon comes from.

But if the particle leaves the top slit and detects it at detector 2, you know which path information, because detector 2 only detects photons from the top slit. Similarly, if the particle leaves the bottom slit and is detected at the detector 3, its path information is also known because the detector 3 can only detect photons from the bottom slit.

When detector 2 detects a photon, detector 1 detects its entangled twins, indicating that it is a particle rather than a wave-because we see a non-interference pattern. The same thing happened to detector 3. However, when detector 4 detects photons, detector 1 detects waveforms. This tells us that when we know the path information, that is, the detector can determine which slit the photon comes from, it is always presented in the form of particles. But when the detector cannot determine which slit the photon comes from, the light behaves like a wave. So far, everything is fine.

But I have left out a small but crucial detail here, which will have important results. This detail is that the path to detector 1 is much shorter than the path to detectors 2, 3, and 4. So photons are always detected on detector 1 and then on other detectors, which is the delay selection aspect of this experiment.

Therefore, when an interference pattern is formed on the detector 1, the path information is erased on the detector 4 after that; when a non-interference pattern is formed on the detector 1, the path information reaches the detector 2 or 3 after that. This shows us that the photon at detector 1 somehow knows in advance which detector its entangled photons will arrive in the future. Therefore, it seems that the future state of photons at detectors 2, 3 and 4 will affect their past states of entanglement pairs at detector 1.

It is absolutely clear in this experiment that the measurement of a particle now can affect the properties of the particle in the past. In other words, decisions made now affect something in the past, and the result seems to precede the cause. But why? I can only say that I don't know, if you know, please tell me, a Nobel Prize waiting for you.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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