(adsbygoogle = window.adsbygoogle || []).push({}); This description you've written depends critically upon the interpretation of quantum mechanics you're assuming. If you make the assumption that the weirdness of quantum mechanics simply doesn't apply to macroscopic objects such as you and me, then you end up with weird conclusions like this, where observations that collapse wave functions tend to lead to nonsensical statements about causality. FallenApple said: ↑Say a photon was emitted from very far away such that it would take billions of years to reach us. According to Feynman's infinite paths, the photon travels every possible path on its way to reach us. The superposition of infinite trajectories collapses when it hits something solid, say an observer on earth for example.

Then the trajectory of the photon is determined upon observation which determines it's past( relative to us) as well, essentially writing billions of years of history for that photon, which is rather astounding.

This is just for one photon, but it could work for many as well, in principle.

Does this translate over to observations of the CMBR? That is, the observations of the photons by us are localizing them to a definite spacetime position in the past? This is very different than saying that we, via CMBR, are observing the past.

It's worth noting that you don't need to understand this as affecting the past, though. But such a wavefunction collapsedoesimpact things far away. In the case of the photon, for example, if the photon you've observed is entangled with another photon traveling in a different direction, then the observation of one member of the pair can be interpreted as having an immediate impact on the far-away photon.

There are a few reasons to believe that this interpretation of what's going on is just not correct. Of course, it's very bizarre: it's stating that quantum mechanics is non-local, which makes nonsense of relativity. The wavefunction is changed inwhosereference frame? Simultaneous events at different locations, after all, are only simultaneous for certain observers. Other observers won't see them as simultaneous. Assuming this non-local behavior, then, also assumes that relativity is all wrong and there really is a preferred reference frame. There's also the problem that when you look at the details of this apparently non-local behavior, it's possible to prove that you can't actually transmit information this way.

The most reasonable conclusion, then, is that this apparent non-locality of quantum mechanics is just an illusion. The simplest explanation for how this could be the case is that wavefunction collapse is itself an illusion. It can be shown that theappearanceof wavefunction collapse will occur in a quantum system even if there is no actual collapse. In this interpretation, the appearance of non-locality disappears entirely: instead of collapsing, the observer's wavefunction is split into two components that can't communicate. One component sees outcome A, and interacts with the entire universe as if outcome A is the only outcome. The same with the component that sees outcome B.

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# B Does observation *Determine* the past?

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