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It's already well know that in quantum mechanics the act of observation affects the outcome of an experiment making the wave function to collapse.Now in order to clear up my confusion about this topic it's possible for the very act of observation to influence the probability of an event to occur?

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The quantum Zeno effect is a good example of this happening. Repeated measurement can change the probability of particle decays.

The way this happens is that each measurement collapses the wave function to one of the measurement eigenstates. If this happens often enough there is not enough time for the system to evolve to a state where the probability of collapse to another state is large, and the system stays roughly where it is. This has been experimentally demonstrated in transitions between electron states and in quantum tunneling.

By changing the timing decay rates can be increased. In fact, this may be a more generic phenomenon than the slowing.

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In short, yes. This is exactly what quantum entanglement is. You prepare for example two photons so that the state is |1,2>+|2,1>, to a normalization factor, 1 and 2 being two orthogonal polarizations. If you measure the polarization of the first photon as being 1, the second one must absolutely, with probability 1, be 2. Before measurement, the probability was 1. So, by observing the first photon, you changed the probability of observing the different polarization of the other photon.

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  • $\begingroup$ This is a very misleading way of putting it, because it blatantly contradicts relativity. The probability distribution is always 50/50, whether or not somebody has measured the other photon. $\endgroup$ – knzhou Oct 18 '17 at 18:44
  • $\begingroup$ It all depends on what you mean by "influence" and "event" and "probability". Does it seem to contradict relativity? Well, it seems to, but it isn't, and that's how the world works. You can see the EPR paradox and the Bell inequality Wikepedia articles for reference. $\endgroup$ – Manuel Fortin Oct 18 '17 at 19:00
  • $\begingroup$ I know what EPR is; you are botching it. The probability distribution stays exactly the same unless somebody tells you the result of the first measurement. $\endgroup$ – knzhou Oct 18 '17 at 19:06
  • $\begingroup$ Ok thanks i've got it,and i also know that EPR is all about correlation and doesn't violate relativity or allow locality violations.But i was also wondering if this type of quantum features (as weird as it sounds even to me) can have some effect on macroscopic system and ,in some way raise the probability of an event to occur for example but i know that maybe im missing something.@manuel fortin @knzhou $\endgroup$ – Andrea Scaglioni Oct 18 '17 at 20:18
  • $\begingroup$ @knzhou "... unless somebody tells you the result of the first measurement" doesn't seem right. The wavefunction collapses on interaction with the detector whether or not anyone with a consciousness knows the result of the measurement. Once the measurement is made, the quantum probability distribution no longer exists. What's left is just one's lack of knowledge. If a game was played yesterday with a 50/50 chance for your team to win, but you didn't watch and don't know the already existing result, you still have a 50/50 chance of guessing, but is it the probability of the game or your guess? $\endgroup$ – safesphere Oct 18 '17 at 21:53

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