Editing Quantum entanglement (section)

=== Paradox ===
{{main|EPR paradox}}
The singlet state described above is the basis for one version of the EPR paradox. In this variant, introduced by [[David Bohm]], a source emits particles and sends them in opposite directions. The state describing each pair is entangled.<ref>{{cite book|first=David |last=Bohm |author-link=David Bohm |title=Quantum Theory |orig-year=1951 |year=1989 |publisher=Dover |edition=reprint |isbn=0-486-65969-0 |pages=611–622}}</ref> In the standard textbook presentation of quantum mechanics, performing a spin measurement on one of the particles causes the wave function for the whole pair to [[wave function collapse|collapse]] into a state in which each particle has a definite spin (either up or down) along the axis of measurement. The outcome is random, with each possibility having a probability of 50%. However, if both spins are measured along the same axis, they are found to be anti-correlated. This means that the random outcome of the measurement made on one particle seems to have been transmitted to the other, so that it can make the "right choice" when it too is measured.<ref name="Zwiebach2022"/>{{rp|§18.8}}<ref name="Griffiths">{{cite book|first1=David J. |last1=Griffiths |author-link1=David J. Griffiths |first2=Darrell F. |last2=Schroeter |title=Introduction to Quantum Mechanics |title-link=Introduction to Quantum Mechanics (book) |edition=3rd |year=2018 |publisher=Cambridge University Press |isbn=978-1-107-18963-8 }}</ref>{{rp|447–448}}

The distance and timing of the measurements can be chosen so as to make the interval between the two measurements [[spacelike]], hence, any causal effect connecting the events would have to travel faster than light. According to the principles of [[special relativity]], it is not possible for any information to travel between two such measuring events. It is not even possible to say which of the measurements came first. For two spacelike separated events {{math|''x''<sub>1</sub>}} and {{math|''x''<sub>2</sub>}} there are [[inertial frame]]s in which {{math|''x''<sub>1</sub>}} is first and others in which {{math|''x''<sub>2</sub>}} is first. Therefore, the correlation between the two measurements cannot be explained as one measurement determining the other: different observers would disagree about the role of cause and effect.<ref>{{cite journal|first=Asher |last=Peres |author-link=Asher Peres |doi=10.1103/PhysRevA.61.022117 |title=Classical interventions in quantum systems. II. Relativistic invariance |journal=Physical Review A |volume=61 |pages=022117 |date=2000-01-18|issue=2 |arxiv=quant-ph/9906034 |bibcode=2000PhRvA..61b2117P }}</ref>