Editing Uncertainty principle (section)

=== Einstein's slit ===
The first of Einstein's [[thought experiment]]s challenging the uncertainty principle went as follows:

{{quote|Consider a particle passing through a slit of width {{mvar|d}}. The slit introduces an uncertainty in momentum of approximately {{mvar|{{sfrac|h|d}}}} because the particle passes through the wall. But let us determine the momentum of the particle by measuring the recoil of the wall. In doing so, we find the momentum of the particle to arbitrary accuracy by conservation of momentum.}}

Bohr's response was that the wall is quantum mechanical as well, and that to measure the recoil to accuracy {{math|Δ''p''}}, the momentum of the wall must be known to this accuracy before the particle passes through. This introduces an uncertainty in the position of the wall and therefore the position of the slit equal to {{math|{{sfrac|''h''|Δ''p''}}}}, and if the wall's momentum is known precisely enough to measure the recoil, the slit's position is uncertain enough to disallow a position measurement.

A similar analysis with particles diffracting through multiple slits is given by [[Richard Feynman]].<ref>Feynman lectures on Physics, vol 3, 2–2</ref>