The outcome of a quantum experiment can change depending on whether or not we choose to measure some property of the particles involved

Light can be thought of as a kind of wave, and when waves emerge from two slits like this they can interfere with each other. If their peaks coincide, they reinforce each other, whereas if a peak and a trough coincide, they cancel out. 

This wave interference is called diffraction, and it produces a series of alternating bright and dark stripes on the back screen, where the light waves are either reinforced or cancelled out.

Now suppose that the quantum particles are sent through the slits one by one, and their arrival at the screen is likewise seen one by one. Now there is apparently nothing for each particle to interfere with along its route – yet nevertheless the pattern of particle impacts that builds up over time reveals interference bands.

The implication seems to be that each particle passes simultaneously through both slits and interferes with itself. This combination of "both paths at once" is known as a superposition state.

But here is the really odd thing.

If we place a detector inside or just behind one slit, we can find out whether any given particle goes through it or not. In that case, however, the interference vanishes. Simply by observing a particle's path – even if that observation should not disturb the particle's motion – we change the outcome.


http://www.bbc.com/earth/story/20170215-the-strange-link-between-the-human-mind-and-quantum-physics