In quantum mechanics, the measurement process plays a special role. Before measurement, it makes no sense to ask whether something is there or what properties it has. Einstein worried about this profoundly. He thought it could not possibly be correct. He reportedly summed up the question by asking whether the moon is there when nobody looks, implying that, certainly, the question is absurd. Common sense says “yes” because what does the moon care if someone looks at it. But quantum mechanics says “no”. Support me on Patreon: https://www.patreon.com/Sabine