In reply to blurty:
I am with you here! Quantum physics is still very mysterious and poorly understood. Feynman famously said “nobody understands quantum mechanics". The standard (“Copenhagen”) interpretation remains controversial. In a conference in 1999, only 4 out of 90 eminent scientists voted in its support.
Quantum physics is not my scientific discipline, but as a layman I find the Copenhagen interpretation confusing and seemingly illogical. These are some of the things it would have us believe:
(a) Everything is everywhere at once
(b) Particles are “entangled” however far apart.
(c) Things don’t really exist until they are observed.
(d) The probability of a particle existing anywhere in the universe is described by a ”wave function”
(e) The wave function has to collapse to a point in order for a particle to exist.
(f) This “collapse of the wave function” only happens when an observer takes a measurement.
(g) Cause and effect are "non-local": a cause can have an instantaneous effect however far way, which violates the speed of light barrier imposed by the theory of Relativity. This is what Einstein called “spooky action at a distance”.
(h) Wave-particle duality obeys Bohr’s principle of “complementarity” - that the wave and particle aspects are complementary but exclusive, i.e. a wave/particle is only observable as either a wave OR a particle.
(i) In the double-slit experiment, a particle displays its wave-like rather than particle (projectile) behaviour: it passes through both slits at once as a wave that interferes with itself.
(j) The wave is not a real physical wave but a mathematical probability function (the “wave function” described by Schrodinger’s equation) that has to collapse to a point in order for the particle aspect to become real.
One alternative interpretation of quantum mechanics that I find much more palatable is the Pilot Wave theory of De Broglie dating from the 1920’s, which was developed further by Bohm after the Second World War. The main element of the Pilot Wave theory is that both the particle and wave aspects of wave-particle duality are real and exist at the same time independently of one another. In the Pilot Wave interpretation of the double-slit experiment, both a real wave AND a real particle exist at the same time. The wave passes through both slits at the same time, whereas the particle passes through one of other of the slits. The wave causes the interference pattern, and the particle’s course is guided by the (pilot) wave. A series of particles fired through the slits gradually builds up a pattern in the target area that reflects the wave’s interference pattern there.
Einstein was a fan of De Broglie and was amongst many scientists who have been deeply troubled by the Copenhagen interpretation of quantum mechanics. In scientific theories, Einstein believed in (a) “realism” - that things exist independently of observers, and (b) “locality” - that cause and effect occur locally, i.e. in the same place, or if separated by a distance with a delay imposed by the speed of light.