Light and quantum physics part ii
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In a Double-Slit Experiment, an actual wave completely passes through two slits at the same time creating an interference pattern. But a particle can only pass through either one of the slits, and so it should just pass through and hit the region on the wall right behind the slits. We would expect the particles to create 2 bright regions.
But instead we observe that the particles pass through and fall in multiple light and dark regions based on a pattern - the interference pattern of a wave passing through both the slits. How do the particles know where on the screen do they need to fall to create the light and dark regions?
(re-read if necessary)
How do we explain this?
Chapter 6 of 10
Neils Bohr Comes To The Rescue
Much like you and I, scientists were also baffled by this observation that particles were creating wave like interference patterns on the screen. After all it was quite difficult to explain it, as we couldn't see any real wave. All we could see was that a particle was thrown towards the slits and a particle was detected on the screen.
So two scientists Neils Bohr and Werner Heisenberg between 1925-1927 came up with a reasonable explanation. They proposed that (rephrased for understanding) -
You can visualise this probability wave as given below.
- We need to give up our assumption, that each individual electron fired towards the slits is a particle throughout its journey to the screen.
- When we are not looking at or observing the electron, it behaves like a wave. The moment we observe the electron (say by putting a screen), the wave collapses into a particle.
- This wave can be thought of as a wave of probabilities. The probability of the electron being at a certain position. In fact, it applies not only to 'position' but also to other properties of the electron like 'velocity (speed with direction)', 'spin' etc.
- When we observe the electron, its probability wave collapses and takes up a definite location from among the various possible locations. Similarly, it takes up a definite value for its other properties like spin, momentum, etc; from a range of possible values.
The height (probability value) of the wave at different points on the XY plane, indicates the likeliness of finding the electron at those different points on the plane. The electron is most likely to be found, upon observation, at the location point with the highest probability - the point right below the tip of the wave. But there is also a chance, that upon observation, it is instead found at some other location with lesser probability.
This idea came to be known as the 'Copenhagen Interpretation' of Quantum Mechanics.
Explanation For Interference Pattern By Electrons
Using Bohr's Copenhagen interpretation, you can now see, how the interference pattern was created on the screen when electrons were fired at the two slits (previous slide). Each electron's probability wave passed through both the slits. Then, just like any other wave, the probability wave broke into two new probability waves at the other side of the slits, which interfered with each other to create a new interference wave with new high and low probabilities of presence. When this new interference wave of probabilities hit the screen (electron was observed), it collapsed taking up one of the probable positions. When more electrons were fired, they did the same thing individually and overall we got to see the interference pattern appear on the screen.
Chapter 7 of 10
"God Does Not Throw Dice"
Einstein didn't agree with Niels Bohr's explanation of the quantum world.
He thought nature had a very defined set of laws that govern exactly how events must happen and that if we understand these laws thoroughly we should be able to exactly predict the outcome. He thought it was absurd that nature would allow particles to be in all possible locations (velocities, spins etc.) when not being observed, and just pick one value of location (velocity, spin etc.) out of many at random when being observed. To this he said - "God does not throw dice."
Chapter 8 of 10
Superposition
Einstein along with two other scientists, Boris Podolsky and Nathan Rosen published a paper where they tried to challenge the Copenhagen Interpretation by putting forth a thought experiment, called the "EPR Paradox".
Applying Bohr's Superposition Idea
They said that if we apply Bohr's idea of a particle being in multiple possible states at the same time when not being observed - "Superposition of possible states" to two particles something really crazy would happen. So let's see what happens.
Create a system of two particles
An electron has a negative charge; while a positron is nothing but an electron with a positive charge instead of negative. When the two particles are created, the positron has to have a positive charge because we started out with a zero overall charge (2 photons) which must be conserved (remain the same) after the production.
Let's say we create two particles an electron and a positron by colliding two gamma ray photons (basically two photons of light) as shown below.
Conservation of Spin
Similar to charge, the spin property of the overall system must also remain unchanged. Let's say we started out with zero spin on the 2 photons, then the overall spin of the electron and positron system must be zero as well. This implies, the spins of the electron and positron individually would have to be opposite each other. If one is 'spin-up', the other one would have to be 'spin-down'.
In the Copenhagen interpretation, Bohr says that when we are not looking at a particle/system, it is not just in one of its possible states, but like a probability wave, it is in all the possible states at the same time. Only when we observe it, the system randomly chooses and collapses its superposition to one of the possible states.
With this understanding, the electron and positron pair act as one system. This system has only two possible states -
- electron UP - positron DOWN
- electron DOWN - positron UP
The pair can't have (UP-UP or DOWN-DOWN) states to conserve the overall spin to zero.
As per superposition, the pair is in both the states at the same time. I know this is weird to understand, so just recollect how the particles were behaving like a probability wave of all possible locations (velocities, spins) in the double-slit experiments. It's the same thing, just applied to a system of two particles.
So we have now created our system of two particles that act as if they are one and their system has two possible states. Such particles acting as a system in multiple superposition states, are said to be Quantum Entangled.
But what's the crazy thing about it?
Chapter 9 of 10
The Crazy Thing - EPR Paradox
Okay so now that we have an entangled pair of particles, an interesting thing about being entangled is that, the particles stay entangled as a system even if we move them away from each other.
(source)
Even if we move the individual particles apart to vast distances in the universe, they keep behaving like a system in superposition of multiple states.
So consider we have such an entangled pair separated by a huge huge distance (light years away) in the universe. The moment we make a measurement of the system, let's say by measuring one of the particles, it obviously collapses the superposition of the system to one of the possible states, like it should as per quantum mechanics. This implies, that instantaneously both the particles collapse to one defined state (spin up or spin down).
Think about this for a moment!
The particle which was put under our measurement device collapsing to a defined spin immediately when measured sounds fine, but the other particle also collapsing to a defined spin immediately at the same time is crazy!! It means that the collapsing effect, was transmitted from the measurement end to the other far away particle instantaneously without any delay.
Violation Of The Most Fundamental Principle - Causality
The effect from a cause cannot travel faster than the speed of light is the fundamental principle called 'Causality'. Most of our classical physics, involving Einstein's Theory of Relativity is based on this fundamental assumption, that nothing can travel faster than the speed of light, even the effect from a cause.
But in the case of entangled particles, the collapsing effect is transmitted to the other end faster than even light - instantaneously. This is the EPR Paradox.
Einstein called it "Spooky action at a distance."
With this thought experiment, Einstein believed that he had proved that the Copenhagen Interpretation was incomplete in thinking that particles could be in a state of superposition of multiple possible states, because as per the EPR Paradox, that would mean that even our most fundamental principle of Causality would be violated.
Experimentation However Proved That Bohr Was Right!
You'd be surprised to know, that much later, in multiple experiments, it was indeed observed that Causality is violated in the case of entangled particles and that the collapsing effect is transmitted instantaneously.
Chapter 10 of 10
Journey So Far, Let's Recap!
If you've reached this far, I'm sure you must be mind-boggled. Understanding these weird quantum phenomenon is not so easy as it challenges our most basic intuitions. So feel free to go back anytime you like and re-indulge in these concepts.
The concept of Superposition - that individual particles and even systems of entangled particles can be in more than one possible states at the same time when not being observed; is fairly complex, that is why we had to go through the other concepts on light being a wave and a particle, to particles behaving likes waves just to get comfortably introduced to the concept of Superposition.
And that's the story of how Quantum Physics came to be.
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