In this series, we are going to study the issue of “quantum entanglement”. While this is a relatively simple issue, we are going to break it down into a few parts. In this part, we are going to cover the issues of realism and locality.
First, we will set the stage for why any of this matters by looking at what “quantum entanglement” is and why we might want to discuss it here on the Metaphysics of Physics website.
Introduction to Entanglement
Quantum entanglement is the physical phenomenon that is said to take place when groups of particles interact in a certain way. It is said that any number of particles can interact in this way, but for the sake of simplicity, usually it is discussed in the context of two interacting particles.
What is this certain way in which they interact? Well, to understand this we must first introduce the idea of a “quantum state“.
Briefly, the “quantum state” is a mathematical description of certain properties of a particle. The mathematics is fairly complicated, so we will not go into detail (there are a lot of vectors and other fairly abstract details which would be more than a little distracting).
Quantum entanglement says that if two particles are “entangled”, that their “quantum states” cannot be described independently of the state of the other particle, even when separated by a large distance.
One might well ask what on Earth that means?
Quantum physicists tend to describe fairly simple ideas in an overly complicated manner. So, let me translate this fairly accurately into laymen’s terms.
If two particles are “entangled”, it means that some of their properties are related to one another. If you measure the value of the property of one particle, then that says something about the property of the other particle.
Let us take a hypothetical example and consider the “spin” of two particles. In quantum mechanics, there is a property called “spin”. We will not go into the complicated issue of what spin is (it is not the same thing as rotation from classical mechanics).
Suppose that if you had two entangled particles, A and B. Suppose you then measure the spin of A to be “up”. In our example, this means that the spin of B must be “down”.
One might wonder what the issue here is?
After all, so what if we can deduce something about particle B by looking at particle A? After all, we know that macroscopic things can be related in this way. If we bring the north poles of two magnets together and magnet A experiences a force pushing it to the left, then we can deduce that there is a force pushing magnet B to the right.
We should not expect any issue with being able to infer things about two different parts of a system. So, again what is the issue here?
Well, the problem is that quantum mechanics tends to make this issue far more nonsensical than it needs to be. Measurements have been performed of “entangled” particles that seem to indicate that particles over very long distances are correlated.
The problem is that this entanglement is said to work instantly!
That if you observe or change the quantum state of any one of the entangled particles, it may potentially change the quantum states of all the other particles it is entangled with.
Instantly! That is, without any physical interaction! As though the particles are somehow linked by some kind of telepathic magic. Or, more accurately as though the mere fact of observing/changing the state of one particle can somehow affect the state of the other particles, again without any physical interaction!
Now, we accept that entangled particles might be able to affect other particles. They just need to interact with the other particles by some physical means. Some non-instantaneous physical means.
We reject the notion that simply observing something can cause it to change/acquire states. Or that simply observing an entangled particle magically influences the quantum states of other particles without requiring any physical interaction.
Now let us look at a few terms, “realism” and “non-locality”. In the next part, we will look further at how quantum physics deals with these concepts.
Realism, as used in physics, is the idea that physical reality exists independently of the human mind. More specifically, as used in quantum mechanics, it is the (allegedly unreasonable) “assumption” that particles have well-defined properties that exist independently of measurement. That is, particles have certain properties regardless of whether we measure those properties.
For instance, an electron will always have a certain “spin” direction, even if nobody bothers to check what it is.
Perhaps an example from the macroscopic world would help to make this more clear. Suppose that we consider a ball. It has various properties, even if nobody is currently observing the ball. It has a radius, it has hardness, it has a velocity (which might be zero), it has various chemical properties. These properties exist even if we stop looking at the ball.
This might seem rather obvious and so it should. What we call the “properties” of the ball are simply various aspects of the ball’s nature. The “properties” of the ball simply describe the ball and its nature and/or what it will do under given conditions. It should be obvious that these properties will exist regardless of measurement. A ball will always have a certain radius or a certain temperature, regardless of whether we are measuring those properties or observing the ball.
The act of observation does not create these properties. We do not create these properties simply by measuring them. Just as we do not create the ball’s radius when we take a measuring tape to it and measure it. We do not create its temperature when we measure its temperature. Measuring a property does not create that property. “Property” refers to some attribute of something and those attributes are simply aspects of something. Aspects which exist whether or not they are being observed.
Now, can we apply this to the quantum world? Of course, we can. We should not expect that just because subatomic particles are very tiny, that they are somehow not subject to basic metaphysical principles. These entities also have attributes that exist independently of the human mind.