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Interpretations of Quantum Mechanics · Acedra
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Interpretations of Quantum Mechanics
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0:00
In this video, we attempt to present a reasonably comprehensive list of the various interpretations of Quantum Mechanics. We include options such as Objective Collapse, Retro-causality, Super-determinism, QBism, Many Worlds, Relational Quantum Mechanics, Pilot Wave, Quantum Logic, and a number of others.
0:29
The most popular interpretation of Quantum Mechanics is the Copenhagen Interpretation.
0:36
The Copenhagen Interpretation states that nature is intrinsically probabilistic, and that particles don't have properties such as position or momentum until they are observed.
0:51
Copenhagen Interpretation states that everything that can be known about a particle is summarized by a wave function.
1:01
According to the Copenhagen interpretation, wave functions are just a mathematical tool for calculating the probabilities of all possible observations,
1:13
and at the moment of observation, the wave function collapses to just a single outcome.
1:22
One of the many problems with the Copenhagen Interpretation is that it never states what exactly counts as an observation, since a wave function can describe systems of many interacting particles.
1:37
We know that a wave function can interact with a detector without collapsing, due to experiments like the delayed choice quantum eraser. The Copenhagen Interpretation fails to explain the following: At what point in time does the wave function collapse into just a single definite outcome, and what causes this collapse to occur?
2:03
There are a number of different approaches to answering these questions.
2:09
One set of approaches are Objective Collapse theories, which state that wave functions collapse on their own, without observation.
2:22
In some Objective Collapse theories, the wave function associated with each individual particle has a very small probability of collapsing on its own at each moment in time. If this particle is entangled with a group of particles, then the moment any one of these particles collapses, this causes the wave function for all the particles to collapse. Therefore, the larger the number of entangled particles, the greater the probability that the entire system as a whole will collapse at any moment.
3:01
In other words, even if the wave function for a single isolated particle is unlikely to collapse for millions of years,
3:10
the wave function for any large object will likely collapse in less than a microsecond, due to the fact that large objects consist of a very large number of entangled particles.
3:27
Other versions of Objective Collapse theories state that it is the interaction with the curvature of space-time which causes a wave function to collapse. In these theories, although an individual particle can be in a superposition of states indefinitely, this is not true for the curvature of space-time, and the greater the superposition of states for the curvature of space-time, the greater the probability that the entire system will collapse at any moment.
4:04
The different types of Objective Collapse theories discussed here are all testable, in that they each make testable predictions for possible future experiments, and these predictions are slightly different from the predictions of traditional Quantum Physics.
4:24
At the time of the making of this video, the technology does not yet exist to perform these experiments with the necessary precision, but this technology may exist in the very near future.
4:46
One of the objections to both the Objective Collapse Interpretation and to the Copenhagen Interpretation is that when a Wave Function collapses, it must collapse everywhere simultaneously, thereby sending information faster than the speed of light.
5:07
However, according to Einstein's Theory of Relativity, different observers can disagree on the simultaneity of different events,
5:18
and if information can travel faster than the speed of light, it would be possible to send information backwards in time.
5:31
This is the essence of the EPR paradox with spooky action at a distance where we have two entangled particles far away from each other. In the EPR paradox, if we measure the spins of two entangled particles, with the detectors rotated relative to each other by 45 degrees, then the two particles measure opposite directions of spins approximately 85% of the time.
6:02
This is a mathematical and logical impossibility if the two particles don't communicate with each other.
6:12
One set of approaches to dealing with this paradox are the Retro-causality interpretations of Quantum Mechanics. The Retro-causality interpretations reject the possibility of instantaneous messages travelling faster than the speed of light, but they accept that messages travel backwards in time. The idea of messages travelling backwards in time is not necessarily that bizarre, since a positron moving forward in time can mathematically be viewed as an electron moving backwards in time.
6:51
In the Retro-causality interpretations, the two particles are communicating with each other not through any spooky action at a distance, but due to the fact that information is being sent both forward and backwards in time along the paths of the particles.
7:12
The particles therefore know in advance whether or not there are detectors present, and what each detector will be measuring, and the particles set their initial conditions accordingly, to avoid any future paradoxes.
7:29
The most well known Retro-Causality theory is the Transactional Interpretation of Quantum Mechanics.
7:38
In the Transactional Interpretation, a wave function travels forward in time, and the complex conjugate of the wave function travels backwards in time.
8:22
The wave function travelling forward in time is what we consider to be the wave function in traditional Quantum Mechanics.
8:31
But, according to the Transactional Interpretation, we also need to take into consideration the wave function travelling backwards in time, as it is the transaction between these two wave functions that causes the universe to decide which path to select, independent of any observer.
9:07
One of the advantages of the Transactional Interpretation is that it explains why the probability of finding a particle at a particular location is proportional to the square of the amplitude of the wave function at that location, whereas the Copenhagen Interpretation just states this principle as an arbitrary assumption.
9:33
The Transactional Interpretation does not make any claims about whether or not the universe is deterministic, since the wave functions travelling backwards in time can be travelling from possible futures, rather than a future that is predetermined. In addition to the Transactional Interpretation, there are a number of other Retro-Causality interpretations.
10:01
Contrary to the name Retro-Causality, we don't actually have to worry about events from the future changing past events that have already occurred, since it is not possible to change something that is already determined. In fact, if the universe is deterministic, then it is not possible to change either the future or the past.
10:27
And this brings us to the Super Determinism interpretation of Quantum Mechanics.
10:37
As with the Retro-Causality interpretations, Super Determinism also resolves the EPR paradox by stating that the particles know in advance what each detector will be measuring, and that the particles set their initial conditions accordingly.
10:56
But in Super Determinism, this is not because messages are being sent backwards in time, but simply because everything in the Universe was already predetermined at the Creation of the Universe, and this information was transmitted to all the particles during the Big Bang.
11:18
Let us now turn to the interpretation of Quantum Mechanics called QBism. QBism is also known as Quantum Bayesianism
11:31
In QBism, the Universe can be intrinsically probabilistic. But according to QBism, the probabilities calculated from quantum theory do not represent an objective probability that all observers agree on, but only our own personal subjective belief about the probability of an outcome. Every time we perform an observation, we update our personal beliefs about the probabilities of all possible future outcomes.
12:03
According to QBism, there is nothing mysterious about a wave function collapsing when we perform an observation, since the wave function only represented our own personal subjective beliefs about the probabilities.
12:19
Since these beliefs are subjective, different people might assign different probabilities to the same event, but there are nevertheless objective rules for the relationships between different probabilities. For example, if we think there is an 80% chance that it will rain, and that there is a 50% chance that a coin will land heads, then logically we should believe that there is a 40% chance that both events will happen together.
12:52
According to QBism, these types of rules for how different probabilities relate to each other are different in Quantum Mechanics than they are in classical probability theory. This is why we when we measure the spins of entangled particles, we get results that seem to be mathematical impossibilities from classical theory. QBism denies the existence of an objective wave function, and the collapse of a wave function is simply viewed as us updating our beliefs about future probabilities.
13:31
Let us now consider the following interpretation of Quantum Physics. This is the possibility that even as we perform observations, the wave function never collapses. This is often referred to as the Many Worlds interpretation, where every possible quantum outcome occurs in a parallel universe.
13:55
This theory is completely untestable, but its supporters argue that there is no reason to assume that the wave function collapses.
14:05
They say that the theory with the fewest assumptions should be the one we select, provided that it fits all the data.
14:14
One of the main criticisms of the Many Worlds interpretation is the following. If a particle has a 61% probability of being observed in location A, and a 39% probability of being observed in location B, the Many Worlds interpretation needs to say that the particle is in location A in 61% of the universes, and that it is in location B in 39% of the universes.
14:49
However, the Many Worlds interpretation can't explain why the split occurred with this particular distribution, unless we add extra assumptions to the theory.
15:02
This does not necessarily make the theory wrong, but adding these extra assumptions takes away the main argument in favor of the Many Worlds interpretation, in that it was thought to require fewer assumptions.
15:21
Let us now turn to the Pilot Wave Theory Interpretation of Quantum Mechanics. Pilot Wave theory is also known as Bohmian mechanics.
15:34
The Pilot Wave Theory states that the Universe is completely deterministic. According to Pilot Wave Theory, all particles always have a definite position and velocity, and the motion of these particles are guided by waves.
15:52
For the Pilot Wave Theory to work, interactions in any one location must be able to be felt everywhere instantly, thereby sending messages faster than the speed of light and violating Einstein's Theory of Relativity.
16:10
The Pilot Wave Theory also has far more additional problems with Einstein's Theory of Relativity than does traditional Quantum Mechanics.
16:22
We now turn to the interpretation that some people prefer not to hear about in a science video. This is the view that consciousness plays an essential role.
16:34
This interpretation says that the wave function is still in a superposition of states after it is measured by a detector, and that the wave function collapses only when it is observed by a conscious observer.
16:59
Claiming that only conscious observers can collapse a wave function poses some new questions.
17:06
Who counts as a conscious observer? What were wave functions doing before any conscious observers evolved in the Universe? And why should the presence of conscious observers make a difference?
17:23
Answering these questions require a fundamentally different set of metaphysical assumptions than we normally make in science.
17:31
One possibility is that we live in a computer simulation, and to save computation time, the simulation does not calculate the exact properties of a particle until it becomes relevant to the observations of one of the players.
17:48
Another possibility is that consciousness is not the result of electrochemical reactions in the brain, but is due to the presence of a soul not described by the known laws of physics, and the wave function collapses only when it is observed by a soul. These types of views have a number of challenges to deal with. For example, according to Einstein's Theory of Relativity, different observers will disagree about which of two events happened first,
18:20
and different observers will therefore disagree about which of them collapsed the wave function. Also, we again have to deal with the EPR paradox involving the spins of entangled particles.
18:40
One interesting approach to addressing these questions is by using another interpretation called Relational Quantum Mechanics.
18:51
In the Relational Quantum Mechanics interpretation, the observer does not need to be a conscious observer, but can be any system. The Relational Quantum Mechanics interpretation states the following:
19:07
In the same way that different observers can disagree about which of two events happened first, different observers can also disagree about whether or not a wave function has collapsed. From the perspective of one observer, another observer can be in a superposition of states.
19:30
Relational Quantum Mechanics deals with the EPR paradox of spooky action at a distance by pointing out that no observer is able to measure the spins of both particles simultaneously. In this interpretation, from the point of view of the observer, the spin of the unobserved particle is still in a superposition of states, until the observer interacts with something that has measured the spin of that particle.
20:03
There are other interpretations of quantum mechanics that have other ways of dealing with the EPR paradox. In the EPR paradox, if we measure the spins of two entangled particles, with the detectors rotated relative to each other by 45 degrees, then the two particles measure opposite directions of spins approximately 85% of the time.
20:28
This is a mathematical and logical impossibility if the two particles don't communicate with each other.
20:36
Some interpretations of Quantum Mechanics deal with this problem by stating that the rules of logic we are familiar with simply do not apply to the Quantum World.
20:47
The argument is that rather than use the rules of logic we are familiar with, we instead have to use Quantum Logic.
20:58
In some versions of Quantum Logic, the probability of the truth of a statement can be represented geometrically using the projection of vectors, thereby giving us the experimental results we observe.
21:12
The classical logic we are familiar with could just be a special case of Quantum Logic, just as Euclidian Geometry is a special case of non-Euclidian Geometry.
21:26
The argument is that Quantum Logic and non-Euclidian Geometry are contrary to our intuition only because our ancestors never previously encountered them anywhere in our evolutionary history.
21:41
Our brains are hard wired to think that the Universe operates according to classical logic and according to Euclidian Geometry, only because that is what helped our ancestors survive. However, rejecting the known laws of logic is a very big step, and we have to think very carefully before making this leap, especially since we have many other interpretations of Quantum Mechanics to choose from.
22:11
In this video, we have tried to provide a reasonably comprehensive list of different types of interpretations of Quantum Mechanics. However, it is not possible to provide a completely comprehensive list, and we offer our apologies if your favorite interpretation was not listed here.
22:31
Detailed information about the actual physics of Quantum Mechanics, as well as the philosophical nature of the paradoxes, are available in the other videos on this channel. Please subscribe for notifications when new videos are ready.
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