Copenhagen vs Many Worlds Interpretation of Quantum Mechanics – Explained simply

This video is sponsored by Blinkist Until about 1900, all we had was classical mechanics. This is the mechanics of Isaac Newton   and James Clerk Maxwell. But after 1900, Max Planck ushered in an era of quantum mechanics.   And it explained the various anomalies in  classical mechanics, such as why electrons  

Do not radiate energy and fall into the nucleus,  as they should according to Maxwell’s equations. Since that time, Quantum mechanics has become  one the most proven and successful theories in   all of science. But, while equations  such as the Schrodinger equation are  

Superb at making predictions and explaining  behavior at quantum scales, when you start   asking the question. “what is actually going on,” this is where the controversy starts. Caltech physicist Sean Carroll says quantum  physicists are like people with iPhones.  

They know how to use it, and can do some great  things with it, but if you ask them what   goes on inside their iPhones, they have no  idea. Similarly, he says, physicists know   how to use the equations of quantum mechanics  to predict all kinds of things, but ask them  

How quantum mechanics actually works, and if they are honest, they will say they really don’t know. But this hasn’t stopped physicists  from speculating what the mechanism is.   These various speculations are known as  interpretations of quantum mechanics. The standard interpretation is called  the Copenhagen interpretation because  

If was devised in Copenhagen,  Denmark by mainly Niels Bohr   and Werner Heisenberg in the 1920s. This is the  interpretation taught to most students in college.   But even a majority of physicists do not agree  that this is the correct interpretation. In fact,   there is no single interpretation  that has a consensus agreement.

So what is the Copenhagen interpretation? What are   the best alternatives interpretations? Why do  we even need an interpretation to begin with?   Those are some great questions, which I  will attempt to answer…coming up right now. The primary challenge of understanding quantum  mechanics is that according to the equations,  

All particles exist in a state of superposition.  That is to say, that the properties of any quantum   particle such as its position, momentum, spin  etc. is not only unknown, but is unknowable   until it is measured. In fact, before it  is measured, the particle is said to be  

In many states at once. It is not here OR  there, it is here AND there at the same   time. It is not spin up OR spin down, it  is spin up AND spin down at the same time. This sounds crazy from our  classical mechanics perspective  

Because we never experience large objects being  in super position. When you hold a tennis ball,   you know exactly where it is and how fast it’s  moving. So, the quantum   mechanical behavior predicted by quantum mechanics does not seem to fit with our world view.

One of the biggest challenges of quantum mechanics is trying to explain this transition from what   is thought to be the behavior of objects at  quantum scales – superposition of multiple states   vs. their classical behavior upon measurement. The various interpretations of quantum mechanics   can be thought of as attempts  to explain this transition.

Most interpretations of quantum mechanics focus on the Schrodinger equation and the wavefunction   to explain quantum behavior. This equation  was developed by Irish-Austrian physicist   Erwin Schrodinger in 1926. It contains a wave  function, represented by the Greek letter psi. German physicist Max Born formulated  the interpretation of psi,  

Which is that the square of the  absolute value of psi represents   the probability of finding a particle in any  one particular state if we were to measure it. The concept of measurement was introduced to   explain what we actually see  when we make an observation.

The fact is that even if it were possible  for us to directly observe quantum particles,   we would never see them being in superposition,  we would only observe them being in one state or   another. We would only see the spin as up or down, not up and down.

Presumably, our observation acts like a measurement that destroys the super position. To give you an intuitive feel, let’s  look at some of the interpretations   in terms of the famous  Schrodinger’s cat experiment. This is a thought experiment proposed  by Erwin Schrodinger to illustrate,  

Ironically, what he felt was the absurdity of  assigning probabilities using his own equation. In this thought experiment, we have a box.  There are 4 things in the box. There is a cat,   a radioactive source – the emission of  radiation is completely random according  

To most theories of quantum mechanics, so this  is the source of quantum mechanical randomness. There is a radiation detector attached to a  hammer, and a vial of poison gas like cyanide.   If the detector detects radiation, the hammer  comes down and will smash the vial of gas and  

The cat will die. If it doesn’t detect radiation,  no gas is released and the cat will stay alive. If we look at this from the quantum mechanical  point of view, there are two possibilities for the   wave function of this system. If we presume that  the quantum system consists of the just the cat,  

Prior to measurement, the wavefunction  of the cat will look something like this:  Where the wave function describes the superposition of the cat being alive and dead. We have one over the square root of two because  there are two probabilities and the square of each  

Probability will be one half, and thus added  together the total probabilities will be one.   The wave function always shows that the  sum of all probabilities will equal one. In the standard, or Copenhagen interpretation, as soon as you open the box and make an observation,  

One of the probabilities comes true,  and the other probability disappears. So let’s say you observe the cat being alive,   that probability is now 100%. And the other  probability of the cat being dead becomes zero,   so that probability goes away. This  is called wave function collapse,  

Meaning the wave function has collapsed to  one state – based on the 50/50 probability. The wave function collapses  as the result of a measurement   by an observer or apparatus external to  the quantum system. A measurement is simply   an interaction of the quantum system  with a classical system. In this case  

It is you the observer opening the box and  measuring whether the cat is alive or dead. The problem with this interpretation is that  it sets two set of rules for how particles   behave – one for before measurement, and one  after measurement. But it doesn’t explain  

How this transition happens. This is often  characterized as the measurement problem. Why did the other probability go away. What is  the mechanism that collapsed the wave function? Bohr might have said, well, it just fits the  data. The data is that we observe only one event,  

So all the other events that we could have  observed no longer exist. We are just interpreting   quantum mechanics based on the data that we can plainly see. Don’t ask me how or why this happens.   Or you can say what Richard Feynman said when  asked the question, “Just shut up and calculate.”

This is just not very satisfying because we,  and, I won’t speak for you, but at least I,   need to know what’s really going on. A popular alternative, the many worlds  interpretation of the same event would be  

That no collapse occurs. The wave function is the  only true nature of reality. It never goes away. This interpretation was formulated by Hugh  Everett in 1957 as a graduate student at   Princeton University. And the followers of  this interpretation, sometimes referred to as  

Everettians, say that this is the simplest and  most basic interpretation of quantum mechanics   because it introduces no other assumptions or  equations, other than the Schrodinger equation. In our cat analogy, the distinction that  the many worlds interpretation makes vs. the  

Copenhagen interpretation is that it says, hey,  you as the observer are also a quantum system,   and you are entangled with the  cat. So the wave function includes   more than the cat. It also includes you,  the observer. It would look more like this,  

Where one part of the wave function is that  the cat is alive, and you observe it as such,   and the other part where the cat  is dead and you observe it dead: That is one wave function. When you opened  the box, the reality that you observed,  

Where the cat is alive, is but one world. However,   there is another world in which you  would have found the cat to be dead. Both worlds where you found the cat alive  and where you found the cat dead exist.  

You just happen to find yourself in one of them. So the question is why do we find ourselves   in the one branch where the cat  is alive, and not the other one? Everettians argue that other versions of ourselves  in the other worlds are asking the same question.  

You just happen to be asking it  in the world you find yourself in.   Every universe is equally  ‘real’ to those living in it. To embrace this interpretation,  you have to accept that many,   perhaps infinite, worlds exist, all  with different quantum outcomes. The problem is that

This does not seem to fit  with our experience, because   we have no inkling of the other versions  of ourselves. Where are the other ones? If we really are entangled with the cat,   then shouldn’t some part of me feel like  I saw the cat alive as well as dead?

But this never happens. There is never a  world where I see the cat both dead and alive,   where half of me saw the cat alive,  and the other half saw the cat dead.   So how does the split of the worlds occur?  Everettians say it is due to decoherence.

So what the heck is decoherence? Quantum  decoherence is the physical process   that is used to describe how quantum states  transition to the one state that we experience. The Copenhagen interpretation treats wave  function collapse as a fundamental process   without explaining the details of how it happens.  Decoherence attempts to explain what appears to be  

Like wave function collapse, but in MWI-speak,   it is a splitting of worlds. No  wave collapse actually occurs. If the universe was composed of only you and  the cat and nothing else, then you and the   cat would be in a coherent superposition.  This would be represented by our original   equation here:

The key realization is that in reality, you  have more than just you and the cat entangled.   Both you and the cat will also be entangled  with your environment because THAT is also   a quantum system. So, for example, the cat  will be entangled with what’s inside the box,  

Atoms of air, photons from  black body radiation, etc. All these objects will be entangled  with the cat. And you will also be   entangled with your environment. The  environment inside the box for a cat   that is alive will be different than  the environment for a cat that is dead.

Why? Because a dead cat’s interaction with the  air molecules and photons will be different,   not only because it’s position will likely  be different, but also other factors such as   heat produced etc. So Psi now looks like this,  where there is an added component of the cat  

And you being entangled with environment 1 in  one case, and environment 2 in the other case: These two environments are completely different. Because the entanglement with the  environment now enters the picture,   the coherent superpositon between  you and the cat is broken.

The Schrodinger equation says that the two parts  of the wave function above are perpendicular to   each other, that have no connection to each other.  This can be interpreted as two separate worlds. It is as if the universe splits into two  separate realities. This is decoherence.

Decoherence is another way of explaining how  quantum superposition gets lost, by interaction   with the environment. You can also think of this  as the quantum nature of the original 2 component   system leaking information into the environment  due to it entanglement with the environment. I  

Made a video about information leak using tennis  balls, if you want to check it out here. The problem is that since there is no  overlap between the branching wave functions,   no communication or connection between the  worlds exists. So it is unclear whether we  

Could ever verify whether the other  worlds exist. The only evidence is   the mathematics of the Schrodinger equation. An  experimental verification may not be possible. Now if the idea of quantum superposition  and randomness makes you uncomfortable,   I want you to relax because there are completely  deterministic interpretations as well.  

One is the de Broglie-Bohm, or pilot wave  theory, also known as Bohmian mechanics.   Another fascinating theory I like is an  Objective collapse theory by Roger Penrose,   who combines principles from general  relativity with quantum mechanics.   And these two, along with some other crazy  interpretation will be the subject of my next  

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