Well, the thing is that the MWI is actually simpler than all the other interpretations, because it simply removes the idea of wave function collapse entirely. Any theory involving wave function collapse is adding something extra to quantum physics that can't be demonstrated experimentally.
Because we live in the macro world, it feels like a single unitary reality is simpler, but actually, the MWI makes fewer assumptions. So I would turn that around and put the burden on the other theories to show that collapse is real, the default position should be MWI.
It is not simpler, since MWI still needs the Born postulate to actually predict the results of quantum experiments. It replaces this idea of the wave function collapsing with a redefinition of what a measurement apparatus / observer is (in MWI, a measurement apparatus exists in a single "branch" of the wave function).
This is especially problematic because QM doesn't predict any particular decomposition of a quantum state into particular classical states. That is, the Schrodinger equation doesn't actually predict that a particle has some amplitude here and some other amplitude there, as is often presented; instead, it predicts that it is described by some vector which can be decomposed in many different ways. You can say it has some amplitude X here and some amplitude Y there, but with any 2 points in space-time (adjusting the amplitudes). Or, it can have some combination of position and spin with amplitude X, and some other combination of position and spin with amplitude Y. You can choose any basis you like for the measurement, and you will get the corresponding answer.
But, for MWI to actually predict experimental results (and the classical world we live in) you not only have to choose to look at a single element of that basis, but you also have to believe that the classical basis is somehow preferred.
>This is especially problematic because QM doesn't predict any particular decomposition of a quantum state into particular classical states.
It does predict it, this decomposition is called decoherence, where a classical state splits into a superposition of several different classical states.
> this decomposition is called decoherence, where a classical state splits into a superposition of several different classical states.
No, that's not what decoherence is. Decoherence is the spread of entanglement over a very large number of untrackable degrees of freedom (usually referred to as the "environment"). This makes interference effects unobservable. But decoherence itself does not involve any "splitting"; the entanglement that decoherence spreads over a very large number of untrackable degrees of freedom has to already be there before decoherence can act on it.
AFAIK the largest molecule to show quantum behavior in experiments is somewhere at 100 atoms. I suppose that's all "environment" you need for a large number of degrees of freedom.
And yes, spread of entanglement does mean splitting. Entangled state is not factorizable, because all states in it are split. This splitting is the result of splitting of the initial state.
> I suppose that's all "environment" you need for a large number of degrees of freedom.
No. The 100 atoms is the largest molecule that we can do experiments on without having decoherence happen and ruining our attempts to observe interference.
The number of degrees of freedom in the environment is many orders of magnitude larger, something like 10^30 or more for a typical experiment.
Only to the extent that MWI is actually a position. My understanding of MWI is: shit happens, and there's no need to understand why specific shit happens because all shit "happens" (by a definition of "happens" that carries basically no meaning) and you should be happy just to prove that the shit that happens is among the full set of shit that could happen. It's the "don't worry your pretty little head over it" theory of reality.
Here's the best poem ever written, or that will ever be written:
Roses are red
Violets are blue
MWI stinks
And so do you.
Don't think it's that great? Well, that's because there exist other versions of me that wrote different words up there, and you just happened to read a version that wasn't that great.
I hereby accept my position as God Poet of the Universe.
Shit happens because that's how the state of matter changes over time, the wave function is the description of the state and the Schrodinger equation is the description of the state's change. That's as complete and happy understanding as it goes.
..And failing to find any evidence of collapse. Hence "can't be demonstrated". I didn't mean "can't be looked for" I meant that attempts to find it have not been successful.
You don't think postulating an unknown number of extra universes with a very poorly defined relationship with each other qualifies as an extra assumption?
The MWI does not postulate extra universes, although that pop science description is unfortunately common even though it is wrong. The MWI just says that the one universe that exists is actually very, very different from what we perceive. (To be clear, I'm not saying this correct description makes the MWI any more palatable. I am just clarifying exactly what kind of unpalatableness the MWI requires.)
One should not be trying to learn physics from Wikipedia, particularly not for something as complicated as QM, even more particularly not for an aspect of QM as contentious as the MWI.
The MWI says that there is one single universal wave function, which evolves in time by unitary evolution forever, and that that universal wave function is all that exists. That is one universe, not an infinity of them. It is just one universe that is nothing like what we think we perceive.
Not the parent, but I 'd say this amounts to how you define a "universe". There is a single wavefunction which says that one "copy" of you observes result A and another "copy" observes result B. Using the phrase "many universes" implies that these two scenarios are considered as "different universes". I personally do not like this phrase and find it misleading.
You are all the "copies", and, under certain conditions, can experience all their observations. Think of yourself more as an aspect of a multifaceted being experiencing the universe through you.
Practice lucid dreaming, so you can maintain a semi-awake meditative state. Practice remembering your dreams -some of them are memories from other aspects like you, others are your subconscious communicating through symbols and situations. Practice asking questions, dreaming, and remembering. Practice reaching the semi-awake meditative state from the awakened end of your day. Realize, what you think of as your body, is the 4D sensory organ of your hyperdimensional self, maintain meditative state, and open your other eyes. The effect is a lot like the collage view of regular stereoscopic vision, except out of the immediate overton-window of aspects of yourself similar enough to you.
Getting to that point can take several years. A quicker option seem to be drugs, based on conversations I had with friends, but I never wanted to artificially taint my perception, so I did it the hard way. My findings are that while there is no fate, there exist enough aspects in universes similar enough to yours, but perhaps chronically shifted, that you can learn from their mistakes. This is known as instinct, "guardian angels", premonition, deja-vue, etc.
Good luck.
If you can receive information from alternative universes, experiments can be devised to prove this, and would indeed answer a great many questions in quantum physics. Physicists and others interested in the field would be extremely grateful. As would cogitative scientists and statisticians, who would need to adjust their experimentally demonstrated explanations for instinct, "guardian angels", premonitions, deja-vue etc.
Those worlds are states in superposition, the term "world" is an allegory to help people understand that those states don't interact due to linearity of the Schrodinger equation, because not all properties of linearity are obvious.
Let's say we generally subscribed to the flat earth theory. But that causes many problems with our actual measurements. So someone proposes the "many earths" theory, where there are many flat earths connected at different angles.
If you could travel between these many earths, you could even end up in one - let's call this mythical land "Australia" - where South was up!
Sounds a bit crazy but some people believe it could really exist.
We perceived flat earth and geocentrism, so not the first time. I can recommend quantum electrodynamics to people who are too attached to corpuscular paradigm.
It is the other way around. The "universes" are there when you solve the equations, and you can either let them be or keep only one by invoking the collapse.
The collapse amounts to just setting some terms to zero.
Because we live in the macro world, it feels like a single unitary reality is simpler, but actually, the MWI makes fewer assumptions. So I would turn that around and put the burden on the other theories to show that collapse is real, the default position should be MWI.