IMO the most embarrassing problem in physics is people pushing non-falsifiable, unscientific theories (multiverse). The fact that some futzing constant needs or doesn't need to be added in order to bring a model in line with empirical observation, in comparison, is quite benign.
That is, if we take the simplest known cosmological model consistent with our evidence, its equations predict space and matter outside our observable universe. That cosmological model is falsifiable in that it makes other predictions besides this one, many of which can be (and have been) tested, but this specific prediction cannot be directly tested.
Similarly, the equations of quantum mechanics predict multiple "worlds" (although I find this term unfortunate: the state of the "multiverse" is always just a single point in Hilbert space, and it only seems like there are multiple "worlds" when we imperfectly try to map that physical state onto the intuitive level of everyday experience). That specific prediction cannot be directly tested, but other predictions of the equations can be, and have been. There is essentially no dispute about the math. The dispute is whether to deny the reality of something which is right there in that math, akin to denying the reality of a universe beyond that which is observable.
The many worlds interpretation is completely unrelated, and it doesn't suffer from this problem of being unfalsifiable even in principle. However, I would also not say that the MWI is a straightforward deduction from the laws of QM. It rejects the most visible effect of those laws (the Born rule, i.e. that they predict probabilities of classical events and not certainties) and replaces it with an infinity of "worlds". Even worse, it still doesn't explain WHY we can only "see" one such "world", while elementary particles can "see" all of them (i.e. it doesn't solve the measurement problem any more than Copenhagen interpretation). And there is also debate on how plausible it is to even define the "probability of a world", so whether MWI is actually consistent with the Born rule is somewhat in doubt.
It's just that there's one of us in each world. The one in this world sees this one. The one in that world sees that one.
That's just one of the many problems that MWI fails to solve.
In e.g. the Copenhagen interpretation, observation plays a special role, because it's the moment of wave function collapse.
In MWI, there is no wave function collapse, so there's no need to identify certain special events called "observations" which are somehow different from the rest of physics. It's just our eyes, and brains, and instruments, interacting and becoming entangled with the other things in the world.
When we "observe" Schrödinger's cat, we entangle ourselves with the cat just as the cat was already entangled with the cesium atom. Now the world is in a superposition of two states: in one, the cat is alive and we see it as alive; in the other, the cat is dead and we see it as dead. This is exactly what the equations say should happen. Nothing special.
Even more interestingly, say you want to predict the outcome of a quantum experiment. In the MWI, that outcome is deterministic, and it is a superposition in Hilbert space. However, in your own space, you still need to compute a probability for that outcome.
What is this a probability of in the MWI? The concept of probability across worlds is not really rigorously defined, especially since you need to arrive at very specific results to be consistent with observations.
I agree that the meaning of probability in the MWI context is an interesting open question. Mathematically, it's the squared amplitude of part of the wave function. But why this corresponds to our personal experience of frequency is something I'm unclear on.
This is exactly the measurement problem, essentially identical between Copenhagen and MWI.
As I understand it, it becomes ever more difficult to maintain a coherent quantum state among larger and larger collections of particles (this is why making large quantum computers is difficult), so for most practical purposes large aggregates of particles behave like a single "world". But there is no sharp dividing line, and the same math applies at all scales.
Unfortunately, this is where the imperfect mapping to which I referred above — between the math and our intuition — becomes misleading, and why I don't like the term "world" to begin with.
But still, the problem I was questioning was different: you are saying "there is one of us in each world". In fact, that is not what QM predicts: in QM, there is only one of us in the entire system described by the wave function, with different amplitudes in different states. A system with 1 electron with some amplitude < 1 in state A and some amplitude < 1 in state B is not the same as a system with 2 electrons, one with amplitude 1 in state A and another with amplitude 1 in state B.
So, if we took a detector that could measure state A or state B, we should expect that it detects state A with some amplitude and, simultaneously, state B with some other amplitude. Instead, MWI postulates that there are 2 detectors, one which measures state A with amplitude 1, and the other state B with amplitude 1 (in fact, it postulates an infinity of detectors, out of which some number measure state A and some measure state B, such that the Born rule predictions by "counting" detectors).
This is the part that remains unexplained.
There is only 1 detector, and it, along with the detected particle, is in a superposition of states.
Again, this is where the language of "worlds" becomes misleading, and why I don't like it.
Copenhagen is adding unnecessary concepts to the interpretation and making it more complex, so, if we are using the scientific method, the burden of proof should be with the backers of Copenhagen.
Instead, the more interesting thing is to stop ignoring the measurement problem and find a theory that actually solves it somehow; until such a thing exists, waxing philosophically about many worlds or un-reality is not really interesting.
Objective Collapse theories, and Bohmian Mechanics (hidden variables / pilot waves) are the only other games in town for being actual working theories. (Copenhagen is not a theory, it is an assertion that one should stop trying.)
Of these, Everettianism is actually the simplest.
That said, the multiverse, often referred to as the Bulk is a horse of different color. It has nothing to do with branches of the wave function in Everettian mechanics. Instead it seems to be a way to explain the fine-tuning problem. If there is no Bulk, and our comoving patch of the universe is representative of the whole shebang, then the conditions at the time of the Big Bang were unimaginably improbable, extraordinarily special, just exactly right to produce the stars in the sky, and us. A single solar mass black hole has more entropy than the entirety of the universe did at the time of the Big Bang.
One way to explain that "problem" is to think that the universe in which our comoving patch exists is a small pocket arising from quantum fluctuations in a much more vast "bulk" of true vacuum that exists at high entropy.
It does seem like a lot of mental gymnastics, and mostly it seems to be directed at elimination of every whiff of the idea that a creator was required for the improbable conditions in the early universe that give us the arrow of time. I'm not sure it's at all useful, nor would it have any predictive power were the idea to be true. Our comoving patch still expands to an infinite nothingness where even the black holes have evaporated away.
Perhaps that is just playing with the meaning of words or semantics to some, but it’s compelling enough for me.
The other description of quantum equations predicting many “worlds” I think is a different and more acceptable proposition. Like we’ve observed the shadow of an object, but can’t figure out how to observe the object directly.
For that you need quantum decoherence. Big creatures like us only ever see one or the other outcome of a two-outcome experiment because we get entangled with the outcome.
No they don’t. If I roll a dice it’ll land on one of 6 faces. That’s a model, in that model I might assume equal probability due to the geometrical symmetry, that’s an assumption. So now I have a problem if all outcomes are equally likely how do I resolve that only one occurs? Well obviously every dice roll must spawn 6 parallel universes... Now nowhere in that model or that assumption does “so there must be 6 parallel worlds spawned when you roll the dice” get predicted by my model. The multiverse/many world is an interpretation, not an outcome of, or even prediction from the model.
There is nothing “right there in the math”. QM makes predictions about probabilistic outcomes, accurate predictions, but there is no requirement for those accurate predictions to be right that there must be multiple parallel worlds.
While I mostly agree with you, the discussion is much more complex than this.
First of all, most of QM is completely deterministic, just like classical and relativistic mechanics. The Schrodinger equation is a linear partial differential equation, it gives you an exact prediction of the state of any system of particles.
However, that prediction turns out to be so wrong on the face of it, it's almost meaningless: the Schrodinger equation predicts that "particles" (wave-packets) have some complex amplitudes of many different mutually-exclusive states, such as being here with amplitude 1+i and being there with amplitude 1-i.
BUT, it turns out that if we interpret these amplitudes of the wave function (well, the squares of their absolute values) as being probabilities of the particle being in those states when measured, then you get extremely accurate predictions.
The final problem then is that the amplitudes -> probabilities step is only correct when predicting the result of a measurement. At the particle level (before the measurement) the particles actually exist in all those states at the same time, and interact with each other in all those states (e.g. particle A may interact with particles B and C at the same time in two different places, modifying both their trajectories).
So, the laws of motion for QM that you need to use are:
1. Particles move and interact according to the Schrodinger equation (e.g. causing interference patterns, interacting in multiple places at the same time), fully deterministically and linearly
2. When you want to predict the outcome of a measurement on a system that has evolved according to 1, you will get a single value with some probability computed as the square of the absolute values of the amplitudes of the possible states. After the measurement, the amplitude of any other state than the one measured will be 0, and this can be plugged in to make further predictions about how it will interact with other particles.
It is this 2nd postulate that feels very artificial, the main subject of the measurement problem. MWI seeks to explain QM without the 2nd postulate - that is why people say that it "derives from the math" (the 2nd postulate can obviously not be derived from the first, as it is a nonlinear change to the system).
Everett spent 4 years doing graduate studies, taking his first physics courses then and coming up with his many-worlds dissertation within a single year (137 pages ’typed’ (according to current historical records) by his later wife Nancy Gordon). He pretty much left physics afterwards, for doing weapons research(!) for the Pentagon.
Many-worlds sounds like what an overconfident ‚pragmatic’ college graduate would come up with even to those who don’t do physics.
Is it just my impression or is it mostly US-Americans who are pushing many worlds into physics?
"In this picture, the curvature of space would constantly fluctuate on extremely small scales, well beyond anything we could hope to measure"
"Her model is based on the idea that extra dimensions, beyond the three of space and one of time that we witness, might be hidden out of sight."
Instead of cosmological constant "novel" approaches relay on something that could not be observed, even in principle. That does not sound like a good approach.
My guess is that they try to make some assumptions, figure out math around it, hope that it will somehow stick nicely, which would give hope that there is something in it.
Indeed, it seems that math "likes" to describe our universe, some mathematical theories (Riemannian geometry, group theory) fit physics wonderfully. The problem is that for physics math is just a language, it is unlikely that language alone will give us answers without understanding physical phenomena itself.
I find multiverses far more plausible than futzing constants, because the universe in general seems to be absurdly big but simple and consistent physics-wise. Smaller but more fiddly/complicated models seem much less plausible, just judging by history.
Philosophically I find the idea of the multiverse probable, but making it science is a separate thing.
edit poor word usage.
A theory involving a multiverse can make observable predictions even if the other regions are not accessible.
I should have said evidence rather than proof, my mistake.
>theory involving a multiverse can make observable predictions even if the other regions are not accessible
All those predictions would serve as evidence for the same theory without involving a multiverse.
"For some reason" is not a theory and it makes no testable predictions.
One with the same evidence as "god did it."
> is not a theory and it makes no testable predictions.
This is true of the multiverse too. It is an attempt to explain why things are like they are rather than a testable theory.
Okay, this is just going in circles. Up above you accepted that multiverse theories did make testable predictions. And they do, which is what makes them science.
edit let's use a concrete example. This thread is about the cosmological constant. I could easily say that concept is causes by multiverse influence on this universe, just as rightly I could say it's god's finger on the scales. Neither now have experimental data because I can say that.
further edit sorry, this is a bit stuck in my head. Look here
https://en.wikipedia.org/wiki/Multiverse
Notice that it is labeled a hypothesis, and the theories that involve evidence predict a multiverse may be possible with these conditions.
The prediction that objects continue to exist when we're not looking at them is not testable in the same sense (you could even say it's unfalsifiable and therefore not scientific); nevertheless most of us tend to believe in it.
We have experimental evidence that certain light patterns have been observed. For any theory that has Jupiter in it, you could make the same theory without involving Jupiter. I don't see any fundamental difference between the evidence for Jupiter and the evidence for the multiverse.
Another universe, basically by definition, has no observable characteristics. We can seemingly only observe our universe, and have no way of knowing if something unobservable is impacting our universe.
Sure, but isn't it the same for multiverse theories? Obviously we don't observe the multiverse directly, but we observe data for which the best explanation also implies a multiverse, just as the best explanation for various observations implies the planet Jupiter.
> Another universe, basically by definition, has no observable characteristics. We can seemingly only observe our universe, and have no way of knowing if something unobservable is impacting our universe.
There's a lot we can't observe or interact with directly; we can't interact with the past, we can't observe the future, we have to real way of interacting with distant galaxies, we can't actually observe inflation at all... a good multiverse theory (of which I'm not sure there are any, but I think they're possible in principle) would be in much the same category as inflation AFAICS: we can't observe the thing itself but we can observe things that seem most likely to be consequences of it.
No, we don't have observational data supporting a theory of a multiverse. With Jupiter, if there were a planet there we would expect x to happen due to our understanding of planetary physics. We then observe, and x happens.
We can't repeat this with a multiverse, as all of our observational data ever provides no insight into how another universe works.
Two people keep saying this, it is easy to prove me wrong here.
>There's a lot we can't observe or interact with directly;
None of which are science. We have past observations, and we can make predictions of the future, but that's basically the extent of their place in science. We do observe distant galaxies, and that's where are understanding of them comes from.
As for inflation, like of space? As we do have experimental data for that, something about how long light takes to reach us. If you're talking economics, very little of it is science.
No we don't - I assume you're thinking of dark energy, which is a separate and (as far as we know) unrelated phenomenon. The evidence for inflation is basically "the cosmic microwave background looks like it was produced by the universe behaving in such-and-such a way"; it had no effects except for very shortly after the big bang, so we can't use it make predictions about the future and observe whether they happen or not. (We can, and have, made "predictions" about things that haven't yet been measured, e.g. inflation was used to predict what the CMB polarisation would be before we launched Planck to measure it - but what we're measuring is something that happened in the very early universe).
So I don't see a lot of difference between that and e.g. a theory that says (with evidence) that our universe is the result of a symmetry-breaking process that would naturally have produced an ensemble of similar universes.
I was talking about the expansion of the universe that is still ongoing.
Inflation has observational data already, the radiation you mention, and much of the work done with particle accelerators is to recreate the conditions of these kinds of events. This is enough to make it science, even if completely wrong.
I thought of perhaps a simpler way to put this. The realm of science is the observable universe. We can use that to develop theories of events in our universe we can't observe, as there are still remnants in the observable universe. We can't develop theories of things that never happened in the observable universe though.
Maybe someday someone will develop some method to observe effects that would show evidence of a multiverse. It hasn't happened, and personally it seems unlikely.
Though, the outlook of getting an answer to "is there really a multiverse out there or is the multiverse just a sequence of simulation runs on some future teenagers quantum computer simulator trying to find the best set of meta-parameters" is probably bleak...
I don't understand this criticism. It's my understanding, the double slit experiment shows interference between universes. What is the alternative? decoherence?, but how do you develop an experiment that result in evidence that decoherence is real?
From a experimental point of view, multiverse have the same evidence that the alternative. From an epistemological point of view, multiverse is a more simple theory. Ergo, until we have more data, we have to prefer the multiverse theory.
Yes, the alternative is that classical mechanics is incomplete and quantum mechanics is necessary. And there has been plenty of experiments supporting this conclusion.
The Big Bang itself is a non-falsifiable, unscientific theory. We cannot prove it happened in the way we think it might have. The Big Bang created a universe of cause and effect, yet has no cause itself. All known laws of physics fail to work in the early moments of our existence.
“Give me one free miracle, and science will explain the rest” (— maybe Terrence McKenna? I can’t remember)
Just because we don't understand it's cause yet doesn't mean we won't, and doesn't mean it didn't have a cause.
There is experimental support for the electroweak transition at the LHC at CERN and Lep. LISA (https://lisamission.org), when and if it happens, will also gather more evidence for and against the electroweak transition at cosmological scales, with somewhat less clear-cut (but still highly supportive) evidence becoming ever more available through indirect tests in the interim.
The CMB photons are almost wholly from the recombination epoch at which entirely ionized atomic nuclei -- mostly single protons but other things up to Beryllium-9 -- first became electrically bound to free electrons. The neutralization of these particles releases photons. In the case of Hydrogen-1 + one electron, the emitted photon is principally at Lyman-alpha (Lya) frequencies. Because of the relative abundance of this pairing during recombination, the Lya spectral lines dominate the CMB.
Redshifting preserves the (relative) structure of the Lya lines, and one result of the expanding post-recombination cosmos is that later absorbers (dust, gas) and emitters (neutral hydrogen excited by quasars and so on) produce Lya line structures that are shifted red to different degrees.
The CMB is the reddest Lya structure, although it is smeared into a (differential of) a blackbody spectrum by Thompson scattering, and kinematic effects make it very nearly uniform (correcting for proper motion) in every direction.
Between the end of the electroweak transition and the start of recombination, there were still many photons. These scattered off the electrically charged free electrons, nuclei, and protons, and it is extremely unlikely that there would have been a spectral structure if that light were measured because of the random lengths of photon free paths. As scattering fell off thanks to a combination of expansion (increasing the distances between charged particles) and neutralization (electrically neutral objects don't cause a relevant photon scattering) the photon mean free path became cosmologically long, terminating the randomization of the momenta of photons. Instead of being boosted up or down by process like (Inverse) Compton Scattering, photons emitted by transitions of free particles to bound systems at lower energy states remained at the emission wavelength subject to redshift from the metric expansion of space.
Cosmologists work with a set of points at the time when photons that were essentially freed from scattering around the time of recombination are now reaching us, and call that the surface of last scattering. The surface of last scattering has a cosmological redshift of z ~ 1100, which places it at about 380 thousand years after the Hot Big Bang.
Additionaly, we have several lines of indirect evidence for the Cosmic Neutrino Background, which was emitted more than 379 thousand years earlier than the surface of last scattering, and long-wavelength gravitational radiation detectors on constellations of spacecraft are planned that will probe gravitational waves that are even older than those relic neutrinos. Among the evidence is structural detail in the Cosmic Microwave Background that is sensitive to the density and varieties (in particular rest masses) of the extremely early Cosmic Neutrinos, because they interact both gravitationally and through chiral currents with the various early pre-electromagnetism plasmas.
So the stress-energy in the cosmos was quite busy evolving well before the Cosmic Microwave Background, generating actual observables (and many reasonably-theorized observables that we just can't probe reliably today). There were several different "plasmas" between the hot big bang and the recombination era, each with very different degrees of freedom, and successively much colder temperatures. Moreover, there were lots of plasmas after the surface of last scattering, for example during "first light" as very bright UV sources reionized neutral hydrogen.
I am therefore with other commenters in this thread: we don't see the big bang happening because we can't directly detect anything before the surface of last scattering, mainly because the scattering thoroughly randomized light-borne observation prior to that, secondarily because we can't detect extremely redshifted neutrinos (we have trouble enough detecting neutrinos from fission reactors we have built on Earth, or solar neutrinos, which have not been redshifted to very low energies), and tertiarily because we haven't yet built big enough networks of gravitational wave detectors. What we do see is relatively late in the pre-stellar history of the universe : it had expanded and cooled enormously by then.
Analogies between the early universe and chemical or even nuclear explosions are generally pretty misleading.
That is there was a bang, it was big, and we can directly observe it.
Rather than an acoustic "bang" which has a moving sharp amplitude peak that decays back to the background as it radiates outwards, the acoustics of the hot big bang are comparable to Gaussian white noise with an almost everywhere-identical intensity that decays everywhere almost-identically (that is, the sound of the big bang is the background). The pressure waves weren't moving through anything like our atmosphere or oceans, it was simply too hot for there to be any molecules at all, and there were so many low-amplitude pressure-waves that one would have to think about trillions of trillions of trillions of simultaneous explosions and implosions, and not just one.
Explosions in a medium like air or water create an overdensity (compared to some global average) shockwave that has a particular focus, not random overdensities and underdensities everywhere. Additionally, an explosion's overdensity shockwave is strongly associated with an increase in temperature -- colloquially, explosions tend to create fireballs that break up structures depending on the strength of the explosion (they can break weak molecular bonds, chemical bonds, the bonds between electrons and nuclei, and the bonds within nuclei, in increasing order of power).
The hot big bang is consistent with a very hot, dense, low entropy phase suddenly expanding and cooling adiabatically everywhere, with everywhere remaining close to the average. With this expansion-driven cooling, new bonds formed, first freezing out neutrinos, quarks and gluons, baryons, hadrons, electroweak particles, weak and electromagnetic particles, and eventually less-ionized and even neutral atoms and molecular hydrogen (H2).
In other words, the hot big bang's expansion produced effects in matter almost exactly opposite what one would get in an explosion.
Finally, "big" in a spatial-extent sense isn't right either, as everything that is near us in our modern galaxy cluster was compacted in to something much smaller than the head of a pin. Indeed, everything we see in our sky (including at least billions of galaxies) was compacted into an area much smaller than our own galaxy.[1]
Perhaps you might consider that the "big bang" was a misleading choice of name put forward on a radio broadcast in 1949, and that trying to promote it from poetic licence (or even just rhetoric) into an analogue of a chemical or nuclear explosion is futile given that Hoyle knew he was mischaracterizing (not just oversimplifying) what was known of the distant universe at the time of the broadcast.
("Big bang" is certainly fancifully poetic, and short, and those two attributes tend to be found in the jargon of theoretical physics. Consider the names of the standard model quarks, or "black hole".)
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[1] Ethan Siegel, a theoretical astrophysicist and science writer, gives some numbers for the spatial volume of our Hubble volume as one approaches the end of the inflationary epoch (and the start of the big bang) at https://www.forbes.com/sites/startswithabang/2017/03/24/how-...
It is not so much that we need it for a model to be in line with empirical observation, it’s that empirical observation gives one value and our models predict another value.
The discrepancy is big and if we can’t make our models work empirical observations, that is really embarrassing.
Regarding non-falsifiable theories; there’s not a physicist that thinks theories should not be tested. A lot of work is done to make these theories testable. Unfortunately, they are often only testsble on open problems since the limit of these theories just reduces to GR for instance.
Is this a not-so-subtle dig at Beacham and other experimentalists who make room for multiverse? Or do I jump to the wrong conclusion?
Two questions for you:
1) Does 'falsifiable' in this context per se mean that there needs to be some particle physics experiment that can be designed to test the theory? Is there no room for theoretical physics? Is Rovelli, for example, per se barking up the wrong tree by this measure?
2) Aren't these theories reasonably falsifiable insofar as they'll eventually result in experiments designed to map out the possible domains of the space? eg, what if we build a much higher energy collider and finally find real evidence of the super-symmetry particles?
Would you treat not finding super-symmetry particles as falsifying those theories? AIUI those theories admit the possibility of such particles but also the possibility of such particles not existing, so they're not actually falsifiable that way.
Any newly observed fact can falsify these theories. It just takes a long time. There’s still tons of open issues on General Relativity too. So far though, it seems to have passed every test.
Would you call GR pseudoscience too then? Since it’ll likely never be fully verified.
Pseudoscience is something labelling itself as science while not adhering to scientific standards. Claiming it's a fact would be a dead giveaway though.
Relativity has been experimentally tested and made accurate predictions, it is impossible for it to be "fully verified."
https://www.lesswrong.com/s/Kqs6GR7F5xziuSyGZ/p/xsZnufn3cQw7...
A famous scientist, Arthur Eddington, did a lot of work on what he called 'Fundamental Theory', an attempt at unifying everything. It went over 'like a submarine with screendoors'.
After I'd looked through the book for a couple of hours, I asked a seasoned quantum theoretician if there was anything particularly wrong about it. 'No'. Then why hadn't I heard of it, why didn't anyone take it seriously? 'Because they didn't like it.'
But yeah more in the world of thoughts than anything else.