Most people probably can follow simple math. If the Universe had a temperature of 10^32 degrees at the Big Bang, and the Heat Death is defined by it being asymptotically close to 0 degrees, then it being currently 2.7 degrees tells us what?

Is it: A) We are pretty much at the end of the journey. Yes siree, 32 orders of magnitude is quite a big drop. We are not even talking nanoseconds to midnight (nano being merely 9 orders of magnitude).

Or: B) Bibble, bibble, bibble. Blub, blub, blub....

No need to justify your extraordinary calculations. You had me when you predicted the demise of the universe as fact. Gutsy move.

Well, if the Big Bang happened once.....

You are very flattering. But its just standard cosmology. You can read all about it yourself.

I know. And you are here to give us the scientific facts.

You have 4 months to live. You have 2 years to live. The universe has 1 trillion years to live. Always with the proclamations.

The universe has 1 trillion years to live. — Rich

Don't you mean that the Heat Death is eternal? That's quite a surprising conclusion if you think about it.

There have been a number of attempts to derive/justify the Born rule, including the self-locating uncertainty approach that Carroll and Sebens develop (I haven't looked at their paper, but they probably cite earlier works in the same vein). Not everyone is convinced that such justifications are (a) not circular, and (b) do not smuggle in assumptions that are not present in the starting interpretation. But adjudicating this debate is way beyond my pay grade.

I just want to take issue with your characterization of probabilistic theories as "acausal." What you are talking about is causal determinism, and the keyword here is determinism. You can, of course, put your foot down and insist that causality necessarily implies determinism, but, as far as your arguments here are concerned, causality may as well equal determinism, because you are not actually talking about any aspect of causality other than it being deterministic. So for your purposes, causality is a redundant concept, since all that you are talking about is determinism. And I suspect that you only bring it up for rhetorical purposes (everyone wants to preserve causality in our theories, right?)

There have been a number of attempts to derive/justify the Born rule, including the self-locating uncertainty approach that Carroll and Sebens develop (I haven't looked at their paper, but they probably cite earlier works in the same vein). Not everyone is convinced that such justifications are (a) not circular, and (b) do not smuggle in assumptions that are not present in the starting interpretation. But adjudicating this debate is way beyond my pay grade. — SophistiCat

That's fine. At any rate, the justification for the Born rule boils down to the following claims:

1. On the Everett interpretation, measurement leads to initial self-locating uncertainty. An observer can have complete knowledge about the relative states of the system, but not which particular state they have just measured. This raises the question of how to quantify their uncertainty in terms of probabilities.

2. If the state amplitudes are equal, the observer should initially be indifferent about which state they have measured. So the states can simply be counted to calculate the probability that a particular state has been measured.

3. If the state amplitudes are not equal, they can be mathematically factored into states that do have equal amplitudes. And again the states can be counted to calculate the probability. The number of factored states exactly tracks the square of the initial amplitude, so it is equivalent to applying the Born rule.

The main assumption is the indifference rule which seems reasonable to me.

I just want to take issue with your characterization of probabilistic theories as "acausal." What you are talking about is causal determinism, and the keyword here is determinism. You can, of course, put your foot down and insist that causality necessarily implies determinism, but, as far as your arguments here are concerned, causality may as well equal determinism, because you are not actually talking about any aspect of causality other than it being deterministic. So for your purposes, causality is a redundant concept, since all that you are talking about is determinism. And I suspect that you only bring it up for rhetorical purposes (everyone wants to preserve causality in our theories, right?) — SophistiCat

It would be great if everyone wanted to preserve causality in their theories but that is what the Copenhagen interpretation explicitly rejects. The idea that the universe is inherently probabilistic implies that the probabilities are a brute fact and inexplicable.

Naturally since the Everett interpretation directly maps the quantum formalism onto the world, then a deterministic formalism leads to a causal (or, if you prefer, deterministic) theory. But it's worth noting that the theory describes and predicts behavior, it does not prescribe it.

Reading through the reader reviews of that title, it seems Deutsch gives pretty short shrift to anyone who doubts the actual reality of parallel universes, which he seems to think is necessary for the concept to actually work. — Wayfarer

Here's the actual challenge Deutsch raises in his book:

Logically, the possibility of complex quantum computations adds nothing to a case that is already unanswerable. But it does add psychological impact. With Shor’s algorithm, the argument has been writ very large. To those who still cling to a single-universe world-view, I issue this challenge: explain how Shor’s algorithm works. I do not merely mean predict that it will work, which is merely a matter of solving a few uncontroversial equations. I mean provide an explanation. When Shor’s algorithm has factorized a number, using 10^500 or so times the computational resources that can be seen to be present, where was the number factorized? There are only about 10^80 atoms in the entire visible universe, an utterly minuscule number compared with 10^500. So if the visible universe were the extent of physical reality, physical reality would not even remotely contain the resources required to factorize such a large number. Who did factorize it, then? How, and where, was the computation performed? — David Deutsch - “The Fabric of Reality”

where are numbers? Any numbers? There might be a vast domain of which the physical universe is simply an aspect, but which is not physical. //edit// Where is 'the realm of possibility'? You might say 'it doesn't exist', but then, there are some things which are in the domain of possibility, and some things which are not. So there are 'real possibilities' - but they don't actually exist anywhere. Which, in the context, is significant, I would have thought.//

↪Andrew M where are numbers? Any numbers? There might be a vast domain of which the physical universe is simply an aspect, but which is not physical. //edit// Where is 'the realm of possibility'? You might say 'it doesn't exist', but then, there are some things which are in the domain of possibility, and some things which are not. So there are 'real possibilities' - but they don't actually exist anywhere. Which, in the context, is significant, I would have thought.// — Wayfarer

Numbers aren't anywhere. Numbers are an abstraction over things (which is more-or-less the Aristotelian view).

Possibilities are also abstractions. In ordinary use, a real possibility is just one that is more likely to eventuate.

Numbers aren't anywhere. — Andrew M

In that case I clearly don't understand what Deutsch is talking about, although that is probably not surprising.

Factoring large numbers requires physical resources (i.e., a computer). If a successful factorization required vastly more physical resources than were available in the visible universe, then where would those resources have come from?

but that's not something that's been done, right? Is that part of the argument for quantum computers? That if you want to do that, then you will need a quantum computer to do it?

It hasn't been done - it's only a theoretical possibility at present (and, no, classical computers couldn't do this). The practical goal right now is to outperform classical computers, so-called quantum supremacy.

in which case, I faill to see the cogency of that example for Deutsch's argument for there being many worlds.

//edit// Where is 'the realm of possibility'? You might say 'it doesn't exist', but then, there are some things which are in the domain of possibility, and some things which are not. So there are 'real possibilities' - but they don't actually exist anywhere. Which, in the context, is significant, I would have thought.// — Wayfarer

The realm of possibility is the future. The difference between a real possibility and an unreal possibility (something said to be possible but actually impossible) is determined by the past. What has already occurred, in the past, determines what is possible in the future. If you take a presentist perspective, neither past nor future "exists", because existence is limited to the present. If you take a dualist perspective you can allow that both the past and the future have actual existence, but there is a substantial difference between the two (substance dualism). Therefore the realm of possibility, remaining always ahead of us in time, in the future, is very real, but since it has not yet received material existence at the present it is apprehended only by the mind, and not the senses

↪Andrew M in which case, I faill to see the cogency of that example for Deutsch's argument for there being many worlds. — Wayfarer

The demonstration would require a quantum computer with about 300 qubits. Either that is an engineering problem that can one day be solved. Or there is some unknown law of the universe that prevents that possibility.

'We have a factoring problem which is so big it can't be solved with any known computer. Therefore in order to solve it we will need a computer that utilises many universes. Therefore there are many universes.'

It would be great if everyone wanted to preserve causality in their theories but that is what the Copenhagen interpretation explicitly rejects. The idea that the universe is inherently probabilistic implies that the probabilities are a brute fact and inexplicable. — Andrew M

Well, again, you are just equivocating between "causal" and "deterministic." What you are really saying is that our theories ought to be deterministic. I disagree. There is no a priori reason why we should prefer determinism. Or indeterminism, for that matter. I don't consider either to be a theoretical virtue, in and of itself. Of course, if one also offered better or more specific predictions, or a more economical description than the other, then it ought to be preferred - but those other advantages obtain independently of the determinism/indeterminism split. Indeed, in the case of the interpretations of quantum mechanics, none of the empirical advantages can be credited to one interpretation and not another, since they all make the same empirical predictions*.

* That may not actually be true - some interpretations seem to make distinct predictions, but they are presently out of reach for empirical investigation.

* That may not actually be true - some interpretations seem to make distinct predictions, but they are presently out of reach for empirical investigation. — SophistiCat

While not a prediction per se, Bohm's version of the Schrodinger equation implies a quantum potential that can act non-locally, at a distance. This implication inspired Bell to formulate his Theorem which has been experimentally tested many times over. In this regard, the causal, non-deterministic model of Bohm's carries some additional weight. Does this sound reasonable?

What you are really saying is that our theories ought to be deterministic. — SophistiCat

Yes I think our theories should be deterministic. But, most importantly, our theories should be explanatory which is how I've used "causal" in this thread. I'm unaware of any non-deterministic theory that meets that criterion.

Consider a simple probabilistic theory about dice. This (well-tested) theory says that any given dice roll will have a 1/6 probability of producing any particular number between 1 and 6. But the theory doesn't explain why dice exhibit that behavior, it just asserts it.

That is precisely the situation with the Copenhagen interpretation and any other interpretations that postulate the Born rule probabilities instead of deriving them. They may make the correct predictions but they don't actually explain anything.

All interpretations of quantum mechanics explain exactly the same observations, so in that sense they are explanatory to exactly the same degree. One could make a case that some of them are more parsimonious than others, but that is never an easy case to make. That said, superficially at least, the Everett interpretation does seem to be more parsimonious than its main rivals. But I realize that things aren't so simple, and not having sufficient expertise, withhold further judgment.

the Everett interpretation does seem to be more parsimonious than — SophistiCat

This implies that creating a new world for every quantum event, or smearing every thing across infinite works comprising some mega-universe of some sort is parsimonious. Others may call such a theory as unimaginably elaborate.

Great thread this.

In response to previous assertions that the reality of multiple universe is 'craziness' is it any more crazy than any of the other interpretations?! (Cat both dead/alive etc)

Also - apokrisis, could you please expand on what you mean by 'epistemic cut.' Thanks

In response to previous assertions that the reality of multiple universe is 'craziness' is it any more crazy than any of the other interpretations?! (Cat both dead/alive etc) — Mike Adams

Much, much crazier than Bohm's straightforward causal non-deterministic interpretation.

Bohm's interpretation implies non-locality which has been experimentally observed at the molecular level. It is also no-deterministic which drives the determinists crazy. As a result they come up with this interpretation which requires an infinitly ever-growing exponentially mega-world interpretation which indeed it's still probabilistic but deterministic in this fantasized infinite-world interpretation. It demonstrates how far materialists-determinists are willing to go to preserve their beliefs. I would say God is much more reasonable.

Yeah, the weirdness objection is the worst of the lot, and does not deserve any respect. Quantum mechanics is weird. The world is weird. Get used to it.

So, the idea that everytime there is a quantum event, which is happening continuously everywhere in the universe, a new world is created for every possibility of that event, seems reasonable. Yes, now we need to discuss plausibility, taken in consideration that the sole reason for such an interpretation is to maintain some possibility of determinism in this mega-worlds ( not even the universe we live in). It's pretty ridiculous, but on this fine forum it is taken seriously. Why? So maintain nice relationships with determinists. Yes, let's consider this as simply weird, when in fact it is preposterous so the determinists can present such an interpretation which has yet to find any kind of definition whatsoever.

MW theory has been taken seriously by many scientists for decades...

(Plus your apparent hatred for determinists is bordering on the hysterical.)