The fine-tuning argument for God says that the universal constants have to be what they are to allow for order that permits life, so someone must have set them to allow for life.
A common rebuttal is to say that there might be a multiverse, each universe having different constants, so, inevitably, one would have the right ones for order, and thus, life.
A common rejoinder is that there is no evidence for such a multiverse.
A common return is that quantum mechanics, per the Many Worlds Interpretation, already suggests, and did so many years ago, that there is a multiverse of sorts.
Now, my question. Does the MWI actually posit universes with different constants, or just it posit universes all with the same constants, with the only differences being in whether subparticles zig or zag?
Tegmarks mathematical multiverse goes beyond quantum physics. It posits (infinitely) many universes with wildly different rules. Some of them have quantum physics, some of them have (something that on surface resembles) classical physics, some of them have a rectangular grid with simple rules, etc.
MWI is about one specific universe we live in, which seems to be governed by a specific set of rules, although there is a possibility that some “constants” in those rules are actually not as constant as they may seem locally. But still, it would be generally the same system of rules everywhere, only with locally different parameters.
Back to the fine-tuning argument, MWI is a response to “is it not a magical coincidence that Earth happens to have the right distance from the Sun?”, the theories about locally different physical ‘constants’ are a response to “is it not a magical coincidence that the constants have the right value, so that e.g. matter can exist?”, and the Tegmark multiverse is a response to “is it not a magical coincidence that the laws of physics are flexible enough to allow you all the previously mentioned excuses?”.
I’ve not heard of Tegmark before. It sounds like if it’s true, then it’s a great answer to the fine-tuning question. Is it all speculation, or is this a hypothesis with some evidence to it?
The multiple universes proposed by Tegmark are causally disconnected; nothing that happens in one of them can in principle influence what happens in the others. So we cannot do experiments with other universes. (This is different from quantum physics, where the parallel branches start connected, and for a short time we can interact with them.)
We could try to provide an indirect evidence. If we can make the theory more specific—what exactly counts as a possible universe, and what measure do the universes have—perhaps we could make statements in the form of “X % of universes have a property P”, and then compare it with the properties of our universe. For example, if we could list 100 properties that universes have with probability 50%, and then find out that our universe has approximately half of them, that could be considered a statistical evidence. Assuming that our universe is a typical one; but statistically speaking, that is what we should assume. Problem is, some of these properties may interfere with a possibility of life, so maybe it should instead be “X % of universes containing life have a property P”… which is easy to say, but I have no idea how to do it (how do you, looking at mathematical equations describing a universe, determine the probability or measure of life in such universe).
Note that this is connected with problems such as “why does anything exist, instead of nothing” and “why these laws of physics, instead of different laws”. Tegmark hypothesis provides a kind of explanation for both: our laws of physics are just the local rules of this universe, other universes have other rules; and there is nothing special about a universe existing, things that are in the same universe exist relatively to each other (and don’t exist relatively to things in other universes).
In some sense, this has similar aesthetics as the Einstein’s relativity—which may sound like a stupid argument, except that some scientific discoveries were indeed made this way: by assuming that more symetric laws are somehow better than the less symetric ones, or that rules using only local information are somehow better than the ones assuming some global state. Or more simply, if we take the progress of science as “there is nothing special about Earth, there are many planets” and “there is nothing special about this random situation, there are many parallel quantum worlds”, it feels like “there is nothing special about this universe existing, there are many alternative universes” continues the pattern.
Which doesn’t mean that everything is (equally likely) possible, just like the theory of relativity doesn’t mean that. I think that Tegmark hypothesis still assumes that more simple universes are in some sense more likely than the less simple ones (which explains why our universe is understandable, sometimes by relatively short equations, as opposed to having zillions of insanely complex rules), and a more serious theory built on this could possibly give us some specific numbers and equations. -- But again, the anthropic principle complicates this: maybe there are universes with laws of physics more simple than ours, but much less friendly to evolution of an intelligent life. This would be difficult to put into an equation.
There are other kinds of multiverse theory that are less speculative. For instance, if “inflation” is right then it seems likely that it produces “eternal inflation”, which yields infinitely many universes that might have different fundamental constants (though the same kind of fundamental laws).
(And of course either MWI or a spatially infinite universe will produce what might as well be a multiverse, if you don’t need the fundamental constants of physics to be different between “universes”.)
The problem is that “quantum mechanics” is not really a theory. It’s a framework, or a language if you will. There’s a classical quantum mechanics, there’s a semi-classical quantum mechanics, there’s quantum field theory, there are various unified field theories, and so on. All of them, although they are very different, can be called quantum mechanics and be the subject of different interpretations. Classical quantum mechanics does not predict different cosmological constants. Semi-classical might, for example, for the value of the inflaton. Other theories might have different derivations altogether.
MWI is an interpretation of quantum mechanics, ergo is applicable to all of the theories mentioned before. You can have MWI in classical, semi-classical, etc. Meaning that it is a rebuttal, although how much good depends on how much credit you give to a theory that predicts different cosmological constants.
But MWI allows for different constants without predicting them, right? It would be a mistake to say that the case and evidence for MWI is evidence for a different-constants system, which itself has much less (almost no?) evidence for it, making this line of reasoning a very weak rebuttal?
No, MWI does not have varying constants. Cosmological inflation seems to be about a changing cosmological constant. I think “eternal inflation” is an elaboration emphasizing more variation of the parameters.
Added: No, that’s not quite right. Inflation is the pretty much empirical claim that the cosmological constant has changed. That strongly implies that other parameters can change. But do they change enough in interesting ways? Chaotic/eternal inflation is a dynamic theory about how the cosmological constant changes. It implies that the universe is very big, so the other parameters have room to run wild.
Equivocating between MWI and inflation, as in your dialogue, is bad.* But asking whether a specific argument is bad is usually a wrong question. I think your dialogue is a confused memory of a coherent argument. Inflation is pretty much accepted and that is definitely evidence for chaotic inflation, although perhaps quite weak evidence. And chaotic inflation is pretty much the right multiverse for the full argument.
* Maybe that is the fault of Tegmark, who calls MWI a multiverse, unlike most people; but, as Viliam notes, he does make some anthropic fine-tuning arguments via MWI and ordinary inflation.
There ought to be one fundamental set of rules. This fundamental set of rules may or may not shake out into different local sets of rules. Assuming that these local rulesets arise from aspects of quantum state, then MWI is capable of realizing an arbitrarily large number of them.
String Theory, for instance, has a mindbogglingly large number of wildly varying possible local rulesets - ‘compactifications’. So, if String Theory is correct, then yes, this is taken care of unless the number of compactifications yielding rules even vaguely like ours is unexpectedly small.
Okay, but the best theory, MWI, does not suggest different constants, and the theory that does is not particularly well thought of, am I understanding this right?
So, this is a bad rebuttal to the fine-tuning argument.
MWI is orthogonal to the question of different fundamental constants. MWI is just wavefunction realism plus no collapse plus ‘that’s OK’.
So, any quantum-governed system that generates local constants will do under MWI. The leading example of this would be String Theory.
MWI is important here because if only one branch is real, then you need to be just as lucky anyway—it doesn’t help unless the mechanism makes an unusually high density of livable rules. That would be convenient, but also very improbable.
Can you clarify? The first part sounds like MWI is irrelevant to the question of fine-tuning of universal constants. Are you saying that if only one Everett branch was real, then it would be unlikely to have things like a planet under the right circumstances for life, but that is accounted for by MWI, since it explores all the permutations of a universe with constants like ours?
If I’m getting this, then that means MWI accounts for things like “why is the earth in the right place” kinds of things, but not “why is the proton this particular mass” kinds of things
Well, if the laws of the universe were such that it were unlikely but not impossible for life to form, MWI would take care of the rest, yes.
BUT, if you combine MWI with something that sets the force laws and particle zoo of the later universe as an aspect of quantum state, then MWI helps a lot—instead of getting only one, it makes ALL† of those laws real.
† or in case of precise interference that completely forces certain sets of laws to have a perfectly zero component, nearly all. Or if half of them end up having a precisely zero component due to some symmetry, then, the other half of these rule-sets… etc. Considering the high-dimensional messiness of these proto-universe-theories, large swaths being nodal (having zero wavefunction) seems unlikely.
The fine-tuning argument for God says that the universal constants have to be what they are to allow for order that permits life, so someone must have set them to allow for life.
A common rebuttal is to say that there might be a multiverse, each universe having different constants, so, inevitably, one would have the right ones for order, and thus, life.
A common rejoinder is that there is no evidence for such a multiverse.
A common return is that quantum mechanics, per the Many Worlds Interpretation, already suggests, and did so many years ago, that there is a multiverse of sorts.
Now, my question. Does the MWI actually posit universes with different constants, or just it posit universes all with the same constants, with the only differences being in whether subparticles zig or zag?
Tegmarks mathematical multiverse goes beyond quantum physics. It posits (infinitely) many universes with wildly different rules. Some of them have quantum physics, some of them have (something that on surface resembles) classical physics, some of them have a rectangular grid with simple rules, etc.
MWI is about one specific universe we live in, which seems to be governed by a specific set of rules, although there is a possibility that some “constants” in those rules are actually not as constant as they may seem locally. But still, it would be generally the same system of rules everywhere, only with locally different parameters.
Back to the fine-tuning argument, MWI is a response to “is it not a magical coincidence that Earth happens to have the right distance from the Sun?”, the theories about locally different physical ‘constants’ are a response to “is it not a magical coincidence that the constants have the right value, so that e.g. matter can exist?”, and the Tegmark multiverse is a response to “is it not a magical coincidence that the laws of physics are flexible enough to allow you all the previously mentioned excuses?”.
I’ve not heard of Tegmark before. It sounds like if it’s true, then it’s a great answer to the fine-tuning question. Is it all speculation, or is this a hypothesis with some evidence to it?
See Mathematical universe hypothesis on Wikipedia.
The multiple universes proposed by Tegmark are causally disconnected; nothing that happens in one of them can in principle influence what happens in the others. So we cannot do experiments with other universes. (This is different from quantum physics, where the parallel branches start connected, and for a short time we can interact with them.)
We could try to provide an indirect evidence. If we can make the theory more specific—what exactly counts as a possible universe, and what measure do the universes have—perhaps we could make statements in the form of “X % of universes have a property P”, and then compare it with the properties of our universe. For example, if we could list 100 properties that universes have with probability 50%, and then find out that our universe has approximately half of them, that could be considered a statistical evidence. Assuming that our universe is a typical one; but statistically speaking, that is what we should assume. Problem is, some of these properties may interfere with a possibility of life, so maybe it should instead be “X % of universes containing life have a property P”… which is easy to say, but I have no idea how to do it (how do you, looking at mathematical equations describing a universe, determine the probability or measure of life in such universe).
Note that this is connected with problems such as “why does anything exist, instead of nothing” and “why these laws of physics, instead of different laws”. Tegmark hypothesis provides a kind of explanation for both: our laws of physics are just the local rules of this universe, other universes have other rules; and there is nothing special about a universe existing, things that are in the same universe exist relatively to each other (and don’t exist relatively to things in other universes).
In some sense, this has similar aesthetics as the Einstein’s relativity—which may sound like a stupid argument, except that some scientific discoveries were indeed made this way: by assuming that more symetric laws are somehow better than the less symetric ones, or that rules using only local information are somehow better than the ones assuming some global state. Or more simply, if we take the progress of science as “there is nothing special about Earth, there are many planets” and “there is nothing special about this random situation, there are many parallel quantum worlds”, it feels like “there is nothing special about this universe existing, there are many alternative universes” continues the pattern.
Which doesn’t mean that everything is (equally likely) possible, just like the theory of relativity doesn’t mean that. I think that Tegmark hypothesis still assumes that more simple universes are in some sense more likely than the less simple ones (which explains why our universe is understandable, sometimes by relatively short equations, as opposed to having zillions of insanely complex rules), and a more serious theory built on this could possibly give us some specific numbers and equations. -- But again, the anthropic principle complicates this: maybe there are universes with laws of physics more simple than ours, but much less friendly to evolution of an intelligent life. This would be difficult to put into an equation.
Speculation.
There are other kinds of multiverse theory that are less speculative. For instance, if “inflation” is right then it seems likely that it produces “eternal inflation”, which yields infinitely many universes that might have different fundamental constants (though the same kind of fundamental laws).
(And of course either MWI or a spatially infinite universe will produce what might as well be a multiverse, if you don’t need the fundamental constants of physics to be different between “universes”.)
The problem is that “quantum mechanics” is not really a theory. It’s a framework, or a language if you will. There’s a classical quantum mechanics, there’s a semi-classical quantum mechanics, there’s quantum field theory, there are various unified field theories, and so on. All of them, although they are very different, can be called quantum mechanics and be the subject of different interpretations.
Classical quantum mechanics does not predict different cosmological constants. Semi-classical might, for example, for the value of the inflaton. Other theories might have different derivations altogether.
But if there is some version with different constants, it’s not MWI, or anything most people have heard of, is that right?
Meaning that this rebuttal to the fine-tuning argument is not a good one.
MWI is an interpretation of quantum mechanics, ergo is applicable to all of the theories mentioned before. You can have MWI in classical, semi-classical, etc.
Meaning that it is a rebuttal, although how much good depends on how much credit you give to a theory that predicts different cosmological constants.
But MWI allows for different constants without predicting them, right? It would be a mistake to say that the case and evidence for MWI is evidence for a different-constants system, which itself has much less (almost no?) evidence for it, making this line of reasoning a very weak rebuttal?
Am I getting this right?
No, MWI does not have varying constants. Cosmological inflation seems to be about a changing cosmological constant. I think “eternal inflation” is an elaboration emphasizing more variation of the parameters.
Added: No, that’s not quite right. Inflation is the pretty much empirical claim that the cosmological constant has changed. That strongly implies that other parameters can change. But do they change enough in interesting ways? Chaotic/eternal inflation is a dynamic theory about how the cosmological constant changes. It implies that the universe is very big, so the other parameters have room to run wild.
So, this is a bad rebuttal to the fine-tuning argument, no?
Equivocating between MWI and inflation, as in your dialogue, is bad.* But asking whether a specific argument is bad is usually a wrong question. I think your dialogue is a confused memory of a coherent argument. Inflation is pretty much accepted and that is definitely evidence for chaotic inflation, although perhaps quite weak evidence. And chaotic inflation is pretty much the right multiverse for the full argument.
* Maybe that is the fault of Tegmark, who calls MWI a multiverse, unlike most people; but, as Viliam notes, he does make some anthropic fine-tuning arguments via MWI and ordinary inflation.
There ought to be one fundamental set of rules. This fundamental set of rules may or may not shake out into different local sets of rules. Assuming that these local rulesets arise from aspects of quantum state, then MWI is capable of realizing an arbitrarily large number of them.
String Theory, for instance, has a mindbogglingly large number of wildly varying possible local rulesets - ‘compactifications’. So, if String Theory is correct, then yes, this is taken care of unless the number of compactifications yielding rules even vaguely like ours is unexpectedly small.
Okay, but the best theory, MWI, does not suggest different constants, and the theory that does is not particularly well thought of, am I understanding this right?
So, this is a bad rebuttal to the fine-tuning argument.
MWI is orthogonal to the question of different fundamental constants. MWI is just wavefunction realism plus no collapse plus ‘that’s OK’.
So, any quantum-governed system that generates local constants will do under MWI. The leading example of this would be String Theory.
MWI is important here because if only one branch is real, then you need to be just as lucky anyway—it doesn’t help unless the mechanism makes an unusually high density of livable rules. That would be convenient, but also very improbable.
Thanks, Luke
Can you clarify? The first part sounds like MWI is irrelevant to the question of fine-tuning of universal constants. Are you saying that if only one Everett branch was real, then it would be unlikely to have things like a planet under the right circumstances for life, but that is accounted for by MWI, since it explores all the permutations of a universe with constants like ours?
If I’m getting this, then that means MWI accounts for things like “why is the earth in the right place” kinds of things, but not “why is the proton this particular mass” kinds of things
Well, if the laws of the universe were such that it were unlikely but not impossible for life to form, MWI would take care of the rest, yes.
BUT, if you combine MWI with something that sets the force laws and particle zoo of the later universe as an aspect of quantum state, then MWI helps a lot—instead of getting only one, it makes ALL† of those laws real.
† or in case of precise interference that completely forces certain sets of laws to have a perfectly zero component, nearly all. Or if half of them end up having a precisely zero component due to some symmetry, then, the other half of these rule-sets… etc. Considering the high-dimensional messiness of these proto-universe-theories, large swaths being nodal (having zero wavefunction) seems unlikely.