Discovering the Shape of Spacetime from the Big Bang: A Scientific Exploration

[Ken G]

Here's problem with Popper's criteria. Quantum mechanics. QM creates only probabilistic predictions, and there is no observation or set of observations that could refute QM. If you observe anything, you could always just say that you were *very* unlucky.

It sounds like you are reading in a black-and-white character to "falsification" that was never intended by a mind as nuanced as Popper's. All he was saying is that confirmations don't mean a thing if there was not an honest chance of refutation. As a perfect example of this, I once heard a person doing experimental tests of special relativity saying that the only reason they were doing the tests was to show that SR was correct. Had they ever gotten a result that got that SR was wrong, they would have figured they did something wrong in the experiment. I had two reactions:
1) then what is the point of doing anything at all, and
2) it certainly doesn't sound like what they were doing could be called science.
I think Popper would have agreed. But I don't think there's any fundamental problem posed by statistical theories-- falsification simply means outcomes that have an "honest" chance of showing a different distribution than the predictions, in a way that you could not just twiddle some arbitrary parameter and recover agreement, and certainly where you would not simply conclude you did something wrong and not publish if you got disagreement. FTL neutrinos are a perfect example of the opposite-- the result was published, and even if the community is not "betting" on it, there is still a need to try and either reproduce the result, or pinpoint the cause of experimental error. Otherwise SR isn't science any more, it is dogma or delusion-- as I suspect Popper would say.

 

[Haelfix]

Theories should be accepted or refuted based on only one thing: experimental results. But these days we are seeing way too much of the mathematical equivalent of rhetoric, in place of the basic skepticism and demand for demonstration that should underpin science.

Sorry, but almost every working scientist will disagree with you here. Partially its b/c people have been fed Popper a little too much. In practise, there are often certain things that are simply not testable, not even in principle.

Even better. Sometimes there are things that are testable, but you just don't have to test b/c you know that it won't work.

For instance, if you told me that you have placed an apple on the surface of the moon, I insist that it would be irrational for me to hop into a NASA rocket to actually falsify the claim.

Yet another thought experiment. Suppose I was to tell you that you had a dollar in coins, that were split in some way under three black jars. I shuffle them, move them around and you open Jar 1 revealing that it has one dollar in change. The point is, you don't have to open Jar 2 and Jar 3. You know that they are empty by elementary logic under the assumption that I haven't cheated in some way.

Something a little more sophisticated, but essentially the same occurs in elementary particle physics. Sometimes, you simply know (really truly) that an undiscovered particle has to be at a certain place. It is that way b/c the mathematics of previous discoveries imply and constrain such and such a thing to be where it is. So of course while an assumption might break down at one point or another (apples might suddenly fall upwards), you can sometimes really know something has to be a certain way.

Indeed, and here is the key. The most primary thing in all of science, is not experimental discovery, it is on the contrary the primacy of logic. The world is and must be logical. Without that starting assumption, no experiment ever conducted has any explanatory power whatsoever.

 

[Ken G]

Sorry, but almost every working scientist will disagree with you here.

Only if they misrepresent the argument as much as you are doing, as will become more clear.

For instance, if you told me that you have placed an apple on the surface of the moon, I insist that it would be irrational for me to hop into a NASA rocket to actually falsify the claim.

What does that have to do with the idea that mathematical rhetoric cannot substitute for observational falsification? The reason we doubt that there could be an apple on the Moon is that we have a vast array of observations that speak to the issue. We have observations of the surface of the Moon that indicate it is rocky and barren. We have a vast array of apple observations that say they grow on trees, which grow in soil, and need water and air. These all constitute experimental data that falsifies the hypothesis. I don't think Popper was saying we can't use our brains.

What's more, you are also arguing that Popper was saying we can't know that certain theories or hypotheses are bad. Nothing that Popper was talking about constitutes a requirement for calling a theory bad-- he was talking about requirements for calling a theory good.

Yet another thought experiment. Suppose I was to tell you that you had a dollar in coins, that were split in some way under three black jars. I shuffle them, move them around and you open Jar 1 revealing that it has one dollar in change. The point is, you don't have to open Jar 2 and Jar 3. You know that they are empty by elementary logic under the assumption that I haven't cheated in some way.

No one is saying you can't use logic, the issue is whether you are basing that logic on experimental evidence.

Something a little more sophisticated, but essentially the same occurs in elementary particle physics. Sometimes, you simply know (really truly) that an undiscovered particle has to be at a certain place. It is that way b/c the mathematics of previous discoveries imply and constrain such and such a thing to be where it is.

And what is "the mathematics of previous discoveries"? It is the conceptual unification of a body of experimental data. It is not a rationalistic argument that "the universe needs to be this way because it makes sense to us for it to be so." That's the difference, right there, between empirical evidence, and rhetoric. Either one can be logical, and mathematical, what distinguishes them is what underpins it. That seems to be to be what Popper was actually talking about, not being an idiot (Popper was fairly well educated as a physicist, after all).

So of course while an assumption might break down at one point or another (apples might suddenly fall upwards), you can sometimes really know something has to be a certain way.

I dropped that reasoning somewhere, on the surface it looks like "although you can't really know something, you can really know it." I'm reminded of Einstein's sage quote: "To the extent math refers to reality, we are not certain; to the extent we are certain, math does not refer to reality." But this is a secondary issue anyway-- no one is saying we shouldn't use mathematical logic as our primary tool for making connections between observations, the issue is whether it can stand entirely on its own, without such observational underpinning, and without making "risky" predictions that could actually be confronted with observation. Science must put a question to nature, not to our own heads, or it is back to the natural philosophy of yore.

Indeed, and here is the key. The most primary thing in all of science, is not experimental discovery, it is on the contrary the primacy of logic. The world is and must be logical. Without that starting assumption, no experiment ever conducted has any explanatory power whatsoever.

That is the mantra of rationalism, but I would argue it is exactly the "false turn" we have made all to many times in the past. When will we learn? Logic is a tool for science, it has no "primacy", any more than paint has "primacy" in art. Saying "the world must be logical" is much like the common erroneous framing of Occam's Razor, "the simplest explanation is most likely correct." I would argue that a far better way to frame both these ideas is, "physics seeks whatever logic we can find in the world", and "the goal is to find the simplest explanation that works." None of the important content of the ideas are lost when framed this way, and they actually become true.

 

[TrinityP]

if the universe is infinite and the 'big bang' didn't come from a singular point and happened everywhere at once. Wouldn't that nullify the whole big bang theory. And wouldn't we then detect certain areas in space moving toward us as others are moving away??

 

[bapowell]

Wouldn't that nullify the whole big bang theory.

Why?

And wouldn't we then detect certain areas in space moving toward us as others are moving away??

Have a look at the balloon analogy sticky: https://www.physicsforums.com/showthread.php?t=261161

 

[phinds]

And wouldn't we then detect certain areas in space moving toward us as others are moving away??

You have that backwards. Try this:

www.phinds.com/balloonanalogy

 

[bapowell]

Try this:
www.phinds.com/balloonanalogy

Shoot, sorry phinds. I should have recommended your page! Next time...

 

[Haelfix]

And what is "the mathematics of previous discoveries"? It is the conceptual unification of a body of experimental data. It is not a rationalistic argument that "the universe needs to be this way because it makes sense to us for it to be so."

In practise it is actually a combination of both experimental and theoretical. For instance, the case for the existence of quarks is untestable directly and in fact untestable in principle. Popper I think would not necessarily approve! However you can measure other quantities in Hadron physics that imply their existence. So the point is you are led to a body of evidence from 3 different areas (indirect, mathematical and logical (what else can it be)) that changes a subjective belief in the validity of a model to the point where it is essentially far over the probability threshold for a discovery. This sort of Bayesian reasoning is quite alien to Karl Popper.

This is completely isomorphic to the reason for the belief in the theory of inflation, and several specific subset models that tend to lead to the case for a multiverse. Namely that there is considerable indirect observational evidence both for inflation proper, but in particular a certain type of inflation that tends to produce the conditions necessary for a multiverse.

It is important to note that there is still a large amount of parameter space in the space of possible inflationary models that typically do not lead to a multiverse, so it is ok to be skeptical. But this is important, if you analyze the properties of those specific models in detail, they tend to be MORE not less contrived and unnatural theoretically. In some sense they all require a miracle to occur at some point in the far past, where a group of extremely unlikely conditions had to be arranged to occur.

 

[Ken G]

In practise it is actually a combination of both experimental and theoretical.

I agree, and so would Popper. The issue is not experiment vs. theory, it is, does good theory need to have an experimental basis, centered on the concept of falsifiability.

For instance, the case for the existence of quarks is untestable directly and in fact untestable in principle. Popper I think would not necessarily approve!

No, his issue would be whether a language around quarks can make "risky predictions." People have a lot of strange ideas about what Popper was saying!

This sort of Bayesian reasoning is quite alien to Karl Popper.

Baloney! Popper understood statistics, and winnowing possibilities based on evidence. Indeed, his point was all about making sure one was really doing that. It's easy to think you are, when you really aren't.

This is completely isomorphic to the reason for the belief in the theory of inflation, and several specific subset models that tend to lead to the case for a multiverse. Namely that there is considerable indirect observational evidence both for inflation proper, but in particular a certain type of inflation that tends to produce the conditions necessary for a multiverse.

According to a small group of multiverse enthusiasts, yes. There were also a small group of Adlerians, and a small group of Marxists, and a small group of Freudians, the leading theorists of their day in those areas, making similar sounding claims, and that is exactly why Popper blew the whistle on them. All he used was basic, solid, scientific skepticism, and he found a way to make that more concrete.

It is important to note that there is still a large amount of parameter space in the space of possible inflationary models that typically do not lead to a multiverse, so it is ok to be skeptical.

Goodness that is hardly the only reason to be skeptical! The other reason to be skeptical is the entire idea that a theory, regardless of "parameter space measure", can indicate the existence of something that cannot pass basic falsification criteria. Which was Popper's point in spades-- he'd have a field day with the multiverse!

But this is important, if you analyze the properties of those specific models in detail, they tend to be MORE not less contrived and unnatural theoretically.

Oh no, we're back to "argument by theoretical naturalness." I thought we put that kind of logic away with the Greeks! Einstein's relativity has a great deal of theoretical naturalness, but that is just part of what makes it a great theory-- not what makes it right. It is agreement with observations that make it useful, and that is the other half of the reason it is a great theory. As soon as someone starts saying "see how beautiful my theory is, it just has to be right", I think "that's that false use of Occam's Razor again." Probably, the theory is wrong, just less wrong than the less unifying theories it replaces. That is why no physical theory can ever dictate to what exists.

 

[Haelfix]

I agree, and so would Popper. The issue is not experiment vs. theory, it is, does good theory need to have an experimental basis, centered on the concept of falsifiability.

Inflation has this, and contrary to what you claim various theories of the multiverse do too.

No, his issue would be whether a language around quarks can make "risky predictions."

I do not understand this sentence.

According to a small group of multiverse enthusiasts, yes.

Hmm?
Large portions of the parameter space for multiverse theories are directly falsifiable. In the end, the fact is while we might not ever measure a multiverse directly unless we get lucky, however there might be plenty of indirect evidence coming from several different places (for instance the CMB, B Modes, gravitational wave detectors and so forth). So much evidence in fact, that it might constrain us into a paradigm whether we like it or not.

There is a reason the plurality of theoretical physicists are in the chaotic inflation/eternal inflation camp. Various predictions were written down back in the 80s that were actually tested by COBE and WMAP.. Various other models of inflation were or have been falsified.

Oh no, we're back to "argument by theoretical naturalness." I thought we put that kind of logic away with the Greeks! Einstein's relativity has a great deal of theoretical naturalness, but that is just part of what makes it a great theory-- not what makes it right.

Ken, when a physicist speaks of naturalness, there is a very specific sense in what they mean as the word has a technical meaning. For instance, there are candidate theories other than the theory of quarks that explain all hadronic data ever observed. The problem is these theories are ruled out by naturalness.

For instance, if you saw an elephant perching on the tip of a cliff such that it was hanging perilously close but not actually tipping over. Your immediate reaction would be that there is a hidden rope or gimmick keeping the creature there. This is the correct attitude! However, I could write down a theory of physics where I tune all possible quantities (coefficient of friction, mass distributions, etc) to a large degree such that the situation would appear stable. But the point is, you must disregard such theories, b/c they are overwhelmingly contrived and unnatural.

This is basically what happens with inflation.

 

[marcus]

It is important to note that there is still a large amount of parameter space in the space of possible inflationary models that typically do not lead to a multiverse, so it is ok to be skeptical. But this is important, if you analyze the properties of those specific models in detail, they tend to be MORE not less contrived and unnatural theoretically. In some sense they all require a miracle to occur at some point in the far past, where a group of extremely unlikely conditions had to be arranged to occur.

That's a rather broad generalization. The possible inflation models that do not lead to multiverse all require a miracle. If a model does not lead to multiverse then it will be MORE contrived. That seems to be what you are trying to suggest.

Do you have a link to a source where some reliable expert makes such a statement?

 

[Ken G]

Inflation has this, and contrary to what you claim various theories of the multiverse do too.

Well that is just exactly the issue. Because you see, it is very much my impression that the case for the multiverse is just exactly how you framed it just one post ago-- that it is primarily based in a desire to be able to tell a good story, rather than actual experimental justification. Using physics to tell plausible stories is of great importance, I am well aware, but that stage comes after one has the empirical support that allows you to feel justified in weaving the tale. That's what separates physicists from charlatans (who can often tell even better stories).

What I'd like to see is a multiverse proponent saying something like "I'm not going to try and argue there is a multiverse on purely rationalistic grounds, because the observations that could falsify or support it just haven't been done yet, but here are the observations we can suggest that would answer the question, and we can't say we have support of the concept until these are done", to which I would say "ah, now that's science." Instead, I hear arguments with very much the flavor of the one you just gave-- "I believe in the multiverse because I can use it to tell a story that seems more natural to me that way," to which I say, "just what Popper was warning against."

I do not understand this sentence.

This is central to understanding Popper. You say Popper wouldn't like quark theory because we can't do an observation that directly images a quark, or some such thing. That's not at all what Popper was saying, he never said you aren't allowed to draw inferences. His central point is that a good theory must be able to make "risky" predictions, which means, predictions that go against your expectations without that theory, predictions that are hard to rationalize. In other words, the goal of a theory is not to make what you already know easier to rationalize, it is to make predictions that people who don't know that theory would be doubtful of. The classic example is relativity-- people who have never heard of relativity are always highly dubious that two different motions between two events would yield different clock readings. So that's a "risky" prediction, stemming from a good theory. But a theory that is capable of explaining any outcome of an unknown experiment is a bad theory, in Popper's view, because the theory has become an excuse for us to imagine we understand what we actually don't.

That's the crux of Popper's point He is saying that if you have theory T, and experiment X, and you say "I don't really care how X comes out, because my theory will be able to make any outcome seem like a natural consequence of my theory, which is how I know my theory is right", then Popper says "then your theory stinks." You need to able to say "my theory makes me suspect outcome X, but if I didn't have my theory, I would have expectd outcome Y." Then you have a theory worth scrutiny, that's what is meant by falsifiability.

Now, since the quark model does make predictions that we would have no reason to expect without that theory, Popper would have been just fine with it. As for the multiverse, I still await the first prediction that looks like "if there's a multiverse, experiment X will give the surprising answer Y, but if not, then we should get the answer Z that you would otherwise expect." I have yet to see a multiverse prediction that can be framed like that, but I do see gobs and gobs of "it has to be right because it just makes so much sense, it fits any universe you can name." That's exactly the lack of falsifiability that Popper complained about with Adlerism, Freudism, and Marxism.

There is a reason the plurality of theoretical physicists are in the chaotic inflation/eternal inflation camp. Various predictions were written down back in the 80s that were actually tested by COBE and WMAP.. Various other models of inflation were or have been falsified.

Let's face it, inflation theory is like string theory today-- it is not actually a theory, it is more like a recipe for creating theories. So although I cannot say that your claim is necessarily true or false, I can say that I have seen little evidence that inflationary models are currently detailed enough that any particular version that you could present and say "now this one really needs a multiverse", I could not simply replace with another inflation model that does all the same things without it. I'm still awaiting the "risky prediction" that makes sense with a multiverse, but not without it (and note that Weinberg's celebrated "prediction" of the amount of dark energy is not a prediction at all-- it is a postdiction, because we already know we are here, so we know there are constraints on dark energy-- even if there is only one universe). All I've seen of the multiverse is a rationalizing agent that allows us to feel better about what we already know is true, though I'm open to some much more convincing evidence, something much more Popperian.

Ken, when a physicist speaks of naturalness, there is a very specific sense in what they mean as the word has a technical meaning. For instance, there are candidate theories other than the theory of quarks that explain all hadronic data ever observed. The problem is these theories are ruled out by naturalness.

Please do expound. I've heard of "Occam's Razor", but your "naturalness" criterion is new to me.

For instance, if you saw an elephant perching on the tip of a cliff such that it was hanging perilously close but not actually tipping over. Your immediate reaction would be that there is a hidden rope or gimmick keeping the creature there. This is the correct attitude! However, I could write down a theory of physics where I tune all possible quantities (coefficient of friction, mass distributions, etc) to a large degree such that the situation would appear stable. But the point is, you must disregard such theories, b/c they are overwhelmingly contrived and unnatural.

This is basically what happens with inflation.

Your "naturalness" sounds a lot like "truthiness" to me. You know Colbert? But all I need for the elephant example is Occam's Razor, which says that the point of physics is to find the simplest and most unifying description among all those that could possibly work. But inflation models have not reached that level yet-- we simply don't have an inflation model that exhibits a satisfactory degree of unification, so saying we seem closer with models that will inflate eternally seems to overlook the possibility that we are just plain barking up the wrong tree, or trees in the case of the multiverse. It just hasn't been substantiated by risky predictions.

 

[twofish-quant]

Well that is just exactly the issue. Because you see, it is very much my impression that the case for the multiverse is just exactly how you framed it just one post ago-- that it is primarily based in a desire for the "universe to be logical" and more "natural", rather than actual experimental justification.

But the heuristic that the universe be "logical" and "natural" is a valid assumption in doing theoretically physics. There's absolutely no way to experimentally exclude the possibility that the laws of physics will change suddenly tomorrow for no reason, since experiments are about the past rather than the future. In order to make future statements, you have to include some sort of "logical" and "naturalness" assumption.

The very fact that people already believe in it proves how far away from actual science it has become

Personally, I think the fact that scientists take the seriously suggests that you need to rethink how obvious your definition of science is.

Instead, I hear arguments with very much the flavor of the one you just gave-- "I believe in the multiverse because my nice pretty theory seems more natural to me that way," to which I say, "just what Popper was warning against."

And hopefully at some point we'll figure out some way of testing those assumptions.

The other thing is that Popper was not a scientist. He was a philosopher. His ideas on what science is and isn't are interesting and worth discussing, but there is no reason to take his views on science as being more valid than those of Pope Benedict or Barak Obama's.

His central point is that a good theory must be able to make "risky" predictions, which means, predictions that go against your expectations without that theory, predictions that are hard to rationalize.

And I agree with that statement. A theory that is testable and predictable is a *better* theory than one that isn't. But that's different than saying that a theory that is untestable and unpredictable is outside the bounds of science. .

That's the crux of Popper's point He is saying that if you have theory T, and experiment X, and you say "I don't really care how X comes out, because my theory will be able to make any outcome seem like a natural consequence of my theory, which is how I know my theory is right", then Popper says "then your theory stinks."

And I agree. Where I disagree is the stronger statement that this theory outside the bounds of science. I also disagree that good and bad are absolute. A theory that has testable predictions is better than one that doesn't. But a theory that doesn't currently make testable predictions is not outside the bounds of science.

Now, since the quark model does make predictions that we would have no reason to expect without that theory, Popper would have been just fine with it.

Such as?

As for the multiverse, I still await the first prediction that looks like "if there's a multiverse, experiment X will give the surprising answer Y, but if not, then we should get the answer Z that you would otherwise expect from this observation." I have yet to see a multiverse prediction that can be framed like that, but I do see gobs and gobs of "it has to be right because it just makes so much sense, it fits any universe you can name." That's exactly the lack of falsifiability that Popper complained about with Adlerism, Freudism, and Marxism.

People are trying. The problem is that you are insisting that people come up with falsifiable predictions *right now*. It can take years, sometimes decades, to come up with these sorts of predictions. If it turns out that we give multiverses a few years, and we can't come up with testable predictions, then there's something wrong and we need to do something else.

The trouble is that it's not obvious whether something is testable or not. And let's suppose we falsify something, then what?

Also Popper chose some bad examples. Most people would consider the collapse of the Soviet Union to be a refutation of Marxism.

The other thing is that you can't cite Popper to settle an argument the same way that Christians cite Jesus. Popper says X. Well, he's wrong. Now what?

But inflation models have not reached that level yet-- we simply don't have an inflation model that exhibits a satisfactory degree of unification, so saying we seem closer with models that will inflate eternally seems to overlook the possibility that we are just plain barking up the wrong tree, or trees.

The big evidence for inflation is the CMB power spectrum.

 

[twofish-quant]

There are a few problems with Popper.

One thing to point out is that Popper was a philosopher and not a scientist. There is *no reason* I can think of to take Popper's ideas on what science is and isn't as some sort of gospel truth, and it turns out that cosmologists and theorists do all sorts of things that just don't fit into the Popperian framework.

Part of the problem is that it's often not obvious what is testable and not-testable and what is falsifiable and not-falsifiable. Much of the point in being a theorist is to figure out what is testable and what isn't, and it can take a while. For example, it turns out that string theory isn't that testable. So you can ask why we wasted 30 years on a theory that doesn't come up with testable predictions, and the answer is that it took 30 years to figure out that it doesn't come up with testable predictions.

Contrast that with inflation. The big evidence for inflation is that it gives us very detailed predictions about the CMB background. The thing about it is that Alan Guth had no clue that this was a consequence of inflation in 1981 and it wasn't clear that this was the situation until the early 1990's.

It takes several years of hard work to see if you can get a testable prediction, and that effort is something I call "doing science." Saying that the multiverse isn't science because we can't get obvious testable predictions is prematurely judging guilt. If you can come up with an argument that multiverse theories *will never* come up with testable predictions, that's something different, but no one has ever come up with something like that.

The second thing is that because testability is considered a "good thing", physicists make a lot of predictions. A lot of time you come up with something that's falsifiable and then it turns out to be wrong. Now what? Well, most of the times you just patch your theory, and then come up with a slightly different theory with the same basic assumptions but a tweak here and there. But Popper thinks that's a bad thing, and makes things "non-science" but that makes no sense to me.

And then you get into problems with "what is a model?" I take general relativity, it doesn't work the cosmological constant set to 0, so I set it to 0.00001 and it works perfectly. Did I just falsify a model? Or not.

And then you get into problems with things like the supersymmetry problem. Supersymmetry predicts a gaggle of particles. If we find one, then suddenly we can argue that supersymmetry is true. However, if it's false, we see nothing, but seeing nothing says nothing about whether it's true or not. I don't think this fits into Popper's framework at all.

Finally, there is a problem which is a huge problem in social sciences. If you adopt Popper's framework, and anything that is not testable is not science, then you have a problem with 'one time events." If you developed a theory on electrons, it's easy because one electron is the same as another electron, so you can run the same experiment a million times. But you can't do that one one time events, like the Great Depression or the collapse of 2007. The trouble with this is that then economists ignore one time events, change their theories to model *only* repeatable events, and then are caught off-guard when something "strange" like the collapse of 2007 happens.

This is a problem with economists because economists try too much to be like physicists, and then end up doing things that they think physicists do (like follow Popper) when physicists don't. You have a lot of issues with dealing with one time astrophysical events (like the big bang or supernova 1987A) and astrophysicists just deal with it. Because astrophysicists think a lot about one time events, and finance people and economists don't, this is why for certain financial problems, banks are more likely to hire an astrophysicist than an economist.

 

[twofish-quant]

Inflation has this, and contrary to what you claim various theories of the multiverse do too.

Yup. Most of the multiverse papers *try* to predict things like the physical constants. The problem with those predictions is that they are rather "weak" predictions.

Something that would be an impressive prediction would be to try to predict something non-trivial about the gravitational wave spectrum. Something else that would be impressive would be if you go up and say "you are wrong." That there is some measurement that we've done, that isn't what it is. You know that a theory is good when it says that measurements are wrong, and they turn out to be wrong.

One other thing is that "Ken G" has stated a "cosmic coincidence principle" which is that any theory that requires a coincidence should be rejected. Based on this, I think that we could argue that people are wrong and that curvature and DE are set up in a way that neither are constant. That's would in fact be an impressive paper if it turns out to be correct. Also, if it turns out to be wrong, it would still be interesting.

This is an example of why theorists *shouldn't* always try to match observations. In fact, it's usually better when doing theory, not to look at the observations too closely, otherwise there is more pressure than useful to make the theory match up with observations.

Science is hard. People would *like* to come up with testable predictions, but sometimes God doesn't cooperate. However, in that situation, it's a *bad* thing to just give up. I dislike anthropic arguments, but if I can't come up with something better, it's a bad thing to convince people that they shouldn't do them.

 

[twofish-quant]

Instead, I hear arguments with very much the flavor of the one you just gave-- "I believe in the multiverse because I can use it to tell a story that seems more natural to me that way," to which I say, "just what Popper was warning against."

To which I would respond, "and that's exactly why Popper gets science wrong."

 

[Ken G]

But the heuristic that the universe be "logical" and "natural" is a valid assumption in doing theoretically physics.

Not necessarily-- all physics requires is the statement that physics will use logic and seek naturalness (which as near as I can tell, is synonymous with unification, which is certainly the goal of physics to find). But none of this makes any claims on the universe, it makes claims on physics-- the other criterion of physics is that it must not make claims on nature prior to their empirical establishment. We certainly have found a great deal that is logical and somewhat natural, but we also have constantly made the error of overinterpreting this fact, throughout history.

There's absolutely no way to experimentally exclude the possibility that the laws of physics will change suddenly tomorrow for no reason, since experiments are about the past rather than the future. In order to make future statements, you have to include some sort of "logical" and "naturalness" assumption.

But again those are just constraints on physics. We don't need to assume anything about the universe, we only need to define what our tools are going to be. One of those tools is going to be making theories that don't have evolving laws if we have no reason to do so.

Personally, I think the fact that scientists take the seriously suggests that you need to rethink how obvious your definition of science is.

I was mostly referring to the irony that if a lot of people believe something before it is tested, then it either did not need to be tested, or the people had no basis for their belief. People seem to want it both ways! (Think "Higgs boson.")

And hopefully at some point we'll figure out some way of testing those assumptions.

I certainly agree that ultimately these issues will come into the realm of experimental testing, at which point whatever works will rise to the fore, so it doesn't matter too much what our current opinions are. I'm really just saying "whoa, we don't really know these things yet, let's wait until we do have the experiments to justify our expectatons." There's a kind of irony in Popper's "risky prediction" idea-- the better theory is the one that makes the predictions that seemed least likely to be right, but ended up being right, than the one that made predictions that seemed inevitably true. So the signpost of a "good theory in the making" is that few are inclined to believe it prior to the experiments that establish it! So in that light, it isn't really saying anything all that good about eternal inflation that many theorists currently believe the predictions it makes will end up being true.

The other thing is that Popper was not a scientist. He was a philosopher. His ideas on what science is and isn't are interesting and worth discussing, but there is no reason to take his views on science as being more valid than those of Pope Benedict or Barak Obama's.

That is blatantly untrue. Philosophers of science are perfectly qualified to make judgements about what makes a good scientific theory, they are exactly the ones who worry about that sort of thing. Yes, some don't understand the laws of physics as much as we'd like, but Popper was not in that class, he was quite knowledgeable in those laws. His job, in a nutshell, was to keep scientists honest and grounded, when they might otherwise tend to enter into a kind of self-perpetuating flight of fancy. I always marvel at how little most physicists understand what philosophy even is, or what philosophers do-- and how often I see the attitude "it's truth if I agree with it, and philosophy if I don't." Indeed, philosophers think that physics is a subset of philosophy, which comes as a big surprise to most physicists but actually it has perfectly good historical precedent.

And I agree with that statement. A theory that is testable and predictable is a *better* theory than one that isn't.

Good, then we have a common ground.

But that's different than saying that a theory that is untestable and unpredictable is outside the bounds of science. .

This is where it gets dicey. I agree with your basic point that science is not "one-stop shopping", it is actually a very diverse and complex interplay of different modes of discovery. So I don't want to cheapen it with some oversimplified template that it has to fit to count as science. I'm just siding with Popper that we should all see red flags when people start generating theories that can explain almost anything. A factory for theories that are flexible enough to make any outcome seem "natural" is not what we need, we need a single theory that makes seemingly unlikely predictions that end up being true. Such a theory does not rationalize what we already know to be true, it tells us something we didn't know we should expect. I don't rule out the possibility that eternal inflation or the multiverse might accomplish that, my objection is that what I mostly see is exactly what Popper warned against-- a stream of rationalizations that seem like they could easily rationalize any outcome at all.

What I want to know is, how is a universe that has life in it and is part of a multiverse, different from a universe that has life in it and is not part of a multiverse? If someone could answer that question for me, I could then test the concept scientifically.

And I agree. Where I disagree is the stronger statement that this theory outside the bounds of science. I also disagree that good and bad are absolute.

Well, the discussion is evolving, and those more strident statements of mine or no longer of any particular usefulness. Instead, we are honing in on just what kinds of requirements we are going to need from the multiverse idea before we can really feel like we are following a scientific course, rather than getting swept away in a current of successful rationalization. I think your references to Popper have really helped crystallize that progress, because this was very much Popper's mission.

Such as?

Here I'll defer to Wikipedia: "The spin 3⁄2 Ω−
baryon, a member of the ground state decuplet, was a prediction of the model. When it was discovered in an experiment at Brookhaven National Laboratory, Gell-Mann received a Nobel prize for his work on the quark model." I don't know chapter and verse of how many particle attributes the model was created to explain, versus how many it predicted and were later found, but I understand that the situation was not all the former-- meeting the criterion of making "risky predictions."

People are trying. The problem is that you are insisting that people come up with falsifiable predictions *right now*. It can take years, sometimes decades, to come up with these sorts of predictions.

Then let them wait before they call the multiverse a scientific hypothesis. I have no crystal ball, I don't know what discoveries the idea will lead to or what risky predictions it might eventually make that hold true. I'm just saying that until it delivers on these important objectives, the jury is still out on whether or not the idea can be considered a scientific model, and not just a way to feel like we can rationalize what observations we already know to be true.

If it turns out that we give multiverses a few years, and we can't come up with testable predictions, then there's something wrong and we need to do something else.

I'm happy with a "wait and see" attitude, as with string theory. What I object to is how all these "in fashion" theories tend to get oversold before they have really delivered on any of the promises we associate with them.

Also Popper chose some bad examples. Most people would consider the collapse of the Soviet Union to be a refutation of Marxism.

I don't think the issue here is whether or not it is possible to discover if Marxism is a viable theory to base an economy on, because Popper wasn't saying Marxism was a bad scientific theory because there was no possible way to falsify it. He was saying that the people who were using Marxism as their theory of choice for interpreting what was happening in the world were not doing so in an honest way-- they had the system rigged such that anything that happened could be interpreted as a confirmation of the theory. No doubt such people could interpret the fall of the Soviet Union as a confirmation of Marxist theory as well, they would point to some flaw in how the Soviet Union was applying Marxism or some such thing! Rationalization is easy, which was Popper's point.

So it's not so much the theory itself, it is the way it is used. I think what Popper was mostly cautioning against is rationalization-- the tendency to interpret facts in the light of a preconception. Instead, the scientist must take the opposite approach, the skeptical approach-- disbelieve everything, and fervently so, and attempt as hard as possible to falsify every theory. Only the theories that survive the onslaught can then be considered good, but it must be clear that the theory could have failed, even should have failed, had it not been onto something crucially important.

The big evidence for inflation is the CMB power spectrum.

But we're talking now about eternal inflation, and the multiverse. In my view, it's fine to expect a generic outcome subject to whatever strict sampling requirements are established by what you already know to be true. That is what we might call the weak anthropic principle.

 

[Ken G]

To which I would respond, "and that's exactly why Popper gets science wrong."

To that I would point out that he is generally regarded as one of the few most influential figures in the philosophy of science in the last century, so he couldn't have gotten it that wrong. More likely, it is actually the oversimplified accounts of Popper's views that are what has been gotten wrong.

 

[twofish-quant]

Not necessarily-- all physics requires is the statement that physics will use logic and seek naturalness (which as near as I can tell, is synonymous with unification, which is certainly the goal of physics to find). But none of this makes any claims on the universe, it makes claims on physics.

Physics does indeed make claims on the universe. These become more obvious when you are in situations where those claims happen to be false. One reason that the "string theory" approach has hit something of a dead end is that it turns out that extreme high energy physics isn't amenable to "naturalness" arguments. That also is why inflation is as complex as it is.

I don't see what's wrong with making claims about the universe.

the other criterion of physics is that it must not make claims on nature prior to their empirical establishment.

And if I do, what's going to happen? Is someone going to revoke my Ph.D.? Are they going to revoke my AAS membership if I do that?

The whole point of theory is to make claims on the universe before you've done empirical work establishing them. It may turn out that those claims are false, but onward or upward.

We certainly have found a great deal that is logical and somewhat natural, but we also have constantly made the error of overinterpreting this fact, throughout history.

There's a difference between making a claim and *believing* a claim. Sometimes appeals to naturalness work. Sometimes they don't. But when you have no idea what to do, then logic and naturalness help you create theories, and if you don't come up with theories (and *wrong* theories) then scientific progress comes to a halt.

I was mostly referring to the irony that if a lot of people believe something before it is tested, then it either did not need to be tested, or the people had no basis for their belief. People seem to want it both ways! (Think "Higgs boson.")

That's not true. Just because I believe something to be true doesn't mean that I don't think it shouldn't be tested. Guess what. I've been known to be wrong. One thing that makes science different from religion is that science admits that it is falliable.

For example, I happen to believe that if you drop a proton and an anti-proton, they will fall at the same rate. That doesn't mean that I don't think we shouldn't do the experiment. Same with the FTL neutrinos. I didn't think that we'd discover FTL neutrinos, but I'm glad that someone checked it. My first reaction to the results was "experimental error" however if you have another group do a different experiment that measures the same thing, that goes out the window.

I believe lots of things. I also believe that much of what I believe is wrong.

Part of the reason I believe things without empirical data is that i have to in order to get through the day. I happen to believe that there isn't a man eating Bengal tiger outside my office door. If I believed that there was a Bengal tiger, I'd behave very differently.

I'm really just saying "whoa, we don't really know these things yet, let's wait until we do have the experiments to justify our expectations."

But you have to make decisions based on current data, and you have to make decisions on *what experiments to make*? Much of the jobs of theorists is to give observers some ideas what to look for. Without some expectation as to what you will find, you can't set up the experiment.

So in that light, it isn't really saying anything all that good about eternal inflation that many theorists currently believe the predictions it makes will end up being true.

Name names. You have this habit of making general statements about "many theorists" without naming them.

You are mistaking assumptions with belief. Just because a theorist writes a paper that outlines the predictions of eternal inflation, doesn't mean that they *believe* it. There are religions that are based on the idea of *belief*, but *belief* in science doesn't work the same way that it does in Protestant Christianity.

The papers on eternal inflation are usually, if you assume X, you'll see Y. That's got nothing to do with *belief* in the religious sense.

Philosophers of science are perfectly qualified to make judgements about what makes a good scientific theory, they are exactly the ones who worry about that sort of thing.

I don't see what makes them more qualified than the Pope or the President.

I'm just siding with Popper that we should all see red flags when people start generating theories that can explain almost anything.

But the examples he gave were bad. Marxism had huge difficulty explaining the collapse of the Soviet Union, and Freudian psychoanalysis would have problems if people turned into vampires and zombies because of it. For that matter, if Jesus Christ came down from the heavens and said that "God is Lutheran" the Pope would have some explaining to do.

The other thing is that you don't know if you can explain almost anything until you do the math. It turns out that string theory has this problem in that it doesn't constrain the value of fundamental constants. This wasn't obvious in 1980. It takes years to work through a theory to figure out exactly what it predicts.

A factory for theories that are flexible enough to make any outcome seem "natural" is not what we need, we need a single theory that makes seemingly unlikely predictions that end up being true.

Sometimes the universe doesn't cooperate with you. You take what you can get.

Also, what is a theory. I take cold dark matter. It doesn't work. I add the cosmological constant. Is this the same theory or a different one?

I don't rule out the possibility that eternal inflation or the multiverse might accomplish that, my objection is that what I mostly see is exactly what Popper warned against-- a stream of rationalizations that seem like they could easily rationalize any outcome at all.

But it *doesn't* rationalize any outcome at all. I remember the excitement when COBE went up and for the first month, they weren't reporting fluctuations. This was exciting because if we didn't detect CMB fluctuations then means we got something *very* wrong. Then we find those fluctuations. Bummer.

Also, you are missing the Dyson paper that argues that eternal inflation is inconsistent with a curvature more than 1e-4. If we find 1e-3 then eternal inflation is dead.

Now it wouldn't kill the general inflation concept. That would require addressing the CMB microwave background and the horizon problem.

What I want to know is, how is a universe that has life in it and is part of a multiverse, different from a universe that has life in it and is not part of a multiverse? If someone could answer that question for me, I could then test the concept scientifically.

First let's try to eliminate different *classes* of multiverses.

A universe that's in a multiverse has cosmological parameters set up so that if you apply those parameters to other universes that have the same class of physical laws and observers, that you end up with the same numbers.

For example

http://arxiv.org/pdf/1202.5037v1.pdf

If eternal inflation is true that most universes will end up with large amounts of inflation, and hence a very small curvature. Which means that if you pick a generic universe, you'll see no curvature. If you do see any curvature, then this is extremely, extremely unlikely, and since there are *no* anthropic reasons why curvature of 0.0000 is preferable to curvature of 0.001, the conclusion is that if you see small curvature than eternal inflation is dead to very high probability.

Instead, we are honing in on just what kinds of requirements we are going to need from the multiverse idea before we can really feel like we are following a scientific course

I think that part of the problem is that you are getting your impressions about what cosmologists are working on from the popular press rather than the professional literature. A lot of the books in the popular press are "GEE WHIZ, aren't multiple universes *COOL*!"

In order to actually do something "real" you have to make long and technical arguments that 99% of the people in popular books will fall asleep reading. A lot of the professional literature is about trying to figure out what can we say, and what we can't from multiverse arguments.

I don't know chapter and verse of how many particle attributes the model was created to explain, versus how many it predicted and were later found, but I understand that the situation was not all the former-- meeting the criterion of making "risky predictions."

It wasn't that risky. What happens is that if you put all of the particles in a chart, you end up with a "hole" in it.

Then let them wait before they call the multiverse a scientific hypothesis.

You are putting the cart before the horse. You state the hypothesis *before* you try to figure out if it's testable.

One thing that *is* known from quantum mechanics is that you get the right numbers if you *assume* that there are multiple universes. Now for most of QM you can end up arguing that these is just a "mathematical trick" and that the alternative universes don't "really" exist. You can call this an "interpretation"

The trouble is that if you argue that the universe is the result of a quantum fluctuation, then you have problems figuring out what's going on.

I'm happy with a "wait and see" attitude, as with string theory. What I object to is how all these "in fashion" theories tend to get oversold before they have really delivered on any of the promises we associate with them.

Oversold to whom? There *is* a huge problem with people like Lawerence Krauss and Stephen Hawking spouting off before things are firmed up, but that's a "how science is popularized" issue and not a "science" issue. One problem is that people that say *I've discovered the secrets of the universe* get more press coverage.

He was saying that the people who were using Marxism as their theory of choice for interpreting what was happening in the world were not doing so in an honest way-- they had the system rigged such that anything that happened could be interpreted as a confirmation of the theory.

1) But then that makes the problem with the people that interpret the ideology rather than the ideology itself.
2) OK, you are a Marxist in 1910, and the revolution hasn't happened and you are clearly wrong. What do you do? You tweak the theory to explain what happened with the minimum of changes. The reason I can't argue that this is a bad thing is that this is exactly what scientists do, when their theories get disproven.

No doubt such people could interpret the fall of the Soviet Union as a confirmation of Marxist theory as well, they would point to some flaw in how the Soviet Union was applying Marxism or some such thing! Rationalization is easy, which was Popper's point.

1) Some people do but most people don't. There aren't too many Marxists in Vienna today.
2) I don't think that scientists are less immune to rationalization than other groups.

So it's not so much the theory itself, it is the way it is used. I think what Popper was mostly cautioning against is rationalization-- the tendency to interpret facts in the light of a preconception. Instead, the scientist must take the opposite approach, the skeptical approach-- disbelieve everything, and fervently so, and attempt as hard as possible to falsify every theory.

Again this *MUST*. This isn't how scientists behave, and I think that it's wrong and even dangerous to think that they do behave this way. If someone argues that they've discovered FTL neutrinos, I'm going to ask for a *LOT* more evidence than if they argue something that's consistent with relativity.

It's a bad idea to pretend that scientists are even-handed or less prone to belief than other people. Interpreting data in light of a preconception is not necessarily a bad thing, and I think it's impossible to interpret data without preconceptions. Pretending that scientist *can* interpret data without preconceptions is bad, because that means that the preconceptions just go underground.

Something that I have seen (although not in physics) is uneven skepticism. If someone assert something you agree with, you let the thing pass, whereas if you assert something they disagree with, they will argue the issue to death and demand evidence that isn't available.

Only the theories that survive the onslaught can then be considered good, but it must be clear that the theory could have failed, even should have failed, had it not been onto something crucially important.

Disagree. No theory is going to match data completely, and it's better to have a theory that's *known to be inaccurate* than something whose predictions are uncertain. There are about half a dozen current observations that LCDM has trouble with, and it's better to know what they are, and than to reject it completely.

Also, bad theories are sometimes good. You figure out that it's X by eliminating A, B, and C.

One reason I dislike Popper is that things are either true or false. That's not the way science works. If something turns out to be "true if you add a fudge factor" that could be useful.

 

[twofish-quant]

To that I would point out that he is generally regarded as one of the few most influential figures in the philosophy of science in the last century, so he couldn't have gotten it that wrong.

Yes he could. It's not as if he is Jesus Christ or the Prophet Muhammed.

For someone that just spend lots of articles talking about how we should be skeptical and shouldn't rationalize, you are being remarkably uncritical about Popper.

Popper's ideas belong into a class of philosophies called logical positivism. One problem with those philosophies is that they state that we shouldn't make statements that are untestable, and then proceed to do just that.

 

[Ken G]

For someone that just spend lots of articles talking about how we should be skeptical and shouldn't rationalize, you are being remarkably uncritical about Popper.

The problem is his ideas are being badly mischaracterized, so we can't even get to the stage of a legitimate criticism. I don't want to get off topic, but the discussion about the shape of spacetime has taken us into the arena of whether or not multiverse ideas currently uphold a standard of empirical support we normally associate with physics, and Popper's views are of course intensely relevant.

Popper's ideas belong into a class of philosophies called logical positivism. One problem with those philosophies is that they state that we shouldn't make statements that are untestable, and then proceed to do just that.

I'm not sure where you are getting these ideas, but they are naive at best. Logical positivism is generally associated with a group of philosophers, physicists, and mathematicians called "the Vienna Circle." Here is what the Stanford Encyclopedia of Philosophy has to say about them:
"It included as members, besides Schlick who had been appointed to Mach's old chair in Philosophy of the Inductive Sciences at the University of Vienna in 1922, the mathematician Hans Hahn, the physicist Philipp Frank, the social scientist Otto Neurath, his wife, the mathematician Olga Hahn-Neurath, the philosopher Viktor Kraft, the mathematicians Theodor Radacovic and Gustav Bergmann and, since 1926, the philosopher and logician Rudolf Carnap. (Even before World War I, there existed a similarly oriented discussion circle that included Frank, Hahn and Neurath. During the time of the Schlick Circle, Frank resided in Prague throughout, Carnap did so from 1931.) Further members were recruited among Schlick's students, like Friedrich Waismann, Herbert Feigl and Marcel Natkin, others were recruited among Hahn's students, like Karl Menger and Kurt Gödel. Though listed as members in the manifesto, Menger and Kraft later wanted to be known only as as sympathetic associates, like, all along, the mathematician Kurt Reidemeister and the philosopher and historian of science Edgar Zilsel. (Karl Popper was never a member or associate of the Circle, though he studied with Hahn in the 1920s and in the early 1930s discussed its doctrines with Feigl and Carnap.) "

Later, we find about Popper: "He did not however, regularly attend meetings of the Vienna Circle and generally considered himself an outsider. Later he claimed to have “killed” logical positivism."

The Wiki on logical positivism makes this point even more clear, where we find:
"A well-known critic of logical positivism was Karl Popper, who published the book Logik der Forschung in 1934 (translated by himself as The Logic of Scientific Discovery, published 1959). In it he argued that the positivists' criterion of verifiability was too strong a criterion for science, and should be replaced by a criterion of falsifiability. Popper thought that falsifiability was a better criterion because it did not invite the philosophical problems inherent in verifying an inductive inference, and it allowed statements from the physical sciences which seemed scientific but which did not satisfy the verification criterion.

Popper's concern was not with distinguishing meaningful from meaningless statements, but distinguishing scientific from metaphysical statements. Unlike the positivists, he did not claim that metaphysical statements must be meaningless; he also claimed that a statement which was "metaphysical" and unfalsifiable in one century (like the ancient Greek philosophy about atoms) could, in another century, be developed into falsifiable theories that have the metaphysical views as a consequence, and thus become scientific."

From these quotes, we find several points are in evidence:
1) logical positivists are not just clueless philosophers who "got science wrong", as you say, but rather include active physicists and mathematicians, which was not untypical of the day.
2) Karl Popper's name does not generally come up under the heading of "logical positivist", and indeed he claimed that his approach led to the "death" of logical positivism.
3) Popper's main objection to positivism is that he did not feel the point was being positive about what we could verify, but rather being able to tell if we have tried hard enough to falsify our theories. This was a much more flexible view of a good scientific theory.
4) Popper seemed to agree with my characterization that an idea that can at first only be regarded as speculation can later on graduate to the status of a scientific theory, at such a time that falsifiability becomes a legitimate possibility.

This last issue is the entire crux of the multiverse question-- is there legitimate falsifiability there, given what we already know what must be true (such as that we are here)? Is there really "risky predictions" being made, that one would expect to be wrong if the multiverse is not a good model? Personally, I have never seen a single one-- and the papers that report on predictions are usually talking about things that could be tested in principle, rather than legitimate tests we can expect to actually carry out, motivated by the theory. A theory that motivates falsifying observations is a good theory, but I just don't see the observations that the multiverse is motivating, that any cosmological picture would not motivate equally well. It's just a theory waiting for an actual purpose, beyond the "warm fuzzy feeling" of successful rationalization.

In regard to a more correct understanding of Popper's views, I would argue that they reveal just how insightful he really was, and how important of a "cautionary tale" he provided for helping keep scientists honest to others in how they sell their theories, and more importantly, honest to themselves.

 

[Haelfix]

That's a rather broad generalization. The possible inflation models that do not lead to multiverse all require a miracle. If a model does not lead to multiverse then it will be MORE contrived. That seems to be what you are trying to suggest.

Do you have a link to a source where some reliable expert makes such a statement?

So the fact that inflation has a finetuning problem has been pointed out many times by Roger Penrose (see the Road to Reality) and Steinhart and is common knowledge in the field. See this paper by Caroll for a summary and some speculations about the possible resolutions: http://arxiv.org/abs/1007.1417

It has been argued by a number of authors that eternal chaotic inflation ameliorates the problem. See:
http://arxiv.org/abs/hep-th/0702178

 

[Ken G]

That last paper seems like a nice summary of many of the issues we have been discussing, and note the abstract includes this:

Although the infinity of pocket universes produced by eternal inflation are unobservable, it is argued that eternal inflation has real consequences in terms of the way that predictions are extracted from theoretical models. The ambiguities in defining probabilities in eternally inflating spacetimes are reviewed, with emphasis on the youngness paradox that results from a synchronous gauge regularization technique. Although inflation is generically eternal into the future, it is not eternal into the past: it can be proven under reasonable assumptions that the inflating region must be incomplete in past directions, so some physics other than inflation is needed to describe the past boundary of the inflating region.

I translate that as saying that just as inflation is not really a theory, but more like a signpost to a theory capable of resolving many conundrums like the flatness and horizon problems, eternal inflation is also not really a theory, but more like a signpost to a theory, that could resolve even more purely metaphysical problems like the "youngness" problem. However, the paper seems clear that these problems have not yet been resolved (in particular, it seems the issue of getting constraints on the current "multiverse" distributionis a lot harder than constraining what the distribution is asymptotically evolving into in the future).

The paper stresses ways to connect with the standards of science in terms of changing our testable predictions, but I would still worry about Popper's "risky" element: a signpost to a theory that is a factory for making predictions that are capable of fitting anything we observe sounds a lot like fitting n independent observables with n theoretical degrees of freedom, i.e., not risky. It still sounds to me that the fundamental motivation for multiverse-oriented models is primarily metaphysical, and I think that should give us all pause as to whether or not the important line between science and metaphysics is being carefully respected in the more grandiose versions of claims on the multiverse.

 

[Haelfix]

Well that is just exactly the issue. Because you see, it is very much my impression that the case for the multiverse is just exactly how you framed it just one post ago-- that it is primarily based in a desire to be able to tell a good story, rather than actual experimental justification.

No! The case for the multiverse already have experimental support. These models were written down in the 80s, and already had a large amount of support by theorists. People only took them seriously after COBE and WMAP verified their detailed predictions. No one would believe them otherwise.

The problem is not that they don't make experimental predictions, its that there is an inverse problem. Namely that there is another model Y that makes the same or almost the same detailed predictions as Multiverse model X. Of course as we get better with understanding model X, the inverse problem diminishes as new predictions are able to be made and so forth.

But anyway the naive statement is then that you should prefer model Y b/c it doesn't include the same superstructure that model X does.

But this is not correct. The reason model X should be preferred over model Y, is that there is a great deal fewer miracles that must occur. The finetuning is considerably reduced and more 'natural', and the extra assumptions are quite reasonable:

Namely that if we believe in inflation to begin with, then we know that our universe is very large. If we believe in a very large universe, than an event that occurred by chance once, could in principle happen again somewhere else. Finally the event that happened once, has a parameter space of possible constants that very generically create a situation where inflation is eternal (the case where inflation is not eternal is of measure zero)!

 

[Haelfix]

I translate that as saying that just as inflation is not really a theory, but more like a signpost to a theory capable of resolving many conundrums like the flatness and horizon problems, eternal inflation is also not really a theory, but more like a signpost to a theorye.

Sure its an infinite set of possible theories, with an infinite dimensional parameter space! The simplest versions (involving a solitary scalar field) still have several undetermined constants that basically must be fitted to experiment and to theoretical constraints (for instance, slow roll conditions).

The problem is experiment currently only gives 2 numbers, which still vastly undetermines the solutions.

Thus theorists have to ask questions about the space of all possible inflationary universes satisfying the experimental constraints.

 

[marcus]

...
Namely that if we believe in inflation to begin with, then we know that our universe is very large. If we believe in a very large universe, than an event that occurred by chance once, could in principle happen again somewhere else. Finally the event that happened once, has a parameter space of possible constants that very generically create a situation where inflation is eternal (the case where inflation is not eternal is of measure zero)!

This is the main weakness in the argument for MV. We do not know that inflation is initiated by a random fluctuation. (IOW, something that would spontaneously recur, given enough space and time.)

 

[Ken G]

No! The case for the multiverse already have experimental support. These models were written down in the 80s, and already had a large amount of support by theorists. People only took them seriously after COBE and WMAP verified their detailed predictions. No one would believe them otherwise.

Well, that description just does not seem at all accurate to me. That sounds more like what happened to the Big Bang model in the 60s, at which point it became the cornerstone of cosmology and began to be taught as "lesson one" in any introductory cosmology course. I don't see any of that as being true for the multiverse. I realize that we have much better constraints on the models now, and multiverse models have survived those better constraints, but as I said above, that may just be (and seems to me is) simply because the multiverse models were flexible enough to accommodate almost any outcome of WMAP. It was only the "risky" models that could not, but that's what made those models real scientific models in the first place. Ironically, it is that they were good science that made them get falsified, whereas the jury is still very much out on that in the case of the multiverse idea.

The problem is not that they don't make experimental predictions, its that there is an inverse problem. Namely that there is another model Y that makes the same or almost the same detailed predictions as Multiverse model X. Of course as we get better with understanding model X, the inverse problem diminishes as new predictions are able to be made and so forth.

Right, that's why Popper stressed the need for "risky" predictions. Adler had a model of how human psychology worked, and the problem with it was not that it made no predictions-- indeed, the whole point of the Adler model was to predict human behavior. The problem was that its predictions were not "risky", they were not predictions you would be inclined to expect to be wrong if you didn't already know they were already examples of human behavior. Hence, any outcome of any experiment on human behavior could be rationalized via Adler's model, but it wasn't granting us new insight, it is was preventing any real insight from being obtained (because the answer was viewed as already known, whereas the goal of scientific research is to find the answer that is being missed). It is the difference between prediction and rationalization-- there's that old joke that an observer shows their result to a theorist, and the theorist says "I can explain that, my theory must be good!" Then the observer says "oops, hang on, I was holding the graph upside down," to which the theoriest resplies "no worries, my theory is so good, I can explain that too." I have a hard time seeing how multiverse thinking is not similarly flexible, for the simple reason that we have so few a priori constraints on the attributes of the putative "distribution" of universes.

What's more, it is also not clear to me that embedding one universe in a distribution that cannot be observed but is said to exist anyway is not purely metaphysical to its core-- how would that model ever be distinguished from a model which simply asserts "any time we have prior knowledge X of the universe, and we want to make a prediction for unknown outcome Y, we can imagine there is some probability distribution Z, informed by X, that is pertinent to Y." I see nowhere in any of that which requires the existence of a multiverse, any more than playing a single hand of poker in a perfectly intelligent way requires that any other hands of poker have ever existed anywhere else. It's nothing but a probabilistic model for how unknowns in nature should be addressed, all else is metaphysics.

But this is not correct. The reason model X should be preferred over model Y, is that there is a great deal fewer miracles that must occur. The finetuning is considerably reduced and more 'natural', and the extra assumptions are quite reasonable:

But none of that requires the multiverse concept. This is getting too far off topic probably, so I am starting a new Cosmology thread on "is the multiverse cosmology or metaphysics?"

Namely that if we believe in inflation to begin with, then we know that our universe is very large. If we believe in a very large universe, than an event that occurred by chance once, could in principle happen again somewhere else. Finally the event that happened once, has a parameter space of possible constants that very generically create a situation where inflation is eternal (the case where inflation is not eternal is of measure zero)!

That was flawed logic, because you used the largeness of the universe after inflation as a reason to expect inflation, but then assumed a largeness of the universe before inflation as a reason to expect inflation to occur many times. The argument makes assumptions that are not in evidence in observational fact, and is therefore metaphysics, not physics.

 

[Ken G]

Sure its an infinite set of possible theories, with an infinite dimensional parameter space! The simplest versions (involving a solitary scalar field) still have several undetermined constants that basically must be fitted to experiment and to theoretical constraints (for instance, slow roll conditions).

I'm glad we agree on this point, and I think this is exactly the place where the insights of Popper are most poignant.

The problem is experiment currently only gives 2 numbers, which still vastly undetermines the solutions.

I can just see what Popper's reaction would be to this: "so you are saying that now we have 2 reasons to believe the theory, but when experiment gives us 100 numbers, and we choose the parameters of our theory to fit those 100 numbers, then we will have 100 reasons to believe our theory."

Thus theorists have to ask questions about the space of all possible inflationary universes satisfying the experimental constraints.

Yeah, my point exactly.

 

[twofish-quant]

The problem is his ideas are being badly mischaracterized, so we can't even get to the stage of a legitimate criticism.

I don't think that I'm badly mis-characterizing his ideas. He seems to think that general relativity is more falsifiable than Marxism and psychoanalysis, and that "tweaking a theory" to make it fit reality weakens it.

I don't agree,

I don't want to get into the situation which happens with Marx in which anytime someone argues that Marx is just wrong, people argue that he is misquoted.

1) logical positivists are not just clueless philosophers who "got science wrong", as you say, but rather include active physicists and mathematicians, which was not untypical of the day.
2) Karl Popper's name does not generally come up under the heading of "logical positivist", and indeed he claimed that his approach led to the "death" of logical positivism.
3) Popper's main objection to positivism is that he did not feel the point was being positive about what we could verify, but rather being able to tell if we have tried hard enough to falsify our theories. This was a much more flexible view of a good scientific theory.
4) Popper seemed to agree with my characterization that an idea that can at first only be regarded as speculation can later on graduate to the status of a scientific theory, at such a time that falsifiability becomes a legitimate possibility.

Points taken, but the way that science was done in the 1920's is very different that the way that it's done today, and people have rather different philosophical assumptions about the world.

Also, I've actually tried to minimize philosophy, because I think that you vastly underestimate how falsifiable the models that have been proposed really are, and if I can convince you of that, then the philosophy is irrelevant.

The other thing is that if someone comes up with reasons to think that multiverse models are *inherently* unfalsifiable, then yes we do have a problem, but I don't think that's the situation.

This last issue is the entire crux of the multiverse question-- is there legitimate falsifiability there, given what we already know what must be true (such as that we are here)?

Too vague.

I think it could be argued that talking about the "multiverse concept" is not a proper scientific theory because it is *too vague*. The "multiverse concept" is probably much too vague to falsify, but it can (and has been used) to generate specific theories that are clearly falsifiable. Most of the time multiverse ideas have been invoked in the professional literature, they refer to "multiverses generated by string theory" and those are subject to falsifiablity (i.e. if string theory is wrong, then those models are wrong).

The same goes with the anthropic principle. For it to work, you have to be in a situation where life is impossible under some set of physical constants. This is not obviously true. For example, if you double the FSC, then human life may be impossible, but if it turns out that you can create something else intelligent, then the anthropic principle is dead.

Personally, I have never seen a single one-- and the papers that report on predictions are usually talking about things that could be tested in principle, rather than legitimate tests we can expect to actually carry out, motivated by the theory.

I've seen several.

Max Tegmark's paper on dimensionality for one. Also, it's an interesting paper, because even though the final result is not highly testable, the fact that dimensionality has this effect is interesting.

This is why I think that *requiring* falsifiablity to label something science is a bad idea. There are clearly things that scientists do that are science that don't involve creating falsifiable models.

In regard to a more correct understanding of Popper's views, I would argue that they reveal just how insightful he really was, and how important of a "cautionary tale" he provided for helping keep scientists honest to others in how they sell their theories, and more importantly, honest to themselves.

The trouble is that if we are really honest, then I'd have to say that some of the things that Popper considers "science" (i.e. general relativity) are in fact harder to test than some of the things that Popper consider "non-science" (i.e. Marxism).

 

[twofish-quant]

Also it would help if you named some names.

I do agree that some popular science writers (Lawrence Krauss, Michio Kaku, and Stephen Hawking) are *way* overselling what is currently being investigated, and part of the problem is that someone that makes scientifically ground claims is going to get less attention that someone that makes extravagant claims.

I *don't* think that this is a problem in the professional literature. Something that I find interesting is that nothing that Krauss has mentioned in his public speeches is part of his professional publication record, and when I read is peer-reviewed papers, it's almost like reading some one else.

 

[twofish-quant]

When experiment gives us 100 numbers, and we choose the parameters of our theory to fit those 100 numbers, then we will have 100 reasons to believe our theory."Yeah, my point exactly.

At which point the theorist will ask for several billion dollars for telescopes that will give them 10000 numbers to fit 100 parameters. This is one reason the CMB *spectrum* is so important. You aren't fitting to a number, you are fitting to a curve, which gives you a huge number of numbers to fit to.

 

[twofish-quant]

I said above, that may just be (and seems to me is) simply because the multiverse models were flexible enough to accommodate almost any outcome of WMAP.

That's not the situation. If you had zero CMB fluctuation, you have a problem. If you have large non-Gaussian CMB fluctuation, you have a problem. If you have unexplained anisotropy, you have a problem. If you have unexplained polarization, you have a problem.

This is a teaching issue, because I don't think that most intro astronomy courses for non-major really go into sufficient detail about the empirical evidence for cosmological models. Part of the problem is that to lots of people, they are *boring*.

Right, that's why Popper stressed the need for "risky" predictions.

Predicting the CMB *spectrum* seems to be a very risky prediction.

I have a hard time seeing how multiverse thinking is not similarly flexible, for the simple reason that we have so few a priori constraints on the attributes of the putative "distribution" of universes.

We can impose constraints, and eliminate *classes* of models.

I see nowhere in any of that which requires the existence of a multiverse, any more than playing a single hand of poker in a perfectly intelligent way requires that any other hands of poker have ever existed anywhere else.

In the case of inflationary models, there is a reason. Inflation asserts that the universe expanded very quickly so that a small region of space expanded to a volume larger than the current observed universe. Eternal inflation asserts that the scalar field is such that most areas are still expanding.

So in the case of eternal inflation, you are *required* to have a large volume outside the observed universe.

Again, I think the reason we are even talking metaphysics is that we aren't being specific enough. We aren't talking about *all possible multiverse models*, we are talking about a *specific* model.

That was flawed logic, because you used the largeness of the universe after inflation as a reason to expect inflation, but then assumed a largeness of the universe before inflation as a reason to expect inflation to occur many times.

It's a testable assumption. If the characteristics of the pre-inflationary universe wasn't uniform, we'd see it in CMB.

 

[Ken G]

At which point the theorist will ask for several billion dollars for telescopes that will give them 10000 numbers to fit 100 parameters. This is one reason the CMB *spectrum* is so important. You aren't fitting to a number, you are fitting to a curve, which gives you a huge number of numbers to fit to.

Not if the curve covers its tracks, which is exactly what a thermal spectrum does, except for the fluctuations-- a much lower-dimensional set of constraints.

 

[Ken G]

That's not the situation. If you had zero CMB fluctuation, you have a problem. If you have large non-Gaussian CMB fluctuation, you have a problem. If you have unexplained anisotropy, you have a problem. If you have unexplained polarization, you have a problem.

Sure, but those would be problems for any cosmology, not just multiverse cosmology. It doesn't single out the multiverse, I wouldn't expect any of those things prior to their being detected. Now, if the multiverse did predict one of those things, then at least it would have been falsifiable. The bottom line is, you really never have any idea if a theory is good until it makes a prediction that you didn't already expect without that theory, and then you test it. When the multiverse idea does that, I will agree it has evidential support. If it never does that, then it will never have evidential support, it will always just be a way to rationalize what we already knew to be true.

This is a teaching issue, because I don't think that most intro astronomy courses for non-major really go into sufficient detail about the empirical evidence for cosmological models. Part of the problem is that to lots of people, they are *boring*.

Possibly there could be more details on the models. But I think the real reason you don't see that is they are just too speculative. That's putting it quite mildly, in fact, for the case of the multiverse.

Predicting the CMB *spectrum* seems to be a very risky prediction.

It was indeed risky to predict a thermal background, and that is a prediction of the Big Bang. Any Big Bang, with our without multiverses. Predicting the fluctuations does not seem to be a risky prediction-- it seems to just be fitting the parameters, knowing that we would be able to fit them given the flexibility to invoke dark matter, dark energy, and any arbitrary scalar potential for the inflation.

Eternal inflation asserts that the scalar field is such that most areas are still expanding.

Which makes the risky prediction that ______? (fill in the blank.)

So in the case of eternal inflation, you are *required* to have a large volume outside the observed universe.

This is a natural feature of the inflation phenomenon, it does not adjudicate multiverse existence. I am inclined to disbelieve the multiverse, yet I'm also inclined to believe in a large universe. It's evidence for inflation, not the multiverse.

Again, I think the reason we are even talking metaphysics is that we aren't being specific enough. We aren't talking about *all possible multiverse models*, we are talking about a *specific* model.

Which specific model? What predictions does it make that are going to get tested any time soon? Can you say "the leading candidate model is X, which makes risky prediction Y, which will be tested by experiment Z, so we'll soon have an answer if just wait"? If you can say that, please do, and if you cannot, isn't that trying to tell you something?

 

[twofish-quant]

Sure, but those would be problems for any cosmology, not just multiverse cosmology. It doesn't single out the multiverse, I wouldn't expect any of those things prior to their being detected. Now, if the multiverse did predict one of those things, then at least it would have been falsifiable.

It would rule out certain *types* of multiverse cosmologies. Eternal inflation for example makes pretty specfic predictions about the CMB background.

The bottom line is, you really never have any idea if a theory is good until it makes a prediction that you didn't already expect without that theory, and then you test it.

But there are times in which a theory is good even if it predicts nothing new. If you are able to turn something into a small set of parameters, that's a good thing to do even if nothing is predicted.

The other thing is that sometimes the universe doesn't cooperate. What do you do if general relativity happens to be right?

Possibly there could be more details on the models. But I think the real reason you don't see that is they are just too speculative.

There are different levels of speculative. Anything inflation related has something is dealing with real data so it's not as speculative as supersymmetry or string theory.

Predicting the fluctuations does not seem to be a risky prediction-- it seems to just be fitting the parameters, knowing that we would be able to fit them given the flexibility to invoke dark matter, dark energy, and any arbitrary scalar potential for the inflation.

Except that you can't. No matter how arbitrary the scalar potentials are and no matter how much dark matter, and dark energy you have, you are going to end up with a power spectrum that is roughly Gaussian. There is a *particular* power spectrum that inflation predicts, and it's hardly a case of "fitting the parameters" because people made the prediction a few years before COBE went up.

Inflation is inconsistent with zero fluctuation because you can't send information faster than the speed of light. This means that two regions that are outside of the light cone, can't talk with each other and they can't come up with the same temperature.

Predicting that there will be fluctuations isn't impressive. Predicting the exact spectrum of the fluctuations is.

Which specific model?

Eternal inflation (which is a class of models)

What predictions does it make that are going to get tested any time soon?

Predicts that curvature is < 1e-5 and that CMB is gaussian at small scales. Also makes predictions for scalar potentials, which will have HEP effects.

Can you say "the leading candidate model is X, which makes risky prediction Y, which will be tested by experiment Z, so we'll soon have an answer if just wait"? If you can say that, please do, and if you cannot, isn't that trying to tell you something?

I can't say the leading candidate model since there are several hundred inflationary models, all with various predictions.

Also "risky prediction" is rather subjective. If you have no clue what the result is going to be then all predictions become risky, because you have zero expectation of what you are doing to expect.

Also for an example of a *falsified* multiverse model, look at cosmological natural selection. Smolin made the prediction that there would be no pulsars with more than 1.6 solar mass, and when we found a 2.0 solar mass pulsar, that theory went bye-bye.