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

[Ken G]

First, there is a mechanism by which inflation can be achieved within GR, and so we already have more than a mere phenomenon.

I would say you can claim that when the mechanism works, when one mechanism emerges from all the possibilities because it is well constrained and absent of any difficulties.

This mechanism does make use of hypothetical scalar fields, which you claim is ad hoc. (So, I must ask, are you equally skeptical of gauge theories and spontaneous symmetry breaking?)

Gauge theories are a unifying way of thinking about a wide class of behavior, and spontaneous symmetry breaking likewise-- it is a unifying principle. These ideas employ scalar potentials for only one reason, AFAIK-- because it is the simplest way to do it. That's it, that's the reason-- not because there is a shred of evidence that approach should work. Now, of course we would always start with the simplest approach, that's looking for the keys under the streetlight first. But it's still no reason to expect it will work, or that it is the "right physics", until there is some much better reason to expect that, based on some success that simply has not yet appeared. The keys have not been found yet, so the search under the streetlight continues, until either the keys are found, or the search moves on to somewhere more difficult. That is how we look for keys, but we don't need to pretend it is some better guided process than that!

Keep in mind that the stress-energy that drives inflation need not be a fundamental scalar, it only needs to have an effective equation of state that satisfies w < -1/3. Yes, we don't know precisely what the source of this stress-energy is, but that does not demote inflation to a mere ad hoc phenomenon.

It's not important to me to be able to label inflation "ad hoc", I'm perfectly happy with "currently speculative details of how it works." I hope inflation works out simply, why wouldn't I. I'm just saying we should not kid ourselves that we have good reason to expect a scalar potential mechanism to turn out to be the correct description of the phenomenon of inflation.

Second, I would argue that there is a wealth of data supporting an early inflationary epoch.

Yes, that's the "phenomenon" we are talking about. The question is, what is a good model of whatever mechanism made that happen?

You claimed earlier than inflation makes not testable predictions beyond that which it was constructed to explain. I'm not sure I agree; first, what would you say inflation was constructed to explain? Flatness? Lack of monopoles? Resolve the horizon problem? I suppose that question can only be answered by learning the intent of the scientists who built the theory. I would say that even if we claim these were the prescribed goals of the theory, the discovery that inflation could generate the seeds of large scale structure was certainly not apparent at its conception.

I'm saying it is appropriate to separate the phenomenon of inflation, which is simply the statement that the universe expanded by the factor X at epoch Y, from any physical theories or mechanisms that can actually accomplish that. Once making that distinction, we can then look for what observations we have that support the phenomenon, and what observations support the mechanism. I don't think that distinction has been clearly made, because the list of successes you cite all sound to me like they stem from the phenomenon itself-- the mechanism is still not accomplishing any of these independent successes, all it is doing is the one thing it was built to do-- to give the phenomenon.

Such a mechanism is not unifying anything, it's not a principle, until it can point to its own successes related to the mechanism independent from the basic phenomenon it is built to produce. Things like reheating and so forth could be examples of the mechanism working, beyond just the phenomenon, but these are exactly the kinds of details that are still being thrashed out, and remain unclear as to whether or not they are going to work. That's the natural state of affairs when a theory is being built, we don't know if we have the right construction to get something that works, so it's fine to try-- but we needn't pretend that we know we have a good mechanism just because we know we have a good phenomenon. That's not bashing the noble effort to look under the streetlight, it's just being realistic about it. Maybe you're right that if I knew better all the things that scalar potentials are doing for us, I'd be more inclined to see that approach to inflation as a unifying principle. But it starts to sound a bit like string theory, which I am also assured is doing wonderful things for our understanding, except that it hasn't really delivered what it claimed it would.

So, we have a hypothesis: that a period of exponential expansion took place in the early universe, driven by a source of stress-energy with the quantum numbers of the vacuum.

That's two hypotheses, one the phenomenon and one the mechanism, and we must not conflate the successes of each. They are important to keep separate.

But there are also observations that address that latter part of the hypothesis -- that a source with the quantum numbers of the vacuum drove the expansion. Such a source can be effectively modeled as a scalar field. We know certain facts about this field: it must have a potential energy that both supports inflation and ends it. We know it must have a shape that supports a sufficient amount of inflation. In its simplest form consistent with these requirements, it also makes predictions: namely, that the density perturbations will be adiabatic, Gaussian, and nearly scale invariant.

OK, now we are indeed talking about the mechanism, but are those predictions unique to the scalar potential approach, or are those behaviors endemic to a wide class of approaches? You say we can "effectively model" the inflation with a scalar potential, but if that is true, why are there so many different ways to do it, each with their own issues? How can we have a unifying principle here, if we cannot even identify which principle is the right one? I think the jury is still out on just how effective that approach can be judged, but those on the inside of the effort might disagree.

And from this approach, I can discover, by observations of the observable universe alone, whether the potential that drove inflation in our Hubble patch is operative elsewhere in the universe. It doesn't matter that I don't know how exactly the inflaton arises from supergravity or some other theory. The data, together a suitable application of Occam's razor, are sufficient.

Then by all means, do what can be done! But until it is done, how do we know what can, or cannot, be done? I never said it's a bad idea, I just said it is speculative as to whether or not it is really going to fulfill its promise. And there's nothing wrong with that, new theory making is speculative, that's just what it is-- but why sell it as something more?

And I must caveat this discussion by saying that I am not a proponent of eternal inflation or of multiverse theories per se; I have no vested interest in their veracity. But I think it is too constrictive to maintain your tact that unless it is strictly observed that it has no place in science. Many things that belong in mainstream science have not been strictly observed but only inferred or implied by the strict consistency of theories that have been supported by other observations.

That particular tack was specifically about the geometry of the universe beyond what we can infer from observations. I'd say it's already pretty clear, whether or not we ever detect any curvature, that the curvature is not going to be enough to constrain the geometry of the rest of the universe without wholesale extrapolation. On what basis can the shape of a nose be extrapolated to the shape of a head? The cosmological principle is not a principle of extrapolation or constraint beyond what we can observationally falsify, it is simply a modeling principle in the domain of what we can.

Edit: but to clarify, I don't see myself as in any position to pass judgment on inflation to people who do it, I'm just saying that a lot of rather grandiose claims get made about inflation but a lot of them seem to come with a rather large portion of faith. It behooves us to be realistic about what we have a right to expect from our theories, and what we might have to accept is more difficult than we'd like! None of this is in any way an attempt to discredit inflation as a useful research direction.

 

[twofish-quant]

The cosmological principle is not a principle of extrapolation or constraint beyond what we can observationally falsify, it is simply a modeling principle in the domain of what we can.

Strongly disagree.

Giordano Bruno was able to deduce the existence of exoplanets centuries before it became observationally possible to falsify them. He published his work in 1584, and it wasn't until the 1960's when it became possible even in principle to falsify their existence.

I don't see how multiverses are any different.

One other thing. This idea that observational falsification is the basis of science is quite new. Popper came up with it in the 1930's, and one problem that I have with discussions of the "scientific method" is that there is this idea that what is science and what isn't is self-evidently obvious and settled.

 

[Ken G]

Giordano Bruno was able to deduce the existence of exoplanets centuries before it became observationally possible to falsify them. He published his work in 1584, and it wasn't until the 1960's when it became possible even in principle to falsify their existence.

The analogy does not hold. The only thing that makes exoplanets important is the simple fact that we can observationally verify their existence! That's exactly what we know we will not be able to do with some extended corner of the universe that has no observable consequences on what we can see, or worse, other universes altogether. If the cosmological principle worked well on the scale of what we see, but started to fail on scales an order of magnitude larger, I don't see how we would ever know that, constraints like that seem pretty much a pipe dream.

I don't see how multiverses are any different.

The difference is fundamental and crucial, it is what makes exoplanets science and multiverses not science (I argue). And it is straightforward: we can observe planets. Science is what we can observe. Yes, we are allowed to draw inferences, assume interactions, etc., but multiverses are not postulated because they interact, or because we can draw inferences about them, they exist simply to make us feel better about being in a seemingly very special universe, when rationalistic thinking about the "laws" of physics don't accommodate specialness very well. Bruno's speculations about planets had nothing to do with that motivation, he just realized the hugely important unifying principle of equating our Sun with other stars. It has only become science since we have been able to observe those planets.

One other thing. This idea that observational falsification is the basis of science is quite new. Popper came up with it in the 1930's, and one problem that I have with discussions of the "scientific method" is that there is this idea that what is science and what isn't is self-evidently obvious and settled.

I don't think it is accurate that a stress on observational falsification is all that new, it's actually incredibly hard to come up with anything that is new in philosophy! But Popper certainly popularized the idea. Anyway, I agree with your central point, that it is not at all obvious what "science" really is in the first place, but that's the whole reason why it's important to be skeptical that multiverse thinking is really science. What science is evolves constantly, and if one is not careful, one's science can evolve into something that is rather a large step backward, into realms where science becomes a way to feel good about what one knows, rather than a prescription for constantly requiring empirical demonstration in order to hold that one knows it.

 

[twofish-quant]

It's not important to me to be able to label inflation "ad hoc", I'm perfectly happy with "currently speculative details of how it works." I hope inflation works out simply, why wouldn't I. I'm just saying we should not kid ourselves that we have good reason to expect a scalar potential mechanism to turn out to be the correct description of the phenomenon of inflation.

We actually do. If you have a vector or tensor potential, then you'll end up with topological defects. What happens is that you have different parts of space go down vector potentials in different directions, so you'll end up with places where the vectors change direction suddenly, and those result in strong signals that we don't see in the CMB.

So whatever caused inflation was largely a scalar potential.

I'm saying it is appropriate to separate the phenomenon of inflation, which is simply the statement that the universe expanded by the factor X at epoch Y, from any physical theories or mechanisms that can actually accomplish that.

I'm not sure I see the point. One thing about astrophysics is that there are lots of examples in which we have a phenomenon with an unknown mechanism. We don't have a good mechanism for supernova, or accretion jets.

Things like reheating and so forth could be examples of the mechanism working, beyond just the phenomenon, but these are exactly the kinds of details that are still being thrashed out, and remain unclear as to whether or not they are going to work.

But the first thing is to establish that something exists. We don't understand the mechanism behind supernova, but we know supernova exist. We don't understand the mechanism behind inflation, but we know it happened.

Maybe you're right that if I knew better all the things that scalar potentials are doing for us, I'd be more inclined to see that approach to inflation as a unifying principle.

This is why I'm so harsh about LCDM and your efforts to get rid of mathematical baggage.

The big evidence for inflation is that if you assume that that there was massive expansion due to a scalar potential, you end up with a fluctuation spectrum. Because of quantum noise, some places have higher density, some places have lower density and this gets expanded by inflation. You can do detailed mathematical calculations about the density spectrum, and voila, it matches what we see when we look at WMAP.

If you try to get rid of this "mathematical baggage" for the sake of simplicity then all of this disappears. At this point inflation just becomes a fairy tale.

But it starts to sound a bit like string theory, which I am also assured is doing wonderful things for our understanding, except that it hasn't really delivered what it claimed it would.

Which is what happens when you get rid of the details. Just to use another analogy. We are *way* past the "earth is round" stage of cosmology. With LCDM, we can see the individual peaks and valleys of the universe. We can make very detailed calculations of the CMB background.

If you get rid of the "useless math baggage", then you also get rid of the ability to make complex and detailed predictions.

OK, now we are indeed talking about the mechanism, but are those predictions unique to the scalar potential approach, or are those behaviors endemic to a wide class of approaches? You say we can "effectively model" the inflation with a scalar potential, but if that is true, why are there so many different ways to do it, each with their own issues?

Because reality is complicated. There's also a tradeoff. One reason that we can use inflation for a lot of things is that it turns out that most of the predictions of inflation are not model dependent, but if the observations are model independent, then you have a plethora of models that fit the observations.

How can we have a unifying principle here, if we cannot even identify which principle is the right one?

Because for a lot of things, the details don't matter. With inflation you end up with two numbers which you then put into LCDM. How you got those two numbers, that doesn't matter.

And there's nothing wrong with that, new theory making is speculative, that's just what it is-- but why sell it as something more?

But it's not that speculative. You get CDM power spectrum out of it.

I'd say it's already pretty clear, whether or not we ever detect any curvature, that the curvature is not going to be enough to constrain the geometry of the rest of the universe without wholesale extrapolation. On what basis can the shape of a nose be extrapolated to the shape of a head?

Because CDM density perturbations can give you the limit of anisotropy, and can give you limits for how much the universe expanded during inflation. If you start with the premise that the fluctuations are due to quantum differences in density, you can calculate how much the universe expanded in order to give the current observations. You can also calculate the limits at which nearby bits could be different which gives you a radius at which you expect things to be isotropic.

What's happening is that you are taking a theory, stripping out the important bits as "useless mathematical baggage" and then complaining that the theory makes no real predictions.

The cosmological principle is not a principle of extrapolation or constraint beyond what we can observationally falsify, it is simply a modeling principle in the domain of what we can.

Exoplanets.

 

[twofish-quant]

The analogy does not hold. The only thing that makes exoplanets important is the simple fact that we can observationally verify their existence!

Yes with the technology that we have in 2012. Not in 1584, and it wasn't until the 1960's that we had anything close to the technology necessary to verify exoplanets.

That's exactly what we know we will not be able to do with some extended corner of the universe that has no observable consequences on what we can see, or worse, other universes altogether.

Who is "we"?

Off the top of my head, I can't think of how to observationally verify multiverse scenarios, but if you were to ask Giordano Bruno in 1584 how he intends to verify the existence of exoplanets, he couldn't tell you either.

Even "build a big telescope" wouldn't work. The optical telescope hadn't been invented until 1600.

If the cosmological principle worked well on the scale of what we see, but started to fail on scales an order of magnitude larger, I don't see how we would ever know that

Stare at the problem for a few hundred years before giving up.

The difference is fundamental and crucial, it is what makes exoplanets science and multiverses not science (I argue)

We weren't able to observe exoplanets until the 1990's. Now if you are making the statement that we will *never* be able to observe multiverses, then I think that's highly, highly premature.

A lot of the research on the idea of multiverses is to figure out what the impact on CMB background would be. We can actually exclude some scenarios based on what we know.

Bruno's speculations about planets had nothing to do with that motivation, he just realized the hugely important unifying principle of equating our Sun with other stars. It has only become science since we have been able to observe those planets.

So exoplanets were "unscientific" until 1990? That seems to me absurd. Also, we'd never even begin to observe exoplanets until we tried, and we couldn't try until we had a theory that described what we were looking for.

I don't think it is accurate that a stress on observational falsification is all that new, it's actually incredibly hard to come up with anything that is new in philosophy! But Popper certainly popularized the idea.

He invented it. There are some obvious problems with Popper's ideas.

 

[Ken G]

Yes with the technology that we have in 2012. Not in 1584, and it wasn't until the 1960's that we had anything close to the technology necessary to verify exoplanets.

I would say that what emerges here is an important distinction between what is science, and what inspires science but isn't itself science. Bruno was not doing science when he speculated the existence of planets, because he was not offering any tests of his idea. But it was clear enough that the suggestion could be turned into science as soon as we had the technology to see that far or that well. Similarly, Edgar Allen Poe was not doing science when (in 1848!) he speculated that the universe was expanding, but he might have inspired the science of cosmology (it is unknown if Friedmann read "Eureka", but it is known he was a Poe enthusiast). Immanual Kant wasn't doing science when he speculated the existence of "island universes" of stars, but he might have helped inspire the scientific pursuit of the study of galaxies. The point is, we can speculate anything we like, whether it be ESP or angels or galaxies, and we can be proven right or wrong, but what makes something science is the act of trying to support or falsify some idea with observations. It's a fine line, but to me the guiding principle is whether we are letting nature answer the question, or if we are pushing our answer down nature's throat. I guess everyone has to make that choice for themselves, in regard to the multiverse speculation.

He invented it.

In looking into it, I have come to agree with you-- Popper really does seem to have arrived at his views, on the importance of falsifiability in the definition of science, entirely through his own experiences with certain theories of his day that were claiming to be science. I think he actually has quite a few extremely good points, and at risk of going further off topic, I'll offer up what I see as a brilliant quote from him, on the topic of the pitfalls of inductive logic when it is allowed to become particularly careless (from http://www.stephenjaygould.org/ctrl/popper_falsification.html), it's just such a gem, and is not completely unrelated to the question of whether the multiverse is science:

'The most characteristic element in this situation seemed to me the incessant stream of confirmations, of observations which "verified" the theories in question; and this point was constantly emphasize by their adherents. A Marxist could not open a newspaper without finding on every page confirming evidence for his interpretation of history; not only in the news, but also in its presentation — which revealed the class bias of the paper — and especially of course what the paper did not say. The Freudian analysts emphasized that their theories were constantly verified by their "clinical observations." As for Adler, I was much impressed by a personal experience. Once, in 1919, I reported to him a case which to me did not seem particularly Adlerian, but which he found no difficulty in analyzing in terms of his theory of inferiority feelings, Although he had not even seen the child. Slightly shocked, I asked him how he could be so sure. "Because of my thousandfold experience," he replied; whereupon I could not help saying: "And with this new case, I suppose, your experience has become thousand-and-one-fold." '

 

[twofish-quant]

I would say that what emerges here is an important distinction between what is science, and what inspires science but isn't itself science. Bruno was not doing science when he speculated the existence of planets, because he was not offering any tests of his idea.

But then you've marked what most theorists do as "non-science." Theorists often do not come up with tests of their ideas, because that's for other people (maybe in several decades) to figure out.

One of the major predictions of general relativity was gravity waves, but Einstein didn't offer any tests for that idea. For that matter, I don't think that Einstein in his papers on special relativity offered *any* experimental tests for it.

The point is, we can speculate anything we like, whether it be ESP or angels or galaxies, and we can be proven right or wrong, but what makes something science is the act of trying to support or falsify some idea with observations.

*Trying*

There's no need that the scientist come up with a way of falsifying the idea *right now*.

Also, you can falsify an idea with theoretical constraints. If you come up with a theory of gravity, and it requires faster than light travel, you are going to have to do a lot of arguing to get that accepted.

As far as Popper's statements, I have problems with 7). The point about patching theories in order to fit observations is something that scientists do all the time. The "conversationist strategm" is something that's a good description of how science is done. You have a model. It doesn't work, you patch the model. Marxism-1919 is different from Marxism-1888, but electroweak theory-2012 is different from electroweak theory-1973. In order to get everything to work, we've had to tweak and retweak the big bang, but that hardly renders it less "scientific."

One thing that come in after Popper was the concept of "paradigms." Popper's world is very brittle, you find one thing wrong with your theory and then what?

The other thing is that it's very odd to say from the point of view of 2012 that Marxism or psychoanalysis are irrefutable. Most people would consider Marxism to have between refuted. Yes it's possible to get swept up by the crowd, but that happens with physics too (witness supersymmetry).

The other problem with Popper's ideas is that taken to the extreme, it makes it impossible to say anything meaningful about people or societies. In physics you *usually* don't have this problem. Most things in physics are not one time events and you can figure out what happens with repeated experiments. This isn't the case with societies, and it's also not the case with cosmology.

 

[twofish-quant]

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.

 

[bapowell]

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.

I would argue that this is true of science in general. All measurements are uncertain, and so are all conclusions. The only difference with quantum mechanics is that the uncertainty is fundamental, but to experimental science, all that matters is that there be uncertainty.

 

[Ken G]

But then you've marked what most theorists do as "non-science." Theorists often do not come up with tests of their ideas, because that's for other people (maybe in several decades) to figure out.

So you are saying that Bruno, Kant, and Poe were astrophysical theorists? After all, not only did they theorize, they were also right. You don't see any "blind squirrel" phenomena there? After all, none of those three were basing their theories on a single shred of observational evidence.

One of the major predictions of general relativity was gravity waves, but Einstein didn't offer any tests for that idea. For that matter, I don't think that Einstein in his papers on special relativity offered *any* experimental tests for it.

At no point did I say that a theorist had to offer experimental tests, I said a theory had to offer experimental tests. I'm sure you see the difference.

Also, you can falsify an idea with theoretical constraints. If you come up with a theory of gravity, and it requires faster than light travel, you are going to have to do a lot of arguing to get that accepted.

Just look at your words! Now theories should be accepted or refuted entirely based on the "amount of arguing" they require? There is always going to be pedagogical issues and a search for consensus, all of which is basically rhetoric, but sadly I think we are indeed seeing a lot these days of pure mathematical rhetoric. (Look at Hawking radiation, for example-- has there ever been an example of a theory so widely accepted as representing a real phenomenon on grounds that involve extrapolation of a theory into wholly untested domains, and with so little likelihood of ever receiving experimental demonstration? Popper would have cringed, I suspect.) 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. It's not necessarily bad, as it's really all we have to go on right now, but it's too oversold, there just needs to be more "truth in advertising" about what is speculation and what has empirical support.

As far as Popper's statements, I have problems with 7). The point about patching theories in order to fit observations is something that scientists do all the time. The "conversationist strategm" is something that's a good description of how science is done. You have a model. It doesn't work, you patch the model. Marxism-1919 is different from Marxism-1888, but electroweak theory-2012 is different from electroweak theory-1973. In order to get everything to work, we've had to tweak and retweak the big bang, but that hardly renders it less "scientific."

I agree (7) is the most questionable, the rest are all pretty rock solid. I think what rescues (7) is what is meant by "ad hoc", albeit this is a difficult word to define clearly. It seems to me that Popper's sentiment here is that a theory that is in a state of "constant backpedalling" is probably a theory that is not worth having, whereas a theory that almost got it right but needed some fixes that did not deviate from the central stance of the theory (so was not "ad hoc") is still a good theory. What I think is missing from (7) is some clear way to "count the unifications" of a theory, such that if you need X patches in a theory that accomplishes Y unifications, this is still science if X < Y. He seems to be complaining more about when X=Y, effectively reducing Y to zero. I think that's the phenomenon he witnessed with some theories of his day that gained a lot of momentum but never really "delivered the goods." It's a cautionary tale we do well to keep an eye on today as well, I wager!

So I see Popper as having two fundamental beefs with theories that he did not consider good science:
1) theories that were so versatile they could explain anything, thereby explaining nothing because they achieved no fundamental unification of the unknowns, and
2) theories that required so many patches to respond to their failings that any unifications they originally promised ended up vanishing in all the patches.
I think those are two mighty good points to bear in mind.

Most things in physics are not one time events and you can figure out what happens with repeated experiments. This isn't the case with societies, and it's also not the case with cosmology.

It is definitely a dicey issue when using physics to do history, as cosmology does, for just this "unrepeatability" problem. But I think in cosmology, you can still apply Popper's basic scheme, you just have to generalize what "repeatability" means. You only get one "trial" to study, that's true, but you can study it in what seem like independent ways-- you can do observations of very different phenomena, that are all predicted by the theory, and in that sense each independent prediction allows "repeatability" in the efforts to falsify it. So probably the stress on "repeatibility" is not so crucial there, it is instead a kind of need for "independent confirmation", which is really what "repeatibility" mostly means anyway.

 

[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.