L. Susskind -- The "Megaverse"

[Perpetual Student]

It's not the case, no.

Please explain.
α_G = 1.75∙10^-45 and is dimensionless according to THIS.

 

[ctamblyn]

Please explain.
α_G = 1.75∙10^-45 and is dimensionless according to THIS.

The quantity α_G defined in that article is not a universal constant as such, since it depends on the masses of the particles involved. E.g. for two electrons you get

α_G(electrons) = 1.75 × 10^-41​

while the value for two protons is

α_G(protons) = 5.91 × 10^-39.​

The strength of the gravitational interaction is characterised universally by either Newton's gravitational constant or, equivalently, the reciprocal of the squared Planck mass, which are dimensionful.

ETA: This is different to what happens in electromagnetism, for example, where the interaction strength is characterised universally by a dimensionless number (the fine structure constant).

 

[Perpetual Student]

The quantity α_G defined in that article is not a universal constant as such, since it depends on the masses of the particles involved. E.g. for two electrons you get

α_G(electrons) = 1.75 × 10^-41​

while the value for two protons is

α_G(protons) = 5.91 × 10^-39.​

The strength of the gravitational interaction is characterised universally by either Newton's gravitational constant or, equivalently, the reciprocal of the squared Planck mass, which are dimensionful.

ETA: This is different to what happens in electromagnetism, for example, where the interaction strength is characterised universally by a dimensionless number (the fine structure constant).

Thanks for that. Upon further review, I see that the above is true.
It seems peculiar to call it a coupling constant, when it is not constant. The Wikipedia article says, "α_G is a fundamental physical constant and a dimensionless quantity, so that its numerical value does not vary with the choice of units of measurement." While that is true, (m_e/m_p)² will not depend on the units used, it does vary by the mass in question. As a layman I find that definition (coupling constant) very peculiar.

 

[Roboramma]

This is a very, very generous reading of his paper, especially if you think that anthropic reasoning is anything other than taking observations into account.

What's "generous" about it? It seems that he made a prediction, and the prediction was later verified by observations.

 

[Fudbucker]

This is a very, very generous reading of his paper, especially if you think that anthropic reasoning is anything other than taking observations into account.

I think I see your point. Let's say the Cosmological Constant had a possible value between 1 and 100. But Weinberg figures that it can't be over 65, because anything over that wouldn't support any kind of life that could measure the value. Ditto, for anything under 45. So he figures it's somewhere around 52 and when we actually measure it, it turns out to be 63, which is still pretty close.

I know that's totally simplified, but would the above really count as a prediction? Like Kwalish is saying (I think), Weinberg just specified the upper and lower bounds that made observing the value even possible. Is it really a prediction if you predict the next square you see will have four corners?

 

[Roboramma]

I think I see your point. Let's say the Cosmological Constant had a possible value between 1 and 100. But Weinberg figures that it can't be over 65, because anything over that wouldn't support any kind of life that could measure the value. Ditto, for anything under 45. So he figures it's somewhere around 52 and when we actually measure it, it turns out to be 63, which is still pretty close.

From my reading, there was no lower bound. It could have been zero, and that's what most people thought it was likely to be. His prediction was that it would be very close to the upper bound, and it was.

Perhaps I misunderstood, but from what Sol said, and looking up first wikipedia and then his original paper (which I pretty much just glanced at), that's my understanding.

 

[Vorpal]

I know that's totally simplified, but would the above really count as a prediction? Like Kwalish is saying (I think), Weinberg just specified the upper and lower bounds that made observing the value even possible. Is it really a prediction if you predict the next square you see will have four corners?

Your analogy has no similarity whatsoever with this situation. Predicting that the next square will have four corners is entirely a priori and based on the meaning of the terms only, while Weinberg actually used observations about that universe. (Unless you think life is not part of the universe?)

This is a very, very generous reading of his paper, especially if you think that anthropic reasoning is anything other than taking observations into account.

Nonsense. One might perhaps have a case that "anthropic reasoning" provides no explanatory power, but whatever resolution that philosophical quandary has, it has nothing to do with validity of predictions. Weinberg argued that a value outside a certain range is inconsistent with observations.

The claim that this is somehow illegitimate implies that there is something special about the observation that life like ours exists that makes it very different from any other observable fact about the universe. In other words, your position implies that humans are somehow that super-special.

 

[sol invictus]

This is a very, very generous reading of his paper, especially if you think that anthropic reasoning is anything other than taking observations into account.

First you asserted it was a "gross mischaracterization", now a "very, very generous reading". The truth is, what Weinberg said is quite clear (as usual - he is very precise).

It's apparent that you have some kind of irrational dislike for the anthropic principle or the idea of a multiverse.

 

[sol invictus]

From my reading, there was no lower bound. It could have been zero, and that's what most people thought it was likely to be. His prediction was that it would be very close to the upper bound, and it was.

That's right. And you have to remember that the "natural" value is 10^123 times greater than the measured value, so at the time Weinberg wrote his paper nearly everyone expected it to be exactly zero.

 

[Kwalish Kid]

First you asserted it was a "gross mischaracterization", now a "very, very generous reading". The truth is, what Weinberg said is quite clear (as usual - he is very precise).

Your first presentation was a gross mischaraterization. The single claim that you later made was a very, very generous reading.

It's apparent that you have some kind of irrational dislike for the anthropic principle or the idea of a multiverse.

I have a dislike for people who want to merely assume that there is a multiverse without providing evidence for a multiverse. Odd that people on this particular board would want to accept a multiverse without such evidence.

One problem is that discussions of this Weinberg paper somewhat equivocate on "anthropic reasoning". In placing boundaries on the cosmological constant, Weinberg uses weak anthropic reasoning. This is essentially taking into account that humans exist in a certain kind of world and that a cosmological theory should not violate this observation.

Weinberg uses the way that the cosmological constant can ruin structure formation to place an upper limit on it. This is fine, weak anthropological reasoning. He then uses the general preference for a flat universe and the fact that a larger value for the cosmological constant allows for a greater age of the universe to propose that the cosmological constant is at the upper limit of the range. This latter motivation takes into account observations that conflict with the age of the universe as determined without the cosmological constant. However, these observations could have later been thrown into doubt and the total energy density of the universe was unknown at the time that Weinberg wrote.

When Weinberg refers to anthropic reasoning at some points in the paper, he may be referring to strong anthropic reasoning, the use of anthropic reasoning (and little else) to infer some principle that forces the universe to be as we see it. That is, he may be referring to the use of theories that provide a large background of different universes or different areas in the universe to explain why certain features exist in our region.

It certainly makes sense to use weak anthropic reasoning as Weinberg did. It doesn't seem to make sense to use strong anthropic reasoning. It seems to me that everything that strong anthropic reasoning purports to infer should be inferred from the results investigation into the relevant physics. This strong anthropic reasoning seems always to be accompanied by hastily adopted probability distributions and very fragile assumptions about the nature of universes, mega- or multi-.

 

[Toontown]

It's apparent that you have some kind of irrational dislike for the anthropic principle or the idea of a multiverse.

I have a dislike for people who want to merely assume that there is a multiverse without providing evidence for a multiverse.

So?

I have a dislike for people who demand that others must provide evidence the demander will accept for everything the demander assumes they are assuming without evidence, simply because the demander does not accept the evidence they are using.

Therefore, based on my superior dislike, I hereby proclaim that I am the thought cop around here, and you will obey my lawful orders.

Odd that people on this particular board would want to accept a multiverse without such evidence.

What evidence is "such evidence" in your mind?

Is it possible that these people you dislike are using evidence you do not recognize as such?

 

[Kwalish Kid]

If I was going to thought cop this board, I'd start with the misogyny. People are free to jump on whatever scientific bandwagon they wish.

 

[Fudbucker]

That's right. And you have to remember that the "natural" value is 10^123 times greater than the measured value, so at the time Weinberg wrote his paper nearly everyone expected it to be exactly zero.

I guess my analogy is wrong. It was a good prediction then.

 

[Fudbucker]

If I was going to thought cop this board, I'd start with the misogyny. People are free to jump on whatever scientific bandwagon they wish.

Wait, what?

 

[Toontown]

If I was going to thought cop this board, I'd start with the misogyny. People are free to jump on whatever scientific bandwagon they wish.

Wait, what?

You know. Woman-hate. That's the real issue here. I wouldn't be surprised if racism has reared it's ugly head as well. And politics. Always politics.

 

[Kwalish Kid]

I guess my analogy is wrong. It was a good prediction then.

That so-called natural predicted value is not really a predicted value. It is a calculation based on quite a few assumptions, including the cutoff for the vacuum energy and that the the vacuum energy of quantum field theory interacts with gravity directly. There is as of yet no natural way to introduce a vacuum energy with a finite contribution on a curved spacetime.

As far as the cosmological constant is concerned, what we observe could be four things:

A) an energy associated with the vacuum state interacting gravitationally
B) a constant of the gravitational equation
C) a specific field (probably scalar) that evolves over cosmological time that extends throughout spacetime
D) some combination of A, B, and C.

It could be that the semi-classical calculation of vacuum energy density is correct and there is a constant of the gravitational equation that almost completely negates is (in this case, the actual cosmological constant would have the opposite sign to what it would otherwise have if it were the only component causing the phenomena that we observe). There are some complicated issues here.

 

[Hercules Rockefeller]

In another universe you all agree.

 

[sol invictus]

Your first presentation was a gross mischaraterization. The single claim that you later made was a very, very generous reading.

There is no substantive difference between my two "presentations", and there is nothing "generous" about my reading. Weinberg is extremely clear. I quotes the most relevant passage - read that, or read the entire paper.

I have a dislike for people who want to merely assume that there is a multiverse without providing evidence for a multiverse.

You seem to be operating under some kind of fundamental misunderstanding of science. You do not have to have evidence for a hypothesis to propose it and take it seriously enough to try to understand it. Instead, science works when people propose hypotheses that are not supported by evidence, use those hypotheses to make testable predictions, and then seek to falsify those predictions.

That's precisely what Weinberg did - he made a falsifiable prediction, and it turned out to be correct. That's scientific evidence for his hypothesis. Conclusive? Of course not, nothing ever is. But refusing to acknowledge the success of that prediction is simply irrational.

 

[Kwalish Kid]

There is no substantive difference between my two "presentations", and there is nothing "generous" about my reading. Weinberg is extremely clear. I quotes the most relevant passage - read that, or read the entire paper.

Having read that paper, I cannot agree with your interpretation nor with your somewhat hyperbolic interpretation of the history surrounding it.

You seem to be operating under some kind of fundamental misunderstanding of science. You do not have to have evidence for a hypothesis to propose it and take it seriously enough to try to understand it. Instead, science works when people propose hypotheses that are not supported by evidence, use those hypotheses to make testable predictions, and then seek to falsify those predictions.

That's precisely what Weinberg did - he made a falsifiable prediction, and it turned out to be correct. That's scientific evidence for his hypothesis. Conclusive? Of course not, nothing ever is. But refusing to acknowledge the success of that prediction is simply irrational.

You can operate under an impoverished view of science and reinterpret Weinberg for yourself to fit this, but I will not. Not only was Weinberg using available observations to constrain the parameter space of cosmology, not simply making a prediction, much of the recent success of cosmology is of a similar vein: gaining success not through failure to falsify but through the demonstration of converging limitations on parameter space through measurement.

If the multiple universe as a hypothesis bears some fruit, this would be great. But it is an incredibly fragile assumption.

 

[sol invictus]

Having read that paper, I cannot agree with your interpretation nor with your somewhat hyperbolic interpretation of the history surrounding it.

And yet, you seem to be incapable of articulating where exactly you disagree. We agree that Weinberg set a bound using (crude) cosmological data. He then made a prediction using anthropic/multiverse logic - that the actual value should be 1-2 orders of magnitude below to the bound.

....we would then conclude that pV must be much greater than the present mass density p0 [and less than ~500 times it]

There is no "interpretation", it's crystal clear. What about that do you fail to understand?

You can operate under an impoverished view of science and reinterpret Weinberg for yourself to fit this, but I will not. Not only was Weinberg using available observations to constrain the parameter space of cosmology, not simply making a prediction, much of the recent success of cosmology is of a similar vein: gaining success not through failure to falsify but through the demonstration of converging limitations on parameter space through measurement.

I can't make any sense of that paragraph.