L. Susskind -- The "Megaverse"

[edd]

I'm not completely convinced you can't come up with some kind of uninformative prior by imposing some limits on fundamental parameters. For example you could require that all fermion masses be below the Planck mass.
Edit: I guess that would be not entirely unrelated to the hierarchy problem in fact.

 

[Kwalish Kid]

But this is a ridiculous argument. We are indeed ignorant of whatever probability distribution might determine the values, so we have no reason to prefer one over another.

You think my argument is ridiculous, but you repeat it, presumably to endorse it.

Very strange.

Assigning a probability of 1 would be like assigning 27 equal spikes (see above) equally distributed by some factorX(1/π). It's not a guessing game; we really don't know!

Are you saying that the fine-tuning problem is that we have to come up with a probability distribution or are you saying, as you said earlier, that the fine-tuning problem is that we are forced, because of ignorance, to assume a certain probability distribution that makes our universe improbable?

 

[Perpetual Student]

Are you saying that the fine-tuning problem is that we have to come up with a probability distribution or are you saying, as you said earlier, that the fine-tuning problem is that we are forced, because of ignorance, to assume a certain probability distribution that makes our universe improbable?

Let's say that you dip your hand into a huge container containing tiny mote sized chips each with some number magnetically encoded. This container has many millions of these chips (we do not know the number), which can be placed in a reader to determine the number encoded. You have no information about the numbers on the chips: they could all be the same number, they could have every number their finite count would allow or anything in between. So, you pick a chip and look at the number. What can you conclude? Are they all that same number, mostly that number?
You do not have enough information to make any determination. You might argue that the best guess is that they are all that same number since that is the only number we have knowledge of -- but with very little certainty.
Similarly, we do not know why the electron has the mass it has, what the alternatives (if any) might have been or why any of the other fundamental constants are what they are or what any alternatives might have been.
So, now we consider that all these fundamental constants give rise to the universe we know, with galaxies, stars, chemistry, planets, and life. How likely does that seem with no knowledge of the possibilities of each of these fundamental constants? That is the fine-tuning question!
If it does not seem to be a problem for some people, so be it. I and many others find it to be a very perplexing and meaningful problem. The megaverse conjecture provides one possible context within which to deal with it.

 

[TubbaBlubba]

Again, what we do know is that the probability of this universe having this set of constants is 1. Any issues (including fine-tuning) about the prior probability arise from conjecture about the process that spawned it.

Once we do have a reasonable conjecture, fine-tuning may be a problem to deal with, but not before.

 

[Kwalish Kid]

Let's say that you dip your hand into a huge container containing tiny mote sized chips each with some number magnetically encoded. This container has many millions of these chips (we do not know the number), which can be placed in a reader to determine the number encoded. You have no information about the numbers on the chips: they could all be the same number, they could have every number their finite count would allow or anything in between. So, you pick a chip and look at the number. What can you conclude? Are they all that same number, mostly that number?
You do not have enough information to make any determination. You might argue that the best guess is that they are all that same number since that is the only number we have knowledge of -- but with very little certainty.
Similarly, we do not know why the electron has the mass it has, what the alternatives (if any) might have been or why any of the other fundamental constants are what they are or what any alternatives might have been.
So, now we consider that all these fundamental constants give rise to the universe we know, with galaxies, stars, chemistry, planets, and life. How likely does that seem with no knowledge of the possibilities of each of these fundamental constants? That is the fine-tuning question!
If it does not seem to be a problem for some people, so be it. I and many others find it to be a very perplexing and meaningful problem. The megaverse conjecture provides one possible context within which to deal with it.

You just wrote a lot without answering my question.

Are you saying that the fine-tuning problem is that we have to come up with a probability distribution or are you saying, as you said earlier, that the fine-tuning problem is that we are forced, because of ignorance, to assume a certain probability distribution that makes our universe improbable?

 

[Perpetual Student]

Again, what we do know is that the probability of this universe having this set of constants is 1. Any issues (including fine-tuning) about the prior probability arise from conjecture about the process that spawned it.

We know nothing about that process nor anything about prior probabilities.

Once we do have a reasonable conjecture, fine-tuning may be a problem to deal with, but not before.

Clearly, some of us feel there is a fine-tuning question in the absence of such a conjecture. You may continue to feel free to ignore it, as you wish.

 

[Perpetual Student]

You just wrote a lot without answering my question.

Are you saying that the fine-tuning problem is that we have to come up with a probability distribution or are you saying, as you said earlier, that the fine-tuning problem is that we are forced, because of ignorance, to assume a certain probability distribution that makes our universe improbable?

Why would we "have to come with a probability distribution"? Do you have a reading comprehension problem? It seems to me my description above makes it clear that we know nothing about any probability distribution of the fundamental constants. Furthermore, we may never have any such knowledge. I don't know and I don't believe anyone else does -- do you? Speculating about the apparent fine-tuned nature of the universe has occupied physicists for some time now. Rather than obsessing about my particular take on this question, perhaps you might read some of the many essays and books written about it by people who have seriously studied this question.

 

[Toontown]

We know nothing about that process nor anything about prior probabilities.

Not nothing. We know the prior probability of this particular universe was not 0.

What we actually know nothing about is what the prior probabilities of other universes may have been.

Seems rather presumptuous to me to assume no other prior probabilities existed. Which seems to me to naturally give rise to a "reasonable conjecture" to the contrary.

Clearly, some of us feel there is a fine-tuning question in the absence of such a conjecture. You may continue to feel free to ignore it, as you wish.

I seem to be having difficulty understanding what the two of you mean by "reasonable conjecture".

There exists conjecture, which I consider reasonable, that this particular universe may have arisen spontaneously out of a multitude of possibilities. To me, it almost seems denialist to consciously avoid such a conjecture.

So it seems to me that, if the only prerequisite to acknowledgement of a fine tuning question is a "reasonable conjecture" which in turn gives rise to the fine tuning question, then we have that prerequisite.

 

[Perpetual Student]

Not nothing. We know the prior probability of this particular universe was not 0.

What we actually know nothing about is what the prior probabilities of other universes may have been.

OK, I agree.

Seems rather presumptuous to me to assume no other prior probabilities existed. Which seems to me to naturally give rise to a "reasonable conjecture" to the contrary.

To my knowledge, we have no knowledge of prior probabilities, whether they existed or not. For all we know, all possibilities are equally likely, there is a narrow spike only for our universe -- or anything in between. That is part of the fine-tuning question.

I seem to be having difficulty understanding what the two of you mean by "reasonable conjecture".

There exists conjecture, which I consider reasonable, that this particular universe may have arisen spontaneously out of a multitude of possibilities. To me, it almost seems denialist to consciously avoid such a conjecture.

So it seems to me that, if the only prerequisite to acknowledgement of a fine tuning question is a "reasonable conjecture" which in turn gives rise to the fine tuning question, then we have that prerequisite.

It is tb's opinion that, "Once we do have a reasonable conjecture, fine-tuning may be a problem to deal with, but not before." This required conjecture, in his view, is a "conjecture about the process that spawned (the universe)." I do not see the need for any such conjecture; the fine-tuning question exists in the absence of knowledge about prior processes or probabilities.

 

[Toontown]

It is tb's opinion that, "Once we do have a reasonable conjecture, fine-tuning may be a problem to deal with, but not before." This required conjecture, in his view, is a "conjecture about the process that spawned (the universe)." I do not see the need for any such conjecture; the fine-tuning question exists in the absence of knowledge about prior processes or probabilities.

OK, I missed that. Thanks.

I agree that the fine tuning question is part and parcel of our general ignorance as to the process(es) that spawned the universe(s). Tb seems to be waiting for the spawning questions to be resolved before bothering with the fine tuning question, if the question remains at that point.

But it seems to me that resolution of the spawning questions will almost certainly resolve the fine tuning question as well. So in that sense, maybe there is merit in not worrying about fine tuning at all, if we expect it to be resolved along with the spawning mystery.

But I see the appearance of fine tuning as a clue, rather than a question. I think it tells more than it asks (if that makes any sense). I don't think it's telling us there is a universe designer hiding out there somewhere. But as we look ever more closely at reality, we keep seeing something that does look as if some designer has been at work. I think that means something.

 

[TubbaBlubba]

OK, I missed that. Thanks.

I agree that the fine tuning question is part and parcel of our general ignorance as to the process(es) that spawned the universe(s). Tb seems to be waiting for the spawning questions to be resolved before bothering with the fine tuning question, if the question remains at that point.

But it seems to me that resolution of the spawning questions will almost certainly resolve the fine tuning question as well. So in that sense, maybe there is merit in not worrying about fine tuning at all, if we expect it to be resolved along with the spawning mystery.

Yes, that's about it as far as I'm concerned..

But I see the appearance of fine tuning as a clue, rather than a question. I think it tells more than it asks (if that makes any sense). I don't think it's telling us there is a universe designer hiding out there somewhere. But as we look ever more closely at reality, we keep seeing something that does look as if some designer has been at work. I think that means something.

I agree that it can be a springboard for ideas. "What if our universe is unlikely? What could it all look like then?", but I simply cannot (no matter however Perpetual Student feels) consider it a problem the same way the unification of GR and QM is a problem, or the question of what will happen to the universe in the future is soimething to be solved.

Incidentally, looking for good examples of unresolved problems, I went to this page: http://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics To my slight surprise fine-tuning is not listed.

 

[edd]

There are several problems on that page that one might call fine tuning problems.

 

[TubbaBlubba]

There's the hierarchy problem, perhaps, on why gravity is so weak. Given a random constant conjecture with many universes, the anthropic fine-tuning idea becomes an explanation.

Still, I can't see how the question here could be "why is gravity tuned for life?" when we don't even have a single theory that explains both gravity and electromagnetism. Even if we did and the discrepancy remained, the question to follow would be "how do constants arise?" And if one suggests a stochastic manner (not unreasonable given how we experience QM, I suppose), THEN the' fine-tuning' may hint at some manner of multiverses.

 

[Fudbucker]

There are several problems on that page that one might call fine tuning problems.

Indeed, if you click on the link for the cosmological constant problem, you get sent to an article about the Anthropic Principle.

 

[Kwalish Kid]

Why would we "have to come with a probability distribution"? Do you have a reading comprehension problem? It seems to me my description above makes it clear that we know nothing about any probability distribution of the fundamental constants. Furthermore, we may never have any such knowledge. I don't know and I don't believe anyone else does -- do you? Speculating about the apparent fine-tuned nature of the universe has occupied physicists for some time now. Rather than obsessing about my particular take on this question, perhaps you might read some of the many essays and books written about it by people who have seriously studied this question.

I have read quite a bit on this subject, which is why I objected when you insulted TubbaBubba for taking the sane approach to the topic. Your take on this subject is what is at issue here, since you feel confident enough on this topic to deride others with statements about this field that you now seem to admit were made in ignorance. Fair enough.

In the literature on the subject, one finds some irresponsible speculation but one also finds attempts to produce probability distributions for the values of certain physical constants. These are generally, if I recall correctly, based upon hypotheses about the nature of physical phenomena at high energies and how this physics changes into the behaviour of phenomena at the energies we are most familiar with. Ideally, if these hypotheses become well-supported theories, they could be used to predict (or, rather, infer) the existence of other spacetime regions or other spacetimes in which there are different values for what we now consider to be physical constants.

 

[Perpetual Student]

It was not my intention to deride or insult and I do not believe I have done so.
In fact, you are the one guilty of communicating with a combative tone.

 

[Fudbucker]

There's the hierarchy problem, perhaps, on why gravity is so weak. Given a random constant conjecture with many universes, the anthropic fine-tuning idea becomes an explanation.

Still, I can't see how the question here could be "why is gravity tuned for life?" when we don't even have a single theory that explains both gravity and electromagnetism. Even if we did and the discrepancy remained, the question to follow would be "how do constants arise?" And if one suggests a stochastic manner (not unreasonable given how we experience QM, I suppose), THEN the' fine-tuning' may hint at some manner of multiverses.

Here's my take on analogizing this:

We find a machine that has a button and an LCD display on it. When you press the button on the machine, the numbers 14142135623730950 pop up. Without pressing the button again, there is at least one thing we can conclude about the machine: it is not randomly spitting out numbers. We can conclude this because the odds of getting anything mathematically interesting from a random process are extremely small. Therefore, the numbers we got were almost certainly not the result of some random process. Or if you don't like numbers, just imagine the machine displays the first paragraph of the Declaration of Independence. We would conclude the same thing: it's not the result of a random process.

So we look the machine over and discover no underlying principle why it would produce the numbers (or letters) that it did. We have no idea what range of numbers (letters) it can produce. Maybe it produces the same set of numbers every time. Maybe the numbers can only vary by 1, plus or minus, or maybe they can vary completely. Our lack of knowledge about the range of possible values doesn't affect our overall conclusion that the numbers were produced by a non-random process.

If the numbers were "set" by some internal programming (they can't vary at all), we would simply wonder why they're set for the exact value of the square root of 2 (or the exact first paragraph of the Declaration), and we're back to square one.

But what happens if we press the button over and over again, and a bunch of random "meaningless" numbers keep appearing? The more times we press it, the less sure we would be that it's all a non-random process. Eventually, after a huge number of button pushes, we would conclude we just got lucky the first time.

In other words, there doesn't seem to be an underlying principle behind the values of the various physical constants we've measured. They just happen to have the values they have. We don't know for sure what range they could possibly have, either. We just know we have this sequence of physical constant values that allows for complex structure to exist in the universe. Any deviation in the values, even by the tiniest bit, results in a totally "failed" universe.

But there's no button to press, to generate new values, so we simply conclude there's a whole bunch of universes out there, and we just happen to be in one of the lucky few that can support life. Hence, the rising popularity of multiverse theory.

 

[Roboramma]

Still, I can't see how the question here could be "why is gravity tuned for life?" when we don't even have a single theory that explains both gravity and electromagnetism. Even if we did and the discrepancy remained, the question to follow would be "how do constants arise?" And if one suggests a stochastic manner (not unreasonable given how we experience QM, I suppose), THEN the' fine-tuning' may hint at some manner of multiverses.

I think string theory does "explain both gravity and electromagnetism", or least, puts them into the same context. It also gives a context in which different universes with different constants makes some sense.

 

[Reality Check]

Still, I can't see how the question here could be "why is gravity tuned for life?" when we don't even have a single theory that explains both gravity and electromagnetism.

I cannot see what not having a theory that explains both gravity and electromagnetism has anything to do with that question.
We do have a theory that explains gravity and a theory that explains electromagnetism.

Why gravity is tuned for life in this universe is a question for cosmology. And there are several possibilities:

• Maybe there is a cosmological model that always produces universe like ours, e.g. inflationary cosmology seems to fit the bill.
• Maybe there are multiple universes (multiverse).
• Maybe the universe tunnels through the parameter space so that life happens under the right conditions (and then is amaze that the right conditions exist!).
• Maybe the universe is one of many bubble universes.
• Maybe it is quantum mechanics: our universe has "selected" the past histories that lead to there being life.

 

[TubbaBlubba]

I cannot see what not having a theory that explains both gravity and electromagnetism has anything to do with that question.

Because we don't understand how they're interconnected, if at all. It could be analogous to, say, "why is the magnetic field active only when moving?" pre-SR.