# Questionable article in Scientific American?

1. Jan 5, 2010

### SW VandeCarr

"Questionable" article in Scientific American?

"Questionable" is my polite assessment. It's highly speculative at best, but specifying distances to "copies" of our galaxy (and ourselves) is cranky as far as I'm concerned. I'm not paying to read the full article. What ever happened to Scientific American? Maybe someone can tell me why such an article would be published in SA. I would like to renew my faith in this journal.

http://www.scientificamerican.com/article.cfm?id=parallel-universes

2. Jan 5, 2010

### marcus

Re: "Questionable" article in Scientific American?

What I hear is people saying they canceled their subscriptions. The wisest course may be not trying to renew your faith in SA. Just accept that the magazines good years are over

3. Jan 5, 2010

### turbo

Re: "Questionable" article in Scientific American?

Not good. The "philosophy" (I shall not call it science) behind claims that the universe branches infinitely, and every possible choice exists is way past woo-woo. Can we agree that we exist on Earth and start to figure out the Universe from there? If not, it's Katy, bar the door.

4. Jan 5, 2010

### SW VandeCarr

Re: "Questionable" article in Scientific American?

That sounds like MWI, which is an interpretation, not a theory. However, in MWI the observer only enters one branch, and the others are ontologically inaccessible. There's no idea of distance between branches. Many physicists do prefer MWI to other interpretations since it is deterministic and gets rid of the observer caused collapse of the wave function. But what's described in this short advertisement is not the MWI. It sounds like bad science fiction to me.

EDIT I want to correct my description of MWI. Copies of the observer each enter one of the branches, but communication between the branches is not possible. I don't subscribe to MWI, not that it matters because I'm not a physicist. For what its worth I take 'the shut up and calculate' (instrumental) position because feel I don't know enough to choose another. I do understand that the calculations are invariably predictive of experimental outcomes.

Last edited: Jan 6, 2010
5. Jan 5, 2010

### SW VandeCarr

Re: "Questionable" article in Scientific American?

Too bad. Another American icon down the tubes!

6. Jan 5, 2010

### DaveC426913

Re: "Questionable" article in Scientific American?

Is it worse science fiction than 75 years ago when they started toying with the idea that no one knew what the Moon did when it wasn't being observed?

Today, we understand that this is just a thought-experiment, an idea in principle, not in practice.

But these are the kinds of postulates that make us question what we always thought was rock-solid reality. We did the same thing when Einstein proposed that space and time were not absolute fixed backgrounds against which the universe played out - that time and space were plastic.

OK, so it's more philosophy than science...

7. Jan 6, 2010

### SW VandeCarr

Re: "Questionable" article in Scientific American?

IMHO yes. First, it's in a once prestigious journal, no place for ultra-speculative whatever. Second, its not philosophy because Tegmark is throwing out numbers. Someone tell me where he gets these numbers.

Last edited: Jan 6, 2010
8. Jan 6, 2010

### sylas

Re: "Questionable" article in Scientific American?

I don't see the problem. Even from your description I guessed what it was about. The author is http://space.mit.edu/home/tegmark/" [Broken], who is a first rate legitimate cosmologist, and the article is from 2003.

Tegmark has written some good popular descriptions of some ideas in cosmology. In this case, he is considering the implications of a flat infinite universe... which is surely a reasonable thing to do. It is not a prediction that the universe is infinite, but a consequence of the universe being infinite and homogenous. The large scale homogeneity holds as far as we can see (the observable universe) and the flatness holds to within measurement accuracies.

What you can do is consider the number of possible "states" for a galaxy. That is, how many different galaxies might exist, given a finite amount of mass arranged in a very large but finite number of ways. Then calculate the density of galaxies that are identifitcal to our own; identical right down to locations of all molecules in the galaxy... and hence to existence of you and I, writing in this forum; or at least one physically indistinguishable from it by any available measurement.

I remember seeing this, and I suspect it is something pretty much like this article available on his home page at MIT: Parallel Universes, (Jan 2003) described as to appear in "Science and Ultimate Reality: From Quantum to Cosmos", honoring John Wheeler's 90th birthday, J.D. Barrow, P.C.W. Davies, & C.L. Harper eds., Cambridge University Press (2003).

In this article he considers 4 difference ways in which "parallel" universes might exist. The first "level 1" is what I have described above and which appears in the start of your link. Consider an infinite flat universe. Then, from an argument based on possible arrangements of matter, we can infer probabilities of something indistinguishable from you in a given volume. From the article:
Level I: A generic prediction of in ation is an infinite ergodic universe, which contains Hubble volumes realizing all initial conditions -- including an identical copy of you about 10^(10^29) m away.

This is a good and thought provoking article by a legitimate and very competent scientist. Recommended reading, as far as I am concerned.

Cheers -- sylas

Last edited by a moderator: May 4, 2017
9. Jan 6, 2010

### Chalnoth

Re: "Questionable" article in Scientific American?

...if a bit eccentric. He's well known for some of his more interesting antics during talks, such as including pictures of other peoples' slides in his own talks at conferences!

Last edited by a moderator: May 4, 2017
10. Jan 6, 2010

### sylas

Re: "Questionable" article in Scientific American?

Sure. Many great physicists are a tad eccentric. With respect the original article, I recommend as a followup this page from Tenmark's own pages, which refers to the article cited in the OP: Welcome to my Crazy Universe. Extract:
Every time I've written ten mainstream papers, I allow myself to indulge in writing one wacky one, like my Scientific American article about parallel universes. This is because I have a burning curiosity about the ultimate nature of reality; indeed, this is why I went into physics in the first place. So far, I've learned one thing in this quest that I'm really sure of: whatever the ultimate nature of reality may turn out to be, it's completely different from how it seems. So I feel a bit like the protagonist in the Truman Show, the Matrix or the 13th Floor trying to figure out what's really going on.

The rest of the page goes on to give more detail and heaps of references for the topics introduced in the Sci Am article. There is also slightly more recent and similar account as The Multiverse Hierarchy, by Max Tegmark, at arXiv:0905.1283v1.

Cheers -- sylas

11. Jan 6, 2010

### S.Vasojevic

Re: "Questionable" article in Scientific American?

Concept mathematically resembles analysis of number Pi. Search through digits of Pi long enough, and you will find any set of numbers. Once you identify set you are looking for, if you continue to search you will find it again, and again, and again...
It is just matter of how far you will have to search. More complex the set, more you need to search.
So, Tegmark is talking about infinite universe, and homogeneous one. His article seems ok to me.
Numbers he is giving are not completely speculative, but they lack proper definition. Article states that there is a twin at galaxy 10 to the 10^28 meters from here. So what is the same as here? Twin, his planet, his solar system, his galaxy, his cluster? Tegmark's estimate would depend on where is the boundary of the repeating structure. Furthermore it should be given with probability.

12. Jan 6, 2010

### DaveC426913

Re: "Questionable" article in Scientific American?

I think he was saying his twin.

13. Jan 6, 2010

### twofish-quant

Re: "Questionable" article in Scientific American?

This is really what I mean when I said that all productive physicists that I've met are half-crackpots. Max Tegmark is a first rate physicist. He has some really crazy ideas about how the universe works, which doesn't bother me because they aren't as crazy as some of the ideas I have. It's really scary to have a bunch of physicists reading each other's papers, because they will try to "out-crazy" each other.

Having said that, I do think that Scientific American has really gone downhill. The important thing about productive physicists is that they are *HALF*-crackpots. It's really important to have your head in the clouds, but it's equally important to have your feet stuck in the ground, and one the problems I have with Scientific American is that the spend to much time on the nutty speculative weird stuff, and not enough time saying "this is nutty speculative and weird and probably wrong."

I haven't read SA in years, since I haven't seen anything worth reading in it.

14. Jan 6, 2010

### SW VandeCarr

Re: "Questionable" article in Scientific American?

Thanks Silas. Now I know where Tegmark got his number an I can feel a bit better about SA. The implications of infinity are strange indeed.

15. Jan 6, 2010

### sylas

Re: "Questionable" article in Scientific American?

The arxiv reference I gave earlier describes the calculation in more detail. The basis for this calculation is essentially the simplest possible ΛCDM model. In this simplest case, the universe is topologically simple, and flat; therefore infinite. It is also assumed that the there is nothing special about this region of space by comparison with others, and so a UPPER bound can be calculated on the frequency at which structures similar to "here" will be found throughout the universe, simply by identifying the number of possible quantum states for a given volume and temperature. The "distance" to your twin is actually an upper bound on the mean distance between the infinite number of twins that would exist throughout this simple infinite universe.

The calculation depends on the size of the volume being twinned. It could be enough to contain just you; or enough to contain a sphere 100 ly in diameter around you (so that you are "twinned" and so also all your perceptions for the next 100 years); or enough to contain the entire observable universe (Hubble volume).

From The Multiverse Hierarchy, by Max Tegmark, at arXiv:0905.1283v1:
This means both that pretty much all imaginable matter configurations occur in some Hubble volume far away, and also that we should expect our own Hubble volume to be a fairly typical one -- at least typical among those that contain observers. A crude estimate suggests that the closest identical copy of you is about ∼ 10^(10^29) m away. About ~ 10^(10^91) m away, there should be a sphere of radius 100 light-years identical to the one centered here, so all perceptions that we have during the next century will be identical to those of our counterparts over there. About ~ 10^(10^115) m away, there should be an entire Hubble volume identical to ours.5

....

5 This is an extremely conservative estimate, simply counting all possible quantum states that a Hubble volume can have that are no hotter than 108K. 10^115 is roughly the number of protons that the Pauli exclusion principle would allow you to pack into a Hubble volume at this temperature (our own Hubble volume contains only about 10^80 protons). Each of these 10^115 slots can be either occupied or unoccupied, giving N = 2^(10^115) ~ 10^(10^115) possibilities, so the expected distance to the nearest identical Hubble volume is N^(1/3) ~ 10^(10^115) Hubble radii ~ 10^(10^115) meters. Your nearest copy is likely to be much closer than 10^(10^29) meters, since the planet formation and evolutionary processes that have tipped the odds in your favor are at work everywhere. There are probably at least 10^20 habitable planets in our own Hubble
volume alone.

By the way, I think the number of protons in our Hubble volume is closer to 10^77. 10^80 is more like the number of all particles, including photons, neutrinos, etc. But one of the nice things about working with double exponentials is that this easily fits into small rounding errors.

Cheers -- sylas

Last edited: Jan 6, 2010
16. Jan 6, 2010

### SW VandeCarr

Re: "Questionable" article in Scientific American?

What are the consequences of a flat infinite universe with more or less large scale uniformly distributed matter? Mathematically you could show that an infinite volume can expand. It can also contract to a smaller but still infinite volume because any non zero fraction of an infinite set is also infinite. But I don't see how you could preserve the Big Bang cosmology, specifically with respect to time. Does it make sense to to talk about the age of an infinite flat universe? Does the mathematical model model the physical universe insofar that it could have an infinite history?

Finally, if a flat infinite universe has an infinite history, would that mean all radiation would have been able to fill the universe? Since there is infinite matter, perhaps there should be infinite radiation too. I think that this could not be the case for humans to exist and contemplate this 'possibility'.

EDIT: I think that superluminary expansion might get around some of these consequences, but expansion from what?

Last edited: Jan 6, 2010
17. Jan 6, 2010

### sylas

Re: "Questionable" article in Scientific American?

A flat infinite universe is one of the simplest ways to have a Big Bang cosmology.

The difficulty is not with the idea of a Big Bang itself, but with the idea of an infinite universe being homogenous. Why would the initial conditions be more or less the same over an infinite universe? This was a problem even with a sufficiently large finite universe. We see galaxies on opposite sides of the sky which (when the simplest Big Bang is extrapolated backwards to the singularity) have never been in causal contact. So why is the background radiation the same all over the sky? We can see that the universe is homogenous on large scales... but WHY is it homogenous?

Inflation solves this. It proposes a very short period of time in the extremely early universe in which expansion was exponential, as if driven by a strong dark energy field that is no longer working. This rapid expansion would mean that regions on opposite sides of the sky and even far beyond would in fact has been in causal contact prior to inflation, and the homogeneity we observe was established prior to inflation, with our Hubble volume only a small part within this much larger homogeneous region.

In an infinite universe, inflation doesn't help with infinitely extended homogeneity. This calculation simply takes it for granted that the universe seems homogeneous on large scales as far as we can see, and considers the implications if that same homogeneity extends through an infinite universe.

Yes, there would be infinite radiation as well. This model proposes an infinite volume which is everywhere filled with the same background cosmic radiation, and the same large scale distribution of galaxies.

The meaning of "Big Bang" in this case is simply that at any time, the large scale density of matter and radiation through the whole infinite universe is everywhere a function of time, but not of place, and the singularity is the moment in time when density diverges to infinite, everywhere.

Cheers -- sylas

18. Jan 6, 2010

### SW VandeCarr

Re: "Questionable" article in Scientific American?

You're too fast Sylas. I was editing my post when you responded. Earlier, in a recent response to a post of mine on the topic "Flat universe?" you stated that an infinite universe was always and will always be infinite. I don't see how this is consistent with a singularity at the beginning of time and a finite history.

Last edited: Jan 6, 2010
19. Jan 6, 2010

### sylas

Re: "Questionable" article in Scientific American?

Imagine a function of time. It should be smooth, monotonically increasing, and with f(0) approaching 0 as t reduces back towards 0. Here's the simplest example of such a function:
$$f(t) = t$$​
For a more realistic function, I'll have to give the differential equation
$$\frac{df}{dt}= \sqrt{0.27 f^{-1} + 0.73 f^2}$$​
The differential equation has a problem when t=0. In fact, it has a singularity. But that's okay. We can just let the differential equation define f for positive values of t, with boundary conditions so that f(t) approaches 0 in the limit as t approaches 0. When f is very small, df/dt is large, so it aligns very closely with
$$f(t) = (1.5 t \sqrt{0.27})^{2/3}$$​
in the neighbourhood of 0.

We call this function "scale factor", and usually add a couple of constants to scale f and t as we choose, but the above functions will do.

Now. Imagine an infinite 3D cartesian space with specks laid out in an infinite 3D grid pattern, one at every point with integer co-ordinates. This represents a static infinite flat space, filled with evenly spaced galaxies.

To apply expansion, suppose that the specks are moving apart from each other, so that this is their position at a time t when f(t) = 1. Give each speck a name (x,y,z) corresponding to its co-ordinates at this time, and at all OTHER times, suppose that the speck is located at (x.f(t), y.f(t), x.f(t)).

Voila. This is expansion. The speck at location (0,0,0) never moves, and all the others move away from it, with a velocity that is the product of their distance when the scale factor was equal to 1, sqrt(x2+y2+z2), and to the function f(t).

Of course, you can also convert the co-ordinates to see where all the specks are relative to any other speck; and curiously, no matter which speck you pick as your origin, the expansion looks exactly the same. Each speck would see itself as the center of the expansion.

Also, as you run time backwards towards 0, you always have a perfectly spaced rectangular grid, with distances of f(t) between the points.

At t = 0, of course, there's a singularity. Every point is at zero; which introduces a strange kind of discontinuity. At every other time, the grid is infinite and expanding.

That's the flat Big Bang model: flat meaning we can use nice simple cartesian co-ordinates like this.

Cheers -- sylas

PS. Sorry for being so fast. I've been monitoring the forum and happened to see your post. Yes, the Big Bang involves "superluminary" expansion, in the sense that the distance between widely separated specks (or galaxies) can be greater than the speed of light; in fact there's no limit on the speed of separation. Ignore this detail; it is not actually in conflict with relativity and why it isn't is a question for another time.

Finding the path of a photon moving between specks in this simple model can be fun; pick your velocity c and just make sure the photon is always moving at this speed relative to any speck it is moving past. Don't worry about anything special in the way of relativistic conversions; they don't matter. The technique outlined in this postscript works.

Last edited: Jan 7, 2010
20. Jan 6, 2010

### SW VandeCarr

Re: "Questionable" article in Scientific American?

Thanks for your detailed responses Sylas. The main issue I have is with what I quoted above. You have this "strange kind of discontinuity" where your scaling factor seems to break down. What's the analogue of this in the standard Big Bang theory - inflation?

In any case, you seem to be going from a finite perspective to an infinite one; something that you said couldn't happen in the "Flat universe?" thread.

While the math seems to work after the strange discontinuity, does the physics work? Looking backwards in time we are going from galaxies to quarks. Are quantum level phenomenon describable in the simple expanding grid you described given the uncertainty of position/momentum?

Last edited: Jan 7, 2010