Talk:Planck scale

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[edit] Comment about Natural Units

You don't need to use natural units to define the Planck scale! They really are two entirely different things! Phys

Ditto for Planck length, Planck temperature, Planck mass, etc. Phys

[edit] Article Expanded to include second definition in common usage

I added a reference for the other common usage of the term, as it arises in physical cosmology. I attempted to use similar structure as particle physics entry, but expand a bit. I'll probably add a third paragraph and some relevant external links, when I have time. There's actually a lot of new data from the cosmology side (WMAP and IceCube) which serves to illuminate Planck Scale dynamics. But the data from RHIC are voluminous, and everbody expects to see exciting stuff when the new collider comes online at CERN. Plus the recent theoretical work of Connes may show us a shortcut to understanding this domain. That is; suddenly there is a lot to say on this topic. I'll carefully add little bits, over time, to this article. JonathanD 16:34, 5 October 2006 (UTC)

[edit] General Info & Experimental Evidence sections added

I added a paragraph with general info about Planck Scale Physics, and a paragraph with some info about experimental attempts to probe the Planck scale domain. I found it deplorable there was so little on this page, prior to my augmentations, and that only the Particle Physics definition had been delineated. That usage was foreign to me, being more a cosmology guy, and I tried to do justice to all parties with this article. It's mostly general info, distilled from a large number of sources. It is my attempt to reflect the 'common knowledge' on this topic. I am prepared to cite and corroborate, but a complete list of references will have to wait. Is it time to delete the 'stub' tag? JonathanD 02:19, 8 October 2006 (UTC)

[edit] Stub Tag Removed

After sectionalizing paragraphs under indexed headings, and reviewing accuracy and completeness, I deleted the Stub tag. JonathanD 23:05, 9 October 2006 (UTC)

[edit] Corrections and re-arrangements made

Hi Jonathan. I like the new stuff you added to the article. I made several corrections and some re-arrangements. Hope you like it. Yevgeny Kats 05:46, 12 October


Thank you Yevgeny, for your contributions and edits. I greatly prefer your new heading for the section Theoretical Ideas. I like some of the other edits too, but question others. It was a noble attempt to relate the two sections at the top of page, but I wonder if it adds confusion. The usage of the term Planck Scale in Astrophysics is overwhelmingly related to its meaning as a lower limit to the Length Scale. If the reader's exposure to Cosmology is mainly from Mathematical Physics, especially String theory or Particle Physics, your edits make perfect sense. People in Astronomy, or Astrophysics, might feel slighted. To them; it is a proven fact that the universe was once immeasurably small, and some of the items you elevated to factual status remain in the realm of the reasonable but unproven assumptions. Let's continue to collaborate, as your experience appears stonger in areas I am weak, and together we'll make this into an A-rated article. JonathanD 16:08, 12 October 2006 (UTC)

[edit] Question about Initial Size of Universe being at Planck Scale

Dear Jonathan, I completely agree with you that the article should be easily accessible to people who come from astrophysics. However, I think it's a misconception on your side saying that the size of the universe was literally of the Planck length. If space is infinite, then it was always infinite, down to the Big Bang. It would be correct to say that the curvature becomes Planckian if the temperature reaches Planck temperature, and then quantum gravity is required. (Note however, that in a recent lecture [1] Hawking suggests that it's not necessarily the case, and that it's possible that the universe has never been in the Planckian regime.) Yevgeny Kats 18:54, 12 October 2006 (UTC)

Hmm.. Well, what I can glean from scanning recent papers on this topic is that the foremost experts feel that the Planck Area (or square of the Planck length) is actually a more fundamental concept and/or state, than the Planck Length, but the universe was once infinitesmal. So far as I can tell; most theorists do indeed assume that space underwent a process of evolution, in terms of the scale metric itself, and that not only the universe, but space itself once occupied a very tiny dimension at the outset. It is believed that the process of Cosmic Inflation is what brought the universe to macroscopic dimensions, with the boundaries of space expanding by a factor of 1050 from an initial size of 10-26 meters in diameter (a hundred billion times smaller than a proton) to approximately one hundred million light years (1024 m) in diameter, over a period of 10-32 seconds. This came after the Planck Era or Planck epoch, and gave rise to the essentially infinite flat space you describe.

There are several theories on the table, that address the problems of understanding this realm. Loop Quantum Gravity, Noncommutative Geometry, String theory, and M-theory all give slightly different answers to this queston, but there is some consensus that the properties of space at the lower extreme of measureable size play an important part, and set the stage for the Big Bang. In my opinion, while saying that the diameter of the universe at its inception was the Planck Length may be inaccurate, it is likely less than one order of magnitude away from the truth. Perhaps 'approximately' should be inserted. I'll look into this further, and share what I find. However, the notion that the universe once had a size near the Planck length (or Planck area), is definitely part of the common usage in Astrophysics today. JonathanD 04:49, 20 October 2006 (UTC)

[edit] More Debate on Initial Size and request for References

Regarding the question of whether the Planck scale is the initial size of the universe, the Astronomy Knowledge Base at the U of Ottowa gives this definition for the Planck Era (seems to say yes)... Planck Era - definition JonathanD 15:22, 20 October 2006 (UTC)

Do you have any books or articles that include a calculation or an argument that shows the following:
(1) The size of the universe at the beginning was approximately the Planck length.
(2) The point at which quantum gravity should become important is when the size of the universe is approximately the Planck length.
I think you won't find any of these statements, which would mean that what you wrote in the article is wrong.
(Do you believe there was some special point in the history of the universe at which it switched from being finite to infinite? Of course not. Then your statement that the universe had a finite size in the past implies that it is finite now as well. I guess you know that this is not necessarily true.)
P.S. I'm familiar with modern cosmology, including inflation and everything else, so you don't need to explain me the basics.
P.P.S. The definition from that website doesn't seem to be correct.
P.P.P.S Maybe you can make your statements correct in some way by talking about the size of the horizon rather than the size of the universe.
Yevgeny Kats 16:45, 20 October 2006 (UTC)

Well gee... The short answer is yes, but maybe no, as there is no definitive theory or measurement which allows us to make absolute statements about the Planck epoch. The lecture by Hawking was interesting, and though he states that the evidence doesn't require a mosaic and allows for the possiblity of a continuum, it hardly constututes proof. Most astrophysics texts do state that the universe itself was unbelievably small at the outset of the Big Bang. My cosmology texts (Silk - The Big Bang, and Seeds - Horizons) do make this statement explicitly, but they are quite out of date, especially after the 'Revolution' in cosmology during the last 5 years. The wikipedia section on The Big Bang does make a similar statement as well, and I've found quite a few references.

The idea of the universe being Planck-sized at its outset seems far less controversial, however, than ascribing a particular number of dimensions to the early universe. That is; we may be able to know how 'big' the universe was, but not the dimensionality of space itself. To some extent; the idea of space having an extent (infinite or otherwise)is meaningless at the Planck scale, as no measurement or observation is possible (due to the limitations of Optical Geometry). Strictly speaking, it may be more correct to speak about the horizon, rather than the extent of space or the size of the universe at the beginning. Also; the dimensionality of space itself can only be measured in relation to objects and/or observers contained therein. Ergo; it is likely that at the Planck scale space has more than 3 dimensions, but they can more properly be spoken of as degrees of freedom. In the Randall-Sundrum formulation, an infinite 5-dimensional space lives on the other side of the brane from our 3-dimensional space, and there is a literal transition from one character of space to another. It is commonly believed that this change in the character of space happens somewhere between the Planck epoch and the end of the decoupling era, with recombination (a term I abhor, because matter wasn't connected beforehand). It is widely believed that the metric of space has evolved, which accounts for expansion, but your belief that the universe was always infinite is also widely held. Some interesting facts relate. Did you know that a 5-d ball (or hypersphere) has a larger content (volume) than one of 6 dimensions? Surface area, on the other hand peaks out in a 7-dimensional space.

More Later, but I value your commentary. It goes to show how much opinions differ, often just down the hallway in the same institution, within the same Physics department. I will seek some definitive references of recent vintage. Truth is - nobody knows what's correct yet, but the article should reflect the breadth (or full range) of commonly held beliefs, or a consensus view, and my statements still match the consensus pretty well. Thanks again. JonathanD 20:41, 20 October 2006 (UTC)

[edit] Accuracy and Relevance of References challenged

Dear Jonathan, the 3 webpages that you bring as evidence do not give any support to your statement. The first webpage doesn't say a word about the size of the universe. (It only says that the Planck length is the scale on which the spacetime is expected to have some structure. This doesn't mean that the size of the universe is Planck length.) The second and the third webpages don't even include the words "Planck length". Yevgeny Kats 23:48, 20 October 2006 (UTC)

Thanks again, Yevgeny, for the lively debate, here goes...

Unique; you and I read the same words, and drew opposing conclusions. The fact is - nobody knows for sure, what the state of the universe is, at the Planck time. It's all largely conjecture based upon extrapolation of known facts. But your queries can be addressed point by point. The CERN web-site states the following. "there was no space to expand through at the beginning of time. Rather, physicists believe the Big Bang created and stretched space itself, expanding the universe." This states pretty directly that an infinite expanse of 3-d space did not exist, during the Planck epoch. In their article for Scientific American (March 2005), Lineweaver and Davis attempt to debunk some of the misconceptions people have about the Big Bang and the expansion of the universe. They state categorically that the Big Bang was not like an explosion in space, but rather an explosion of space itself. Read the technical version of this analysis at

One of their suggested sources has been a rich source of information for me, as well..

Rather than settling the score, or giving us an 'answer,' these sources and the WMAP website link add fuel to the fire, strengthening both our arguments, but calling some of our supposed facts into question. Nothing I've read, so far, conflicts with the view that the universe is Planck-sized at the Planck time, but it would be impossible to prove, whether or not it's true. Still; the idea that the universe was 10-26 meters in diameter at the outset of inflation, and underwent expansion prior to that time, is a fairly well accepted part of Inflationary Universe theories. The only way we 'know' that universe once occupied the Planck scale (dimensionally) is that it appears to have had the Planck energy at the outset, and this corresponds to a Planck-sized area of space. Rather than trying to run the clock backwards, however, we need to learn what makes it run forward, in the first place. To put it plainly; there is no way to prove the universe was Planck-sized, but there is compelling evidence, and no clear evidence to the contrary. I find our debate on this interesting and curious, because the term Planck Scale is one that I have almost always encountered as a lower limit on size, and as that dimension the universe was supposed to have occupied at the beginning of the Big Bang. The usage as a measure of energy is relatively foreign and unfamiliar. That is; I regard Planck scale to be primarily a term for the infinitesmal size of the universe at its outset, and the notion that Planck scale refers to a level of energy as a secondary definition, that follows naturally from the correspondence of small size with high energies. One of the first times I encountered the term Planck scale in the context of energy, was in this wiki article before I added to it. I don't know how you can categorically reject the Astronomy Knowledge Base definition, if you have any familiarity with cosmology whatsoever. But I acknowledge that all of the cosmology textbooks have had to be re-written at least twice, in the past 10 years. Things may have changed, in terms of the current consensus.

Still, I'd put the length scale definition first on the page, and derive the energy scale from that concept, as it is far more common in my experience, to speak of the Planck Scale that way. But for now, I'll leave that be as it is.

I'll look for some definitive references from recent texts or papers. I think John Baez has some comments on this subject that should help settle our dispute, and he is considered an authority. More Later, JonathanD 03:28, 21 October 2006 (UTC)

[edit] Assumptions, References, and confusion of Scale with Size

Regarding the idea of the Universe beginning with size at the Planck Scale - Here we go: I have conducted considerable research, and the story goes like this. For most of the last 40-50 years, the idea that the universe began with a size near the Planck length has been a central assumption of cosmology. This assumption has been the basis of many theories and equations, rather than the result of same. Perhaps the central equation justifying this assumption has been the relation setting the Schwartzchild radius equal to the Compton length, as they are roughly equal at the Planck scale. I am uncertain whether it was Planck or Hubble, who first voiced this view, but it quickly became a part of the standard Big Bang theory. More recent theories have called this assumption into question, but it is still a popular view among cosmologists and theoretical physicists, as it is so convenient in explaining the convergence of force magnitudes. Supersymmetry, universe on a wall theories, and some inflationary scenarios posit that the unification of forces may happen at a larger distance scale, typically near the electroweak boundary, which would allow for the possibility that the universe could be significantly larger at its inception, but there is no proof of this. I did a search on arxiv.org for papers containing the words Planck Scale and I came up with a lot more info. BTW - the majority of those papers employed the usage of Planck scale as a distance measurement, and not one of energy. Especially helpful is the following paper, which seems to explicitly state that my assumption is correct, or is widely believed in Physics today. "In the latest approaches such as Quantum Strings (or M-theory) or Quntum Gravity theories, it is generally accepted that the Planck Scale defines a minimum scale for the universe" (Several references cited here). physics/0509026 Is that good enough for you? I have a bunch more, if you still have questions, or dispute my orginal wording. I note it's been changed already, but no harm is done. I may elect to restore the reference, however, if it has been deleted entirely, as I believe my claim is proven factual. JonathanD 21:34, 21 October 2006 (UTC)

Jonathan, there is a difference between a scale and a size. I'll give you an example: suppose you have a piece of material and you want to describe its elastic properties. Suppose you're applying all kind of stresses on the material, and describe how it deforms. If you're trying to curve it such that the radius of the curvature is comparable to the size of an atom, your usual description of material's elasticity will break down. And it will be completely meaningless to discuss a situation is which the radius of curvature is several orders of magnitude smaller than the size of the atom. In this example, the size of the atom is the minimal scale of the elastic material. Similarly, when people talk about the minimum scale for the universe, they talk about the situation in which the curvature radius of the universe is of the order of the Planck length. It doesn't mean that the size of the universe is equal to the Planck length when such curvature exists, exactly like it would be incorrect to say that the size of the elastic material is the size of the atom when you curve the material on such scale. (In conclusion, you haven't presented yet any reference that says that the size of the universe at the beginning was of the order of the Planck length.) Yevgeny Kats 22:43, 21 October 2006 (UTC)

[edit] Why is Planck length related to Planck energy via Uncertainty Principle?

A question for Yevgeny: Why is the Planck length related to Planck energy by the uncertainty principle? Isn't it much simpler to use the wavelength of a photon? JonathanD 22:09, 21 October 2006 (UTC)

The description using the wavelength of a photon is roughly the same as the uncertainty principle argument. However, there is nothing special about the photon, it works for any particle. Yevgeny Kats 22:43, 21 October 2006 (UTC)

I disagree, as no other particle known decreases in size with an increase of energy. The dimension of sub-atomic particles is fixed by a balance of forces (the strong and weak force, specifically). Your statement that it "works for any particle" seems almost absurd, but I invite you to explain. JonathanD 03:18, 22 October 2006 (UTC)

You are right that for composite particles the relation works only up to the scale of their compositeness. Still, there are many particles that you can use besides the photon: all the gauge bosons, quarks, leptons, etc. Yevgeny Kats 04:30, 23 October 2006 (UTC)

[edit] Claim that References were Rejected without Counter-Evidence

Regarding the initial size of the universe, I did present the following reference, which you chose to reject out of hand.

You have not cited any references which refute this, and I think perhaps you won't find any. Several knowledgeable parties have stated that anyone would be crazy to make a definitive statement regarding the universe at the Planck time, but some go on to describe what is sensible. I have discovered some dissenting opinions about my original statement, but only after considerable research. I invite you to find your own counter-example, or to cite any credible reference which you feel refutes my statement about the initial size of the universe being at or near the Planck scale in size. JonathanD 03:36, 22 October 2006 (UTC)

P.S. - I am continuing my search for an acceptably definitive reference to confirm my statement. I will make it known, once I have found a text of recent enough vintage (5 years old or less) to be current. So far as your arguments go, they may raise doubts, but do not disprove my point. I will not correct your mis-statements until I have an alternative which is more precisely correct, however. JonathanD 04:12, 22 October 2006 (UTC)

[edit] References Section will be added

I will be creating a section for citation of References to support claims made in this article. I would ask all editors to the Planck Scale entry to pay careful attention to syntax. The references are embedded in the body of the article, but will appear in the References section.

Thanks,

JonathanD 21:46, 22 October 2006 (UTC)

[edit] Planck-sized elegant universe

JonathanD wrote on my talk page:

Thank you Yevgeny, for taking me to the wall on this issue, and making me prove my point.... However; it seems I was exactly right all along. In "The Elegant Universe" Brian Greene echoes almost precisely my original statement (inserting the conditional term 'known' universe) in the Chapter reflections on cosmology (pp. 350 in the paperback) he states. "In a cosmological context, this occurred (breakdown of QFT) when the whole of the known universe fit within a Planck-sized nugget". A few pages later the sub-Heading reads "In the Beginning There was a Planck-sized Nugget" and he takes the time to explain this in detail. Continuing on that page, he states "At the beginning moment of the universe, all of the dimensions of string theory are on a completely equal footing--they are completely symmetric--all curled up into a multidimensional Planck-sized nugget." Q.E.D. !!! User:JonathanD 19:23, 22 October 2006 (UTC)
Dear Jonathan, I agree that it is a plausible picture in the framework of string theory, and should be presented as such. However, as you probably know, the details of how to apply string theory to cosmology are not known (yet), and this picture is just a reasonable guess. (Furthermore, there are serious directions of research which do incorporate string theory, but in which the Planck-scale cosmology is different or even non-existent, like was described in Hawking's seminar in CERN that I referred to earlier on this talk page). In conclusion, it wouldn't be appropriate to present this statement as an established truth (and definitely not as a definition!), but as a plausible scenario in the framework of string theory (and possibly in a number of non-stringy models). Yevgeny Kats 05:08, 23 October 2006 (UTC)
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