- Planck scale
In

particle physics andphysical cosmology , the**Planck scale**is anenergy scale around 1.22 × 10^{28}eV (which corresponds by themass–energy equivalence to thePlanck mass 2.17645 × 10^{−8}kg) at which quantum effects ofgravity become strong. At this scale, the description of sub-atomic particle interactions in terms ofquantum field theory breaks down (due to the non-renormalizability of gravity). That is; although physicists have a fairly good understanding of the otherfundamental interactions or forces on the quantum level,gravity is problematic, and cannot be integrated withquantum mechanics (at high energies) using the usual framework of quantum field theory. For energies approaching the Planck scale, an exact theory ofquantum gravity is required, and the current leading candidate isstring theory , or its modernized formM-theory . Other approaches to this problem includeLoop quantum gravity andNoncommutative geometry . At the Planck scale, the strength of gravity is expected to become comparable to the other forces, and it is theorized that all the fundamental forces are unified at that scale, but the exact mechanism of this unification remains unknown.The term

**Planck scale**can also refer to alength scale or time scale.The

Planck length is related toPlanck energy by theuncertainty principle . At this scale, the concepts of size and distance break down, asquantum indeterminacy becomes virtually absolute. Because theCompton wavelength is roughly equal to theSchwarzschild radius of ablack hole at the Planck scale, a photon with sufficient energy to probe this realm would yield no information whatsoever. Any photon energetic enough to precisely measure a Planck-sized object could actually "create" a particle of that dimension, but it would be massive enough to immediately become a black hole (a.k.aPlanck particle ), thus completely distorting that region of space, and swallowing the photon. This is the most extreme example possible of theuncertainty principle , and explains why only aquantum gravity theory reconcilinggeneral relativity withquantum mechanics will allow us to understand the dynamics ofspace-time at this scale. Planck scale dynamics is important for cosmology because if we trace the evolution of the cosmos back to the very beginning, at some very early stage the universe should have been so hot that processes involving energies as high as the Planck energy (corresponding to distances as short as the Planck length) may have occurred. This period is therefore called the Planck era orPlanck epoch .**Theoretical ideas**The nature of reality at the Planck scale is the subject of much debate in the world of

physics , as it relates to a surprisingly broad range of topics. It may, in fact, be a fundamental aspect of the universe. In terms of size, the Planck scale is unimaginably small (many orders of magnitude smaller than a proton). In terms of energy, it is unimaginably 'hot' and energetic. Thewavelength of aphoton (and therefore its size) decreases as itsfrequency or energy increases. The fundamental limit for a photon's energy is thePlanck energy , for the reasons cited above. This makes the Planck scale a fascinating realm for speculation bytheoretical physicists from various schools of thought. Is the Planck scale domain a seething mass of virtual black holes? Is it a fabric of unimaginably fine loops or aspin foam network? Is it interpenetrated by innumerableCalabi-Yau manifolds,cite book |last = Greene |first = Brian | authorlink = Brian Greene |title = "The Elegant Universe " |pages = pp. 207–208 |id = ISBN 0-375-70811-1] which connect our 3-dimensional universe with a higher dimensional space? Perhaps our 3-D universe is 'sitting' on a 'brane ' [*cite paper*] which separates it from a 2, 5, or 10-dimensional universe and this accounts for the apparent 'weakness' of gravity in ours. These approaches, among several others, are being considered to gain insight into Planck scale dynamics. This would allow physicists to create a unified description of all the fundamental forces.

first = Nima

last = Arkani-Hamed

authorlink = Nima Arkani-Hamed

coauthors =Savas Dimopoulos ,Gia Dvali , Nemanja Kaloper

title = Manyfold Universe

date = 1999-11-17

url = http://arxiv.org/abs/hep-ph/9911386

accessdate = 2007-07-20**Experiments probing the Planck Scale**Experimental evidence of Planck scale dynamics is difficult to obtain, and until quite recently was scant to non-existent. Although it remains impossible to probe this realm directly, as those energies are well beyond the capability of any current or planned

particle accelerator , there possibly was a time when the universe itself achieved Planck scale energies, and we have measured the afterglow of that era with instruments such as theWMAP probe, which recently accumulated sufficient data to allow scientists to probe back to the first trillionth of a second after theBig Bang , near theelectroweak phase transition. This is still several orders of magnitude away from thePlanck epoch , when the universe was at the Planck scale, but planned probes such asPlanck Surveyor and related experiments such asIceCube expect to greatly improve on current astrophysical measurements. Recently; results from theRelativistic Heavy Ion Collider have pushed back the particle physics frontier to discover the fluid nature of thequark-gluon plasma , and this process will be augmented by theLarge Hadron Collider coming online soon atCERN , pushing back the 'cosmic clock' for particle physics still further. This may add to our understanding of Planck scale dynamics, and sharpen our knowledge of what evolves from that state. No experiment current or planned, however, will allow us to precisely probe or completely understand the Planck scale. Nonetheless, we have already accumulated enough data to narrow the field of workable inflationary universe theories, and to eliminate some theorized extensions to theStandard Model .**ub-Planck physics****Sub-Planck**refers to hypothetical, speculative, and conjectural physics beyond or smaller than the**Planck scale**.**ee also***

Planck units

*Planck epoch

*Planck particle

*Quantum gravity **References****External links*** [

*http://www.phys.unsw.edu.au/einsteinlight/jw/module6_Planck.htm The Planck scale: relativity meets quantum mechanics meets gravity*] from 'Einstein Light' at UNSW

* [*http://csep10.phys.utk.edu/astr162/lect/cosmology/planck.html The Planck Era*] from U of Tennessee Astrophysics pages

* [*http://math.ucr.edu/home/baez/planck/ Higher-Dimensional Algebra and Planck-Scale Physics*] byJohn C. Baez

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