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In particle physics and physical cosmology, the Planck scale is an energy scale around 1.22 × 1028 eV (which corresponds by the mass–energy equivalence to the Planck mass 2.17645 × 10−8 kg) at which quantum effects of gravity become strong. At this scale, the description of sub-atomic particle interactions in terms of quantum field theory breaks down (due to the non-renormalizability of gravity). That is; although physicists have a fairly good understanding of the other fundamental interactions or forces on the quantum level, gravity is problematic, and cannot be integrated with quantum mechanics (at high energies) using the usual framework of quantum field theory. For energies approaching the Planck scale, an exact theory of quantum gravity is required, and the current leading candidate is string theory, or its modernized form M-theory. Other approaches to this problem include Loop quantum gravity and Noncommutative 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 a length scale or time scale.

Quantity SI equivalent
Planck time 5.39121 × 10−44 s
Planck mass 2.17645 × 10−8 kg


The Planck length is related to Planck energy by the uncertainty principle. At this scale, the concepts of size and distance break down, as quantum indeterminacy becomes virtually absolute. Because the Compton wavelength is roughly equal to the Schwarzschild radius of a black 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.a Planck particle), thus completely distorting that region of space, and swallowing the photon. This is the most extreme example possible of the uncertainty principle, and explains why only a quantum gravity theory reconciling general relativity with quantum mechanics will allow us to understand the dynamics of space-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 or Planck 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. The wavelength of a photon (and therefore its size) decreases as its frequency or energy increases. The fundamental limit for a photon's energy is the Planck energy, for the reasons cited above. This makes the Planck scale a fascinating realm for speculation by theoretical 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 a spin foam network? Is it interpenetrated by innumerable Calabi-Yau manifolds, which connect our 3-dimensional universe with a higher dimensional space? Perhaps our 3-D universe is 'sitting' on a 'brane' 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.

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 the WMAP probe, which recently accumulated sufficient data to allow scientists to probe back to the first trillionth of a second after the Big Bang, near the electroweak phase transition. This is still several orders of magnitude away from the Planck epoch, when the universe was at the Planck scale, but planned probes such as Planck Surveyor and related experiments such as IceCube expect to greatly improve on current astrophysical measurements.

Results from the Relativistic Heavy Ion Collidermarker have pushed back the particle physics frontier to discover the fluid nature of the quark-gluon plasma, and this process will be augmented by the Large Hadron Collidermarker coming online soon at CERNmarker, pushing back the 'cosmic clock' for particle physics still further. This is likely to add to the understanding of Planck scale dynamics, and sharpen the knowledge of what evolves from that state. No experiment current or planned will allow the precise probing or complete understanding of the Planck scale. Nonetheless, enough data have already been accumulated to narrow the field of workable inflationary universe theories, and to eliminate some theorized extensions to the Standard Model.

Sub-Planck physics

Sub-Planck refers to conjectural physics beyond or smaller than the Planck scale.

The Elegant Universe by Brian Greene discusses briefly the strange world of the sub-Planck and how it "creates" the quantum universe by its averages. In his later work, The Fabric of the Cosmos, Greene states that "the familiar notion of space and time do not extend into the sub-Planckian realm, which suggests that space and time as we currently understand them may be mere approximations to more fundamental concepts that still await our discovery.”

It should be made clear that sub-Planck physics is not a science because the body of knowledge is not sufficient to support a scientific method. First evidence of this limit was found in string theory in 1989 by Amati, Ciafaloni and Veneziano when they showed that distances smaller than Planck scale cannot be probed.

See also



References

  1. D Amati, M Ciafaloni, G Veneziano, “Can spacetime be probed below the string size?”, Phys Lett B216 (1989) 41.


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