Everything about Planck Scale totally explained
In
particle physics and
physical cosmology, the
Planck scale is an
energy scale around 1.22 × 10
28 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 can't 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's 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.
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 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's 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've 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. Recently; results from the
Relativistic Heavy Ion Collider 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 Collider coming online soon at
CERN, 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've already accumulated enough data 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 hypothetical, speculative, and 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 don't 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.”
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