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