| Topic: |
Science > Philosophy |
| User: |
"Frederick" |
| Date: |
30 Nov 2003 03:50:29 PM |
| Object: |
On the Weakness of Gravity |
THEORETICAL PHYSICS: ON THE WEAKNESS OF GRAVITY
ScienceWeek http://www.scienceweek.com
The following points are made by Jonathan L. Feng (Science 2003
302:795):
1) Of the four known fundamental forces -- gravity,
electromagnetism, and the weak and strong forces -- gravity is by
far the weakest. The reasons for this weakness have long remained
enigmatic. Recent proposals suggest, however, that the weakness
of gravity may be evidence for extra spatial dimensions.
Experiments ranging from tabletop tests of Newtonian gravity to
searches for microscopic black holes in kilometer-scale detectors
are now putting these ideas to the test.
2) The importance of gravity in everyday life results not from
its strength but from its universality: Objects cannot be
gravitationally neutral, and all bodies with mass attract. Yet as
an interaction between elementary particles, gravity is extremely
weak. For example, the gravitational attraction between two
protons is 35 orders of magnitude weaker than their
electromagnetic repulsion. This holds for protons separated by
any distance r, because both gravitational and electromagnetic
forces are proportional to 1/r^(2).
3) The observed weakness of gravity may, however, not be an
intrinsic property of gravity, but may instead be an effect of
extra spatial dimensions. This possibility is based on a simple
consideration. Suppose that our three-dimensional (3D) world is
merely a subspace of a higher-dimensional space, and that gravity
propagates freely in all dimensions, but that all other forces
are confined to our three dimensions. In contrast to the familiar
three dimensions, the extra dimensions are curled up in small
circles of circumference (L). Hence, moving a distance L in the
direction of any of the extra dimensions brings one back to one's
starting place.
4) Now suppose that at some separation distance (r < L), gravity
is strong, that is, comparable to electromagnetism. As r
increases, the electromagnetic force drops as 1/r^(2). However,
the gravitational field spreads out in all available spatial
dimensions, and the gravitational force therefore decreases much
more rapidly as 1/r^(2+n), where n is the number of extra
dimensions. This rapid drop continues until r > L, at which point
the extra dimensions become less and less important and gravity
recovers its 1/r^(2) behavior.
5) If this picture is correct, then gravity is not intrinsically
weak: It is as strong as electromagnetism at small length scales.
It appears weak at the relatively large distances of common
experience only because its effects are diluted by propagation in
extra dimensions. The distance at which the gravitational and
electromagnetic forces might have equal strength is unknown, but
a particularly interesting possibility is that it is 10^(-19) m,
the distance at which the electromagnetic and weak forces are
known to unify to form the electroweak force (1-5).
References (abridged):
1. N. Arkani-Hamed, S. Dimopoulos, G. R. Dvali, Phys. Lett. B
429, 263 (1998)
2. E. G. Adelberger et al, http://arXiv.org/abs/hep-ex/0202008
(2002)
3. S. Cullen, M. Perelstein, Phys. Rev. Lett. 83, 268 (1999)
4. L. J. Hall, D. R. Smith, Phys. Rev. D 60, 085008 (1999)
5. S. B. Giddings, S. Thomas, Phys. Rev. D 65, 056010 (2002)
Science http://www.sciencemag.org
--------------------------------
THEORETICAL PHYSICS: THE WEAKNESS OF GRAVITY
According to Newton's law of gravitation, there is a force of
attraction between any two massive particles in the Universe.
This force of attraction, as stated by Newton, may be expressed
as a simple relationship involving the masses of the two
particles and the distance between them. When the two masses are
unit masses and the distance between their centers of mass is
unit distance, then the force of attraction is equal to what is
called the "gravitational constant", usually denoted as "G". The
gravitational constant is usually regarded as a true universal
constant independent of place or time, but in some cosmological
models it is proposed that the gravitational constant decreases
with time as the Universe expands.
The following points are made by Frank Wilczek (Physics Today
2001 June):
1) Gravity dominates the large-scale structure of the Universe,
but only by default: matter arranges itself to cancel
electromagnetism, and the *strong and weak forces are
intrinsically short range. At a more fundamental level, gravity
is extremely weak: acting between protons, gravitational
attraction is approximately 10^(-36) times weaker than electrical
repulsion. The author asks: "Where does this outlandish disparity
come from? What does it mean?"
2) The author points out that these questions greatly disturbed
Richard Feynman (1918-1988). In 1963, in Feynman's famous paper
on quantizing general relativity, the paper in which he first
described his discovery of the "ghost particles" that eventually
played a crucial role in understanding modern *gauge field
theories, Feynman noted that the correct problem is to understand
what determines the size of gravitation.
3) Wilczek points out that the same question drove Paul Dirac
(1902-1984) to consider, 30 years before Feynman, the radical
idea that the fundamental "constants" of nature are time
dependent, so that the weakness of gravity could be related to
the great age of the Universe. Dirac's argument was that the
expansion rate of the Universe suggests that it began with a bang
approximately 10^(17) seconds ago. On the other hand, the time it
takes light to cross the diameter of a proton is approximately
10^(-24) seconds, which provides a ratio, 10^(-41), which is not
so far from the mysterious 10^(-36). But the age of the Universe,
of course, changes with time, so if the numerological coincidence
is to abide, something else -- the relative strength of gravity
or the size of protons -- will have to change in proportion.
Since there are powerful experimental constraints on such
effects, Dirac's idea is not easy to reconcile with standard
modern theories of cosmology and fundamental interactions,
theories which are extremely successful.
4) Wilczek discusses the dimensionless number N = Gm^(2)/hc,
where (G) is Newton's constant, (m) is the mass of the proton,
(h) is Planck's constant, and (c) is the speed of light.
Substituting measured values, we find N is approximately 3 x
10^(-39), and Wilczek notes: "This is what we mean when we say
gravity is extravagantly feeble." But the real problem, Wilczek
suggests, is to understand the smallness of (N).
5) Wilczek then proposes that an understanding of the smallness
of N can be derived from an understanding of the smallness of the
mass of the proton, and in particular from the constraints
imposed by the *theory of quantum chromodynamics on the coupling
constants between quarks, the fundamental components of the
proton. Essentially, the inter-quark coupling force increases
with distance between quarks, which provides a powerful
constraint on the size and mass of the proton. The smallness of
N, therefore, is an apparent natural consequence of the theory of
quantum chromodynamics.
Physics Today http://www.physicstoday.org
--------------------------------
Notes:
strong and weak forces: The weak forces are the forces
responsible for the change of neutrons and protons into each
other in radioactive processes and in the stars. The strong
forces are the forces that hold quarks together inside protons
and neutrons, and that hold protons and neutrons together inside
atomic nuclei.
gauge field theories: Quantum field theory is the mathematical
fusion of quantum mechanics with special relativity theory, and
there are essentially 2 branches: quantum electrodynamics
(applicable to charged particles involved in electromagnetic
interactions) and quantum chromodynamics (applicable to nuclear
particles involved in strong force interactions). In this
context, a "gauge theory" is any quantum field theory which has
the property of "gauge symmetry", i.e., the equations describing
the field do not change when some operation is applied to all
particles everywhere in space. In general, fields with gauge
symmetry can be remeasured (regauged) from different baselines
without affecting their properties. Quantum electrodynamics and
quantum chromodynamics are examples of gauge theories.
theory of quantum chromodynamics: Quantum chromodynamics (QCD) is
a theory that describes the strong interaction (strong nuclear
force) in terms of quarks and antiquarks and the exchange of
massless "gluons" between them. The "chromo-" in chromodynamics
derives from the use of designated "color" attributes of quarks,
the various "colors" labels for various quark properties.
--------------------------------
ON QUANTUM GRAVITY
The following points are made by J.F. Hawley and K.A. Holcomb
(citation below):
1) Gravity is by far the weakest force in the Universe: in the
hydrogen atom, the electromagnetic force between the proton and
electron is about 10^(40) times as great as the gravitational
force between them. This is fairly representative of the
difference in scales between the quantum and gravitational
realms, and accounts for our ability [in cosmology] to separate
the two theories without ambiguity. Yet they must inevitably
meet. Near a singularity, the curvature of space-time must be so
great that the scale of gravity becomes comparable to that of the
other fundamental forces. To describe such a state, we must find
a theory of quantum gravity. Moreover, quantum mechanics has
already been applied to the explanation of the other three
forces, the electromagnetic force and the strong and weak
interactions; should not gravity be similar?
2) It might seem as though the challenge of developing quantum
gravity should not be so great. After all, special relativity and
quantum mechanics were united in the 1920s by the British
physicist Paul A.M. Dirac. The most significant result of Dirac's
theory was its requirement that antiparticles exist, a prediction
that was confirmed in 1932 by the discovery of the positron (the
anti-electron). The Dirac theory is now well-established as the
special relativistic quantum mechanics. More than 70 years later,
however, general relativity has still not been successfully
incorporated into a consistent quantum formulation.
Adapted from: J.F. Hawley and K.A. Holcomb: Foundations of Modern
Cosmology. Oxford University Press 1998, p.441. More information
at:
http://www.amazon.com/exec/obidos/ASIN/01951049768/scienceweek
ScienceWeek http://www.scienceweek.com
--
Best,
Frederick Martin McNeill
Poway, California, United States of America
mmcneill@fuzzysys.com
http://www.fuzzysys.com
http://members.cox.net/fmmcneill/
*************************
Phrase of the week :
"One of the symptoms of approaching nervous
breakdown is the belief that one's work is
terribly important."
-- Bertrand Russell (1872-1970)
:-))))Snort!)
*************************
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