7,722 research outputs found
Life, The Universe, and Nothing: Life and Death in an Ever-Expanding Universe
Current evidence suggests that the cosmological constant is not zero, or that
we live in an open universe. We examine the implications for the future under
these assumptions, and find that they are striking. If the Universe is
cosmological constant-dominated, our ability to probe the evolution of large
scale structure will decrease with time ---presently observable distant sources
will disappear on a time-scale comparable to the period of stellar burning.
Moreover, while the Universe might expand forever, the integrated conscious
lifetime of any civilization will be finite, although it can be astronomically
long. We find that this latter result is far more general. In the absence of
possible exotic and uncertain strong gravitational effects, the total
information recoverable by any civilization over the entire history of our
universe is finite, and assuming that consciousness has a physical
computational basis, life cannot be eternal.Comment: 23 pages, latex, submitted to Ap.
Universal Limits on Computation
The physical limits to computation have been under active scrutiny over the
past decade or two, as theoretical investigations of the possible impact of
quantum mechanical processes on computing have begun to make contact with
realizable experimental configurations. We demonstrate here that the observed
acceleration of the Universe can produce a universal limit on the total amount
of information that can be stored and processed in the future, putting an
ultimate limit on future technology for any civilization, including a
time-limit on Moore's Law. The limits we derive are stringent, and include the
possibilities that the computing performed is either distributed or local. A
careful consideration of the effect of horizons on information processing is
necessary for this analysis, which suggests that the total amount of
information that can be processed by any observer is significantly less than
the Hawking-Bekenstein entropy associated with the existence of an event
horizon in an accelerating universe.Comment: 3 pages including eps figure, submitted to Phys. Rev. Lett; several
typos corrected, several references added, and a short discussion of w <-1
adde
Simulations of Electron Acceleration at Collisionless Shocks: The Effects of Surface Fluctuations
Energetic electrons are a common feature of interplanetary shocks and
planetary bow shocks, and they are invoked as a key component of models of
nonthermal radio emission, such as solar radio bursts. A simulation study is
carried out of electron acceleration for high Mach number, quasi-perpendicular
shocks, typical of the shocks in the solar wind. Two dimensional
self-consistent hybrid shock simulations provide the electric and magnetic
fields in which test particle electrons are followed. A range of different
shock types, shock normal angles, and injection energies are studied. When the
Mach number is low, or the simulation configuration suppresses fluctuations
along the magnetic field direction, the results agree with theory assuming
magnetic moment conserving reflection (or Fast Fermi acceleration), with
electron energy gains of a factor only 2 - 3. For high Mach number, with a
realistic simulation configuration, the shock front has a dynamic rippled
character. The corresponding electron energization is radically different:
Energy spectra display: (1) considerably higher maximum energies than Fast
Fermi acceleration; (2) a plateau, or shallow sloped region, at intermediate
energies 2 - 5 times the injection energy; (3) power law fall off with
increasing energy, for both upstream and downstream particles, with a slope
decreasing as the shock normal angle approaches perpendicular; (4) sustained
flux levels over a broader region of shock normal angle than for adiabatic
reflection. All these features are in good qualitative agreement with
observations, and show that dynamic structure in the shock surface at ion
scales produces effective scattering and can be responsible for making high
Mach number shocks effective sites for electron acceleration.Comment: 26 pages, 12 figure
Old Galaxies at High Redshift and the Cosmological Constant
In a recent striking discovery, Dunlop {\bf \it et al} observed a galaxy at
redshift z=1.55 with an estimated age of 3.5 Gyr. This is incompatible with age
estimates for a flat matter dominated universe unless the Hubble constant is
less than . While both an open universe, and a universe
with a cosmological constant alleviate this problem, I argue here that this
result favors a non-zero cosmological constant, especially when considered in
light of other cosmological constraints. In the first place, for the favored
range of matter densities, this constraint is more stringent than the globular
cluster age constraint, which already favors a non-zero cosmological constant.
Moreover, the age-redshift relation for redshifts of order unity implies that
the ratio between the age associated with redshift 1.55 and the present age is
also generally larger for a cosmological constant dominated universe than for
an open universe. In addition, structure formation is generally suppressed in
low density cosmologies, arguing against early galaxy formation. The additional
constraints imposed by the new observation on the parameter space of vs
(where ) are derived for both
cosmologies. For a cosmological constant dominated universe this constraint is
consistent with the range allowed by other cosmological constraints, which also
favor a non-zero value.Comment: latex, 10 pages, including two embedded postscript figure
The Original Understanding of the Seventh Amendment Right to Jury Trial
I ought to be very angry with my friend Akhil Amar. His new book, The Bill of Rights: Creation and Reconstruction, strengthens, develops, and popularizes his strikingly original claim that the meaning of our Bill of Rights must be sought in the understanding of the people who enacted the Fourteenth Amendment, rather than that of James Madison and his contemporaries. If Akhil carries the day on this question-and I find his arguments quite powerful, my ongoing research into the original meaning of the Bill will be of interest only to antiquarians
Adjusting to Life in Mexico â One Coupleâs Adaptation Process and Suggestions for Coping
In this paper I present challenges which confronted Sharon Beckett, my partner, and me as we underwent the process of adapting to life in Mexico during a three month stay. Some of the issues are unique to a couple with our respective back grounds, but, many are applicable to any couple or individual traveling to Mexico or other foreign countries. I discuss some guidelines and principles of interacting which proved helpful to us in Mexico and also offer some suggestions for coping to couples or individuals going abroad. The primary source of data for the paper is a daily journal which I kept in Mexico throughout our stay
Representing the Community: A Look at the Selection Process in Obscenity Cases and Capital Sentencing
- âŠ