This is NOT an original post. I am just re-posting the content of the original blog.
In 1900, the British
physicist Lord Kelvin is said to have pronounced: "There is nothing new to
be discovered in physics now. All that remains is more and more precise
measurement." Within three decades, quantum mechanics and Einstein's
theory of relativity had revolutionized the field. Today, no physicist would
dare assert that our physical knowledge of the universe is near completion. To
the contrary, each new discovery seems to unlock a Pandora's box of even
bigger, even deeper physics questions. These are our picks for the most
profound open questions of all.
What is dark energy?
No matter how astrophysicists
crunch the numbers, the universe simply doesn't add up. Even though gravity is
pulling inward on space-time — the "fabric" of the cosmos — it keeps
expanding outward faster and faster. To account for this, astrophysicists have
proposed an invisible agent that counteracts gravity by pushing space-time
apart. They call it dark energy. In
the most widely accepted model of dark energy, it is a "cosmological
constant": an inherent property of space itself, which has "negative
pressure" driving space apart. As space expands, more space is created,
and with it, more dark energy. Based on the observed rate of expansion,
scientists know that the sum of all the dark energy must make up more than 70
percent of the total contents of the universe. But no one knows how to look for
it.
What is dark matter?
Evidently, about 84 percent
of the matter in the universe does not absorb or emit light. "Dark
matter," as it is called, cannot be seen directly, and it hasn't yet been
detected by indirect means, either. Instead, dark matter's existence and
properties are inferred from its gravitational effects on visible matter,
radiation and the structure of the universe. This shadowy substance is thought
to pervade the outskirts of galaxies, and may be composed of "weakly
interacting massive particles," or WIMPs. Worldwide, there are several
detectors on the lookout for WIMPs, but so far, not one has been found.
Why is there an arrow of time?
Time moves forward because a
property of the universe called "entropy," roughly defined as the
level of disorder, only increases, and so there is no way to reverse a rise in
entropy after it has occurred. The fact that entropy increases is a matter of
logic: There are more disordered arrangements of particles than there are
ordered arrangements, and so as things change, they tend to fall into disarray.
But the underlying question here is, why was entropy so low in the past? Put
differently, why was the universe so ordered at its beginning, when a huge
amount of energy was crammed together in a small amount of space?
Are there parallel universes?
Astrophysical data suggests space-time
might be "flat," rather than curved, and thus that it goes on
forever. If so, then the region we can see (which we think of as "the
universe") is just one patch in an infinitely large "quilted
multiverse." At the same time, the laws of quantum mechanics dictate that
there are only a finite number of possible particle configurations within each
cosmic patch (10^10^122 distinct possibilities). So, with an infinite number of cosmic patches, the particle
arrangements within them are forced to repeat — infinitely many times over.
This means there are infinitely many parallel universes: cosmic patches
exactly the same as ours (containing someone exactly like you), as well as
patches that differ by just one particle's position, patches that differ by two
particles' positions, and so on down to patches that are totally different from
ours.
Is there something wrong with that
logic, or is its bizarre outcome true? And if it is true, how might we ever
detect the presence of parallel universes?
Why is there more matter than antimatter?
The question of why there is
so much more matter than its oppositely-charged and oppositely-spinning twin, antimatter,
is actually a question of why anything exists at all. One assumes the universe
would treat matter and antimatter symmetrically, and thus that, at the moment
of the Big Bang, equal amounts of matter and antimatter should have been
produced. But if that had happened, there would have been a total annihilation
of both: Protons would have canceled with antiprotons, electrons with
anti-electrons (positrons), neutrons with antineutrons, and so on, leaving
behind a dull sea of photons in a matterless expanse. For some reason, there
was excess matter that didn't get annihilated, and here we are. For this, there
is no accepted explanation.
What is the fate of the universe?
The fate of the universe strongly
depends on a factor of unknown value: Ω, a measure of the density of matter and
energy throughout the cosmos. If Ω is greater than 1, then space-time would be
"closed" like the surface of an enormous sphere. If there is no dark
energy, such a universe would eventually stop expanding and would instead start
contracting, eventually collapsing in on itself in an event dubbed the
"Big Crunch." If the universe is closed but there is dark energy, the spherical universe
would expand forever.
Alternatively, if Ω is less than 1,
then the geometry of space would be "open" like the surface of a
saddle. In this case, its ultimate fate is the "Big Freeze" followed
by the "Big Rip": first, the universe's outward acceleration would
tear galaxies and stars apart, leaving all matter frigid and alone. Next, the
acceleration would grow so strong that it would overwhelm the effects of the
forces that hold atoms together, and everything would be wrenched apart.
If Ω = 1, the universe would be flat,
extending like an infinite plane in all directions. If there is no dark energy,
such a planar universe would expand forever but at a continually decelerating
rate, approaching a standstill. If there is dark energy, the flat universe
ultimately would experience runaway expansion leading to the Big Rip.
Que sera, sera.
How do measurements collapse quantum wavefunctions?
In the strange realm of electrons,
photons and the other fundamental particles, quantum mechanics is law.
Particles don't behave like tiny balls, but rather like waves that are
spread over a large area. Each particle is described by a
"wavefunction," or probability distribution, which tells what its
location, velocity, and other properties are more likely to be, but not what
those properties are. The particle actually has a range of values for all
the properties, until you experimentally measure one of them — its location,
for example — at which point the particle's wavefunction "collapses"
and it adopts just one location.
But how and why does measuring a
particle make its wavefunction collapse, producing the concrete reality that we
perceive to exist? The issue, known as the measurement problem, may seem
esoteric, but our understanding of what reality is, or if it exists at all,
hinges upon the answer.
Is string theory correct?
When physicists assume all
the elementary particles are actually one-dimensional loops, or
"strings," each of which vibrates at a different frequency, physics
gets much easier. String theory allows physicists to reconcile the laws
governing particles, called quantum mechanics, with the laws governing
space-time, called general relativity, and to unify the four fundamental forces
of nature into a single framework. But the problem is, string theory can only
work in a universe with 10 or 11 dimensions: three large spatial ones, six or
seven compacted spatial ones, and a time dimension. The compacted spatial
dimensions — as well as the vibrating strings themselves — are about a
billionth of a trillionth of the size of an atomic nucleus. There's no
conceivable way to detect anything that small, and so there's no known way to
experimentally validate or invalidate string theory.
Is there order in chaos?
Physicists can't exactly
solve the set of equations that describes the behavior of fluids, from water to
air to all other liquids and gases. In fact, it isn't known whether a general
solution of the so-called Navier-Stokes equations even exists, or, if there is
a solution, whether it describes fluids everywhere, or contains inherently
unknowable points called singularities. As a consequence, the nature of chaos
is not well understood. Physicists and mathematicians wonder, is the weather
merely difficult to predict, or inherently unpredictable? Does turbulence
transcend mathematical description, or does it all make sense when you tackle
it with the right math?
