http://www.thebigview.com/spacetime/quantumtheory.html
Quantum theory
evolved as a new branch of theoretical physics during the first few decades
of the 20th century in an endeavour to understand the fundamental properties
of matter. It began with the study of the interactions of matter and
radiation. Certain radiation effects could neither be explained by classical
mechanics, nor by the theory of electromagnetism. In particular, physicists
were puzzled by the nature of light. Peculiar lines in the spectrum of
sunlight had been discovered earlier by Joseph von Fraunhofer (1787-1826).
These spectral lines were then systematically catalogued for various substances,
yet nobody could explain why the spectral lines are there and why they would
differ for each substance. It took about one hundred years, until a plausible
explanation was supplied by quantum theory.
In contrast to
Einstein's Relativity, which is about the largest things in the universe,
quantum theory deals with the tiniest things we know, the particles that
atoms are made of, which we call "subatomic" particles. In contrast
to Relativity, quantum theory was not the work of one individual, but the
collaborative effort of some of the most brilliant physicists of the 20th
century, among them Niels Bohr, Erwin Schrödinger, Wolfgang Pauli, and Max
Born. Two names clearly stand out: Max Planck (1858-1947) and Werner
Heisenberg (1901-1976). Planck is recognised as the originator of the quantum
theory, while Heisenberg formulated one of the most eminent laws of quantum
theory, the Uncertainty Principle, which is occasionally also referred to as
the principle of indeterminacy.
Around 1900, Max
Planck from the University of Kiel concerned himself with
observations of the radiation of heated materials. He attempted to draw
conclusions from the radiation to the radiating atom. On basis of empirical
data, he developed a new formula which later showed remarkable agreement with
accurate measurements of the spectrum of heat radiation. The result of this
formula was so that energy is always emitted or absorbed in discrete units, which
he called quanta.
Planck
developed his quantum theory further and derived a universal constant, which
came to be known as Planck's constant. The resulting law states that the
energy of each quantum is equal to the frequency of the radiation multiplied
by the universal constant: E=f*h, where h is 6.63 * 10E-34 Js. The discovery
of quanta revolutionised physics, because it contradicted conventional ideas
about the nature of radiation and energy.
To understand the
gist of the quantum view of matter, we have to go back to the 19th century's
predominant model of matter. Scientists at the time believed -like the Greek
atomists- that matter is composed of indivisible, solid atoms, until
Rutherford proved otherwise.
The British
physicist Ernest Rutherford (1871-1937) demonstrated experimentally that the
atom is not solid as previously assumed, but that it has an internal
structure consisting of a small, dense nucleus about which electrons circle
in orbits.
Niels
Bohr (1885-1962) refined Rutherford's model by introducing different orbits in which electrons spin around
the nucleus. This model is still used in chemistry. Elements are
distinguished by their "atomic number", which specifies the number
of protons in the nucleus of the atom. Electrons are held in their orbits
through the electrical attraction between the positive nucleus and the negative
electron. Bohr argued that each electron has a certain fixed amount of
energy, which corresponds to its fixed orbit. Therefore, when an electron
absorbs energy, it jumps to the next higher orbit rather than moving
continuously between orbits. The characteristic of electrons having fixed
energy quantities (quanta) is also known as the quantum theory of the atom.
The above model
bears a striking similarity with the Newtonian model of our solar system.
Electrons revolve around the nucleus, just as planets revolve around the Sun.
It is therefore not surprising that physicists tried to apply classical
mechanics to the atomic structure. The forces between nucleus and electrons
were equated with the gravitational forces between celestial bodies. This
idea worked quite well for the hydrogen atom, the simplest of all elements,
but it failed to explain the behaviour of more complex atoms.
The idea that
energy could be emitted or absorbed only in discrete energy quanta seemed
odd, since it could not be fitted into the traditional framework of physics.
The quantum behaviour of electrons in atoms contradicted not only classical
mechanics, but also Maxwell's electromagnetic theory, which required it to
radiate away energy while orbiting in a quantum energy state. Even Max
Planck, who was a conservative man, initially doubted his own discovery. The
traditional view was that energy flows in a continuum like a smooth, unbroken
stream of water. That there should be gaps between the discrete entities of
energy seemed wholly unreasonable. In fact, Planck's idea only gained
credence when Einstein used it in 1905 to explain the photoelectric effect. -
After all, if matter is not infinitely divisible, why should energy be?
In the course of
time, physicists descended deeper into the realm of the atom. Bohr's atom
model was remarkably successful in describing the spectrum of the hydrogen
atom by using Planck's formula to relate different energy levels of electrons
to different frequencies of light radiation. Unfortunately, it did not work
well for more complex atoms, and so a more sophisticated theory had to be
developed. The problem seemed to be rooted in the assumption that an electron
rotates around the nucleus like a massive object revolves around a centre of
gravity. De Broglie, Schrödinger, and Heisenberg showed that classical
mechanics had to be abandoned in order to describe the subatomic world
adequately. In an inference not less dramatic than Planck's discovery of quanta,
they stated that particles don't really have a trajectory or an orbit, much
less do they behave like a ball that is shot through a corridor or is whirled
around on the end of a cord.
Just as light is
thought to have a dual nature, sometimes showing the characteristic of a
wave, and sometimes that of a particle (photon), quantum theory attributes a
similar dual wave-particle nature to subatomic particles. Electrons that
orbit around the nucleus interact with each other by showing interference
patterns, not unlike those of wave interference. If the velocity of the
electron is thought of as its wavelength, the crests of neighbouring electron
waves amplify or cancel each other, thereby creating a pattern that
corresponds to Bohr's allowed orbits.
Bohr's
model of the atom was superseded by the probability cloud model that
describes physical reality better. The orbital clouds are mathematical
descriptions of where the electrons in an atom are most likely to be found,
which means the model shows the spatial distribution of electrons. The
(simplified) picture to the left shows electron probability clouds in a water
molecule.
Even cloud models are only approximations. The computation of the actual
distribution of electrons in an atom is extremely laborious and the result is
too complicated to be illustrated in a single layer 3D model.
The nature of
electrons seems odd. Seemingly they exist in different places at different
points in time, but it is impossible to say where the electron will be at a
given time. At time t1 it is at point A, then at time t2 it is at point B,
yet without moving from A to B. It seems to appear in different places
without describing a trajectory. Therefore, even if t1 and A can be
pinpointed, it is impossible to derive t2 and B from this measurement. In
other words: There seems to be no causal relation between any two positions.
The concept of causality cannot be applied to what is observed. In case of
the electrons of an atom, the closest we can get to describing the electron's
position is by giving a number for the probability of it being at a
particular place. Moreover, particles have other "disturbing"
properties: They have a tendency to decay into other particles or into
energy, and sometimes -under special circumstances- they merge to form new
particles. They do so after indeterminate time spans. Although we can make
statistical assertions about a particle's lifetime, it is impossible to
predict the fate of an individual particle.
Can we derive any
new knowledge about the universe from quantum physics? After all, the entire
universe is composed of an unimaginable large number of matter and energy. It
seems to be of great importance to understand quantum theory properly in view
of the large-scale structure of the cosmos. For example, an interesting
question in this context is why the observable matter in the universe is
packed together in galaxies and is not evenly distributed throughout space.
Could it have to do with the quantum characteristics of energy? Are quantum
effects responsible for matter forming discrete entities, instead of
spreading out evenly during the birth of the universe? The answer to this
question is still being debated.
If cosmological
conclusions seem laboured, we might be able to derive philosophical insights
from quantum physics. At least Fritjof Capra thinks this is possible when he
describes the parallels between modern physics and ancient Eastern philosophy
in his book The Tao of Physics. He holds that in a way, the essence of modern
physics is comparable to the teachings of the ancient Eastern philosophies,
such as the Chinese Tao Te Ching, the Indian Upanishads, or the Buddhist
Sutras. Eastern philosophies agree in the point that ultimate reality is
indescribable and unapproachable, not only in terms of common language, but
also in the language of mathematics. That is, science and mathematics must
fail at some stage in describing ultimate reality. We see this exemplified in
the Uncertainty Principle, which is elucidated in the following section.
The oriental
scriptures agree in the point that all observable and describable realities
are manifestations of the same underlying "divine" principle.
Although many phenomena of the observable world are seemingly unrelated, they
all go back to the same source. Things are intertwined and interdependent to
an unfathomable degree, just as the particles in an atom are. Although the
electrons in an atom can be thought of as individual particles, they are not
really individual particles, because of the complicated wave relations that
exist between them. Hence, the electron cloud model describes the atomic
structure more adequately. The sum of electrons in an atom cannot be
separated from its nucleus, which has a compound structure itself and can
neither be regarded a separate entity. Thus, in the multiplicity of things
there is unity. Matter is many things and one thing at the same time.
The Eastern
scriptures say that no statement about the world is ultimately valid
("The Tao that can be told is not the eternal Tao." Tao Te Ching,
Verse 1), since not even the most elaborate language is capable of rendering
a perfect model of the universe. Science is often compared to a tree that
branches out into many directions. The disposition of physics is that it
follows the tree upward to its branches and leaves, while meta-physics
follows it down to the root. Whether the branches of knowledge stretch out
indefinitely is still a matter of debate. However, it appears that most
scientific discoveries do not only answer questions, but also raise new ones.
The German
philosopher, FriedrichHegel formulated an idea at the beginning of the 19th
century that describes this process. He proposed the dialectic triad of
thesis, antithesis, and synthesis, in which an idea (thesis) always contains
incompleteness and thus yields a conflicting idea (antithesis). A third point
of view (synthesis) arises, which overcomes the conflict by reconciling the
truth contained in both, thesis and antithesis, at a higher level of
understanding. The synthesis then becomes a new thesis, generates another
antithesis, and the process starts over. In the next section, we shall see
how 20th century physics embodies Hegel's dialectical principle. We will also
take a close look at the philosophical implications of Heisenberg's
Uncertainty Principle.
At a time when
Einstein had gained international recognition, quantum theory culminated in
the late 1920’s statement of the Uncertainty Principle, which says that the more precisely the position of a particle is
determined, the less precisely the momentum is known in this instant, and
vice versa. The above phrasing of the principle is a succinct
version of the mathematically precise uncertainty relation that Heisenberg
published in 1927. Since the momentum of a particle is the product of its
mass and velocity, the principle is sometimes stated differently, however,
its meaning remains the same: The act of measuring one magnitude of a
particle, be it its mass, its velocity, or its position, causes the other
magnitudes to blur. This is not due to imprecise measurements. Technology is
advanced enough to hypothetically yield correct measurements. The blurring of
these magnitudes is a fundamental property of nature.
Click on this button to hear
Heisenberg explaining his uncertainty principle. (.au, 176 kb)
The uncertainty
relation describes the "blur" between the measurable quantities of
a particle in mathematical terms. Like much of the math in quantum theory, it
is not for the faint of heart, which is to say it is completely
unintelligible to most people. Therefore we restrict ourselves to a brief
account on the underlying ideas and how they developed into the
"Copenhagen Interpretation", which Niels Bohr and Werner Heisenberg
jointly elaborated as a complete and consistent view of quantum mechanics
(the Copenhagen Interpretation refers to Bohr's place of birth).
Around 1925 there
were two competing mathematical theories that both attempted to explain
electron orbits. Matrix mechanics developed by Heisenberg interprets the
electron as a particle with quantum behaviour. It is based on sophisticated
matrix computations, which introduce discontinuities and quantum jumps. In
contrast, wave mechanics developed by Erwin Schrödinger interprets the
electron as an energy wave. Because wave mechanics entails more familiar
concepts and equations, it quickly gained popularity among scientists.
Schrödinger and
Heisenberg were no too fond of each other's competing works. Schrödinger says
about matrix mechanics: "I knew of [Heisenberg's] theory, of course, but
I felt discouraged, not to say repelled, by the methods of transcendental
algebra, which appeared difficult to me, and by the lack of
visualisability." Heisenberg's comment on wave mechanics was: "The
more I think about the physical portion of Schrödinger's theory, the more
repulsive I find it. [...] What Schrödinger writes about the visualisability
of his theory 'is probably not quite right,' in other words it's crap."
Despite the
differences, Schrödinger published a proof in 1926, which showed that the
results of matrix and wave mechanics are equivalent; they were in fact the
same theory. According to the Copenhagen Interpretation, the wave and
particle pictures of the atom, or the visual and causal representations, are
"complementary" to each other. That is, they are mutually
exclusive, yet jointly essential for a complete description of quantum
events. Obviously in an experiment in the everyday world an object cannot be
both a wave and a particle at the same time; it must be either one or the
other, depending on the situation. In later refinements of this interpretation,
the wave function of the unobserved object is a mixture of both, the wave and
particle pictures, until the experimenter chooses what to observe in a given
experiment.
The German
physicist Werner Heisenberg (1901-1976) received the Nobel Prize in physics
in 1932 for his work in nuclear physics and quantum theory. The paper on the
uncertainty relation is his most important contribution to physics.
Heisenberg
impressed his teachers with his ambition and brilliance. He never produced
other grades than straight A's, except on one occasion: During his doctorate,
professor Wien of the university of Munich gave him an F in experimental physics, because he handled the laboratory
equipment clumsily. Reportedly this left Heisenberg so disconcerted that he
did not speak to anyone for days.
Fate had it that a few years later, Heisenberg demonstrated the very
limitations of experimental physics, which unquestionably constituted a
setback for its advocates, including Professor Wien.
The notion of the
observer becoming a part of the observed system is fundamentally new in
physics. In quantum physics, the observer is no longer external and neutral,
but through the act of measurement he becomes himself a part of observed
reality. This marks the end of the neutrality of the experimenter. It also
has huge implications on the epistemology of science: certain facts are no longer
objectifiable in quantum theory. If in an exact science, such as physics, the
outcome of an experiment depends on the view of the observer, then what does
this imply for other fields of human knowledge? It would seem that in any
faculty of science, there are different interpretations of the same
phenomena. More often than occasionally, these interpretations are in
conflict with each other. Does this mean that ultimate truth is unknowable?
The results of
quantum theory, and particularly of Heisenberg's work, left scientists
puzzled. Many felt that quantum theory had somehow "missed the
point". At least Albert Einstein did so. He was an outspoken critic of
quantum mechanics and is often quoted on his comment regarding the
Uncertainty Principle: "The Old One (God) doesn't play dice." He
also said: "I like to believe that the moon is still there even if we
don't look at it." In particular, Einstein was convinced that electrons
do have definite orbits, even if we cannot observe them. In a conversation
with Heisenberg he said:
Heisenberg:
"One cannot observe the electron orbits inside the atom. [...] but since
it is reasonable to consider only those quantities in a theory that can be
measured, it seemed natural to me to introduce them only as entities, as
representatives of electron orbits, so to speak."
Einstein:
"But you don't seriously believe that only observable quantities should
be considered in a physical theory?"
"I thought
this was the very idea that your Relativity Theory is based on?"
Heisenberg asked in surprise.
"Perhaps I
used this kind of reasoning," replied Einstein, "but it is nonsense
nevertheless. [...] In reality the opposite is true: only the theory decides
what can be observed."
(translated from
"Der Teil und das Ganze" by W. Heisenberg)
We can easily see
the rift between Einstein's intuitive and Heisenberg's empirical approach.
Although Einstein's argumentation appears tricky, it is clear that he
believes in a reality independent of what we can observe, which is in essence
the view of realism. Kant's "thing in itself" comes to mind. - In
contrast, Heisenberg believes that reality is what can be observed. If there
are different observations, there must be different realities, which depend
on the observer. Insofar Heisenberg can be regarded as an advocate of
philosophical idealism, which states that the objects of perception are
identical with the ideas we have about them. The idealist view denies that
any particular thing has an independent real essence outside of
consciousness.
The two
philosophies seem incompatible at first. Heisenberg is in good company with
famous contenders of idealistic positions, such Plato, Schopenhauer, and
Husserl, but so is Albert Einstein. If we take Heisenberg's view for granted,
strict causality is broken, or better: the past and future events of
particles are indeterminate. One cannot calculate the precise future motion
of a particle, but only a range of possibilities. Physics loses its grip. The
dream of physicists, to be able to predict any future event in the universe
based on its present state, meets its certain death.
If we regard
reality as that which can be observed by all, we have to find that there is
no objective movement of an electron around the nucleus. This viewpoint would
imply that reality is created by the observer; in other words: if we take
Heisenberg literally, the moon is not there when nobody is looking at it.
However, we must consider the possibility that there is a subatomic reality
independent of observation and that the electron may have an actual
trajectory which cannot be measured. The moon may be there after all. This
conflict is the philosophical essence of the Uncertainty Principle.
Relativity and
quantum theory are inconsonant up to the present day, despite great efforts
in creating a unified theory capable of accommodating both views. After
having published his papers on Relativity, Einstein dedicated the rest of his
life to working on such a unified field theory, yet without success. The
physicists who followed his lead developed a new model called string theory
during the 1970s and 1980s. String theory was successful to some extent in
providing a mathematical model that integrates the strong and the weak
nuclear forces, electromagnetism, and gravitation. In spite of this, it
cannot yet be called a breakthrough, because (1) the theory has not been
corroborated thoroughly by observational evidence; and (2) there is not one,
but five competing string theories. The latter point has recently been
addressed by M-theory, a theory that unites existing string theories in 11
dimensions.
We shall leave the
problem of theoretical unification to the physicists and instead briefly
consider a philosophical unification of Relativity and quantum theory. Is
this possible? Contemplating the subatomic realm seems like a Zen exercise.
The nuclear reality embodies duality and multiplicity, such as is evident in
the complicated structure of atoms and particles. It transgresses the narrow
world of opposites. We have to realise that in spite of the different parts
and components, the subatomic world in actuality is an undivided whole, where
the boundary between the observer and the observed is blurred. Object and
subject have become inseparable, spatial and temporal detachment is an
illusion. When the American physicist J.R. Oppenheimer (1902-1967) describes
the structure of probability clouds, he almost sounds like a Zen Master: "If
we ask, whether the position of the electron remains the same, we have to say
no. If we ask, whether the position of an electron changes with the course of
time, we have to say no. If we ask, whether the electron is in a state of
rest, we have to say no. If we ask, whether the electron is in motion, we
have to say no."
In the beginning,
the Earth was flat. At least it appeared so to its first observers, hunters
and gatherers, and members of early civilisations. Not totally unreasonable,
one would think, because the curvature of our planet's surface is not
immediately apparent. Yet we know, and it must have been not totally
inconceivable even to the archaic tribesmen, that our senses occasionally
deceive us. The Earth being flat brings about the problem that it must end
somewhere, unless we imagine it to extend infinitely. Infinity is a rather
unfathomable conception and, hence, right down to the Middle Ages people were
afraid of the possibility of falling off the Earth's boundaries.
What lies beyond
these boundaries was largely unknown and open to speculation. The starry
heavens were a source of endless wonder and inspiration. Peoples from all
parts of the world created their own myths, inspired by the skies and the
celestial bodies. Their cosmogonies can be seen as an attempt to explain
their own place in the universe. Six thousand years ago, the Sumerians
believed that the Earth is at the centre of the cosmos. This belief was later
carried into the Babylonian and Greek civilisations.
According to the
history books, it was the Greeks who first put forward the idea that our
planet is a sphere. Around 340 BC, the Greek philosopher Aristotle made a few
good points in favour of this theory in On the Heavens. First, he argued that
one always sees the sails of a ship coming over the horizon first and only
later its hull, which suggests that the surface of the ocean is curved.
Second, he realised that the eclipses of the Moon were caused by the Earth
casting its shadow on the moon. Obviously, the shadow would not always appear
round, if the Earth was a flat disk, unless the Sun was directly under the
centre of the disk. Third, from their travels to foreign countries, the
Greeks knew that the North Star appears higher on the northern firmament and
lower in the south. Aristotle explained this correctly with the parallactic
shift that occurs when moving between two observation points on a spherical
object. Among the Greeks, the heliocentric system was proposed by the
Pythagoreans and by Aristarchus of Samos (ca. 270 BC). However, Aristotle
dismissed the case for heliocentrism.
The influence of
Aristotle was significant. Around 150 AD, Claudius Ptolemaeus (Ptolemy)
elaborated Aristotle's ideas into a complete cosmological model. He thought
that the Earth was stationary at the centre of the universe and that the Sun,
the stars, and all planets revolve around it in circular orbits, hence, the
model is sometimes referred to as the geocentric system. Ptolemy was aware
that the postulation of perfect circular orbits contradicted observation,
because the planets' motion, size and brightness varied with time. To account
for the observed deviations, he introduced the idea of epicycles, smaller
circular orbits around imaginary centres on which planets were supposed to
move while describing a revolution around Earth. This enabled astronomers to
make reasonably accurate predictions about the movement of the celestial
bodies, and consequently the Ptolemaic model was a great success. The system
was later adopted by the Christian Church and became the dominant cosmology
until the 16th century.
Ptolemy's model of the universe was that of
an onion with the Earth at its centre and stars arranged in layers around it.
The outer layer was thought to be like a crystal to which the fix stars were
attached. The hypothesis of epicycles accounted for the observable
deviations.
In 1514 the Polish
astronomer Nicolaus Copernicus (1473-1543) put forward an alternative model,
referred to as the heliocentric system, in which the Sun is at the centre of
the universe, and all planets, including Earth, revolve around it. The
further apart a planet is from the Sun, the longer it takes to complete a
revolution. Copernicus said that the ostensible movement of the Sun is caused
by the Earth rotating around its north-to-south axis. The heliocentric system
got rid of Ptolemy's obscure epicycles, whose main weakness was that they did
neither account for the observed backward motion of Mars, Jupiter, and
Saturn, nor for the fact that Mercury and Venus never moved more than a certain
distance from the Sun. Unfortunately, the Copernican system was not
inherently simpler than the geocentric system; and it did not immediately
render more accurate calculations of the planet's motion.
The end of the
Ptolemaic theory came with the invention of the telescope. With the help of
this device, Galileo Galilei (1564-1642) discovered the four largest Jupiter
moons. The existence of these moons demonstrated beyond doubt that not all
celestial bodies revolve around the Earth, contrary to Ptolemy’s theory.
Galileo confirmed the Copernican model and thus initiated a scientific
revolution of great importance, much to the discontent of the Roman Catholic
Church. Unsurprisingly, Galileo struggled with church authorities during much
of his lifetime. In 1594 the German astronomer Johannes Kepler (1571-1630)
refined the heliocentric model in his book Mysterium Cosmographicum by
showing that planets move on elliptical, rather than circular orbits. Kepler
also prepared the idea of gravity by explaining that the Sun exerts a force
on planets that diminishes inversely with distance and causes them to move
faster on their orbits, the closer they come to the Sun. This theory finally
allowed predictions that matched observations.
Kepler’s model
became the accepted 17th century cosmology, until Isaac Newton further
refined Kepler's notion of the forces between celestial bodies. Newton postulated the law of
universal gravitation that applied to all bodies, whether in space or on
Earth, and he supplied the mathematical foundation for it. According to Newton, bodies attract each other
proportionally with their size and inverse proportionally with the square of
the distance between them. He went on to demonstrate that according to this
law, planets move on elliptical orbits, as previously assumed by Kepler.
Unfortunately, one consequence of this theory is that the stars of the
universe attract each other and thus must eventually collapse onto each other.
Newton was not able to give a plausible explanation for why
this did not happen.
To counter this
paradox, it was inferred that the universe is infinite in space, and thus
contains an infinite number of evenly distributed stars, which would on the
whole create a gravitational equilibrium. This assumption, however, would
still imply instability. If the balance is disturbed in one region of space,
the nearest stars collapse and the gravitational pull of the resulting more
massive body draws in the next cluster of stars. Clusters would collapse like
a house of cards and eventually draw in the entire universe. Today we know
that this is not the case, because the universe is not static as Newton thought. The cosmos is in a
state of expansion and therefore, gravitational collapse is prevented.
The question of
whether the universe has boundaries in time and space has captivated the
imagination of mankind since early times. Some would say the universe had
existed forever, while others would say that the universe was created and
thus had a beginning in time and space. The second thesis immediately raises
the question what exists beyond its temporal and spatial bounds. Could it be
nothingness? But then, what is nothingness? The absence of matter, or the
absence of space and time itself? The German philosopher Immanuel Kant
(1724-1804) dealt intensively with this question. In his book Critique of
Pure Reason he came to the conclusion that the question cannot be answered
reliably within the limits of human knowledge, since thesis and antithesis
are equally valid. Kant thought instead of time and space as fundamental
aspects of human perception.
Fast forward:
Despite Kant's doubts thereto, it appears that modern cosmology has answered
the above question. The universe we can observe is finite. It has a beginning
in space and time, before which the concept of space and time has no meaning,
because spacetime itself is a property of the universe. According to the Big
Bang theory, the universe began about twelve to fifteen billion years ago in a violent
explosion. For an incomprehensibly small fraction of a second, the universe
was an infinitely dense and infinitely hot fireball. A peculiar form of energy
that we don't know yet, suddenly pushed out the fabric of spacetime in a
process called "inflation", which lasted for only one millionth of
a second. Thereafter, the universe continued to expand but not nearly as
quickly. The process of phase transition formed out the most basic forces in
nature: first gravity, then the strong nuclear force, followed by the weak
nuclear and electromagnetic forces. After the first second, the universe was
made up of fundamental energy and particles like quarks, electrons, photons,
neutrinos and other less familiar particles.
About 3 seconds
after the Big Bang, nucleosynthesis set in with protons and neutrons
beginning to form the nuclei of simple elements, predominantly hydrogen and
helium, yet for the first 100,000 years after the initial hot explosion there
was no matter of the form we know today. Instead, radiation (light, X rays,
and radio waves) dominated the early universe. Following the radiation era,
atoms were formed by nuclei linking up with free electrons and thus matter
slowly became dominant over energy. It took 200 million years until
irregularities in the primordial gas began to form galaxies and early stars
out of pockets of gas condensing by virtue of gravity. The Sun of our solar
system was formed out of such a pocket of gas in a spiral arm of the Milky
Way galaxy roughly five billion years ago. A vast disk
of gas and debris swirling around the early Sun gave birth to the planets,
including Earth, which is between 4.6 and 4.5 billion years old. This is -in
short- the history of our universe according to the Big Bang theory, which
constitutes today's most widely accepted cosmological viewpoint.
A number of
different observations corroborate the Big Bang theory. Edwin Hubble
(1889-1953) discovered that galaxies are receding from us in all directions.
He observed shifts in the spectra of light from different galaxies, which are
proportional to their distance from us. The farther away the galaxy, the more
its spectrum is shifted towards the low (red) end of the spectrum, which is
in some way comparable to the Doppler effect. This redshift indicates
recession of objects in space, or better: the ballooning of space itself.
Today, there is convincing evidence for Hubble's observations. Projecting
galaxy trajectories backward in time means that they converge to a
high-density state, i.e. the initial fireball.
If two intelligent life forms in two different galaxies look at each other’s
galaxy, they perceive the same thing. The light of the other galaxy appears
redshifted in comparison to nearer objects. This is caused by ballooning
space that stretches the wavelength of emitted light. The magnitude of this
effect is proportional to the distance of the observed galaxy.
According to the
Copernican cosmological principle, the universe appears the same in every
direction from every point in space, or in more scientific terms: The universe
is homogeneous and isotropic. There is overwhelming evidence for this
assertion. The best evidence is provided by the almost perfect uniformity of
the cosmic background radiation. This observed radiation is isotropic to a
very high degree and is thought to be a remnant of the initial Big Bang
explosion. The background radiation originates from an era of a few hundred
thousand years after the Big Bang, when the first atoms where formed. Another
piece of evidence speaking in favour of Big Bang is the abundance of light
elements, like hydrogen, deuterium (heavy hydrogen), helium, and lithium. Big
Bang nucleosynthesis predicts that about a quarter of the mass of the
universe should be helium-4, which is in good agreement with what is
observed.
On basis of our
understanding of the past and present universe, we can speculate about its
future. The prime question is whether gravitational attraction between
galaxies will one day slow the expansion and ultimately force the universe into
contraction, or whether it will continue to expand and cool forever. The
current rate of expansion (Hubble Constant) and the average density of the
universe determine whether the gravitational force is strong enough to halt
expansion. The density required to halt expansion (=critical density) is 1.1
* 10^-26 kg per cubic meter, or six hydrogen atoms per cubic meter; the
relation "actual density" / "critical density" is called
Omega. With Omega less than 1, the universe is called "open", i.e.
forever expanding. If Omega is greater than 1 the universe is called
"closed", which means that it will contract and eventually collapse
in a Big Crunch. In the unlikely event that Omega = 1, the expansion of the
universe will asymptotically slow down until it becomes virtually
imperceptible, but it won't collapse.
Some scientists
think it not impossible that the universe is oscillating between eras of
expansion and contraction, where every Big Bang is followed by a Big Crunch.
Stephen Hawking (born 1942) pointed out the possibility that such an
oscillating universe must not necessarily start and end in singularities,
i.e. questionable points in spacetime where physical theories, such as
General Relativity, break down while energy and density levels approximate
infinity. Although everything points towards Big Bang, the future reversal
and contraction of the universe is rather uncertain. Big Crunch is at most a
hypothesis, because only about 1/100th of the matter needed for Omega=1 can
be observed.
In spite of this,
galaxies and star clusters behave as if they would contain more matter than
we can see. It is almost as if these objects were engulfed by invisible
matter. This "dark matter" that cannot be accounted for is one of
the open questions in cosmology. Dark matter makes is thought to make up 23%
of the universe.
Today, most
cosmologists believe there is not enough matter in the universe to halt and
revert expansion. Robert Caldwell of Dartmouth University has recently suggested a
third alternative for the fate of the universe. His Big Rip scenario is based
on astronomical observations made in the late 1990s according to which a
mysterious force, labelled dark energy, is responsible for the expansion of
the universe. Dark energy makes up 73% of the universe. If the rate of
acceleration increases, there will be a point in time at which the repulsive
force becomes so strong that it overwhelms gravity and the other fundamental
forces. According to Caldwell, this will happen in 20 billion years. "The
expansion becomes so fast that it literally rips apart all bound
objects," Caldwell explains. "It rips apart
clusters of galaxies. It rips apart stars. It rips apart planets and solar
systems. And it eventually rips apart all matter." Even atoms would be
torn apart in the last 10-19 seconds before the end of time. –Whether or not
this scenario will become true is to be decided by future research. Until
then, the field is open to speculation.
Physics has
answered many questions about space, time, and matter. Thanks to
technological advances, we have been able to look deeper and deeper into the
large-scale structure of the universe and the small-scale structure of
matter. From the invention of the telescope to the time of particle
accelerators, insight and understanding have grown. Yet, there are still many
unsolved mysteries. The contemporary models of matter, space, and time are
incomplete and our picture of the world still has holes. Some of today's most
challenging questions in physics are:
There seems to be a
halo of mysterious invisible material engulfing galaxies, which is commonly
referred to as dark matter. Scientists infer the existence of dark
(=invisible) matter from the observation of its gravitational pull, which
causes the stars in the outer regions of a galaxy to orbit faster than they
would if there was only visible matter present. Another indication is that we
see galaxies in our own local cluster moving towards each other.
The Andromeda
galaxy -about 2.2 million light years away from the Milky Way- is speeding
toward us at 200,000 miles per hour. This motion can only be explained by
gravitational attraction, even though the mass we observe is not nearly great
enough to exert that kind of pull. It follows there must be a large amount of
unseen mass causing the gravitational pull -roughly equivalent to ten times
the size of the Milky Way- lying between the two galaxies.
Astronomers have no
idea what the dark matter is that supposedly makes up 23% of all matter in
our universe. Black holes and massive neutrinos are two possible
explanations. Dark matter must have played an important role in galaxy
formation during the evolution of the cosmos. But, even taking into account
all known and suspected black holes, there seems to be much more matter out
there than we can presently see or extrapolate.
Dark energy is
perhaps even more mysterious than dark matter. The discovery of dark energy
goes back to 1998 when a 10-year study of supernovae took an astonishing
turn. A group of scientists had recorded several dozen supernovae, including
some so distant that their light had started to travel towards Earth when the
universe was only a fraction of its present age. The group's goal was to
measure small changes in the expansion rate of the universe, which in turn
would yield clues to the origin, structure, and fate of the cosmos. Contrary
to their expectation, the scientists found that the expansion of the universe
is not slowing, but accelerating.
The acceleration is
supposedly due to the anti-gravitational properties of the so-called dark
energy. While the exact nature of this energy is presently unknown,
scientists agree that dark energy is the dominant constituent of our
universe, which means that it is larger than the sum of visible and dark
matter. Einstein already postulated an anti-gravitational force at the
beginning of the 20th century. He acknowledged that the observed matter would
lead to gravitational collapse, and hence, introduced a cosmological constant
to bring Relativity into line with observation. After it was discovered by
Hubble that the universe is expanding, Einstein called his cosmological
constant the greatest blunder of his life.
Yet, at the
beginning of the 21st century it seems that anti-gravity is coming back with
vengeance. A possible explanation is that the energy content of a vacuum is
non-zero with a negative pressure. This negative pressure of the vacuum would
grow in strength as the universe expands and it would cause the expansion to
accelerate. If the acceleration does not stop, this will lead to the Big Rip
scenario suggested by Caldwell, in which the universe will
be literally torn apart by the anti-gravitational force in several billion years.
Stephen Hawking
says in the foreword of The Cosmos Explained (Cambridge, July 28, 1997):
"At the Big Bang, the universe and time itself came into existence, so
that this is the first cause. If we could understand the Big Bang, we would
know why the universe is the way it is. It used to be thought that it was
impossible to apply the laws of science to the beginning of the universe, and
indeed that it was sacrilegious to try. But recent developments in unifying
the two pillars of twentieth-century science, Einstein's General Theory of
Relativity and the Quantum Theory, have encouraged us to believe that it may
be possible to find laws that hold even at the creation of the universe. In
that case, everything in the universe would be determined by the laws of
science. So if we understood those laws, we would in a sense be masters of
the universe."
It is uncertain
whether mankind is able to develop such a theory in the near future, and it
may be even more questionable whether this knowledge would indeed help us to
become masters of the universe, as Stephen Hawking connotes. Obviously it is
difficult to speculate on a theory that has not been developed yet. The
theory might as well have no practical value at all. The great 20th century
physical theories showed us that complexity and abstraction are growing,
while intelligibility and practical applicability are decreasing. From a
unified physical theory we can expect a more complete picture of matter,
space, and time and a better understanding of the beginning of the universe.
It may satisfy our curiosity in view of some big philosophical questions. Any
practical value beyond this is rather uncertain.
The theory of
gravity as formulated by Einstein is incompatible with the rules of quantum
mechanics. Physicists encounter serious difficulties when trying to construct
a quantum version of gravity. In the later years of his life, Einstein tried
but failed to devise a theory that unifies gravity with quantum theory. In
the 1960s, the weak nuclear force was united with electromagnetism to form
the electroweak theory, which was subsequently verified in particle accelerator
experiments. The next step is to create a model that unites the other
fundamental forces.
Theorists are
working on such a model, which they call grand unified theory (GUT). It
amalgamates electromagnetism with the weak and strong nuclear interaction, but
omits gravity. From GUT we expect the answer to why particles have the masses
we observe. Although we observe the masses of electrons, protons, and
neutrons generated through what is called "electroweak breaking,"
we don't know how this breaking mechanism works. GUT should be able to
interpret the electroweak breaking process and thus provide an explanation
for the mass of a particle.
Beyond GUT, there
is a theory that accounts for all four fundamental forces in nature,
including gravity. The greatest endeavour of physics is to draw hitherto
unrelated and incompatible theories together into a single unified theory.
The advantage of such a system is obvious: It would account for all currently
known phenomena without leaving theoretical holes and it may point towards
future areas of study. It is hypothesised that such a theory could create a
new fundamental understanding of nature. String theory, supersymmetry, and
M-theory are some candidates currently considered.
Presently it is not
known whether quarks and leptons are elementary or compound particles. It
seems that physicists have become more careful with announcing the
fundamentality of particles after having learned that atoms, atom cores, and
finally protons and neutrons are divisible. What is more, quarks and leptons
are so small that they may be thought of as geometrical points in space with
no spatial extension at all. This is perhaps not as miraculous as it first
sounds, because after having learned from Rutherford's model that the volume of
an atom is mostly made of "empty" space, it would not be too
surprising to find out that matter is in fact nothing but empty space.
While the commonly
accepted standard model of matter provides a very good description of the
phenomena observed in experiments, the model is still incomplete. It can
explain the behaviour of particles fairly well, but it cannot explain why
some particles exist as they do. For example, it has been impossible to
predict the mass of the top quark accurately from theoretical inference until
it was determined experimentally. As mentioned before, the standard model of
matter does not provide any mathematical model that allows us to calculate the
observed mass.
Another question
concerns the fact that there are three families of quarks and leptons. Of the
three families (or generations) of particles, only the first is stable,
namely that of up/down quarks, e-neutrinos, and electrons. There seems to be
no need for the other two generations in the natural world, yet they exist.
Theoretical physics has no explanation for the existence of the two unstable
generations. Likewise, the question why there is hardly any antimatter in the
observable universe remains unaccounted for. Since there is an almost perfect
symmetry between matter and antimatter, one would expect some regions of the
universe to be composed of matter and others of antimatter, yet almost all
mass we can observe is composed of conventional matter.
Andrei Linde at
Stanford has brought forward the cosmological model of a multiverse, which he
calls the "self-reproducing inflationary universe." The theory is
based on Alan Guth's inflation model, and it includes multiple universes
woven together in some kind of spacetime foam. Each universe exists in a
closed volume of space and time. Linde's model, based on advanced principles
of quantum physics, defies easy visualisation. Quite simplified, it suggests
quantum fluctuations in the universe's inflationary expansion period to have
a wavelike character. Linde theorises that these waves can "freeze"
atop one another, thus magnifying their effect.
The stacked-up
quantum waves can in turn create such intense disruptions in scalar fields
-the underlying fields that determine the behaviour of elementary particles-
that they exceed a critical mass and start procreating new inflationary
domains. The multiverse, Linde contends, is like a growing fractal, sprouting
inflationary domains, with each domain spreading and cooling into a new
universe.
If Linde is
correct, our universe is just one of the sprouts. The theory neatly straddles
two ancient ideas about the universe: that it had a definite beginning, and
that it had existed forever. In Linde's view, each particular part of the
multiverse, including our part, began from a singularity somewhere in the
past, but that singularity was just one of an endless series that was spawned
before it and will continue after it.
Some physicists
believe that a complete physical model can explain everything we observe.
They hold that once the fundamental laws are known and powerful computers
allow us to compute models of the world by applying these laws, we can
eventually deduce explanations for all phenomena. In other words, physics can
lead us to understanding ultimate reality. Is this really possible?
One may doubt it.
Even if we give physicists credit for their remarkable discoveries, we have
to realise that their research takes place in an isolated field of knowledge.
Physics does not concern itself with issues outside its own domain. For
example, the subjects of biology, life, and chemistry, as well as the
phenomena of mind and consciousness cannot be explained in physical terms. In
addition, the following fundamental questions arise:
1. Physics deals
only with what can be measured. A complete physical model must therefore
necessarily produce a materialistic view of reality. Although materialists
usually deny the possibility that phenomena exist which cannot be measured or
somehow quantified, they may actually exist.
2. There are limits
to what can be measured, as demonstrated by the Uncertainty Principle.
3. The materialist
view is generally allied with reductionism. Materialists often claim that
high-level phenomena, such as biological or psychological phenomena, can be
reduced to physical phenomena. However, this is far from being obvious. For
example, there is no generally accepted reductionist theory of consciousness.
Reductionism fails in most practical cases. For example, it is practically
impossible to describe the process of DNA replication in terms of
subatomic properties.
4. Advanced
physical models are abstract to the degree of being unintelligible to most
people. Modern physics is based on higher mathematics and can hardly be put
into common language, much less can it be imagined. The multidimensional
worlds of Relativity and string theory, for example, are elusive to plastic
imagination. The value of any science depends on how useful its models are
for the thoughts and actions of humanity as a whole, hence, its usefulness
leans partly on intelligibility.
Light is a
phenomenon that has particle and wave characteristics. Its carrier particles
are called photons, which are not really particles, but massless discrete
units of energy.
The speed of light
is 299,792,458 m/s in a vacuum. The symbol used in Relativity for the speed
of light is "c", which probably stands for the Latin word
"celeritas", meaning swift.
The speed of light
is constant by definition in the sense that it is independent of the
reference frame of the observer. Light travels slightly slower in a
transparent medium, such as water, glass, and even air.
No. In Relativity,
c puts an absolute limit to speed at which any object can travel, hence,
nothing, no particle, no rocket, no space vehicle can go at faster-than-light
(=superluminal) speeds. However, there are some cases where things appear to
move at superluminal speeds, such as in the following examples: 1. Consider
two spaceships moving each at 0.6c in opposite directions. For a stationary
observer, the distance between both ships grows at faster-than-light speed.
The same is true for distant galaxies that drift apart in opposite directions
of the sky. 2. Another example: Consider pointing a very strong laser on the
moon so that it projects a dot on the moon's service and then moving the
laser rapidly towards Earth, so that it points on the floor in front of you.
If you accomplish this in less than one second, the laser dot obviously
travelled at superluminal speed, seeing that the average distance between the
Earth and the Moon is 384,403 km.
The schoolbook
definition would be: Matter is what takes up space and has mass. Matter as we
know it is composed of molecules, which themselves are built from individual
atoms. Atoms are composed of a core and one or more electrons that spin
around the core in an electron cloud. The core is composed of protons and
neutrons, the former have a positive electrical charge, the latter are
electrically neutral. Protons and neutrons are composed of quarks, of which
there are six types: up/down, charm/strange, and top/bottom. Quarks only
exist in composite particles, whereas leptons can be seen as independent
particles. There are six types of leptons: the electron, the muon, the tau
and the three types of neutrinos. The particles that make up an atom could be
seen as a stable form of locked up energy. Particles are extremely small,
therefore 99.999999999999% (or maybe all) of an atom's volume is just empty
space. Almost all visible matter in the universe is made of up/down quarks,
electrons and (e-)-neutrinos, because the other particles are very unstable
and quickly decay into the former.
An electron in a
hydrogen atom moves at about 2.2 million m/s. With the circumference of the
n=1 state for hydrogen being about 0,33x10-9 m in size, it follows that an
n=1 electron for a hydrogen atom revolves around the nucleus 6,569,372 billion times in just one
second.
Not really. Fist of
all, quarks always appear in composite particles, namely hadrons (baryons and
mesons), then there is antimatter, and finally there are the four fundamental
forces.
The existence of
antimatter was first predicted in 1928 by Paul Dirac and has been
experimentally verified by the artificial creation of the positron (e+) in a
laboratory in 1933. The positron, the electron's antiparticle, carries a
positive electrical charge. Not unlike the reflection in a mirror, there is
exactly one antimatter particle for each known particle and they behave just
like their corresponding matter particles, except they have opposite charges
and/or spins. When a matter particle and antimatter particle meet, they
annihilate each other into a flash of energy. The universe we can observe
contains almost no antimatter. Therefore, antimatter particles are likely to
meet their fate and collide with matter particles. Recent research suggests
that the symmetry between matter and antimatter is less than perfect.
Scientists have observed a phenomenon called charge/parity violation, which implies
that antimatter presents not quite the reflection image of matter.
The four
fundamental forces are gravity, the electromagnetic force, and the weak and
strong nuclear forces. Any other force you can think of (magnetism, nuclear
decay, friction, adhesion, etc.) is caused by one of these four fundamental
forces or by a combination of them. Electromagnetism and the weak nuclear
force have been shown to be two aspects of a single electroweak force.
Gravity is the
force that causes objects on Earth to fall down and stars and planets to
attract each other. Isaac Newton quantified the gravitational force: F =
mass1 * mass2 / distance². Gravity is a very weak force when compared with
the other fundamental forces. The electrical repulsion between two electrons,
for example, is some 10^40 times stronger than their gravitational
attraction. Nevertheless, gravity is the dominant force on the large scales
of interest in astronomy. Einstein describes gravitation not as a force, but
as a consequence of the curvature of spacetime. This means that gravity can
be explained in terms of geometry, rather than as interacting forces. The
General Relativity model of gravitation is largely compatible with Newton, except that it accounts
for certain phenomena such as the bending of light rays correctly, and is
therefore more accurate than Newton's formula. According to
General Relativity, matter tells space how to curve, while the curvature of
space tells matter how to move. The carrier particle of the gravitational
force is the graviton.
Electromagnetism is
the force that causes like-charged particles to repel and oppositely-charged
particles to attract each other. The carrier particle of the electromagnetic
force is the photon. Photons of different energies span the electromagnetic
spectrum of x rays, visible light, radio waves, and so forth. Residual
electromagnetic force allows atoms to bond and form molecules.
The strong force
acts between quarks to form hadrons. The nucleus of an atom is hold together
on account of residual strong force, i.e. by quarks of neighbouring neutrons
and protons interacting with each other. Quarks have an electromagnetic
charge and another property that is called colour charge, they come in three
different colour charges. The carrier particles of the strong nuclear force
are called gluons. In contrast to photons, gluons have a colour charge, while
composite particles like hadrons have no colour charge.
Weak interactions
are responsible for the decay of massive quarks and leptons into lighter
quarks and leptons. It is the primary reason why matter is mainly composed of
the stable lighter particles, namely up/down quarks and electrons.
Radioactivity is due to the weak nuclear force. The carrier particles of the
weak force are the W+, W-, and the Z bosons.
The photon, gluon,
and the graviton carrier particles are thought to be massless and having no
electrical charge. Only the W and Z particles, mediators of the weak nuclear
force, are massive, and the W+ and W- particles carry charge. Force carrier
particles can only be absorbed or produced by a matter particle which is
affected by that particular force. These particles allow us to explain
interactions between matter.
Today's most widely
accepted cosmology, the Big Bang theory, states that the universe is limited
in space and time. The current estimate for the age of the universe is 13.7 billion years. This figure was
computed from the cosmic microwave background (CMB) radiation data that the
Wilkinson Microwave Anisotropy Probe (WMAP) captured in 2002.
The Big Bang model
is singular at the time of the Big Bang. This means that one cannot even
define time, since spacetime is singular. In some models like the oscillating
universe, suggested by Stephen Hawking, the expanding universe is just one of
many phases of expansion and contraction. Other models postulate that our own
universe is just one bubble in a spacetime foam containing a multitude of
universes. The "multiverse" model of Linde proposes that multiple
universes recursively spawn each other, like in a growing fractal. However,
until now there is no observational data confirming either theory. It is
indeed questionable, whether we will ever be able to gain empirical evidence
speaking in favor these theories, because nothing outside our own universe
can be observed directly. Hence, the question can currently not be answered
by science.
The universe is
constantly expanding in all directions, therefore its size cannot be stated.
Scientists think it contains approximately 100 billion galaxies with each
galaxy containing between 100 and 200 billion star systems. Our own
galaxy, the Milky Way, is average when compared with other galaxies. It is a
disk-shaped spiral galaxy of about 100,000 light-years in diameter.
This question is
based on the popular misconception that the universe is some curved object
embedded into a higher dimensional space, and that the universe is expanding
into this space. There is nothing whatsoever that we have measured or can
measure that will show us anything about this larger space. Everything that
we measure is within the universe, and so we see neither edge nor boundary
nor centre of expansion. Thus the universe is not expanding into anything
that we can see or measure.
If the universe
were infinitely old, and infinite in extent, and stars could shine forever,
then every direction you looked would eventually end on the surface of a
star, and the whole sky would be as bright as the surface of the Sun. This is
known as Olbers's paradox, named after Heinrich Wilhelm Olbers [1757-1840]
who wrote about it in 1823-1826. Absorption by interstellar dust does not
circumvent this paradox, since dust reradiates whatever radiation it absorbs
within a few minutes, which is much less than the age of the universe.
However, the universe is not infinitely old, and the expansion of the
universe reduces the accumulated energy radiated by distant stars. Either one
of these effects acting alone would solve Olbers's paradox, but they both act
at once.
This question is essentially answered by Special Relativity. When talking
about the distance of a moving object, we mean the spatial separation now,
with the positions of us and the object specified at the current time. In an
expanding universe, this distance is now larger than the speed of light times
the light travel time due to the increase of separations between objects, as
the universe expands. It does not mean that any object in the universe
travels faster than light.
|