The first record of the motion of the planets were made in clay table by the Babylonians. One of the tablet is called the Venus tablet and records the motion of Venus. (see here for more details)
The Greek visited Babylon and brought back their knowledge. The Earth was thought to be at the center of the solar system. This is called the geocentric model. This model stayed for 2,000 years. This
view didn't fit the observations so Ptolemy introduced "
epicycles " that were circular orbits on top of circular orbits. Aristotle's view of the universe sticked for 2000 years unti Copernicus, GAlilelo and Kepler challenged it. Aristotle didn't base his description of the Universe on observations but on his beliefs : - The Earth is at the center of the solar system (only 6 planets were known) - planets and Sun move along circles because this is the perfect shape according to Greeks. - stars are glued on a christal sphere. -
the natural state of object is rest. (but Galileo=if there is no
friction, an object will keep going at the same speed along same
direction) - objects can' accelerate - vacuum is not possible - heavier object fall at a larger speed than lighter ones.
Note: Sometimes in Sciences or Economy or other field we end up with an unnecessary complex model called " epicycle " Usually when a theory is too complex and full of patches (like string theory) it is a sign that something is wrong. The best theories are the ones with the fewer free parameters. (or independent variables like force x mass = acceleration force and mass are the free parameters and the theory is very simple F = ma ) NOTE: some fundamentalists still think Galileo was wrong !!!
Copernicus, Jordano Bruno, Galileo; heliocentric model Come along Copernicus and the heliocentric model(see history of Physics) the The sun is now at the center of the solar system. But epicycles are still present. ( 1543 death of Copernicus) :note :June 2010 Nicolaus
Copernicus, the Polish priest and astronomer who proposed that the
Earth orbits the Sun, has been reburied with honors at Frombork
Cathedral in Northern Poland, five centuries after he advanced
his then-controversial theory. His remains had previously lain in an
unmarked grave beneath the cathedral's floor. The reburial is the
culmination of a six-year effort, initiated by a local bishop, to
locate the astronomer's remains and identify them using forensic
reconstruction and DNA analysis See washngton post
Foundation of Physics starts with Astronomy,
Kepler/Tycho Brahe and elliptical orbits
Next comes Johannes Kepler who stole the best naked eyes observations (Mars position around the EArth) available at time from Tycho Brahe. See the story. Tycho Brahe was the greatest observer of the pre-telescopic era The long computations almost drove Kepler to madness (he said) He derives his 3 laws of planetary motion. The orbits are ellipses and the epicycles have disappeared. he is able to prediction the motion of Mercury in front of the Sun. His model fits the observations but Sciences was waiting for Newton to derive these laws using the universal gravitational law, his 3 laws of motion and calculus that he had invented. keplers laws Galileo Galilei
He sees the mountains on the Moon with his newly invented telescope. The moons of Jupiter, the star clusters of the milky way. He supports the heliocentric model and writes a book in Italian. See the story From now on, theories have to be supported by the facts and the observations. Newton , Physics is born and Physics rules Newton derives Kepler's law using his 3 laws of motion. The second law says F = ma acceleration of an object = net force acting on it / its mass first find the acceleration then integrate 2 times to get the position of an object like a planet or a projectile. Newton introduced the universal law of gavitational attraction. The Moon orbits the Earth because of the force of gravity and its initial speed. It is constantly falling toward the Earth but can't reach it. The same law applies to objects on EArth. Therefore, Newton unifies motion in the heaven and motion on EArth. Physics
seems to be able to explain and predict all the natural
phenomena. Philosophically it means you don't have free will. It seems : 1) The Universe is governed by universal laws that apply in all domains. 2) laws are mathematical in nature. Every thing can be explain and predicted
Halley's comet The comet was recorded for the first time (1066) in the " tapisserie of Bayeux", Bayeux , fRance. See photos and story here. The tapestry tells the story of William the conqueror who invaded England and took the crown. The
comet was observed by a German astronomer, Peter Apian, in 1531
and reported that its tail always stretched away from the Sun. Apian’s observations, printed in a book with beautiful hand-coloured drawings, made a great impression. Comet tails do indeed flee from the Sun, no matter in which direction the comet is travelling, but the full explanation had to await twentieth-century physics. 76 years later Kepler observed the comet.
in 1695 Halley understood that the comet is a periodic phenomenon that appears every 76 years. He predicts the next appearance. (was dead by then). The halley's comet comes from the Kuiper belt
Note: In 1577 Tycho Brahe, the greatest observer of the pre-telescopic era, made a breakthrough that was literally shattering. From his observatory on the Danish island of Ven, Tycho demonstrated that the bright comet of 1577 lay far beyond the Moon and in the realm of the planets, in contradiction to the teachings of Aristotle. more on the comet Willam Herschel He gave up music for astronomy. Built huge telescopes and discovered Uranus and the milky way. + other objects like comets. Here to learn more
general relativity At the end of 19th century some problems with Newtonian Physics arose. Comes quantum mechanics and general relativity. General relativity is an extension of Newton's theory of relativity for large masses. For example, Newtonian Physics could not account for the motion of Mercury. Newton's Physics explains gravity by an invisible force pulling instantaneously on masses, taking grab on them. The Sun pulls on the EArth and forces the EArth into Orbit. Einstein says gravity does not act instantaneously. Gravity travels at the speed of light. So if the Sun was to disappear it would take 8 minutes for the EArth to move in a straight line (gravity is no more). Also, Einstein explained how the curvature of space and time accounts for gravity. The Sun curved the space around it and the Earth moves in one fold of the fabric of space. The space pushes on Earth and prevents it to escape : The space-time is like a trampolie, the Sun like a bowling Bowl and the EArth a marble stuck in a curve.
watch that movie
(from NOVA) go to : 6:29 Einstein's theory triumphed when the light from a star was observed to be curved during a solar eclipse. lNo matter ca go faster than the speed of light and the speed of light does not depend on the frame of reference it is measured in ! unlike time/energy/length Einstein's big ideas : - The speed the light does not depend on the frame of reference it is measured in ! (see below) - E = mc2 Energy and mass are interchangeable. As you increase the speed , you increase the energy and therefore the mass (inertia). Mass becomes infinite at he speed of light. - length, time , momentum , energy depends on the frame of reference you are measuring in
-------------------PAUSE------------------------ llight is made of particle waves = photons. Each photon is a bundle of energy. photons have no mass. But they have energy proportional to their frequency. Einstein + Max Plank = Quantum mechanics. photo electric effect shoewd that Einstein was right. photons= bunch of bundles of energy
galaxies, universe is expanding , big bang 20s 1927 Edwin Hubble using the observatory at Mt Wilson understands that some of these fuzzy clouds observed in the background of stars are in fact far away galaxies. With the new telescopes scientists could have a better look at the sky. White nebulae were spotted but could not be explained. Edwin Hubble understand these nebulae are in fact galaxies.
By looking at the redshift of galaxies he theorized that the Universe is expanding at a constant rate. Using the Cepheid stars (variable stars) Hubble computes the distance to these galaxies and their receding speed. Farther are the galaxies, faster is their relative speed. The Universe is expanding. So it was smaller in the past/ but how small ? watch the movie George Gamow and the big bang George Gamow comes with the theory of the big bang. The Universe startes with an epxlosion that created not only hydrogen and helium but also space and time. The theory of the Big Bang is developed by Georges Gamow and others. He described the very early Universe are being a " ylem" which means soup in yiddish.
his team from Princeton university predicts the Cosmic Microwave Background Radiation.
radio waves from the center of the milky way are detected 30s Jansky from the Bell laboratory detects radio waves coming from the center of the Milky Way . (he was studying the halo of gas in the center the galaxy) in the constellation of the sagittarius. Later, a super massive black hole was identified to be the source of the radio waves. Radio astronomy was born. NOTE: Since the late 1970s, astronomers have kept watch on an object at the Milky Way's core known as Sagittarius A-star. When
viewed through a radio telescope, Sagittarius A-star appears so bright
that many believe only a black hole's gravity could fuel it Most of that light scatters into the interstellar dust that lies between us and the Milky Way's core, but radio waves and x-rays can penetrate the shroud of dust to reach the earth. dark matter
Dark matter was postulated by Fritz Zwicky in 1934, to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequently, other observations have indicated the presence of dark matter in the universe, including the rotational speeds of galaxies,gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. note: Zwicky also understood how neutron stars worked. He is famous for calling his colleagues " spherical bastards " dark matter = the speeds of galaxies are larger than expected.
Fritz Zwicky
dark matter in 3D
here another evidence for dark matter: the Bullet Cluster without sound gravitationnal lensing
cosmic microwave background radiation 60s watch the movie: evidence for the big bang from discovery channel radiation from 300,000 years after the Big Bang was detected . The signal is in the micro wave range because it was stretched while the universe was expanding. In 1965,Arno Penzias and Robert Woodrow Wilson at the Crawford Hill location of Bell Telephone Laboratories in nearby Holmdel Township, New Jersey had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments. They detect the signal but don't know what it is. They
contacted George Gamow from the University of Princeton (and team) who
predicted this signal to be the left over of the Big Bang and was about to build a device to detect it. Wilson and Penzias got the Nobel Prize but unfortunately they didn't share it with Gamow
and coe who predicted it and explained it. there is a funny story about
the couple of pigeons who were nesting in the antenna. Wilson and Penzias thought the drops from the pigeons were the cause of the microwave signal. So they killed the pigeons. that didn't stop the signal. Later a satellite and a probe (cobe, WMAP) were sent to get a better record of the signal and we have refined image of the early Universe with the seeds of the galaxies. form WMAP
Gamma rays burst 69 to 71 / 1 burst per day During the Cold war American were " listening" to Gamma rays that could have came from soviet union nuclear tests. (see movie Dr.strangelove with Peter Sellers) They recorded Gamma burst but coming from space. Gamma rays burst are still not well understood. The long ones come probably from exploding supernovae very far away (in time). The short ones are more mysterious. They maybe due to colliding neutron stars? pulsars? see animation gamma rays are also spew out by supermassive black holes. short video swift is a space craft that studies the BRBs with 3 instruments. 1 detect the GRB and the others collect the after glow in X rays and UV/visible. More here
90s exoplanets are found. WE can't detect Earth like planets yet but we can detect Jupiter's like planet/ Extrasolar planets were first confirmed to be orbiting the pulsarsPSR B1257+12 and PSR B1620-26. Aleksander Wolszczan was the first one to find 2 exoplanets around a pulsar in 1992. These discoveries were announced in 1992 and 1993 respectively. The first extrasolar planet to be found orbiting a main sequence star, 51 Pegasi b, was discovered in 1995. 100s of exoplanets have been found. Geoff Marcy from Berkeley University has found many of them/ He found a 5 planets system in the constellation of cancer. This is the largest number of exoplanets found around 1 star. including a planet taking 2.8 days to go around. see here for more about the 5 planets solar system Aleksander Wolszczan was the first one to find 2 exoplanets around a pulsar in 1992.
There
are different ways to detect an exoplanet/ The method the most used is
by detecting the wobble of the star due to the motion of the planet. The
planet pulls on the star a little and the induced motion of the star
can be detected by " looking " at the blueshift/redshift of the star. It
works best if the orbit of the star is edge on. Another method is the
transit one. When the planet travels in fromt of the star, the light of
the star is dimmer. The brightness is recorded. Other methods are used too. applet
2 teams in cAlifornia , Supernova Cosmology Project at Lawrence Berkeley National Laboratory and High-z Supernova Search Team found accelerating expansion of the universe. Among the Sicientists Saul Perlmutter and Alex Filippenko (best lecturer @ Berkeley) It was done by observing the type IA supernovae. If the Universe is expanding, it is pulled a some kind of energy/force. Since
scientists don't know what it is, it is called dark energy. Einstein
called that " anti gravity" the cosmological constant. He thought he was wrong and called this constant the greatest blunder of his life.
