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Scientists
Albert Einstein
1879-1955 
The German physicist Albert Einstein revolutionized our concepts of space and time with his theories of relativity.
In 1905 Einstein published hisspecial theory of relativity.The foundation of the theory is the observation that light moves at a constant speed as measured by all observers, whatever their state of motion. One consequence is that measurements of distance and time are not absolute quantities but vary relative to the motion of an observer. Another is that energy (E) can be converted into mass (m), and vice versa, according to the formula
E = mc2, where c is the speed of light.
Ten years later, Einstein’sgeneral theory of relativityextended this work to include acceleration and gravity—stating that the two are equivalent. Einstein showed that gravity is actually the warping of space by matter. One consequence is that light will follow a curved path when it passes a massive body. Observations of a small apparent shift in the positions of stars near the sun during a solar eclipse in 1919 showed that the theory was correct. General relativity was later used to interpret Hubble’s discovery of the expanding universe.
Isaac Newton
1642-1727
Legend has it that Isaac Newton, an English mathematician and physicist, was led to discover the law of universal gravitation when he saw an apple fall from a tree. He showed that the same force of gravity that pulls an apple to the ground also holds the moon in orbit. In other words, orbital motion and falling are exactly the same thing.
Galileo Galilei
1564-1642
The Italian astronomer and physicist Galileo is often credited for inventing the telescope. In truth, a Dutch optician invented the telescope before 1609, but as soon as Galileo learned about this important new instrument, he set about making one of his own.
Galileo worked to improve his telescope and turned it on the sky. In a matter of months he made some astonishing discoveries, including mountains and valleys on the moon, the phases of Venus, and the four big moons orbiting Jupiter. These observations supported the Copernican view of the solar system. Galileo also used the telescope to discover that the Milky Way is composed of countless stars too faint to be seen with the unaided eye.
Galileo’s popular advocacy of the Copernican system provoked the opposition of the Roman Catholic Church, and he was accused of being a heretic. In 1633 he was forced to denounce the Copernican system and was sentenced to house arrest for the rest of his life.
Johannes Kepler
1571-1630
Armed with Tycho Brahe’s very accurate observations of the planets and his own painstaking measurements, the German astronomer Johannes Kepler discovered what we now call Kepler’s laws of planetary motion. These three laws precisely describe the motions of the planets around the sun.
Kepler’s first law states that planets move in elliptical orbits around the sun. This overturned the ancient dogma that the planets move in perfect circles. His second law describes how a planet travels faster in its orbit when it is closer to the sun. The third law describes how the orbital periods of the planets increase with the size of their orbits.
Kepler’s laws were based on the Copernican view of a heliocentric universe.
Tycho Brahe
1546-1601
Before the invention of the telescope, the Danish astronomer Tycho Brahe revolutionized astronomy by establishing the importance of accurate observations. He invented an improved sextant and used it to make precise observations of the positions of stars and planets.
Brahe completely recalculated Ptolemy’s astronomical tables, which contained many errors, and catalogued over one thousand stars during his lifetime. He built
In 1572 Brahe made careful observations of a “new star” (actually a supernova) that appeared suddenly in the constellation Cassiopeia. He showed that the object was not in the Earth’s atmosphere but was actually beyond the orbit of the moon. This discredited the prevailing theory that the heavens are static and unchanging.
Nicolaus Copernicus
1473-1543
Before the 16th century, the common belief was that the sun and all the other planets revolved around the Earth. This theory had been developed by the ancient Greek astronomer Ptolemy around A.D. 150, but the Polish astronomer Copernicus found that the Ptolemaic system could not explain the observed motions of the planets.
In 1543, he published a book calledThe Revolutions of the Heavenly Orbs, which argued that the Earth was not the center of the universe but was one of the planets and, like them, revolved around the sun. This model of the solar system is known as the Copernican system.
Copernicus also said that the Earth spins on its axis once per day, which accounts for why the sky appears to revolve around the Earth.
Copernicus’s theory explained why superior planets—those further from the sun than Earth—sometimes appear to be moving backward (inretrogrademotion) with respect to the stars, while those closest to the sun—Mercury and Venus—always appear to move in only one direction. The Earth is moving in a faster orbit around the sun than the more distant planets—periodically “passing” them, making them appear to move backward. But Mercury and Venus are moving in even faster orbits closer to the sun. Copernicus was right about the basic arrangement of the solar system, but he had no real proof.
Edwin Hubble
1889-1953
One of the most important figures in modern cosmology is the American astronomer Edwin Hubble. He discovered a universe of galaxies outside the Milky Way and showed that the entire universe is expanding.
In the early 20th century, most astronomers believed that the universe did not extend beyond our Milky Way galaxy. Then in the early 1920s, Hubble pointed the largest telescope in the world (on
Hubble continued to find galaxies at greater and greater distances He developed the system for classifying galaxies by their structure, which is still used today.
Hubble also discovered that the distant galaxies are moving away from each other and the greater the distance between two galaxies, the faster they are moving away. This simple proportionality between the distance and the speed of galaxies moving away from each other, set forth in 1929, is now known as Hubble’s law. It shows that the universe is expanding.
Georges Lemaître
1894-1966
Georges Lemaître, a Belgian cosmologist and Catholic priest, was one of the first scientists to use Einstein’s general theory of relativity to describe the universe as a whole. In 1927, he showed how the expansion of the universe, observed by Hubble, was a natural consequence of the equations of general relativity.
Lemaître also thought about the expansion of the universe in a radically new way. If we could imagine tracing the expansion backward in time, we would see it getting smaller and smaller. Wouldn’t there be a limit to how small it could get? In 1931, Lemaître proposed that at some point in the distant past, the universe was compressed into a tiny object which he called the “cosmic egg.” This cosmic egg would have exploded—launching the expansion of space itself that we still see today. This theory later became known as thebig bangand provided the first scientific description of how the universe began.
Arno Penzias (1933- )
and Robert Wilson (1936- )
In 1964, Arno Penzias and Robert Wilson decided to use an ultrasensitive microwave antenna to study natural radio emissions from the Milky Way. But what they found instead was an annoying background static. It was there all the time, no matter where they pointed their antenna. They assumed that the problem was with their equipment and spent months trying to eliminate all possible sources of the static, including pigeon droppings inside their giant horn-shaped antenna. Finally they realized that the constant static really must be coming from the sky.
Penzias and Wilson then learned from other astronomers that the microwave background radiation was an expected consequence of the big bang theory. According to the theory, the universe was born in a very hot dense fireball. As it expanded, the fireball cooled. Today, after billions of years of expansion and cooling, the energy of the original fireball remains only as a very faint glow of microwave radiation coming from all directions in the sky.
The discovery of the cosmic microwave background radiation was the first evidence to support the big bang theory, and it convinced most astronomers that the theory was correct. In 1978, Penzias and Wilson shared the Nobel prize in physics for their discovery.
