**ALBERT EINSTEIN (1879-1955)**

*Legacy*

- Reformulated the concept of time and space (E = mc² => special relativity)
- Time is not an absolute quantity but appears to flow at a different rate depending on relative motion

- Opened the road to quantum mechanics
- Light “hits” like a particle
- Light waves have “quantized” and “discrete” energies, depending on their wavelengths

- Presented a revised theory of relativity
- General relativity: space is curved
- Foundation of modern cosmology

*Einstein’s World*

- Reality of atoms and molecules in hot debate
- Light poorly understood: “What was the medium light traveled in?”
- Phenomena of radiation
- Absorption lines in the Sun were observed, but could not be explained

Einstein helped clear these mysteries and began the era of modern physics.

*Einstein’s Early Life and Career*

Born in Ulm, German Empire in 1879, Albert Einstein excelled in physics and mathematics but failed in other subjects. Einstein dropped out of high school in 1895 and restarted school in Aarau, Switzerland, where he studied Maxwell’s works (~1870), which stated that electricity and magnetism obeyed the same set of physical laws — hence, electromagnetism. Einstein discovered that the velocity of light remained constant no matter the media. Although Einstein was brilliant, he irritated professors as he was too independent. In 1902, Einstein became a patent office clerk at the Swiss Patent Office in Bern. By 1905, Einstein had written six scientific papers, three of which explored the existence of molecules and the “kinetic theory.” For his other three papers, one published in March explained his light-quantum hypothesis (light hits like a particle), a fundamental step of quantum mechanics. For this, Einstein received a Nobel Prize in 1921. Another paper published in June was Einstein’s first paper on Special Relativity that explored light contraction and time dilation approaching the speed of light. In September of 1905, Einstein published his second paper on special relativity, in which he included the famous equation E = mc².

* General relativity includes gravity, while special relativity does not.

**Special Relativity**

- The laws of physics are the same in all uniformly moving reference frames, or in all directions
- In any uniformly moving reference frame, the velocity of light (c) is the same whether emitted by a body at rest or a body in motion

*Time Dilation*: Time itself doesn’t tick at the same rate approaching the speed of light; instead, the time synchronization veers off; so approaching the speed of light, time appears to tick much slower.

*Length Contraction*: The lengths of moving objects are contracted when viewed by a stationary viewer

**Mass and Energy**

- The mass of a moving body increases compared to its “rest mass” because it takes a bigger force to accelerate

- Acceleration: speed gained in a given time
- An object accelerating up is smaller because of time dilation; acceleration is harder the more massive the object is

- Energy is responsible for powering stars, nuclear decay, and nuclear energy

*Einstein’s Impact*

- At first, the scientific community met Einstein’s special relativity theory with silence, but Max Planck, who won the Nobel Prize for explaining black body radiation, realized the importance of Einstein’s work and publicized it; from 1906, scientists took notice and visited Einstein to talk about science
- Einstein’s scientific circles grew stating 1908; became associate professor in 1911 and a professor of the Swiss Federal Institute of Technology in 1912
- Einstein’s findings demanded a new way of thinking as Newton’s Law of Gravity was only valid from speeds much smaller than light
- Einstein named the “birth of special relativity” “The Step”

**1907**: The Equivalence Principle – gravity corresponds to acceleration**1911**: Bending of light in a gravitational field, a consequence of the Equivalence Principle, could be checked with astronomical observations**1912-1915**: Extend relativity to objects moving in an arbitrary way with respect to one another**1915**: General Relativity “Gravity curves space”: there’s no need for the “force” of gravity; all motion is along “straight lines” in curved space-time and matter tells space how to move*Evidence*: starlight bends around the Sun; Mercury’s orbit will precede at a different rate than Newton predicted

**1919**: Arthur Eddington leaders solar eclipse expedition and confirms special relativity**1929**: Edwin Hubble observes expansion of the Universe- Friedmann said that Einstein’s equations supported an expanding Universe, but Einstein proposed the “cosmological constant” to keep the Universe static