# Unit 1905, Relativity

**Relativity Concepts**

- A frame of reference is loosely approximated by the term "point of view." In physics, your frame of reference is the perspective you would use to measure physical quantities.
- An inertial frame of reference is one which is not accelerating. It is either moving at constant velocity, or stationary. Note: It may have occurred to you that the earth is not an inertial frame (it is rotating, for instance). For our purposes, it comes close enough to an inertial frame, and we will treat it as such.
- The Special Theory of Relativity is called special because it applies only for inertial reference frames, not universally.
- The Special Theory's first postulate: The laws of physics are the same in all inertial reference frames. This was not new, but came from Galileo and Newton. Note: A postulate is a claim.
- In Einstein's day, scientists were concerned because measurements of the speed of light did not fit in with Galilean-Newtonian Relativity. Specifically, light did not obey relativity of velocity, but was always measured to have the same speed.
- Physicists in Einstein's day were concerned with the problem of the ether. Light was thought to travel through a difficult-to-detect substance called the ether, which filled empty space. However, all attempts to detect the ether or its effects failed.
- The Special Theory's second postulate: Light propagates through empty space with a definite speed
*c*independent of the speed of the source or observer. - The Special Theory's 2nd Postulate resolves the problem of the ether. There is no ether. The speed of light is a constant.
- A consequence: Time and other physical quantites are relative (depend on your frame of reference).
- Einstein showed that time is relative using thought experiments (remember the train and lightning striking the two poles).
- Since Einstein's time, every experiment (and there have been many) has shown that the theory is correct. For example, flying an extremely precise atomic clock on a jet produces a measurable disagreement with a clock that stayed behind. The clock that was moved will be some nanoseconds slower than the stationary clock.
- The effects of Special Relativity are not easily measurable at everyday speeds. The effects become more pronounced as moving objects approach the speed of light, relative to an observer.
- Clocks moving relative to an observer are measured by the observer as running more slowly than clocks at rest. This is often called time dilation.
- Objects moving relative to an observer are measured by the observer as shrinking along the axis of motion.
- The mass of a moving object increases relative to a stationary observer.
- One way of looking at Special Relativity is to view space-time as fused. There are three dimensions of space, and one of time, that make up the observable universe.
- One way of looking at time dilation is this: Imagine you are stationary in space. You continue to move through time at the speed of light. Now begin moving through space. Your speed through time is reduced by a proportional amount. The faster you move through space, the slower you move through time. If you
*could*move through space at the speed of light, you would not move through time at all. Time would stop for you.- As Einstein worked through the application of Special Relativity to mass, momentum, and energy, he discovered the relation E=mc
^{2}. This is a statement of the equivalence of mass and energy. One way to look at this is that mass is "congealed" energy. Nuclear processes of fission and fusion (as in the atomic bombs, and the sun, respectively) are products of the conversion of mass into energy.

- As Einstein worked through the application of Special Relativity to mass, momentum, and energy, he discovered the relation E=mc
- Einstein sought to make his Special Relativity apply to all of physics. His General Theory of Relativity is the extension of relativity to non-inertial frames of reference.
- Einstein's principle of equivalence states that there is no way to determine a difference between acceleration and a gravitational field. Note: Remember the thought experiment of a weightless astronaut in a rocket that begins accelerating at 9.8 m/s
^{2}. The floor accelerating upward is equivalent to (indistinguishable from) the astronaut falling on earth. - General Relativity predicts that gravity bends light. Note: Another thought experiment --- A beam of light enters the accelerating rocket perpendicular to the direction of the rocket's motion. As it heads for the opposite wall the rocket increases speed. The beam of light is seen to follow a curved path from the rocket ship, just as a projectile does on earth.
- General Relativity has stood the test of time. In 1919 light was photographed bending around the sun, and subsequent experiments have verified General Relativity to a high degree of precision.
- Not a concept, so to speak, but be aware of some of the context of Einstein's life and times as presented in the video "Einstein Revealed." Think about how Einstein was (and was not) a product of his times, and also how the times became a product of Einstein's thought (to a certain extent).