Mr. BaldwinPHYSICS 5/23/2014Relativity Aim: What does it mean to travel at light speed? Do Now: What happens when an object’s length (dimensions) as it approaches the speed of light?

Relativity Theory The days come and go like muffled veiled figures from a distant friendly party, but they say nothing, and if we do not use gifts they bring, they carry them as silently away. Ralph Waldo Emerson

It’s only relative. Einstein’s theory of relativity is actually fairly simple. It is based on two very simple ideas. –Principle of relativity –Principle of the constancy of lightspeed To understand this theory one needs the mental flexibility to follow an argument wherever it might lead.

Constancy of Lightspeed: Strange but true. No matter how fast you move, a light beam always passes you at a speed c, relative to you. –Einstein concluded that every observer ought to observe every light beam to move at speed c, regardless of the observer’s motion. The Principle of the Constancy of Lightspeed –The speed of light in empty space is the same for all nonaccelerated observers, regardless of the motion of the light source or of the observer. The principle is valid only for nonaccelerated observers.

Newton’s Laws and Mass-Energy Equivalence Accepting Einstein’s postulates requires some major changes in how we think about space and time. For example, does Newton’s second law of motion still apply when objects are moving at large velocities? F = ma =  p /  t In order to maintain conservation of momentum, Einstein redefined momentum as p =  mv

Mechanical energy also must take on a new meaning: KE =  mc 2 - mc 2 The quantity mc 2 = E 0 is called the rest energy. Accelerating an object increases its energy above its rest energy. Therefore its total energy would be E =  mc 2.

The rest energy term E 0 = mc 2 indicates a startling equivalence between mass and energy. Einstein’s famous equation correctly predicts the amount of energy released in nuclear reactions. –Rest-mass energy is transformed into kinetic energy in nuclear fission and fusion.

A Bunsen burner adds 1000 J of heat energy to a beaker of water. What is the increase in the mass of the water? a)1000 kg b)1110 kg c)1.11 x kg d)1.11 x kg

The Relativity of Time Time is the measure of events that goes through periodic or identical repetitions. A light clock: two parallel facing mirrors with a light beam placed between them. –Distance between them 150,000 km –Time from one mirror to the next and back is 1 sec The relativity of time is called time dilation –Time is relative to the observer –Objects that travel near the speed of light experiences an increase in time as observed by an outside observer.

Time Travel: You Can’t Go Home Again The twin paradoxThe twin paradox: in the accelerated reference frame relativity theory does not apply. Twin that leaves and returns ages slower. A light year (l.y.) is the distance light travels in one year 1 l.y. = 9.45 x m

Mr. BaldwinPHYSICS 2/26/09Relativity Aim: What happens to objects when they travel at light speed? Do Now: 1.What happens to the length of an object when it travels near the speed of light? 2.What happens to the mass of an object when it travels at the speed of light? 3.What happens to a clock on a spaceship traveling at a fraction of light speed when viewed from Earth?

Time Dilation

The Relativity of Space Space: “what is measured by rulers” –At high speeds space-time becomes distorted (non- relative) for observers Time & space are tangled up with each other! –Measurements parallel to the direction of motion becomes contracted (shortened); while measurements perpendicular to the direction of motion remain unchanged. This distortion of space is called length contraction.

Length Contraction Problem: Maia’s spaceship is 100 m long and 10 m high as measured by Maia. She passes Marshall at 60% of lightspeed (0.6C). As observed by Marshall, what are the dimensions of Maia’s space craft?