Special Theory of Relativity

Chapter 10

The Man Who’s Name is Synonymous With Genius

• Though he was known to slack off in school, Albert Einstein changed the direction of physics with his theories in 1905.

• He spent his free time pondering about motion and the speed of light.

• There is a thin line between Genius and Insanity, I think it’s safe to say he walked that line in a silly way.

lolThis pretty much sums it up.

Old Relativity According to Galilean and Newtonian physics

• A person in a moving train throws a ball • The person in the train observes the

ball to move at V1.

• The person out side the train would observe the ball to move at V1 + Vof the train.

• Light, since it has velocity c, would also move in this scenario if replaced with a ball at a velocity equal to c+V of the train.

Principle of Relativity

• This is how relative motion has been accepted for centuries, but Einstein sat around wondering how things would be observed as one approaches the speed of light.

• Every nonaccelerated observer observes the same laws of nature therefore, no experiment performed within a sealed room moving at an unchanging speed and direction can tell you whether you are stationary or in motion.

Light Speed is the Limit• The reason Galilean and Newtonian Relativity

does not apply when considering the speed of light is because Light speed is constant

c= 300,000,000 m/s It doesn’t matter how fast the light source is

moving because light and other electromagnetic waves always travel at c.

• The speed of light (and of other electromagnetic radiation) in empty space is the same for all nonaccelerated observes, regardless of the motion of the light source or of the observer.

Flashlight in a rocket ship.

No matter how fast the light source is going, light still moves at the constant velocity c.

Ether

• Theory about what makes up space. • If there is ether, and light’s speed is

relative to that, there should be different speeds measured depending on the movement of the observer through the ether as well.

• Michelson-Morley experiment in 1887 compared the speed of a beam of light with that of another facing different directions in the ether. What did they find?

Nothing

• Light stayed the same speed no matter the direction as the earth moves through the conceptualized ether.

• Light is nonmaterial as it is an electromagnetic wave which contrasts once again with the Galilean and Newtonian view of the world.

Relativity of Time• The person inside a moving spaceship will

see the light move up and then back down as it is reflected off of a mirror.

• The person outside the spaceship would see the path of the light at different points in space in a sort of upside-down “u” shape. Just like when someone tosses a ball up in a train.

• If C is constant, yet the outside observer observes the light cover a greater distance, than the inside observer does, something must be relative. The speed is still the same, but the time it took to cover the distance is what is relative in this scenario.

Time Travel is Not SiFi Anymore!!

If the time is relative, a person traveling nearly the speed of light will observe and experience time as normal for them, but this time will be longer for someone back on earth. By the time the person gets back, Their biological age will be younger than it would have been on earth. This is the Twin Paradox.

This has been experimentally verified with atomic clocks moving at very fast speeds. The speeding clock runs slower than the stationary clock.

Space and Mass are Relative too!!

• It’s hard to believe, but length will contract parallel to motion and mass (amount of inertia it posses as opposed to rest-mass- the amount of stuff)increases. To the observer in the ship, things are normal, but everything outside of their frame of reference would be shorter and larger as relativistic mass is concerned.

• This is were we get that light is a limiting speed. As matter approaches light speed the mass would reach infinity and the equation would require one to divide by zero. These are impossible therefore matter cannot achieve the speed of light.

Energy-Mass Equivalency• E=mc² The most famous equation ever!

As mass increases energy increases and visa versa. This also means that a change in mass= change in energy/ c²

• Remember, mass here means amount of inertia.

• Energy has mass ; that is, energy has inertia. Mass has energy; that is, mass has the ability to do work. Quantitatively the relationship between any system of mass and energy is E=mc²

Post Newtonian World View

• How we expect the world to work and how experiments and mathematics show the world to work are different. People are still stuck in a Newtonian world view.

• Atoms are made of fields and motion. But where is our matter?

• Matter is energy so everything we call matter is made of fields and motions. This is enough to make Newton’s head spin in his grave.

Quiz

1. Velma’s normal ball-throwing speed is 20m/s. She is in a train moving at 70m/s east and throws the ball to the rear. Relative to Velma, the speed of the ball is

A)50m/s eastB)50m/s westC)20m/s east D)20m/s west

2. In the preceding question, the speed and direction of the ball relative to Mort, outside of the train isA)50m/s eastB)50m/s westC)20m/s eastD)20m/s west

3. Velma moves away from mort at .75c. She turns on two lasers, on pointed forward and the other backward. How fast does mort observe the lasers to move?A) .25c and 1.75cB)c and cC).25c and .75cD).75c and.25c

4. It is physically possible for your mother to leave earth after you were born and return

A)Before you were bornB)Before she was bornC)Younger than youD)Older than youE)Older than she was when she left

5. Which of these is a form of matter?

A)Red lightB) Invisible carbon dioxide gasC)Electron beam that creates the

picture on the tv. D)A gamma ray

Answers

1. D2. B3. B4. C, D, E5. B & CBe sure to do more exercises and

review questions at the end of the book.