IB Physics 12 Mr. Jean January 7 th, 2014

The Plan: Video clip of the day Review of Time Dilation Length Contraction Space Time

Time Dilation Formula:

Time Dilation Examples:

Key points to relativity: Observations are relative to motion. The speed of light never changes. It always travels the same distance per second relative to the motion. Observers must see light at a constant velocity. Time on a moving object slows down in comparison to a stationary observer.

A Light second is a light second

Both spaceships fire their lasers at the same time. The bottom spaceship is moving quickly from left to right.

The lasers travel outwards at the same speed. The speed of light. We would see both beams of light travelling side by side at the same speed.

The crew of the stationary ship sees that the light travels 1 light second per for each 12 light seconds. What about the moving ship?

If we agree that the speed of light is a constant then, we must disagree with the time scales and distance scales.

The problem with Time Dilation: Unfortunately time dilation does not do enough to explain this. We need to show how distance is also influenced by speed.

Since the bottom spaceship is moving time dilation does occur; however the spaceship also becomes squished due to length contraction. The spaceship is physically squished compared to the stationary observer.

Only 9 seconds actually pass on the moving spaceships clock due to time dilation. Due to length contraction our ruler is no longer accurate.

With length contraction we now see that the lasers distance travelled was 9 light seconds per second. Relative to the people on the moving space, light is travelling at light speed.

Length Contraction: Two observers that move at a constant speed with respect to each other, disagree on the time that it takes for an event to occur. They also disagree on the length of an object or the distance between two points.

Suppose that a spaceship travels from planet A to planet B. Observers on A and B both measure the distance between the planets as L 0. We shall show that the astronaut measures a different distance between the planets.

From the observers' point of view, the spacecraft, or anything else that is moving, also contracts. Are you serious?!… (yes)

Spaceship Moving at the 10 % the Speed of Light

Spaceship Moving at the 85 % the Speed of Light

Spaceship Moving at the 99 % the Speed of Light

Spaceship Moving at the % the Speed of Light

Length Contraction:

Length Contraction Example:

Understanding Space-Time:

Let’s say that a car is travelling east at 100km/h. All of the motion is in the eastward direction. There is no change in the northern position.

What happens at 100km/h NE?

All visual information is travelling through the universe (space and time) at the speed of light. If you are a stationary object you are travelling at the maximum speed of time.

As you start moving through space your time begins to slow down. It is replaced by your motion in space. Similar to moving North East instead of due east

If you can travel at the speed of light through space/time there would be no progress in time. If you can go faster than the speed of light you can go backwards in time.

Thus with space-time, time dilation, and length contraction we can predict what happens at incredibly fast speeds.

To do: Start working on “Visiting the Relatives”