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Einstein’s postulates
1. The laws of nature are the same for everyone.
2. The speed of light in a vacuum is constant for all observers.
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Principle of relativity
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Accelerated motion
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Discussion
Jackie claims that she is at rest and you are moving. But you claim that you are at rest and she is accelerating away from you. Assuming Jackie cannot tell your engines are running, how can she prove that you are accelerating?
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Inertial forces
Fictitious forces felt by an object trying to move in a straight line at a constant velocity but unable to do so because its reference frame is being accelerated.
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Einstein’s happiest thought of his life
If you fall in a gravitational field, you will not feel gravity. You will feel weightless.
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Discussion
If the space shuttle orbits at a distance of 20 miles from the Earth’s surface, according to Newton’s universal law of gravity, Earth’s gravitational force is a little less (because the distance from Earth’s center is 20 miles larger) than on the Earth’s surface. Yet, astronauts are weightless. How can this be?
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Discussion
If you are in a car, which is in an accident, how does everything in the car move that isn’t tied down?
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Accelerated motion in a gravitational field (freefall) can cancel gravity.
Accelerated motion far from any gravitational field, feels just like gravity.
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The equivalence principle
The effects of gravity are exactly equivalent to the effects of acceleration.
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Discussion
You are driving in a car with a bunch of helium balloons in the back seat. When you accelerate at a stop light, which way to the balloons move? Use the equivalence principle to explain.
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Discussion
If you planted some seeds in a pot that you rotated on a turntable outside in the sunshine, in which direction would the plants grow?
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Discussion
How does the equivalence of accelerated motion and gravity make even accerated motion relative?
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Spacetime diagrams
We can try to visualize spacetime by collapsing the 3 dimensions of space onto one or two axes and plotting time on the remaining axis.
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Spacetime diagrams
time
future
past
present
x
y
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Discussion
Draw a spacetime diagram of your path through spacetime during this class.
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Spacetime diagrams
time
future
past
present
x
y
You sitting in class
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Discussion
If an object is moving at a constant velocity, what does it’s world line look like on a spacetime diagram?
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Me on the way to work
time
x
y
Getting speeding tickethours
miles
Speeding
Traveling the speed limit
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Discussion
If an object is accelerating, what does it’s world line look like on a spacetime diagram?
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Stopping at a red light
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What does the Earth’s orbit look like on a three dimensional spacetime diagram?
Discussion
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What is the shortest distance between two points?
Discussion
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Discussion
What is Newton’s first law of motion?
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Newton’s 1st law of motion
In the absence of net external forces, an object will travel the shortest possible path through spacetime.
The shortest path is the one for which time ticks the fastest.
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Twin Paradox
time
x
y
years
Light years
Cen
Leave Earth
Return toEarth
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Discussion
If you were to fly a plane from Philadelphia to Beijing, approximately at the same latitude, you would just follow a straight path due west? Why or why not?
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Discussion
What would the world-line of a skydiver look like on a spacetime diagram?
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Gravity is not a mysterious “force” of nature between two masses as viewed by Newton. Instead, mass warps or curves spacetime.
Objects falling in a gravitational field are following geodesics, the shortest path between two points in a curved spacetime.
General Relativity
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Discussion
Imagine a time (before 500 BCE) when people thought the Earth was flat. Two explorers set out in exactly opposite directions, one east, one west. What do they discover?
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Consider being in outer space. Because you did not study your astronomy, you firmly believe that spacetime is flat. You decide to explore the universe by sending out two probes in exactly opposite directions. To your great surprise, the probes send back pictures of one another!
Where can we do this experiment and get this result in out lifetime?
Discussion
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Gravity slows time
Clocks tick more slowly in a gravitational field.
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Both clocks remain at rest relative to each other regardless of the acceleration.
Both observers agree that the clock in front is ticking faster than the clock in back.
Not the Same as SR time dilation
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Discussion
If you shine a laser into the window of a rocket that is accelerating past you, draw the path of the laser as it would appear to those inside the rocket.
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Discussion
Because this is the same as being in a gravitational field, what can you say about the path of light near the surface of the Earth?
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Light always takes the shortest path between two points. In a gravitational field, where spacetime is curved, light follows a geodesic, a curved path.
Gravity bends light rays
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Gravitational lens
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Discussion
Explain physically why light bends in a gravitational field. What is happening to the light to make it curve?
Hint: How does a lens made of glass bend light?
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Discussion
Using the fact that gravity slows time, explain why a ball thrown in a gravitational field will follow a curved path?
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Discussion
You’re on the top floor of the Empire State building. On the ground floor there is a clock that you can see that flashes very second. However, if you time the flashes, you find that they arrive a intervals greater than a second. Explain why.
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Discussion
If time moves more slowly on a star that has a higher surface gravity, how will this effect this the light we see emitted from the surface of the more massive star compared to that of the less massive star?
Hint: Think about how the light we see is actually created.
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Gravitational redshift
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Discussion
If you are in a closed room without windows, according to the equivalence principle is there any way you can tell the difference between falling freely in Earth’s gravitational field and moving at a constant velocity far away from any gravitational field?
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Discussion
Suppose you are locked in a closed room where you do not feel your weight. This time I allow you to have two billiard balls. You place the balls side-by-side in front of you. If your closed room is moving at a constant velocity how will the balls move?
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Discussion
Suppose you are locked in a closed room where you do not feel your weight. This time I allow you to have two billiard balls. You place the balls side-by-side in front of you. If your closed room is falling freely in a gravitational field how will the balls move?
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Discussion
Suppose you place the balls on top of each other in front of you how will they move?
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Tidal forces
Free-fall makes the downward pull of gravity disappear, but tidal forces remain.
As you fall toward Earth feet first, your feet feel a slightly stronger gravitational pull and accelerate a little bit faster than your head. Furthermore, your right side will be pulled slightly toward your left side while your left side will be pulled slightly toward your right side.
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Discussion
Does the existence of tidal forces invalidate the equivalence principle? How can we make tidal forces go away?
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Tidal Forces are the key
Tidal forces show you how space is warped
Space is stretched radially
Space is squeezed tangentially
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General Theory of Relativity
Gravity is the same as the effects of acceleration, caused by a warping of spacetime
Slows the passage of time
Bends the path of light
Causes light to be redshifted