Introducing Relativity

Ross Phillips, PLC

New Course

• 2005 Einstein’s Relativity Detailed Study in Unit 3.

• 2004 many of the pilot schools are preparing to teach relativity.

• Common sense is the collection of prejudices acquired by age 18 (Einstein)

Daunting

• “Einstein’s contribution has a name for being difficult, but this is quite wrong. Einstein’s contribution is very easy to understand, but unfortunately it rests on the theories of Galileo and Newton which are very difficult to understand!”

• Renowned relativist and pioneer of the Steady State Theory Hermann Bondi

Movement Unit 2: “compare the accounts of the action of forces

by Aristotle, Galileo and Newton”.

• Aristotle: forces cause and maintain motion• Galileo: objects will keep moving forever if no

other force acted on them

• Newton: agreed with Galileo but introduced the idea of action at a distance ie. force acts instantaneously between two masses, even if separated by a great distance like the Sun and the Earth.

Why did the chicken cross the road?

• Aristotle: It is the nature of chickens to cross roads.

• Newton: Chickens at rest tend to stay at rest, chickens in motion tend to cross roads

• Einstein: Whether the chicken crossed the road or the road moved beneath the chicken depends on your frame of reference.

Newton was no Newtonian.

• God was intervening to keep all the masses in the universe from collapsing into one big mass

• He believed in absolute space, that was at rest and centred on the centre of mass of the Solar System.

• He believed in absolute time.

Prior to Galileo• the Earth was stationary, Aristotle, supported

this belief. • the Moon is not left behind as it would be if the

Earth moves through space.

• used balls rolling along ramps, and his observations of Jupiter to argue against Aristotle.

Galileo

Father of Physics• you can move without knowing that you are

moving!

• the principle of relativity states that the laws of physics are the same in all inertial reference frames.

• an idea of the world as an orderly place that could be understood with a set of laws.

• The Parable of the Ship http://physics.syr.edu/courses/modules/LIGHTCONE/galileo.html

Maxwell

• developed Michael Faraday’s field idea.

• these fields radiated at the speed of light thus were not instantaneous.

• Einstein later developed the field theory for gravity in his general relativity.

Discussion Topics

• What familiar examples demonstrate the principle of relativity?

• In what ways could you state the principle of relativity?

• How was Aristotle’s relativity different to Galileo’s?

Inertial Earth?

• we are on a body that is hurtling through space but we cannot tell!

• It was obvious to the person in the street that the Earth was not moving – but it is, (or is it?)

• due to the principle of relativity, and the nature of the Earth’s movement, that movement is difficult to detect.

Can Measure Acceleration, not Velocity

• How could you measure the speed of the Earth?

• Stellar aberration and parallax are due to revolving around the Sun

• Foucault’s Pendulum and weather patterns confirm that the Earth is rotating about its axis

Michelson-Morley Expt

• Attempt to measure the speed of Earth through the aether

• If successful would violate the principle of relativity as it applied to mechanics

• http://galileoandeinstein.physics.virginia.edu/more_stuff\flashlets\mmexpt6.htm

Einstein’s Postulates

• “the laws of electrodynamics and optics will be valid for all frames of reference for which the laws of mechanics hold good”.

• “light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body”.– Einstein A. (1905) On the electrodynamics of moving bodies.

The Challenge

• You do not really understand something unless you can explain it to your grandmother. (Einstein)

• I tried to imagine the easiest way God could have done it. (Einstein)

What do we mean by “the speed of light”?

• Einstein defines the time it takes for light to travel from A to B to be equal to the time it takes for light to travel from B to A (clocks stationary to A and B)

• c is simply twice the distance from A to B divided by the time it takes light to go from A to B and back again.

Implications

• Einstein’s explanation upset the Newtonian concept of time and space as demonstrated by

• http://casa.colorado.edu/~ajsh/sr/paradox.html#c 2

Time Dilation

L0

t0=2L0/c vt vt

• Were both headings the same length? Yes and no.

• They are the same length at rest relative to each other

Length? The separation of two ends at an instant in time.

L0

2L0=ct0

L + vt1

L+vt1 =ct1

L L - vt2

L-vt2=ct2

t1+t2=t

Sketch the letter L if it is moving near the speed of light:

To the right To the left

Up the page Diagonally

Wisdom?

• Definition of Theoretical Physics?– If the facts don’t fit the theory, change the facts.

(Einstein)

• Trouble finding questions?– One of Einstein’s students said to him: “The

questions in this year’s exam are the same as last year’s!” “True,” Einstein said, “but this year all the answers are different.”

Role play

• In groups, develop a little skit demonstrating what you might look like to observers if you were in a spaceship passing at near the speed of light. Be prepared to discuss the limitations of your skit.

• Students observing are asked to show where the performance inaccurately depicts relativity.

Creative Writing

• Write a science fiction story about a space ship that travels to another star system at close to light speed. In your story, describe the trip from the point of view of those on board and contrast that with the point of view of those on Earth (a post modern novel!)

Argumentative writing topics

• We can never travel to other stars because it is too far. True or false?

• Length contraction violates the principle of relativity. Comment on this statement.

• People travelling fast look fatter because they appear to have more mass. True or false?

Spreadsheet task: design a relativity calculator

Links• http://www.glenbrook.k12.il.us/gbssci/

phys/Class/Bboard.html#einstein

• http://members.tripod.com/conduit9SR

• Http://www.goshen.edu/~reubendb/relativity

• Http://www.pbs.org/deepspace/classroom/activity4.html#objectives a rather challenging activity designed for high schools

Demonstration (Einstein’s inspiration)

• Electric current induced a magnet moving near a wire

• Moving a wire through a fixed magnetic field induces a current in the wire

• Before Einstein these were distinct phenomena• http://www.physics.ucla.edu/demoweb/

demomanual/modern_physics/special_relativity/special_relativity.html

Let Einstein Do the Talking

• http://www.time.com/time/time100/poc/einstein_audio.html

Free Software/ Links

• http://saintmarys.edu/rtarara/software.html – Great simulations

• http://www-ed.fnal.gov/data/phy_sci/relativity/student/chalbegin.shtml 2 3 4– Great data logging task

Books• Einstein’s dreams, Alan Lightman (short

stories)• The Principle of Relativity, Einstein, Lorentz,

Weyl, Minkowski (original papers)• Special Relativity, A. P. French• Relativity and its Roots, Banesh Hoffmann• Relativity and Common Sense, Hermann

Bondi