Introducing Relativity

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Introducing Relativity

• Ross Phillips, PLC

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New Course

• 2005 Einsteins 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)

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Daunting

• Einsteins contribution has a name for being
  difficult, but this is quite wrong. Einsteins
  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

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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.

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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.

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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.

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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
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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/cou
  rses/modules/LIGHTCONE/galileo.html

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Maxwell

• developed Michael Faradays 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.

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Discussion Topics

• What familiar examples demonstrate the principle
  of relativity?
• In what ways could you state the principle of
  relativity?
• How was Aristotles relativity different to
  Galileos?

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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 Earths movement, that movement is
  difficult to detect.

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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
• Foucaults Pendulum and weather patterns confirm
  that the Earth is rotating about its axis

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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/mor
  e_stuff\flashlets\mmexpt6.htm

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Einsteins 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.

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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)

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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.

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Implications

• Einsteins explanation upset the Newtonian
  concept of time and space as demonstrated by
• http//casa.colorado.edu/ajsh/sr/paradox.htmlc
  2

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Time Dilation
L0
t02L0/c
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• Were both headings the same length? Yes and no.
• They are the same length at rest relative to each
  other

Length Contraction
Length Contraction
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Length? The separation of two ends at an instant
in time.
Lvt1 ct1
L-vt2ct2
t1t2t
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• Sketch the letter L if it is moving near the
  speed of light
• To the right To the left
• Up the page Diagonally

L
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Wisdom?

• Definition of Theoretical Physics?
• If the facts dont fit the theory, change the
  facts. (Einstein)
• Trouble finding questions?
• One of Einsteins students said to him The
  questions in this years exam are the same as
  last years! True, Einstein said, but this
  year all the answers are different.

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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.

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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!)

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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?

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Spreadsheet task design a relativity calculator
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Links

• http//www.glenbrook.k12.il.us/gbssci/phys/Class/B
  board.htmleinstein
• http//members.tripod.com/conduit9SR
• Http//www.goshen.edu/reubendb/relativity
• Http//www.pbs.org/deepspace/classroom/activity4.h
  tmlobjectives a rather challenging activity
  designed for high schools

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Demonstration (Einsteins 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/mod
  ern_physics/special_relativity/special_relativity.
  html

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Let Einstein Do the Talking

• http//www.time.com/time/time100/poc/einstein_audi
  o.html

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Free Software/ Links

• http//saintmarys.edu/rtarara/software.html
• Great simulations
• http//www-ed.fnal.gov/data/phy_sci/relativity/stu
  dent/chalbegin.shtml 2 3 4
• Great data logging task

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Books

• Einsteins 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