There are two theories in modern science that I think anyone intending to comment intelligently on science in general should be aware of and have at least some basic knowledge of. One is Quantum Theory which I'll post on in another thread, the other is Relativity which I want to post on here.
My aim is to give an introduction to Relativity suitable for any interested layperson and written from the perspective of a non-scientist (myself). I would also hope that questions and problems that people may have could be fielded in this thread by myself, where capable, and by other posters who I know are knowledgeable in this field. I will cover relativity in 3 sections – Part 1 (this posting) will be on the history of relativity theory. Part 2 will cover the Special Theory of Relativity and part 3 will cover the General Theory of Relativity.
Part 1. Relativity - a brief history.
Contrary to popular belief, the theory of relativity was not 'invented' by Einstein and nor is it a recent development. The story starts as far back as Aristotle in ancient Greece. Aristotle proposed that objects on the Earth have a natural tendency to slow down until stopped which is their natural condition.
One method of showing this, which will be used throughout this paper, is the spacetime diagram. A spacetime diagram simply shows the position in space of an object on one axis (normally the y, or vertical, axis) and the time on the other axis (normally the x, or horizontal, axis).
Aristotle's idea can be illustrated by the following spacetime diagram which shows an object fist accelerating and then coming to a stop.
Galileo (1564 to 1642) challenged this view by asking - what happens to a moving object on-board a moving ship. This can be expressed by the following spacetime diagram:
Obviously the object is still moving in the final state. Galileo proposed that absolute velocity is not a useful measure and that only the relative velocity is important. This allows us to redraw the spacetime diagram:
You can imagine the diagram drawn on the side of a pack of cards and then tapping the deck down at an angle whilst keeping the edges straight. This process is known as a Galilean Transformation and this concept represents the birth of relativity - known as Galilean Relativity.
Notice that time is unaffected and two observers (one sat on a moving object on-board the ship and another stationary with respect to the ship, for example) in this transformation can agree on time and distance. The thing which is not agreeable by the two observers is velocity. If the observer on the moving object measures his velocity it only makes sense when quoted relative to the velocity of the ship, not some arbitrary point on land. Thus velocity is only sensible when quoted 'relative' to another observer. This allowed Newton to later develop the theory of gravity.
Over 200 years after Newton came Maxwell who developed a theory of electromagnetism using what we now call the Maxwell Equations. Without going into detail, these equations showed that electromagnetic waves (of which light is an example) move at an absolute speed (ie it is the same for all observers,. regardless of their own motion with regard to the source of the wave). This obviously contradicted Galilean Relativity and could not be explained by simply 'tapping the card deck'. Einstein considered the problem and attempted to fit Maxwell's equations into the standard picture of Newtonian physics. This led to the Special Theory of Relativity. A few terms will be used which need explaining before we consider this theory.
The Special Theory of relativity (1905) simply states that :
Although the theory of relativity fit well with the laws that govern electricity and magnetism, it wasn't compatible with Newton's law of gravity. This law said that if you changed the distribution of matter in one region of space, the change in the gravitational field would be felt instantaneously everywhere else in the universe. Not only would this mean you could send signals faster than light (something that was forbidden by relativity), but it also required the Universal notion of Time that relativity had abolished. It wasn’t until 1916 that Einstein worked out a more complete theory (General Theory of Relativity) that included gravity in the relativistic model.
Today the theories (Special and General) of Relativity have been shown accurate by many different tests - these will be dealt with more specifically in parts 2&3.
References
http://www.astro.ucla.edu/~wright/relatvty.htm
http://archive.ncsa.uiuc.edu/Cyberia/NumRel/GenRelativity.html
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/General_relativity.html
http://csep10.phys.utk.edu/astr161/lect/history/einstein.html
http://www2.slac.stanford.edu/vvc/theory/relativity.html
My aim is to give an introduction to Relativity suitable for any interested layperson and written from the perspective of a non-scientist (myself). I would also hope that questions and problems that people may have could be fielded in this thread by myself, where capable, and by other posters who I know are knowledgeable in this field. I will cover relativity in 3 sections – Part 1 (this posting) will be on the history of relativity theory. Part 2 will cover the Special Theory of Relativity and part 3 will cover the General Theory of Relativity.
Part 1. Relativity - a brief history.
Contrary to popular belief, the theory of relativity was not 'invented' by Einstein and nor is it a recent development. The story starts as far back as Aristotle in ancient Greece. Aristotle proposed that objects on the Earth have a natural tendency to slow down until stopped which is their natural condition.
One method of showing this, which will be used throughout this paper, is the spacetime diagram. A spacetime diagram simply shows the position in space of an object on one axis (normally the y, or vertical, axis) and the time on the other axis (normally the x, or horizontal, axis).
Aristotle's idea can be illustrated by the following spacetime diagram which shows an object fist accelerating and then coming to a stop.
Galileo (1564 to 1642) challenged this view by asking - what happens to a moving object on-board a moving ship. This can be expressed by the following spacetime diagram:
Obviously the object is still moving in the final state. Galileo proposed that absolute velocity is not a useful measure and that only the relative velocity is important. This allows us to redraw the spacetime diagram:
You can imagine the diagram drawn on the side of a pack of cards and then tapping the deck down at an angle whilst keeping the edges straight. This process is known as a Galilean Transformation and this concept represents the birth of relativity - known as Galilean Relativity.
Notice that time is unaffected and two observers (one sat on a moving object on-board the ship and another stationary with respect to the ship, for example) in this transformation can agree on time and distance. The thing which is not agreeable by the two observers is velocity. If the observer on the moving object measures his velocity it only makes sense when quoted relative to the velocity of the ship, not some arbitrary point on land. Thus velocity is only sensible when quoted 'relative' to another observer. This allowed Newton to later develop the theory of gravity.
Over 200 years after Newton came Maxwell who developed a theory of electromagnetism using what we now call the Maxwell Equations. Without going into detail, these equations showed that electromagnetic waves (of which light is an example) move at an absolute speed (ie it is the same for all observers,. regardless of their own motion with regard to the source of the wave). This obviously contradicted Galilean Relativity and could not be explained by simply 'tapping the card deck'. Einstein considered the problem and attempted to fit Maxwell's equations into the standard picture of Newtonian physics. This led to the Special Theory of Relativity. A few terms will be used which need explaining before we consider this theory.
- Spacetime. Space and time are treated as a 4 dimensional whole. 3 coordinates specify a position in space and a 4th coordinate specifies the position in time.
- Spacetime Frame of Reference. This is the 4 coordinates which specify the position of an object in spacetime. To go back to the analogy of the ship, an observer sitting on a deck-chair sliding along the deck of the ship would be in one frame of reference, the ship another, someone on land yet another.
- Inertial Frame (of reference). This is a fame of reference which is not accelerating and in which, therefore, any un-accelerated object moves in a straight line.[/i]To go back to the ship analogy, the deck chair, ship and land would all be inertial frames of reference providing that the deck chair was not accelerating with respect to the ship and the ship was not accelerating with respect to land.
The Special Theory of relativity (1905) simply states that :
- The laws of physics apply the same in all inertial frames of reference
Although the theory of relativity fit well with the laws that govern electricity and magnetism, it wasn't compatible with Newton's law of gravity. This law said that if you changed the distribution of matter in one region of space, the change in the gravitational field would be felt instantaneously everywhere else in the universe. Not only would this mean you could send signals faster than light (something that was forbidden by relativity), but it also required the Universal notion of Time that relativity had abolished. It wasn’t until 1916 that Einstein worked out a more complete theory (General Theory of Relativity) that included gravity in the relativistic model.
Today the theories (Special and General) of Relativity have been shown accurate by many different tests - these will be dealt with more specifically in parts 2&3.
References
http://www.astro.ucla.edu/~wright/relatvty.htm
http://archive.ncsa.uiuc.edu/Cyberia/NumRel/GenRelativity.html
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/General_relativity.html
http://csep10.phys.utk.edu/astr161/lect/history/einstein.html
http://www2.slac.stanford.edu/vvc/theory/relativity.html
