Physics Help Forum Examples of Einstein's Theory of Relativity

 Special and General Relativity Special and General Relativity Physics Help Forum

 Dec 1st 2014, 03:21 AM #1 Junior Member   Join Date: Dec 2014 Posts: 2 Examples of Einstein's Theory of Relativity Einstein's theory of Relativity introduces three concepts..... Space contracts and time dilates Space time Invariant light speed I know that the space travelling twin sibling is an example of time dilation.........Can anyone of you give examples of other concepts and explain detaily....... Last edited by Smrithi; Dec 1st 2014 at 05:45 AM.
Dec 2nd 2014, 01:12 AM   #2
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 Originally Posted by Smrithi Einstein's theory of Relativity introduces three concepts..... Space contracts and time dilates Space time Invariant light speed I know that the space travelling twin sibling is an example of time dilation.........Can anyone of you give examples of other concepts and explain detaily.......
Yes. You neglected to include the first principle of relativity which is

The Principle of Relativity - The laws of physics are the same (i.e. invariant) in all inertial frames of reference.

Also mass increases and it can be used to show that E = mc[sup]2[/sup], i.e. the mass-energy relation.

 Dec 2nd 2014, 05:35 AM #3 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 In my view, the key points of the theory are: 1) The Principle of Relativity - The laws of physics are the same (i.e. invariant) in all inertial frames of reference. 2) Invariant Light Speed. 3) The concept of a 4 dimensional SpaceTime. All the rest (e.g. time dilation) is a consequence of these 2 basic postulates, (Invariant Laws, Constant Speed of Light) expressed in the context of a 4 dimensional SpaceTime. __________________ You have GOT to Laugh !
Dec 2nd 2014, 05:43 AM   #4
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 Originally Posted by MBW In my view, the key points of the theory are: 1) The Principle of Relativity - The laws of physics are the same (i.e. invariant) in all inertial frames of reference. 2) Invariant Light Speed. 3) The concept of a 4 dimensional SpaceTime. All the rest (e.g. time dilation) is a consequence of these 2 basic postulates, (Invariant Laws, Constant Speed of Light) expressed in the context of a 4 dimensional SpaceTime.
Your "3)" is something that pre-dates relativity theory. It is present in all of physics since physicists began using three dimensional geometry and time. What special relativity theory does is take the way that we conceptualize space and time (through the ideal processes of measurement and the ideal Newtonian mechanics and Maxwell's electrodynamics behavior of physical objects at a specific location) and use the other two principles to show that one cannot make a determination of the nature of spatial coordinates without at the same time making a determination of the nature of time coordinates. This ends us implying that a 4D spacetime geometry is the underlying geometry of a 3D space + 1D time geometry that can be produced in a number of ways. This then forces us to write physical laws in certain ways to ensure that they can be expressed in a way that allows them to be applied to any (special) system of coordinates such that the laws can be easily translated from one system to another.

(General relativity then accepts this behavior at the level of geometry for adjacent points, but allows complete freedom of coordinate choice. This then requires additional restrictions on the way that physical laws are expressed so that they can be applied to any system of coordinates.

So I would say that the underlying principles of special relativity are:
1) The Principle of Relativity - Changing the system of coordinate one used, as long as they are coordinates in which one can do physics, does not change physics.
2) Invariant Light Speed - the speed of light between coordinate points is the same in all coordinate systems in which one can do physics. [This relies also on the assumption of most, if not all, of Maxwell's electrodynamics.]
3) Those coordinate systems in which it is possible to do physics are those coordinate systems in which Newtonian mechanics correctly describes motion.

Last edited by StarsBaby; Dec 2nd 2014 at 05:59 AM.

Feb 8th 2015, 09:27 PM   #5
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 Originally Posted by StarsBaby Your "3)" is something that pre-dates relativity theory.
That's not quite true. The notion of 4-d spacetime came only a few years after posted his relativity paper. So while it "literally" predates relativity, it doesn't do so by very much.

Other concepts introduced by relativity is

1) Mass and energy are related to each other by the expression E = mc[sup]2[/sup]

2) Inertial mass is a function of speed, momentum and stress

4) Relativity tells us that the electric field and magnetic field are parts of one single field called the EM field. I.e. a magnetic field in one frame can mean an electric field in another frame. An electric field in one frame can mean a magnetic field in another frame.

5) Relativity tells us that the simultaneity of two events is observer dependent

Sep 16th 2015, 08:02 PM   #6
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 Originally Posted by MBW In my view, the key points of the theory are: 1) The Principle of Relativity - The laws of physics are the same (i.e. invariant) in all inertial frames of reference. 2) Invariant Light Speed. 3) The concept of a 4 dimensional SpaceTime. All the rest (e.g. time dilation) is a consequence of these 2 basic postulates, (Invariant Laws, Constant Speed of Light) expressed in the context of a 4 dimensional SpaceTime.
I neglected to mention a couple of points when this thread was active. In SR one has to keep in mind that mass is defined as the quantity m such that

p = mv (p is momentum which is a conserved quantity)

or

m = |p|/|v|

Force is defined as

F = dp/dt

People often make the mistake of thinking that force is defined as F = ma which isn't true. F = dp/dt is how Newton defined force. F = ma is an equality established by Euler.

 Sep 17th 2015, 04:38 AM #7 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,354 I would add Einstein's "happiest thought" - the equivalence principle of GR which says that the gravitational force as experienced by an observer standing on a massive body like the Earth is the same as the pseudo-force experienced in an accelerating frame. If you are in an enclosed box with no windows (like an elevator car) you have no way of differentiating the effects of gravity from the effects of acceleration of that car.

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