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Lorentz and Poincaré developed Special Relativity as way of understanding how Maxwell's equations for electromagnetism could be valid in different frames of reference. Einstein famously published an explanation of Poincaré's theory<ref>http://www.fourmilab.ch/etexts/einstein/specrel/www/ "On the Electrodynamics of Moving Bodies"</ref> in terms of two assumptions (postulates):

Lorentz and Poincaré developed Special Relativity as way of understanding how Maxwell's equations for electromagnetism could be valid in different frames of reference. Einstein famously published an explanation of Poincaré's theory<ref>http://www.fourmilab.ch/etexts/einstein/specrel/www/ "On the Electrodynamics of Moving Bodies"</ref> in terms of two assumptions (postulates):

# ''The [[speed of light]] is constant for all (inertial) observers, regardless of their velocities relative to each other.''

# ''The laws of physics are identical in all inertial reference frames.''

#''The [[speed of light]] is constant for all (inertial) observers, regardless of their velocities relative to each other.''

In layman's terms, these two assumptions can be restated as:

As an example of the first of these, if one performs an experiment&mdash;say, with a pendulum of some similar apparatus&mdash;in a railroad car traveling uniformly at 50 miles per hour, one would find the results of the experiment (positions, speeds, etc.) to be consistent with the laws of physics in the railroad car.  An outside stationary observer, looking through the window, would also find the results consistent with the laws of physics in his stationary reference frame.  He would simply have to subtract 50 miles per hour from the velocities of the objects in order to get values that agree with the values obtained by the observer on the train.  But the two observations would each be consistent with the laws of physics.  This is all straightforward and obvious to modern people, and has been since the time of Galileo.  This principle is now called "Galilean relativity".  Another way of stating this is:

# It is impossible ever to transmit information faster than the speed of light.<ref>This assumption is commonly restated in this manner.  For example, a discussion of hypothetical [[tachyons]] talks "about using tachyons to transmit information faster than the speed of light, '''in violation of Special Relativity'''."[http://www.math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html] However, there is some question whether the Theory of Special Relativity really restricts faster-than-light communication of information.</ref>

*''There is no "preferred" frame of reference.''

# The laws of physics are identical, without any variation, in every location throughout the universe.

Neither observer can claim that his frame of reference is "right".  Of course, the observer on the train would think that the Earth is moving at 50 miles per hour under the wheels.

# The laws of physics are identical, without any variation, no matter how fast something is traveling (in the absence of acceleration).

Or, in more concise, clearer terms, these assumptions are this:

The second postulate is what makes relativity what it is.  Using the above principle, if someone in the railroad car shines a beam of light forward, he would observe it moving at a speed of 670,000,000 miles per hour.  One would expect, according to the first postulate, that an observer standing on the ground would measure its speed as 670,000,050 miles per hour.  But the second postulate says that the stationary observer measures the same speed, 670,000,000 miles per hour.  While this seems paradoxical, many experiments, such as the [[Michelson-Morley experiment]] show that it is nevertheless true.  Special Relativity explains the apparent inconsistency between these postulates.

#there is no [[action at a distance]] (because that would make observations dependent on the frame of reference)

#space and time are completely symmetric throughout the universe (because otherwise frames of reference would not be interchangeable)

 

#No material object of nonzero mass can travel at or greater than the speed of light.  (Objects of zero mass, such as photons, travel at exactly the speed of light.)

#Information can not travel faster than the speed of light.<ref>This assumption is commonly restated in this manner.  For example, a discussion of hypothetical [[tachyons]] talks "about using tachyons to transmit information faster than the speed of light, '''in violation of Special Relativity'''."[http://www.math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html]</ref>  This is sometimes stated as:

#There is no "action at a distance", because that would make observations dependent on the frame of reference.  See [[Action at a distance]] for a discussion of this point.

When the assumptions are stated clearly as above, the weaknesses in the theory are more apparent.  There “is” action at a distance in [[quantum entanglement]] and apparently also in gravity, as no gravitons can be found. However, no information has yet been transmitted via quantum entanglement, so while non-locality violates the spirit of relativity it is consistent with it if relativity is limited to the transmission of information. [[Quantum field theory]], an attempt to partially reconcile [[quantum mechanics]] with relativity, is incomplete at best.  As to the second assumption, it is contrary to the [[arrow of time]], which illustrates the lack of symmetry in time.  Logical defects include the incoherence of relativistic mass (see discussion below) and the lack of relativistic constraints near the beginning the universe (see above).

When the assumptions are stated clearly as above, the weaknesses in the theory are more apparent.  There “is” action at a distance in [[quantum entanglement]] and apparently also in gravity, as no gravitons can be found. However, no information has yet been transmitted via quantum entanglement, so while non-locality violates the spirit of relativity it is consistent with it if relativity is limited to the transmission of information. [[Quantum field theory]], an attempt to partially reconcile [[quantum mechanics]] with relativity, is incomplete at best.  As to the second assumption, it is contrary to the [[arrow of time]], which illustrates the lack of symmetry in time.  Logical defects include the incoherence of relativistic mass (see discussion below) and the lack of relativistic constraints near the beginning the universe (see above).