Twin Slit Experiment
The Structure of Time
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The intention behind this page is to provide some plain language background
and rationale for Einstein's Special Theory of Relativity.
The Case For Relativity
Special Relativity has a strong basis in the observed behavior of electrodynamic
phenomena, (the interactions of moving charges and electromagnetic waves).
While I am not clear as to the exact history, it would seem that the
primary feature of physics that leads to (and underpins) the Special
Theory of Relativity theory is electromagnetic field theory and in particular,
Maxwell's equations and the Lorentz transformation.
Maxwell's equations very precisely describe electromagnetic phenomena
including electromagnetic waves and magnetic induction. In doing
so, they accurately describe phenomena that are fundamental to the operation
of a vast portion of modern electro-technology.
Maxwell's equations can be derived in two ways,
1) (in hindsight) by applying special relativity to electrostatic
phenomena (this is nowadays shown in textbooks), or
2) by working from the characteristics of electrodynamic interactions.
In the latter case, the Lorentz transformation (central to special
relativity) can be derived from the characteristics of Maxwell's equations,
as was done in the later years of the 19th century.
The Lorentz transformations deal with a feature of Maxwell's equations
and that is, that the speed of an electromagnetic wave is constant and
independent of the velocity of the source and receiver/observer. The transformations
were discovered in this manner by several people (including Lorentz) toward
the end of the 19th century, well before Einstein published his first
paper on relativity in 1905.
My understanding is that when the Lorentz transformations were first
discovered, they were thought only to apply to electromagnetic phenomena,
and it was Albert Einstein who took the step of applying the transformation
to general mechanics and coming up with The Special Theory of Relativity.
The key point is that special relativity is fundamental to the relationships
of electrodynamics and any alternative theory, would still need to encompass
The Context for Special Relativity
Einstein's special theory of relativity addresses the apparently
paradoxical situations that occur because of the constancy of the speed
of light and other electromagnetic radiation,(
such as gamma rays, x-rays, radio waves and microwaves
) as per
in Maxwell's equations
The constancy of the speed of light (labeled "c" as in the formula
E=mc2) leads to paradoxes if one
presumes space and time to have a classical (Galilean/Newtonian) structure.
For example, two observers moving at different velocities while observing
the same source of light will measure the speed of the incoming light
as being the same (c), no matter what their difference in their relative
velocities, or the velocity of the source.
This contradicts classical (commonsense) expectations. A classical framework
predicts that the difference between the velocity measurements of two different
observers exactly corresponds to the difference between their own relative
Every measurement that we make regarding the propagation of light
and electromagnetic radiation, confirms the predictions of Maxwell's equations
and contradicts classical predictions. Or alternatively, we have never
been able to do an experiment that shows the speed of light and other electromagnetic
radiation contradicting relativity.
What is Special Relativity
Einstein's theory addresses the relative speed paradox by allowing that
time and space could occur differently for different observers in a particular
way such that the apparent differences in their observations are reconciled.
In essence, to measure speed one needs to measure two intervals, one
of distance and the other of time. Now, take two observers moving
with respect to each other.
If their clocks run at different rates and/or their rulers are of different
lengths, then when they measure the speed of an object their results will
not correspond to classical ‘additive’ predictions of relative velocities.
The particular way that
is consistent with Maxwell's equations and that is; to apply the Lorentz
transformations to determine how the time and space of one observer relates
to that of another.
The conclusion is that Space and time are not absolute and depend on
an observer's motion with respect to the phenomena being observed.
When you apply the Lorentz transformations in this way then you have
a set rules for transforming time and space consistent with the conditions
The stunning feature of Special Relativity is that it exactly predicts
the equivalence between energy and mass
that has been found to occur in nuclear particle interactions and atomic
All observers measure the speed of light as being the same, irrespective
of their relative motion,
Time and space occur in the same way for observers at rest with respect
to each other,
The mechanics of Special Relativity is almost identical to that of
Classical Mechanics when applied in a context in which velocities are much
less than the speed of light.
Issues With Relativity
The main difficulty with Special relativity is that is is limited in scope.
Special relativity provides a set of relationships and conservation laws
(mass/energy & momentum) that allow us to reproduce relativistic counterparts
to classical particle dynamics as well as electromagnetic interactions.
For example, through conservation laws, it can tell us the outcomes of
an interaction (say, between a photon and an electron as in the Compton
effect) but provides no way to represent an electron or a photon.
Another area where it says little, is in the domain of acceleration and
gravity. For this reason, Einstein spent much time during the later part of
his career working on a theory of General Relativity, a broader theory that
encompasses special relativity and provides a means to include gravity and
While also being highly successful (predicting the curvature of space-time,
bending of light by gravity and many other phenomena), this broader theory
still lacks a fundamental ingredient that is also lacking in Special Relativity
and classical theories. That is, how to describe the interface between an
object that is the source of a field (electrical or graviational) and the
In other words, General Relativity breaks down in matter. It can describe
the dymanics of bodies or light under the effect of gravity, but cannot
describe the source of gravity - effectively, that fact that particles with
mass have a gravitational field is taken as a basic premise, there is no
description as to why.
Finally, there is no equivalent space/time treatment of electromagnetic
fields such as is provided for gravity by General Relativity. While
there is current work on the combined problem of providing a description that
encompasses both (electro-gravity) there is still much contention.
Why be Skeptical of Ether Theories
Around the turn of the 19th century a common view of electromagnetic
waves phenomena was that the waves propagated through a medium called
the "luminiferous ether (aether)". Nowadays, this theory looks difficult
to reconcile with relativity and Maxwell's equations because it is difficult
to explain the constancy of the speed of light and the relativistic nature
of magnetic interactions.
Taking a straightforward view, if electromagnetic phenomena propagated
through some medium like sound propagates through air, then the velocity
would be fixed with respect to the medium and observers moving with respect
to the medium would get different velocities as per classical expectations.
A number of experiments, (notably the Michelson Morely Interferometer)
have shown (to very high precision) that the local velocity of light
is independent of the motion of the earth. The north/south velocity and
east/west velocities are the same and are not affected by the rotation
of the earth.
This result is consistent with relativity and rules out some simple
ether theories, but not more elaborate ones (e.g. the ether is dragged
by the earth).
However, the point to consider is that the result is consistent with
relativity, not that it "disproves" ether theories.
So if ether is not so simply disproved, then why are so many scientists
highly skeptical of ether theories?
I think that the main reason is that many current ether theories seek
to disclaim Special Relativity,
(if the theories encompassed relativity then there is no such problems)
and that raises two major issues.
The first problem with disclaiming relativity is that the prediction of
the mass/energy (formula E=mc2) is lost and the equivalence
needs to be accounted for in some other way, this is often omitted.
The second is often not addressed by proponents of Ether theories, and
that is the reciprocal nature of electrodynamic phenomena, particularly
magnetic fields. (I note that Einstein used this property as one basis for
arguing for relativity in his 1905 paper presenting Special Relativity.)
In a nutshell, according to Maxwell's equations, if you take two moving
charges, and look at the magnetic fields present, then the issue as to
which charge (or both) generates a magnetic field depends on which frame
of reference you adopt. Basically the structure of magnetic fields is totally
dependent on which frame of reference you choose, and is in accordance
with Special Relativity.
If the fields were propagated in an "ether" then this reciprocity would
disappear and electrodynamics would not work as is currently understood.