The de Sitter Effect
In the early 1900s were two competing theories about
the nature of light. One held that it was ballistic
and the other that it was relativistic.
The ballistic theory described light as behaving like
projectiles and postulated its measured speed would
vary according to the velocity of the emitting source.
For example, if a source were moving toward a target,
the time taken for light to arrive would decrease and
an observer at the target would measure the light to
be moving at c+v,
where v is the source velocity.
Conversely if the source were moving away, travel time
would increase and the observer would measure the
light to be moving at c-v.
Then there was the relativistic theory, which held
that light will always appear to move at the same
speed and the time taken to reach a destination will
not depend on the velocity of the source. In other
words, an observer will always measure the light to be
moving at c, regardless of v.
Two key individuals represented opposing sides of the
debate. Walter Ritz argued for the ballistic theory
and Willem de Sitter argued for the relativistic one.
De Sitter suggested that if the ballistic theory was
true then binary stars, i.e. a pair of stars orbiting
each other, would appear different. If two such stars
were in orbit and the Earth was roughly in their orbit
plane then the light from these stars would take
differing amounts of time to reach here.
The above diagram explains this. Here
two stars orbit clockwise with velocity v.
The light from the top star moves toward earth at
velocity c+v and
the light from the bottom star at c-v.
Thus the top star’s light takes slightly less time to
reach earth. If the stars were nearby this would be of
little importance because v is small
relative to c and the stars would
always appear at opposite sides of their orbit. But
over a long distance this time difference becomes
large. As a result the light signals may arrive out of
sequence and we would observe the stars to be not
opposite each other. In an extreme case we might see
both stars on the same side of an orbit.
De Sitter studied a number of binary systems but in
each case did not observe any such resequencing.
Further studies by other astronomers have confirmed
his findings. This is known as the de Sitter double
There can be little doubt that the
findings of de Sitter and others confirm that the
light from both stars travelled to earth at the same
speed. And this certainly appears as impressive
evidence that light moves at a constant speed
regardless of its emitting source. But how could that
If the light came from two separate sources and had to
move at the same speed relative to an arbitrary
distant receiver, then how would the light ‘know’ how
much to adjust its velocity before it reaches that
receiver? And if there were observers on either side
of the star system, how could it make such adjustments
to suit both observers?
Could this mean the inverse-logic of relativity
theory, in which the observer determines the light’s
speed, is correct after all? Or could there be a more
I believe there is and I’ll demonstrate with a simple
Through the looking glass
Consider the below diagram showing a
laser and glass block.
The laser beam starts at the left,
passes through the glass, then exits the right of it.
We know that light slows down as it passes through a
transparent medium. The amount of slow-down is
determined by the reciprocal of the refractive index
of the medium. In the case of glass it’s around 1.52.
But let’s simplify things by making it 1.43. This
makes the slow-down amount 0.7, i.e. 70% of light
So as the beam moves through the glass it is going at
0.7c. But what is this speed relative
to? To the glass of course. Once the beam enters the
glass it starts moving from atom-to-atom within the
glass. Each atom becomes a new launch point for the
light and that is what the beam moves relative to. To
make a weak analogy, it is like someone running first
on dry land then through waist-deep water: the water
is the medium that determines current speed, not the
The beam then exits the block and returns to its full
speed. It is now travelling at c.
But relative to what – the laser or the glass? Again:
the glass. The beam can no longer be influenced by the
laser since it left that long ago. The final layer of
atoms in the glass represents the beam’s most recent
launch point so they are what determine the beam’s
We’ll now complicate the situation a little as shown:
Now there are two lasers. One is
standing motionless and the other is moving toward the
glass. On the other side of the glass is an observer
who will monitor the beams.
Assuming the ballistic theory of light is correct, the
light from the moving laser will strike the glass at a
slightly higher velocity. For arguments sake we’ll say
the laser is going at 0.1c. So the
two beams will hit the glass – one at c
and the other at 1.1c.
The beams strike the glass. Then what? They both slow
down of course. But by how much: does the ‘motionless’
beam slow to 0.7c and the ‘moving’
beam to 0.8c?
Answer: they both slow to 0.7c. The
beam is now inside the glass and is moving relative to
it. The initial speed of the laser can no longer have
any effect on the current beam speed because, as
before, the beam is now moving from atom to atom
within the glass. Those atoms are what control the
The beams then reach the other side of the glass and
exit. The beams now go back to full speed: c.
But relative to what – the lasers? No, the glass of
course! Like the earlier example, the original beam
speed is no longer important. The beams exiting the
glass now move with identical speed.
This is not to say the beams will be identical in all
aspects. The beam from the moving laser strikes the
glass at a higher velocity and its light waves will
appear to have a frequency 10% higher. This frequency
will be preserved throughout the process. And the
observer will see the moving laser beam as having a
higher frequency – a Doppler shift! But the
final velocity of both beams will be the same: c.
Now let’s apply this analogy to our binary stars.
The above diagram shows the binary stars
with their motion as before and Earth in their orbit
plane. Now imagine we could place a sheet of glass
just in front of the orbiting stars.
The light from these stars would strike the glass at
varying velocities: sometimes faster than light and
sometimes slower. But the instant it hit the glass it
would slow down to 0.7c. It would
then exit the glass and leave at speed c.
Just as with the moving lasers, this exit speed would
be relative only to the glass and not the rotating
stars. The light from both stars would then continue
all the way to Earth, each at the same speed.
Mister, got any gas?
Okay, so there are no giant sheets of
glass in space. This is just an analogy. But what if
there was something else? Glass is not the only
refractive material. There are plenty of others. Of
which we are in right now: air. So what if there were
was large wall of air in front of the stars – would
that have the same effect? It certainly would.
Okay there is no air-wall either. That is also an
analogy. But there may be something else – gas. We
know the space in our solar system is not a complete
vacuum. We also know our sun throws out a lot of
particles, which we call solar wind. So what if those
binary stars threw out a lot of solar wind, or rather,
stellar wind? And what if this caused them to be
surrounded by a weak layer of gas – would that cause
their light to slow down and then resume to full and
identical speed upon exiting?
It stands to reason that it would. Once the light has
left this surrounding gas layer it would continue
toward earth at the same speed. Astronomers would
observe the star rotation sequence to be the same as
it would be observed close to the stars. That is, the
stars would appear perfectly synchronised.
Although, their light will not be identical. As with
the moving lasers, the stars will have a Doppler shift
which will increase and decrease in frequency, and
this frequency shift will be preserved all the way to
So there you have it, a straightforward explanation.
No strange relativity requirements. Just an
interstellar gas medium. I’ll have more to say on this
topic in a later chapter on cosmology.