The notion that everything "travels through spacetime at the speed of
light" was popularized by Brian Greene, but it only works if you choose a
rather odd definition of "speed through spacetime", one which I haven't
seen any other physicists make use of. See my post #3 on the thread at where I quote
Greene's explanation of the math behind his statement and explain why this
terminology seems counter-intuitive and not particularly illuminating to me.

In any case, you haven't really addressed the basic argument in SR that
there is no single objective present--the principle of the "relativity of
simultaneity". In relativity there are different inertial reference frames,
and the two basic postulates of relativity are that the laws of physics
must work exactly the same in each inertial frame (so if you were in a
windowless room moving inertially in space, there'd be no experiment you
could do that would give different results depending on what inertial frame
you were at rest in), and the speed of light must be measured to be c in
every inertial frame--see

>From these two postulates you can derive the fact that different frames
must judge simultaneity differently. For example, suppose I am standing on
a space station watching you travel by me on a spaceship, with one wall
transparent so I can see what's going on inside your ship. Suppose you set
off a flash of light at the exact center of your ship, and I measure the
time it takes in my frame for the light from the flash to hit the front and
back walls of your ship. At the moment the flash happened it was
equidistant from the front and back wall, but since the ship is moving
forwards in my frame, the back wall is moving *towards* the photons that
are heading in the direction of the back wall, while the front wall is
moving *away* from the photons that are heading in the direction of the
front wall. So, if both sets of photons move at the same speed relative to
*my* frame, I must conclude that the photons will reach the back wall at an
earlier time than the photons reach the front wall.

On the other hand, in your rest frame the ship is simply at rest, so
neither wall is moving towards or away from the point where the flash
happened, and it's still true that both walls are equidistant from the
flash. So if both sets of photons move at the same speed as measured in
your frame, then it must be true that in your frame the photons reach the
front and back walls simultaneously. So, in this way we can see that the
basic postulates imply that different frames cannot agree about the
simultaneity of events that happen at different locations in spacetime,
like photons hitting two different walls (though all frames do agree about
events that coincide in space and time, like two twins comparing ages at
the moment they reunite). There's a good youtube video illustrating a
somewhat similar argument involving lightning hitting two ends of a moving
train at

Anyway, the upshot is that in SR, if Alice and Bob are moving away from
each other at some significant fraction of lightspeed, you can have a
situation where in Alice's frame, the event of her 40th birthday happens
simultaneously with the event of Bob's 32nd birthday, but in Bob's frame
the event of Alice's 40th birthday is simultaneous with his own 50th
birthday. Unless you are claiming that the same "present moment" can
include both the event of Bob celebrating his 32nd birthday and the event
of him celebrating his 50th birthday, it seems that your notion of a
"privileged present moment" must pick one frame's definition of
simultaneity out as the "correct" one while others are incorrect. But all
of relativistic physics is derived from the basic postulates which say the
laws of physics are the same in all frames, so unless the equations derived
this way are fundamentally incorrect, there can be absolutely no
experimental way to distinguish one frame as more correct than any other.
So, the only way you can have a "true present" compatible with the
experimental accuracy of relativity is to say the there is some kind of
"metaphysically preferred" definition of simultaneity which has no
experimental consequences whatsoever. This wouldn't contradict any known
physics, but it seems kind of ad seems a lot more parsimonious to
assume metaphysics lines up with physics in this case, so that a lack of
any physically preferred definition of simultaneity would imply a lack of a
metaphysically preferred definition too, which would mean the philosophy of
time known as "eternalism" or "block time" (see ) would have
to be favored over the philosophy known as "presentism" which you seem to
advocate (see

Finally, note that special relativity already has built into it some notion
that there are "two types of time"--these are coordinate time in inertial
frames, and "proper time" for each observer. If the two twins in the twin
paradox reunite and one has experienced 40 years since they departed while
the other has experienced 50 years, those numbers represent the "proper
time" along each twin's worldline between departing and reuniting. But the
events "Twin 1 turns 40 and meets up with Twin 2" and "Twin 2 turns 50 and
meets up with Twin 1" happen at the same point in spacetime, so no matter
which inertial frame you use, both events will be assigned the same
coordinate time in that frame. So, in this sense it is already meaningful
in relativity to say that these two events happen at the "same time" even
though they happen at different proper times for each twin--but this
explanation makes perfect sense in a "block time" framework, there's no
need to adopt presentism in order to understand it.


On Tue, Dec 24, 2013 at 8:26 PM, Edgar L. Owen <> wrote:

> Liz states that "Special relativity shows that there is no such thing as
> a "common present moment". but this is incorrect.
> Actually special relativity shows exactly the opposite. In my book I
> explain how this works. It is well known, though little understood, that
> everything without exception continually travels through spacetime at the
> speed of light according to its own comoving clock. I call this the STc
> Principle. This is a well known consequence of special relativity but
> actually as I point out in my book this is an even more fundamental
> Principle than Special Relativity and Special Relativity is properly a
> consequence of it and can be derived from it.
> What the STc Principle says is that the total velocity through both space
> and through time of everything without exception is = to the speed of
> light. This is the reason that time slows on a clock moving with some
> relative spatial velocity, as Special Relativity tells us.
> It also demonstrates that the speed of light is properly understood as the
> speed of TIME. That's what c really is. Light just happens to move entirely
> in space according to its own comoving clock, therefore its entire
> spacetime velocity is in space only.
> Anyway it is precisely this STc Principle that puts both the arrow of time
> and a privileged present moment on a firm physical basis. Why? Because it
> requires that everything must be in one particular place in spacetime (the
> present moment) and moving at the speed of light (the arrow of time).
> So exactly contrary to your statement, it is precisely special relativity,
> properly understood, that puts both the arrow of time and a common present
> moment on a firm physical basis.
> This insight simultaneously solves two of the big problems of the
> philosophy of science, the source of the arrow of time, and the reason for
> a common present moment, though no one seems to have recognized this prior
> to my exposition in 1997 in my paper 'Spacetime and Consciousness'.
> Edgar
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