TVF's statement from:
http://groups.google.com/groups?dq=&...ent.com#link12

[TVF]: When you leave the abstract mathematical world and convert
to a 3-space, proper time context, GR does have classical forces and
potentials in its field interpretation. While it is true that the
geometric interpretation ("gravity is just geometry") tried to do away
with those, it does so at the expense of the causality principle (no
cause and effect to produce 3-space acceleration because curvature
alone
is incapable of initiating motion) and at the expense of the "no
creation ex nihilo" principle (by requiring that the new momentum
acquired by target bodies arise from nothing). Perhaps these points
were
a factor in the preference that Einstein, Dirac, and Feynman displayed
for the field interpretation over the geometric interpretation of GR.

Robert's reply from:
http://groups.google.com/groups?dq=&...ent.com#link13

[Roberts]: That's nonsense. In GR spaceTIME curvature is certainly
capable of
"initiating motion": this is clearest for gravitation in a static
manifold, where the timelike Killing vector is the natural time
coordinate, massive objects' trajectories are proportional to the
timelike Killing vector, and the 3-space in which they are at rest is
orthogonal to it[#]. But timelike geodesics are NOT proportional to
the
timelike Killing vector -- thus a freefalling test particle initially
at
rest wrt the natural 3-space MUST start to move (wrt that 3-space)[%].
The idea holds for all particles in freefall, in all manifolds....

[#] Example: Schwarzschild coordinates on Schw. spacetime, r2M;
the timelike Killing vector is d/dt, and {r,\theta,\phi}
define the 3-space orthogonal to it.
[%] Timelike geodesics "fall". If this weren't true, GR would
have been a non-starter from the beginning -- you REALLY
need to learn the basics of GR....

[...] A test particle (of constant mass) following its
geodesic has constant 4-momentum (i.e. there is no "new momentum") --
that's what the geodesic equation says, literally.
--------------------------------------------------------------------------

Well, well, well, what do we have here?

Let's start by quoting Reichenbach, p42, from "The Direction of Time":

" [...] It remains true that Einstein's theory presupposes a causal
definition of time, even if it does no require a definition of time
direction."

It seems that GR has replaced the notion of direction of time and
associated notion of causality found in Newtonian Mechanics and other
Realist's interpretations with an order of time and an Anti-realist
view.

In this context, the classical causality principle that requires an
antecedant cause for every effect is now replaced with a causal
definition of time itself. It turns out that the remarkable invariance
of GR Roberts has been so loud to advertise did not come at no
expense. And the problem now can be stated by a layman like myself as
follows:

If there is no prefered reference frame and relationalism of 4-space
is all that there is in GR, what determines the order of time?

Is it maybe Mach's surrounding universe what determines causality in
GR and consequently the time order? (or vice versa)

Obviously, GR has an incomplete metaphysical foundation and proposes
something indistinguishable from magic, unless its foundation is
clearly established and shown to provide grounds for empirical
determination of time order in the context of 4-space only. Comments
like: "It is just a MODEL, usually made by Roberts and his likes,
cannot be accepted by men of reason on face value. The
counter-argument is that such MODEL may be compromising common sense
of reality at the expense of a mathematically elegant MODEL.

I would kindly ask Roberts to show what makes the order of time in GR
such that an object in free fall hits the ground instead of flying to
outer space. Unless one assumes the proper order of time a priori,
both these events are equivalent in GR. In contrast, in NM, the
direction of motion is established by postulating, an independent of
space, absolute time that evolves in the positive time axis. Although
the equations of motion in NM are time reversible, a universal
absolute time provides a metaphysical ground of causality ans
ultimately direction. In modern layman terminology, NM is sufficiently
grounded on the metaphysical concepts of absolute time and time rate
of change of momentum (force).

Therefore, since 4-momentum is always zero in GR and there is no
direction of time, where does the order of time come from? Magic?

Undeniable wrote:
[quotes from the two people named in the subject, followed by a lot
of gobbledygook unrelated to either]


It turns out that the remarkable invariance
of GR Roberts has been so loud to advertise did not come at no
expense.

The "invariance" of GR is there from the beginning, and is one of its
primary features. The "expense" you seem to be thinking of is merely
your own misunderstandings....


And the problem now can be stated by a layman like myself as
follows:
If there is no prefered reference frame [...], what determines the order of time?

The local geometric structure of spacetime. (See below for more on this)


Is it maybe Mach's surrounding universe what determines causality in
GR and consequently the time order? (or vice versa)

No. In GR all physics is local.


I would kindly ask Roberts to show what makes the order of time in GR
such that an object in free fall hits the ground instead of flying to
outer space.

Huh???? An object in free fall near the earth can either hit the ground
or fly to outer space, depending on its INITIAL CONDITIONS. This is no
different from Newtonian mechanics....

If one releases an object from rest above the earth, it falls to the
ground. In the time-reversed situation, the object leaps up from the
ground and comes to rest in one's hand -- this of course occurs because
of the remarkable set of initial conditions of the object and earth's
surface that cause the surface to literally throw it up with just the
right velocity so it comes to rest in one's hand. This applies in both
NM and GR. Of course if one releases the object from rest in the
time-reversed situation, it falls to the ground.


Unless one assumes the proper order of time a priori,
both these events are equivalent in GR.

You have an utterly crazy meaning of "equivalent" -- one that's totally
wrong. But still, one must "assume the order of time" (i.e. put it in
via initial conditions).


In contrast, in NM, the
direction of motion is established by postulating, an independent of
space, absolute time that evolves in the positive time axis.

In which of the three laws is this mentioned? -- in NM the orientation
of the time axis is every bit as arbitrary as it is in GR. Like
Newtonian mechanics and classical electrodynamics, GR is invariant over
time reversal. That is, in any coordinate patch one can make the
coordinate transform t = -t and the equations of GR remain unchanged.

Of course in general the SOLUTIONS of the equations are not
invariant over t = -t. But if one can interchange the initial
conditions appropriately then they are.

More generally, in a given 4-d spacetime manifold, one can interpret one
timelike direction as "future" and the other as "past". But the
assignment of these labels is arbitrary. Note that once those two labels
are applied to any point in the manifold, they are determined EVERYWHERE
within the manifold (i.e. at each and every point). There are of course
exactly two ways to apply those labels.


Although
the equations of motion in NM are time reversible, a universal
absolute time provides a metaphysical ground of causality ans
ultimately direction.

Hmmmm. You seem to have a different "NM" than the rest of us -- in which
of Newton's three laws is this mentioned?

[Oh, yes, some authors expand Newton's theory with such baggage,
and a large literature is devoted to it. But to a working
physicist that is all irrelevant. And uninteresting. As was
said in another context, physics is not metaphysics.]


Therefore, since 4-momentum is always zero in GR and there is no
direction of time, where does the order of time come from? Magic?

Where on earth do you get this stuff???? Before commenting on GR you owe
it to your readers (and YOURSELF) to understand what GR actually says,
rather than making stuff up. In GR, 4-momentum is manifestly not "always
zero", for any timelike object -- in fact, it is IMPOSSIBLE for a
timelike object to have zero 4-momentum. And that's irrelevant to time
order, anyway (in GR, time order is part of the local structure of
spacetime, utterly independent of any objects or their 4-momenta).


Just as in Newtonian Mechanics, the "time order' you are so confused
about comes from INITIAL CONDITIONS. The equations of neither NM nor GR
impose a "direction of time", it must be imposed from outside. It seems
your prattling about "absolute space and absolute time" is merely a
long-winded way of saying that, but one that brings along A LOT of
extraneous baggage not actually present in NM. Or GR.

Of course statistics and thermodynamics do indicate a "direction
of time", for both NM and GR, in the usual way....


Tom Roberts

"Tom Roberts" wrote in message ...
Undeniable wrote:

[snip]

Therefore, since 4-momentum is always zero in GR and there is no
direction of time, where does the order of time come from? Magic?

Where on earth do you get this stuff????

Undeniable is Louis Savain, aka Nemesis.

Dirk Vdm

Tom Roberts wrote in message ...
Undeniable wrote:
[quotes from the two people named in the subject, followed by a lot
of gobbledygook unrelated to either]


It turns out that the remarkable invariance
of GR Roberts has been so loud to advertise did not come at no
expense.

The "invariance" of GR is there from the beginning, and is one of its
primary features. The "expense" you seem to be thinking of is merely
your own misunderstandings....


And the problem now can be stated by a layman like myself as
follows:
If there is no prefered reference frame [...], what determines the order of time?

The local geometric structure of spacetime. (See below for more on this)


No, it does not.


Is it maybe Mach's surrounding universe what determines causality in
GR and consequently the time order? (or vice versa)

No. In GR all physics is local.

I would kindly ask Roberts to show what makes the order of time in GR
such that an object in free fall hits the ground instead of flying to
outer space.

Huh???? An object in free fall near the earth can either hit the ground
or fly to outer space, depending on its INITIAL CONDITIONS. This is no
different from Newtonian mechanics....

If one releases an object from rest above the earth, it falls to the
ground. In the time-reversed situation, the object leaps up from the
ground and comes to rest in one's hand -- this of course occurs because
of the remarkable set of initial conditions of the object and earth's
surface that cause the surface to literally throw it up with just the
right velocity so it comes to rest in one's hand. This applies in both
NM and GR. Of course if one releases the object from rest in the
time-reversed situation, it falls to the ground.


This problem is not about initial conditions one one of time
reversibility, i.e. a time direction problem.


Unless one assumes the proper order of time a priori,
both these events are equivalent in GR.

You have an utterly crazy meaning of "equivalent" -- one that's totally
wrong. But still, one must "assume the order of time" (i.e. put it in
via initial conditions).


The order of time must be established and maintained throughout the
path. Just specifying the initial condition, specifies a given event,
which is not enough to determine the order of time. You need two
events and causality to determine the direction of time and three
events to determine the order.

What are your three events, which given an initial condition determine
the time order? Remember, time direction is not presupposed in GR,
like it is in Newtonian Mechanics because of its Metaphysical ground
of absolute space and time.


In contrast, in NM, the
direction of motion is established by postulating, an independent of
space, absolute time that evolves in the positive time axis.

In which of the three laws is this mentioned? -- in NM the orientation
of the time axis is every bit as arbitrary as it is in GR. Like
Newtonian mechanics and classical electrodynamics, GR is invariant over
time reversal. That is, in any coordinate patch one can make the
coordinate transform t = -t and the equations of GR remain unchanged.

Look, it is not mentioned in the laws, it is what makes stating the
laws possible,or in other words, those laws are based on the implicit
assumption, or are founded on, of the concept of absolute space and
time, or in modern layman terminology, Substantival space and time.


Of course in general the SOLUTIONS of the equations are not
invariant over t = -t. But if one can interchange the initial
conditions appropriately then they are.

Any equation of motion based on second derivative of space wrt to time
is time reversible.


More generally, in a given 4-d spacetime manifold, one can interpret one
timelike direction as "future" and the other as "past". But the
assignment of these labels is arbitrary. Note that once those two labels
are applied to any point in the manifold, they are determined EVERYWHERE
within the manifold (i.e. at each and every point). There are of course
exactly two ways to apply those labels.


Although
the equations of motion in NM are time reversible, a universal
absolute time provides a metaphysical ground of causality ans
ultimately direction.

Hmmmm. You seem to have a different "NM" than the rest of us -- in which
of Newton's three laws is this mentioned?

You're playing word games. This assumption makes stating the laws
possible.

[Oh, yes, some authors expand Newton's theory with such baggage,
and a large literature is devoted to it. But to a working
physicist that is all irrelevant. And uninteresting. As was
said in another context, physics is not metaphysics.]


Now you are trying to undermine the issue. Specifically the fact that
GR (and SR) propose a timeless universe (no time flow) with also no
direct means of establishing the time order a priori, given the
initial conditions. Time order is determined a posteriori.

It is so simple, but you won't admit it. As soom as one introduces a
postulate which determines time direction, the concept of 4-space, or
spacetime, falls on his face. So simple...

You are hidding your face in the sand. There is no time direction in
GR and time order is unspecified a priori. It is a magical process.
Every time you solve those equations of motion you assume it's there.


Therefore, since 4-momentum is always zero in GR and there is no
direction of time, where does the order of time come from? Magic?

Where on earth do you get this stuff???? Before commenting on GR you owe
it to your readers (and YOURSELF) to understand what GR actually says,
rather than making stuff up. In GR, 4-momentum is manifestly not "always
zero", for any timelike object -- in fact, it is IMPOSSIBLE for a
timelike object to have zero 4-momentum. And that's irrelevant to time
order, anyway (in GR, time order is part of the local structure of
spacetime, utterly independent of any objects or their 4-momenta).


Just as in Newtonian Mechanics, the "time order' you are so confused
about comes from INITIAL CONDITIONS. The equations of neither NM nor GR
impose a "direction of time", it must be imposed from outside. It seems
your prattling about "absolute space and absolute time" is merely a
long-winded way of saying that, but one that brings along A LOT of
extraneous baggage not actually present in NM. Or GR.

Of course statistics and thermodynamics do indicate a "direction
of time", for both NM and GR, in the usual way....


The only attempt to establish the direction of time based on
probabilities was made by Reichenbach (although Prigogine also tried
in a similar way) and considered unsuccessful or incomplete.

Just let me know your opinion on this: Is physical reality just
relativized in SR (3-space + time + relativity of simultaneity) and
observer dependent, or indeed 4-space observer independent?

I know you'll say that you don't care and you just look at it as a
MODEL. If you say that, you'll be hidding your face in the sand once
more. Religions first start as a social MODEL and slowly transform as
beliefs in some transcendental truth. Christianity is a good example.
GR is probably the next one to come, with whatever implications may
bring with it, which you either underestimate of unware of.

One thing is certain. You know Relativity well...I'm just learning...


Tom Roberts

Let us take a simple statement such as, "in relativity there is no time
flow or direction of time", or thoughts to that effect. I hear it
repeated many times, including this thread. I am not sure relativity
says that.

Take the example of a black hole. It grows from a small black hole with
the mass of 1 star and can grow to the mass of a billon stars. BHs grow
be accreting more and more mass. Stars masses flow in, they do not at
the same speed flow out. Relativity does not permit this.

What am I missing in suggesting the above stated seeming contradiction?

I also quite clearly see causality at work.