Re: The speed of gravity revisited

On Apr 30, 3:33 am, "Tom Van Flandern" <to...@xxxxxxxxxxxxxxxx> wrote:
[This replies to Mike, Koobee Wublee, and Steve Carlip.]

"Mike" writes:
[Mike]: There is nothing in Newton's law, F = dp/dt, that says forces
propagate at any speed.

To truly understand physics, we must place its laws onto a foundation
based on logic. In the discipline known as "deep reality physics", there is
only one assumption: "no miracles allowed".

The premises that arise from logic alone rather than from experiments or
observations or philosophy or mathematics include these:

** Causality: Every effect has an antecedent, proximate cause.
** Creation: No creation ex nihilo; and no demise ad nihil

For the reasoning behind these and the other principles of physics, see
Ref. [1].

As this applies to your statement, when a force is applied to a target
body to change its momentum, that new momentum cannot arise from nothing
because that is creation ex nihilo. The momentum must be delivered by the
force. That means the force itself must have momentum (the product of mass
and velocity), which necessitates that the force propagates.

Students often object "What about the force of gravity holding a rock on
the ground?" (or some similar example). Even a rock is mostly empty space
and can be crushed into a much smaller volume. Gravity forces all its
free-to-move entities (such as atomic nuclei or vibrating molecules) to
fall. They do so until they collide with a neighbor entity, and they pass
along their momentum when they do so. That momentum continues to exist and
is passed from entity to entity until it reaches any point of rock-contact
with the ground. The ground is held in place by electrostatic forces and
sends the received momentum back into the rock, which pushes back on the
entities. As long as the force of gravity continues, this cycle of
exchanging momentum back and forth continues also, and is responsible for
the pressure the rock continually applies to the ground. If there were no
gravity to make rock constituents fall, the rock could apply no pressure to
another body. Even with gravity, if all rock constituents could be held
immobile (say, suspended from above by tiny "strings"), the rock could apply
no pressure. It is the (usually unobserved) motion of free rock constituents
within the rock that creates any pressure the rock applies and we measure as
"weight".

The downward pressure caused by gravity exists in every atom of the rock
on the ground, and it flattens or reshapes the rock to the extent that it
can. Only the back-force from the ground prevents the whole rock from moving
downward. So the net force on the rock and its net momentum are both zero.
But individual atoms or molecules are continually moving or vibrating to the
extent they can within the rock in response to these active forces and the
momentum they carry.

[Mike]: You keep talking about "speed of gravity" but you never justified
why gravity must have a speed in the first place.

Gravitation originates in a source mass and affects a target body at
some distance. Because action at a distance without intermediaries is a
logical impossibility, the force of gravity must propagate from the source
mass to the target body and transfer momentum to the latter by contact.

[Mike]: action_at_a_distance is sufficient for NM for all practical
purposes

Yes, but that is a form of miracle because it violates the "proximate"
part of the causality principle. As Newton himself said: "That one body may
act upon another at a distance through a vacuum, without the mediation of
anything else, by and through which their action and force may be conveyed
from one to the other, is to me so great an absurdity, that I believe no man
who has in philosophical matters a competent faculty of thinking, can ever
fall into it."

In other words, it requires a miracle. What is your justification for
allowing action at a distance as an explanation for any real, physical
process? Do you propose to allow miracles, or can you describe a way (that
no one else can) for one body to act on another without something passing
between them? Think about it.


We agree that miracles are not allowed in Nature. But,
human minds believe in miracles. Miracle is a relative
concept. If you switch on a light bulb, an uneducated
neanderthal specimen would exclaim "oh, miracle!",
but you know that's not a miracle at all, only a device
that follows strict physical laws. Thus, miracles have a lot
to do with scientific knowledge. The laws of nature that
you 'know' might be insufficient to explain certain kind of
phenomena, so you would consider those phenomena
as miracles. Suppose now, that you are that uneducated
neanderthal with respect to an overly advanced alien,
coming to you with an extraordinary device. He switches
on that device and you start to levitate. You would exclaim
"wow, miracle!", but the alien knows it is not a miracle at all,
just natural laws technological application.


.

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