[geordief]

So equivalence is singular. Now that is interesting and not trivial.

Do you want to say any more? How is equivalence singular? Do you mean equivalence is a limit?

[Bill S]

I think it is because ,if you release two objects at a  two different distances from the Sun (as an example of a body with gravitational attraction) and both initially stationary wrt the Sun, then the object which is nearer to the Sun  will accelerate more quickly than the one that is further away -and so the distance between the two objects  will increase over time.

Yes; that was my thinking.  Gravity decreases as a square of distance; so the nearer one is more strongly attracted.

It is neat, question is would it be possible to measure it in such a small volume?

That's why I included the " sufficiently sensitive measuring device".  []

[evan_au]

Gravity decreases as a square of distance; so the nearer one is more strongly attracted

This is the origin of "spaghettification" - in an extreme gravitational gradient (eg near a black hole), your feet would be attracted much more strongly than your head, so you get turned into spaghetti.

[zx16]

I thought the source of Gravity was supposed to be a single, unique,  "God-Particle"  -  the Higg's Boson?

But the Higg's is already postulated as coming in different varieties - "colours" or "flavours" or "spin", or whatever.

Physicists achieved their best work by discovering the Electron, the Proton, and the Neutron.
These are particles which seem firmly grounded in physical reality.

But later phantasms like the so-called "Higg's Boson",  I don't believe in at all.  It's just a fairy-tale.  Doesn't  everyone know it really?

[Bill S]

But later phantasms like the so-called "Higg's Boson",  I don't believe in at all.  It's just a fairy-tale.  Doesn't  everyone know it really?

May I suggest that you try to make three small adjustments to your thinking; then start the understanding process again.  I suggest this, not as any sort of expert, but as one who has had to go through that process, more than once.

1. Get rid of the “God particle” idea.  It seems to lead to all kinds of off-track thinking. It was such a bad idea, from the start, that its originator tried to blame it on his publisher.

2. Give some serious thought to what you mean by physical reality.

3. Ask yourself why you “don't believe in [that] at all”; just to be sure your belief is based on science, rather than prejudice.

[jeffreyH]

So equivalence is singular. Now that is interesting and not trivial.

Do you want to say any more? How is equivalence singular? Do you mean equivalence is a limit?

Equivalence is only valid for a singularity. That is an infinitesimally small point which is immune from tidal forces.

[geordief]

So equivalence is singular. Now that is interesting and not trivial.

Do you want to say any more? How is equivalence singular? Do you mean equivalence is a limit?

Equivalence is only valid for a singularity. That is an infinitesimally small point which is immune from tidal forces.

Which point within the ship did Feynman mean? Any infinitesimal point or  one both  strategically positioned  and infinitesimal ?

[jeffreyH]

So equivalence is singular. Now that is interesting and not trivial.

Do you want to say any more? How is equivalence singular? Do you mean equivalence is a limit?

Equivalence is only valid for a singularity. That is an infinitesimally small point which is immune from tidal forces.

Which point within the ship did Feynman mean? Any infinitesimal point or  one both  strategically positioned  and infinitesimal ?

The exact centre of gravity seems to be a good bet to me. Since objects are treated as point sources.

[zx16]

But later phantasms like the so-called "Higg's Boson",  I don't believe in at all.  It's just a fairy-tale.  Doesn't  everyone know it really?

May I suggest that you try to make three small adjustments to your thinking; then start the understanding process again.  I suggest this, not as any sort of expert, but as one who has had to go through that process, more than once.

1. Get rid of the “God particle” idea.  It seems to lead to all kinds of off-track thinking. It was such a bad idea, from the start, that its originator tried to blame it on his publisher.

2. Give some serious thought to what you mean by physical reality.

3. Ask yourself why you “don't believe in [that] at all”; just to be sure your belief is based on science, rather than prejudice.

Gravity seems to be causing so much trouble to physics.  Could this be, because "Gravity" isn't really  a "force" at all.  It's just an in-built tendency of matter to gather together.
Without any force involved.  Matter just intrinsically "wants" to be with matter.  As a kind of a natural Universal  "homing" instinct.  Like the proverbial "birds of a feather, flock together"
.
All  matter, however widely scattered it is, and whether it's composed of grains of dust, or planets, or stars, or entire galaxies, will naturally move together, until it's reached "home".

This was old Aristotle's teleological explanation.  Was he right after all?

[evan_au]

Could this be, because "Gravity" isn't really  a "force" at all.  It's just an in-built tendency of matter to gather together.
Without any force involved.

You then have to ask "so how strong is this tendency?".

And you discover that this "tendency" increases proportional to the mass of either object, and decreases in proportion to the square of the distance between their centres.

And this "tendency" can be measured in units of Newtons - which is a unit of Force.

So gravity can be considered qualitatively as a "Tendency", and quantitatively as a "Force".
And if you follow Einsteins' General Relativity, you may consider it causatively as a "Curvature in Spacetime".
Nobody yet has a consistent quantum theory of gravity, but probably someone will eventually describe it in terms of an "Exchange of Gravitons".

[geordief]

Could this be, because "Gravity" isn't really  a "force" at all.  It's just an in-built tendency of matter to gather together.
Without any force involved.

You then have to ask "so how strong is this tendency?".

And you discover that this "tendency" increases proportional to the mass of either object, and decreases in proportion to the square of the distance between their centres.

And this "tendency" can be measured in units of Newtons - which is a unit of Force.

So gravity can be considered qualitatively as a "Tendency", and quantitatively as a "Force".
And if you follow Einsteins' General Relativity, you may consider it causatively as a "Curvature in Spacetime".
Nobody yet has a consistent quantum theory of gravity, but probably someone will eventually describe it in terms of an "Exchange of Gravitons".

Are there any theories as to the mechanism that might be involved if "gravitons" were involved?

What might be their range ,for example?

[evan_au]

Are there any theories as to the mechanism that might be involved if "gravitons" were involved?
What might be their range ,for example?

Gravitons are a hypothetical particle that carries the gravitational force; the individual particles have very low energy, which makes confirming their existence quite a challenge.

They are most commonly thought to be massless (zero "rest mass"), travel at the speed of light, and have an infinite range (provided the general expansion of the universe does not carry distant objects away faster than c).

However, there are a number of alternative theories about the graviton, and at present no real way to distinguish between them.

See: https://en.wikipedia.org/wiki/Graviton

[Yahya]

Einstein said " everything should be as simple as it can be, but not simpler " it is just as simple as this : there is equivalence between the energy contained inside matter ( mass/energy )  and the sum of all  possible potential energy it can exert outside this mass until infinity.

[zx16]

Einstein said " everything should be as simple as it can be, but not simpler " it is just as simple as this : there is equivalence between the energy contained inside matter ( mass/energy )  and the sum of all  possible potential energy it can exert outside this mass until infinity.

I wouldn't put too much trust in what Einstein said. He was just an early 20th-century theorist.  Isn't it time to move on?

[Yahya]

yes , Einstein intelligence is a constant, but the equation also inversely proportion to years taken! but his wisdom never vanish.

[McQueen]

james  Muirhead asked the Naked Scientists:What is the source of the force of gravity or what is it that reaches up from the Earth to pull the apple downward? What do you think?

That is the very question that Isaac Newton spent a  life time trying to solve, unfortunately although he was  able to come up with possible hypotheses for the phenomena of gravity there were never any supporting empirical proofs that could be proved by experiment. Hence his famous quote:' Hypotheses non finga ' (I make no hypotheses.). The hypotheses that Newton had in mind was one involving the existence of an aether because  he could not comprehend the fact that gravity apparently acted at a distance with no intervening medium in between:

“It is inconceivable that inanimate brute matter should, without the mediation of something else which is not material, operate upon and affect other matter without mutual contact, as it must be, if gravitation in the sense of Epicurus, be essential and inherent in it. And this is one reason why I desired you would not ascribe innate gravity to me. That gravity should be innate, inherent, and essential to matter, so 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 another, 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. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left open to the consideration of my readers."

However, he considered that even action at a distance was preferable to the vortice theory put forward by Leibniz and Huygens:

“For since celestial motions are more regular than if they arose from vortices and observe other laws, so much so that vortices contribute not to the regulation but the disturbance of the motions of planets and comets; and since all phenomena of the heavens and of the sea follow precisely, so far as I am aware, from nothing but gravity acting in accordance with the laws described by me; and since nature is very simple, I have myself concluded that all other causes are to be rejected and that the heavens are to be stripped as far as may be of all matter, lest the motions of planets and comets be hindered or rendered irregular. But if, meanwhile, someone explains gravity along with all its laws by the action of some subtle matter, and shows that the motion of planets and comets will not be disturbed by this matter, I shall be far from objecting.”

Today it is Einstein's version of gravity as illustrated by General Relativity that captures the imagination of physicists. Yet Einstein's theory of General Relativity is very far from being perfect and it is hard to imagination future generations of astronauts on a Mars mission relying solely on General Relativity to navigate to Mars or anywhere else in the Solar system or for that matter the Universe, which in effect speaks for itself.  As someone once said of the General Theory of Relativity:

“Gravity is the most familiar force. We are subject to it every day of our lives. Newton gave us his ‘law of gravity,’ which describes its effect but doesn’t explain it. “I frame no hypotheses,” he wrote. Einstein wasn’t so prudent when he introduced his “postulates.” Unfortunately, his unreal geometry doesn’t explain gravity either. The usual demonstration using heavy steel balls on a rubber sheet to represent ‘gravity wells’ relies on gravity as its own explanation!”New Scientist, 5th. July 1973.
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[evan_au]

it is hard to imagine future generations of astronauts on a Mars mission relying solely on General Relativity to navigate

General Relativity is already used for multiple gravitational slingshots, so it will be fine for a "simple" Earth-Mars transfer orbit.

Probably the main difference with a manned mission is that a manned mission occasionally vents gases or liquids into space, an orbital disturbance that must be corrected.

Getting safely through the atmosphere of Mars has always been a challenge, but this requires a better understanding of hypersonic airflow, not an improvement on General Relativity.

[rmolnav]

#56 evan_au said:
"General Relativity is already used for multiple gravitational slingshots, so it will be fine for a "simple" Earth-Mars transfer orbit".
[/size]Could you please why G. R. is necessary to gravitational slingshots?[/color]
[/size]I consider that, similarly to original "slingshots" (arm-wrist-hand powered sling), they can be explained within the limits of Newton´s Mechanics. [/color]

[evan_au]

it is hard to imagine future generations of astronauts on a Mars mission relying solely on General Relativity to navigate...
...I consider that, similarly to original "slingshots" (arm-wrist-hand powered sling), they can be explained within the limits of Newton´s Mechanics.

I agree with this. Provided you stay well outside the orbit of Mercury, the difference between Newton & Einstein's model of gravity is so small that it will not affect orbital calculations.

Functionally, Newton's model of gravity is a subset of Einstein's model of gravity, in weak gravitational fields (such as most of the Solar System).
- So orbital planning could rely solely on general relativity
- But they may choose to use Newton's gravity because the calculations are a tiny bit faster on a computer

[yor_on]

"Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase."


There are two variables to the pebbles falling Bill, well, as I see it. One is about the ships density, the 'material' as it might be, I would prefer 'unobtanium' of a 'infinite rigidity' for this one. Then it is the source of the ships gravity (the bottom of the ship) creating the ever increasing displacements in time. The origin of those displacements are its 'center of mass' as described from a inside so I would expect those pebbles to increasingly separate just as they should, if let go from some separation of elevation, on Earth. The first argument you use is one where you assume that the rockets center of mass (engine) is evenly distributed over the whole 'floor' of the rocket, right? I actually wonder there whether the rockets 'center of mass' won't be situated in a 'middle' of that floor too?