Gravity question

[Z28Jerry]

As stated I have a "artificial" gravity question. I have always been told that the size/density of a body determines it's gravitational pull. So people say that some of those sci-fi movies where the ships are the size of a small planet would flood the earth with tidal waves and whatnot because they are large enough to have thier own gravitational field?

So, if we built a spacecraft large enough, it would have it's own gravity?

I know it would be slight because the ship would have very little mass/denisty, but it would have a -little- gravity? So, when the space shuttle is in orbit, does it have a tiny amount of gravity? Maybe not measurable with our instrumentation, but still there anyway?

I have phrased my question poorly, but I think you can get the gist of meaning, thanks in advance.

Jerry

Oh, how about an object on a an object, what about that? If a skyscraper were made of lead (suspend arguments concerning foundation issues, lol). Even though it is on a body (the Earth) that has enough mass/density to have a large gravitational field, would the lead skyscraper also have it's own gravity?

[Ken G]

As stated I have a "artificial" gravity question.

Actually the gravity you are talking about would not really be "artificial", it's perfectly real, but I know what you mean.

I have always been told that the size/density of a body determines it's gravitational pull.

Substitute: mass.

So, if we built a spacecraft large enough, it would have it's own gravity?

Absolutely, anything with mass would certainly have its own gravity. Your keyboard is tugging on you right now. But yes, it is so minute as to be unmeasureable. Roughly speaking, since we are talking about Earthlike densities in general here, the gravity at the surface of any object is roughly proportional to the distance across the object, i.e., the linear size of the object. (OK a space shuttle has a big cavity in it, so you wouldn't measure the whole length of the shuttle for this calculation, but you get the idea.) So if you are comparing to Earthlike gravity, you are comparing the object size to the diameter of the Earth. So the surface gravity of any spaceship would be pretty small even if it was a huge ship. Tidal effects over the size of the whole Earth are another story, they do tend to be a lot easier to get, and I could believe that a huge spaceship like a small moon could produce important tidal effects on the oceans if the ship got close enough to Earth.

Oh, how about an object on a an object, what about that?

It makes no difference where the object is relative to other massive bodies, a mass will always produce gravity. For gravities that are not huge, the effects are simply additive.

[Elyk]

Every object has it's own gravity. There's gravity between you and your computer and everything around you. We just can't feel it..probably because were either so used to it or because its so slight we cannot feel it. So, if you build anything massive enough it would eventually pull things closer to it. It would have to be huge to do that though...

[Kaptain K]

FWIW, the gravitational effects of mountains can and have been measured. A plumb-bob on the plains east of Denver will be slightly off of vertical toward the nearby mountains.

[Bob]

Cavendish in the 1700's determined the gravitational constant with an apparatus that fit in his lab easily and used two lead weights weighing only a few hundred pounds.
I heard an interview with a scientist from the University of Washington who built an apparatus to detect gravity waves. To maintain calibration he has to account for the gravity produced by the dozens of cars which fill the parking structure next to the physics building during the day but leave at night.

[Nereid]

FWIW, the gravitational effects of mountains can and have been measured. A plumb-bob on the plains east of Denver will be slightly off of vertical toward the nearby mountains.

FWIW, the gravitational attraction of masses as small as 1 gram (and maybe 1 µg) has been measured :surprised

These experiments were done in part to see if there was a short-distance deviation from the inverse-square law of Newton (none has - so far - been found).

[north]

FWIW, the gravitational attraction of masses as small as 1 gram (and maybe 1 µg) has been measured :surprised

These experiments were done in part to see if there was a short-distance deviation from the inverse-square law of Newton (none has - so far - been found).

how close together(or distance apart) were these masses i wonder before they attracted each other?

[Nereid]

how close together(or distance apart) were these masses i wonder before they attracted each other?

IIRC, the experiments were o several kinds, but typical distances were µ, or small multiples.

[north]

IIRC, the experiments were o several kinds, but typical distances were µ, or small multiples.

first what is u again be darned if i can remember.( although i know it is very small)

but the reason i asked is that since electromagnetic force is much stronger than gravity wouldn't the attraction have more to do with the magnetic fields and atomic rotation than with gravity, persay? if gravity waves are detected are these gravity waves not moving OUT from the mass not inwards? how does a wave that is moving OUTWARD, attract a mass towards itself?

[Nereid]

first what is u again be darned if i can remember.( although i know it is very small)

but the reason i asked is that since electromagnetic force is much stronger than gravity wouldn't the attraction have more to do with the magnetic fields and atomic rotation than with gravity, persay? if gravity waves are detected are these gravity waves not moving OUT from the mass not inwards? how does a wave that is moving OUTWARD, attract a mass towards itself?

micron (= 10^-6 metre), approx the width of a human hair.

The wonderful thing about electrostatic interactions is that they can be reduced essentially to zero, by 'grounding'.

For magnetic fields it's a little more tricky, but not much more (Faraday cage, or similar).

IIRC, the most difficult part of these experiments involved the software/math (the 'test mass' was on a silicon cantilever, excavated by techniques similar to those used to make the chips that are in the PC you use to communicate with BAUT) and residual gas in the evacuated chamber.

[north]

micron (= 10^-6 metre), approx the width of a human hair.

The wonderful thing about electrostatic interactions is that they can be reduced essentially to zero, by 'grounding'.

i was't talking of electrostatic interactions. i was tlking about the electric current in the atom its self. if you completely took the electric current and therefore the electromagnetism out of the atom what would then happen? it would dissolve.

For magnetic fields it's a little more tricky, but not much more (Faraday cage, or similar).

Faraday cage??

IIRC, the most difficult part of these experiments involved the software/math (the 'test mass' was on a silicon cantilever, excavated by techniques similar to those used to make the chips that are in the PC you use to communicate with BAUT) and residual gas in the evacuated chamber.

still regardless, if these gravity waves are found it would actually contradict the contemparay view that gravity draws things in. gravity waves produce a paradox for contemparay physics. how does gravity draw matter in, if the gravity waves are moving away from the center of the mass that produced these gravity waves??

[Nereid]

I think you're making this much more complicated that it need be north.

Remember Newton?

F = Gm₁m₂/d²

Cavendish's experiment involved masses ~1-100 kg and separations ~cm-m; Newton's ~10²²- 10³⁰ kg, and ~10⁸-10¹² m (the solar system, per Kepler's results). Since Cavendish, the mass-distance plane in which the inverse square nature of gravity has been tested has been expanded considerably.

The kinds of 'electric current in the atom its self' considerations were investigated in modern times by Loránd ("Ronald") Eötvös (no such variations of the kind to which you allude were found), and, more recently, with far greater sensitivity, by the University of Washington (the 'Eot-Wash' tests).

'Gravity waves' (a.k.a. gravitational radiation) are a prediction of Einstein's theory of General Relativity (which is needed to account for good observations in certain 'large mass/small distance' parts of the mass-distance plane); they have not yet been observed (but GR has passed every test to which it's been subject, so far, with flying colours). I don't understand the relevance of your question here (other than that it seems poorly worded - there is no such 'paradox for contemporary physics').

[Jeff Root]

FWIW, the gravitational attraction of masses as small as 1 gram
(and maybe 1 µg) has been measured

These experiments were done in part to see if there was a short-
distance deviation from the inverse-square law of Newton (none
has - so far - been found).

IIRC, the experiments were o several kinds, but typical
distances were µ, or small multiples.

I read of that experiment a couple of years ago, but didn't
read the details. I agree about the short distance aspect of
the experiments but find it hard to believe that the masses
could be that small. On the other hand, if the distances are
small, how could the masses not be small?

micron (= 10-6 metre), approx the width of a human hair.

The symbol µ for "micron" is still widely used, but is outdated
and considered nonstandard. The SI metric unit symbol is µm and
the correct name is "micrometre" or "micrometer". It is one
millionth of a metre, or one thousandth of a millimetre.

The width of a human hair is about 0.1 mm, or 100 µm.

-- Jeff, in Minneapolis

[Jeff Root]

(the 'test mass' was on a silicon cantilever, excavated by
techniques similar to those used to make the chips that are
in the PC you use to communicate with BAUT)

That's what I read, too. Was the other mass the Earth?

-- Jeff, in Minneapolis

[north]

I think you're making this much more complicated that it need be north.

Remember Newton?

F = Gm₁m₂/d²

Cavendish's experiment involved masses ~1-100 kg and separations ~cm-m; Newton's ~10²²- 10³⁰ kg, and ~10⁸-10¹² m (the solar system, per Kepler's results). Since Cavendish, the mass-distance plane in which the inverse square nature of gravity has been tested has been expanded considerably.

i'm not discussing whether Kepler or Newton were wrong or right( in which for one Kepler did not get into the physics of why, just the geometry and Newton just called this attraction gravity but that he himself had no defintion of what gravity was). what i'm trying to point out is that the mechanism of gravity are electromagnetic and therefore electric current based.

The kinds of 'electric current in the atom its self' considerations were investigated in modern times by Loránd ("Ronald") Eötvös (no such variations of the kind to which you allude were found), and, more recently, with far greater sensitivity, by the University of Washington (the 'Eot-Wash' tests).

i'm not sure what variations you are referring too, that i have mentioned??

'Gravity waves' (a.k.a. gravitational radiation) are a prediction of Einstein's theory of General Relativity (which is needed to account for good observations in certain 'large mass/small distance' parts of the mass-distance plane); they have not yet been observed (but GR has passed every test to which it's been subject, so far, with flying colours). I don't understand the relevance of your question here (other than that it seems poorly worded - there is no such 'paradox for contemporary physics').

yes there is a paradox, for if these "gravity waves" exist and since the source of the gravity waves are from within a mass and therefore move outward then the observations are the result of a different force.

how gravity waves move out but pull matter in?? name any wave that moves out from the source of the wave, but pull something in??

[Jeff Root]

the reason i asked is that since electromagnetic force is much
stronger than gravity wouldn't the attraction have more to do with
the magnetic fields and atomic rotation than with gravity, persay?

The fact that electromagnetic forces are so much stronger than
gravitational force makes measuring gravitational force between
small masses very difficult and tricky. What the experimenter
has to do is eliminate all unbalanced electromagnetic forces
in the experimental apparatus. That is done, as Nereid said,
through use of Faraday cages and grounding.

Faraday cage??

Do a web search, or use an online dictionary, or Wikipedia:
http://en.wikipedia.org/wiki/Faraday_cage

if gravity waves are detected are these gravity waves not
moving OUT from the mass not inwards? how does a wave that is
moving OUTWARD, attract a mass towards itself?
...

still regardless, if these gravity waves are found it would
actually contradict the contemparay view that gravity draws
things in. gravity waves produce a paradox for contemparay
physics. how does gravity draw matter in, if the gravity waves
are moving away from the center of the mass that produced these
gravity waves??

First, the term "gravity wave" has a specific meaning which
is different from what you are talking about. The same goes
for the term "gravitational wave", which is yet another thing.
But I know what you mean, and I'm not sure there is a term
for it. Best to say "the force of gravity", I think.

Your question may be stated as "How can a force caused by an
object be toward that object?" Or, "How do attractive
forces attract?" Gravity is not special in this regard. You
can as well ask how a positive electric charge attracts a
negative electric charge, or how a magnet attracts iron.

I can't answer that question, but I think the problem is one
of conceptualization, rather than a problem of physics. Some
ideas seem obvious, while others don't, even if they are equally
true. A force of attraction is unintuitive, and thus difficult
to accept. But it poses no problem for physics.

-- Jeff, in Minneapolis

[Jeff Root]

what i'm trying to point out is that the mechanism of gravity
are electromagnetic and therefore electric current based.

That is about as wrong as possible. Gravity is one of the four
known fundamental forces. Electromagnetism is another. The two
are similar in some ways, but are entirely separate.

Gravitational force is caused by energy, and is always attractive.

Electromagnetic forces are caused by electric charges, and are
attractive between opposite charges and repulsive between like
charges.

-- Jeff, in Minneapolis

[Thanatos]

I sense a departure from real science here. Nereid gave the correct explanation.

[Nereid]

I read of that experiment a couple of years ago, but didn't
read the details. I agree about the short distance aspect of
the experiments but find it hard to believe that the masses
could be that small. On the other hand, if the distances are
small, how could the masses not be small?

Here is a webpage describing one such experiment; the key part (for our discussion): "We put a small test mass on the end of a cantilever, and below this mass, we oscillate a pattern of heavy and light masses. The gravitational force between this oscillating drive mass pattern and the test mass excites the cantilever at its resonant frequency. The amplitude of the motion is measured using laser interferometry; consequently, we can determine the gravitational force between the drive and test masses. By varying the distance between masses, we can discover how the gravitational force scales with distance."

The symbol µ for "micron" is still widely used, but is outdated
and considered nonstandard. The SI metric unit symbol is µm and
the correct name is "micrometre" or "micrometer". It is one
millionth of a metre, or one thousandth of a millimetre.

The width of a human hair is about 0.1 mm, or 100 µm.

I sit corrected; thanks Jeff (it seems human hairs vary quite a bit in thickness; by ~50% around a mean of ~90 µm!)

[Nereid]

i'm not discussing whether Kepler or Newton were wrong or right( in which for one Kepler did not get into the physics of why, just the geometry and Newton just called this attraction gravity but that he himself had no defintion of what gravity was). what i'm trying to point out is that the mechanism of gravity are electromagnetic and therefore electric current based.

Well, that seems to be about as pure an ATM idea as you can get!

If you would like to present this idea, please do so in the ATM section of BAUT, not here in Q&A. (Of course, if I'm wrong, and you have a couple of good references from peer-reviewed journals to show that the 'the mechanism of gravity are electromagnetic', then I'll happily allow further discussion here!

i'm not sure what variations you are referring too, that i have mentioned??

One idea that is well worth testing is 'to what extent does "the force of gravity" depend upon the atomic composition of the masses?' For example, if I have two objects whose mass is the same, but which differ considerably as to the ratio of neutrons to protons (for example), then does that make a difference?

At first glance, this might seem somewhat circular (e.g. how do you tell if the masses are the same, except by gravity?), so some ingenuity goes into the experimental design.

The results are pretty unequivocal - mass is mass is mass; along the way these experiments probably rule out large classes of your 'the mechanism of gravity are electromagnetic' idea (though they more strong constrain 'the mechanism of gravity is the strong force' better).

yes there is a paradox, for if these "gravity waves" exist and since the source of the gravity waves are from within a mass and therefore move outward then the observations are the result of a different force.

how gravity waves move out but pull matter in?? name any wave that moves out from the source of the wave, but pull something in??

I see that Jeff has already addressed this.

[Z28Jerry]

That's why I love this place, I learn more in a single thread like this than in a whole YEAR in public school. Really.

[Tim Thompson]

Faraday cage??

• Faraday Cage (Wikipedia)
• How does a Faraday Cage work? (Physlink.com, "Ask the Experts")

A Faraday cage will eliminate all outside electromagnetic wave interference. A friend of mine lived for many years under the forest of broadcast antennae on Mt. Wilson above Los Angeles, and had to wrap a Faraday cage around his VCR to make it work.

still regardless, if these gravity waves are found it would actually contradict the contemparay view that gravity draws things in. gravity waves produce a paradox for contemparay physics. how does gravity draw matter in, if the gravity waves are moving away from the center of the mass that produced these gravity waves??

Nonononononno. So imagine a universe in which there aree two and only two masses, fixed in space, and that in this universe general relativity is the correct theory of space time. Now let one of the masses get pushed from where it is, to some other place, so that the distance it moves is short compared to the distance between the two masses. The Newtonian "force" of gravity between these two masses is GM₁M₂/r². The distance "r" between them has changed. How long will it take before the second mass feels the effect of the change in "r" and a different force?

In an old Newtonian view of the universe, it would happen instantaneously. In the general relativistic model, the change in force will propagate between the two masses at the speed of light, so it will take r/c seconds (r is the distance & c is the speed of light) for the effect of the change to arrive at the second mass.

That's what a gravitational wave is, in general relativity, the propagation through spacetime, of a change in gravitational force. Observation of the spindown time of a binary pulsar is consistent with the general relativistic prediction of energy lost to the system via the radiation of gravitational waves. Using a fully general relativistic form for the force will not change the result, which is based not on the "force" equation, but rather on the fact (in general relativity) that gravity takes time to get from one place to another.

Pedantic Aside:
A gravity wave is a pressure wave in an atmosphere, whereas a gravitational wave is the wave propagation of a gravitational field. It is common enough for professionals to use the words "gravity wave" as synonymous with "gravitational wave", that I am probably fighting a losing battle here, but I'm a linguistic "purist", so I like to see the right words used for the right things.

• Self-Consistency of Relativistic Observables with General Relativity in the White Dwarf-Neutron Star Binary PSR J1141-6545; M. Bailes, et al., The Astrophysical Journal Letters 595(1): L49-L52, September 2002
• Pulsar timing observations and tests of general relativity in double-neutron-star binaries; I.H. Stairs, The Ninth Marcel Grossmann Meeting, July 2000, proceedings published in 2002.
• Measurement of Relativistic Orbital Decay in the PSR B1534+12 Binary System; I.H Stairs, et al., The Astrophysical Journal 505(1): 352-357, September 1998

[Ken G]

A friend of mine lived for many years under the forest of broadcast antennae on Mt. Wilson above Los Angeles, and had to wrap a Faraday cage around his VCR to make it work.

Yikes, I suspect there's a lot of hysteria about EM effects on the human body, but still I'm not sure I'd want to spend years in a place where my VCR needed a Faraday cage unless I had one for my whole house!

Observation of the spindown time of a binary pulsar is consistent with the general relativistic prediction of energy lost to the system via the radiation of gravitational waves. Using a fully general relativistic form for the force will not change the result, which is based not on the "force" equation, but rather on the fact (in general relativity) that gravity takes time to get from one place to another.

But surely the rate of spindown depends on system parameters like mass and period, so I'm not sure what you mean that it is not based on the force equation.

Hear hear on the distinction between gravity waves and gravitational waves, that's going to cause lots of confusion in the near future if we don't make the distinction now when we have a chance!

[mugaliens]

Someone said gravity travels at the speed of light. Another said that it's instantaneous. What's the right answer, and how do we know?

[Tim Thompson]

But surely the rate of spindown depends on system parameters like mass and period, so I'm not sure what you mean that it is not based on the force equation.

What I meant was that the general conclusion, that there should be "gravitational waves", depends on the finite speed of gravity. Of course, a computation of the expected spin down rate requires a fully general relativistic treatment of the dynamics. In that case, the Newtonian approximation would never do.

Someone said gravity travels at the speed of light. Another said that it's instantaneous. What's the right answer, and how do we know?

The instantaneous propagation of gravity is an element in Newtonian mechanics, although Newton himself did not believe that this should be the case, and it somehwhat bothered him. Nowadays, Tom Van Flandern claims instantaneous propagation of gravity, but he's just about the only one who believes his argument. General relativity requires that gravity travels at the speed of light. Observation, as I outlined above, is consistent with this theory, and so in the absence of compelling evidence to the contrary, I assume it is correct that gravity travels at the speed of light.

[Tensor]

Nowadays, Tom Van Flandern claims instantaneous propagation of gravity, but he's just about the only one who believes his argument.

Tim, do you know if Tom Van Flandern has done any calculations, using his ideas, based on PSR 1913+16's rate of inspiral and how it matches with his predictions vs GR? I would think if he has, and it matches, he would be trumpeting that fact. If he has and it doesn't match, has he admitted it? And, if he hasn't done the calculations, why not and has anyone challenged him on this?

[mugaliens]

Thanks, Tim, and makes sense because can't send info faster than light. Move a rock and info would be sent faster than light if gravity were instantaneous.

[north]

Gravitational force is caused by energy, and is always attractive.

Jeff, in Minneapolis

Jeff and Nereid what is this energy?? thats the tough part is it not? to say energy, is the cause of Gravitational force, is so easy but to define it and to describe or suggest its essence( where does this energy come from) is not easy.

that is why i have suggested the ideas i have. what frequency for instance does this "gravitational wave" have? and is this predicted by GR? apparently not, otherwise we would have found it.

what if the gravitational wave is caused by the vibrational wavelength of all the consituents the mass its self. that is the sum of all vibrations(atomics) within the mass its self produce( once all the peaks and troughs are accouted for) a fundamental wave length, characteristic of this or that certain mass?

does electric-current its self actually vibrate at a certain wave length??

[Tim Thompson]

what frequency for instance does this "gravitational wave" have? and is this predicted by GR? apparently not, otherwise we would have found it.

Yes, the frequency & amplitude of gravitational waves are predicted by general relativity, depending very much on the details of the source. Supernovae & colliding neutron stars, & etc., they all have different spectral energy distributions. They have not been found yet, because the amplitudes are so small that we have not been able to build instruments sensitive enough to detect them.

It is only now that we are on the threshold of being able to do this, and it is anticipated that LIGO, or LISA, or one of the other upcoming experiments will detect gravitational waves.

[north]

Yes, the frequency & amplitude of gravitational waves are predicted by general relativity, depending very much on the details of the source. Supernovae & colliding neutron stars, & etc., they all have different spectral energy distributions. They have not been found yet, because the amplitudes are so small that we have not been able to build instruments sensitive enough to detect them.

It is only now that we are on the threshold of being able to do this, and it is anticipated that LIGO, or LISA, or one of the other upcoming experiments will detect gravitational waves.

what then are these frequences and amplitudes specificly?