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Author Topic: Dark Matter - Is gravity really constant?  (Read 9433 times)

Offline Geezer

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Dark Matter - Is gravity really constant?
« on: 14/08/2009 19:51:35 »
Gravitational force results from matter distorting space. I suppose this creates a sort of "gradient" in space that allows objects to "fall" towards each other. Our measurements of this effect are very precise. However, many of these measurements are based on observations in our general locality.

Are there experiments that confirm the gravitational force in interstellar, or intergalactic, space is the same as the force we observe in this locality?

Is it possible that gravitational force is attenuated when there is a lot of distortion of space because of a lot of matter? Could the gravitational effect be greater where space is less distorted by matter?


 

Offline glovesforfoxes

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Dark Matter - Is gravity really constant?
« Reply #1 on: 14/08/2009 20:56:28 »
Quote
Is it possible that gravitational force is attenuated when there is a lot of distortion of space because of a lot of matter?

well no - things with larger mass exert more distortion on space - gravity on the moon is weaker than earths, gravity on jupiter is stronger than earths. i feel i may have missed something here though, perhaps i have misunderstood? ???
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #2 on: 14/08/2009 22:44:32 »
Gravitational force certainly depends on mass. I'm suggesting that two masses might experience different attractive forces at different places in the cosmos and that the difference is a function of the amount of matter in the general vicinity, in other words, the uniformity of space.

Space in our solar system must be significantly distorted in order to maintain the planets in orbit around our Sun. The forces involved are truly colossal. Therefore, our space is anything but uniform.

Presumably, as we move into interstellar space, space becomes more uniform because there is a lot less matter to distort space there, and as we move into intergalactic space it becomes yet more uniform because there is almost no matter there.

So, if we accept that "gravity" is a manifestation of an interaction between matter and space, why would we assume that there is a gravitational constant throughout space?

If gravity is more effective when space is more uniform can we eliminate the need for dark matter?
 

Offline glovesforfoxes

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Dark Matter - Is gravity really constant?
« Reply #3 on: 14/08/2009 23:37:05 »
ah, i see. then i honestly have no idea, though i'd be interested to hear the answer from someone who did :)
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #4 on: 15/08/2009 03:57:08 »
Me too.

I would like to understand if there is evidence to support that gravity is invariant throughout space.

On the other hand, if we accept that gravity is only a manifestation of an interaction between matter/mass and space, why would we assume that "gravity" is constant in all space?

If it is not constant, what effect might that have on our perceptions regarding dark matter?
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #5 on: 15/08/2009 06:08:57 »
I think the basis for the existence of "dark matter" is based on the "gravitational force" exerted between bodies. However, if we accept that there is no direct gravitational force between bodies and the observed phenomena are a function of how the bodies influence space, then it would seem reasonable to expect that gravitational effects that result from matter/mass interactions with space are rather complex.
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #6 on: 15/08/2009 06:26:38 »
In other words, could the existence of "Dark Matter" be based on a false premise? Yikes! Would that be an unpopular idea.
 

Offline LeeE

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Dark Matter - Is gravity really constant?
« Reply #7 on: 15/08/2009 16:09:39 »
Weeell...  gravity can't be attenuated but it can be summed and negated.  Probably the easiest illustration is to consider the gravitational force at the center of a spheroid volume i.e. at the center of a planet.  The gravitational 'force' generated by the planet is greatest at its surface and reduces as you tunnel down towards its center.  At the center, the net gravitational force is zero.

An everyday (if you live by the sea) example of how gravity can appear to change over time, is the way that our ocean and sea tides vary according only to the alignment of the major astronomical bodies in our system, primarily the Moon and Sun, not because any masses or forces are changing.

When you start looking at things on the scale of galaxies though, you can tend to view entire solar systems and even multiple star systems i.e. binary and trinary systems etc. as point masses because the distances between each system is so great.  It's in this context that Dark Matter seems to be necessary.  However, Dark Matter doesn't have to be some exotic form of matter - it could just be lots of brown dwarf, super-Jupiter like, sub-stars, which never acquired enough mass to ignite and which would be very difficult to detect, even within our own galaxy.
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #8 on: 15/08/2009 18:07:08 »
Do we have evidence that gravitational attraction is the same in interstellar space as it is is in our solar system? Actually, there is quite a lot of evidence to suggest that it is significantly different! Gravity is a consequence of the distortion of space. Matter does not directly attract other matter. If space is significantly distorted by a lot of matter, as it must be in our solar system, would it not be reasonable to think that the gravitational effect might be at least slightly different where space is less distorted.

I say "less distorted" because, even intergalactic space must be slightly distorted.

I think it's going to take an awful lot of brown dwarfs to account for the "missing mass", but as we are getting a lot better at detecting dark objects in space, and if there are a very large number of brown dwarfs, it should not be long before we start to find some.

Alternatively, we might be missing something here - not just some matter! Our observations might really be telling us our model of gravity could do with a little recalibration. Is that any more outlandish than the notion that space is chock full of dark matter? Sometimes it is good to take the observed evidence at face value and challenge the accepted theory rather than assume the accepted theory is completely accurate.
 

Offline glovesforfoxes

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Dark Matter - Is gravity really constant?
« Reply #9 on: 15/08/2009 18:47:35 »
i'm sure someone with much more knowledge of physics has considered that, geezer.. though there is a psychological bias to make evidence fit a theory rather than refute it, science's purpose is to do exactly the opposite. don't take it that i'm trying to shut you up though geezer, just making a sort of side point :P
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #10 on: 15/08/2009 19:10:26 »
Glovesforfoxes:

As you have no way of knowing how much I do, or do not know about physics, I have to assume you are referring to those with much more knowledge of physics than yourself.

I have worked with a lot of very smart scientists and engineers for over forty years. Sometimes, they are so close to the problem that they can't see the wood for the trees. Conventional wisdom isn't always so wise.
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #11 on: 15/08/2009 19:33:07 »
Foxglove:

So, what is your take on "Dark Matter"?

Do you believe we can only account for a mere fraction of matter while most of it remains invisible, or are you just standing on the sidelines holding up score cards?
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #12 on: 16/08/2009 01:01:26 »
Come to think of it, the term "Dark Matter" presupposes the solution for the problem by assuming that our understanding of gravity is correct and, therefore, there must be a lot of invisible matter in space.

A less misleading term might be something like "Universal Gravitational Anomaly". Introducing terms like "dark matter" when there is no evidence that any such a thing exists is nothing more than a red herring, regardless of how many people have fallen for it.
 

Offline krytie75

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Dark Matter - Is gravity really constant?
« Reply #13 on: 16/08/2009 01:10:42 »
Hi Geezer.

In answer to your original question of can objects of similar mass experience different gravitational forces, I think the answer is no.  I base this on an explanation by Stephen Hawking in his book A Brief History of Time.

It's all to do with that crazy bit of physics that causes two objects of different mass to fall at the same speed in a vacuum.  Consider two planets of equal mass. One of Newton's theories "states that every body attracts every other body with a force that is proportional to the mass of each body."  Hence if you double the mass of one of the planets, it will exert twice as much force on the other.  However, having twice the mass also means half the acceleration (as twice as much energy is required to move it), so these two effects exactly cancel each other out.  If gravity was not constant across the universe then this principal would not hold true.

I believe it has been recorded to be correct by astronomical observation.

If anyone knows if this is correct or if I'm talking utter rubbish I'd be happy to know.

Ta! Jon

P.S. The sections of A Brief History of Time I'm talking about can be found in the first few pages of chapter 2 'Space and Time'. A complete copy of the book can be found here... http://www.nt.ntnu.no/users/lale/e_book/stephenHawking-ABriefHistoryOfTime.pdf
« Last Edit: 16/08/2009 01:25:48 by krytie75 »
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #14 on: 16/08/2009 01:52:17 »
Hi Geezer.

In answer to your original question of can objects of similar mass experience different gravitational forces, I think the answer is no.  I base this on an explanation by Stephen Hawking in his book A Brief History of Time.

It's all to do with that crazy bit of physics that causes two objects of different mass to fall at the same speed in a vacuum.  Consider two planets of equal mass. One of Newton's theories "states that every body attracts every other body with a force that is proportional to the mass of each body."  Hence if you double the mass of one of the planets, it will exert twice as much force on the other.  However, having twice the mass also means half the acceleration (as twice as much energy is required to move it), so these two effects exactly cancel each other out.  If gravity was not constant across the universe then this principal would not hold true.

I believe it has been recorded to be correct by astronomical observation.

If anyone knows if this is correct or if I'm talking utter rubbish I'd be happy to know.

Ta! Jon

P.S. The sections of A Brief History of Time I'm talking about can be found in the first few pages of chapter 2 'Space and Time'. A complete copy of the book can be found here... http://www.nt.ntnu.no/users/lale/e_book/stephenHawking-ABriefHistoryOfTime.pdf

Bodies may not actually attract each other directly. That is what we perceive and measure, but the current theory is much more interesting. Bodies have mass, and that mass actually warps space, so much so that the bodies "fall" toward each other. In effect, space is pushing the bodies towards each other.

I will check out the link. Thanks!
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #15 on: 16/08/2009 05:28:39 »
krytie75:

I did check out the link, but I didn't see anything to refute or support my notion. I did learn that the Earth actually travels is a straight line as it orbits the Sun. The Sun distorts space so much that a "straight" line actually forms a circular path around the Sun. (I am not making this up!) That's what holds our planet in orbit.

I suppose we could test this if we had a really powerful "laser" (sorry). If we point it in just the right direction the beam will bend all the way around the Sun and come right back at us. It will take about 35 minutes to show up of course, and aiming it would be tricky, to say the least.
 

Offline glovesforfoxes

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Dark Matter - Is gravity really constant?
« Reply #16 on: 16/08/2009 13:35:03 »
i'm just standing on the sidelines, but not with scorecards. i've already said i know nothing about this topic, i was just offering a reminder that there are many researchers working on physics. it is possible that they can't see the wood for the trees, but i find this unlikely considering there are physicists all over the world in every continent (and therefore one system of thinking is not influencing them all, besides, of course, the scientific method)
 

Offline syhprum

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Dark Matter - Is gravity really constant?
« Reply #17 on: 16/08/2009 15:00:05 »
The earth orbits the Sun because of its modest speed of 29.87 Km/s, Photons shot off at 300,000 Km/s would only be deflected by a tiny amount.
« Last Edit: 16/08/2009 17:29:14 by syhprum »
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #18 on: 16/08/2009 17:21:24 »
The earth orbits the Sun because of its modest speed of 29.87 Km/s, Photons shot of at 300,000 Km/s would only be deflected by a tiny amount.

That does seem a lot more likely!
 

Offline Turveyd

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Dark Matter - Is gravity really constant?
« Reply #19 on: 16/08/2009 22:24:57 »
Geezer,  There is no distortion,  the earth is traveling in a straight line,  but at the same time its in a free fall being pulled towards the Sun nothing more,  the shuttle orbits the earth in the same way.

If you shoot a Laser into space,  the laser will go in a straight line through space no curving around,  radio waves couldn't be sent / received to space craft if this was not the case.

Dark Matter to me, is ludicrous it's a fix for wrongly calculating the mass in the universe,  the mass we can't ever possibly calculate by adding up all of the light in the sky,  laughable.

Its true,  on the earth 2 objects of different weight fall at the same speed in a vacuum,  but the planets are at different distances,  further from the sun lower the gravitational effect,  the there is the speed there traveling at ( in a straight line ) to consider

 

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Dark Matter - Is gravity really constant?
« Reply #20 on: 16/08/2009 23:34:05 »
Nothing is traveling in a straight line - not even light, in a Universe where there are objects with mass. Most things are "as near as dammit", of course. But not a straight straight line.
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #21 on: 17/08/2009 01:17:17 »
Well, yes and no.

According to Hawking - A brief History of Time - quote:

“In general relativity, bodies always follow straight lines in four-dimensional space-time, but they nevertheless appear to us to move along curved paths in our three-dimensional space. The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space time, it appears to us to move along a circular orbit in three-dimensional space.”
End quote.

Photons have zero mass, (perhaps because they are only quanta of energy propagating through space-time) so they are not influenced my mass. However, objects with mass do distort space, and therefore photons are influenced, indirectly, by mass. If I understand correctly, this is why “gravitational lensing” works.
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #22 on: 17/08/2009 05:11:14 »
i'm just standing on the sidelines, but not with scorecards. i've already said i know nothing about this topic, i was just offering a reminder that there are many researchers working on physics. it is possible that they can't see the wood for the trees, but i find this unlikely considering there are physicists all over the world in every continent (and therefore one system of thinking is not influencing them all, besides, of course, the scientific method)

Glovesforfoxes:

Consider that if a certain patent examiner in Switzerland had assumed that physicists would figure everything out, our world would be a very different place.

Now, I freely admit that I'm no Einstein, but perhaps, if I can provoke just a little lateral thinking, it might help someone figure out one of the greatest mysteries in science. I may embarrass myself in the process, but nothing ventured...

BTW, the idea that gravity is "different" in our locality has been suggested by physicists to explain the "missing matter" in the past. However, this is not generally accepted (correctly, I think) because it would require that there is something "special" about our part of the universe. I am not suggesting there is. I am questioning whether the amount of matter in a volume of space can in some way alter the gravitational effect within that volume. This would require that space has some novel properties, but as there is little agreement regarding what space actually is, and as we know matter does interact with space, why would we assume the interaction does not expose novel properties?

Perhaps there are experiments that demonstrate my question has no merit. Fine with me. I'm just hoping someone can point them out.
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #23 on: 17/08/2009 18:23:35 »
"Its true,  on the earth 2 objects of different weight fall at the same speed in a vacuum"

No disagreement.

For clarification, here's an experiment that might help to explain my question:

On Earth, take two spheres. For convenience, they can be of equal mass. Measure the apparent attractive force between them. (Include tests to ensure they are not being attracted by electrostatic force, magnetic force, air movement etc.)

Transport the same two spheres deep into intergalactic space. Repeat the measurements.

Same results as on Earth? Case closed! Keep looking for dark matter and other explanations.

It's a simple enough experiment, just hopelessly impractical. But perhaps a more practical experiment already exists.
 

Offline Turveyd

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Dark Matter - Is gravity really constant?
« Reply #24 on: 17/08/2009 19:08:06 »
The spheres in a vacuum,  on earth or deep space would move the same correct but....

The Earth is a Sphere,  moving in a fixed direction,  but being pulled towards the sun at the same time,  if you stopped the earth from moving relative to the sun then it would free fall straight into the sun ( Lets not try it )

If you sped the earth up then the orbit and distance from the sun would increase and eventually we'd fly off into deep space,  then freeze.

The spheres / objects within the air given the same aerodynamic properties then the heavier object will reach a higher top speed as it's mass will counteract the resistance from the air.

Easy!!

 

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