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Author Topic: Expansion as analogy to gravity - can it be falsified irrefutably?  (Read 3869 times)

Offline sasam

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A friend of mine - also a science enthusiast - mentioned some guy's weird theory about universal expansion a while ago to me with great enthusiasm. Upon closer inspection I noticed that the theory had a few proponents in the past, but never seemed to make mainstream science.

Now this is not only the expansion that we are used to - namely that the fabrick of the universe stretches outwards. This proposal goes further to state that all matter may in fact be expanding at a constant rate. No observer would actually experience such expansion, since the observer would also be expanding.

The proposal now is that what we experience as an attractive force between objects can be the effect created by the objects actually expanding towards each other.

After my initial negative reaction towards this preposterous suggestion I became intrigued to think of how such a model could be analogous to our understanding of gravity. It must be said that it is very difficult (for me in any case) to think myself into an ever expanding spatial framework. For example - if a small sphere is travelling in a straight line past a big sphere while both expand at the same rate, the smaller object will obviously "bend" towards the bigger object. What I struggle to contemplate is whether it is possible for this smaller object (moving in a straight line)to actually circle around the bigger object for a specific relative speed and direction.

What I can see - however - is that there is a definite analogy with the idea of a gravitational force. I also get the impression that this proposal can not be argued away as easily as one may think.
 
The critique I've seen on the net for this proposal all centred on the weak add-on arguments surrounding this proposal, as well as on the authority (and cheekyness) of the proposer and not on the basis of the proposal itself. As a matter of fact - there was some interest in how one would go about developing a mathematical model for this proposal.

Much can be said and detail can be delved into, but all that I want to know is whether this proposal can be irrefutably falsified? Perhaps using ping pong balls in stead of lead balls in the Cavendish experiment? Perhaps finding the so called "gravity particle"? It will lay my mind at rest that I need not take the "lift acceleration" analogy to gravity quite so literally in that our earth constantly pushes upwards instead of us being dragged down. (Doesn't make for a good night's rest)

 


 

Offline Bored chemist

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"Perhaps using ping pong balls in stead of lead balls in the Cavendish experiment?"
Been done- sort of. The effect Cavendish measured has been verified using other materials.
What I'm not sure of is whether or not this refutes the original postulate.

 

Offline yor_on

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"if a small sphere is travelling in a straight line past a big sphere while both expand at the same rate, the smaller object will obviously "bend" towards the bigger object."

Why and how?

Assume that both expand, and I presume that the same should go for the 'space' between them? Why would the spheres be 'attracted/bend' more, and how?

If I assume only matter expanding, then it seems as 'bye bye space' :)
Can't see why that would change 'gravity's' proportions unless you assume that gravity is unsymmetrical, like this maybe? Nonsymmetric gravitational theory.

Or, what am I missing here?
The mass/distance if both grow proportionally (& space) shouldn't make them gravitate towards each other, if anything the opposite I guess, as gravity weakens with inverse square of the distance?

Anyone that want to make some calculations on it?
==

And thinking some more of it. It can't be true anyway as we know that 'space' between the objects/galaxies are expanding faster. That should then mean that you would have a disproportional growth where 'space' would grow faster, but not inside the galaxies? How would the idea explain that?
« Last Edit: 25/03/2011 01:11:56 by yor_on »
 

Offline Phractality

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Cosmologists prefer comoving coordinates. These are equivalent to measuring chains, whose individual links don't expand. Each link is stationary relative to comoving objects around it. Each time a link is added to the chain, there is a bit of slack, but the slack soon vanishes as all the links move away from the origin. When there is no more slack, a new link must be added to prevent the chain from snapping.

In non-expanding Euclidean coordinates, where the grid lines of space are defined by non-expanding perfectly rigid measuring tapes, the expansion of physical space is equivalent to a gravity hill centered on the origin of the coordinate system. In such a system, the ends of the measuring tapes would experience tension due to their acceleration relative to the physical space around them. At modest distances, up to a million or so light years from the origin, the shape of the gravity hill is parabolic. At greater distances, relativity alters that shape to some kind of hyperbolic trig function.

In comoving coordinates, the changing distance from the origin is considered to be "only apparent" velocity and acceleration. In non-expanding Euclidean coordinates the velocity and acceleration are real, and subject to the rules of relativity. Both coordinate systems are valid. They are like different kinds of graph paper; like lin-lin paper vs. log-log paper.

This is just my opinion, and I fear I may be shouted down by the mainstream advocates.
 

Offline yor_on

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« Last Edit: 25/03/2011 13:15:13 by yor_on »
 

Offline yor_on

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Most problems with how to see 'gravity' seems to come from that there has to be 'something' doing it, right? And if you want to see 'something' then quanta is near to use. Or a 'field'. Of those two I like 'fields' better, but what if everything is a field? SpaceTime as a 'field'? Then you would need to invent descriptions for the differences we see, wouldn't you? Space, matter etc.

So what would you call it? Densities? But that seem to assume that space too has a 'density'. Now those of you into QM will point out that it has :) but you still have to explain the classical view, which, if you want to be fair, is the one we really 'see'.

I'm not sure I like densities. I think we should use the word 'field' as in a Jello of four properties, or maybe two, three? gravity and 'distance' and 'time'. Of those three you can put gravity and distance as one and the same. So then we have two gravity and time. Those two and the 'field', or do they make the 'field'?

what happens with a particle when time is reversed?
=

As if 'gravity' would be the background  from where our 'particles' emerge, to dance under the causality chain of 'times arrow'. ( I better move this one to a more appropriate place huh :)
« Last Edit: 25/03/2011 14:48:00 by yor_on »
 

Offline sasam

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Reply to yor_on.

In a hypothetical universe where everything is expanding you can have either space and objects expanding at the same rate or at different rates. The "bending in" effect of an expanding small sphere in a straight line past an expanding large sphere is purely geometrical and assumes the space inbetween just as "nothing". This would be equivalent - I guess - to the objects expanding faster than space - which we know are "something" and thus also subject to expansion.

If - as you say - space is expanding faster than objects, then we should see objects caught up with each other in orbiting motion moving away from each other over time. Do I understand that this is the reason that (say) the moon is slowly moving away from earth or is there a more mundane reason?

Would it be correct to state: If it can be shown that space always expands faster than objects then the so called "Object expansion analogy to the force of gravity" would be effectively falsified?

In any case it would seem that we are then larger than our ancestors :) Our time machine should effectively then also be a reduction / enlargement machine apart from the fact that we will need to track the future and past motion of our position.
 

Offline yor_on

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No, I didn't say that 'space is expanding faster'. As we think of it now it's space between the galaxies that expands only. Inside the galaxy 'gravity' and matters bonding prohibit that expansion. I only pointed out that if everything, in a hypothetical universe expanded, I think gravity would lessen between the objects, even though they might increase in size. That as gravity weakens with the inverse square of the distance. So it can't be right.

As for why the moon orbits away from the Earth? It has to do with 'conservation of angular momentum'. They separate due to the 'tidal forces' created between them, creating Earths bulges/tides.
 

Offline Phractality

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Reply to yor_on.

In a hypothetical universe where everything is expanding you can have either space and objects expanding at the same rate or at different rates.

Expansion has to be relative to the measure of distance. The unit of measure is the meter, which is now defined as "1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum." You could, of course, define a different unit of distance which expands at the rate of H₀ relative to the meter. If that is your measure of space, then everything from meter sticks to galaxies will be shrinking. 

The "bending in" effect of an expanding small sphere in a straight line past an expanding large sphere is purely geometrical and assumes the space inbetween just as "nothing". This would be equivalent - I guess - to the objects expanding faster than space - which we know are "something" and thus also subject to expansion.

A lot of things do expand faster than space. When a typical object sits in sunlight, it's thermal expansion rate is several orders of magnitude faster than H₀; but when it cools down, it shrinks. When it returns to the starting temperature, it also returns to its starting size. Even the expansion of the moon's orbit due to tidal friction is faster than H₀. H₀ ≈ 2.5 x 10^-18/s. Multiply that by the diameter of the galaxy, and you get about 2.5 km/s. The galaxy is moving thru space (relative to the CMB) at about 627 km/s. So even though new space is being created inside the galaxy, it is minor compared to the galaxy's motion thru space.

If - as you say - space is expanding faster than objects, then we should see objects caught up with each other in orbiting motion moving away from each other over time. Do I understand that this is the reason that (say) the moon is slowly moving away from earth or is there a more mundane reason?

The moon moves away because tidal friction transfers Earth's angluar momentum of rotation to the moon's angular momentum due to its orbit. This effect equals about 1.65 x 10^-15/s, which is about 660 times faster than H₀. The rate is determined by the dynamics of the Earth moon system; it has nothing to do with the expansion of space.

Would it be correct to state: If it can be shown that space always expands faster than objects then the so called "Object expansion analogy to the force of gravity" would be effectively falsified?

Expansion and contraction of objects are governed by a whole different set of principles. They are independent of the expansion of space. I don't know what the "Object expansion analogy to the force of gravity" is.

In any case it would seem that we are then larger than our ancestors :) Our time machine should effectively then also be a reduction / enlargement machine apart from the fact that we will need to track the future and past motion of our position.

Turning the clock back brings us closer to the most distant galaxies, but the galaxies, stars, planets, etc., are not made smaller.
 

Offline sasam

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Thanks for the replies. I've received ample food for thought.
 

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