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Author Topic: Why is it assumed that gravity is always attractive?  (Read 17794 times)

Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #25 on: 17/06/2011 07:56:57 »
Attractive might be the wrong word Mike. You could also think of it as 'gravity' having a 'direction'. That 'direction' gives you a 'up' and a 'down' biologically. Magnets can 'attract' or 'repel'. Gravity just is.

Gravity does have a direction.  Gravity is the major source of entropy and a major arrow of time.

Saying "gravity just is", is the same as admitting that you don't understand it.
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #26 on: 17/06/2011 10:53:30 »
Mike physics does not admit to root causes, it advances understanding by generalising and modelling at a more basic level but there is no "real answer".  Everything is explained in terms of something else and you have to allow some things to be agreed without explanation.

- This is the classic feynmann clip explaining levels of understanding (in this case magnetism). 

Unfo Yoron is right - whilst we can explain gravity in terms of einstein's curved space time - and we are looking to explain it via qft and exchange of virtual gauge bosons; each of these, and other explanations, have axiomatic foundations that must be accepted without proof. At the end of the day something just is
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #27 on: 17/06/2011 18:26:16 »
imatfaal

I appreciate your dialogue but would like you to answer the two points in my previous post where I didn't understand what you were saying and asked for clarification.
Thanks
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #28 on: 17/06/2011 22:09:16 »
The whole point of the gedanken is to take the antimatter/matter particle pair crunch them together - increase energy of photons produced, remake the pair and the excess energy gained by the photons falling towards earth is matched by the kinetic energy required to move the the particle pair back up to original level.  this balance works for attractive.

if we assume that antimatter is repulsive of matter then you will get the particles again with enough kinetic energy to get them back to the original higher level BUT now the repulsive antimatter particle gains additional kinetic energy as it accelerates outwards - so instead of having two particle at rest (which is the starting situation) you have the matter at rest and the antimatter with increased velocity and accelerating outwards.  So you have gone from two stationary particles at beginning of experiment to one stationary and one fast moving at the end; that is the failure of conservation of energy.
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #29 on: 18/06/2011 09:11:16 »
The whole point of the gedanken is to take the antimatter/matter particle pair crunch them together - increase energy of photons produced, remake the pair and the excess energy gained by the photons falling towards earth is matched by the kinetic energy required to move the the particle pair back up to original level.  this balance works for attractive.

if we assume that antimatter is repulsive of matter then you will get the particles again with enough kinetic energy to get them back to the original higher level BUT now the repulsive antimatter particle gains additional kinetic energy as it accelerates outwards - so instead of having two particle at rest (which is the starting situation) you have the matter at rest and the antimatter with increased velocity and accelerating outwards.  So you have gone from two stationary particles at beginning of experiment to one stationary and one fast moving at the end; that is the failure of conservation of energy.

At the start of the experiment both particles had gravitational potential energy.  The matter particle to fall to Earth, the antimatter particle to fly off into space.

Both particles when they reach the top of the tower have exactly the same energy as at the start of the experiment.
At the start of the experiment the antimatter particle had gravitational potential energy that was trying to expel it from its position in the gravity well at the top of the tower.  The newly created antimatter at the top of the tower contains exactly the same amount of gravitational potential energy.

Energy is conserved.
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #30 on: 18/06/2011 09:43:58 »
Mike - this is why I cannot be bothered to explain things to you - you don't read answers.  I gave you an answer in terms of KE (the two particles are at same position as before the experiment so ignore PE) - think it through rather than try and move the goal posts.  Try changing frames of reference to an arbitrarily high distance (U->0) if you still cannot get you head round it. 

A little humility when challenging ideas and gedanken proposed and held by professional physicists (and I don't mean me) would do you a world of good.  The physics faq is maintained by a group of academics intimately involved in this subject every day - whilst it is not sacrosanct, it is worth making sure you understand them before you say they are wrong.   

I know maths is not your thing - but this is not a difficult one to work through.  The only difficulty would be the changing acceleration with distance - if you use a small enough distance you can use uniform acceleration without lose of accuracy. 
« Last Edit: 18/06/2011 09:47:31 by imatfaal »
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #31 on: 18/06/2011 15:18:22 »
I have no wish to offend anyone but I have no problem in challenging something I strongly believe to be wrong.  All I am trying to do is engage in a debate and either be persuaded through argument that I am wrong or if that does not happen then my ideas will probably be reinforced.

The whole point of the gedanken is to take the antimatter/matter particle pair crunch them together - increase energy of photons produced, remake the pair and the excess energy gained by the photons falling towards earth is matched by the kinetic energy required to move the the particle pair back up to original level.  this balance works for attractive.

if we assume that antimatter is repulsive of matter then you will get the particles again with enough kinetic energy to get them back to the original higher level BUT now the repulsive antimatter particle gains additional kinetic energy as it accelerates outwards - so instead of having two particle at rest (which is the starting situation) you have the matter at rest and the antimatter with increased velocity and accelerating outwards.  So you have gone from two stationary particles at beginning of experiment to one stationary and one fast moving at the end; that is the failure of conservation of energy.

Ordinary gravity, can be thought of as kinetic energy.  Repulsive gravity is the opposite, as kinetic energy is gained from acceleration in a gravitational field, so gravitational potential energy is lost.  The accelerating antiparticle is gaining kinetic energy at the cost of loosing gravitational potential energy the higher it climbs out of the gravity well.  The two cancel and the total energy remains the same.  The accelerating antiparticle is not gaining energy.

Energy is conserved.

Honest, I am not being awkward, I just want to get at the truth.
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #32 on: 18/06/2011 17:38:39 »
Ordinary gravity, can be thought of as kinetic energy.  Repulsive gravity is the opposite, as kinetic energy is gained from acceleration in a gravitational field, so gravitational potential energy is lost.  The accelerating antiparticle is gaining kinetic energy at the cost of loosing gravitational potential energy the higher it climbs out of the gravity well.  The two cancel and the total energy remains the same.  The accelerating antiparticle is not gaining energy.
No gravity CANNOT be thought of a kinetic energy - this is just nonsense.

Forget Potential Energy.  This is why:

1. PE is proportional to distance from centre of mass (CofM) - no matter whether that of antimatter or matter
2. Particles are x metres from CofM at begining of experiment
3. Particles are x metres from CofM at end of experiment
4. Particles mass is same as at beginning and position is same --> PE must be same at beginning and end of experiment.
5. There is no change between PE beginning and end whether Anti-M is repulsive or not.
 
But - Kinetic Energy varies.  This is why
1. There is a FoR in which the two particles start at rest - ie zero kinetic energy
2. At the end of the experiment the two particles return to this point
3. But by the end the matter particle is at rest - whereas the anti-matter (if repulsive) has a velocity & kinetic energy
4. So there is a change in Kinetic Energy iff anti-matter is repulsive under gravity

This is a closed system and you have ended up with more energy than you started with

I have laid it bit by bit - Tell me which bit of the above is wrong.
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #33 on: 19/06/2011 06:15:15 »
Ordinary gravity, can be thought of as kinetic energy.  Repulsive gravity is the opposite, as kinetic energy is gained from acceleration in a gravitational field, so gravitational potential energy is lost.  The accelerating antiparticle is gaining kinetic energy at the cost of loosing gravitational potential energy the higher it climbs out of the gravity well.  The two cancel and the total energy remains the same.  The accelerating antiparticle is not gaining energy.
No gravity CANNOT be thought of a kinetic energy - this is just nonsense.

Forget Potential Energy.  This is why:

1. PE is proportional to distance from centre of mass (CofM) - no matter whether that of antimatter or matter
2. Particles are x metres from CofM at begining of experiment
3. Particles are x metres from CofM at end of experiment
4. Particles mass is same as at beginning and position is same --> PE must be same at beginning and end of experiment.
5. There is no change between PE beginning and end whether Anti-M is repulsive or not.
 
But - Kinetic Energy varies.  This is why
1. There is a FoR in which the two particles start at rest - ie zero kinetic energy
2. At the end of the experiment the two particles return to this point
3. But by the end the matter particle is at rest - whereas the anti-matter (if repulsive) has a velocity & kinetic energy
4. So there is a change in Kinetic Energy iff anti-matter is repulsive under gravity


This is a closed system and you have ended up with more energy than you started with

I have laid it bit by bit - Tell me which bit of the above is wrong.


Ordinary gravity, can be thought of as kinetic energy.Sorry this was an over simplification. Everywhere in the universe is within a gravitational field, therefore all mass is moving.  In accelerating it gains kinetic energy, the source of which is gravitation.  An object gains kinetic energy when accelerating in a gravitational field but looses gravitational potential energy.

I think we both agree that when both particles have returned to their starting points energy has been conserved.  The problem as you see it is the antimatter particle has velocity and must therefore be gaining kinetic energy beyond that point.

This seems to be what we disagree upon:-
Gravity adds kinetic energy to an object accelerating in a gravitational field at the cost of the gravitational potential energy of the object weakening.  This is the same for matter and antimatter.

3) Yes the antimatter particle has velocity and has gained kinetic energy but the kinetic energy gain is at the cost of loosing the equivalent amount of potential energy. 
4)Overall the total energy is the same.


An apple falling to the ground gains kinetic energy but looses gravitational potential energy.

 

Offline Geezer

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Why is it assumed that gravity is always attractive?
« Reply #34 on: 19/06/2011 07:27:32 »
Sorry for butting in, but

Quote
Everywhere in the universe is within a gravitational field, therefore all mass is moving.

does not seem right at all.

Just because matter is in a gravitational field, there is no guarantee that it is moving, and matter can be moving regardless of the gravitational field that it is in.
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #35 on: 19/06/2011 09:30:30 »
Mike - one last try! 

Gravitational Potential Energy is proportional to ONLY the masses, the separation and the force.
True or False?

The particles are in exactly the same place, have exactly the same mass, and the graviational force is the same  at both beginning and end of experiment. 
True or False?

The particles have the same Potential Energy at beginning and end.
True or False?

The fact that the antimatter particle (if g is repulsive) has too much kinetic - does NOT mean that this energy can only have come from lost potential of the same amount!  We have shown above that PE remains the same.  The fact that KE has increased shows that Energy has not been conserved.  if you insist on this form of analysis then you must calculate the PE at the moment of re-creation of the two particles - this is difficult!


Geezer is correct, I am currently sitting at my desk putting off my phd research, I am in a gravitational field with a force of approx 9.8ms-2, to the best of my knowledge I am not moving
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #36 on: 20/06/2011 18:22:55 »
Sorry for butting in, but

Quote
Everywhere in the universe is within a gravitational field, therefore all mass is moving.

does not seem right at all.

Just because matter is in a gravitational field, there is no guarantee that it is moving, and matter can be moving regardless of the gravitational field that it is in.

If you define any object in space time as a fixed reference frame then every thing else in the universes is moving in space time relative that reference frame.  Even a fixed in space reference frame is still moving in time. Gravity, alone is enough to ensure movement.  Unless objects are physically, fixed in position they will move.  As you sit in your chair, at your workstation in your room you are blissfully unaware of movement but moving you are.  The Earth is rotating on its axis.  The Earth and Moon rotate upon a common axis.  The Earth circles the Sun.  Our solar systems position within on of the spiral arms of our galaxy is believed to rotate within that arm.  The galaxy rotates.  The Milky Way and Andromeda galaxies are approaching each other.  Space expands and galaxies are believed to be receding from each other.
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #37 on: 20/06/2011 18:24:04 »
Mike - one last try! 

Gravitational Potential Energy is proportional to ONLY the masses, the separation and the force.
True or False?
True

The particles are in exactly the same place, have exactly the same mass, and the graviational force is the same  at both beginning and end of experiment. 
True or False?
True

The particles have the same Potential Energy at beginning and end.
True or False?
True

The fact that the antimatter particle (if g is repulsive) has too much kinetic - does NOT mean that this energy can only have come from lost potential of the same amount!  We have shown above that PE remains the same.  The fact that KE has increased shows that Energy has not been conserved.  if you insist on this form of analysis then you must calculate the PE at the moment of re-creation of the two particles - this is difficult!
KE, has either not increased (when the two particles are back at their original starting points KE has not increased) or has increased at the expense of gravitational potential energy GPE
Where I have answered true to the above points, I mean true when the particles are back in their original starting positions.

Geezer is correct, I am currently sitting at my desk putting off my phd research, I am in a gravitational field with a force of approx 9.8ms-2, to the best of my knowledge I am not moving


Two Apples.

Let us consider two apples of equal mass.  One is hanging on the tree.  The other is upon the ground.  They both have the same rest mass energy  (E=mc2) but do not have the same gravitational potential energy, GPE.  The apple on the tree has more GPE that the one on the ground as it is higher in the gravity well.

If we substitute the apple of the ground with one of equal mass but made of antimatter then the apple on the ground wants to fly off into space.

When the apple on the tree falls to the ground, it gains kinetic energy, this energy has to come from somewhere.  The source of this kinetic energy is GPE.  GPE is converted into an equal amount of kinetic energy.  Energy is conserved.

When the apple on the ground is released, it initially gains kinetic energy as it accelerates upwards.  This gain of kinetic energy is at the loss of GPE.  (for an antimatter apple the gravitational field is stronger at the bottom of the gravity well).

Each apple when covering the same distance from their respective starting points convert the same amount of GPE into the same amount of kinetic energy.  Energy is conserved.

The antimatter apple as it flies away is in a weakening gravitational field, therefore its acceleration becomes negative.  In which case, it is not gaining kinetic energy.

In contrast, the matter apple as it falls accelerates as it is in an increasing gravitational field and hence gains kinetic energy

The above argument also applies to the two particles in the original experiment.
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #38 on: 20/06/2011 18:44:34 »
Mike - you keep saying this, it is wrong. 

PE is ONLY proportional to distance, mass, and gravitational attraction. 

Either tell me this is wrong, or agree with it.

If you give me a straight answer (ie one word) then I can build on that to explain where you are wrong - cos you are wrong.  If you insist on long and rambling replies which ignore my posts I am gonna call it quits
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #39 on: 21/06/2011 05:56:13 »
Mike - you keep saying this, it is wrong. 

PE is ONLY proportional to distance, mass, and gravitational attraction. 

Either tell me this is wrong, or agree with it.

If you give me a straight answer (ie one word) then I can build on that to explain where you are wrong - cos you are wrong.  If you insist on long and rambling replies which ignore my posts I am gonna call it quits

You keep saying it is wrong but don't say exactly why it is wrong and I have never intentially ignored any of your posts.  I do meticuously try to answer all points that you raise.

I have already agreed that PE is only proportional to distance, mass and the gravitational force whether attractive or repulsive. Saying that PE is only proportional to distance, mass and gravitational attraction is wrong.

My reply was not long and rambling, it was concise and to the point.  You have ignored all of the points that I have made.

My argument is twofold:-
For an object to gain kinetic energy through free-fall in a gravitational field then the energy must come from the conversion of GPE to KE.

A matter object in free-fall in a gravitational field is continuously falling deeper within the gravity well.  This is the reason why its kinetic energy is continuously increasing and why it continues to accelerate.
True or false?

A antimatter object in free-fall in the same gravitational field is being expelled from the gravity well.  The gravitational field is continuously weakening, not increasing.
True or false?

For an object to gain kinetic energy from a gravitational field it has to accelerate.  No acceleration, no gain in kinetic energy.
True or false?

I really do not know how I can put it simpler that that.

What precisely, is it that you do not agree with?
 

Offline Geezer

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Why is it assumed that gravity is always attractive?
« Reply #40 on: 21/06/2011 07:39:20 »
Ah yes! The dreaded PE/KE debate  :D

At this risk of introducing a gigantic poisson rouge, or at least providing a common target that you might both agree to aim at, a body does not actually have any potential energy because of its position.

The "energy" is not in the body at all. It's in the gravitational field.

It's not really very different from the situation of a bow and arrow. (I'm sure the clever reference back to the previously mentioned target has not gone unnoticed.) When the archer pulls back the string, the potential energy is stored in the bow, not the arrow.

Admittedly it is a bit more complicated than that because the energy stored in the gravitational field is also a consequence of the position of the mass within it, but the energy that is converted into kinetic energy was not stored within the object, so it had to be stored in the field.

Exactly what this has to do with antimatter, I do not know, but perhaps it might help.
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #41 on: 21/06/2011 09:59:43 »
Mike the reason I don't contest most of your points is that individually they are correct - but you put them together to reach a fallacious conclusion.

1. The whole point of this experiment is that in both the repulsive scheme and the attractive scheme the particles end up in the same position. 
2. When a particle, whether anti-matter or matter, whether attracted or repelled by gravity, is in the same position as before it has the same potential energy.
3. Thus from the beginning to the end of the experiment although the PE changes it must return to the same amount.

If you compare the beginning with the end the PE is the same in both attractive and repulsive cases - but in the repulsive case the KE has increased - this is a failure of the conservation law.


I promise you the answer is in the above sentences - especially number 2

Quote
I think we both agree that when both particles have returned to their starting points energy has been conserved.
- No we don't agree.  The PE is the same - the Total energy is increased; this is the nub of the problem

Quote
Yes the antimatter particle has velocity and has gained kinetic energy but the kinetic energy gain is at the cost of loosing the equivalent amount of potential energy.
  And I think your problem is that you are working with the assumption that because an antimatter particle in a repulsive scheme has gained KE - its PE at the end must be reduced from the beginning.  This is not the case.  The extra energy; which can be thought of as either KE at the end or (if you insist) an anomolously high PE in the middle of the experiment when the antimatter particle is recreated (and is then converted to KE); this is the problem and why the gedanken shows that it is more likely that a-m is attracted.


 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #42 on: 21/06/2011 10:06:35 »
Ah yes! The dreaded PE/KE debate  :D
Perhaps variant of the strain; this is more a book-keeping exercise.

Where the energy is stored and how it is manifest is not really important - as long as you agree on where zero is and keep to a sensible FoR the energy could just as well be thought of a attendant pink fairies.

New Sig!
 

Offline JP

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Why is it assumed that gravity is always attractive?
« Reply #43 on: 21/06/2011 20:12:42 »
This is a bit beyond the discussion here, but I thought it was interesting:
http://iopscience.iop.org/0295-5075/94/2/20001/

Under CPT symmetry (http://en.wikipedia.org/wiki/CPT_symmetry), general relativity apparently predicts that matter and antimatter gravitationally repel.  I suspect this is highly theoretical at this point, since it's arbitrarily applying a conservation law from quantum mechanics to general relativity, but it's definitely interesting.
 

Offline Geezer

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Why is it assumed that gravity is always attractive?
« Reply #44 on: 21/06/2011 23:08:40 »

New Sig!
 

Yes - I thought it was time to upgrade to something a bit more scientific.
 

Offline KineticFlow

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Why is it assumed that gravity is always attractive?
« Reply #45 on: 22/06/2011 12:07:00 »
Hi, I found this discussion really facsinating, thanks. 


imatfaal, I'm trying to understand your explanation.  Please could you help? 


Firstly, are you suggesting the following, and please could you clarify where I may have mis-understood? 

(1) Take a positron and an electron, which are 'stationary'
(2) They may combine, and would turn into light
(3) The light may move through a gravity field, and then its frequency would change:  blue shift if moving towards a (positive?) mass
(4) The light can then turn back into a positron and electron, which are now both moving faster than before because the frequency of the light increased. 
(5) Under the original assumption, the particles may easily be returned to their starting positions without changing the system's kinetic energy

Next, are you saying the following (is this right)? 
(a) At stage 5 the total gravitational potential energy of the system is the same as at stage 1
(b) The particles are moving faster, so Total kinetic energy has increased
(c) No other forms of energy would have been changed
(d) a b and c would only imply that the total energy of the system has increased, which violates the principle of conservation of energy. 


Lastly, may I ask a few questions?  These will demonstrate my complete lack of knowledge, sorry! 


(i) If the pair were to be re-combined and moved back to where they started in the form of light, then would the light start at the higher frequency again (higher because of the additional kinetic energy)?  And would the frequency drop this time (as it moved in the opposite direction to before through the field; red shift on the way out instead of blue shift on the way in)?  And therefore if they re-appear at the original position by this method would they be 'stationary' again?

(ii) How can we move the pair back to their starting positions, without changing the total kinetic energy of the system? 
[Edit:  I re-read the description by Supercryptid, but I'm still struggling with this bit: "The net result is that you can change their height above the Earth's surface with no net change in the energy of the system."
(Should it be re-worded "..kinetic energy of the system."?)
How can this be achieved?  Does this mean that the energy to move them back out through the field is zero?  So they pull against each other in the field, and a small tap sends them off and an equal and opposite one to stop them when they get there?  ]

(iii) So to get to (5), have we assumed that both the inertial mass and gravitational mass of the positron are negative?  If so, would negative mass affect how the kinetic energy of the system is measured at stages (4) and (5)? 
May a positron in motion have negative kinetic energy under our assumption, and would this resolve the apparent violation in conservation of energy? 
Or did we get there without needing negative inertial mass?  But then confirming they are still to be treated as the same once again resolves that situation. 

(iv) Is it OK to ignore the magnetic potential energy between the positron and the electron? (I think it is, because the electron and positron start and finish the same distance apart - is that right?) 

(v) Couldn't half the light go in the wrong direction, and need to be reflected back by a mirror or something?  If so, would that move the mirror a bit, or heat it, or change the frequency of the light, and is that relevant when adding up all the energy in the system? 
Or can all the light go where we want it, without adding components to the system?  Can it still do so on a macro scale? 


Good luck with ignoring your phd research, perhaps my questions will help with that!  I'm busy ignoring a piece of coursework for my accountancy exams right now  :D
« Last Edit: 22/06/2011 16:22:32 by KineticFlow »
 

Offline KineticFlow

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Why is it assumed that gravity is always attractive?
« Reply #46 on: 23/06/2011 10:58:45 »
Hmm, I hope I didn't kill the thread >.<

Anyway, just wondering some more: 

At the bottom (stage (4)), if the particles now have kinetic energy, that means they're moving.  They are magnetically attracted (or in any case, being treated as linked somehow in order to get them back in stage (5)), so does that mean we have a spinning or pendulum-ing gravitic di-pole? 
What happens to the spinning motion as we move them back to where they started?  (eg Might the spinning be dampened or changed somehow by the act of moving through the field)? 
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #47 on: 23/06/2011 14:01:39 »
Mike - tied up at work, but I will revert eventually
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #48 on: 23/06/2011 14:21:29 »
Ah yes! The dreaded PE/KE debate  :D

At this risk of introducing a gigantic poisson rouge, or at least providing a common target that you might both agree to aim at, a body does not actually have any potential energy because of its position.

The "energy" is not in the body at all. It's in the gravitational field.

It's not really very different from the situation of a bow and arrow. (I'm sure the clever reference back to the previously mentioned target has not gone unnoticed.) When the archer pulls back the string, the potential energy is stored in the bow, not the arrow.

Admittedly it is a bit more complicated than that because the energy stored in the gravitational field is also a consequence of the position of the mass within it, but the energy that is converted into kinetic energy was not stored within the object, so it had to be stored in the field.

Exactly what this has to do with antimatter, I do not know, but perhaps it might help.

Geezer
a body does not actually have any potential energy because of its position.
I would argue that all bodies have potential energy when they are within a gravitational field and everything within the universe is within a gravitational field.

The "energy" is not in the body at all. It's in the gravitational field.
That's in accordance with relativity but the field is due to the various gravitating masses.  An object does not just fall to Earth because of the 'warp' of space time due to the Earth but because of the 'warp' of space time due to the combined 'warp' of space time due to the Earth and the object.  When two masses combine into one object 'it' experiences more time dilation than do the two masses individually.  If you follow this argument (which is part of relativity) through to it's logical conclusion you end up with the universe being contained within a black hole.
However, I digress from the main argument which I will address in a reply to imatfaal.
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #49 on: 23/06/2011 23:10:03 »
Hi, I found this discussion really facsinating, thanks. 


imatfaal, I'm trying to understand your explanation.  Please could you help? 


Firstly, are you suggesting the following, and please could you clarify where I may have mis-understood? 

I will take it through bit by bit
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(1) Take a positron and an electron, which are 'stationary'
(2) They may combine, and would turn into light
(3) The light may move through a gravity field, and then its frequency would change:  blue shift if moving towards a (positive?) mass
(4) The light can then turn back into a positron and electron, which are now both moving faster than before because the frequency of the light increased. 
(5) Under the original assumption, the particles may easily be returned to their starting positions without changing the system's kinetic energy

1,2 yep
3 there is at moment only positive mass
4 yep
5 this is the bit we need to be exact.  the kinetic energy they have upon re-creation they have enough kinetic energy to JUST make it back to initial position.
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Next, are you saying the following (is this right)? 
(a) At stage 5 the total gravitational potential energy of the system is the same as at stage 1
(b) The particles are moving faster, so Total kinetic energy has increased
(c) No other forms of energy would have been changed
(d) a b and c would only imply that the total energy of the system has increased, which violates the principle of conservation of energy. 
a - yes once the particles have returned to original position they should have same total energy as long as no external energy source
b - no.  if energy is conserved then the particles are not moving once they get back to original place.
c - once they have returned to original situation then should be back to same

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Lastly, may I ask a few questions?  These will demonstrate my complete lack of knowledge, sorry! 
(i) If the pair were to be re-combined and moved back to where they started in the form of light, then would the light start at the higher frequency again (higher because of the additional kinetic energy)?  And would the frequency drop this time (as it moved in the opposite direction to before through the field; red shift on the way out instead of blue shift on the way in)?  And therefore if they re-appear at the original position by this method would they be 'stationary' again?
This is all gedanken - but they should have same kinetic energy
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(ii) How can we move the pair back to their starting positions, without changing the total kinetic energy of the system? 
[Edit:  I re-read the description by Supercryptid, but I'm still struggling with this bit: "The net result is that you can change their height above the Earth's surface with no net change in the energy of the system."
(Should it be re-worded "..kinetic energy of the system."?)
How can this be achieved?  Does this mean that the energy to move them back out through the field is zero?  So they pull against each other in the field, and a small tap sends them off and an equal and opposite one to stop them when they get there?  ]
You do change the the Kinetic Energy - when they are recreated they have JUST enough KE to get back to the inital position before they grind to a halt (for attractive model)
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(iii) So to get to (5), have we assumed that both the inertial mass and gravitational mass of the positron are negative?  If so, would negative mass affect how the kinetic energy of the system is measured at stages (4) and (5)? 
May a positron in motion have negative kinetic energy under our assumption, and would this resolve the apparent violation in conservation of energy? 
Or did we get there without needing negative inertial mass?  But then confirming they are still to be treated as the same once again resolves that situation. 
Negative mass / negative energy : difficult problem.  All totally hypothetical.  But yes quite possible; this gedanken relies on all else being normal.  But as JP said above there is good theoretical ideas that the whole this is above rubbish.
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(iv) Is it OK to ignore the magnetic potential energy between the positron and the electron? (I think it is, because the electron and positron start and finish the same distance apart - is that right?) 
yes
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(v) Couldn't half the light go in the wrong direction, and need to be reflected back by a mirror or something?  If so, would that move the mirror a bit, or heat it, or change the frequency of the light, and is that relevant when adding up all the energy in the system? 
Or can all the light go where we want it, without adding components to the system?  Can it still do so on a macro scale? 

Yes - it is a gedanken.  the light will go in opposite directions

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Good luck with ignoring your phd research, perhaps my questions will help with that!  I'm busy ignoring a piece of coursework for my accountancy exams right now  :D
Unfo I don't need any luck to manage that! Good luck with those exams and coursework.
 

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Why is it assumed that gravity is always attractive?
« Reply #49 on: 23/06/2011 23:10:03 »

 

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