Why is it assumed that gravity is always attractive?

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Offline MikeS

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Antimatter can be thought of as ordinary matter going backwards in time.  Either it is or it isn't, the odds are 50/50.  Mainstream science, assumes matter and antimatter to gravitationally attract each other.  Why is this assumption so overwhelmingly strong?  Does this dogma have any valid scientific reasoning behind it?

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Offline Geezer

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Why is it assumed that gravity is always attractive?
« Reply #1 on: 12/06/2011 09:56:00 »
Antimatter can be thought of as ordinary matter going backwards in time.  Either it is or it isn't, the odds are 50/50.  Mainstream science, assumes matter and antimatter to gravitationally attract each other.  Why is this assumption so overwhelmingly strong?  Does this dogma have any valid scientific reasoning behind it?

No. Only a gigantic amount of empirical evidence.
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Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #2 on: 12/06/2011 09:58:35 »
Like what?

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Offline Bored chemist

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Why is it assumed that gravity is always attractive?
« Reply #3 on: 12/06/2011 14:17:05 »
Like what?
Like what goes up must come down.
Like the fact that if you drop something it falls.
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Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #4 on: 12/06/2011 16:16:43 »
I have to say, but there is absolutely no empirical evidence for antimatter's gravitational attraction - theory (and an assumption that it behaves as does matter) tells us that it has a normal positive mass/energy and thus will be attracted in the same way as matter.  Although I do think that Mike has overstated the time reversal aspect - admittedly on Feynmann diagrams antiparticles go the wrong way, I am not sure that is quite the same as a complete acceptance of time reversal in all matters.  And Mike - no it isn't a 50/50 yes or no question.
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Offline Geezer

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Why is it assumed that gravity is always attractive?
« Reply #5 on: 13/06/2011 01:43:55 »
I'm pretty sure I didn't read the question properly! I retract my statement  [:I]
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Offline JP

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Why is it assumed that gravity is always attractive?
« Reply #6 on: 13/06/2011 07:20:48 »
Either it is or it isn't, the odds are 50/50. 

No.  Having two possible outcomes doesn't make the odds 50/50.

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Offline Phractality

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Why is it assumed that gravity is always attractive?
« Reply #7 on: 13/06/2011 07:48:08 »
I have no strong opinion on the question of whether antimatter has anti-gravity.
If I had to guess, I'd guess that it does, but I prefer to wait for experimental results.

The gravitational attraction between two protons is something like 10^44 times greater weaker than the electrostatic repulsion. Since gravity is so weak, we shall need to capture a large quantity of antimatter with absolutely zero electric charge and release it in a vacuum with no electrostatic or magnetic field to see which way it accelerates in Earth's gravity. If the antimatter has an excess or deficit of even one electron charge, the results will be meaningless. [Thanks imatfaal for catching my bass ackwards comparison.]

I don't think anyone has mentioned the question of whether inertial mass and gravitational mass are necessarily the same. I can accept negative gravitational mass, but negative inertial mass is paradoxical. The harder you push a negative mass away from you, the faster it accelerates toward you.
« Last Edit: 14/06/2011 02:54:15 by Phractality »
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Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #8 on: 13/06/2011 07:58:28 »
The arrow of time lets assume for the sake of simplicity can only have two directions and this was the point I was making in saying 50/50.  This is self evident.  I assume that when it is mentioned that the odds are not 50/50 then you are referring to a statistical probability of evidence.

It does seem to me that believing that matter and antimatter are both gravitationally attractive is little more than an assumption.  What actual evidence is there to back up this assumption?  Physics is almost completely based on this idea, it is so fundamental that it has to be based on something.

Phrac you posted while I was replying
"but negative inertial mass is paradoxical"
Yes, but only in our universe.  In an antimatter universe, it is no longer negative in respect to its universe, so there is no paradox.

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Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #9 on: 13/06/2011 11:07:34 »
The arrow of time lets assume for the sake of simplicity can only have two directions and this was the point I was making in saying 50/50.  This is self evident.  
This is also wrong
Quote
Physics is almost completely based on this idea, it is so fundamental that it has to be based on something.
No it is not.  Some portions of cosmology may be - but not physics per se.  And once you start talking about anti-matter universes you are out of the comfort zone - speculations about another universe will always remain just that; our universe is a portion of that which we can measure and observe.  There is a massive paradox as Fract pointed out - positing a different universe does not solve or remove that paradox; antimatter exists in this universe.  We have now been able to contain anti-hydrogen for macro periods (1000s of seconds) and will soon be able to start making qualitative and quantitative measurements.  However, as Fract said (or meant to he got signs wrong way around) the gravitational attraction is far too weak to be measured in the lab for many years to come
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Offline Bored chemist

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Why is it assumed that gravity is always attractive?
« Reply #10 on: 13/06/2011 19:19:42 »
It's not my field but I suspect that the behaviour of the universe would be rather different if antimatter had negative mass.
Bulk antimatter is rare but virtual particles are not. So, for example, Hawking radiation would be rather different.
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Offline Supercryptid

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Why is it assumed that gravity is always attractive?
« Reply #11 on: 13/06/2011 21:49:03 »
If antimatter repels normal matter gravitationally, then that would allow for a violation of the conservation of energy (which is a big no-no).

Imagine an electron-positron pair in the gravitational field of the Earth. They both have equal mass, but one is attracted towards the Earth while the other is repelled away from it. In this sense, the pair has no weight, since they cancel out one another's gravitational effects. 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.

Now imagine that you put the pair at a high altitude and allow it to self-annihilate to produce a pair of gamma ray photons. Then you move those gamma ray photons down closer to the Earth. When light travels in towards a gravitational source, it's frequency increases and it gains energy (blue-shifting). Think of it as the opposite of what happens when light travels away from a dense object like a neutron star (red-shifting).

Once you return to the same height that you originally had the electron-positron pair at (before you moved it up high), you allow the gamma ray photons to create a new electron-positron pair. But wait, this electron-positron pair has a higher energy state than it did before due to the blue-shifting of the gamma rays. You can repeat the process and create a pair with even more energy than that, and so on. Where is this extra energy coming from?

If both matter and antimatter are attracted gravitationally, then this problem is avoided.

Also, light (which is neither matter nor antimatter) is attracted by gravity (i.e. gravitational lensing). So why would antimatter be different from both matter and light in this sense?
« Last Edit: 13/06/2011 21:51:06 by Supercryptid »
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Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #12 on: 14/06/2011 10:21:55 »
SuperC - nice argument, not entirely convinced, but cannot see the flaw.
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Offline Phractality

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Why is it assumed that gravity is always attractive?
« Reply #13 on: 15/06/2011 00:36:56 »
If antimatter repels normal matter gravitationally, then that would allow for a violation of the conservation of energy (which is a big no-no).

Imagine an electron-positron pair in the gravitational field of the Earth. They both have equal mass, but one is attracted towards the Earth while the other is repelled away from it. In this sense, the pair has no weight, since they cancel out one another's gravitational effects. 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.

Now imagine that you put the pair at a high altitude and allow it to self-annihilate to produce a pair of gamma ray photons. Then you move those gamma ray photons down closer to the Earth. When light travels in towards a gravitational source, it's frequency increases and it gains energy (blue-shifting). Think of it as the opposite of what happens when light travels away from a dense object like a neutron star (red-shifting).

Once you return to the same height that you originally had the electron-positron pair at (before you moved it up high), you allow the gamma ray photons to create a new electron-positron pair. But wait, this electron-positron pair has a higher energy state than it did before due to the blue-shifting of the gamma rays. You can repeat the process and create a pair with even more energy than that, and so on. Where is this extra energy coming from?

If both matter and antimatter are attracted gravitationally, then this problem is avoided.

Also, light (which is neither matter nor antimatter) is attracted by gravity (i.e. gravitational lensing). So why would antimatter be different from both matter and light in this sense?
"Energy is conserved," is an oversimplefication of the conscept of energy conservation. The energy of a closed system is conserved in any inertial reference frame, but the system has different amounts of energy relative to different reference frames.

The photonic energy that is released by annihilation IN THE REFERENCE FRAME OF THE PAIR AT THE TIME AND PLACE WHERE ANNIHILATION OCCURS is equal to the mass of the particle pair, including the mass equivalent of any kinetic energy they had in that reference frame prior to collision. In a different reference frame the mass-energy may vary, but the rest mass is constant.

The rest mass of a particle is the mass of the particle in a reference frame which is stationary relative to the particle.
« Last Edit: 15/06/2011 00:38:33 by Phractality »
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Offline Supercryptid

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Why is it assumed that gravity is always attractive?
« Reply #14 on: 15/06/2011 01:20:10 »
Quote
SuperC - nice argument, not entirely convinced, but cannot see the flaw.
The argument was thought up by Philip Morrison, not me, just to make sure he gets the proper credit: http://en.wikipedia.org/wiki/Philip_Morrison

Quote
"Energy is conserved," is an oversimplefication of the conscept of energy conservation. The energy of a closed system is conserved in any inertial reference frame, but the system has different amounts of energy relative to different reference frames.

The photonic energy that is released by annihilation IN THE REFERENCE FRAME OF THE PAIR AT THE TIME AND PLACE WHERE ANNIHILATION OCCURS is equal to the mass of the particle pair, including the mass equivalent of any kinetic energy they had in that reference frame prior to collision. In a different reference frame the mass-energy may vary, but the rest mass is constant.

The rest mass of a particle is the mass of the particle in a reference frame which is stationary relative to the particle.
What about in the reference frame of a scientist on the ground doing the measurements? He should still see an increase in the energy of the system.
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Offline yor_on

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Why is it assumed that gravity is always attractive?
« Reply #15 on: 15/06/2011 01:35:41 »
It's a nice idea Super :)

Still, any blueshift must be a relation, in this case between a gravitationally accelerating Earth at one gravity, and whatever light infalling. And if you instead move it upwards, it will to a stationary observer Earth appear as 'red shifted'

The idea of a light quanta is that its intrinsic energy never varies, what varies are the relations it will have toward another object, depending on mass and motion. Waves is another description of light in where they are seen to quench as well as reinforce each other, when you have two or more interfering, but a light quanta can only be a photon. And the relative red and blueshift will always need to be a relation.

Although you are right in that if we let it 'accelerate/blue shift' toward a black hole, standing at the event horizon observing it, it would create something stronger in a interaction than if leaving it. So maybe you can argue it?

You might use it as a proof of 'energy' being stored in the 'space' of a static gravitational field? But that is what the stress energy tensor speaks about too ,if I got it right. And I agree that the conservation of energy shouldn't allow your scheme, at least if we presume that you expect to get more energy out of it than you put in. But it's a nice thought experiment.
==

What it seems to hinge on is the idea of 'matter waves' being so incredibly 'short' so that when we move a rest mass upwards (from Earth) it shouldn't be noticeable. Never thought of it that way actually, and I'm not even sure if you can argue it as 'matter waves'?

If matter was 'waves' the red and blue shift must be as true for it as it is for light, don't you agree. At least there should be a equivalence. Nice one.
« Last Edit: 15/06/2011 01:49:18 by yor_on »
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Offline mpc755

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Why is it assumed that gravity is always attractive?
« Reply #16 on: 15/06/2011 02:58:54 »
Gravity is not an attractive force.

Aether has mass. Aether physically occupies three dimensional space. Aether is physically displaced by matter. Aether displaced by matter exerts force toward the matter.

Force exerted towards matter by aether displaced by matter is gravity.

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Offline MikeS

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Offline imatfaal

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Why is it assumed that gravity is always attractive?
« Reply #18 on: 15/06/2011 10:31:40 »
MPC - please don't jump in and take a thread off-topic like that.  Mike's question was on a strange remote possibility of a reversal of sign of gravity in the case of anti-matter - and it should be debated with as close to mainstream ideas as possible.
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Offline MikeS

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Why is it assumed that gravity is always attractive?
« Reply #19 on: 16/06/2011 06:18:47 »
This is an extract from the thought experiment, available in full here. 
http://www.desy.de/user/projects/Physics/ParticleAndNuclear/antimatter_fall.html

"Our argument, which is an adaptation of the "Morrison argument" described in [2], will only make two basic assumptions.  It will assume that energy is conserved.  And it will assume that "fundamental constants," such as the inertial mass of the proton or electron, or the speed of light, do not vary with height above the Earth.  It will also use well-tested experimental results (such as the fact that E=m c2 has been observed to hold to high accuracy).  As explained above, when this equation is coupled with gravitational redshift experiments, it shows that antimatter must fall down with an acceleration within 0.04% of that of ordinary matter.

Start with a chunk of matter and a chunk of antimatter, each of mass m (mass is always understood here to mean inertial mass), at the top of a tower of height L.  These "chunks" could, for example, be a proton and antiproton, which have been experimentally observed to have the same inertial mass to within one part in 100 thousand million.  If we combine these two chunks, they form a photon (actually a bunch of photons).  If we measure the energy of these photons locally (by, for example, looking at their frequency), special relativity tells us that we will see.

Now let's have the person at the bottom of the tower take these photons and turn them back into chunks of matter and antimatter, each of mass m (for example, a proton and antiproton).  By special relativity, we know that the energy 2mc2 is just enough energy to create chunks of matter and antimatter, each of mass m.  But the photons have some extra energy, 2m gphoton L.  This gives the matter and antimatter some extra energy (manifested as kinetic energy).  We want to use this extra energy to move the matter and antimatter back to the top of the tower.  This extra energy must be just enough to move them back to the top of the tower, or else energy would not be conserved.  In other words, this cycle takes us back to the exact same conditions that we started with, so we had better not have lost or gained energy in carrying it out.

So how much energy does it take to move these guys back to the top of the tower?  Well, the matter has an inertial mass m , and "feels" an acceleration gmatter.  So it feels a force m gmatter, and to move it a distance L requires energy m gmatter L. Similarly, it takes an energy m gantimatter L to move the antimatter to the top of the tower.  To conserve energy, these two energies must add up to be the same as the extra photon energy, so we need"


Let's have a look at what is happening in this experiment.
Firstly lets consider what would happen if matter and antimatter gravitationally attract:-
The photons gain gravitational energy and are blue shifted.
At the bottom of the tower they are combined and form a matter particle plus a antimatter particle.  These two particles contain less gravitational potential energy as the bottom of the tower is deeper within the gravity well.  The excess energy left over from their creation is just enough to propel them back to their starting point at the top of the tower.
Energy has been conserved

Now let's consider what would happen if matter and antimatter gravitationally repel:-
The photons gain gravitational energy and are blue shifted.
At the bottom of the tower they are combined and form a matter particle plus a antimatter particle.
The matter particle contains less gravitational potential energy as before. so the surplus energy will be available to propel it back up the tower.
The antimatter particle to be created at the base of the tower requires an input of energy as it is further within the gravity well that it is trying to escape.  So its creation has used the energy provided by the blue shift of the photons.
Both particles return to the top of the tower.  In so doing the antimatter particle looses gravitational potential energy.
Energy has been conserved.

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Offline imatfaal

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« Reply #20 on: 16/06/2011 11:01:40 »
Mike - the upshot of that gedanken is that antimatter is attracted to matter.  You will note that this has "kinda" been checked experimentally with the deflection of neutrinos and antineutrinos being the same
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Offline MikeS

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« Reply #21 on: 16/06/2011 17:56:16 »
This gedanken is one of the few barriers remaining to support the idea that matter and antimatter gravitationally attract each other.  The argument being that if matter and antimatter were gravitationally attractive then it would violate the conservation of energy.  I am arguing that it does not.  My argument is very simple and I believe difficult to fault.  It requires nothing new and is based on known facts.  No doubt lots of people would like to prove me wrong.  That's fair enough, I have no objection to being proven wrong.  If you don't understand my argument or you think there is a flaw in my argument then please tell me what is it.

Mike - the upshot of that gedanken is that antimatter is attracted to matter.  You will note that this has "kinda" been checked experimentally with the deflection of neutrinos and antineutrinos being the same

the upshot of that gedanken is that antimatter is attracted to matter.
Yes but it's wrong and I have explained why it is wrong. 
Excerpt from my last post.
The antimatter particle to be created at the base of the tower requires an input of energy as it is further within the gravity well that it is trying to escape.  So its creation has used the energy provided by the blue shift of the photons.
Both particles return to the top of the tower.  In so doing the antimatter particle looses gravitational potential energy.
Energy has been conserved.

If there is something amiss with my argument then please explain what it is.

You will note that this has "kinda" been checked experimentally with the deflection of neutrinos and antineutrinos being the same
They are far from certain that they have actually detected antineutrinos so that experiment is meaningless and it's not likely to be repeated any time soon..

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Offline imatfaal

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« Reply #22 on: 16/06/2011 18:40:16 »
Mike I have tried to explain simpler notions to you - you refuse to compromise or see that you could possibly be wrong.  The creation of particle pairs will have same amount of excess energy independent of the height above centre of mass of planet - it follows that the two forms you have suggested will have same xs energy, thus if energy is conserved only the attractive model works
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Offline yor_on

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« Reply #23 on: 16/06/2011 22:39:22 »
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.
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Offline MikeS

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« Reply #24 on: 17/06/2011 07:46:33 »
Mike I have tried to explain simpler notions to you - you refuse to compromise or see that you could possibly be wrong.  The creation of particle pairs will have same amount of excess energy independent of the height above centre of mass of planet - it follows that the two forms you have suggested will have same xs energy, thus if energy is conserved only the attractive model works

I am quite happy to compromise if proved wrong.
The creation of particle pairs will have same amount of excess energy independent of the height above centre of mass of planet - it follows that the two forms you have suggested will have same xs energy, thus if energy is conserved only the attractive model works[/color]

Could you please explain what you mean by this.

I really don't understand why you say only the attractive model works, I would be grateful for an explanation.

Unless I am missing something from the two things you said that I don't understand, the point I was trying to prove is that in both attractive gravity and repulsive gravity, energy is conserved.  I have tried to prove this because one of the main arguments about gravity between matter and antimatter being repulsive is that mainstream believes that repulsive gravity violates the conservation of energy.  I have tried to show that it does not.


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Offline MikeS

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« 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.

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Offline imatfaal

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« 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.

http://www.youtube.com/watch?v=wMFPe-DwULM  - 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
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Offline MikeS

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« 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

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Offline imatfaal

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« 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.
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Offline MikeS

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« 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.

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Offline imatfaal

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« 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 »
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Offline MikeS

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« 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.

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Offline imatfaal

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« 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.
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Offline MikeS

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« 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.


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Offline Geezer

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« 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.
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

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Offline imatfaal

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« 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
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Offline MikeS

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« 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.

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Offline MikeS

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« 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.

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Offline imatfaal

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« 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
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Offline MikeS

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« 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?

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Offline Geezer

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« 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.
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

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Offline imatfaal

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« 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.


There’s no sense in being precise when you don’t even know what you’re talking about.  John Von Neumann

At the surface, we may appear as intellects, helpful people, friendly staff or protectors of the interwebs. Deep down inside, we're all trolls. CaptainPanic @ sf.n

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Offline imatfaal

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« 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!
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Offline JP

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« 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.

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Offline Geezer

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« 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.
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

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Offline KineticFlow

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« 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 »
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Offline KineticFlow

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« 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)? 
Age to age succeeds, blowing a noise of tongues and deeds, a dust of systems and of creeds ~ Alfred Tennyson

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Offline imatfaal

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« Reply #47 on: 23/06/2011 14:01:39 »
Mike - tied up at work, but I will revert eventually
There’s no sense in being precise when you don’t even know what you’re talking about.  John Von Neumann

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Offline MikeS

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« 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.

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Offline imatfaal

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« 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.
There’s no sense in being precise when you don’t even know what you’re talking about.  John Von Neumann

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