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Author Topic: Why is it assumed that gravity is always attractive?  (Read 17754 times)

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?


 

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.
 

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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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.
 

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.
 

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]
 

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.
 

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 »
 

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.
 

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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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.
 

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 »
 

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.
 

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 »
 

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.
 

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 »
 

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.
 

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.
 

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.
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« 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
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« 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..
 

Offline imatfaal

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Why is it assumed that gravity is always attractive?
« 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
 

Offline yor_on

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Why is it assumed that gravity is always attractive?
« 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.
 

Offline MikeS

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Why is it assumed that gravity is always attractive?
« 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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Why is it assumed that gravity is always attractive?
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