Post by: mxplxxx on 29/07/2015 11:27:33
Post by: alancalverd on 29/07/2015 12:13:52
The opposite of gravity is hilarity. Unfortunately (perhaps fortunately) there is no such force.
Post by: evan_au on 29/07/2015 12:49:54
Quote from: mxplxxx
The planets and the Sun are likely in stable orbits because they both attract and repel each other with the same force.If there were equal and opposite forces between the Earth and Sun, the net force would be zero, and the Earth would sail off into space. You would not really call this an orbit.
This could be done if Earth and Sun were given an enormous positive static electric charge, big enough to counteract the gravitational attraction between Earth & Sun. Just don't do it while I'm around, as it would be a hair-raising experience, like sitting on a giant Van Der Graaf generator (promptly followed by the departure of all Earth's atmosphere).
Post by: Bill S on 29/07/2015 13:41:23
Gravity: that which keeps everyone down.
Grabity: that which enables the few to rise above the masses.
Post by: Bored chemist on 29/07/2015 19:29:40
Post by: jeffreyH on 30/07/2015 00:31:52
Post by: PmbPhy on 30/07/2015 02:40:40
Quote from: alancalverd
The opposite of gravity is hilarity.Wrong. The opposite of gravity is antigravity.
Quote from: alancalverd
Unfortunately (perhaps fortunately) there is no such force.There most certainly is, Alan. Jeff has it right! Ya gotta love him! :)
Quote from: jeffreyH
Surely it is two words. Dark energy.
The most obvious example is dark energy which causes the accelerating rate of expansion of the universe. Simply put, that's gravitational repulsion. This can be caused in one of two ways. One way is the presence of a positive cosmological constant. The other is the presence of a large enough negative pressure whose magnitude is greater than the mass density.
Have you ever heard of a vacuum domain wall? They're mentioned in Peebles text Principles of Physical Cosmology. You can read about them on Wikipedia at: https://en.wikipedia.org/wiki/Domain_wall
See also http://physicsworld.com/cws/article/news/2013/jan/17/physicists-seek-cosmic-domain-walls
Alexander Vilenkin of Tufts University has done work on these structures. He's a well-known cosmologist.
See: http://adsabs.harvard.edu/abs/1983PhLB..133..177V
If you'd like I can e-mail you a paper on the subject. Or you can look in Peebles text which you can download from http://bookos-z1.org/
Essentially its a plane with zero thickness (singularity) which repels anything placed in its gravitational field.
Post by: mxplxxx on 30/07/2015 07:03:58
Post by: alancalverd on 30/07/2015 10:03:58
Except of course that positrons and antiprotons, whilst being exact inverse analogs, are very rare. So maybe the universe is populated with gravitons and antigravitons and we just don't see the latter very often?
Post by: evan_au on 30/07/2015 12:22:08
Quote from: alancalverd
maybe the universe is populated with gravitons and antigravitons and we just don't see the latter very often?Just as the photon is its own anti-particle, the hypothetical graviton (https://en.wikipedia.org/wiki/Graviton) would also be its own anti-particle.
Post by: PmbPhy on 30/07/2015 13:17:23
Quote from: mxplxxx
Anti-gravity is science fiction isn't it.Term antigravity to some people refers to canceling the effects of gravity. To others it means a repulsive gravitation. But in either case that's what general relativity allows.
Quote from: mxplxxx
It seems to me that it is the repulsive force that is expanding the universe and that is the opposite of gravity.Correct.
Quote from: mxplxxx
But it don't have a name!Sure it does. It too is called gravity just like electrical attraction and repulsion are both called electric.
Post by: PmbPhy on 30/07/2015 13:20:35
Quote from: alancalverd
We can measure gravitational attraction at a mesoscopic level, in the laboratory, geologically, and by making orbital calculations. Dark energy doesn't seem to invade our calculations - any such contribution must be orders of magnitude less than gravitation (though it must be admitted that G is probably the least precisely known of the fundamental constants). This makes it rather different from the common forces and antiparticles, which generally mirror one another exactly.Until we know the source of dark energy I doubt we can say that with any confidence. If dark energy is a result of negative mass and there are particles yet to be discovered which have negative mass then your assumption would be wrong.
Quote from: alancalverd
Except of course that positrons and antiprotons, whilst being exact inverse analogs, are very rare.Antimatter has nothing to do with gravitational repulsion or antigravity.
The terminology of "gravitational repulsion" is now finding its way into the textbooks. See:
http://www.eftaylor.com/exploringblackholes/ExpandCosmos150518v2.pdf
Post by: alancalverd on 30/07/2015 13:45:13
Now since we don't see random bursts of photons as microscopic bits of the visible universe disappear (or do we? there's an awful lot of cosmic gamma radiation about!) it seems that the annihilation of positive and negative mass does not generate a signal. So if we could measure a mass independently of any other mass, we would see it gradually and quietly depleting as it encountered stray particles of cosmic negative mass.
Post by: jeffreyH on 30/07/2015 21:57:09
so with negative mass we get 
so that the sign of the force is reversed. This would result in a repulsive force. Whether or not negative mass has any realistic meaning is a matter of philosophical debate. It could simply be a mathematical convenience that produces the correct results. Define mass in definitive terms. It is like saying define energy in definitive terms.
Post by: PmbPhy on 30/07/2015 23:13:35
Quote from: jeffreyH
We haveYou just showed that if there exists matter which has (precisely stated) negative active gravitational mass (nagm) then according to the expression that you yourself wrote which describes what is observed, matter is repelled from that matter. That is the real meaning of it. So why are you saying that its a matter of philosophical debate?
Quote from: jeffreyH
It could simply be a mathematical convenience that produces the correct results. Define mass in definitive terms. It is like saying define energy in definitive terms.It's not a mathematical convenience. nagm is something whose existence depends on observation only. If its observed to exist then its already been observed to repel normal matter. The math is just a description. Please never forget that, i.e. math is only a description of nature. All too many people make the mistake that physicists let the math fool them into thinking things are true when they aren't, i.e. too many people make the mistake of thinking that we physicists don't know how to use math properly.
Post by: jeffreyH on 01/08/2015 15:32:38
. This has the sign of the force inherent in the mass. Dark energy would then have the form 
. When we think of electric charge a negative sign balances the books. The big difference between electromagnetism and gravity in this model is that for EM opposites attract and like repel. This is reversed for gravitation where like signs attract and opposites repel. What this actually means I am still thinking about.
Post by: mxplxxx on 27/08/2015 21:19:11
Whilst I am not at all sure how each scale of the universe comes into being, I suspect strongly it is related to the quantum of energy. Each scale will come with a base quantum of energy that is related to the “frozen” time of the scale. This will maybe result in something like an exponential set of scales (.ie. Scale 2, scale 4, scale 16 etc.). This will also likely relate to Planck’s constant H and the quantum’s probability wave. Lots of food for thought here.
There is also related to the concept of “normal” that is experienced by entities at each scale of the universe. Each entity will have an experience of time and distance that is identical in terms of a universal “normal”. This in turn may mean that time and distance are related to perspective. The universe may exists in a near-infinitely small point (or not at all!!!)! The universe itself may see itself as living to 100 years when you and I see it as existing for 14 billion years.
Post by: PmbPhy on 27/08/2015 23:13:55
Quote from: mxplxxx
Maybe we could call the expansionary force the X-force.Why not simply extend the term antigravity to include gravitational repulsion? It makes more sense given the name, i.e. the "anti" in antigravity.