# Anti grav

1. Aug 10, 2006

### Mrbrit

Hi

Please bear with me on this as I’m doing aerospace engineering at college level in the UK, so my knowledge on atomic physics is none to great.

What I would like to know is about gravity in a way and yes this has something to do with Quantum physics as well but this is also to do with atoms so I put it here lol

in theory it is said there is such a thing has negative mass, I know it has something to do with the electrons being positrons but I’m still not quite sure what this is, I would like to find out about this because it is said to be anti gravity because of its reaction like a magnet to the electron so there for it repels the gravity created from the electrons or something like that.

As I said my atomic and quantum physic is not my best point lol

Anyone care explaining this to me?

2. Aug 10, 2006

### tarbag

The positron has a positive charge and the same mass as an electron. Its mass have nothing with the antimatter, which is still an assumption.

3. Aug 10, 2006

### quetzalcoatl9

I'm not sure, but one thing that comes to mind is the energy equation of special relativity for a particle:

$$E^2 = m^2 c^4 + p^2 c^2$$

for a particle at rest, this reduces to the more well known form:

$$E = m c^2$$

(you can also use this to deduce that a photon mass no mass, yet has momentum, and the de Broglie relation to boot)

but this involves taking a square root of both sides, in which case (mathematically) you get a + and a - solution. This has been postulated to mean that a) there are particles that can move faster than light as long as they _always_ move faster than light (but such particles, to my knowledge, have never been detected) and/or b) there can be negative mass.

I think that the relativistic form of the Schrodinger equation (Dirac equation) led to the knowledge of positronic matter, which has not only been detected but is made regularly in labs around the world in small quantities.

4. Aug 11, 2006

### LURCH

I think this may be a question on negative energy, which is said to have negative mass and predicted to be gravitationaly repulsive, and often gets confused with antimatter (whch is what a postitron is). In the attempt to make clear that negative energy is something completely different from antimatter, researchers often make the problem worse by allways bringing up the two terms together. So the relationship between negative energy (negative mass) and an antiparticle like the positron, is that nobody ever writes about negative energy without writing about the fact that it's not antimatter. AFAIK, that is the only relationship between the two.

5. Aug 11, 2006

If future experiments at CERN show that antimatter has negative mass then new physics will result. Until experiments are conducted all is speculation concerning relationship of antimatter, antigravity, negative mass.

6. Aug 12, 2006

### Mrbrit

ok ok so you proved my Atomic and Quantum physics are a bit rusty :rofl: but still all I wanted to know is what is negative mass and what relation it has to anti-gravity?

7. Aug 12, 2006

### ZapperZ

Staff Emeritus
I'm sure what you're asking here did not suddenly come to you out of nowhere. So can you please give a citation of the source of where you got your information from? This will allow us to check and see if (i) you read the wrong information or (ii) you interpreted what was written wrongly. It is very difficult to tackle something like this based on a perceived understanding of something.

Regarding "negative mass", look at an air bubble in water. That's negative mass to you.

And I'm not trying to be funny either. In condensed matter physics, there's negative "mass" all over the place (see how "effective mass" is defined), including positive "holes" as charge carriers. But sorry, no "anti-gravity", at least not in the way you imagined.

Zz.

8. Aug 12, 2006

### Mrbrit

Hang on a second what you just said is negative weight not mass lol

And I know this because I’m doing aerospace engineering at college; I work with lift and weight all the time. A air bubble floats to the top of water because it is much less dense then water there for it will float to the top of it from the lift created this is due to a unequal pressure. It still has a mass and it is defo not negative.

My source is my college tutor who told me something vague about it, wasn’t quite sure what he was talking about so I came on here to ask.

9. Aug 12, 2006

### ZapperZ

Staff Emeritus
You may be surprised, charge carriers CAN have a negative effective mass from the way it is defined via the dispersion relation. But that is besides the point. These things do not define "anti gravity", whatever that is.

Zz.

10. Aug 12, 2006

### Mrbrit

well I think that’s not true because if it had a negative mass it would effective not be a bubble because it would not able to sustain its self as a mass being that the mass effectively is the amount of molecules in the object and how dense these are. If it was negative it was make a vacuum and suck the water inside of because of a different in pressure, but as we both know this does not happen but anyway back to my negative mass I have looked on the net and found this

http://www.daviddarling.info/encyclopedia/N/negative_mass.html

It makes some sense but still don't give the maths etc. any ideas?

11. Aug 12, 2006

### ZapperZ

Staff Emeritus
Then you obviously haven't come across a solid state text and made the connection when I specifically mentioned about the effective mass of charge carriers. Look at this link

http://psi.phys.wits.ac.za/teaching/Connell/phys284/2005/lecture-07/lecture_07/node5.html

...and pay attention to the definition of effective mass. You'll notice that at the top of the band, the dispersion has a negative curvature, which means the second derivative is negative. This produces a negative effective mass where the charge carrier, for all practical purposes, has a negative mass!

And guess where this is all happening! In your semiconductors and modern electronics that you are using right at this very second!

Zz.

12. Aug 12, 2006

To ZapperZ,

1. The hole has a +ve charge (as opposed to -ve--my addendum)
2, The energy of the hole is equal and opposite to that of the missing electron
3. The effective mass of the hole is equal and opposite to that of the missing electron. (The hole mass will be the curvature of the band near the top of the band.)
4. The momentum of the hole is -k (as opposed to k--my addendum)

So, my question is, why not the the gravity of the hole is opposite to gravity--e.g., "anti-gravity" ? Seems like all other fundamental properties have contrary qualities, why not gravity ?

13. Aug 12, 2006

### ZapperZ

Staff Emeritus
But that was why I asked earlier if one considers an air bubble in water as having the property of "antigravity"?

Note that in such a many-body system, gravity play zero role in the dynamics of a hole. Just look at the Hamiltonian of the many-body system and you'll find that nowhere is there any inclusion of gravitational potential. Thus, the holes may have negative masses, but where would such "antigravity" effects come in?

Zz.

14. Aug 13, 2006

### Mrbrit

you know what, i'm going else where to ask, seem like you just wanna look smart and not answer my question because you do not know, i'm talking mass not effective mass, e.g. the mass of a object when it is standing still which it does say in the link, but seems you did'nt even look at it

15. Aug 13, 2006

### ZapperZ

Staff Emeritus
Actually, I did! You are not the first person to come here and asked things like this.

If you look at the webpage you gave, notice HOW one describes something with "negative mass". You can't just say something has a negative mass and neglect to describe how it would behave! That was why I asked you if you would consider an air bubble as having a "negative mass", because this "antigravity" property and how you define it is crucial in figuring out how things are defined (look at the webpage you cited and see if right upwards in gravity WAS a property of a negative mass)! Would you consider a gravity shielding as "antigravity"? No? Well, some people do! Just ask those Podkletnov fanatics. This was why I wanted to know what exactly do you mean "negative mass" and "anti gravity", the latter is even the topic that you have created for this thread.

However, it seems that asking for clarification and presenting an analogous scenario somehow equates me trying to look smart. Oy vey!

Zz.

16. Aug 14, 2006

### Andrew Mason

Negative mass is a very interesting concept.

Think of the electron as having negative mass but positive charge.

The forces that affect an electron are the coulomb force, gravity and the weak nuclear force. If it has positive charge but negative mass, the electron is repelled by the nucleus but, because it has negative mass, it accelerates in the direction opposite to the force. With gravity, the same thing occurs.

So how do we know that electrons have positive mass and negative charge and not the other way around?

AM

Last edited: Aug 14, 2006
17. Aug 17, 2006

### quetzalcoatl9

I also find it interesting that the functional form of both gravity and electrostatics are the same form (inverse squared) and yet we feel a gravitational pull from the Sun but not an electrostatic pull. The reason: charges can be positive or negative, the sun and earth are (as far as I know) neutral, but more importantly there is no negative mass to speak of. Hence gravity is always attractive - gravity always wins! In fact, once things become massive enough, electrostatics can't quite keep things apart and we wind up with neutron stars, black holes, MECOs, etc, the exotically dense forms of matter. We can make EM circuits and control charged things quite well - like in my computer that I'm using right now - but cannot control gravitation with that kind of ease.

Long story short - the existence of positive and negative terms in the Coulombic potential mean that we can easily control them. We need not resort to gravitational arguments to make things hard - even charge neutral things are hard: hence neutron detectors rely upon He3 reaction releasing a proton in order to detect scattered neutrons since a proton can be easily handled.