# Negative mass and black holes

1. Aug 9, 2007

### jumpjack

Looking at gravitational and electrical interation relations, I had an idea:
F = kQq/r^2
F = GMm/r^2

Couldn't it exist negative mass?
First formula says that electrical charges reject each other if they have same sign, and they pull if they've different sign.
We know that masses with same sign attracts; so, second formula "should say" that masses with different sign reject.

How could we demonstrate negative masses do exist?
How would a "negative mass black hole" behave?!?

2. Aug 9, 2007

### CompuChip

Well, mass is a measure for translational inertia (recall Newton's second law, F = m a - the more mass the more force you need for the same acceleration). If the mass were negative, this would mean you could pull on the object and it would act like an object with positive mass after you had pushed it.
Moreover, if the mass were negative, the rest energy of an object would be negative (according to $E_\mathrm{rest} = m c^2$ - although on second thought, I think it was $E_\mathrm{rest}^2 = (m c^2)^2$ ), objects would fall against a gravitational field, two objects with a negative mass would still attract each other, but an object with negative mass would be rejected by an object of positive mass.

Only problem is that all objects we encounter in nature are built of constituents with positive mass (energy) and since the models we have are quite accurate without introducing negative mass there is no need for it either.

3. Aug 9, 2007

### Gib Z

Mass is currently defined to be a positive quantity. When certain computations bring about a negative mass, we take the absolute value of the number.

What you do not understand is that Those equations refer to forces of different natures. The first one says particles repel if the CHARGE's are different. The second one has nothing to do with charge! It's a completely different force that comes about for a different reason.

4. Aug 9, 2007

### arivero

I remember this argument from usenet. At the end of the day, the minus sign in the mM will cancel with the minus sign of F=ma and the acceleration will be the same. Of course this is just the statement about gravity, that an object orbit around a gravity field does not depend on the mass of the object. It is the argument allowing for zero mass orbits, in general relativity.

On the other hand, when one goes deeper to Quantum Fields, there is an statement about spin 2 exchanges I can not locate now, so I do not remember what happens with different sign of charges in this case, it could well to be that spin 2 is attractive both for like and unlike charges. Has anybody read about this? If it were, the sign in F=ma would not cancel.

In any case, we know that the limit situation m=0, works as predicted, following an attractive orbit. I can not imagine how the m=0 approach from m>0 could be attractive and the approach from m<0 could be repulsive.

5. Aug 9, 2007

### jumpjack

What about Dark Energy and Dark Matter, which look like an "escamotage" to get real world "fit" to our theories?
When I hear about DE and DM, I feel like hearing about ether while talking about radio transmissions, i.e. it looks like a misunderstanding of physical phenomena.
Could negative mass be a solution to the issue of "need" for DM and DE?

6. Aug 9, 2007

### jumpjack

Sure.
It's just the formulas which are quite similar.
Too similar to be just a coincidence... maybe...

7. Aug 9, 2007

### ZapperZ

Staff Emeritus
Please do not confuse "research front" research with facts. These things are still being studied, so it is highly premature to make any kind of conclusion. It is certainly highly speculative to conclude the existence of "negative mass" simply based on such observations.

Stick to your original question and premise (which had nothing to do with General Relativity in the first place and thus, nothing to do with "dark energy"). And in case you missed it, please review the PF Guidelines before proceeding any further.

Zz.

8. Aug 9, 2007

### jumpjack

I'm not concluding, I'm supposing!

9. Aug 7, 2008

### SdogV

Someone who is foillowing up on the "supposing" of negative mass is Edmond S. Miksch (google him) from his original concepts of 2005.:

My idea was:
Look at the symmetry!:
Given:
(1) gravitational force law, F is proportional to product of masses divided by distance between squared
(2) Mass 1 plus mass 2 = 10
(3) Distance between is 1.

M 1 M 2 Force
5 .... 5 . ...25
6 .... 4 ..... 24
7 ....3 .... 21
8 .....2 ....16
9 ....1 ....9
10 .....0 ....0 >.... -2, >>... -8, ....>>> -18, >>>>.... -32.........
11 .....-1 ....-11
12 ....-2 .... -24
13 ....-3 ....-39
14 ....-4 .....-56
15 -5 -65

(Kind of unusual to see electron shells pop out, no?)

Last edited: Aug 7, 2008
10. Aug 8, 2008

### CompuChip

Very unusual, if:
- premises 2 and 3 didn't seem so arbitrary, why not [itex]m + M = \pi, r^2 = \sqrt{14 e}[/tex]?
- I'd see the electron shells pop out, which I don't

so ...

11. Aug 8, 2008

### Andy Resnick

If you had a lump of negative mass, it would pair with a positive lump and accelerate off to infinity. That is, negative mass is not stable in this universe.

12. Aug 8, 2008

### Defennder

Why does that happen?

13. Aug 8, 2008

### Andy Resnick

From:

http://en.wikipedia.org/wiki/Exotic_matter

'At first glance it would appear that a negative mass would accelerate away from a positive mass, but because such an object would also possess negative inertial mass it would accelerate in the opposite direction from F. Furthermore, it can be shown that if both masses are of equal but opposite mass, Bondi pointed out then the combined system of positive and negative particles will accelerate indefinitely without any additional input into the system.'

14. Aug 9, 2008

### rbj

that wasn't the case when i took a Semiconductor Device Physics course 3 decades ago in college. holes behaved like negatively charged particles with negative mass and we did not toss the sign of the mass without also taking out the sign of the charge to compensate for it.

i think the OP understands that the forces come from different actions. what he/she might not understand is that when keeping the Equivalence Principle in mind, that a hypothetical negative mass will still move toward the other mass, if it's positive (a negative mass accelerates in the opposite direction of the force applied). but the other mass will always be repelled from a negative mass.

negative masses, if they were to exist, always repel other masses, whether they are positive or negative. positive masses always attract other masses, whether they are positive or negative. you can get an interesting perpetual motion machine by positioning in free space a planet of positive mass, M, some distance from a equal-sized planet of negative mass -M. one planet will be chasing the other planet who is always trying to lose the first.

Last edited: Aug 9, 2008
15. Aug 9, 2008

### Defennder

rbj, I don't think they were talking about the concept of negative mass in semiconductor physics. It's supposed to be negative "effective mass" to be precise, and which takes into account the interaction of the electrons with the lattice structure.

16. Aug 9, 2008

### rbj

okay, "effective mass" then. the point was a negatively charged particle with negative "effective mass" moved in an E-field the same direction that a positively charged particle did with the equivalent positive mass.

17. Aug 9, 2008

### Staff: Mentor

You can always make 2 and 3 true by appropriate choice of units. What is more of a problem is that mass is not quantized that way as far as I know.

18. Aug 9, 2008

### SdogV

Since the Planck Mass equals square root of (hc/G).... perhaps we SHOULD be considering the negative root in our model(s) somewhere..

Re: CompuChip comment:

Aren't mass, distance (and time) measurements arbitrary, i.e. based on a scaled reference. Why make them transcendental, like pi and e?

Ah well.

19. Aug 9, 2008

### Staff: Mentor

Last edited by a moderator: Apr 23, 2017
20. Aug 9, 2008

### CompuChip

Exactly my point. In your post you produced a table with some numbers which - if I understand you correctly - are supposed to have some connection to electron shells. But I don't see why you would choose the masses as you did, and in fact you can choose them to be anything and get out a different set of numbers which may or may not be linked to something else.

But maybe I misunderstood your original post.