<p>It would be rather difficult to make negative mass consistent with the rest of physics and not have a bunch of weird stuff happen. According to general relativity, mass causes a curvature in spacetime, and objects follow that curvature. So whatever effect negative mass has on spacetime, and thus on positive mass objects, would also be the effect it has on the movement of negative mass objects. If negative masses repel positive masses, then they also repel negative masses.</p>
<p>Now, you may think that in the gravitational force formula, the force is the product of the two masses, so two negative masses would give a positive force (where &quot;positive&quot; means &quot;directed towards the other mass&quot;). However, there's also the fact that acceleration is force divided by mass, so a positive force divided by a negative mass would give a negative acceleration. As the answer given in James K's comment notes, this means that given a negative and positive mass, both of them would experience a negative force, which for the positive mass would mean repulsion, but for the negative mass would mean attraction.</p>
<p>If we try to make an atom out of particles that are identical to protons and electrons, except that they have negative mass, then the electrical force would still be a positive force as it is with normal atoms, but this would result in the particles being repelled. The strong force, which normally keeps nucleuses together, would instead push them apart. So coming up with a way of making matter, rather than just isolated particles, would be rather nontrivial. You could have a negative-proton and a negative-position (that is, a particle that's like an electron except its charge is positive and its mass is negative) or a negative-anti-neutron and a negative electron making an analog of a neutron atom, but a helium atom has four particles in its nucleus, so this tactic wouldn't work for them. Even single baryons would be problematic, since the force that holds quarks together would be repulsive. Moreover, since like charges would be attracted and opposite would be repelled, particles would segregate into clumps of all-positive charges and other clumps of all-negative.</p>

CC BY-SA 4.0
2021-12-23T03:27:31.837Z
`+1` I think it's a great question and this is a great answer, and I think it's good to keep the question open and not block further answers and [possibly loose visibility](https://meta.stackexchange.com/q/366884/303080) by closing. So I've added to the question to tie it in to Astronomy better. I realize we should generally avoid modifying questions after an answer is posted, but I'm wondering if the change is okay with you? If not, we can roll it back. *Thanks!*
uhoh
CC BY-SA 4.0
2021-12-23T03:30:34.453Z
Related in Physics SE: [What is the Current Status of Measurement of the Gravitational Mass of Antimatter?](https://physics.stackexchange.com/q/241060/83380)
uhoh
CC BY-SA 4.0
2021-12-23T00:06:02.023Z
2021-12-23T00:06:02.023Z
Acccumulation
<p>In simple Newtonian gravity, uniform mass density does not give rise to any gravitational field (avoiding arguments as to whether the universe is finite or infinite or expanding or stationary). So, within a universe of constant mass density, any mass concentrations give rise to negative (converging) fields and likewise mass voids give rise to positive (diverging fields).  This can be examined by observing the motion of a small test mass which will be attracted to a mass concentration but repelled from a mass void. In this scenario, mass voids behave as negative masses. <br>
The net result is that two mass concentrations or two mass voids will tend to be attracted to each other, whereas a mass concentration and a mass void will tend to be mutually repelled.</p>
<p>See also: <a href="https://physics.stackexchange.com/questions/591103/is-the-force-due-to-gravity-between-two-objects-immersed-in-a-fluid-correctly-gi">https://physics.stackexchange.com/questions/591103/is-the-force-due-to-gravity-between-two-objects-immersed-in-a-fluid-correctly-gi</a></p>

CC BY-SA 4.0
2021-12-23T04:58:34.657Z
2021-12-23T04:58:34.657Z
Roger Wood
<p>Is there law/rule in physics that precludes the existence of a negative mass, wherein similar negative masses attract but positive and negative masses repel?</p>
<p><strong>To keep this on-topic in Astronomy SE:</strong> Has this ever come up in the context of Astronomy? Do cosmologists consider the possibility? Has anyone <em>looked</em> for signs of negative mass, perhaps on galactic scales?</p>

CC BY-SA 4.0
2021-12-22T14:15:10.050Z
This is a nice question, but it probably should belong on [Physics SE.](https://physics.stackexchange.com)
WarpPrime
CC BY-SA 4.0
2021-12-22T15:47:48.617Z
[Not a direct answer, but related](https://astronomy.stackexchange.com/a/21852/11143). In general, one can imagine all sorts of possible physics (magnetic monopoles, a new type of force, strings, etc.) but whether that physics is true is determined by observations. Observations don't support the existence of negative mass.
zephyr
CC BY-SA 4.0
2021-12-22T17:59:02.997Z
zephyr: I think we should be thinking in two directions... sometimes observations look for a theory, and sometimes a theory says possible, but we require more observations to confirm it. I believe the positron did go this second path in the past. regards.
Riad
CC BY-SA 4.0
2021-12-22T18:37:08.117Z
Of course, it already has an answer on physics https://physics.stackexchange.com/questions/44934/does-matter-with-negative-mass-exist  (tl;dr this doesn't exist)
James K
CC BY-SA 4.0
2021-12-23T03:25:47.873Z
**voting to leave open** here because I think it's a net positive to have one good question and answer(s) about negative mass here in Astronomy SE. Rather than close and block anyone from adding an answer, let's leave this one open! I've modified the question accordingly, and notified the current answer author asking them if it looks okay. Otherwise we can just roll back.
uhoh
CC BY-SA 4.0
2021-12-23T03:32:44.837Z
@JamesK which doesn't stop the boffins from checking! [What is the Current Status of Measurement of the Gravitational Mass of Antimatter?](https://physics.stackexchange.com/q/241060/83380) Thus "hasn't been found and would pose a real theoretical challenge if it was" is better than "doesn't exist".
uhoh
CC BY-SA 4.0
2021-12-23T09:36:42.803Z
@uhoh  Sure, the physicists do this.  The was no astronomy mentioned in the question.  The OP's question has been asked on [physics.se]  answered on physics. I don't see how tagging an astronmy tail to it improves the question.
James K
CC BY-SA 4.0
2021-12-23T13:52:56.973Z
@JamesK it improves it *as an Astronomy SE question*
uhoh
CC BY-SA 4.0
2022-01-11T20:33:12.243Z
I am aware of one research that suggest the existence of negative masses. https://phys.org/news/2018-12-bizarre-dark-fluid-negative-mass.html#:~:text=Dark%20matter%20may%20be%20an,matter%20that%20we%20can%20measure.&text=Negative%20masses%20are%20a%20hypothetical,all%20other%20material%20around%20them.
Riad
<gravity><cosmology><mass>
Are there negative masses that attract each other but repel positive masses?
Riad
# Are there negative masses that attract each other but repel positive masses?

(Asked by: username_0 on 2021-12-22)

Is there law/rule in physics that precludes the existence of a negative mass, wherein similar negative masses attract but positive and negative masses repel?

**To keep this on-topic in Astronomy SE:** Has this ever come up in the context of Astronomy? Do cosmologists consider the possibility? Has anyone *looked* for signs of negative mass, perhaps on galactic scales?

**Tags:** gravity, cosmology, mass

> **Comments:**
> **username_1**: This is a nice question, but it probably should belong on [Physics SE.](https://physics.stackexchange.com)  
> **username_2**: [Not a direct answer, but related](https://astronomy.stackexchange.com/a/21852/11143). In general, one can imagine all sorts of possible physics (magnetic monopoles, a new type of force, strings, etc.) but whether that physics is true is determined by observations. Observations don't support the existence of negative mass.  
> **username_0**: username_2: I think we should be thinking in two directions... sometimes observations look for a theory, and sometimes a theory says possible, but we require more observations to confirm it. I believe the positron did go this second path in the past. regards.  
> **username_3**: Of course, it already has an answer on physics https://physics.stackexchange.com/questions/44934/does-matter-with-negative-mass-exist  (tl;dr this doesn't exist)  
> **username_4**: **voting to leave open** here because I think it's a net positive to have one good question and answer(s) about negative mass here in Astronomy SE. Rather than close and block anyone from adding an answer, let's leave this one open! I've modified the question accordingly, and notified the current answer author asking them if it looks okay. Otherwise we can just roll back.  
> **username_4**: @username_3 which doesn't stop the boffins from checking! [What is the Current Status of Measurement of the Gravitational Mass of Antimatter?](https://physics.stackexchange.com/q/241060/83380) Thus "hasn't been found and would pose a real theoretical challenge if it was" is better than "doesn't exist".  
> **username_3**: @username_4  Sure, the physicists do this.  The was no astronomy mentioned in the question.  The OP's question has been asked on [physics.se]  answered on physics. I don't see how tagging an astronmy tail to it improves the question.  
> **username_4**: @username_3 it improves it *as an Astronomy SE question*  
> **username_0**: I am aware of one research that suggest the existence of negative masses. https://phys.org/news/2018-12-bizarre-dark-fluid-negative-mass.html#:~:text=Dark%20matter%20may%20be%20an,matter%20that%20we%20can%20measure.&text=Negative%20masses%20are%20a%20hypothetical,all%20other%20material%20around%20them.  

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## 2 answers

### Answer by username_5 on 2021-12-23. Score: 2

It would be rather difficult to make negative mass consistent with the rest of physics and not have a bunch of weird stuff happen. According to general relativity, mass causes a curvature in spacetime, and objects follow that curvature. So whatever effect negative mass has on spacetime, and thus on positive mass objects, would also be the effect it has on the movement of negative mass objects. If negative masses repel positive masses, then they also repel negative masses.

Now, you may think that in the gravitational force formula, the force is the product of the two masses, so two negative masses would give a positive force (where "positive" means "directed towards the other mass"). However, there's also the fact that acceleration is force divided by mass, so a positive force divided by a negative mass would give a negative acceleration. As the answer given in username_3's comment notes, this means that given a negative and positive mass, both of them would experience a negative force, which for the positive mass would mean repulsion, but for the negative mass would mean attraction.

If we try to make an atom out of particles that are identical to protons and electrons, except that they have negative mass, then the electrical force would still be a positive force as it is with normal atoms, but this would result in the particles being repelled. The strong force, which normally keeps nucleuses together, would instead push them apart. So coming up with a way of making matter, rather than just isolated particles, would be rather nontrivial. You could have a negative-proton and a negative-position (that is, a particle that's like an electron except its charge is positive and its mass is negative) or a negative-anti-neutron and a negative electron making an analog of a neutron atom, but a helium atom has four particles in its nucleus, so this tactic wouldn't work for them. Even single baryons would be problematic, since the force that holds quarks together would be repulsive. Moreover, since like charges would be attracted and opposite would be repelled, particles would segregate into clumps of all-positive charges and other clumps of all-negative.

> **Comments:**
> **username_4**: `+1` I think it's a great question and this is a great answer, and I think it's good to keep the question open and not block further answers and [possibly loose visibility](https://meta.stackexchange.com/q/366884/303080) by closing. So I've added to the question to tie it in to Astronomy better. I realize we should generally avoid modifying questions after an answer is posted, but I'm wondering if the change is okay with you? If not, we can roll it back. *Thanks!*  
> **username_4**: Related in Physics SE: [What is the Current Status of Measurement of the Gravitational Mass of Antimatter?](https://physics.stackexchange.com/q/241060/83380)  

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### Answer by username_6 on 2021-12-23. Score: 1

In simple Newtonian gravity, uniform mass density does not give rise to any gravitational field (avoiding arguments as to whether the universe is finite or infinite or expanding or stationary). So, within a universe of constant mass density, any mass concentrations give rise to negative (converging) fields and likewise mass voids give rise to positive (diverging fields). This can be examined by observing the motion of a small test mass which will be attracted to a mass concentration but repelled from a mass void. In this scenario, mass voids behave as negative masses.   
The net result is that two mass concentrations or two mass voids will tend to be attracted to each other, whereas a mass concentration and a mass void will tend to be mutually repelled.

See also: <https://physics.stackexchange.com/questions/591103/is-the-force-due-to-gravity-between-two-objects-immersed-in-a-fluid-correctly-gi>


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