If you held a -1kg 4”diameter sphere in you hand, standing out in the open on the surface of Earth, and let go of it, what would it do?

It would accelerate downwards at 9.8 m/s^2, just like any other object. The force on it would be upwards, but it would respond to that force by accelerating in the opposite direction.

The better question is, what would it be doing before you let go of it? It’d be very difficult to keep a negative-mass object in place, because most everyday forces would result in positive feedback.

While we’re waiting on the experts… Eta. Nijaed by the experts…

Chronos, your answer assumes that a mass must be negative both inertially and gravitationally. Could a mass have a negative gravitational “mass charge” and have positive inertial?

On that note I recall reading that a negative mass object would not only drop through the floor but accelerate more and more quickly as it passed through increasingly dense matter. Not the first thing your intuition expects from the concept of negative mass.

I don’t understand this. Whether or not it has positive inertia, why would it have more positive feedback than a 1kg sphere?

**OldGuy**, it is of course possible to hypothesize something whose inertial mass is not equal to its gravitational mass, but it would fly in the face of everything we think we know about physics. Every experiment ever performed has found them to be identical, and Einstein’s general relativity requires that that not just be a coincidence, but a fundamental feature of how gravity works.

**EastUmpqua**, picture an (ordinary) mass hanging on the end of a spring. If you perturb the mass downwards slightly (stretching the spring), the upwards force from the spring will increase, so the net force on the mass will now be upwards, so it’ll accelerate upwards back to where the force was smaller. The net result is an oscillation.

But now picture a negative mass on a spring. Now, if we stretch the string downwards, the upwards force from the spring will still increase, so the net force will still be upwards… but because F = ma, an upwards force acting on a negative mass will result in a downwards acceleration. So the mass will accelerate downwards, which will stretch the string further, which will increase the force even more, which will result in more downwards acceleration, and so on.

And at some level, all contact forces act like springs.

I understand this, but doesn’t negative mass by itself do that? Could negative mass even fit into general relativity. And it’s no surprise no experiment has found anything as we’ve never seen negative mass.

Negative inertial mass seems ridiculous, which I understand doesn’t mean it can’t be physically true as much of quantum mechanics seems equally ridiculous

Mass is the resistance to change in momentum, there is no “negative mass” from that perspective. This only seems to be a paradox because we view gravity as a force, which it is NOT. Watch this video about frames of references, I have linked to a point in time where they will demonstrate why inertial forces are only apparent but watching the entire video will help here.

https://www.youtube.com/watch?v=bJMYoj4hHqU&t=810s

There could be a Universe where inertial mass is not equal to its gravitational mass but it would not lead to the effects that are described above. The ideas around “negative mass” above are purely based on basic human intuitions failing you.

While our language won’t let me properly describe it, *dark energy* is a relativistic effect which does lead to odd behavior but you won’t be able to build an intuition around the implicaitons if you keep assuming that your intuitions are not false.

Gravity is *not* a force that attracts to each other through a force, that is just a way that we can visualize it and the math to *approximate* the effects. Under relativity massive bodies will move towards each other because spacetime is curved by the mass, it is not due to an attractive force.

“Spacetime tells matter how to move; matter tells spacetime how to curve”

To clarify, negative mass doesn’t exist, but there was a recent paper that introduced a new and confusing term.

The way to think about the “negative mass” they were describing is first to rename it something different to get rid of the confusion, I am going to call it “foo”

“foo” is a property when you apply a force to something the object that experiences “foo” counter to your intuition, seems to push against that force.

This property of “foo” can be visualized by thinking of a car tire rotating at a high rate of speed. Let us pretend that the tire is so well balanced and so smooth that you can’t see that it is rotating that fast. If you were to kick it you wouldn’t be able to tell why your foot was pulled in the direction of rotation.

While you may decide that this resulting force, which is not in the direction of your kick is because it had some type of “sideways mass” you would be incorrect, the tire is just spinning and the sum of the force vectors was not in line with your foots movement.

Personally I would think you were a bit of a jerk if you named this “sideways mass”, but the fact that you named it that is a done deal, but it doesn’t change the property that we typically describe that is named “mass”. You just introduced a new confusing term.

Are you sure about this, because the way I read F=mA, if m is negative either force OR acceleration must be negative (or both imaginary), both can not be.

- Force is a vector.
- Negative mass is not a thing related to that formula.
- Newtonian mechanics has been superseded for over a century.

Consider that there have been literally zero Nobel Prize awarded for someone working on Newtonian physics. If something doesn’t work out under the Newtonian model you probably need to at books with a published after 1726.

I get that schools still tend to teach that Newtonian model is an absolute truth but not even Newton thought he had everything figured out. We have learned a lot over the past 300 years.

Scalar numbers can have sign, but not direction. So not sure how that plays out here, it’s just a just sayin’.

Mass is simply the property where an object resists changes to momentum.

Both momentum and angular velocity are conserved quantities under modern theories.

Explain what “force” except for the fictional force of *gravity*, which is not a real force you are even talking about here.

Outside of trivial problems and school books the concept just doesn’t work.

But feel free to describe what you think is going on but please at least use a tensor to describe it. Feel free to fill complete to fill in your answer below to help me understand.

Tensor τ[sub]ij[/sub] =

Here is another way of thinking of it.

Travel back in time to independence hall on July 4, 1776. Now try to accurately explain the internet to the people there only using words that they know, and without changing the definitions of words as they use them.

The mathematical vocabulary to explain effects like dark energy don’t even exist in Newtonian Mechanics. To explain these ideas you have to move forward to a more modern form of language.

Note if you are interested in this the tools do exist to learn the math, but the language used here is math.

Here is one lecture that will show partially why you will need to at least use tensors to understand this and if the math is beyond your current abilities khan academy and other resources can help you.

The mathematical vocabulary of Newton is simply not sufficient to understand these more modern concepts. If you can commit the time to learning the math I promise you will be amazed by the beauty you find. But you have to make that effort and let go of the idea that your intuition is enough.

IMHO it is worth investing the time.

You can try calling it “foo” - but I’ll fight that.

Sure. So if we call up our positive direction, then if we put an upward (positive) force on a piece of negative mass, then its acceleration will be downward (negative).

As to whether negative mass can exist at all, well, we’ve never seen any evidence of it, and it’s the sort of thing that, if it could exist, we’d expect that we would see evidence of it. But nobody’s ever conclusively proven that it can’t exist, so physicists still occasionally play around with the idea, just in case.

That’s what General Relativity says, but physicists also talk about gravitons. Don’t they treat virtual gravitons much the same way they treat virtual photons as force carriers.

Realistically, physicists don’t treat virtual gravitons in much of *any* way, because we don’t have any idea how to, yet. A working quantum theory of gravity is pretty much the Holy Grail of physics.