Negative Mass | Hacker News Reader

Negative Mass

students.tools

78 pointsthunderbong42 comments

Previous discussion: https://news.ycombinator.com/item?id=35669340 (97 points | 84 days ago | 68 comments)

There are some things with an negative effective mass, for example some electrons inside a semiconductor. It's easier to think about them as holes with a positive effective mass. More details https://en.wikipedia.org/wiki/Effective_mass_(solid-state_ph...

Anyway, the part in the site about friction is wrong. For example, holes inside semiconductors slow down and follow the conservation of energy law (it's not friction, but it's close enough). They don't get faster and faster like in the simulation. The weird behavior of the simulation is not realistic.

How come when the negative-mass ball encounters the force of the red ball pushing against it, it moves towards the red ball, but when the negative-mass ball encounters the force of a wall pushing against it, it moves away from the wall? Shouldn’t it also move towards the wall, like it did with the ball?

If we are talking about negative mass, it may be that mass and inertia become more clearly different things. Something with "negative" mass might be "pushed" by gravity but still retain normal inertia when touching other types of matter. So the two balls would still bounce off each other as normal, but the gravity interaction between the two would become weird and result in a net gravitational force moving both in one direction.

Because this simulation is nonsense. I don’t mean that pejoratively, just that you can’t get consistent answers from an inconsistent or in this case, under-defined, premise.

Most of this can be understood with two observations: negative mass has negative kinetic energy and its momentum points opposite the direction of velocity. So if a system composed of negative mass ‘loses’ energy or momentum to the environment it actually gains (negative or absolute) momentum. It’s all very unintuitive of course because it is technically nonsense.

Perhaps because the wall is made from negative mass (?)

Why doesn't the negative-mass ball fly off the ground rather than experiencing friction against it? Why is the negative-mass planet attracted to the positive-mass sun?

Edit: Now I realized that while the gravitational force will act in the opposite direction, it will cause acceleration in the same direction because… negative mass. So these two things are not wrong.

There's a lot of issues with these kinds of examples. The theoretical math/physics examples like these make me laugh. Like you stated, why is the negative mass object affected by gravity at all? The concept of parallel universes just because the math works out, but only if you change something to a negative value. That's fun to talk about over a couple of beers while puff puff passing, but people start to take things seriously and then we get all sorts of problems.

> then we get all sorts of problems.

Like what?

String theory

The bit about the balloons in the first two pages no sense to me, so figured it wasn't worth continuing.

When they were filled with air they exterted an upward force on the spring? Why!? And then when the air was removed, the force was downward? By "air" did the author mean "helium" or something? Or are we underwater or on Venus or something?

Also the meaning of "release" was poorly defined. Like... just draw me a free body diagram so I can tell what you're trying to say.

The air was removed from the surrounding environment.

? On which step?

And what's in the balloon, to explain the forces?

Presumably helium as you said.

ooh ok. So helium balloons in air vs helium balloons in a vaccum. Just missing some critical information on the page.

Did someone confirm negative mass? I wouldn't hold your breath on trying to understand negative mass. It is akin to time travel. I'm not saying it isn't possible to understand it, there are probably better things to waste your time with.

When I was an undergraduate I tried to develop a coordinate system for mass, where the center of mass of the universe was the origin, and the axes where the three fundamental forces. Then I found out from a professor that John Wheeler, who was very much alive at that time, had attempted it and I kinda gave up after that.

Later, in graduate school I learned about Wheelers geometrodynamics ("mass without mass"). Talk about paradoxes. I don't know how you resolve all of those paradoxes. I'm guessing that is why he gave up.

If negative mass were confirmed it would be a seismic shift in physics. I'm getting old, and don't keep up anymore. Let me know if it was confirmed, or what other research has been done to substantiate the idea.

It has never been confirmed or even really hinted at by either theory or experiment, but it is fun to do Newton's laws using negative mass and note that they all still work out (e.g. conservation of energy and momentum still hold) even when the macroscopic phenomena become very weird.

At least at the level of isolated gravitational systems (where one can define the so-called ADM mass of a spacetime), mass cannot be negative under pretty general assumptions:

https://en.m.wikipedia.org/wiki/Positive_energy_theorem

https://www.januscosmologicalmodel.com/negativemass

Fun website, but I’m not quite sure what their point is. Are they trying to say that negative mass can have a meaning sometimes? Or is it just « let’s run a simulation on those equations using a negative mass ans see what happens » ?

I have the feeling that the second one.

That approach is not totally unjustified though. As an example, consider the complex numbers. They were first used to solve cubic polynomials, and people basically said "Let's pretend the square root of a negative number is something I can calculate with, obeying the general rules of calculus", and they did. Then they used those successfully to calculate real roots of cubic polynomials.

And where would you introduce "negative mass"? Involved, according to the presentation, in phenomena showing "creation of energy"?

Well... The website seems like it just messes with things. But I remember an article claiming that phonos of sound in atmosphere behave like they have negative mass. Cause the sound wave would travel faster in the denser layers of air from the bottom, making the sound wave to go a little upwards, like a refraction.

Phonons can have a negative effective mass - in other words, you can mostly ignore the effect of the background environment if you simply change the mass to be something else (which can be negative).

That’s very different than having a negative actual mass.

If mass turns negative enough to overcome friction, the collisions from Brownian motion would amplify and we would explode. If negative mass balls in this simulation are made from ordinary matter they should explode, or at least dissolve into atoms and disperse.

I'm not sure about negative mass, but negative energy density can be achieved with the Casimir effect.

https://en.wikipedia.org/wiki/Casimir_effect

My understanding is that this is not really negative energy, rather only energy lower than the quantum background. There is still an absolute zero energy as a hard floor, akin to how negative temperatures just mean lower than an agreed norm and one cannot have true negative temperature.

There are actual negative temperatures (Kelvin scale!). One example are lasers in which a population inversion facilitates lasing.

https://en.wikipedia.org/wiki/Laser

https://en.wikipedia.org/wiki/Population_inversion

https://en.wikipedia.org/wiki/Negative_temperature#Lasers

(Negative thermodynamic temperature has very unintuitive effects in general, so maybe read all of the last article to get an idea)

On the subject of negative stuff.

You can also have a negative temperature even on absolute scales.

I.e. A temperature that is below absolute zero, although it is hotter than positive temperatures.

https://en.wikipedia.org/wiki/Negative_temperature

Negative mass is an example of how we see the world thru our numbers system and believe it covers all possibilities. Our numbers can go negative, so we believe that it predicts a "secret" reality. If our numbers were discontinuous around some values, we would search for similar oddities in the reality.

I wish they dove deeper into the equations governing what was happening on screen.

Is the concept somewhat along the (very simplified) lines of:

- make observations of a phenomenon

- derive formulas that allow further observations to be predicted

- all observations are positive numbers

- apply negative numbers to the formulas and the outputs still seem to reconcile with our models

They should illustrate how it can be used to make an Alcubierre drive.

Is there a continuous version? Or at least a table of content?

the planet one irritates me.

I expected the orbit of the negative-mass planet to look different, somehow. It's strange and non-intuitive that it would be have just like regular mass with regard to the star it's orbiting, but differently with regard to another object in orbit.

Certainly I would have expected the orbit to go higher the faster the planets moved? Is that not the usual effect of increasing speed?

I wonder if it’s using some derived equation for the motion of the planets (re: their orbit) that assumes positive mass, so that part of the simulation understands negative mass but part does not?

or they've simply fallen into the common trap of thinking planets orbit in fixed tracks.

> Certainly I would have expected the orbit to go higher the faster the planets moved? Is that not the usual effect of increasing speed?

That rather depends on what you mean. The usual effect of increasing speed in a circular orbit at any given moment is to change it to an elliptical orbit, where the apoapsis is further away from the center of gravity and the object in question moves slower. And if you increase speed again at the apoapsis, you can make the orbit circular again, but the object will be moving much slower than it was to start. Thus two accelerations, both prograde, have the net effect of substantially reducing the prograde speed.

For any given circular orbit, though, lower means faster.

Exactly. Given the previous examples, i would expect the negative mass planet to curve outward on a hyperbolic trajectory due to the repulsive force generated by the suns's gravity

I think the theory for the normal orbit is that the force given by Newtonian gravitation points outward rather than inward, but then the effect of an outward-pointing force on an object of negative mass is to cause an inward-pointing acceleration.

As other people have pointed out, that's inconsistent with other aspects of the simulation, and maybe just means that the author happened to use two equations involving mass in the simulation and the two negative signs cancelled each other out. Whereas using an odd number of equations involving mass would cause a physically noticeable different result.

I actually wonder if one could derive an explicit contradiction in Newtonian dynamics by allowing negative masses. That is, show that there is no trajectory that actually satisfies all of the equations in this case. (But that might also depend on which equations we think are fundamental. E.g., if we break conservation of energy, is that just an interesting consequence that that law doesn't apply anymore, or is that a contradiction?)

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