[HN Gopher] Negative Mass
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Negative Mass
Author : thunderbong
Score : 67 points
Date : 2023-07-15 15:01 UTC (1 days ago)
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| axblount wrote:
| 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
| sandworm101 wrote:
| 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.
| _Microft wrote:
| 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)
| gus_massa wrote:
| 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.
| foobarbecue wrote:
| 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.
| bmacho wrote:
| Is there a continuous version? Or at least a table of content?
| xwdv wrote:
| They should illustrate how it can be used to make an Alcubierre
| drive.
| akomtu wrote:
| 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.
| codeduck wrote:
| the planet one irritates me.
| pavel_lishin wrote:
| 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.
| codeduck wrote:
| 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
| schoen wrote:
| 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?)
| exceptotron wrote:
| 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?
| sclarisse wrote:
| > 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.
| codeduck wrote:
| or they've simply fallen into the common trap of thinking
| planets orbit in fixed tracks.
| Ecco wrote:
| 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 >> ?
| MetalLord wrote:
| 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.
| Armisael16 wrote:
| 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.
| deely3 wrote:
| I have the feeling that the second one.
| red_trumpet wrote:
| 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.
| mdp2021 wrote:
| And where would you introduce "negative mass"? Involved,
| according to the presentation, in phenomena showing
| "creation of energy"?
| Waterluvian wrote:
| 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
|
| ?
| dorkwood wrote:
| 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?
| sandworm101 wrote:
| 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.
| AbrahamParangi wrote:
| 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.
| functoid wrote:
| 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.
| amelius wrote:
| Perhaps because the wall is made from negative mass (?)
| xigoi wrote:
| 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.
| dylan604 wrote:
| 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.
| deviantbit wrote:
| 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.
| Analemma_ wrote:
| 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.
| codethief wrote:
| 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
| StackOverlord wrote:
| https://www.januscosmologicalmodel.com/negativemass
| pavel_lishin wrote:
| I wish they dove deeper into the equations governing what was
| happening on screen.
| hirundo wrote:
| 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.
| psychphysic wrote:
| 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
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