[Physics]Negative Mass

[Gilly]

I was watching a programme on Stephen Hawking the other day about his theories regarding black holes.

It was pretty interesting, with discussion aimed around hypothesizing and theorizing about what happens at the edge of a black hole. Why they give off radiation, why no light escapes from them, why they're not just destroying everything in their vicinity, etc.

The thing that got me was the explanation of how and why they give off radiation.

At the event horizon (and everywhere else in space) it is said that particles are appearing and disappearing constantly. These particles are in pairs. The theory states that one of the pair falls into the black hole and one of the pair escapes. The one escaping being of positive mass, the other being of negative mass.

Now, it might just be the way the documentary described it, but I can't get my head around how a particle can have negative mass. I know mass doesn't strictly equal weight... but I'm still not getting it.

I have heard of, read up on, and vaguely understand the concept of antimatter and dark matter. Is that what this would have to be?

 

[Mat]

The only way I can understand it is by using the sheet rubber idea whereby an object of positive mass creates the distortion that leads to a gravitational pull whereas an object of negative mass would lead not to a 'dimple' in the rubber sheet but a 'hill' or 'spike'. This would have course suggest a presence of anti mavity but I dont know whether that hold true.

 

[Burnsy2023]

I have heard of, read up on, and vaguely understand the concept of antimatter and dark matter. Is that what this would have to be?

From my understanding, it's antimatter. The BBC4 documentry 'Atom' described this a bit better then the Hawking Documentry although I'm still not grasping it as well as I'd like to

Burnsy

 

[gumbald]

It's been a while since I did any Physics, or read any Hawking, so I won't attempt to justify an answer. Might dig out his book though for some light reading I'm sure it's explained in there.

 

[lsg1r]

please note I'm rusty (and could have forgotten about all of it) on this - been a while since I did A-Level physics

iirc the only way you can have a negative mass is when theres 2 points of attraction and its measured in relation to another object of mass moving in the opposite direction.
so imagine the newtons cradle (the swinging ball thing) - one is positive one is negative sort of idea.

Another option is a spaceship taking off needs a negative mass (by force of acceleration) in relation to mavity (acceleration in the opposite direction)

The elastic explaination is pretty good too.

 

[Gilly]

So negative mass is merely perspective? You could only have negative mass in relation to something else?

Wouldn't particles either side of an event horizon cease to be in relation to each other?

 

[Stan_Lite]

According to Wikipedia:

From Newton's law:

F = ma

Thus it can be seen that an object with negative inertial mass would be expected to accelerate in the opposite direction to that in which it was pushed, which is arguably a strange concept. No **** Sherlock!

Thus objects with negative gravitational mass (both passive and active), but with positive inertial mass, would be expected to be repelled by positive active masses, and attracted to negative active masses. If all such negative matter were like this, then mavity would work similarly to the electric force except that like masses would attract and unlike masses would repel.

Sort of explains the concept in a manner in which I can just about understand. The article does go on to say that negative mass is purely hypothetical and no evidence has been found for it's existence.

 

[Minto]

negative mass is a bit of a misnomer in this context. Strictly speaking the pair particle production results in a particle-antiparticle pair, only one of which escapes the black hole. The particle-antiparticle have opposite charge, and in a manner of speaking "anti-symmetrised" wave functions - meaning rotation and momentum and afew other things are the opposite of each others - , but they always both have the same, positive mass.

edit:
Note the reason that one escapes is because it is produced outside the event horizon and with enough momentum to escape the black hole. The antiparticle in this case will have the same momentum but in the opposite direction, and so will be captured. See diagram:

<----------------*^---|EH|--------->(Black Hole)

Where EH is the even horizon, from within which nothing (well, unless the black hole is rotating) escapes, * and ^ are the particle-antiparticle pair, and the arrows represent the momenta of the particles.

Edit 2:
the reason * isn't decelerated by the oppositely charged ^ (that is if they are charged at all - depends what is pair produced) is that once ^ passes within the EH the photons which communicate the ElectroMagnetic force cannot escape the black hole to cause any force on *

 

[Stan_Lite]

Pay no attention. Misread the OP.

Actually, no I didn't.

Thinking about it, from what the Wiki article says, things would work oppositely from that described in the OP. Surely, if the black hole is exerting an attractive force on the two particles, it would attract the particle with positive mass and repel the particle with negative mass

 

[Amleto]

The mass of antimatter particles isn't known to be negative. Most people think it is positive according to the wiki link.

 

[Minto]

Most people think it is positive according to the wiki link.

If by most people you mean 'the entire scientific community' and by think you mean 'know'

Negative mass is a mathematical abstraction, antimatter is flying around Switzerland all day long and we can measure it's positive mass.

 

[benjo plz.]

The mass of antimatter particles isn't know to be negative.

Negative mass != to antimatter mass. I'm guessing the difference is the rest mass is the same for matter and antimatter (eg positrons have the same rest mass as electrons) but the relative mass is opposite?

 

[Amleto]

I didn't say antimatter mass was negative mass. I suggested otherwise.

'Relative mass'

 

[benjo plz.]

relative mass

this, but i'm wrong on that. But just to confuse you, antimatter particles aren't the same as negative mass particles.

 

[Minto]

relative mass is dodgey ground, most physics degrees now introduce it and then tell you never to use it, and to work with a different interpretation as it's easy to get confused with relative mass.

 

[benjo plz.]

relative mass is dodgey ground, most physics degrees now introduce it and then tell you never to use it, and to work with a different interpretation as it's easy to get confused with relative mass.

If you're easily confused, physics probably isn't for you . To be honest, most theories at the moment are dodgy ground, in ten years time, most of the stuff taught today will either be proven wrong or shown to only scrape the surface.

 

[Minto]

If you're easily confused, physics probably isn't for you . To be honest, most theories at the moment are dodgy ground, in ten years time, most of the stuff taught today will either be proven wrong or shown to only scrape the surface.

In ten year time we might have more accurate models, or a more complete single theory, but that doesn't make what we know now wrong any more than general relativity makes Newton wrong. Yes GR is required in certain special circumstances, but Newtonian mechanics is still right under it's (albeit slightly modified) assumptions and limits.

Anyway, thats enough of that derailing!

 

[lopkinfop]

its not that antimatter has negative mass but if it were to come in contact with a 'normal' particle it would cancel it out.

i.e. a positron + electron = nothing

 

[daz]

relative mass is dodgey ground, most physics degrees now introduce it and then tell you never to use it, and to work with a different interpretation as it's easy to get confused with relative mass.

My lecturer said that relativistic mass = old way of explaining SR to lay people and only bad text books refer to it.

 

[Coran]

Indeed. As described by Minto, in pair production, both the particle and anti-particle have positive mass. They will have opposite charge and a few other properties which the programme probably didn't go into, most importantly helicity. This is a rather interesting idea. Positive or negative helicity correspond to something being left or right handed (respectively). In classical mechanics, if you built a machine one way, then built an exact reflection of it, so that everything was on the opposite side, it would still work. In quantum mechanics, this isn't the case. A left handed particle is different to a right handed particle. The helicity controls whether certain decays can happen or not, as it is a value which must be conserved.

Just to throw something more into the mix, quarks (combinations of which make protons, neutrons and more) and leptons (e.g. electrons) lead to charge. Then there are various particles which lead to force and there are different types of force particle for each kind of force (electromagnetic, the strong force, weak force). Note, I didn't say mavity. The standard model does not include mavity and this is why people are working on what we call a Grand Unified Theory.

Now, perhaps the most interesting idea is that alone, all these particles would be massless. No mass at all. The Higgs-Boson particle is believed to give them all their mass. However, this is the only particle predicted by the standard model of particle physics which hasn't been observed. We expect to know whether or not it exists within the next few years though as there are some rather large experiments going on these days.

Anyway, I really should get on and do something now.