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Author Topic: If you feed an existing black hole with nothing but electrons will it get full?  (Read 7253 times)

Offline MikeS

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If you feed an existing black hole with nothing but electrons will it get full?


 

Offline Geezer

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How would you maintain a positive potential on the black hole?
 

Offline MikeS

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Aaaaaah..... but the question was hypothetical.

Space is a vacuum so how about an electron gun firing at the black hole.  (Essentially an open ended cathode ray tube, without the screen)
 

Offline Geezer

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Space is a vacuum so how about an electron gun firing at the black hole.  (Essentially an open ended cathode ray tube, without the screen)

Won't work. Electrons only travel towards the screen because of the potential difference between the gun and the screen.
 

Offline MikeS

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But the electron gun can be designed with a hole in the anode that the electrons pass through.
http://matter.org.uk/tem/electron_gun/electron_sources.htm
Some particle accelerators work on the same principle.
http://en.wikipedia.org/wiki/Linear_particle_accelerator
http://en.wikipedia.org/wiki/File:Lineaer_accelerator_en.svg

 

Offline Geezer

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Still won't work. Electrons are too wimpy. They won't get very far.

You can accelerate them in an accelerator, but they are no longer electrons.
 

Offline CliffordK

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You would probably have to make two guns, and two black holes.

Shoot electrons into one black hole.
Shoot protons or positrons into a second black hole.

A few questions.
Does the electron retain its charge, either as an electron, or combining with a proton to form the equivalent of a neutron?  Probably.  The electron is considered to be an elementary particle.

Could one generate an electro-repulsive field equal to the gravity of a black hole?

Coulomb's law, and the equation for the force due to gravity seem similar.  One might be able to calculate the force of gravity on a particle with the mass of an electron, as well as the maximum force of repulsion due to Coulomb's law.  My guess is that the force of electrostatic repulsion may overwhelm the force of gravity, but  you may need to replace much of the matter in the black hole with pure electrons.

Note, the force of repulsion between two isolated electrons is greater than the force of gravitation attraction between the two electron particles.
« Last Edit: 08/01/2012 08:38:44 by CliffordK »
 

Offline MikeS

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clip
Still won't work. Electrons are too wimpy. They won't get very far.


If you position the electron gun just outside the event horizon, surely the overwhelming gravity of the black hole will suck them in?  The black hole could have massive charge but the electrons outside the event horizon will not 'feel' that charge.

Another way to generate the electrons would be through pair production.

But really this is missing the point of the question.
If you feed an existing black hole with nothing but electrons will it get full?
 

Offline Soul Surfer

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Black holes can have a residual charge so I suppose that the excess of electrons would make it less attractive to negatively charged particles and because of the relative weakness of gravity to electric charge on our scale it would only need a proportionately smaller number of electrons to counteract the gravitational field for negatively charged particles at various distances but conversely it would make it much more attractive to positively charged particles and have no effect for neutral particles and no effect on the existence of the event horizon.
 

Offline Geezer

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If you position the electron gun just outside the event horizon, surely the overwhelming gravity of the black hole will suck them in?  The black hole could have massive charge but the electrons outside the event horizon will not 'feel' that charge.


Yes - I guess (I really am guessing!) it would just be a function of the electron's mass. I suppose the BH would become rather negatively charged if you delivered enough electrons to it. Perhaps it might result in a rather severe case of "space lightning".
 

Offline MikeS

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Black holes can have a residual charge so I suppose that the excess of electrons would make it less attractive to negatively charged particles and because of the relative weakness of gravity to electric charge on our scale it would only need a proportionately smaller number of electrons to counteract the gravitational field for negatively charged particles at various distances but conversely it would make it much more attractive to positively charged particles and have no effect for neutral particles and no effect on the existence of the event horizon.

But in a black hole it is not weak, it is immense.
 

Offline JP

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Compared to the electromagnetic force at distances relatively far from the singularity, it isn't immense.  If you replaced the sun with an equivalently-massed black hole, it's gravity at the earth would be the same as the sun's.  If that sun were all electrons, the repulsive force on an electron on the earth would be immense compared to the gravitational pull.  It's only when you get relatively close to the singularity that the pull of gravity dwarfs all the other forces.

So if you're firing electrons in from the outside, they have to overcome the repulsion, which is much stronger at that distance than the gravitational attraction.
 

Offline CliffordK

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The black hole could have massive charge but the electrons outside the event horizon will not 'feel' that charge.

There is nothing in gravity, or the definition of a black hole that would indicate that a electrical, or magnetic forces from within a black hole would not be felt outside of it.

and because of the relative weakness of gravity to electric charge
But in a black hole it is not weak, it is immense.

True, but particle for particle, electric charge repulsion is much greater than gravitational attraction.

If you managed to replace all the mass in a black hole with an equivalent mass of just protons, or just electrons, it would not be stable, and would certainly create an intense electrical field outside of the black hole.

Now, your premise was to start with a pre-existing black hole.  But, say you added enough mass of 100% electrons to double the mass of the black hole, then the electric field would be immense. 

You could not keep that many electrons within the black hole...  Thus it would be "full".
 

Offline Geezer

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Compared to the electromagnetic force at distances relatively far from the singularity, it isn't immense.  If you replaced the sun with an equivalently-massed black hole, it's gravity at the earth would be the same as the sun's.  If that sun were all electrons, the repulsive force on an electron on the earth would be immense compared to the gravitational pull.  It's only when you get relatively close to the singularity that the pull of gravity dwarfs all the other forces.

So if you're firing electrons in from the outside, they have to overcome the repulsion, which is much stronger at that distance than the gravitational attraction.

But surely you could ground the BH and reference it back to your electron gun. All you'd need is a longish wire......................
.
.
(thinking)
.
.
Never mind.
 

Offline MikeS

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The black hole could have massive charge but the electrons outside the event horizon will not 'feel' that charge.

There is nothing in gravity, or the definition of a black hole that would indicate that a electrical, or magnetic forces from within a black hole would not be felt outside of it.


If you managed to replace all the mass in a black hole with an equivalent mass of just protons, or just electrons, it would not be stable, and would certainly create an intense electrical field outside of the black hole.

Now, your premise was to start with a pre-existing black hole.  But, say you added enough mass of 100% electrons to double the mass of the black hole, then the electric field would be immense. 



That's interesting!  Does everyone agree? 
I always understood that nothing could cross the event horizon from inside to outside.
If space time is so highly curved at the event horizon that not even light can escape how can charge escape be felt outside?
If the charge is felt outside of the event horizon could this not in some sense radiate away energy/mass?
It's also telling you something (information) about the inside of the black hole, isn't this also forbidden?
« Last Edit: 09/01/2012 11:34:10 by MikeS »
 

Offline MikeS

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The black hole could have massive charge but the electrons outside the event horizon will not 'feel' that charge.

There is nothing in gravity, or the definition of a black hole that would indicate that a electrical, or magnetic forces from within a black hole would not be felt outside of it.

and because of the relative weakness of gravity to electric charge
But in a black hole it is not weak, it is immense.

True, but particle for particle, electric charge repulsion is much greater than gravitational attraction.

If you managed to replace all the mass in a black hole with an equivalent mass of just protons, or just electrons, it would not be stable, and would certainly create an intense electrical field outside of the black hole.

Now, your premise was to start with a pre-existing black hole.  But, say you added enough mass of 100% electrons to double the mass of the black hole, then the electric field would be immense. 

Compared to the electromagnetic force at distances relatively far from the singularity, it isn't immense.  If you replaced the sun with an equivalently-massed black hole, it's gravity at the earth would be the same as the sun's.  If that sun were all electrons, the repulsive force on an electron on the earth would be immense compared to the gravitational pull.  It's only when you get relatively close to the singularity that the pull of gravity dwarfs all the other forces.

So if you're firing electrons in from the outside, they have to overcome the repulsion, which is much stronger at that distance than the gravitational attraction.



But electrical charge is subject to the inverse square law (with distance) the same as gravity.  In which case why would charge repulsion at a distance dominate gravity?
« Last Edit: 09/01/2012 12:33:01 by MikeS »
 

Offline Pmb

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If you feed an existing black hole with nothing but electrons will it get full?
Even if the electrons could somehow magically get into the black hole it cannot get full. In fact the black hole would just get larger. Besides, when we toss things into a black hole, as we observe it from outside that matter never gets past the event horizon. It merely looks like it does because the grav field redshifts light comming from the object to wavelengths so long it disappears.
 

Offline Soul Surfer

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MikeS consider a proton and an electron in a hydrogen atom these have a gravitational attraction as well as an electrostatic attraction at any separation the electrical attraction between the two particles is about 10^40 times greater than the gravitational attraction.
 

Offline MikeS

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MikeS consider a proton and an electron in a hydrogen atom these have a gravitational attraction as well as an electrostatic attraction at any separation the electrical attraction between the two particles is about 10^40 times greater than the gravitational attraction.

Let's assume that electrons approach the event horizon at a rate of insufficient density that the repulsive charge (outside the event horizon) is overcome by the intense gravitation.

Let me refer back to this post
The black hole could have massive charge but the electrons outside the event horizon will not 'feel' that charge.

There is nothing in gravity, or the definition of a black hole that would indicate that a electrical, or magnetic forces from within a black hole would not be felt outside of it.


If you managed to replace all the mass in a black hole with an equivalent mass of just protons, or just electrons, it would not be stable, and would certainly create an intense electrical field outside of the black hole.

Now, your premise was to start with a pre-existing black hole.  But, say you added enough mass of 100% electrons to double the mass of the black hole, then the electric field would be immense. 



That's interesting!  Does everyone agree? 
I always understood that nothing could cross the event horizon from inside to outside.
If space time is so highly curved at the event horizon that not even light can escape how can charge escape be felt outside?
If the charge is felt outside of the event horizon could this not in some sense radiate away energy/mass?
It's also telling you something (information) about the inside of the black hole, isn't this also forbidden?

I understand that 'charge' is conserved in a black hole but does that imply that charge is felt outside the event horizon?  (see last paragraph above)
« Last Edit: 11/01/2012 08:31:33 by MikeS »
 

Offline MikeS

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If you feed an existing black hole with nothing but electrons will it get full?
Even if the electrons could somehow magically get into the black hole it cannot get full. In fact the black hole would just get larger. Besides, when we toss things into a black hole, as we observe it from outside that matter never gets past the event horizon. It merely looks like it does because the grav field redshifts light comming from the object to wavelengths so long it disappears.

But would it?  Should not the charge stop more electrons entering?

That's just how it appears to us.  The reality is that matter is still being sucked into the black hole regardless of whether we can see it or not.
 

Offline imatfaal

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Quote
That's interesting!  Does everyone agree? 
I always understood that nothing could cross the event horizon from inside to outside.
If space time is so highly curved at the event horizon that not even light can escape how can charge escape be felt outside?
If the charge is felt outside of the event horizon could this not in some sense radiate away energy/mass?
It's also telling you something (information) about the inside of the black hole, isn't this also forbidden?


I understand that 'charge' is conserved in a black hole but does that imply that charge is felt outside the event horizon?  (see last paragraph above)

Gravity is felt o/s the EH - this gives us a measure of mass of the black hole ie information about it.  I do not think we can assume that charge would not also be felt.
 

Offline MikeS

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imatfaal

Can I extrapolate from the above that you think charge would be felt?

Granted gravity is felt outside the EH, that's why space-time is so extremely curved at the event horizon of a black hole that not even light can escape.
Charge would have to propagate faster than the speed of light to stand any chance of escaping.
The EH effectively acts as a shield.
You can think of charge as being similar to the lines of force surrounding a magnet.  These 'lines of force' within the event horizon are so curved they cannot penetrate the EH.
I really don't see any mechanism whereby charge can penetrate (from inside to out) the event horizon.

I would be very interested to hear by what mechanism you think that charge could be felt outside the event horizon?
« Last Edit: 11/01/2012 11:59:14 by MikeS »
 

Offline Soul Surfer

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As I understand it all the fields propagate through all space and the electric and magnetic fields are no exception to this the effect is that the event horizons for a charged black hole could well be in different places for positively charged negatively charged and neutral particles.
 

Offline JP

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I did a bit of searching around the web to find a decent answer to that, Soul Surfer.  It seems there are two effects that allow the field of a charged black hole to interact with matter outside of itself.

The first is that, using only classical results, in-falling matter gives off a field.  Since any external observer would see matter take infinitely long to fall in, I suspect you would get a field from that falling electron forever(?)  But the field would get weaker and weaker and isn't the full answer.

Invoking quantum mechanics, the electrostatic force is mediated by virtual photons.  Virtual particles can cross the event horizon (e.g. Hawking radiation), so a charged black hole can in fact interact via electromagnetism with other stuff. 

All the explanations I found were a bit hand-wavy and I don't really have the time to pick up a book on GR and try to figure it out in more detail, but it seems plausible...
« Last Edit: 12/01/2012 07:10:46 by JP »
 

Offline Geezer

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There's an easy enough way to find out. All you have to do is bombard a BH with charged particles for a bit then see if the field strength in its vicinity (outside the event horizon) has intensified.

Seems simple enough......
 

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