Ask a physicist anything. (6)

[mzungu]

Hehe, well, they don't really matter The super-massive black holes make up typically less than 2% of the mass of the stars in the galaxy, and the stars alone are already a pretty small percentage of the total mass of any given galaxy.

I'm also not so sure that they are the binding force that allows galaxies to form. I'm pretty sure that galaxies tend to form wherever you have a sufficiently large clump of dark matter, and supermassive black holes are a natural consequence of the way that gas clouds fall into the potential well provided by said dark matter.

Finally, our strongest evidence for dark matter, in my opinion, stems from the cosmic microwave background, which was emitted before any galaxies ever formed. Basically, before the CMB was emitted, normal matter, which interacts with photons, experienced pressure. So when it fell into a potential well, it would bounce due to that pressure. Dark matter simply fell in, with no pressure to bounce it back out. This leads to very different-looking sound waves for the two sorts of matter in the early universe, and we see the imprint of those sound waves on the CMB. This gives us our most sensitive way of measuring the ratio between dark matter and normal matter, and pretty conclusively rules out nearly all alternative theories for dark matter.

Could this explain the spontaneous appearance of matter and antimater particles that constantly happens in this universe? After all Dark matter must be also a transformation of energy, is it not? Has it reverted back to a pure energy state? It obviously has left like you say its imprint in the CMB.

Could it have appeared or caused a big bang in another parallel universe?

I know that my questions sound very philosophical and that there are many unknowns but even a philosophical question may point to the right direction sometimes?

 

[Chalnoth]

Could this explain the spontaneous appearance of matter and antimater particles that constantly happens in this universe?

I don't see how they're related. Whenever you have an interaction (say, two electrons colliding) where the center of mass energy is large enough to produce new particle/anti-particle pairs, there is a chance that you'll get a new particle/anti-particle pair. Dark matter can contribute to these interactions, but because it is so weakly-interacting, it isn't going to do it often.

After all Dark matter must be also a transformation of energy, is it not? Has it reverted back to a pure energy state? It obviously has left like you say its imprint in the CMB.

There is no such thing as "pure energy". Energy is a property of matter. Everything out there has some amount of mass energy and some amount of kinetic energy. Dark matter is made up of stuff that has mass and is weakly-interacting (similar to neutrinos, but more massive). It also can't have very much kinetic energy to explain what we see.

Could it have appeared or caused a big bang in another parallel universe?

This doesn't make sense to me.

 

[mzungu]

I don't see how they're related. Whenever you have an interaction (say, two electrons colliding) where the center of mass energy is large enough to produce new particle/anti-particle pairs, there is a chance that you'll get a new particle/anti-particle pair. Dark matter can contribute to these interactions, but because it is so weakly-interacting, it isn't going to do it often.


There is no such thing as "pure energy". Energy is a property of matter. Everything out there has some amount of mass energy and some amount of kinetic energy. Dark matter is made up of stuff that has mass and is weakly-interacting (similar to neutrinos, but more massive). It also can't have very much kinetic energy to explain what we see.

OK! but is not matter a property of energy? At the planck level is not everything just energy? Surely at such quantum levels there cannot be matter? I mean after all at such scales even time breaks down and without time matter cannot exist. Wow weird stuff this QM!

This doesn't make sense to me.

Just a thought that if the parallel universe theory is sound then could not black holes be acting like a wormhole from one universe into another? Yeah I know it sounds crazy and highly implausible but QM is even more strange so don't give it a second thought. Thanks for the feedback!

 

[Eudaimonist]

I'd like to know how particle physicists use the word "energy". What is "energy"?


eudaimonia,

Mark

 

[Chalnoth]

OK! but is not matter a property of energy? At the planck level is not everything just energy? Surely at such quantum levels there cannot be matter? I mean after all at such scales even time breaks down and without time matter cannot exist. Wow weird stuff this QM!

No, it's absolutely the other way around: energy is a property of matter. The idea of energy without matter just doesn't make sense. Now, that matter doesn't necessarily mean something you can hold. It could be photons or neutrinos or gravitons. But the very idea of energy is nonsensical without some sort of matter that has that energy.

This may become a bit more clear when I point out that energy and momentum are two different aspects of the same thing. Basically, in the same way that time and space are two components of the same construct, space-time, energy and momentum are two components of the same construct, energy-momentum.

Now, think of what momentum is: it is the product of mass and velocity. Does it make sense for something to be "pure momentum"?

As far as we know about physics at the Planck scale, this is unchanged.

Just a thought that if the parallel universe theory is sound then could not black holes be acting like a wormhole from one universe into another? Yeah I know it sounds crazy and highly implausible but QM is even more strange so don't give it a second thought. Thanks for the feedback!

In principle, yes. But in practice, how would the connection be formed? There would need to be some sort of interaction between the universes, but black holes in our own universe form as a result of the collapse of very massive stars. It doesn't make much sense for there to be some sort of interaction with the other universe, else the other universe would, I would tend to think, be quite detectable.

 

[Chalnoth]

I'd like to know how particle physicists use the word "energy". What is "energy"?


eudaimonia,

Mark

Well, there are a number of ways to get at it.

One way is to just define it as it is done in relativity: energy is the sum of kinetic energy (the energy of motion) and mass energy (the energy internal to the object). You can change kinetic energy by changing how fast an object is moving. You can change mass energy by adjusting the internal energy of the object. For example, if I have a potato, I can increase its mass by heating it up (though in practice this isn't going to be measurable). With atomic nuclei you can sometimes increase their mass by smacking them with radiation that has high enough energy (the nucleus shifts to a different configuration with higher total energy....though it later relaxes and releases the energy you gave it).

Another way to look at energy is a bit more abstract, through Noether's theorem. Noether's theorem is a neat little mathematical proof that demonstrates that if the laws of physics that govern a certain system are the same no matter how you change a certain quantity, then there is a conservation law associated with that change. For instance, if the system behaves the same when you move it to a different location, then momentum is conserved. If the system behaves the same when you rotate it, then angular momentum is conserved. If the system behaves the same at one time as it does at another time, then energy is conserved.

So since most of our laws of physics obey this symmetry, energy is conserved in all short-distance interactions (General Relativity is the exception here, and that is a whole other discussion). By this definition, energy is just whatever is conserved when you have a system that behaves the same at one time as another. It turns out that the stuff that is conserved in this way is the stuff I mentioned above: the combination of mass (internal) energy and kinetic (motion) energy.

 

[hasone]

Odd. I was always under the impression that the term 'matter' was reserved for things that have mass. Wikipedia seems to think it's not a consistently defined term.

I suspect (but certainly don't know) that a definition of matter is not particularly relevant to solving physical equations and characterizing physical systems, so it's currently not super-important.

 

[mzungu]

Well, there are a number of ways to get at it.

One way is to just define it as it is done in relativity: energy is the sum of kinetic energy (the energy of motion) and mass energy (the energy internal to the object). You can change kinetic energy by changing how fast an object is moving. You can change mass energy by adjusting the internal energy of the object. For example, if I have a potato, I can increase its mass by heating it up (though in practice this isn't going to be measurable). With atomic nuclei you can sometimes increase their mass by smacking them with radiation that has high enough energy (the nucleus shifts to a different configuration with higher total energy....though it later relaxes and releases the energy you gave it).

Another way to look at energy is a bit more abstract, through Noether's theorem. Noether's theorem is a neat little mathematical proof that demonstrates that if the laws of physics that govern a certain system are the same no matter how you change a certain quantity, then there is a conservation law associated with that change. For instance, if the system behaves the same when you move it to a different location, then momentum is conserved. If the system behaves the same when you rotate it, then angular momentum is conserved. If the system behaves the same at one time as it does at another time, then energy is conserved.

So since most of our laws of physics obey this symmetry, energy is conserved in all short-distance interactions (General Relativity is the exception here, and that is a whole other discussion). By this definition, energy is just whatever is conserved when you have a system that behaves the same at one time as another. It turns out that the stuff that is conserved in this way is the stuff I mentioned above: the combination of mass (internal) energy and kinetic (motion) energy.

Today I learnt something new! Thank you!

 

[Wiccan_Child]

Odd. I was always under the impression that the term 'matter' was reserved for things that have mass. Wikipedia seems to think it's not a consistently defined term.

I suspect (but certainly don't know) that a definition of matter is not particularly relevant to solving physical equations and characterizing physical systems, so it's currently not super-important.

Matter is the usual 'stuff' that 'things' are made of. Every type of particle of matter has a counterpart called 'antimatter', and not all of these types have mass. Photons, for instance, don't have mass (or, if they do, it's so small as to be undetectable by modern technology).

 

[mzungu]

To continue the black hole saga: Can information be destroyed? If not then it cannot go beyond the event horizon and thus what is beyond the event horizon is zero time and basically nothingness?

 

[Chalnoth]

Odd. I was always under the impression that the term 'matter' was reserved for things that have mass. Wikipedia seems to think it's not a consistently defined term.

I suspect (but certainly don't know) that a definition of matter is not particularly relevant to solving physical equations and characterizing physical systems, so it's currently not super-important.

Right, often times people define matter differently depending upon the context. Here I'm taking a more quantum mechanical route. If I went a more General Relativistic route, it would be almost identical, except that gravitons wouldn't contribute. Since we're talking about fundamental physics, I don't think it makes sense to use most of the other definitions

Although I suppose I should give a nod to one of the other potential definitions: matter is made up of fermions, while non-matter is made up of bosons. The primary difference between fermions and bosons is that fermions take up space, while you can stack an infinite number of bosons in the same box. Fermions include things like electrons and the quarks that make up protons and neutrons. While bosons are the force carriers: photons, gluons, and the W and Z bosons. So the idea here is that you have matter, and you have the stuff that governs interactions between bits of matter. But this still doesn't allow for "pure energy", because you still need either a bit of ordinary matter or a force carrier to have the energy. There is no "freely floating" energy: it's a property of some type of particle.

 

[Wiccan_Child]

To continue the black hole saga: Can information be destroyed? If not then it cannot go beyond the event horizon and thus what is beyond the event horizon is zero time and basically nothingness?

A man once said that black holes have no hair. What he meant was that black holes have only mass, charge, and spin - any other property of the matter that goes into a black hole, such as structure, is lost. In this sense, information is lost. Not all information, but information nonetheless.

There's the idea that, by analysing Hawking radiation streaming from a black hole, we can decode it any work out what matter when in - quite why you'd want to do that is beyond me, but it's a possible way that information isn't lost, that it's somehow emitted as Hawking radiation.

 

[Chalnoth]

To continue the black hole saga: Can information be destroyed? If not then it cannot go beyond the event horizon and thus what is beyond the event horizon is zero time and basically nothingness?

Well, information can be lost in the sense that it can become randomized to the point that it cannot be recovered through any practical means. But as long as the laws of physics are what is known as "unitary", then it can't be completely destroyed.

Now, all of the laws of physics that we know of today are unitary. This means that if you know perfectly the full configuration of a system at one time, then you can evolve the system forward in time to its future configuration, then evolve it backward in time to its past configuration and get the configuration you started with. Physicists tend to believe that the fundamental laws of physics are likely to be unitary because this is all wrapped up with the idea of determinism: if the laws of physics are to explain things, they kind of need to be deterministic. But if they aren't unitary, then it is going to be very hard to have determinism.

Now, theorists aren't wed to the idea that the laws of physics must be unitary, but so far all of the ones we have tested experimentally are, and so are all of our best candidate theories for what may lie beyond the physics we have tested.

So as this relates to black holes, I believe the current understanding is that the information about the stuff that goes into the black hole is encoded in the horizon, and comes out of the black hole in the Hawking radiation, so that if you formed a black hole by having a bunch of radiation converge at one point in the same exact way that the Hawking radiation left the black hole, then running the clock backward long enough will cause the black hole to spit out the kitchen sink that you originally threw into it.

 

[mzungu]

But this still doesn't allow for "pure energy", because you still need either a bit of ordinary matter or a force carrier to have the energy. There is no "freely floating" energy: it's a property of some type of particle.

So what is Gravity? Would a Graviton be mass less? How does energy move in a pure vacuum? Unless such a thing as a pure vacuum does not exist then how does energy in the form of lets say microwaves move in such a medium if there are no mass particles to convey this energy?

 

[Chalnoth]

So what is Gravity? Would a Graviton be mass less? How does energy move in a pure vacuum? Unless such a thing as a pure vacuum does not exist then how does energy in the form of lets say microwaves move in such a medium if there are no mass particles to convey this energy?

Well, in quantum mechanics as currently defined, there simply isn't any gravity to speak of. In General Relativity, as typically defined, the gravitational fields just don't carry any energy.

However, when we learn more about quantum gravity, I'd be willing to bet that we'll be able to define an energy for the graviton in much the same way we define an energy for the photon. This is because even though there isn't any energy in the gravitational fields in General Relativity, you can set up systems where energy will be transferred through gravitational fields from one object to another. In these situations, GR basically just says, "in between, the gravitational fields are configured in this way," and doesn't bother to define an energy for those fields. Nevertheless, the energy of one system can drop while the energy of another system rises, and the configuration of space-time in between can be seen as the cause of this change in energy.

As for the microwaves, well, those are photons. Photons have no trouble moving through space. They don't need any massive particles to convey their energy. By "matter" I certainly do not mean "stuff with mass". I mean any sort of quantum mechanical particle, whether that has mass or not.

 

[mzungu]

As for the microwaves, well, those are photons. Photons have no trouble moving through space. They don't need any massive particles to convey their energy. By "matter" I certainly do not mean "stuff with mass". I mean any sort of quantum mechanical particle, whether that has mass or not.

OK! But photos are affected by mass and gravitational effects. So if in a pure vacuum where only gravity can play a role then if we totally remove gravity (I suspect this is impossible) then would photos still be able to travel? I have this nagging feeling that it all boils down to what exists at the planck level that allows for light to travel even in a vacuum devoid of any matter!
What intrigues me is what is at the planck level. Surely there cannot be infinite smaller particles? It must eventually all boil down to pure energy and even that cannot be in the form of matter!
Dear me am I making any sense?

 

[Chalnoth]

OK! But photos are affected by mass and gravitational effects. So if in a pure vacuum where only gravity can play a role then if we totally remove gravity (I suspect this is impossible) then would photos still be able to travel?

Well, if you change the laws of physics completely, there is no way to say what will or will not happen.

I have this nagging feeling that it all boils down to what exists at the planck level that allows for light to travel even in a vacuum devoid of any matter!

But it's not devoid of any matter. Photons are a sort of matter.

What intrigues me is what is at the planck level. Surely there cannot be infinite smaller particles? It must eventually all boil down to pure energy and even that cannot be in the form of matter!
Dear me am I making any sense?

At this point, no, I'm not understanding a thing you're saying.

 

[Lillen]

Why do the starsky appear diffrent in diffrent districts of my home town? the starsky I observed as young is not the same as now when i live in another district then before. The former sky include more stars, and diffrent set of star images!

 

[Wiccan_Child]

Why do the starsky appear diffrent in diffrent districts of my home town? the starsky I observed as young is not the same as now when i live in another district then before. The former sky include more stars, and diffrent set of star images!

I disagree. Though there is some variation in the exact hemisphere of sky you observe, the stars themselves are almost entirely constant. Maybe as you get older you are recognising constellations that you didn't recognise before, or maybe pollution has diminished allowing you to view more stars - but the stars themselves haven't moved. You've only ever viewed the same half of the sky (the other half is occluded by the Sun during the daytime).

 

[Eudaimonist]

Loopy or stringy or neither? Which way do you lean?


eudaimonia,

Mark