9 ms·

The WHIM is actually extremely non-dense: 1-10 particles per cubic meter. It turns out that matter can be extremely hot without being dense at all, as long as t

by isaacg 6y ago

The WHIM is actually extremely non-dense: 1-10 particles per cubic meter. It turns out that matter can be extremely hot without being dense at all, as long as there's nothing to cool the matter off, as is the case in the space between galaxies. Matter is easy to see when it emits light (like stars) or absorbs light (like uncharged matter). But charged plasma does neither, so it's hard to detect. Fortunately, the WHIM has a prism-like effect, where it smears out light travelling through it, so we can detect it.

a1369209993 6y ago
> Matter is easy to see when it emits light (like stars) That's the part I don't get; if the intergalactic medium is that hot, shouldn't it be emitting huge amounts of blackbody radiation? Supposedly ionization interferes with that, but even black holes emit blackbody radiation (hawking radiation), so that seems like a insuffient explaination.
- simonh 6y ago
  Plasma can behave differently from regular matter in this respect.
- piannucci 6y ago
  Blackbody radiation describes an electromagnetic field in thermal equilibrium. Typically, we associate this radiation with a particular source, or boundary; but the blackbody spectrum itself is a Maxwell fact, not a matter fact. Real systems have many degrees of freedom, some matter, some Maxwell. In this particular example, the Maxwell degrees of freedom have a temperature of 3K, while the matter degrees of freedom have a much higher temperature. Clearly, then, the matter is not in equilibrium with the electromagnetic field. How can this be? Equilibration is a process that proceeds through a large number of interactions that each exchange energy between two thermal systems (or subsystems). If energy transfer is prevented, for instance by the conservation of mass and momentum, or because no post- states of an appropriate energy are available for one subsystem to evolve into, then the systems may remain far from equilibrium relative to each other. I’d have to do some homework to verify this, but I think a correct statement is that free massive charged particles don’t gain or lose energy in net from passing radiation; and conversely, they cannot radiate. This is guaranteed by Lorentz symmetry, btw: if you examine a million-kelvin electron in a frame of reference moving alongside it, you’d agree there’s no way it can be radiating: there’s a frame in which it is stationary, and stationary charges don’t radiate. If it isn’t radiating in one inertial frame, it isn’t radiating in any inertial frame. Only during collisions between charged particles is there a possibility for the matter and Maxwell degrees of freedom to exchange energy: and these are very rare in tenuous plasma. (We would call these “three-body interactions” because the photon counts as one.) [edit to add] Now one might object that charged particles are always interacting, because their fields fall off like 1/r^2; they don’t just end. Indeed! But one of the effects of being in a plasma is that charges are “screened”. You can think of that as meaning the net effect of many-body interactions can be described as altering the “effective” field of each particle to fall off much faster, like e^-r.
  - TeMPOraL 6y ago
    > If it isn’t radiating in one inertial frame, it isn’t radiating in any inertial frame. Maybe this is the answer to a question I've been harboring in the back of my head since high school: if moving charges radiate, they should lose energy, and thus eventually disappear. Almost nothing ever is stationary, so how come all the charges haven't radiated away? A quick googling now (I don't know why I never bothered to do that before) pointed me to https://physics.stackexchange.com/questions/65339/how-and-why-do-accelerating-charges-radiate-electromagnetic-radiation https://physics.stackexchange.com/questions/65339/how-and-wh..., which would suggest that it's not movement but acceleration that makes charges radiate. But again, almost nothing ever is stationary, nor is it ever in truly uniform motion, so how come all the charges haven't radiated away?
    - throwaway2048 6y ago
      The gas is so not-dense that it's particles effectively never collide, so its acceleration never really changes, so it never radiates.
      - a1369209993 6y ago
        For some reason this (along with piannucci's explaination) really helped it click for me - while the gas has a very high blackbody spectrum temperature, emitting hard radiation when it's particles collide at high speed, it effectively has very little surface area from which to emit that blackbody radiation.
      - deleted 6y ago
        [deleted]
    - ummwhat 6y ago
      You ask a very good fundemental question. For things like planets in orbit (under constant acceleration) there really is radiating gravitational waves which carry energy and in principle the orbits must decay over time. Even in particle accelerators the electrons moving in a circle emit radiation which slows them down and must be compensated for. So why don't atoms give off a ton of radiation quickly while the electron decays and crashes into the nucleus? The answer is on that scale it no longer makes sense to think of an electron as a tiny planet orbiting another tiny planet. The rules of quantum mechanics apply, and quantum mechanics says the orbits are quantized and there are no tinier orbits to decay into. So the full answer is that there only seems to be a paradox because your notion of when things should and shouldn't radiate is an approximate truth which only holds for "normal" cases (neither microscopic nor cosmological).
- supernova87a 6y ago
  See others' answers about the equilibrium required for BB radiation -- maybe also the density of this matter is not high enough to be a normal BB radiator, or "thermalized"? Anyway, the density of this matter could be so low that the amount of energy being emitted/received here per unit area (e.g. per square degree of sky) is too low to be detected by equipment. Also note that the radiation is getting redshifted at every distance, so the signal would be further "smeared" and weakened compared to if it were all at a narrow frequency range.
  - supernova87a 6y ago
    By the way, this got me thinking for a long time without knowing the answer: Can single atoms undergo BB radiation? I would think no, but would love to know when it transitions to yes. Articles on BB radiation kind of gloss over this part, only discussing the bulk phenomenon.
- mjevans 6y ago
  The lack of density is the cause of confounding behavior. A normal human perspective on 'hot' or 'cold' relates to collections of matter at the pressure and density that we live within. There is ample material (mass) and opportunity for energy exchange (conductivity), so if something is relatively hot or cold there is a large difference in potential energy and much opportunity to 'correct' that difference by equalizing energy. The WIMP gas they describe would be __highly energetic__ (hot), but also extremely sparse (vacuum). For any given atomic element present it's rather hot. Yet the actual energy density within a given cubic meter of space, relative to some idealized vacuum, is nearly zero compared to the dense muggy energy in a cubic meter of even frigid polar arctic night air.
  - varjag 6y ago
    I assumed it was some sort of giant intergalactic "clouds"/bands? If this is the case how it remains localized enough given the particle velocities over (assuming) billions of years? There has to be some sort of containing field but it can't be its own gravity. Or is WIMP basically uniformly distributed, high energy component of interstellar medium?
    - carterac 6y ago
      I believe the gravity keeps it from diffusing out too much. The high temperature creates a pressure that keeps it from collapsing into balls. Not an expert though!
      - varjag 6y ago
        Well it was said this is plasma with densities of like 10 particles per m3 so there is hardly any pressure. We know what kind of gravity it takes to keep particles this hot together: this is how we have stars. But it's hard to see how gravity can keep an object this vast, diffuse and energetic together.
    - sulam 6y ago
      Note, you've both used WIMP here, but that's another thing (weakly interacting massive particle -- a candidate theory for dark matter). You want the acronym WHIM, for warm hot interstellar medium.
- ncmncm 6y ago
  At a few nuclei per cubic meter, the particles can hardly be interacting at all, even though, being ions, they interact without actually getting very close to one another, unlike particles in a gas. Moreover, the "million-degree" figure is actively misleading. Temperature is really a measure of average particle speed, in a reference frame where they have zero average velocity. (This is why rocket exhaust as it exits the bell is said to have low temperature and pressure -- the particles are moving very fast, but in one direction.) In the intergalactic medium, if the atoms are fully ionized, the electrons are the ones moving like bats out of hell. The atoms, mostly hydrogen, are at least 1836 times more massive ("but graceful for their size"), so going much, much slower. The average of the two motions defines the temperature. The electrons' paths bend when they get close to another electron or ionized atom, and emit synchrotron radiation that is just at the right energy to be absorbed by another electron not far away. Since the density is so low, blackbody radiation that escapes must be very dim. Orbital x-ray telescopes might see a fog if they are sensitive enough, but the photons it picks up probably originate in much nearer interactions. Anyway that is my guess.
  - titzer 6y ago
    > The atoms, mostly hydrogen, are at least 1836 times more massive ("but graceful for their size"), so going much, much slower. The average of the two motions defines the temperature. Isn't it the case though that thermal energy is not the same as inertial energy of motion? I.e. that "hot" atoms can be essentially "vibrating" in place much faster and more violently, and not necessarily moving through space?
    - ncmncm 6y ago
      I think heat energy distributes itself in a system so that each particle carries as much energy as each other, whatever its mass. Molecules can carry energy in spinning and vibrational modes that electrons can't, which means the electrons have to go even faster, for any temperature. (Electrons have constant "spin" that varies only by direction.)
- titzer 6y ago
  Hawking radiation is not blackbody radiation. It is caused by pair production, where pairs of particles are produced at random by quantum fluctuations near the event horizon. One of the pair of particles falls into the black hole and the other escapes as Hawking radiation.
- mehrdadn 6y ago
  Not a physicist, but "hot" just means high average kinetic energy ("moving really fast"). So imagine a single particle moving really fast (even close to the speed of light if you want)—why that mere fact cause it to emit any kind of radiation? That would require losing energy for no reason, but from its perspective, it's already perfectly stationary. Losing energy just because you (an observer) happen to be moving with high speed in the opposite direction wouldn't really make sense. The conclusion being that blackbody radiation isn't just due to temperature, but also interactions between particles. (Which explains why it's a topic under statistical mechanics. It's an emergent property from large numbers of particles.)

by isaacg 6y ago

a1369209993 6y ago

> Matter is easy to see when it emits light (like stars) That's the part I don't get; if the intergalactic medium is that hot, shouldn't it be emitting huge amounts of blackbody radiation? Supposedly ionization interferes with that, but even black holes emit blackbody radiation (hawking radiation), so that seems like a insuffient explaination.

simonh 6y ago
Plasma can behave differently from regular matter in this respect.
piannucci 6y ago
Blackbody radiation describes an electromagnetic field in thermal equilibrium. Typically, we associate this radiation with a particular source, or boundary; but the blackbody spectrum itself is a Maxwell fact, not a matter fact. Real systems have many degrees of freedom, some matter, some Maxwell. In this particular example, the Maxwell degrees of freedom have a temperature of 3K, while the matter degrees of freedom have a much higher temperature. Clearly, then, the matter is not in equilibrium with the electromagnetic field. How can this be? Equilibration is a process that proceeds through a large number of interactions that each exchange energy between two thermal systems (or subsystems). If energy transfer is prevented, for instance by the conservation of mass and momentum, or because no post- states of an appropriate energy are available for one subsystem to evolve into, then the systems may remain far from equilibrium relative to each other. I’d have to do some homework to verify this, but I think a correct statement is that free massive charged particles don’t gain or lose energy in net from passing radiation; and conversely, they cannot radiate. This is guaranteed by Lorentz symmetry, btw: if you examine a million-kelvin electron in a frame of reference moving alongside it, you’d agree there’s no way it can be radiating: there’s a frame in which it is stationary, and stationary charges don’t radiate. If it isn’t radiating in one inertial frame, it isn’t radiating in any inertial frame. Only during collisions between charged particles is there a possibility for the matter and Maxwell degrees of freedom to exchange energy: and these are very rare in tenuous plasma. (We would call these “three-body interactions” because the photon counts as one.) [edit to add] Now one might object that charged particles are always interacting, because their fields fall off like 1/r^2; they don’t just end. Indeed! But one of the effects of being in a plasma is that charges are “screened”. You can think of that as meaning the net effect of many-body interactions can be described as altering the “effective” field of each particle to fall off much faster, like e^-r.
- TeMPOraL 6y ago
  > If it isn’t radiating in one inertial frame, it isn’t radiating in any inertial frame. Maybe this is the answer to a question I've been harboring in the back of my head since high school: if moving charges radiate, they should lose energy, and thus eventually disappear. Almost nothing ever is stationary, so how come all the charges haven't radiated away? A quick googling now (I don't know why I never bothered to do that before) pointed me to https://physics.stackexchange.com/questions/65339/how-and-why-do-accelerating-charges-radiate-electromagnetic-radiation https://physics.stackexchange.com/questions/65339/how-and-wh..., which would suggest that it's not movement but acceleration that makes charges radiate. But again, almost nothing ever is stationary, nor is it ever in truly uniform motion, so how come all the charges haven't radiated away?
  - throwaway2048 6y ago
    The gas is so not-dense that it's particles effectively never collide, so its acceleration never really changes, so it never radiates.
    - a1369209993 6y ago
      For some reason this (along with piannucci's explaination) really helped it click for me - while the gas has a very high blackbody spectrum temperature, emitting hard radiation when it's particles collide at high speed, it effectively has very little surface area from which to emit that blackbody radiation.
    - deleted 6y ago
      [deleted]
  - ummwhat 6y ago
    You ask a very good fundemental question. For things like planets in orbit (under constant acceleration) there really is radiating gravitational waves which carry energy and in principle the orbits must decay over time. Even in particle accelerators the electrons moving in a circle emit radiation which slows them down and must be compensated for. So why don't atoms give off a ton of radiation quickly while the electron decays and crashes into the nucleus? The answer is on that scale it no longer makes sense to think of an electron as a tiny planet orbiting another tiny planet. The rules of quantum mechanics apply, and quantum mechanics says the orbits are quantized and there are no tinier orbits to decay into. So the full answer is that there only seems to be a paradox because your notion of when things should and shouldn't radiate is an approximate truth which only holds for "normal" cases (neither microscopic nor cosmological).
supernova87a 6y ago
See others' answers about the equilibrium required for BB radiation -- maybe also the density of this matter is not high enough to be a normal BB radiator, or "thermalized"? Anyway, the density of this matter could be so low that the amount of energy being emitted/received here per unit area (e.g. per square degree of sky) is too low to be detected by equipment. Also note that the radiation is getting redshifted at every distance, so the signal would be further "smeared" and weakened compared to if it were all at a narrow frequency range.
- supernova87a 6y ago
  By the way, this got me thinking for a long time without knowing the answer: Can single atoms undergo BB radiation? I would think no, but would love to know when it transitions to yes. Articles on BB radiation kind of gloss over this part, only discussing the bulk phenomenon.
mjevans 6y ago
The lack of density is the cause of confounding behavior. A normal human perspective on 'hot' or 'cold' relates to collections of matter at the pressure and density that we live within. There is ample material (mass) and opportunity for energy exchange (conductivity), so if something is relatively hot or cold there is a large difference in potential energy and much opportunity to 'correct' that difference by equalizing energy. The WIMP gas they describe would be __highly energetic__ (hot), but also extremely sparse (vacuum). For any given atomic element present it's rather hot. Yet the actual energy density within a given cubic meter of space, relative to some idealized vacuum, is nearly zero compared to the dense muggy energy in a cubic meter of even frigid polar arctic night air.
- varjag 6y ago
  I assumed it was some sort of giant intergalactic "clouds"/bands? If this is the case how it remains localized enough given the particle velocities over (assuming) billions of years? There has to be some sort of containing field but it can't be its own gravity. Or is WIMP basically uniformly distributed, high energy component of interstellar medium?
  - carterac 6y ago
    I believe the gravity keeps it from diffusing out too much. The high temperature creates a pressure that keeps it from collapsing into balls. Not an expert though!
    - varjag 6y ago
      Well it was said this is plasma with densities of like 10 particles per m3 so there is hardly any pressure. We know what kind of gravity it takes to keep particles this hot together: this is how we have stars. But it's hard to see how gravity can keep an object this vast, diffuse and energetic together.
  - sulam 6y ago
    Note, you've both used WIMP here, but that's another thing (weakly interacting massive particle -- a candidate theory for dark matter). You want the acronym WHIM, for warm hot interstellar medium.
ncmncm 6y ago
At a few nuclei per cubic meter, the particles can hardly be interacting at all, even though, being ions, they interact without actually getting very close to one another, unlike particles in a gas. Moreover, the "million-degree" figure is actively misleading. Temperature is really a measure of average particle speed, in a reference frame where they have zero average velocity. (This is why rocket exhaust as it exits the bell is said to have low temperature and pressure -- the particles are moving very fast, but in one direction.) In the intergalactic medium, if the atoms are fully ionized, the electrons are the ones moving like bats out of hell. The atoms, mostly hydrogen, are at least 1836 times more massive ("but graceful for their size"), so going much, much slower. The average of the two motions defines the temperature. The electrons' paths bend when they get close to another electron or ionized atom, and emit synchrotron radiation that is just at the right energy to be absorbed by another electron not far away. Since the density is so low, blackbody radiation that escapes must be very dim. Orbital x-ray telescopes might see a fog if they are sensitive enough, but the photons it picks up probably originate in much nearer interactions. Anyway that is my guess.
- titzer 6y ago
  > The atoms, mostly hydrogen, are at least 1836 times more massive ("but graceful for their size"), so going much, much slower. The average of the two motions defines the temperature. Isn't it the case though that thermal energy is not the same as inertial energy of motion? I.e. that "hot" atoms can be essentially "vibrating" in place much faster and more violently, and not necessarily moving through space?
  - ncmncm 6y ago
    I think heat energy distributes itself in a system so that each particle carries as much energy as each other, whatever its mass. Molecules can carry energy in spinning and vibrational modes that electrons can't, which means the electrons have to go even faster, for any temperature. (Electrons have constant "spin" that varies only by direction.)
titzer 6y ago
Hawking radiation is not blackbody radiation. It is caused by pair production, where pairs of particles are produced at random by quantum fluctuations near the event horizon. One of the pair of particles falls into the black hole and the other escapes as Hawking radiation.
mehrdadn 6y ago
Not a physicist, but "hot" just means high average kinetic energy ("moving really fast"). So imagine a single particle moving really fast (even close to the speed of light if you want)—why that mere fact cause it to emit any kind of radiation? That would require losing energy for no reason, but from its perspective, it's already perfectly stationary. Losing energy just because you (an observer) happen to be moving with high speed in the opposite direction wouldn't really make sense. The conclusion being that blackbody radiation isn't just due to temperature, but also interactions between particles. (Which explains why it's a topic under statistical mechanics. It's an emergent property from large numbers of particles.)