Okay, now I've RTFA, and it is one of the worst science articles I've ever read outside New Scientist or Conservapedia. Let us delve in:
But the isotope, helium-3, like many rare Earth elements, has been in high demand with only limited supply.
Helium is not a rare earth element. I have a feeling this line was inserted just to pitch the link below it.
The gas is part of the leftovers that come from cooking up a hydrogen bomb: you know two parts uranium; one part tritium
No idea where that ratio came from. It's not true and irrelevant.
While there are other ways of decaying tritium without needing to build a bomb to do it...
Is the author fully ignorant of nuclear physics or is she gearing up for some kind of scam where she sells "Tritium Decayers" to the government?
But if a patient takes a breath of helium-3, the resulting MRI is so bright it looks as though the patient inhaled a light bulb.
Not as bad, but misses a great opportunity to explain HOW He3 helps lung imaging. He-3 doesn't exist in any significant quantities in the body, so you can tune the MRI to look for that nucleus and bam, you can see the shape of whatever you fill with it.
Until the FDA approves the recycled helium for humans...
The FDA needs to approve this? That's odd, I wonder why. Too bad you didn't explain why or tell us what stage of approval its in.
For a party that suddenly saw the balloons all pop, despite the warnings, everyone jumped.
There are large amounts of He3 being made in heavy water reactors that is not being collected. Until now there has been little motivation to go through the trouble and expense of modifying these reactors to extract it, but it's not THAT hard. At some point it will just be done and then we'll be fine. This is only a short-term problem.
DNRTFA, of course.
Very useful post, but it doesn't address my concern. The geometry and coherency are not the issue, it is the stimulated emission. I believe your description of a traditional laser is not quite right: once the initial spontaneous light starts passing through the rod it induces stimulated emission from the other electrons. It is true that mirrors are not necessary, as in the NIF system for example.
As I understand it the electrons going through the undulators are not undergoing stimulated emission, but are emitting synchrotron radiation as they are accelerated by the undulator's magnetic field. If the electrons are bunched they emit in phase and thus the light is coherent. You're saying that somehow the previously emitted photons stimulate emission from other electrons? What energy do the electrons have that they can emit in stimulated emission? I don't see how it can be their kinetic energy because that would violate conservation of momentum.
Please do explain, because if I'm wrong I'd be quite fascinated to learn what's really going on.
"Stimulated emission" has a specific meaning in physics: an electron (or in theory some other charged particle) is in an excited state, and a photon passing by causes that electron to drop down to a lower energy level and emit a photon of its own. In the case of the FEL we're certainly doing something to the electrons to make them emit photons, but it's not "stimulated emission" in the correct sense.
The nature of stimulated emission is what requires population inversion or something else similar. Population inversion just refers to getting all the electrons in the system up into that excited state. Perhaps you don't have to go to the extreme, but the point is you get the electrons to store up energy, then de-excite them by passing photons nearby.
Unfortunately the true nature of FEL's isn't universally known outside of the x-ray field. I'm a nuclear physicist and went to a talk on FELs and what they can do for nuclear physics, and afterwards about half the physicists I talked to from the audience didn't know, even after the talk, exactly how the FELs worked. The name "laser" gave them pre-conceptions about the fundamental principles that must govern the FEL so that when the description of its operation came all that was left was confusion.
I also looked at that wikipedia article before making my previous post. It seems okay for wikipedia, but am considering making a note on the page about this. I may want to consult with laser experts first though. Maybe that's why I posted on slashdot instead.
I am a physicist-in-training (grad student), and when I first heard of "free electron lasers" I was extremely impressed, because getting electrons into a multi-keV energy state that can lase without atoms involved sounded nearly impossible. Turns out it is, because these are not actually "lasers" the sense of Light Amplification by Stimulated Emission of Radiation. There is no population inversion as in real lasers.
The name of this specific FEL, the "Linac Coherent Light Source", is a much more correct name. They shoot electrons through a wiggler, and as they wiggle they emit coherent photons. Coherency is they key property of laser light, but the name refers to the method of light creation more than the actual output. I don't know why the x-ray community has felt the need to use this misleading name.
Tribes / Tribes 2 are the greatest FPS's ever made (reasonable people can disagree about the relative ranking of those two). Their popularity was hindered by lack of advertisement and a steep learning curve. If a major studio did a remake right (made by people who love it) and advertised it well, then maybe it could really take off.
For any of you who haven't tried it or haven't played for a while and want to get back in, there's still a breath of life. Go to TribesNext, download their client and patch and you can play Tribes 2.
It really is like no other game. If you have never seen it I suggest the videos below that show off its epicness. I don't know how anyone can see these and not want to play it forever.
Once you've played it no other FPS will match up unless it is made in Tribes' image. "For once you have tasted flight you will walk the earth with your eyes turned skywards, for there you have been and there you will long to return. "
I used to do simulations of galaxy collisions, and dark matter is the easiest part to put into the simulation because it is assumed to be collisionless. It just goes wherever the gravitational potential points it (of course, calculating that gravitational potential can be somewhat complicated if you want to smooth over the rough spots caused by the finite number of particles simulated). The hard part is the gas, which has much more complicated physics due to the collisions, heating, cooling, and radiation on top of the gravitational physics. This also removes the scale-independence from dark-matter-only simulations. These are some of the reasons why early galaxy simulations were largely of elliptical galaxies, because they contain negligible amounts of gas.
Yes, and the difference in isotope is very important here. Si-32 is a beta emitter, which is the type of decay that one might possibly expect to be affected by neutrinos if they had any effect at all, because neutrinos are emitted along with the beta. Ra-226 and Pu-238 are both alpha emitters, which makes the seasonal variation in Ra-226 even stranger because neutrinos are not involved at all in alpha decay.
Double blind studies exist to control for the placebo effect. Unless trees are aware that they are being experimented on, this is not an issue (they're already "blind"). This is more like a physics experiment than a medical one: you need to control all the variables and only change one at a time, and use objective measures to determine the effect. Leaf area and shoot length sound like excellent measures in this regard. If there is an opportunity for experimenter bias to creep into any measurement (for example if they are recording something a bit more subjective like bark quality or growth symmetry), then having the person measuring the result be "blind" may be appropriate.
Indeed an interesting mix of scales. However, just so nobody gets confused, they aren't just accelerating one proton at a time. The LHC website claims there will be 2835 bunches each of 10^11 protons in each ring. Suddenly that 1 microjoule (each proton brings to one collision) becomes 565 megajoules (carried by entire beams)
That's the same energy as a 14 ton truck moving at mach 1. Stay out of the way.
What do the hot/cold water exchange generators do? I would expect that pumping cold water from the ocean warms up the ocean...but that would be putting energy INTO the water instead of extracting it. So I'm a little confused. Lets just say it 'changes the ocean temperature'.
Yes, it does indeed heat the water. The way a heat engine works is to move heat from a hot reservoir to a cold one, extracting some useful work in the process. For another example, boil some water and use the steam to turn a turbine. What comes out the end is steam or hot water, vs the cold water you put into the system before you boiled it.
You can't have a perfectly efficient (no waste heat) heat engine, but the larger the temperature difference the more efficient it is.
Running a heat engine backwards is how you do refrigeration. Your AC takes some heat out of your (cold) house, but dumps more than that much heat though the radiator to the (hot) outside air. The extra heat comes from the electricity you used to run the thing. In principle you could run the AC backwards, heating your house and producing electricity (as long as it's hotter outside than inside). This is how the hot/cold water exchange generators work.
Let alone, if we have nearly unlimited electricity what will we do with the heat?
The energy in the wind would eventually dissipate as heat anyway (friction with land, ocean waves, etc). This is why the wind doesn't keep getting strong and stronger without limit. Wind power would actually lead to less net heat emission (0) than most other forms of energy production: fossil fuels and nuclear release energy previously stored away as something other than heat, geothermal speeds the heat release from the mantle, and solar decreases the earth's albedo. Hydro is less obvious... it seems heat neutral but it may even increase the albedo by forming new lakes.
To put this all in perspective, the world power consumption is something like 15 TW. The total amount of solar power incident on the earth is about 130,000 TW.
Bolden is black, just like Obama. That's pretty cool.
Maybe it's just your ignorance, but how many of you didn't realize we had astronauts of many different races and creeds?
Where the hell did this comment come from, and why is it being modded up? To whose ignorance are you referring? Why is it particularly cool that the NASA administrator and the president are the same color? Would it be less cool if he were some other color? Can you rank for us the various color permutations in order of coolness?
Parent didn't say "the electrons travel that far", he said "Electrons on a circuit can't propagate a voltage any further/faster than that", which is perfectly correct. Everything you said is true, but either you misunderstood parent's statement or
Wait...was this a troll?
applies to you.
For the education of those reading, the velocity of propagation of electric signals is typically something like 40%-95% of the speed of light, depending on the geometry and composition of the medium and its surroundings.
Not to burst the joke, but I think Doc Brown and company were saying "gigawatt". The soft "g" sound is a perfectly valid pronunciation. The prefix "giga-" has the same Greek origin as "giant".
You're still thinking binary. In Quantum, there are lots of possibilities in between possible and impossible. Only the extremes of which are possible and impossible.
This gets modded "insightful"? No, things are either possible or impossible, even "In Quantum", whatever that means. People who don't know physics shouldn't post declarations about it.
Clearly all this proves is that we really don't know that much about what's going on in the universe.
I'm getting tired these kinds of posts every time something unexpected is observed. Yes, this observation tells us that our knowledge is not perfect. However, these claims that every contradiction between experiment and theory means that scientists don't know very much aren't just wrong but irresponsible, because people believe them.
The vital point I need to make here is that our finite knowledge is not "all this proves". This proves that 50 solar mass stars can supernova before they shed their hydrogen atmospheres. Now we can take that new piece of knowledge and develop new and better theories about stellar evolution. To just throw are hands up and say "all this proves is that we don't know much" is to overlook a valuable opportunity to advance science.
I presume that you mean a power failure resulting in the elevator freefalling for 110 floors. This is easy to avoid in a system such as this where on the car they probably have magnets placed near conducting plates along the elevator shaft. If power were to fail and the car start falling, the magnets would induce eddy currents in the conductors, which would create an opposing magnetic field, slowing the motion of the car.
This has been used routinely with great success at amusement parks in those drop-towers where you are lifted several stories in the air, then dropped freely most of the way down only to be stopped in the last few meters. There is no active breaking system to stop you: if you look closely you can see metal plates sticking out of the tower that pass behind the car, where magnets are positioned to pass very close to the plates (I'm talking specifically about the one in Six Flags Great America).
You can play with this yourself by dropping a strong magnet down a vertical copper pipe. It will move very slowly, even if it is only barely touching the sides.
I'm not saying that the elevator is a good idea, just that it is probably safe.
I've done some work in the past couple of years on simulations of galaxy collisions, and can speak a little on the value of this type of research.
Large-scale structure simulations such as this one do have specific uses. They do not claim to reproduce the current universe in all its complexity, but can be used to test theories on its composition. When doing simulations like these, one makes certain assumptions in order to test them. They seem to have assumed that dark matter is made of non-relativistic (cold) particles that only interact gravitationally. They also would have had to assume an initial distribution of dark matter that has small density fluctuations. So by comparing the results of this simulation with observations of the real universe, one can get an idea of how accurate our theories of dark matter behavior and initial conditions are.
A common theory that is often assumed to be true is the Cold Dark Matter (CDM) cosmology, where all dark matter is assumed to be relatively heavy particles that are moving much more slowly than the speed of light. When you do this kind of simulation, a large number of dwarf galaxies are created, several times more than are observed in the real universe. This is a strong indication that the CDM assumption is flawed, that there is at least small portion of the dark matter that is "hot", or relativistic, as if there were a large number of high energy neutrinos, or some similar particle.
could such things possibly be closer to the center than we thought? Would this explain what we currently think is the gravity of a central black hole?
We are quite sure that 3.6 million solar masses are in an extremely compact region, so small that if it were multiple objects, they would very quickly collide and merge into one. Here's the observational evidence. Note that some of the data is projected into the future.
shit. I screwed up. I went back and put in the U*236 in the wrong part. That should be the intermediate step in the second set of reactions, the fission ones.
That's what I get for posting so early in the morning.
What I object to, though, is the insinuation that we are the ones splitting the nuclei of the radioactive elements. These things are radioactive precisely because of their tendency to decay and in fact split themselves. They don't even split into other elements. You can't turn uranium into gold, for example, even though it ought to be a straightforward process of splitting off the required number of protons from each atom (if the "we're splitting atoms" camp claims are correct).
We use the heat generated by the decay of radioactive elements to fuel our generators. We do nothing like smashing atoms into smaller bits.
Just a pet peeve of mine whenever I see a nuclear power article.
And a pet peeve of mine is people posting on slashdot in an authoritative fashion when they know nothing about what they're saying.
It is true that Uranium does decay naturally and emit radiation. This decay, however, is the emission of one or very few particles, rather than splitting the nucleus into two large pieces:
U-235 -> U*236 -> Th-231 + alpha
U-238 -> U*236 -> Th-234 + alpha
In nuclear reactors used for power production on Earth, we use the neutrons emitted in radioactive decay to split nuclei of Uranium-235. These two new nuclei are indeed new atoms. A couple common fission processes are:
n + U-235 -> Xe-140 + Sr-94 + 2n
n + U-235 -> La-139 + Mo-95 + 2n
The masses of the two nuclei that come off tend to be between 72 and 160 AMU. Gold is not typically produced, as it's atomic mass is 197 AMU--too heavy to be made in the usual U-235 fission. I think that spontaneus fission might occur, but if it does it is at a much lower rate than is useful.
Energy derived solely from radioactive decay without any fission is sometimes used, but to my knowledge only on deep-space probes such as Voyager and Cassini. IIRC they use the natural heat decay of Plutonium, which is produced from U-238 in reactors.
Slashdot Mirror
Okay, now I've RTFA, and it is one of the worst science articles I've ever read outside New Scientist or Conservapedia. Let us delve in:
Helium is not a rare earth element. I have a feeling this line was inserted just to pitch the link below it.
No idea where that ratio came from. It's not true and irrelevant.
Is the author fully ignorant of nuclear physics or is she gearing up for some kind of scam where she sells "Tritium Decayers" to the government?
Not as bad, but misses a great opportunity to explain HOW He3 helps lung imaging. He-3 doesn't exist in any significant quantities in the body, so you can tune the MRI to look for that nucleus and bam, you can see the shape of whatever you fill with it.
The FDA needs to approve this? That's odd, I wonder why. Too bad you didn't explain why or tell us what stage of approval its in.
wat
There are large amounts of He3 being made in heavy water reactors that is not being collected. Until now there has been little motivation to go through the trouble and expense of modifying these reactors to extract it, but it's not THAT hard. At some point it will just be done and then we'll be fine. This is only a short-term problem. DNRTFA, of course.
Thanks!
Very useful post, but it doesn't address my concern. The geometry and coherency are not the issue, it is the stimulated emission. I believe your description of a traditional laser is not quite right: once the initial spontaneous light starts passing through the rod it induces stimulated emission from the other electrons. It is true that mirrors are not necessary, as in the NIF system for example.
As I understand it the electrons going through the undulators are not undergoing stimulated emission, but are emitting synchrotron radiation as they are accelerated by the undulator's magnetic field. If the electrons are bunched they emit in phase and thus the light is coherent. You're saying that somehow the previously emitted photons stimulate emission from other electrons? What energy do the electrons have that they can emit in stimulated emission? I don't see how it can be their kinetic energy because that would violate conservation of momentum.
Please do explain, because if I'm wrong I'd be quite fascinated to learn what's really going on.
Thanks for the comment.
"Stimulated emission" has a specific meaning in physics: an electron (or in theory some other charged particle) is in an excited state, and a photon passing by causes that electron to drop down to a lower energy level and emit a photon of its own. In the case of the FEL we're certainly doing something to the electrons to make them emit photons, but it's not "stimulated emission" in the correct sense.
The nature of stimulated emission is what requires population inversion or something else similar. Population inversion just refers to getting all the electrons in the system up into that excited state. Perhaps you don't have to go to the extreme, but the point is you get the electrons to store up energy, then de-excite them by passing photons nearby.
Unfortunately the true nature of FEL's isn't universally known outside of the x-ray field. I'm a nuclear physicist and went to a talk on FELs and what they can do for nuclear physics, and afterwards about half the physicists I talked to from the audience didn't know, even after the talk, exactly how the FELs worked. The name "laser" gave them pre-conceptions about the fundamental principles that must govern the FEL so that when the description of its operation came all that was left was confusion.
I also looked at that wikipedia article before making my previous post. It seems okay for wikipedia, but am considering making a note on the page about this. I may want to consult with laser experts first though. Maybe that's why I posted on slashdot instead.
I am a physicist-in-training (grad student), and when I first heard of "free electron lasers" I was extremely impressed, because getting electrons into a multi-keV energy state that can lase without atoms involved sounded nearly impossible. Turns out it is, because these are not actually "lasers" the sense of Light Amplification by Stimulated Emission of Radiation. There is no population inversion as in real lasers.
The name of this specific FEL, the "Linac Coherent Light Source", is a much more correct name. They shoot electrons through a wiggler, and as they wiggle they emit coherent photons. Coherency is they key property of laser light, but the name refers to the method of light creation more than the actual output. I don't know why the x-ray community has felt the need to use this misleading name.
The USS Jimmy Carter already exists, as an attack submarine. wikilink
Tribes / Tribes 2 are the greatest FPS's ever made (reasonable people can disagree about the relative ranking of those two). Their popularity was hindered by lack of advertisement and a steep learning curve. If a major studio did a remake right (made by people who love it) and advertised it well, then maybe it could really take off.
For any of you who haven't tried it or haven't played for a while and want to get back in, there's still a breath of life. Go to TribesNext, download their client and patch and you can play Tribes 2.
It really is like no other game. If you have never seen it I suggest the videos below that show off its epicness. I don't know how anyone can see these and not want to play it forever.
Tribes 1: Legacy Part 1, Part 2, Part 3
Once you've played it no other FPS will match up unless it is made in Tribes' image. "For once you have tasted flight you will walk the earth with your eyes turned skywards, for there you have been and there you will long to return. "
I used to do simulations of galaxy collisions, and dark matter is the easiest part to put into the simulation because it is assumed to be collisionless. It just goes wherever the gravitational potential points it (of course, calculating that gravitational potential can be somewhat complicated if you want to smooth over the rough spots caused by the finite number of particles simulated). The hard part is the gas, which has much more complicated physics due to the collisions, heating, cooling, and radiation on top of the gravitational physics. This also removes the scale-independence from dark-matter-only simulations. These are some of the reasons why early galaxy simulations were largely of elliptical galaxies, because they contain negligible amounts of gas.
Yes, and the difference in isotope is very important here. Si-32 is a beta emitter, which is the type of decay that one might possibly expect to be affected by neutrinos if they had any effect at all, because neutrinos are emitted along with the beta. Ra-226 and Pu-238 are both alpha emitters, which makes the seasonal variation in Ra-226 even stranger because neutrinos are not involved at all in alpha decay.
Double blind studies exist to control for the placebo effect. Unless trees are aware that they are being experimented on, this is not an issue (they're already "blind"). This is more like a physics experiment than a medical one: you need to control all the variables and only change one at a time, and use objective measures to determine the effect. Leaf area and shoot length sound like excellent measures in this regard. If there is an opportunity for experimenter bias to creep into any measurement (for example if they are recording something a bit more subjective like bark quality or growth symmetry), then having the person measuring the result be "blind" may be appropriate.
That's the same energy as a 14 ton truck moving at mach 1. Stay out of the way.
Yes, it does indeed heat the water. The way a heat engine works is to move heat from a hot reservoir to a cold one, extracting some useful work in the process. For another example, boil some water and use the steam to turn a turbine. What comes out the end is steam or hot water, vs the cold water you put into the system before you boiled it. You can't have a perfectly efficient (no waste heat) heat engine, but the larger the temperature difference the more efficient it is. Running a heat engine backwards is how you do refrigeration. Your AC takes some heat out of your (cold) house, but dumps more than that much heat though the radiator to the (hot) outside air. The extra heat comes from the electricity you used to run the thing. In principle you could run the AC backwards, heating your house and producing electricity (as long as it's hotter outside than inside). This is how the hot/cold water exchange generators work.
Wikipedia has an excellent article describing each of the Legrangian points and why each of them is pseudo-stable.
The energy in the wind would eventually dissipate as heat anyway (friction with land, ocean waves, etc). This is why the wind doesn't keep getting strong and stronger without limit. Wind power would actually lead to less net heat emission (0) than most other forms of energy production: fossil fuels and nuclear release energy previously stored away as something other than heat, geothermal speeds the heat release from the mantle, and solar decreases the earth's albedo. Hydro is less obvious... it seems heat neutral but it may even increase the albedo by forming new lakes.
To put this all in perspective, the world power consumption is something like 15 TW. The total amount of solar power incident on the earth is about 130,000 TW.
Where the hell did this comment come from, and why is it being modded up? To whose ignorance are you referring? Why is it particularly cool that the NASA administrator and the president are the same color? Would it be less cool if he were some other color? Can you rank for us the various color permutations in order of coolness?
applies to you.
For the education of those reading, the velocity of propagation of electric signals is typically something like 40%-95% of the speed of light, depending on the geometry and composition of the medium and its surroundings.
Not to burst the joke, but I think Doc Brown and company were saying "gigawatt". The soft "g" sound is a perfectly valid pronunciation. The prefix "giga-" has the same Greek origin as "giant".
This gets modded "insightful"? No, things are either possible or impossible, even "In Quantum", whatever that means. People who don't know physics shouldn't post declarations about it.
I'm getting tired these kinds of posts every time something unexpected is observed. Yes, this observation tells us that our knowledge is not perfect. However, these claims that every contradiction between experiment and theory means that scientists don't know very much aren't just wrong but irresponsible, because people believe them.
The vital point I need to make here is that our finite knowledge is not "all this proves". This proves that 50 solar mass stars can supernova before they shed their hydrogen atmospheres. Now we can take that new piece of knowledge and develop new and better theories about stellar evolution. To just throw are hands up and say "all this proves is that we don't know much" is to overlook a valuable opportunity to advance science.
This has been used routinely with great success at amusement parks in those drop-towers where you are lifted several stories in the air, then dropped freely most of the way down only to be stopped in the last few meters. There is no active breaking system to stop you: if you look closely you can see metal plates sticking out of the tower that pass behind the car, where magnets are positioned to pass very close to the plates (I'm talking specifically about the one in Six Flags Great America).
You can play with this yourself by dropping a strong magnet down a vertical copper pipe. It will move very slowly, even if it is only barely touching the sides.
I'm not saying that the elevator is a good idea, just that it is probably safe.
I've done some work in the past couple of years on simulations of galaxy collisions, and can speak a little on the value of this type of research.
Large-scale structure simulations such as this one do have specific uses. They do not claim to reproduce the current universe in all its complexity, but can be used to test theories on its composition. When doing simulations like these, one makes certain assumptions in order to test them. They seem to have assumed that dark matter is made of non-relativistic (cold) particles that only interact gravitationally. They also would have had to assume an initial distribution of dark matter that has small density fluctuations. So by comparing the results of this simulation with observations of the real universe, one can get an idea of how accurate our theories of dark matter behavior and initial conditions are.
A common theory that is often assumed to be true is the Cold Dark Matter (CDM) cosmology, where all dark matter is assumed to be relatively heavy particles that are moving much more slowly than the speed of light. When you do this kind of simulation, a large number of dwarf galaxies are created, several times more than are observed in the real universe. This is a strong indication that the CDM assumption is flawed, that there is at least small portion of the dark matter that is "hot", or relativistic, as if there were a large number of high energy neutrinos, or some similar particle.
shit. I screwed up. I went back and put in the U*236 in the wrong part. That should be the intermediate step in the second set of reactions, the fission ones. That's what I get for posting so early in the morning.
It is true that Uranium does decay naturally and emit radiation. This decay, however, is the emission of one or very few particles, rather than splitting the nucleus into two large pieces:
U-235 -> U*236 -> Th-231 + alpha
U-238 -> U*236 -> Th-234 + alpha
In nuclear reactors used for power production on Earth, we use the neutrons emitted in radioactive decay to split nuclei of Uranium-235. These two new nuclei are indeed new atoms. A couple common fission processes are:
n + U-235 -> Xe-140 + Sr-94 + 2n
n + U-235 -> La-139 + Mo-95 + 2n
The masses of the two nuclei that come off tend to be between 72 and 160 AMU. Gold is not typically produced, as it's atomic mass is 197 AMU--too heavy to be made in the usual U-235 fission. I think that spontaneus fission might occur, but if it does it is at a much lower rate than is useful.
Energy derived solely from radioactive decay without any fission is sometimes used, but to my knowledge only on deep-space probes such as Voyager and Cassini. IIRC they use the natural heat decay of Plutonium, which is produced from U-238 in reactors.