Seriously, anyone here who lives in Norway (especially Oslo) should look for meteorites in their yards, on their roofs, etc. It is very common for meteors to break up as they reenter, and so it is very common, having found one large meteorite in an area, to find others nearby.
So the thing that surprises me is - how does the US government get involved in IANA and various TLDs? The only TLD they should be bothered about is.us. I guess one could make an argument for.com,.org,.net and others, but there too, they are assigned to non-US organizations as well. While the US may have 'invented the internet', its management as a worldwide resource has to be free of any country's government, even if the bulk of that organization's activities happen within that country.
Which is why it puzzles me that the government should be in any way involved in the relationship b/w ICANN and IANA.
It has always been involved, and there has always been this connection. IANA was set up by Jon Postel under a US Government contract and transferred to ICANN under a US Government contract (the one with the canceled RFP, to be specific).
Sergei Korolev died in 1966, and the N1 was finally canceled in 1976. So, by "a number of years ago" you mean "decades ago." If the Soviets could plan in (say) 1970 to land on Mars 47 years later, I don't see why the Chinese couldn't plan now to land on Mars in (say) 2049, which would be the Centennial of their revolution. And, if they pursue this goal, I think they could, in that time frame, pull it off.
Yeah, it was apparently totally by accident, as they just hadn't been able to separate enough Lithium-6 for a pure test. One wonders that, if they had waited for pure Lithium-6, they would have ever figured this out.
Just imagine if nuclear weapons were in the hands of a civilian group
The movies imply the President, who is a civilian, has to authorise their use. Is that not the case?
It came out much later that Curtis LeMay (first head of SAC) had never instituted the locks to prevent use of nuclear weapons by SAC without Presidential authority (either that, or the codes were set to something like "000"). So, shades of "Dr Strangelove," the President did not have to authorize their use. They could have done it all on their own.
Things are supposed to be different now, but of course it is hard for an outsider to check.
So your proposal is not to have any security, and hope for the best? How do you suppose that would be viewed in retrospect after the next (inevitable) terrorist attack on a plane?
About the same as with security.
I am old enough to remember before there was security on airplanes at all. There were some bombings etc., and yet people didn't freak out. Security was instituted to prevent hijackings to Cuba, which threat seems to have passed. So, from that standpoint it might be better to have no security, so that after an outrage, "something can be done."
An airplane is delicate enough to a single bullet that I can see measures to prevent weapons being taken on board, say, about the same as for entering the US Congress (i.e., the classic magnetometer / X ray combination). Anything more than that I think is unnecessary.
The TSA security system makes no sense because it is not based on any recognizable threat model. All those units are not protecting against planes being used as weapons (changed procedures and attitudes did that), nor are they keeping the body count lower. In a crowded airport with a unified security system (Washington-Dulles, Atlanta, Chicago-Midway all come to mind) you can have many more people assembled together at security than on any one flight. Any detonation there would almost certainly have a higher body count and would also close the entire airport, probably for days or weeks (as it would take out the only checkpoint). And, of course, it would certainly make many people more nervous about flying.
It is thus impossible to regard the TSA as other than security theater, designed to hassle the traveling public enough to reassure them, and buy enough gear to enrich former directors.
The way I prefer to state this is that "Dark Matter" is a sign that there is something we don't know about the physics of large scale matter. We know that there is missing physics, but we don't know where it is. If it is in some new "Cold Dark Matter," (or CDM) the missing physics is in quantum field theory. If, as in Milgrom's MOND hypothesis, the problem is with gravity on large scales or weak accelerations, the missing physics is in gravitation. Your idea would put the missing physics in extra-dimensions, i.e., brane theory, which I regard as perfectly plausible. The point is, we don't know. All we really do know is that there is a hole in the system. We call that hole "Dark Matter," but at present there is no proof that this is necessarily due to non-interacting particles (i.e., CDM).
Note that this can get tricky. Dark Matter cannot be baryons (i.e., normal matter, like us) because that would violate various constraints from nucleosynthesis. However, there may be some missing baryonic matter, so in some cases evidence for dark matter may not _require_ Dark Matter or CDM. Also, if you are going to explain Dark Matter as some new gravitational type field, that may also require new particles, and so may have a CDM type analogue. And, of course, if you are mucking around with field theories you may conceivably change the nucleosynthesis constraints.
Now, in cases such as this (specifically the original case, the Bullet Cluster), there have been claims that the separation of Dark Matter and visible matter invalidates MOND. Milgrom disagrees.
My personal feeling is that we are too distant and, even more importantly, have much too short an observational history, to be able to do pure physics with galaxy clusters, and that a proof or disproof of CDM or MOND or brane theory will have to come elsewhere, in the lab, in solar system observations, or through observations of gravitational waves. (CDM would have the normal gravitational waves of General Relativity, while the relativistic version of MOND, TeVeS, would not. Brane theories predict different deflection / delay of light due to matter than General Relativity. Both can be tested here, in the solar system.)
It is still working because it was serviceable, and was serviced. (It is no longer scheduled to be, so it won't last too much longer.) That is a rare thing at NASA, which tends to be dominated by people who would rather spend money building things.
I would argue that wheels (for transportation) are more or less useless without roads, so that the important invention was the road network. If that is true, it is no accident that the wheel was invented during the first period of large empires, as they built road networks, nor that wheel use tended to die out after the fall of the Roman Empire, when the Roman Roads fell into ruin.
In theory the cores of galaxies should be rotating faster than the periphery, however observation contradicts this. So the hypothesis was postulated that there was additional 'dark' matter surrounding galaxies which could cause the periphery to rotate faster. If Abell 520 has had its dark matter removed, its periphery should be rotating in accordance with standard gravitational theory, rather than as effected by invisible dark matter. Its pretty simple really.
Falsifiable ? Yes, but probably not this way. First off, A520 is a cluster of galaxies, not a single one. The dark matter orbiting the galaxy core is going to be tightly bound to that galaxy, and won't be stripped by a cluster collision. And (see my post below), anyway it's not the stars, but the gas that gets separated from the dark matter.
This should not be too confounding. Suppose you have two galaxies collide. The dark matter will sail right through the other galaxy, affected only by the overall gravity. The stars will almost never hit each other, so the vast majority of them will be affected only by the overall gravity too. The gas and dust will not - dust is subject to radiation pressure, and gas (plasma) magnetic fields. Once the gas and the dark matter become separated, there is no guarantee they will ever get back together. As the paper says :
One of the key tools for studying merging clusters is the comparison among the distributions of the three cluster constituents: galaxies, hot plasma, and dark matter. For example, in merging clusters the intracluster medium suffers from ram pressure and lags behind galaxies and dark matter, which are believed to be effectively collisionless. The contrast between collisional and collisionless components becomes highest when we observe merging clusters at their core pass-through, when both the medium velocity and the effect of ram pressure stripping are largest.
This doesn't sound like much of an actual threat. If you can't physically access the machine, what good does having its "algorithms" do you ? What, is Elon Musk going to carry this up to the ISS on the Dragon and take over the air handling system ?
Why is it important ? Because it is built into the structure of spacetime as we understand it. It would mean that special relativity was wrong (or, rather, incomplete, as SR is very solidly tested) and thus, that General Relativity was also incomplete (as it has SR as a limit, and is also well tested). It would also probably mean something similar for Quantum Field Theory, as that is built on top of SR. The ramifications would spread throughout most of physics.
Also, depending on how it occurs, superluminal motion could reveal a host of very strange things, not the least of which would be time travel.
Are there any experiments whose results can only be explained by light being the fastest thing in the universe?
That's probably not the best way to think about it. It's not like saying that Porsches are faster that Hondas, which could be changed if Honda builds a faster car. This is more like saying that distance is equal to velocity multiplied by time, and then finding a make of car where that didn't seem to be true. You would naturally expect either the speedometer or the odometer to be in error, but if they are not...
Suppose, for the sake of argument (I know the chances are between slim and none, I suspect that the speed limit on the universe is to prevent game-breaking exploits of the universe itself) that FTL neutrinos are possible. How fast do they need to travel before you can send messages to the past?
One of the interesting things to come out of this is that you can have bi-metric theories with superluminal motion and not have causality violations (time travel) or tachyons. Basically, these things happen because you are Lorentz transforming something going faster than light. In a bimetric theory, there are actually two conversions between time and distance (i.e., "light" speeds, although light itself only goes at one of them). Matter is sensitive to one metric, the matter metric (thereby avoiding violations of the weak equivalence principle, or WEP, which is tightly constrained for normal matter), while gravitational waves follow the other, gravity metric. It turns out that there are no good WEP limits for neutrinos. so you could imagine neutrino's following the other, gravity, metric. Thus, they get superluminal motion, but without the complications.
So, to answer your question, it may be that no matter how fast they are, you can't send anything into the past.
This is the best description I have seen in English of the debacle (see the first comment).
Crucial bit : We do not know how crooked the plug actually was at the time of our measurements last year. Sub-sequentially we do not know the actual time delay. So, they just don't know.
Slashdot Mirror
if it was his cottage that the meteorite had crashed through.
Given what some falls sell for, he might be anyway, as he might make a tidy profit.
Seriously, anyone here who lives in Norway (especially Oslo) should look for meteorites in their yards, on their roofs, etc. It is very common for meteors to break up as they reenter, and so it is very common, having found one large meteorite in an area, to find others nearby.
So the thing that surprises me is - how does the US government get involved in IANA and various TLDs? The only TLD they should be bothered about is .us. I guess one could make an argument for .com, .org, .net and others, but there too, they are assigned to non-US organizations as well. While the US may have 'invented the internet', its management as a worldwide resource has to be free of any country's government, even if the bulk of that organization's activities happen within that country.
Which is why it puzzles me that the government should be in any way involved in the relationship b/w ICANN and IANA.
It has always been involved, and there has always been this connection. IANA was set up by Jon Postel under a US Government contract and transferred to ICANN under a US Government contract (the one with the canceled RFP, to be specific).
FWIW, that was not the original headline.
Sergei Korolev died in 1966, and the N1 was finally canceled in 1976. So, by "a number of years ago" you mean "decades ago." If the Soviets could plan in (say) 1970 to land on Mars 47 years later, I don't see why the Chinese couldn't plan now to land on Mars in (say) 2049, which would be the Centennial of their revolution. And, if they pursue this goal, I think they could, in that time frame, pull it off.
And Lewis Strauss, the Republican politician who headed the AEC ? Below slime.
Yeah, it was apparently totally by accident, as they just hadn't been able to separate enough Lithium-6 for a pure test. One wonders that, if they had waited for pure Lithium-6, they would have ever figured this out.
The movies imply the President, who is a civilian, has to authorise their use. Is that not the case?
It came out much later that Curtis LeMay (first head of SAC) had never instituted the locks to prevent use of nuclear weapons by SAC without Presidential authority (either that, or the codes were set to something like "000"). So, shades of "Dr Strangelove," the President did not have to authorize their use. They could have done it all on their own.
Things are supposed to be different now, but of course it is hard for an outsider to check.
I read the complete transcripts of "The Matter of J. Robert Oppenheimer" (700 pages IIRC) and have talked to various people who were involved.
Teller was a slime and his students were right to shun him.
Teller destroyed the career of Robert Oppenheimer for no damn good reason, after which his own graduate students shunned him.
I have no interest in anything to do about him.
So your proposal is not to have any security, and hope for the best? How do you suppose that would be viewed in retrospect after the next (inevitable) terrorist attack on a plane?
About the same as with security.
I am old enough to remember before there was security on airplanes at all. There were some bombings etc., and yet people didn't freak out. Security was instituted to prevent hijackings to Cuba, which threat seems to have passed. So, from that standpoint it might be better to have no security, so that after an outrage, "something can be done."
An airplane is delicate enough to a single bullet that I can see measures to prevent weapons being taken on board, say, about the same as for entering the US Congress (i.e., the classic magnetometer / X ray combination). Anything more than that I think is unnecessary.
The TSA security system makes no sense because it is not based on any recognizable threat model. All those units are not protecting against planes being used as weapons (changed procedures and attitudes did that), nor are they keeping the body count lower. In a crowded airport with a unified security system (Washington-Dulles, Atlanta, Chicago-Midway all come to mind) you can have many more people assembled together at security than on any one flight. Any detonation there would almost certainly have a higher body count and would also close the entire airport, probably for days or weeks (as it would take out the only checkpoint). And, of course, it would certainly make many people more nervous about flying.
It is thus impossible to regard the TSA as other than security theater, designed to hassle the traveling public enough to reassure them, and buy enough gear to enrich former directors.
So God is universal, omnipresent, and doesn't interact directly with people at all.
Sounds like the Unitarian-Universalists were right !
Wasn't it standard Imperial procedure to jettison their garbage there ?
The way I prefer to state this is that "Dark Matter" is a sign that there is something we don't know about the physics of large scale matter. We know that there is missing physics, but we don't know where it is. If it is in some new "Cold Dark Matter," (or CDM) the missing physics is in quantum field theory. If, as in Milgrom's MOND hypothesis, the problem is with gravity on large scales or weak accelerations, the missing physics is in gravitation. Your idea would put the missing physics in extra-dimensions, i.e., brane theory, which I regard as perfectly plausible. The point is, we don't know. All we really do know is that there is a hole in the system. We call that hole "Dark Matter," but at present there is no proof that this is necessarily due to non-interacting particles (i.e., CDM).
Note that this can get tricky. Dark Matter cannot be baryons (i.e., normal matter, like us) because that would violate various constraints from nucleosynthesis. However, there may be some missing baryonic matter, so in some cases evidence for dark matter may not _require_ Dark Matter or CDM. Also, if you are going to explain Dark Matter as some new gravitational type field, that may also require new particles, and so may have a CDM type analogue. And, of course, if you are mucking around with field theories you may conceivably change the nucleosynthesis constraints.
Now, in cases such as this (specifically the original case, the Bullet Cluster), there have been claims that the separation of Dark Matter and visible matter invalidates MOND. Milgrom disagrees.
My personal feeling is that we are too distant and, even more importantly, have much too short an observational history, to be able to do pure physics with galaxy clusters, and that a proof or disproof of CDM or MOND or brane theory will have to come elsewhere, in the lab, in solar system observations, or through observations of gravitational waves. (CDM would have the normal gravitational waves of General Relativity, while the relativistic version of MOND, TeVeS, would not. Brane theories predict different deflection / delay of light due to matter than General Relativity. Both can be tested here, in the solar system.)
Likewise, an alien species looking at Earth is probably seeing a bunch of dinosaurs, and we all know how THAT turned out.
Actually, we don't. The birds may outlast us yet.
It is still working because it was serviceable, and was serviced. (It is no longer scheduled to be, so it won't last too much longer.) That is a rare thing at NASA, which tends to be dominated by people who would rather spend money building things.
I would argue that wheels (for transportation) are more or less useless without roads, so that the important invention was the road network. If that is true, it is no accident that the wheel was invented during the first period of large empires, as they built road networks, nor that wheel use tended to die out after the fall of the Roman Empire, when the Roman Roads fell into ruin.
In theory the cores of galaxies should be rotating faster than the periphery, however observation contradicts this. So the hypothesis was postulated that there was additional 'dark' matter surrounding galaxies which could cause the periphery to rotate faster.
If Abell 520 has had its dark matter removed, its periphery should be rotating in accordance with standard gravitational theory, rather than as effected by invisible dark matter. Its pretty simple really.
Falsifiable ? Yes, but probably not this way. First off, A520 is a cluster of galaxies, not a single one. The dark matter orbiting the galaxy core is going to be tightly bound to that galaxy, and won't be stripped by a cluster collision. And (see my post below), anyway it's not the stars, but the gas that gets separated from the dark matter.
This should not be too confounding. Suppose you have two galaxies collide. The dark matter will sail right through the other galaxy, affected only by the overall gravity. The stars will almost never hit each other, so the vast majority of them will be affected only by the overall gravity too. The gas and dust will not - dust is subject to radiation pressure, and gas (plasma) magnetic fields. Once the gas and the dark matter become separated, there is no guarantee they will ever get back together. As the paper says :
One of the key tools for studying merging clusters is the comparison among the distributions of the three cluster constituents: galaxies, hot plasma, and dark matter. For example, in merging clusters the intracluster medium suffers from ram pressure and lags behind galaxies and dark matter, which are believed to be effectively collisionless. The contrast between collisional and collisionless components becomes highest when we observe merging clusters at their core pass-through, when both the medium velocity and the effect of ram pressure stripping are largest.
This doesn't sound like much of an actual threat. If you can't physically access the machine, what good does having its "algorithms" do you ? What, is Elon Musk going to carry this up to the ISS on the Dragon and take over the air handling system ?
Not as often as they did before 1933.
Why is it important ? Because it is built into the structure of spacetime as we understand it. It would mean that special relativity was wrong (or, rather, incomplete, as
SR is very solidly tested) and thus, that General Relativity was also incomplete (as it has SR as a limit, and is also well tested). It would also probably mean something similar for Quantum Field Theory, as that is built on top of SR. The ramifications would spread throughout most of physics.
Also, depending on how it occurs, superluminal motion could reveal a host of very strange things, not the least of which would be time travel.
Are there any experiments whose results can only be explained by light being the fastest thing in the universe?
That's probably not the best way to think about it. It's not like saying that Porsches are faster that Hondas, which could be changed if Honda builds a faster car.
This is more like saying that distance is equal to velocity multiplied by time, and then finding a make of car where that didn't seem to be true. You would naturally expect either the speedometer or the odometer to be in error, but if they are not...
Suppose, for the sake of argument (I know the chances are between slim and none, I suspect that the speed limit on the universe is to prevent game-breaking exploits of the universe itself) that FTL neutrinos are possible. How fast do they need to travel before you can send messages to the past?
One of the interesting things to come out of this is that you can have bi-metric theories with superluminal motion and not have causality violations (time travel) or tachyons. Basically, these things happen because you are Lorentz transforming something going faster than light. In a bimetric theory, there are actually two conversions between time and distance (i.e., "light" speeds, although light itself only goes at one of them). Matter is sensitive to one metric, the matter metric (thereby avoiding violations of the weak equivalence principle, or WEP, which is tightly constrained for normal matter), while gravitational waves follow the other, gravity metric. It turns out that there are no good WEP limits for neutrinos. so you could imagine neutrino's following the other, gravity, metric. Thus, they get superluminal motion, but without the complications.
So, to answer your question, it may be that no matter how fast they are, you can't send anything into the past.
This is the best description I have seen in English of the debacle (see the first comment).
Crucial bit : We do not know how crooked the plug actually was at the time of our measurements last year. Sub-sequentially we do not know the actual time delay. So, they just don't know.