Gravitational lensing produces small (arcsecond) deviations. These things are only localized to arcminute scale, so graviational lensing is negligible.
I can't agree with this enough, especially the pushups. I've been doing ~10 high-rep pushups sets as part of workout I do a few times a week. (I started with the program at hundredpushups.com, but now I roll my own based on the terminal workout.) I alternate sets of pushups with sets of (weightless) squats, roughly 30.
Especially at higher reps, these are *plenty* to get your heart rate up, so you get in your cardio. High-rep pushups sets are essentially a plank position held fr >30s, so you get some core. Pushups & squats hit nearly as many muscles as possible for the time spent, so this will also give you a good base if you decide to hit up a gym at some point.
Thanks for your armchair dismissal of many life works.
Fwiw, I'm an astrophysicist and I'll take the time to correct one point. Supermassive black holes weigh in around 10^9 solar masses. Galaxy masses are >10^12 solar masses. Furthermore, when accreting at the maximum (Eddington) rate, black holes grow exponentially. It's not difficult to grow a 10^9 solar mass black hole in 10^9 years. Really the only hard part is getting the gas close enough to the BH to accrete; this problem and the hierarchical merger of black holes are active research areas.
I have CAPSLOCK mapped to ESC, which makes switching modes in vim/pentadactyl much easier on the fingers. (Especially on my ThinkPad, where the ESC is up in Ultima Thule.)
Well, there is clearly enough circumstellar (circumpulsar?) material to generate the pulsar wind nebula detected in X-rays, but as with all collisionless shocks, this material must be tenuous.
As I noted below, there is surely radio emission -- we are just not positioned to see it. The implication is that gamma ray emission originates higher in the pulsar magnetosphere than radio emission, so it paints a broader "beam" on the sky as the pulsar rotates.
The neutron star surface does emit thermal X-rays because it is hot. However, the electromagnetic radiation originates outside the neutron star, in its magnetosphere.
Pulsars have extremely strong magnetic fields and rotate anywhere from 1-to-1000 times a second. Just like an electric generator, this produces huge electromagnetic fields, and these accelerate electrons to very high energies indeed. These electrons than bang into photons and give them a large chunk of energy in a process called inverse Compton scattering, and we get gamma rays.
(This is the so-called leptonic channel; it is also possible some gamma rays are produced via pions, but the origin of the energy is the same: the huge electromagnetic fields generated by this spinning magnetic dipole.)
No, gamma rays from pulsars are much higher energy than those associated with nuclear transitions (typical scale: 1 MeV; pulsar emission spectra peak at 1 GeV, 1000 times greater).
Pulsars have extremely strong electromagnetic fields and are hence able to accelerate electrons up to very high energies. These electrons then scatter low energy photons upwards in energy to the gamma ray regime.
To answer GP's question, observing radio-quiet pulsars like this on in CTA1 tells us more about the gamma ray emission mechanism. Several different models exist, and the primary difference is where in the pulsar's magnetosphere gamma rays are created. In the polar cap model, gamma rays originate in a small patch near the magnetic pole, the same place as the radio emission. So, if gamma rays predominantly come from the polar cap, we shouldn't see radio-quiet pulsars. Hence, this pulsar favors an emission model with gamma rays from higher altitude, in the so-called outer gaps and slot gaps.
The radio emission is believed to come from a small patch near the surface of the neutron star called the polar cap. Hence, to see it, the pulsar has to be aligned just so with the line of sight to the Earth. Gamma ray emission appears to originate at a higher altitute, so there are more orientations of the pulsar with respect to the line of sight where we can see gamma rays. It's a geometrical effect.
The above is a fantastic text describing all necessary physics (from radiation transport to nuclear reaction rates) to understand stellar processes and, by extension, other astrophysical systems. The author has a consummate understanding of the field and his prose is insightful. In my opinion, the dated nuclear physics are actually a boon, as this book has one of the few comprehensible treatments of "nuclear physics you can use" I have found.
Additionally, I like Schutz and D'Inverno's texts for introductions to General Relativity, and Stuart and Shapiro is dense but delicious for learning about compact objects.
These are suggested from a physicist's perspective; I'm sure there are plenty of good classics known to an astronomy grad student that are outside of my purview.
Anyway, best of luck!
It seems like the thrust of the article is not so much about the technology but about the sudden decision by so many institutions to make 9-figure investments in accelerators. Good news for applied physicists, though!
Except this is a clear violation of the GZK cut-off, as demonstrated by other cosmic ray experiments. This is one of the prime phenomenological manifestations of many theories of quantum gravity, since they all involved some modification of the photon dispersion relation at high energies, neatly circumventing the GZK cut-off, which is based on the good old E^2=(pc)^2+m^2c^4 relation. (This, in the rest frame, is Einstein's famous E=mc^2.) Anyway, the fact that these particles exist, and are clearly extra-galactic, is of prime value in itself.
The best way to determine the matter content of the universe is through observations of the cosmic microwave background (CMB). The properties of the plasma that emitted the CMB are well known and be used to predict temperature anisotropies (variations) in it. These show up as peaks and troughs at different angular scales. We know approximately how far away the CMB is in terms of redshift (z ~ 1100... really far!), so these angular measurements give us a distance scale. In a curved universe, the peaks and troughs appear at different angles, whereas those observed are consistent with a flat universe. A flat universe MUST have a certain energy density, but the observed baryon density only accounts for about 4% of that.
This could all be accounted for by dark matter save for the observations of Type 1A supernovae which indicate accelerating expansion, and this requires domination by a state of matter with negative pressure, and this is what's been coined dark energy.
Because they're not really wild hypotheses at all. You can OBSERVE the rotation curves of galaxies and see they don't match up with the estimates of the matter content. SOMETHING is there, so your only real quibble might be with the cryptic name 'dark matter'. Likewise, SOMETHING is causing the universe to expand, as shown by observations of standard candles such as Type 1A supernovae.
These are things that can be and are published in scientific journals. Whereas the only real observable evidence for the phenomena you mentioned are documentaries:/
Dark matter is not chemically interesting since, by definition, it doesn't interact with normal matter. Hence, it's unlikely to be 'useful' in any current fashion!
As far as whether it's dangerous -- if dark energy is a cosmological constant, it's a property of spacetime, and you are in a sense exposed to it right now. As for dark matter, again, it's something that would pass right through you, much like neutrinos.
There is NO question that that expansion of the universe is accelerating. According to General Relativity, the ONLY way this can be happening is if the universe is dominated by a species with a negative pressure. If you're not happy with the name dark energy, call it 'quintessence', although this term has come to be applied to non-cosmological-constant dark energy, i.e., that provided by scalar fields in false vacuums, etc.
On the contrary, very large bodies are extremely well-approximated by Newton, as it is the slow-velocity, weak field limit of General Relativity. There is already good photographic evidence for dark matter in the form of colliding galaxies (do your Google work), and current observational evidence points pretty strongly towards dark energy in the form of a cosmological constant. While it's true we don't know what that means, it's not just a fudge factor.
Large black holes are located at the center of galaxies, and their mass can be determined by examining rotation curves, etc. They are not dark matter candidates. Primordial black holes are not massive enough. There is some possibility that dark matter could be non-luminous dust, but there are some limits placed on observations of the comsic microwave background, which would have had to travel over 13 billion light years through such dust without being significantly attenuated.
The 'size' of the universe is an ill-defined question. We can only observe what's in our past light cone, and it is *that* universe which suffers from a budget shortfall of matter/energy.
Slashdot Mirror
Gravitational lensing produces small (arcsecond) deviations. These things are only localized to arcminute scale, so graviational lensing is negligible.
Especially at higher reps, these are *plenty* to get your heart rate up, so you get in your cardio. High-rep pushups sets are essentially a plank position held fr >30s, so you get some core. Pushups & squats hit nearly as many muscles as possible for the time spent, so this will also give you a good base if you decide to hit up a gym at some point.
Thanks for your armchair dismissal of many life works. Fwiw, I'm an astrophysicist and I'll take the time to correct one point. Supermassive black holes weigh in around 10^9 solar masses. Galaxy masses are >10^12 solar masses. Furthermore, when accreting at the maximum (Eddington) rate, black holes grow exponentially. It's not difficult to grow a 10^9 solar mass black hole in 10^9 years. Really the only hard part is getting the gas close enough to the BH to accrete; this problem and the hierarchical merger of black holes are active research areas.
I have CAPSLOCK mapped to ESC, which makes switching modes in vim/pentadactyl much easier on the fingers. (Especially on my ThinkPad, where the ESC is up in Ultima Thule.)
lim x--> inf of x/x is "infinity/infinity" and evaluates to 1.
lim x--> inf of e^x/x is also "infinity/infinity" but evaluates to infinity.
They lost commercial power due the big storm system that went through the DC area.
Well, there is clearly enough circumstellar (circumpulsar?) material to generate the pulsar wind nebula detected in X-rays, but as with all collisionless shocks, this material must be tenuous.
Sorry, its high luminosity is more than offset by its distance of kiloparsecs. The relevant quantity is flux, which goes like luminosity/distance^2.
As I noted below, there is surely radio emission -- we are just not positioned to see it. The implication is that gamma ray emission originates higher in the pulsar magnetosphere than radio emission, so it paints a broader "beam" on the sky as the pulsar rotates.
Pulsars have extremely strong magnetic fields and rotate anywhere from 1-to-1000 times a second. Just like an electric generator, this produces huge electromagnetic fields, and these accelerate electrons to very high energies indeed. These electrons than bang into photons and give them a large chunk of energy in a process called inverse Compton scattering, and we get gamma rays.
(This is the so-called leptonic channel; it is also possible some gamma rays are produced via pions, but the origin of the energy is the same: the huge electromagnetic fields generated by this spinning magnetic dipole.)
Pulsars have extremely strong electromagnetic fields and are hence able to accelerate electrons up to very high energies. These electrons then scatter low energy photons upwards in energy to the gamma ray regime.
To answer GP's question, observing radio-quiet pulsars like this on in CTA1 tells us more about the gamma ray emission mechanism. Several different models exist, and the primary difference is where in the pulsar's magnetosphere gamma rays are created. In the polar cap model, gamma rays originate in a small patch near the magnetic pole, the same place as the radio emission. So, if gamma rays predominantly come from the polar cap, we shouldn't see radio-quiet pulsars. Hence, this pulsar favors an emission model with gamma rays from higher altitude, in the so-called outer gaps and slot gaps.
The radio emission is believed to come from a small patch near the surface of the neutron star called the polar cap. Hence, to see it, the pulsar has to be aligned just so with the line of sight to the Earth. Gamma ray emission appears to originate at a higher altitute, so there are more orientations of the pulsar with respect to the line of sight where we can see gamma rays. It's a geometrical effect.
Additionally, I like Schutz and D'Inverno's texts for introductions to General Relativity, and Stuart and Shapiro is dense but delicious for learning about compact objects.
These are suggested from a physicist's perspective; I'm sure there are plenty of good classics known to an astronomy grad student that are outside of my purview. Anyway, best of luck!
Actually, I do like the equations quite a bit, and the whole building is attractive and pleasant. But I'd prefer a gallery to our nutty art nod!
And all we have at the physics building as that damned huge outdoor peanut.
It seems like the thrust of the article is not so much about the technology but about the sudden decision by so many institutions to make 9-figure investments in accelerators. Good news for applied physicists, though!
Hmm, on reading further, I see that the spectrum of the high energy particles is actually consistent with GZK. So, no evidence for QG after all.
Except this is a clear violation of the GZK cut-off, as demonstrated by other cosmic ray experiments. This is one of the prime phenomenological manifestations of many theories of quantum gravity, since they all involved some modification of the photon dispersion relation at high energies, neatly circumventing the GZK cut-off, which is based on the good old E^2=(pc)^2+m^2c^4 relation. (This, in the rest frame, is Einstein's famous E=mc^2.) Anyway, the fact that these particles exist, and are clearly extra-galactic, is of prime value in itself.
Posting on slashdot is more fun than studying for exams, if only marginally!
This could all be accounted for by dark matter save for the observations of Type 1A supernovae which indicate accelerating expansion, and this requires domination by a state of matter with negative pressure, and this is what's been coined dark energy.
These are things that can be and are published in scientific journals. Whereas the only real observable evidence for the phenomena you mentioned are documentaries :/
As far as whether it's dangerous -- if dark energy is a cosmological constant, it's a property of spacetime, and you are in a sense exposed to it right now. As for dark matter, again, it's something that would pass right through you, much like neutrinos.
There is NO question that that expansion of the universe is accelerating. According to General Relativity, the ONLY way this can be happening is if the universe is dominated by a species with a negative pressure. If you're not happy with the name dark energy, call it 'quintessence', although this term has come to be applied to non-cosmological-constant dark energy, i.e., that provided by scalar fields in false vacuums, etc.
On the contrary, very large bodies are extremely well-approximated by Newton, as it is the slow-velocity, weak field limit of General Relativity. There is already good photographic evidence for dark matter in the form of colliding galaxies (do your Google work), and current observational evidence points pretty strongly towards dark energy in the form of a cosmological constant. While it's true we don't know what that means, it's not just a fudge factor.
The 'size' of the universe is an ill-defined question. We can only observe what's in our past light cone, and it is *that* universe which suffers from a budget shortfall of matter/energy.