That way you would saturate the pixels in the bright portions of your image and lose detail there (big white spot). The point of log-response is exactly to avoid that: hight S/N ratio for dark regions and no saturation in bright regions of the picture.
what do you mean by "normal distances" ? One nanosecond corresponds to 30 cm (in vacuum), i.e. 300 m longer route corresponds to 1 ms additional lag (even without velocity factor). If you take into account that network topology near leaf nodes is usually star-like, you can get easily several hundred meters longer routes than line-of-sight.
Yes. At the University of Heidelberg, Germany, Physicists have developed a log-response CCD chip (covering 6 decades of intensity). They want to use it as the frontend sensor for their "tactile vision substitution system" (a machine enabling blind people to "see" with their fingers). The Log response is achieved by operating the sensor transistors in their non-linear range (very crude description, it's been a long time ago since I attended a talk about that project). Links to publications can be found following the above link.
The idea is not to resolve sub-atomic features with visible light in way a classical optical microscopy works. The short wavelength light is needed to trigger the processes in nuclei which require high energy photons (MeV scale) and the short pulse duration is needed have the required time resolution, otherwise you would only see the final result without knowing how the system got there. Experiments with ultrashort light pulses are usually done in the so called Pump-Probe technique: With a first short light pulse (Pump) you trigger some process that you want to investigate in your sample, then after a certain time delay you shine a second light pulse (Probe) on your sample to "look" at state of the process. Doing many such Pump-Probe cycles with varying time delay between the pulses, you can "see" how the triggered process evolves (whereby "look" and "see" is not to be taken literally). In order to be able to do this your light pulses have to be shorter than the timescale of the process that you want to investigate. Chemical reactions and motion of electrons take place in the femtosecond regime, requiring fs-Lasers; nuclear reactions take place on the zeptosecond regime, requiring zs-Lasers.
In the PRL they give explicit examples of experiments to do with zs pulses:
photonuclear reactions (i.e. nuclear reactions triggered by photons): neutron photoproduction on Be (requires 1.7 MeV gamma photons), photofission of uranium nuclei.
The proposed Lasetron would produce very short bursts of very high energy photons (gamma rays), and thus make possible the study of time-resolved nuclear reactions, like those mentioned above (however, for the uranium experiment you need urianium ion with gamma factor approx 100, i.e. an accelerator like RHIC).
A different solution, which is very common for long running processes, is to use savepoints, i.e. save the state of the process regularly to a file at suitable points of the algorithm. Once your process dies or you killed it, you can restart from that savepoint. If your state information is very large, you can stretch the save interval to reasonable long times, e.g. several hours. Typically you don't mind to lose some hours of calculations due to an occasional power outage.
Of course this solution is not as general as the "process cryogenics" you describe, but it's also easier to implement because you have more information about the problem.
Which Max Planck Institute in München ? There are 11 MPIs in München! Anyway, the decision whether to use Linux or not is usually made on the workgroup level (at least at the MPI where I was).
Re:Need to shine a little experimental light.
on
Black Holes Disputed
·
· Score: 1
I wouldn't agree to that. The article states:
In the meantime, they are trying to figure out how they
could tell ordinary-sized black holes and gravastars apart. The differences might be subtle -- after all, in isolation, they're both dark and the gravitational fields outside a black hole event horizon and the gravastar shell would be the same. But a good guess would be that gravastars would shine more brightly, since matter falling onto one would be turned into radiation. Black holes would gobble all the matter, but a gravastar would let its energy escape.
The next step is to identify the telltale signs of a gravastar, Mottola said. "It is the only way to convince the skeptical-including ourselves-that nature really behaves this way." Yet physicists aren't even sure what black holes look like.
In the preprint they state as "Observational Consequences":
...Explosive bursts in which a finite fraction of atoms are ejected have been observed in attractive BEC's in the laboratory [11]. The remnants are left in an excited oscillatory state afterwards. In the present case the shell with the maximally stiff eq. of state p = \rho would be expected to produce an outgoing shock front which would accelerate everything in its vincinty to ultra-relativistic energies, producing ultrahigh energy particles, gamma rays and gravitational radiation. Such stellar hypernovae may be the engines of gamma ray bursters and active galactic nuclei, as well as efficient cosmic ray accelerators. The spectra of gravitational radiation should bear the imprint of the fundamental frequencies of vibration of the gravstar.
[11] E.A.Donley et.al., Nature, 412 295 (2001)
I'd say, they have some ideas how to proceed, but to really nail them down experimentally we need numbers or qualitative differences.
As things are, these gadgets seem to survive in the open atmosphere just fine.
My question is: do they also work in vacuum ? I ask because mechanical parts in vacuum have always the problem that they stick to each other (esp. metals) because there is no lubricating water film on their surfaces.
but if antimatter does tend to fall into the hole and the real matter tends to escape, then the antimatter particle would vaporize some matter in the black hole and the tendency for the Hawking radiation would be to evaporate the black hole, no?
No, once matter or anti-matter has passed the event-horizon, nothing can come back - not even the pair annihilation radiation. Hawking radiation occurs at the border of the event horizon, where it is just possible for the radiation/particle to escape. Pair annihilation inside the black hole also doesn't affect it's apparent mass to the outside observer, because the annihilation radiation itself also "generates" gravity (as does every from of energy).
The reason why Hawking radiation reduces the mass of a black hole is that the energy for the pair production at the event horizon is taken from the black hole, but one partner of the pair manages to escape (otherwise it's not called Hawking radiation) thereby reducing the mass/energy of the black hole.
One option would e.g. be to shoot a mass from moon at the vehicle (or a Laser beam, but let's stick with the mass as it has another advantage).
When the vehicle catches the mass, it is accelerated in to direction that the mass was flying. Of course you need to aim very accurately and you can only accelerate in the direction moon-vehicle. However, once the vehicle has capured the mass, it can eject it in a arbitrary direction, i.e. accelerate in a arbitrary direction.
That way you can circumvent the rocket equation, i.e. you accelerate a constant mass mainly consisting of payload and catcher (no fuel). If you want to re-eject the captured mass you need an adequate energy source, of course.
For the decelerating part of the mission you need either a second mass driver at the destination or conventional means to brake.
advantages of this scheme:
circumvents rocket equation
you can convert the kinetic energy of the incoming masses into electrical energy onboard the vehicle
disadvantages:
need a huge gun that can be aimed very well in the desired directions (huge because you need very high mass velocities: v_mass >> v_vehicle ~ several km/s)
need a decelrating "gun" that can catch the masses reliably
acceleration only in direction massdriver->vehicle, unless you have a second gun at vehicle
alters moon orbit (no problem if vehicle small compared to moon or missions average out in all directions)
Of course you can also shoot particles at the vehicle (think ion drive exhaust aimed at the vehicle rather than machine gun).
The supermassive black hole at the center of the Galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high).
Also very interesting is the part about chandra's hardware. It's not at all easy to make optics for x-rays.
but with spin, you have to destroy its state to read it. you cannot just look at an electron and know it's spin, once you look and know what its spin was, it then changes to a random state. kinda wierd, but that's what nature
You are thinking about a single-electron-spin transistor, however the article is not about single-eletron effects. Second, spin doesn't necessarily change randomly: say you want to distinguish up/down (z) polarization (as opposed to -x/+x or -y/+y polarization). If your electron is in either up or down and you read its z-component (up/down) it get's projected on up/down, i.e. it is not changed. Of course you don't know its x and y components but you're not interested in those anyway. If you read x or y components (i.e. project to x or y eigenstates) then of course you lose the z-polarization, but you don't have/want to do that.
check out this list. these are not games, but very interesting nevertheless.
What I would really like to see, is relativistic correct space simulations (think of Elite or Final Frontier) esp. including the effects of local nature of time. I wonder, nobody mentioned this apparent application/problem of physics in games. Playing such a game would really improve the grasp of special relativity for us.
Xenon is more than likely going to impinge on many of the ISS surfaces and experiments, simply because the plume goes everywhere.
I thought the Xe is expelled at 100000 km/s from the drive, how can it form a plume around the station then? To make a focused plume just collimate it.
As Zaak suggested the continuous operation of a ion engine would not only remove (or reduce) the tiny acceleration caused by atmospheric drag but also the interruption of microgravity experiments due to periodic reboosts.
You are exactly right about the importance of energy density. However, one has to remember that high energy density also means high danger (that's exactly the reason why nuclear power is considered dangerous: because tiny amounts of radioactives carry high amounts of energy). So the current favoured direction in energy generation/conversion is towards low energy density technologies: solar power, wind power, etc. and extracting high amounts of energy by using many devices.
The current combustion engine design is somewhat of a compromise: it has relatively high energy density but it's dangers (gas explosions) are considered bearable.
Actually the important parameter is not bare power (Watts) but fluence=power/area (Watts/cm^2).This is important in this case because over the large distances you have enormous widening of the beam.
Second thing important is the temporal structure of your beam. It's a large difference whether you deposit 10 W continous wave or whether you have 10 W average power but concentrate that in very short pulses.
Taking both points into account you can reach with todays femtosecond pulse Lasers a peak fluence of many Terawatts/cm^2 (of course only for short times on very small spots). Btw. you can cram such a laser setup in 1 or 2 m^3 if you really want. That doesn't mean you can use it for SDI, but you can definitely burn holes with it (in the lab).
The billing mechanism should track for and eliminate charges...
that's a nice idea on the first look, but the question is:
would it work reliably? could you check that it works reliably? (question: do you check every call on your phone bill? good luck checking the 100k pageviews on your monthly ISP bill)
would it be worth to complain if it doesn't? (hey, I lost 10 pennys being charged non-authorized pages...is it worth a phone-call? a lawyer?)
If it's not worth complaining about some lost pennys, wouldn't you think that some people would try to steal just a few pennies from a large number of people?
just one problem with your "polarized light" and "modulated HF signals": their wavelength exceeds even todays structure sizes. Guess why people are working on UV lithography: visible light has too high wavelength (hundreds of nanometers). "normal" HF has even higher wavelength (e.g. 3 GHz = 10 cm, remember: visible light is several hundred THz). If you want to use EM radiation for small (1 nm) structures you have to options: use near field optics (keyword: SNOM, Scanning Near field Optical Microscope) or use X-rays.
While the "Basics of Spaceflight" page is really nice, the "porkchop" page is quite lame - it wastes 5 pages to convince people that porkchop plots are nice and important tools, but when they show one of the plots on page 2 (!) they don't even explain what one can see there. So I'll try to do it myself:
The plot is for the two 2005 Earth-to-Mars transfer opportunities, using ballistic transfer (i.e. no propulsion during the flight except for orbital departure/insertion)
The horizontal axis is obviously the launch time from Earth, while the vertical axis is arrival time on Mars.
The two possible "windows" to Mars for 2005 seem to be the centers of the two blue coutour plots (if we only knew what the blue "C3L" lines mean!).
The red lines seem to be lines of constant travel time (labelled "TTIME"), so the lower window is for the shorter route (200 days TTIME as opposed to 400 days TTIME).
The remaining questions are: What are the blue lines ? (very essential as they define the "windows". Maybe arrival velocity?) What are the magenta and green lines ?
Slashdot Mirror
That way you would saturate the pixels in the bright portions of your image and lose detail there (big white spot). The point of log-response is exactly to avoid that: hight S/N ratio for dark regions and no saturation in bright regions of the picture.
what do you mean by "normal distances" ? One nanosecond corresponds to 30 cm (in vacuum), i.e. 300 m longer route corresponds to 1 ms additional lag (even without velocity factor). If you take into account that network topology near leaf nodes is usually star-like, you can get easily several hundred meters longer routes than line-of-sight.
It has been done before, maybe not for RGB sensors, but certainly for solar cells: Triple Junction Solar Cells.
Yes. At the University of Heidelberg, Germany, Physicists have developed a log-response CCD chip (covering 6 decades of intensity). They want to use it as the frontend sensor for their "tactile vision substitution system" (a machine enabling blind people to "see" with their fingers). The Log response is achieved by operating the sensor transistors in their non-linear range (very crude description, it's been a long time ago since I attended a talk about that project). Links to publications can be found following the above link.
The idea is not to resolve sub-atomic features with visible light in way a classical optical microscopy works. The short wavelength light is needed to trigger the processes in nuclei which require high energy photons (MeV scale) and the short pulse duration is needed have the required time resolution, otherwise you would only see the final result without knowing how the system got there. Experiments with ultrashort light pulses are usually done in the so called Pump-Probe technique: With a first short light pulse (Pump) you trigger some process that you want to investigate in your sample, then after a certain time delay you shine a second light pulse (Probe) on your sample to "look" at state of the process. Doing many such Pump-Probe cycles with varying time delay between the pulses, you can "see" how the triggered process evolves (whereby "look" and "see" is not to be taken literally). In order to be able to do this your light pulses have to be shorter than the timescale of the process that you want to investigate. Chemical reactions and motion of electrons take place in the femtosecond regime, requiring fs-Lasers; nuclear reactions take place on the zeptosecond regime, requiring zs-Lasers.
In the PRL they give explicit examples of experiments to do with zs pulses:
photonuclear reactions (i.e. nuclear reactions triggered by photons): neutron photoproduction on Be (requires 1.7 MeV gamma photons), photofission of uranium nuclei.
The proposed Lasetron would produce very short bursts of very high energy photons (gamma rays), and thus make possible the study of time-resolved nuclear reactions, like those mentioned above (however, for the uranium experiment you need urianium ion with gamma factor approx 100, i.e. an accelerator like RHIC).
A different solution, which is very common for long running processes, is to use savepoints, i.e. save the state of the process regularly to a file at suitable points of the algorithm. Once your process dies or you killed it, you can restart from that savepoint. If your state information is very large, you can stretch the save interval to reasonable long times, e.g. several hours. Typically you don't mind to lose some hours of calculations due to an occasional power outage.
Of course this solution is not as general as the "process cryogenics" you describe, but it's also easier to implement because you have more information about the problem.
Which Max Planck Institute in München ? There are 11 MPIs in München! Anyway, the decision whether to use Linux or not is usually made on the workgroup level (at least at the MPI where I was).
In the preprint they state as "Observational Consequences":
I'd say, they have some ideas how to proceed, but to really nail them down experimentally we need numbers or qualitative differences.
As things are, these gadgets seem to survive in the open atmosphere just fine.
My question is: do they also work in vacuum ? I ask because mechanical parts in vacuum have always the problem that they stick to each other (esp. metals) because there is no lubricating water film on their surfaces.
but if antimatter does tend to fall into the hole and the real matter tends to escape, then the antimatter particle would vaporize some matter in the black hole and the tendency for the Hawking radiation would be to evaporate the black hole, no?
No, once matter or anti-matter has passed the event-horizon, nothing can come back - not even the pair annihilation radiation. Hawking radiation occurs at the border of the event horizon, where it is just possible for the radiation/particle to escape. Pair annihilation inside the black hole also doesn't affect it's apparent mass to the outside observer, because the annihilation radiation itself also "generates" gravity (as does every from of energy).
The reason why Hawking radiation reduces the mass of a black hole is that the energy for the pair production at the event horizon is taken from the black hole, but one partner of the pair manages to escape (otherwise it's not called Hawking radiation) thereby reducing the mass/energy of the black hole.
last time they tried to use degenerative braking they didn't get back their spacecraft as promised. So they switched back to the old scheme.
When the vehicle catches the mass, it is accelerated in to direction that the mass was flying. Of course you need to aim very accurately and you can only accelerate in the direction moon-vehicle. However, once the vehicle has capured the mass, it can eject it in a arbitrary direction, i.e. accelerate in a arbitrary direction.
That way you can circumvent the rocket equation, i.e. you accelerate a constant mass mainly consisting of payload and catcher (no fuel). If you want to re-eject the captured mass you need an adequate energy source, of course.
For the decelerating part of the mission you need either a second mass driver at the destination or conventional means to brake.
advantages of this scheme:
disadvantages:
Of course you can also shoot particles at the vehicle (think ion drive exhaust aimed at the vehicle rather than machine gun).
Also very interesting is the part about chandra's hardware. It's not at all easy to make optics for x-rays.
but with spin, you have to destroy its state to read it. you cannot just look at an electron and know it's spin, once you look and know what its spin was, it then changes to a random state. kinda wierd, but that's what nature
You are thinking about a single-electron-spin transistor, however the article is not about single-eletron effects. Second, spin doesn't necessarily change randomly: say you want to distinguish up/down (z) polarization (as opposed to -x/+x or -y/+y polarization). If your electron is in either up or down and you read its z-component (up/down) it get's projected on up/down, i.e. it is not changed. Of course you don't know its x and y components but you're not interested in those anyway. If you read x or y components (i.e. project to x or y eigenstates) then of course you lose the z-polarization, but you don't have/want to do that.
check out this list. these are not games, but very interesting nevertheless.
What I would really like to see, is relativistic correct space simulations (think of Elite or Final Frontier) esp. including the effects of local nature of time. I wonder, nobody mentioned this apparent application/problem of physics in games. Playing such a game would really improve the grasp of special relativity for us.
The most stripped down version - or the most advanced - depending how you see it, is probably this one.
Xenon is more than likely going to impinge on many of the ISS surfaces and experiments, simply because the plume goes everywhere.
I thought the Xe is expelled at 100000 km/s from the drive, how can it form a plume around the station then? To make a focused plume just collimate it.
As Zaak suggested the continuous operation of a ion engine would not only remove (or reduce) the tiny acceleration caused by atmospheric drag but also the interruption of microgravity experiments due to periodic reboosts.
You are exactly right about the importance of energy density. However, one has to remember that high energy density also means high danger (that's exactly the reason why nuclear power is considered dangerous: because tiny amounts of radioactives carry high amounts of energy). So the current favoured direction in energy generation/conversion is towards low energy density technologies: solar power, wind power, etc. and extracting high amounts of energy by using many devices.
The current combustion engine design is somewhat of a compromise: it has relatively high energy density but it's dangers (gas explosions) are considered bearable.
Actually the important parameter is not bare power (Watts) but fluence=power/area (Watts/cm^2).This is important in this case because over the large distances you have enormous widening of the beam.
Second thing important is the temporal structure of your beam. It's a large difference whether you deposit 10 W continous wave or whether you have 10 W average power but concentrate that in very short pulses.
Taking both points into account you can reach with todays femtosecond pulse Lasers a peak fluence of many Terawatts/cm^2 (of course only for short times on very small spots). Btw. you can cram such a laser setup in 1 or 2 m^3 if you really want. That doesn't mean you can use it for SDI, but you can definitely burn holes with it (in the lab).
Thanks! What temperatures and what exhaust velocity does your exhaust gas reach ? Why do you not use LOX instead of NOX ?
I surfed your site for several hours now, it's great! A pity that I don't live in the UK.
What is the difference between HPR and your type of rocket?
that's a nice idea on the first look, but the question is:
just one problem with your "polarized light" and "modulated HF signals": their wavelength exceeds even todays structure sizes. Guess why people are working on UV lithography: visible light has too high wavelength (hundreds of nanometers). "normal" HF has even higher wavelength (e.g. 3 GHz = 10 cm, remember: visible light is several hundred THz). If you want to use EM radiation for small (1 nm) structures you have to options: use near field optics (keyword: SNOM, Scanning Near field Optical Microscope) or use X-rays.
The remaining questions are: What are the blue lines ? (very essential as they define the "windows". Maybe arrival velocity?) What are the magenta and green lines ?