If you could create a vacuum in the tunnel in theory you would have access to zero-cost travel - just drop goods down the hole and they would appear at the other end an hour or so later (assuming they didn't hit the sides of the tunnel, creating friction). However, unless you wanted to travel between the 2 poles, you would be better off creating curved tunnels as any object that is dropped would not naturally travel in a straight line due to conservation of angular momentum which would make falling objects veer to the east. This means that it would not be efficient to return to your initial start point through the same tunnel. For this reason, the pendulum effect you describe would be slightly different - it would not be as pleasant on the way back as you can't help but hit the walls.
With such a transport system you would reach speeds in excess of 10,000 km/hr, and with a network of tunnels could travel free to just about any line of longitude, (with multiple legs). Travelling between latitudes would be more of a problem though as you can only fall to the equivalent latitude in the opposite hemisphere.
If the tunnels were bored sufficiently accurately it should be possible for humans to travel using such a method, with little risk of hitting the walls (which would no doubt be a bit painful at 10,000km/hr). It's probably advisable to wear a spacesuit though.
I haven't studied physics for a number of years now so please bear with me if I'm a bit rusty. However, I'm interested to learn.
i.e. we have the inescapable thermodynamic effects where the solar sails will heat up rather than convert energy with 100% efficiency. This energy that is converted into heat doesn't get us electricity, and doesn't help the solar sail, either.
I would have thought that any heat would be eventually re-emitted as infra-red, otherwise the sail would just get hotter and hotter. My suggestion is that the IR emitted from the PV cell would be reflected by the base sail, pass through the PV cell (perhaps after multiple absorption- emission-reflection stages) and thereby contribute to the acceleration.
My figure of 2 in the momentum calculation assumes that the photons hit at a 90 degree angle, which is of course not always going to be true. There will be inefficiencies associated with that, of course. But assuming 90 degrees, then the figure of 2 is in fact *due* to conservation of energy. This assumes the photon is reflected elastically, and does not change frequency. AFAIK, photons do not change frequency (red-shift) during reflections, except to a very small extent (refer to Raman scattering).
I was also assuming 90 degrees. I suggest that the very small redshift can be the *only* cause (if you can call it that) of the increased kinetic energy of the spacecraft i.e. the collisions are not elastic - if they were, the spacecraft would not gain energy nor momentum. If the figure 2 is exact, then the photon has the same momentum p = hv/c, and thus the same energy after the reflection as before so the spacecraft cannot have gained energy.
Your scenario of two ships using a single photon to accelerate indefinitely ignores things like the blackbody spectrum of the universe, quantum mechanics, Casimir forces, etc. If we ignore those for a moment, then yes the two ships would accelerate for forever, but as they approach the speed of light (which would take a *very* long time), the apparent wavelength of the photon in question would appear to be longer and longer (due to relativistic distortions), and thus the momentum boost they would get on each reflection would get smaller and smaller
If we ignore the quantum mechanical effects and deal with the classical situation, I still can't see how the spacecraft can gain energy without a corresponding decrease in the energy of the photon, even taking relativity into account. Start with a scenario with an observer between 2 stationary spacecraft. The observer fires a photon at one craft - the photon will hit craft 1, impart momentum upon reflection, then pass past the observer to hit craft 2 and return to the observer. Surely any red-shift observed at this point is not a relativistic effect as the observer has not moved? Alternatively, if there is no red-shift, where has the increased energy of the 2 spacecraft come from?
Thanks for your patience - time for me to look up Casimir forces etc;-)
But, as we all know, solar sails work both by exploiting photon pressure, and solar wind (particles emitted by the sun), so the situation is maybe not that bad.
AFAIK, solar wind effects are negligible compared to photon pressure but the situation may not be as bad as you suggest but for a different reason - the relatively poor efficiency of solar power generation.
Much light entering a photoelectric cell is wasted, through reflection, insufficient energy of some photons, and other reasons. As long as this wasted light is reflected back by the base layer, there would be an additional push to the sail as you describe, though presumably a lot less than with a reflective sail only.
Also, a minor point about photon pressure - you mention that that the momentum change of a photon by a solar sail is twice the original the original momentum. This is not strictly true as it would contradict the conservation of energy principle - imagine 2 spacecraft with sails facing one another - they could accelerate to arbitrarily high speeds by bouncing a photon from one sail to the other. In reality the reflected photon loses a little bit of energy (and momentum) when reflected and this is the energy imparted to the sail. Thus the reflected photon is slightly red-shifted (E=hv where v = frequency).
I'd recommend that/.ers read the report before being overly critical as it appears to be reasonably well written, and includes a description of the methodology they used. If there is any bias in the report it should be straightforward to try to reproduce the findings and see where there are errors.
My main difficulty with the report (assuming the findings are valid) is that it's selective in it's scope - it looks at days of risk between when vulnerabilities are found, and when these vulnerabilities are fixed - but doesn't look at what exploits actually exist in the wild, so it shouldn't be used to conclude that running a Windows server is less risky than a Linux server. In fact, the authors of the report acknowledge that there are other factors that should be considered. From the report:
To get a full view of Security Risk, one has to get a view of two factors:
Vulnerability of software, systems or networks (whichever is appropriate), and
Threats against those vulnerabilities
Of the two factors, our own experience leads us to believe that the latter is more difficult to quantify and predict in an objective manner. This is an exciting and open field and we strongly encourage others to consider this as an area for thoughtful research. However, given that there are research opportunities in both areas, we have chosen to try and make progress in studying and measuring the vulnerability factors first; this is a critical precursor to other threat-based metrics.
Unfortunately, many commentators will ignore this part of the report and will simply conclude that Windows is more secure than Linux.
However if the report's analysis does hold water, albeit for the limited scope of the report, surely this is something that we should be concerned about. It should be possible for an independent analysis based on the methodology in the report, and I would like to see criticisms of the methodology/analysis rather than the fact the report was funded by Microsoft.
Perhaps we need somethings like an "Internet Security Awareness Week" in the UK.
As part of their public broadcasting responsibility the BBC could broadcast daily programs that highlight the problems that exist and explain in laymans terms what can be done about them. This could make pretty good TV if there were interviews with people/companies who had been adversely affected by hackers/viruses/worms etc - definitely a lot more interesting than the reality TV rubbish that we get at the moment. And perhaps people could be encouraged to get help from a "knowledgable friend " to give their computer a once-over.
I took a bit of time to read the paper and there's some interesting stuff there.
The clock skew for a particular device seemed to be reasonably constant over time and location (+/- 0.5 microsecond/sec) and nearly all devices had skews within the range -100 microseconds/sec to +100 microseconds/sec. This suggests the technique would only be useful for identification purposes when there are less than 100 or so candidate devices. Of course, this figure would go up substantially if the technique can be combined with other measurements (e.g. absolute clock time).
When considering applications of the technique, the author states "For forensics, we anticipate
that our techniques will be most useful when arguing that a given device was not involved in a recorded event."
A number of posters have mentioned that the technique can be fooled by adding a random number to each timestamp. This won't work due to the way the author estimates clock skews (the slope of actual time plotted against reported system time) - what is needed is an adjustment to each timestamp that is proportional to the system uptime.
And OS did make a difference - RH9 and Win XP on a particular laptop led to clock skews of -58 and -85 respectively.
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With such a transport system you would reach speeds in excess of 10,000 km/hr, and with a network of tunnels could travel free to just about any line of longitude, (with multiple legs). Travelling between latitudes would be more of a problem though as you can only fall to the equivalent latitude in the opposite hemisphere.
If the tunnels were bored sufficiently accurately it should be possible for humans to travel using such a method, with little risk of hitting the walls (which would no doubt be a bit painful at 10,000km/hr). It's probably advisable to wear a spacesuit though.
They don't appear to be joking.
i.e. we have the inescapable thermodynamic effects where the solar sails will heat up rather than convert energy with 100% efficiency. This energy that is converted into heat doesn't get us electricity, and doesn't help the solar sail, either.
I would have thought that any heat would be eventually re-emitted as infra-red, otherwise the sail would just get hotter and hotter. My suggestion is that the IR emitted from the PV cell would be reflected by the base sail, pass through the PV cell (perhaps after multiple absorption- emission-reflection stages) and thereby contribute to the acceleration.
My figure of 2 in the momentum calculation assumes that the photons hit at a 90 degree angle, which is of course not always going to be true. There will be inefficiencies associated with that, of course. But assuming 90 degrees, then the figure of 2 is in fact *due* to conservation of energy. This assumes the photon is reflected elastically, and does not change frequency. AFAIK, photons do not change frequency (red-shift) during reflections, except to a very small extent (refer to Raman scattering).
I was also assuming 90 degrees. I suggest that the very small redshift can be the *only* cause (if you can call it that) of the increased kinetic energy of the spacecraft i.e. the collisions are not elastic - if they were, the spacecraft would not gain energy nor momentum. If the figure 2 is exact, then the photon has the same momentum p = hv/c, and thus the same energy after the reflection as before so the spacecraft cannot have gained energy.
Your scenario of two ships using a single photon to accelerate indefinitely ignores things like the blackbody spectrum of the universe, quantum mechanics, Casimir forces, etc. If we ignore those for a moment, then yes the two ships would accelerate for forever, but as they approach the speed of light (which would take a *very* long time), the apparent wavelength of the photon in question would appear to be longer and longer (due to relativistic distortions), and thus the momentum boost they would get on each reflection would get smaller and smaller
If we ignore the quantum mechanical effects and deal with the classical situation, I still can't see how the spacecraft can gain energy without a corresponding decrease in the energy of the photon, even taking relativity into account. Start with a scenario with an observer between 2 stationary spacecraft. The observer fires a photon at one craft - the photon will hit craft 1, impart momentum upon reflection, then pass past the observer to hit craft 2 and return to the observer. Surely any red-shift observed at this point is not a relativistic effect as the observer has not moved? Alternatively, if there is no red-shift, where has the increased energy of the 2 spacecraft come from?
Thanks for your patience - time for me to look up Casimir forces etc ;-)
AFAIK, solar wind effects are negligible compared to photon pressure but the situation may not be as bad as you suggest but for a different reason - the relatively poor efficiency of solar power generation.
Much light entering a photoelectric cell is wasted, through reflection, insufficient energy of some photons, and other reasons. As long as this wasted light is reflected back by the base layer, there would be an additional push to the sail as you describe, though presumably a lot less than with a reflective sail only.
Also, a minor point about photon pressure - you mention that that the momentum change of a photon by a solar sail is twice the original the original momentum. This is not strictly true as it would contradict the conservation of energy principle - imagine 2 spacecraft with sails facing one another - they could accelerate to arbitrarily high speeds by bouncing a photon from one sail to the other. In reality the reflected photon loses a little bit of energy (and momentum) when reflected and this is the energy imparted to the sail. Thus the reflected photon is slightly red-shifted (E=hv where v = frequency).
My main difficulty with the report (assuming the findings are valid) is that it's selective in it's scope - it looks at days of risk between when vulnerabilities are found, and when these vulnerabilities are fixed - but doesn't look at what exploits actually exist in the wild, so it shouldn't be used to conclude that running a Windows server is less risky than a Linux server. In fact, the authors of the report acknowledge that there are other factors that should be considered. From the report:
To get a full view of Security Risk, one has to get a view of two factors:
Of the two factors, our own experience leads us to believe that the latter is more difficult to quantify and predict in an objective manner. This is an exciting and open field and we strongly encourage others to consider this as an area for thoughtful research. However, given that there are research opportunities in both areas, we have chosen to try and make progress in studying and measuring the vulnerability factors first; this is a critical precursor to other threat-based metrics.
Unfortunately, many commentators will ignore this part of the report and will simply conclude that Windows is more secure than Linux.
However if the report's analysis does hold water, albeit for the limited scope of the report, surely this is something that we should be concerned about. It should be possible for an independent analysis based on the methodology in the report, and I would like to see criticisms of the methodology/analysis rather than the fact the report was funded by Microsoft.
The story I think you're referring to was posted a few days ago - Imax Theaters Demur On Controversial Science Films
... that the poster feels it's ok to repost someone elses story that argues against internet piracy.
As part of their public broadcasting responsibility the BBC could broadcast daily programs that highlight the problems that exist and explain in laymans terms what can be done about them. This could make pretty good TV if there were interviews with people/companies who had been adversely affected by hackers/viruses/worms etc - definitely a lot more interesting than the reality TV rubbish that we get at the moment. And perhaps people could be encouraged to get help from a "knowledgable friend " to give their computer a once-over.
We broadcast spam to billions of nodes and we don't even have a firewall to protect us from a DOS attack on service Earth.
There's a good description of the unique chemistry of carbon, and the reasons why silicon-based life is unlikely here
The clock skew for a particular device seemed to be reasonably constant over time and location (+/- 0.5 microsecond/sec) and nearly all devices had skews within the range -100 microseconds/sec to +100 microseconds/sec. This suggests the technique would only be useful for identification purposes when there are less than 100 or so candidate devices. Of course, this figure would go up substantially if the technique can be combined with other measurements (e.g. absolute clock time).
When considering applications of the technique, the author states "For forensics, we anticipate that our techniques will be most useful when arguing that a given device was not involved in a recorded event."
A number of posters have mentioned that the technique can be fooled by adding a random number to each timestamp. This won't work due to the way the author estimates clock skews (the slope of actual time plotted against reported system time) - what is needed is an adjustment to each timestamp that is proportional to the system uptime.
And OS did make a difference - RH9 and Win XP on a particular laptop led to clock skews of -58 and -85 respectively.