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User: IWannaBeAnAC

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  1. Re:How much does it weigh in space? on The ISS Marks 10 Years In Space · · Score: 1

    In both situations, you have the same mass and any experiment you do to measure the strength of the local gravitational field will give the same reading.

    Oops, I meant to say, in both situations, you have the same weight. Obviously, the mass is the same too, but the weight is the important one.

  2. Re:How much does it weigh in space? on The ISS Marks 10 Years In Space · · Score: 4, Insightful

    I take it you don't count gravity as a "force" where you come from?

    Ahh, this is where relativity and the equivalence principle come in. According to the equivalence principle, there is no experiment that you can do on the ISS that can distinguish whether it is currently in orbit around the Earth, or instead in deep space (not anywhere near any significant masses), or any other variant of 'free fall'. Well, obviously you can look out the window, but that is not what I mean: you cannot determine the force on you due to gravity by doing any kind of experiment with masses etc. (This experiment is easy to do on the surface of the earth, you just need a set of scales!).

    Without gravity applying a "force" to the ISS, it would move in what is commonly known as a "straight line".

    Yeah true, that is entirely correct in the Newtownian view. But an entirely equivalent way of viewing the motion of the ISS is that it is moving 'straight', but the spacetime surrounding the Earth is curved. This view is easier to grasp if you imagine actually being on the ISS - from this point of view (which is an accelerating reference frame, in Newton's picture), there is no net force acting on it. That is, there is no experiment that you can do to measure the local strength of Earth's gravity.

    Another example of motion (acceleration) being equivalent to gravity: Suppose I applied a force to you by putting you in a car and accelerating very quickly down the freeway, you would feel the forces acting upon you. If you attempted to measure the force of gravity at the same time (for example, by using a set of scales, or an accelerometer) there is no way you can distinguish whether I am accelerating you down the freeway, or if you are actually stationary but have been suddenly transported to another planet where the gravity is stronger. This is the Equivalence Principle (I am too lazy to put in the wikipedia link, but the article is reasonably accurate and worth reading).

    In orbit, the acceleration of the motion precisely cancels out with the force due to gravity. The net force experienced by the astronauts is zero and they are weightless. The equivalence principle states that this situation is precisely equivalent to being in empty space with no gravitational masses anywhere nearby in the sense that it is not possible to distinguish these cases by measuring the local gravitational field. In both cases, it is zero. Similarly, it is impossible to distinguish, by measuring the local gravitational field, between the two cases of (1) a stationary object on the Earth, experiencing a weight of mass*9.81m/s^2, and (2) a rocket ship in free space with the engines on and accelerating at 9.81m/s^2. In both situations, you have the same mass and any experiment you do to measure the strength of the local gravitational field will give the same reading. [*]

    [*] Actually, you can tell, but it is very subtle: on the Earth, the gravitational force is towards the center of the earth, so of you move slightly to the left the direction of the gravitational force changes slightly. But in an accelerating rocket, the force is uniform. These are called 'tidal forces', and with a careful experiment you could measure it. You would see this, for example, in an elevator that is accelerating downwards at exactly the rate of the acceleration due to gravity (so that objects in the elevator were weightless and floating around), and you placed two objects some distance apart, the tidal forces would tend to push them together. This is because they are both accelerating towards the center of the earth, rather than straight down. It is easier if you imagine doing this experiment on a very small and very heavy asteroid. If you put an elevator nearby to the asteroid, and draw lines of force radiating outwards from the center of the asteroid, the lines of force that pass through the elevator are not quite parallel.

  3. Re:How much does it weigh in space? on The ISS Marks 10 Years In Space · · Score: 1

    No, you are forgetting to add together the acceleration due to gravity PLUS the acceleration of the motion. According to your definition, since you are currently standing on the surface of the Earth and are not accelerating, then your weight is currently zero. This is obviously not correct!

    For a stationary object on the Earth's surface, the force due to gravity is F = mass * 9.81m/s^2. You are not accelerating, therefore this is the net gravitational force that is acting on you.

    When you are in freefall, no matter what the origin of this is (falling from a plane, in the few seconds before air resistance is significant, or in orbit, or whatever), you are accelerating at exactly the rate to cancel out the force of gravity and the net force on you is zero. See, for example, Wikipedia article on Weightlessness:

    Weightlessness is a phenomenon experienced by people during free-fall. Although the term zero gravity is often used as a synonym, weightlessness in orbit is not the result of the force of gravity being eliminated or even significantly reduced (in fact, the force of the Earth's gravity at an altitude of 100 km is only 3% less than at the Earths surface). [...] Long periods of weightlessness occur on spacecraft outside a planet's atmosphere, provided no propulsion is applied and the vehicle is not rotating. Weightlessness does not occur when a spacecraft is firing its engines or when re-entering the atmosphere, even if the resultant acceleration is constant. The thrust provided by the engines acts at the surface of the rocket nozzle rather than acting uniformly on the spacecraft, and is transmitted through the structure of the spacecraft via compressive and tensile forces to the objects or people inside. [...] Weightlessness in an orbiting spacecraft is physically identical to free-fall [...].

  4. Re:How much does it weigh in space? on The ISS Marks 10 Years In Space · · Score: 1
    Instead of embarrasing yourself with another mis-statement, why not just read wikipedia, for example?

    If there was a net force on the ISS, then it would not remain in a steady orbit!

  5. Re:How much does it weigh in space? on The ISS Marks 10 Years In Space · · Score: 2, Informative

    No, that is wrong. It is the MASS that is the same in orbit as on the surface of the Earth. The mass of an object is a measure of the amount of matter in it, and this does not change. The weight of an object, however, measures the force due to gravity that is acting upon it. The force of gravity is trying to accelerate you downwards towards the center of the earth, but the Earth's surface is holding you in place[*]. If you step on some scales, then you notice this by your body pushing down and registering a force on the scales. In freefall however, your weight is zero. You could think of it as the set of scales is accelerating at the same rate you are, so you cannot exert a force on it.

    The second half, about falling 'around' the Earth, is basically correct. Another analogy is to think of whirling a rock on a piece of string around your head. The rock is going to tend to fly away from you, but it cannot because the tension in the string is holding it in a circle. If you replace the tension in the string with a gravitational attraction instead, then you get the same effect but without the string.

    [*] To be more precise, unless you are in freefall, then you are standing on a solid object which is stopping you from accelerating - ie. the solid ground is exerting a force on your feet that exactly cancels out the gravitational force that is acting on your entire body, so overall you do not move, BUT the forces are being applied in different places. Gravity acts upon your entire body, trying to pull every atom in your body with equal force, but the restoring force of the ground that is stopping you from falling into the center of the Earth is acting only on your feet. Hence despite the fact that your overall motion is zero, you still feel a residual gravitational force acting on the rest of your body, so if you raise your arm, gravity is acting upon it and is pushing it back down. The Equivalence Principle states that gravity and acceleration are equivalent, so that the effect of accelerating in a rocket ship in free space at 9.81m/s^2 is precisely equivalent to being on the surface of the earth, so you would weigh the same in this situation. Conversely, if you are at freefall in the vicinity of Earth (and being in orbit is one example where you are in freefall), then you are, in effect, accelerating in such a way as to exactly cancel out the gravitational force and your weight is zero.

  6. Re:How much does it weigh in space? on The ISS Marks 10 Years In Space · · Score: 1

    Huh? How did this get modded insightful? It is completely wrong. There is this thing called the Equivalence Principle, that is one of the axioms of Einsteinian Gravity. It says, among other things, that a body in freefall has no net force acting upon it, and therefore has zero weight.

    If objects at the ISS really did have a weight of 85% of what they weigh on the surface of the Earth, where do you think they got all those bits of news footage of people and things floating around in the cabin?

  7. Re:At least he's honest. on Ballmer "Interested" In Open Source Browser Engine · · Score: 4, Insightful

    Do you have any reason to believe that the Trident codebase is anything other than a steaming pile of horseshit?

    Back in the original browser wars, both the Netscape and Microsoft browsers were evolving very rapidly, with lots of quite fundamental changes. At the end, when the Netscape code was open-sourced, it turned out to be (no surprise) a big mess that took several years to sort into shape. Meanwhile, Microsoft sat on their monopoly and did NOTHING to the browser, until they were forced into evolving again once Firefox started to seriously dent their market-share.

    The current state is that the Mozilla code has been substantially rewritten and is now in pretty good shape, but Microsoft are stuck hacking away at the old crud.

  8. Re:Show attached block devices on (Useful) Stupid Unix Tricks? · · Score: 1

    Wow, that is awesome. I think I will be using this all time time! Thanks!

  9. Re:Show attached block devices on (Useful) Stupid Unix Tricks? · · Score: 2, Interesting

    I also stuck to tcsh for a very long time, even after reading that document. Then one day, I finally sat down and finally learned how to do redirections properly in bash, and how to set environment variables ('export', instead of 'setenv'), and I never looked back. Bash is so much better, I can't believe now how long it took to make the change. Aside from much better redirection syntax, other bash-isms I often use are $(command) syntax as an alternative to backticks, for i in {1..10}; do .... ; done loops, and lots of other stuff. But all of the shells have a syntax that to me seems very old fashioned and confusing. I wish there was a much more modern shell, perhaps with a python-like syntax, or even just a cleaned up bash. I never remember all the places where you need to insert spaces after reserved words in bash.

  10. Re:Do we need regulation? on Behind the Cogent-Sprint Depeering · · Score: 2, Insightful

    Whose customers? The customers most affected are Cogent's customers, not Sprint's. If Sprint is to be believed, they have plenty of notice to Cogent before they disconnected them. But Cogent clearly failed to notify their customers, or make alternative arrangements.

    Did you mean that Sprint should have notified Cogent's customers? How are they supposed to do that?

  11. Re:Encryption is good for security, bad for perfor on Resisting the PGP Whole Disk Encryption Craze · · Score: 1

    You obviously never used any of the big DoD supercomputers. Have a look at the (old, by now) ASCI Red configuration here. It has two interfaces, one for classified jobs and one for non-classified jobs.

  12. Re:Encryption is good for security, bad for perfor on Resisting the PGP Whole Disk Encryption Craze · · Score: 1

    Well, OK, but an overhead of 1% is hardly measurable to begin with. If you run it on a dual core machine and halve the overhead, the additional speedup is 1.01 / 1.005 = 1.005. Is it a good investment, to buy a second CPU core to get 0.5 of one percent improvement?

  13. Re:Encryption is good for security, bad for perfor on Resisting the PGP Whole Disk Encryption Craze · · Score: 2, Interesting

    I wasn't even talking about desktops though, I was talking about compute servers! I have used a few clusters at LANL, and yeah they have separate classified and unclassified machines (or sometimes, sections of machines) that are partitioned off for classified work, but even the classified part never (as far as I know) uses whole-disk encryption. The original question specifically said that they were intending to encrypt their servers as well.

  14. Re:Encryption is good for security, bad for perfor on Resisting the PGP Whole Disk Encryption Craze · · Score: 4, Informative

    That is interesting - if the overhead was really 1%, then why even bother with optimizations for multi cores?

    The other thing I cannot understand is why anyone would want to run whole-disk encryption on a compute server. Even the US DoD machines that are used for classified research do not do this!

  15. Re:Pointless on Windows 7 Beta Screenshots Leaked · · Score: 4, Informative

    Is SFU and SUA what the developers in Microsoft use to do real work?

    Yes. For example, when MS bought Hotmail and changed the servers from FreeBSD to Windows, they used SFU (including ssh and rsync) to do the remote administration. There was a leaked MS memo discussing this, it was on Slashdot back in the day. Here, in fact.

  16. Re:Enforcing the license? on Open Source Licenses For Academic Work? · · Score: 1

    Loss of professional reputation is, IMO, worse than this. Something that would definitely happen if you happen to use someone's work without properly attributing it to them.

    Not really, this sort of thing goes on all of the time. I suspect you are not an academic? Or perhaps your field is different? (I'm not being snide here, different fields really do have different cultures of citing people, I have discovered.) Often, the background material for a publication is a quite large set of partially overlaping works, and it is only practical to cite a subset of them. Some research groups hate each other and never cite each others papers. Sometimes (and especially with software) people simply forget to add the citation.

    Sometimes missing citations (whatever the cause) get caught in the referee process, but often they do not. It is rarely harmful to the person who missed the citation. I have had it happen to me that someone deliberately did not cite my work, because their paper overlapped to such a degree that it would not have been publishable. The referee didn't catch it. The result is that their paper gets cited a lot more than mine as it was published in a better known journal. My friends all know the story and are sympathetic, but no one else knows about it, and if they did they probably wouldn't care much.

  17. Re:Creative Commons Attribution on Open Source Licenses For Academic Work? · · Score: 1

    In the end, there is very little difference between a 'use' restriction, and an 'output' restriction. Except for purely personal use (which is basically unenforceable anyway), to get the same effect as a restriction on the output, you just put a restriction on the use: "you are only allowed to use this software if you agree to do X", where "X" is something like "cite the authors of the software in any publications that make use of data obtained with the software".

    In quantum chemistry, there is a very widely used software package called Gaussian. (It is partly for this software that Walter Kohn got the Nobel prize, but I believe he hasn't been associated directly with Gaussian for a while now). The license conditions for Gaussian are very restrictive. At every installation that uses Gaussian, users must agree to a set of terms before they use it, which are of this form:

    1. I am not a member of a research group developing software competitive to Gaussian 03.
    2. I will stop using Gaussian 03 if there is a change in my situation that would bring into question my status with respect to point 1. above.
    3. I will not copy the Gaussian 03 software, nor make it available to anyone else.
    4. I will acknowledge Gaussian Inc. in published works to which Gaussian 03 calculations contributed.

    Now that is a draconian license!

  18. Try the ALPS license on Open Source Licenses For Academic Work? · · Score: 1

    Algorithms and Libraries for Physics Simulations has two licenses, one for the basic libraries and one for applications. They both require citations as a condition of use. Library license and applications license.

    The important bit is common to both licenses:

    This license grants permission to use, reproduce, display, distribute, execute and transmit the Software, and to prepare derivative works of the Software, and to permit others to do so for non-commercial academic use, all subject to the following conditions:
    1. In any scientific publication based wholly or in part on the Software, the use of the Software must be acknowledged and the publications listed in the accompanying CITATIONS.txt document must be cited.

    Aside from this clause, the library and application licenses are based (loosly) on the (version 2) LGPL and GPL respectively.

  19. Re:Creative Commons Attribution on Open Source Licenses For Academic Work? · · Score: 4, Insightful

    If you accept the basic premise of EULA's, which specify a contract, then that contract can do basically anything it likes, within the law.

    Existing EULA's can and do put restraints on what you are allowed to do with an application. Consider for example 'student' versions of software, such as Matlab, Visual C++, etc, that cannot be used for commercial or research purposes. Also a lot of scientific software has a 'no commercial use' clause.

  20. Re:Moreno and Brown on IAU Names Fifth Dwarf Planet Haumea · · Score: 1

    Well, Moreno's claim is that they never really used the information in the logs, but only looked at it to confirm that they were studying the same object. It is bloody suspicious though, the standard procedure in that case would be to just email Brown and ask him. And I agree, not citing Brown is poor form, irrespective of any other circumstances. But what would Moreno have cited? You cannot cite an abstract that contains no information, nor can you cite an observing log on the internet. You can only cite a publication, and Brown hadn't published yet. Presumably the IAU and/or Brown have already tried to determine if the orbit information submitted by Moreno could be traced back to the data in Brown's logs (eg, by calculating the orbit from the exact figures that Moreno accessed from the logs, and seeing if it agreed to the N'th decimal place with the figures submitted to the IAU), but couldn't find a smoking gun.

    I tend to the opinion that, if there had been a fraud, there would be more evidence for it. For example, the 2003 observations that Moreno talks about, these either happened or didn't happen. There must be some documentation on that, and presumably the IAU investigated this.

  21. Re:Moreno and Brown on IAU Names Fifth Dwarf Planet Haumea · · Score: 1

    That isn't quite right, the first announcement from Moreno's group occurred before they did the new observations. You really should read the timeline to get it straight before making posts, otherwise you are just wasting people's time. Moreno's group sent the announcement to the IAU the day after the first access of Brown's observing logs. It was only after this announcement that Moreno accessed Brown's logs again and requested the additional observations. After that, Moreno sent a follow-up email to the IAU with the new observations, and also some more of their 2003 data. This is suspicious, but it is also possible that Moreno's group simply hadn't finished analyzing all the 2003 data by the 27th, but were desperate to claim the discovery nevertheless. That would be impolite, but not fraudulent.

    The whole dispute would have been avoided if Brown's group had contacted the IAU in the first instance, instead of just publishing an abstract for a forthcoming paper that had no official status.

  22. Re:Moreno and Brown on IAU Names Fifth Dwarf Planet Haumea · · Score: 1

    Papers authored by me on the physics arxiv: http://arxiv.org/find/cond-mat/1/au:+McCulloch_I/0/1/0/all/0/1
    Papers authored by my dad on the physics arxiv (he is retired now, most of his papers pre-date the internet): http://arxiv.org/find/astro-ph/1/au:+McCulloch_P/0/1/0/all/0/1

  23. Re:Moreno and Brown on IAU Names Fifth Dwarf Planet Haumea · · Score: 1

    You fool, I already read the timeline; once, at the time of the original event, and once again before I wrote my post. From there I hunted around until I got to http://web.gps.caltech.edu/~mbrown/, and from there to http://www.mikebrownsplanets.com/ .

    FWIW, my Dad is an astronomer. I'm just a condensed matter physicist. I don't have any inside information though, nor does the old man. For you to make such accusations, I would hope that you do have such information. But since you have never deigned to show the proof, I can only assume you are full of it.

  24. Re:Moreno and Brown on IAU Names Fifth Dwarf Planet Haumea · · Score: 1

    I'm intrigued by your choice of word to be intrigued about.

  25. Re:Moreno and Brown on IAU Names Fifth Dwarf Planet Haumea · · Score: 0

    Oops, there was supposed to be a link to Brown's blog in my comment, as a citation for the statement that "Brown himself admits there is no firm evidence for this.". This comes from http://www.mikebrownsplanets.com/