In Israel (at least in Jerusalem) they used to have a really neat system (I haven't been there for a while now). You had two ways to pay:
1. Previously buy parking tickets with scratch-able time and date fields. Scratch off the date and time from a previously bought ticket. Put ticket in window. Each ticket is good for 1/2 an hour, so if you need more time, simply use more tickets.
2. Previously acquire and "charge" an electronic card/display gizmo. After parking press a button on it, this starts "eating up" time and showing that it is doing this on a the display. Place device in window. When you get back to the car, stop timer.
I guess the device could be hacked, but it's just like any subway card with a big fat display and it knows how to eat up credit at a particular rate.
The main down-side is that there has to be a single price of parking in the municipal area. The other downside is that you have to buy these things in advance. But the ease of use are quite amazing, and with the electronic device, you do not have to guess the amount of time you need, you pay what you use.
Take a bunch of people and have them learn a new topic. Have half the people denied access to wikipedia (but full access to britannica.com) and vice versa with the other half. Give them 1 hour to learn about the topic
Then test them on the topic and see who is better "educated".
Possibly do it double blind so that the people who grade them are denied access to both britannica.com and wikipedia, and do not know what source of knowledge the person had.
Unfortunately the figures, equations, and tables came out as "Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com".....if someone can suggest where I should upload the PDF, I'd do that too.
======
Letter
Nature Physics 4, 716 - 720 (2008) Published online: 31 August 2008 | doi:10.1038/nphys1056
Linked and knotted beams of light
William T. M. Irvine1,2 & Dirk Bouwmeester2,3
Abstract
Maxwell's equations allow for curious solutions characterized by the property that all electric and magnetic field lines are closed loops with any two electric (or magnetic) field lines linked. These little-known solutions, constructed by Rañada1, are based on the Hopf fibration. Here we analyse their physical properties to investigate how they can be experimentally realized. We study their time evolution and uncover, through a decomposition into a spectrum of spherical harmonics, a remarkably simple representation. Using this representation, first, a connection is established to the Chandrasekharâ"Kendall curl eigenstates2, which are of broad importance in plasma physics and fluid dynamics. Second, we show how a new class of knotted beams of light can be derived, and third, we show that approximate knots of light may be generated using tightly focused circularly polarized laser beams. We predict theoretical extensions and potential applications, in fields ranging from fluid dynamics, topological optical solitons and particle trapping to cold atomic gases and plasma confinement. Introduction
The concept of field lines whose tangents are the electric or magnetic field is typically used to visualize static solutions of Maxwell's equations. Propagating solutions often have simple field-line structures and so are not usually described in terms of field lines. In the present work, we study a propagating field whose defining and most striking property is the topological structure of its electric and magnetic field lines.
An intriguing configuration for field lines is to be linked and/or knotted. Two closed field lines c1(tau), c2(tau) are linked if they have non-vanishing Gauss linking integral3, 4, 5, 6,
Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com
whereas for a single field line c(tau) the self-linking number, L(c,c), is a measure of knottedness. The linking integral L can also be computed visually by projecting the field lines onto a plane and subsequently counting the crossings in an oriented way3. For example, the lines in Fig. 1a have linking number 1, but do not form a knot, whereas the blue and orange field lines in Fig. 4 below are knotted and linked to each other. In the case of magnetic or electric fields, averaging the linking integral over all field-line pairs together with the self-linking number over all field lines gives rise to the magnetic and electric helicities4, 5:
Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com
where B:=nablatimesA and E:=nablatimesC in free space. Figure 1: Construction of the Hopf fibration. Figure 1 : Construction of the Hopf fibration.
aâ"c, Left column: A torus can be constructed out of circles (fibres) in such a way that no two circles cross and each circle is linked to every other one. a,b, Each circle in such a configuration wraps once around each circumference of the torus. c, By nesting such tori into one another, the whole of three dimensional space, including the point at Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com (U
I don't know what the poster is high on. Jajah is simply a way to call long-distance for cheap. I use it all the time for calling international and long distance (I hate talking on the cell-phone for a long time).
The way that they work is that they call both you and your party and connect the call via VOIP. However, you foot the bill with a credit card. I tried many other calling-card companies, Skype, and whatnot. So far, Jajah is pretty good, and darn cheap.
Sure, you could sign up and put your friends number, but it will not charge any money to them. My only complaint is that you can only change your phone numbers 3 times so if you move often (as I have over the last few months) you might have to open a new account.
They even give you a few $$$ to spend BEFORE they ask for your credit card number! so you can try them out for "free".
Basically if you have enough documents you will be able to find two with the same hash. Not to be mistaken with the difficulty of finding a document with the same hash as a GIVEN document. This cannot be avoided regardless of the hash.
To foil a birthday attack, NEVER agree to sign a given document. One must ALWAYS add some random junk to the document (clear text or hidden). This should be implemented in sofetware that helps in signing, but if the option isn't given, you can add a clear text "random" string.
Slashdot Mirror
In Israel (at least in Jerusalem) they used to have a really neat system (I haven't been there for a while now). You had two ways to pay:
1. Previously buy parking tickets with scratch-able time and date fields. Scratch off the date and time from a previously bought ticket. Put ticket in window. Each ticket is good for 1/2 an hour, so if you need more time, simply use more tickets.
2. Previously acquire and "charge" an electronic card/display gizmo. After parking press a button on it, this starts "eating up" time and showing that it is doing this on a the display. Place device in window.
When you get back to the car, stop timer.
I guess the device could be hacked, but it's just like any subway card with a big fat display and it knows how to eat up credit at a particular rate.
The main down-side is that there has to be a single price of parking in the municipal area. The other downside is that you have to buy these things in advance. But the ease of use are quite amazing, and with the electronic device, you do not have to guess the amount of time you need, you pay what you use.
I propose a test between encyclopedias:
Take a bunch of people and have them learn a new topic. Have half the people denied access to wikipedia (but full access to britannica.com) and vice versa with the other half. Give them 1 hour to learn about the topic
Then test them on the topic and see who is better "educated".
Possibly do it double blind so that the people who grade them are denied access to both britannica.com and wikipedia, and do not know what source of knowledge the person had.
http://pdfmenot.com/view/http://pdfmenot.com/store_local/996c794f28d65c93d38d6cb60ff50d2f.pdf
Unfortunately the figures, equations, and tables came out as "Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com".....if someone can suggest where I should upload the PDF, I'd do that too.
======
Letter
Nature Physics 4, 716 - 720 (2008)
Published online: 31 August 2008 | doi:10.1038/nphys1056
Linked and knotted beams of light
William T. M. Irvine1,2 & Dirk Bouwmeester2,3
Abstract
Maxwell's equations allow for curious solutions characterized by the property that all electric and magnetic field lines are closed loops with any two electric (or magnetic) field lines linked. These little-known solutions, constructed by Rañada1, are based on the Hopf fibration. Here we analyse their physical properties to investigate how they can be experimentally realized. We study their time evolution and uncover, through a decomposition into a spectrum of spherical harmonics, a remarkably simple representation. Using this representation, first, a connection is established to the Chandrasekharâ"Kendall curl eigenstates2, which are of broad importance in plasma physics and fluid dynamics. Second, we show how a new class of knotted beams of light can be derived, and third, we show that approximate knots of light may be generated using tightly focused circularly polarized laser beams. We predict theoretical extensions and potential applications, in fields ranging from fluid dynamics, topological optical solitons and particle trapping to cold atomic gases and plasma confinement.
Introduction
The concept of field lines whose tangents are the electric or magnetic field is typically used to visualize static solutions of Maxwell's equations. Propagating solutions often have simple field-line structures and so are not usually described in terms of field lines. In the present work, we study a propagating field whose defining and most striking property is the topological structure of its electric and magnetic field lines.
An intriguing configuration for field lines is to be linked and/or knotted. Two closed field lines c1(tau), c2(tau) are linked if they have non-vanishing Gauss linking integral3, 4, 5, 6,
Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com
whereas for a single field line c(tau) the self-linking number, L(c,c), is a measure of knottedness. The linking integral L can also be computed visually by projecting the field lines onto a plane and subsequently counting the crossings in an oriented way3. For example, the lines in Fig. 1a have linking number 1, but do not form a knot, whereas the blue and orange field lines in Fig. 4 below are knotted and linked to each other. In the case of magnetic or electric fields, averaging the linking integral over all field-line pairs together with the self-linking number over all field lines gives rise to the magnetic and electric helicities4, 5:
Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com
where B:=nablatimesA and E:=nablatimesC in free space.
Figure 1: Construction of the Hopf fibration.
Figure 1 : Construction of the Hopf fibration.
aâ"c, Left column: A torus can be constructed out of circles (fibres) in such a way that no two circles cross and each circle is linked to every other one. a,b, Each circle in such a configuration wraps once around each circumference of the torus. c, By nesting such tori into one another, the whole of three dimensional space, including the point at Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, or to obtain a text description, please contact npg@nature.com (U
The way that they work is that they call both you and your party and connect the call via VOIP. However, you foot the bill with a credit card. I tried many other calling-card companies, Skype, and whatnot. So far, Jajah is pretty good, and darn cheap.
Sure, you could sign up and put your friends number, but it will not charge any money to them. My only complaint is that you can only change your phone numbers 3 times so if you move often (as I have over the last few months) you might have to open a new account.
They even give you a few $$$ to spend BEFORE they ask for your credit card number! so you can try them out for "free".
yfarjoun.
Hmmm, That's crap. I had no idea. Thanks. Y
See http://en.wikipedia.org/wiki/Birthday_paradox for details.
Basically if you have enough documents you will be able to find two with the same hash. Not to be mistaken with the difficulty of finding a document with the same hash as a GIVEN document. This cannot be avoided regardless of the hash.
To foil a birthday attack, NEVER agree to sign a given document. One must ALWAYS add some random junk to the document (clear text or hidden). This should be implemented in sofetware that helps in signing, but if the option isn't given, you can add a clear text "random" string.
Yossi