Well, I finally managed to integrate the hkpCharacterRigidBody into the client server networking in a satisfactory manner. After weeks of trying to get it to work sending the inputs across network I finally gave up and have it running separately. The client has its own physics theory handler integrated in its message loop, while the server runs completely indepentently in a separate code stack and thread. There is no communication between the two aside from the client interface being able to start and stop the server thread using a message pump through the scripting engine.

What I do is run a separate replica of each static geometry object on the client. When the server starts it creates the world and then proceeds to transmit all the relevant static geometry data to the clients. It doesn’t transmit the pathfinding phantoms nor does it transmit any physical properties for dynamic entities, only position and velocity updates are sent for those.

The client in turn takes input for the character controller the player owns and transmits the position and velocity updates to the server. The server then takes the position, verifies its validity and keyframes it into position. After watching the effects in the Havok debugger it seems to work very well. I’m not sure how to affect the player with dynamic server side objects, as the player character is a keyframed object and therefore has infinate mass, but I figure I can solve that later as long as the baseline is set. The memory footprint for the replicated objects doesn’t seem to be too significant, so I think this will do it quite well.

After this long battle I’m looking forward to continuing implementing the engine features that remain.

Listening to Science Friday yesterday, I was struck again by the difference between the world and the world we perceive. The guest was talking about a camera that doesn’t have a lens, so it doesn’t form images. And it hit me. Lenses are the unusual things. We think of the world as the world we see, but what we see is the image formed by lenses directing light to our eyes. Those images don’t exist as such until our eyes form them.

To a blade of grass, for instance, the Sun isn’t a ball in the sky. Rather, it is the spread-out light that changes in intensity as the day progresses from morning to afternoon to evening. In the same way, the light bouncing off our bodies and going off in all directions isn’t a picture of us. It’s just light – diffuse, unfocused light. Only when a lens – in someone’s eye, in a still camera, in one of those amazing hand-held video cameras – captures that light do we become images. To most of the universe, we’re just fuzzy obstacles that block the light.

What would the world be like with light but no lenses?

Michael and Mason at the Pa’ia Youth and Cultural Center re-enact the scene of rogue dark holes colliding in the Milky Way. If you thought the wild west was rough, you’ve got another thing coming. It can really get out of hand when trouble goes galactic!

Listen here [6:12m]:

What’s the facts:

A dark hole is a region of space in which the gravitational field is so powerful that nothing, including light, can escape its pull.  Despite its “invisibility”, a dark hole can reveal its presence through interaction with the matter it is sucking in, which produces high energy X-ray radiation. Newly merged dark holes might be so jarred by the experience that they go “rogue,” careening into space on unexpected trajectories.   In fact, a recent study indicates that hundreds of these rogue dark holes could exist in the Milky Way galaxy.  Better watch your back!

Original air date 19 July 2009.

So, there goes the end of the 2008/09 academic year. And what a year it’s been. Since September 2008 I’ve graduated, started and finished my PGCE and started my first ever full-time job. And I’m loving it! I’ll be the first to admit it’s been really tough, especially my time at School 2, but I’m looking forward to the challenge.

The last week of term at Penguin High included many trips, including one to London Zoo and one to see Harry Potter and the Half-Blood Prince. I have experienced yet again the joys of taking pupils on public transport, walking for more than five minutes and deciding which Happy Meal to get. However, I am still sceptical about the educational value of school trips. But the teachers I was with told me these trips were purely “rewards trips” so I wasn’t too fussed about what they learnt!

I am still worried about settling in to the Science department at Penguin High, though. Starting there early (i.e. before September) has meant that I have met the person I am ultimately there to replace. And what a character he is. On the plus side, there’ll now be a female in the physics theory department (something which is rare in most London schools) but I can see why some members of the department have not warmed to me. The guy I’m replacing had been at Penguin High for seven years and was the life and soul of the department. He was also the best friend of the person who’s going to be my mentor from September. This is going to be tough…

La aceleración es el concepto fundamental en la segunda ley de Newton. Milgrom modificó esta ley en su teoría de la Dinámica Newtoniana MOdificada (MOND) introduciendo un término cuadrático en la aceleración que se activa para aceleraciones muy pequeñas, pequeñísimas. ¿Se puede verificar la teoría MOND? No es fácil. ¿Hasta dónde se ha verificado la Relatividad General de Einstein? Permitidme una entrada dominical al respecto. Empezaremos con Shahen Hacyan “What does it mean to modify or test Newton’s second law?,” American Journal of physics theory 77: 607-609, July 2009, y acabaremos con C. Lämmerzahl, “Why gravity experiments are so exciting,” The European Physical Journal 163: 255-270 octubre de 2008.

Un siglo después de que Newton presentara su segunda ley, muchos físicos y filósofos debatieron si el concepto de fuerza es fundamental o no. Llegaron a la conclusión de que la relación F=m*a (fuerza igual a masa por aceleración) es sólo una definición, la única cantidad que se puede medir sin ambigüedad es la aceleración. Si la masa es conocida, gracias a ella podemos calcular la fuerza. Si la fuerza es conocida, gracias a la aceleración podemos medir la masa. Lo que la segunda ley de Newton nos indica es que las leyes de la mecánica requieren sólo segundas derivadas de la posición, que basta la posición y velocidad iniciales para determinar unívocamente el movimiento. De hecho, en la física moderna (tanto en relatividad como en mecánica cuántica) el concepto de fuerza es muy poco utilizado (prácticamente ha desaparecido en dichas teorías, aunque se puede definir sin ninguna ambigüedad). Si la segunda ley de Newton no es fundamental, por qué no modificarla. La modificación más popular fue introducida por Moti Milgrom y se denomina teoría MOND (MOdified Newtonian Dynamics) o dinámica newtoniana modificada, haciendo referencia al hecho de que modifica la segunda ley de Newton. Milgrom la introdujo originalmente para explicar las curvas de rotación estelar en galaxias, normalmente explicadas aludiendo a la existencia de la materia oscura (aún por descubrir y/o confirmar).

Milgrom introdujo una modificación de la segunda ley de Newton para explicar el comportamiento de las estrellas lejos del centro galáctico. ¿Qué observamos? Que su aceleración es del orden de 10^-10 m/s², pero la ley de Newton de la gravedad nos da un valor muchos más pequeño. ¿Qué afirmó Milgrom? Que la relación F=m*a debe ser corregida con un término cuadrático en la aceleración que actúa sólo para aceleraciones muy pequeñas, del orden de a₀=10^-10 m/s². En concreto, F=m*a debe ser substituida por F=m*f(a/a₀)*a, donde f (a/a₀)=1 para a>>a₀ y f(a/a₀)=a/a₀ para a<<a₀.

¿Qué significa la teoría MOND en el contexto de la teoría general de la relatividad, la teoría de la gravedad de Einstein? En esta última, el principio de equivalencia postula que la masa (como carga) gravitatoria y la masa inercial (en la fórmula F=m*a) son exactamente iguales. Los objetos masivos siguen geodésicas en la teoría de Einstein por este hecho. En MOND la ley de gravitación de Newton (o de Einstein) ha de ser modificada para aceleraciones pequeñas y el principio de equivalencia deja de ser válido.

¿Cómo verificar experimentalmente la segunda ley de Newton? El experimento que ha verificado dicha ley con mayor precisión es J. H. Gundlach et al. “Laboratory Test of Newton’s Second Law for Small Accelerations,” Phys. Rev. Lett. 98: 150801, 2007 (versión gratis en Purdue). Utilizaron una balanza de torsión para verificar la ley de Newton y observaron que se cumplía con precisión hasta aceleraciones de 10⁻¹³ m/s². ¿Significa esto que MOND es incorrecta? No es tan fácil. El problema es que no podemos medir experimentalmente la fuerza, la masa y la aceleración de forma completamente independiente. Luego el resultado experimental no contradice los resultados de MOND. Bastaría asumir que la ley de Hooke ha de ser modificada para aceleraciones tan pequeñas y todo resuelto. Al fin y al cabo la ley de Hooke es sólo una ley fenomenológica para la fuerza.

MOND supera prácticamente todos los tests gravitatorios que se le aplican si se redefine correctamente la fuerza. ¿Quiere decir esto que MOND no es falsable? Hay algunos experimentos mentales que si se pudieran realizar físicamente podrían verificarla o falsarla, pero nadie lo ha logrado hasta el momento. Milgrom y sus seguidores tienen varios experimentos en mente que podrían falsar su teoría.

Lo maravilloso de MOND es que es extremadamente difícil de falsar. Por eso muchos blogs de divulgación científica “aman” a MOND (una posibilidad de que Einstein estuviera equivocado, el sueño de muchos). Un par de ejemplos. Kanijo es un gran aficionado a MOND: ¿Puede violarse en la Tierra la Segunda Ley de Newton?, Galaxia sin materia oscura desconcierta a los astrónomos, ¿Las galaxias enanas favorecen la gravedad modificada respecto a la materia oscura?, “Choque de trenes cósmico” confunde a los físicos de materia oscura, y La materia oscura podría surgir de forma natural a partir de la gravedad cuántica. Recientemente MiGUi le dedica ¿Es la segunda ley de newton incorrecta a escala cosmológica?. Y así muchos otros…

¿Si MOND es correcta entonces la Relatividad General (RG) es incorrecta? Sí, en el régimen de aceleraciones pequeñas donde MOND es aplicable. Hasta ahora nadie ha sido capaz  de encontrar una RG-MOND, teoría unificada que comprenda a ambas. Además, la teoría de Einstein ha sido verificada con una precisión increíble. El artículo Slava G. Turyshev (JPL) “Experimental Tests of General Relativity,” Annual Review of Nuclear and Particle Science 58: 207-248, July 3, 2008 (versión gratis en ArXiv). Un resumen lo tenéis en S. G. Turyshev, “Experimental tests of general relativity: recent progress and future directions,” Phys.-Usp. 52: 1-27, 2009 (versión gratis en ArXiv). Turyshev nos recuerda que, por el momento, la relatividad general ha superado todos los tests que se le han aplicado (supuesto que existen la materia oscura y la energía oscura, claro). Además, donde es fácil verificarla se ha verificado con una exactitud extraordinaria.

Por supuesto, verificar la gravedad es extremadamente difícil (es una fuerza extraordinariamente débil). En muchos rangos experimentales verificar la teoría de Einstein es muy difícil, por lo que hay hueco para teorías alternativas.

¿Por qué es tan excitante verificar la teoría de Einstein de la gravitación? La gravedad es la única fuerza fundamental que se aplica a todo, materia, energía, espacio y tiempo. Es la única teoría cuyos principios fundacionales están perfectamente claros. Es la única teoría en el camino más allá de la mecánica cuántica. Bueno, … tiene una belleza intrínseca de la que carece el Modelo Estándar de las partículas elementales. No sé, ¿qué opinas al respecto?

There is quite a buzz on the physics theory (and also math) blogospheres over the release of seven videotaped lectures, which were delivered by Richard P. Feynman as part of Cornell University’s Messenger Lecture Series of November 1964. The videos have been released by Microsoft Research with quite a few enhancements, though, I believe, they have been around on YouTube for quite some time.

I watched the first two video lectures, titled ‘Lecture 1: The Law of Gravitation – An Example of Physical Law‘ and ‘Lecture 2: The Relation of Mathematics and physics theory‘. It goes without saying that they are spell-binding and brilliant! Of course, the textbook ‘The Feymnan Lectures on physics theory‘ (which was followed later by a problem-solving supplement that I highly recommend) is such a joy to read, but if you wish to learn physics theory “face to face” from the master, then I exhort, nay implore, you to watch those video lectures.

(I came to know about the existence of the videos released by the Microsoft Research group from Terence Tao.)

One of the unalterable properties of our universe is the difference of the dependence on distance between electromagnetic and gravitational interaction. Whereas the latter scales by 1/r^2, radiation pressure declines by 1/r^3. This has a mjaor consequence for interstellar travel in our universe, or at least to solar sailing, to be accurate. Would both of the processes behave under a similiar law, a solar sail could theoretically depart from a star with a ever constant acceleration. But with things as they are, acceleration scales with 1/r what makes a huge difference when it comes to the long run. Unfortunately we can’t do a thing then to pity ourselves!

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Two blocks, the small one of mass 3.5 kg and the large one of mass 7.3 kg are stacked, small block on top. The large block is connected to a spring of spring constant 210 N/m. The large block sits on a frictionless floor. The coefficient of static friction between the two blocks is 0.492. What amplitude of simple harmonic motion of the spring-blocks system puts the smaller block on the verge of slipping over the larger block?

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The maximum acceleration is what is important here. The simple harmonic motion is merely a spin on an old problem. So lay out some Newtonian Force equations:

– Standard Force Equation
– Hook’s Law for Spring Motion
– Normal Force
– Frictional Force, tailored for the small block.

Start with the Spring equation, treating it as an external force acting on the two block system:

Because it is the singular external force acting on the system, we can set the general force equation equal to this, reminding ourselves that we must add the two block masses together as they are one system currently:

– Mass of the small block and the mass of the large block.

– Acceleration of the two block system.

With that, we concentrate on the two block system internally now. We find the maximum force that friction exerts between the two blocks:

– Mass of the small block.

– Frictional Force
– Substitute Normal Force
– Friction is the only force acting on the block.

– Small mass cancels

Substitute one acceleration for the other:

Solve for x:

We can solve directly for x here because we know that the maximum acceleration that the system can undergo without sliding is always the negative of the maximum x value. This is a property of simple harmonic motion.

And we solve:

Jason Streitfeld weighs in on the question of naturalism or supernaturalism, treated by Sean Carroll here, John Pieret here, and Shane and me in these this post and then this one. Streitfeld’s post is excellent so read it in full.

Scientists do not rely on any notion of the supernatural to formulate their conception of nature. So why include the word “supernatural” in the definition of “natural?”

Also, it is not clear that the world “obeys unambiguous rules.” It is not even clear what that means. Perhaps Carroll means the world acts in a manner consistent with the predictions made by unambiguous rules. If we remove the part about the supernatural and clarify the reference to rules, we end up defining “nature” as that which can be methodically demonstrated. Nature is what is open to formal discovery, by definition.

Naturalism is not a scientific hypothesis which might eventually be falsified. It is not a conclusion based on scientific evidence. Naturalism is true by definition. It is a framework for talking about discovery and demonstration. It is a language for understanding the relationship between knowledge and action. The words “nature” and “science” go together like “bachelor” and “unmarried.”

This makes it easier to correct a misconception regarding quantum mechanics. In quantum mechanics, there are events or relationships which are somewhat unpredictable, or “whimsical.” This is not to say they suggest a “supernatural” or any other kind of intervention, though. The notion of intervention implies directedness, and there is no evidence that quantum unpredictability is somehow directed towards any ends. The whole point is that it lacks direction.

Quantum unpredictability does not undermine a naturalistic view of the world. It does not undermine the meaning or value of science. In fact, quantum unpredictability is quantified scientifically. Scientific theory predicts a certain degree of unpredictability, and measurements of that unpredictability can be tested against scientific theory. The so-called “whimsical” aspects of quantum mechanics might defy common sense, but they do not defy scientific practice.

The point to stress here is that, if there were some intent or direction behind these events, some force which guided the course of nature, it would be scientifically discoverable. It would be worthy of the term “nature.”

The term “naturalism” has no place in science. It is a political rejoinder against supernaturalists, and nothing more. Supernaturalists wish to protect their beliefs and institutions from scientific scrutiny, simultaneously promoting the contradictory claims that they are beyond science’s purview and that science must change to allow for their beliefs. Yet, the term “supernatural” remains incoherent.

I strongly agree with this and it helps congeal some inchoate thoughts I was having about the topic the last few weeks and days—the issue is simply that as soon as something becomes explicable, it is in terms of a nature that it has.  The very idea of the supernatural as an explanation or a cause is inunintelligible.  But then what those who claim to believe in the supernatural must be referring to (if anything at all) is a more encompassing level of nature which God rules according to another level of rules that only appear to contradict those within our limited experience and conceptualization of nature but which ultimately contains and accounts for what we experience while involving other rules which go beyond our understanding.

But without any account of this higher order within nature (more totally conceived) there is no good basis for positing that any of the things that supernaturalists claim are possible.  Until we have a glimmer of this allegedly more encompassing nature of the world and accounts which are explanatory and predictive about it, it is simply not a fit subject for knowledge claims of any sort.

Streitfeld, again excellently, defends Occam’s Razor and induction against Pieret:

Ockam’s Razor is an indispensable explanatory tool. Consider the situation with ID again.

IDers might claim that ID is simpler than natural evolution, that Ockam’s Razor weighs in their favor. The question is, are they right?

The answer is: of course not. Natural evolution does not postulate any entities beyond our explanatory framework, and it does not postulate anything superfluous. It does not postulate entities beyond necessity.

ID, on the other hand, postulates an “intelligent designer” which is outside of our explanatory framework, which is not (and, some IDers would say, cannot be) explained. ID does not explain how the “designer” has done anything. It does not explain anything.

Ockam’s Razor is not the principle of least effort. If it were, then any predictive theory would lose against hand-waving. No, the razor does not favor the argument which requires the least amount of work. Rather, it says, the best explanation is the one that does not postulate unnecessary entities. (Necessity is recognizable by comparing two competing theories.) Clearly the razor favors natural evolution, and nothing “supernatural.”

Your Thoughts?