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Branko Toic
Sysadmin by choice, developer in making...
Sysadmin by choice, developer in making...

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This fictional story put a smile on my face. Art mimics life. I can really relate to it, and the writing style is so exaggerated and sarcastic I couldn't help but chuckle.

+Asbjørn Grandt might like this one.

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This is one of the best information security presentations I've seen in a long time.  It's comprehensive and entertaining.

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How Does Gravity Escape a Black Hole?

Here’s the deal: nothing can travel faster than light. A black hole traps everything including light. So how does gravity escape a black hole? It’s a great question, and a perfectly reasonable one given most people’s understanding of gravity. The answer is that gravity doesn’t work the way you probably think it does.

The most common way to think of gravity is as a force between two masses. For example, the Earth exerts a gravitational force on the Moon, and the Moon pulls back on the Earth in return. This “force model” of gravity is what Newton used to develop his law of universal gravity, which stood as the definite theory of gravity until the early 1900s, and is still used to this day. But built into this model of gravity are some assumptions that we can explore by playing the “what if?” game.

Suppose we had a universe with a single mass. Imagine empty space extending as far as you like, with a single mass in the center (which we’ll call Bob). Would such a mass have gravity? If gravity is a force of one object on another object, then the answer would be no. There’s no other mass for Bob to pull on, so there’s no gravitational force. If we add another mass to our universe (call this one Alice), then Bob and Alice would each exert a force on each other, and gravity would exist. But gravity would only exist between Bob and Alice, and nowhere else in our empty universe.

One of the problems with this force model is that it requires masses to exert forces on other masses across empty space. This “action at a distance” problem was resolved in part by Pierre-Simon Laplace in the early 1800s. His idea was that a mass must reach out to other masses with some kind of energy, which he called a field. Other masses would sense this field as a force acting upon them. So if we again imagine our Bob mass in a lonely universe, we would say that Bob has a gravitational field surrounding it, even if there were no other masses in the universe. This eliminates the need for action-at-a-distance, because when we put our Alice mass into the universe, it simply detects whatever gravitational field is at its location, and experiences a force. We know the gravitational field is due to Bob some distance away, but Alice simply knows there is a gravitational field at its location.

Both the force model and field model of Newtonian gravity give the same predictions, so experimentally there’s no real way to distinguish one from the other. However fields are often an easier concept to work with mathematically, and fields are also used to describe things like electricity and magnetism, so we generally think of Newtonian gravity as a field.

But this raises another question. Suppose in our Bob and Alice universe we suddenly shift Bob’s position. How long will it take for Alice to recognize the change? In other words, if we change the position of Bob, at what speed does the change propagate through the gravitational field? When Laplace looked at this idea he found that changes in a gravitational field had to happen instantly. The “speed of gravity” would have to be infinite. For example, if gravity travelled at the speed of light, the Earth would try to orbit the point where the Sun was 8.3 minutes ago (the time it takes light to travel from the Sun to Earth). As a result, Earth’s orbit would become unstable over time.

At the time, the idea of gravity acting at infinite speed wasn’t seen as a problem. In fact it was used as an argument against alternative gravity ideas proposed at the time. But in the early 1900s Einstein developed his special theory of relativity, which (among other things) required that nothing could travel faster than light. If that’s the case, then there’s something wrong with our theory of gravity. By 1915 Einstein had developed a new model of gravity known as general relativity, which satisfied both Newton’s gravitational model and special relativity.

Decay of a pulsar orbit compared to general relativity (dotted line).
According to theory, for example, when two large masses such as neutron stars orbit each other, they should produce gravitational waves that radiate away from them. These gravitational waves should travel at the speed of light. There have been experimental attempts to detect such gravitational waves, but they have been unsuccessful so far. We have, however, found indirect evidence of gravitational waves. By observing a binary pulsar, we have observed its orbit decay slightly over time. This orbital decay is due to the fact that gravitational waves carry energy away from the system. The rate of this decay matches the prediction of general relativity perfectly. Since this rate of decay depends crucially on the speed of gravitational waves, this is also indirect confirmation that gravitational waves move at the speed of light.

But if gravity moves at the speed of light, doesn’t that mean that planetary orbits should be unstable? Actually, no. When Laplace studied finite-speed gravity, he considered only the effect of the speed of gravity, which is what leads to his result, but in special and general relativity, the finite speed of light leads to other effects, such as time dilation due to relative motion, and the apparent change of mass due to relative motion. Mathematically these effects arise because of a property known as Poincaré invariance. Because of this invariance, the time delay of gravity and the velocity dependent effects of time and mass cancel out, so that effectively masses are attracted to where a mass is. This canceling effect means that for orbital motion it is as if gravity acts instantly.

But wait a minute, how can a gravitational field have a finite speed and act instantly at the same time? A gravitational field can’t, but in general relativity gravity is not an energy field.

Since long before Newton, it was generally assumed that objects and energy fields interacted in space at particular times. In this way, space and time can be seen as a background against which things happen. Space and time were seen as a cosmic grid against which anything could be measured. In developing special relativity, Einstein found that space and time couldn’t be an absolute background. In Newton’s view, two events seen to occur at the same time will be seen to be simultaneous for all observers. But Einstein found that the constancy of light required this concept of “now” to be relative. Different observers moving at different speeds will disagree on the order of events. Rather than a fixed background, space and time is a relation between events that depends upon where and when the observer is.

This principle carried forward into Einstein’s theory of gravity. In general relativity gravity is not an energy field. Instead, mass distorts the relations between space and time. If we go back to our earlier example, if we place mass Bob in an empty universe, the relations of space and time around it are distorted. When we place mass Alice nearby, the distortion of spacetime around it means that moves toward mass Bob. It looks as if Alice is being pulled toward Bob by a force, but it’s actually due to the fact that spacetime is distorted.

As physicist John Wheeler once said, “Spacetime tells matter how to move; matter tells spacetime how to curve.”

This is how gravity can seem to act instantly while gravitational waves seem to travel at the speed of light. Gravity isn’t something that travels through space and time. Gravity is space and time.

A black hole is an extreme distortion of space and time due to a very dense mass. Such a spacetime distortion can prevent light and matter from ever escaping. But the spacetime distortion is also gravity. It doesn’t need to escape the black hole, because it is the black hole.

That’s the thing about science. Sometimes a simple question will pull you toward an unexpected answer.

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What If Light Had No Speed Limit?

What would the universe be like if the speed of light were infinite? It might seem like a silly question, since the speed of light clearly isn’t infinite, but questions like these are a good way to explore how different aspects of a physical model are interrelated.

For example, in our universe light is an electromagnetic wave. It not only has a speed, but a wavelength. If you think of a wave as an oscillation, then at infinite speed light would have no time to oscillate. So infinite light can’t be a wave. Since the wavelength of light determines its color, that would also mean it has no color. But it gets worse because in classical physics light is produced when electromagnetic waves cause the charges in atoms and molecules to oscillate. Without waves, atoms can’t be induced to emit light, the universe would be a sea of darkness.

But real light actually has both wave and particle aspects, so let’s suppose that for infinite light it’s just some kind of particle so we can still have light and color without all that meddling wave business. What else would change?

Relativity is an obvious choice. Einstein’s theory of relativity depends upon a finite speed of light. With an infinite light speed, all those fun things like time dilation are thrown out the window. So is Einstein’s most famous equation, E = mc2. The main consequence of this equation is that matter can be transformed into energy and vice versa. It’s central to things like nuclear fusion, which powers the stars and creates the heavy elements. Stars could still be powered by gravitational contraction, but they would only last for a million years rather than billions of years. They also wouldn’t have any mechanism to explode as supernovae, so there would be no way to make new stars from old ones.

Since Einstein’s theory of gravity is a generalization of special relativity, it goes away too. Our model of the universe, beginning with a big bang and expanding through dark energy, depends upon Einstein’s theory. Without it the universe look very different. No dark energy, possibly no big bang.

Of course this is all just a game of pretend. If you made different assumptions about physical phenomena you would derive different effects. We have no way of knowing what an infinite light speed universe would really be like. But what this shows is just how interconnected different aspects of a physical model actually are. Any tweak to the model has consequences that can ripple into widely different areas, or even cause an entire model to collapse.

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SCiO is based on the proven near-IR spectroscopy method,” writes Consumer Physics. “The physical basis for this material analysis method is that each type of molecule vibrates in its own unique way, and these vibrations interact with light to create a unique optical signature.”

“With every scan, SCiO learns more about the world around us, so we can all get smarter,” the Israel-based developers continue. “Our development team has taught SCiO some exciting things, like to tell how much fat is in any salad dressing, how much sugar is in a particular piece of fruit, how pure an oil is and lots more.”

Read more: World's first pocket spectrometer lets you measure the molecular makeup of anything | Inhabitat - Sustainable Design Innovation, Eco Architecture, Green Building

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"docker" functionality implemented in 100 lines of bash, very nice hack.  Shows just how powerful btrfs is, along with the 'ip' tool.
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