In this week's "Ask a Physicist," we tackle a general relativistic paradox: If time slows down near the event horizon of a black hole, how does anything ever fall in?

I've been enjoying reading all of your questions to "Ask a Physicist." As an added twist in the coming weeks, I'd be interested in hearing any questions you have about physics and cosmology in the news, especially those along the lines of, "Is this real, or just bullshit?" As always, please send your queries to askaphysicist@io9.com

Today's question comes to us from David Sirola who asks:

If a black hole warps space-time to such a degree to slow and stop time, how can anything ever disappear past the event horizon (or whatever point t=0)? It would seem to me in my superficial understanding, that ultimately, after all the fun at the outer edges of the hole, that nothing ever really happens, since happens implies a time/cause/effect relationship.

What am I missing here?

Let's get one thing out of the way from the outset: Black holes are awesome. They are the only major disturbance of space-time which have the advantage of actually being known to exist. Almost every large galaxy, including our own, seems to have a supermassive black hole at the center.

And black holes are ridiculously simple objects — or at least the non-rotating ones are, which are the only ones I'm going to talk about here. They basically consist of an infinitely compact "singularity" at the center and an outer boundary known as an "event horizon" from which nothing can escape (and here's where I'm supposed to use an ominously spooky voice) not even light. These guys are tiny, astronomically speaking. Were our sun to become a black hole, it would be smaller in radius than the city of Philadelphia. Even the 3 million solar mass black hole at the center of the Milky Way could comfortably fit inside the orbit of Mercury.

Okay, you probably knew all of that. I still need to dispel a few myths before we get into the hardcore space-warping.

1. Black holes don't suck.

   Suppose the sun were to suddenly turn into a black hole. Would you notice? Sure you would. The sun would blink out of existence and you'd quickly freeze to death. But in your dying moments, you'd no doubt be struck by the fact that J.J. Abrams lied to you. Rather than get pulled into the black-hole sun, the earth would just keep orbiting that seemingly empty point in the sky, exactly as it always had. Only icier.

2. You can't actually see them.

   Black holes are called that because they don't give off any light. I don't want to get into a nerd-fight here, partly because my mom says I'm not allowed, but mostly because things will go more smoothly if I anticipate a few objections. Somebody is likely to point out that we do, indeed, "see" black holes in the form of quasars in other galaxies. But this isn't quite right. What you're really seeing is hot, glowing gas falling onto the black hole or even larger glowing gas clouds surrounding the whole shebang. And by the way, with the exception of giant radio jets, we can't even generally resolve these clouds. When you see detailed accretion disks in news stories about black holes, that's somebody using MS Paint or whatever they use these days to make artist's conceptions.

   Let me further anticipate a black-belt level nerd who might introduce an even better possibility: Hawking Radiation. This is one of the coolest ideas in astrophysics, and one that most physicists believe, even though we've never observed it. Near the event horizon of black holes, particles and antiparticles are constantly being created in pairs. Every now and again, one of the particles escapes and creates some radiation (and takes with it some of the mass-energy of the black hole). But here's the deal: Hawking radiation is far too dim, and far too long of wavelength to ever be seen directly.

Now that we've got the lay of the land, we can get into the question of what's so special about the event horizon.

One of the major predictions of general relativity is that time runs slower near massive bodies than far away. On earth, we don't notice this effect, since the effect is only about 1 part in a billion. However, if you were of a stout enough constitution (we're talking, like 18+2, or something like that) you could hang out on a neutron star where the effect is more like 20% or more. Hang out for a few years, and even more time will have passed far away. What you've done here is built a (pretty crappy) time machine into the future. Also, this would be a one-way trip.

Black holes do it one better, and at the event horizon the time distortion effect literally becomes infinite. There's the paradox. If time slows down infinitely near the surface, presumably it takes stuff longer and longer to get closer and closer to the event horizon. How does anything ever actually fall in?

Let's imagine you have a friend who you didn't mind sacrificing for science. Suppose he decided to jump feet first into a black hole. What do you see when he crosses the event horizon?

Well, first off, you're not going to see him cross the event horizon at all because he's going to be torn to shreds by tidal forces long before-hand. He's also going be squeezed by the strong gravitational forces until he's ripped apart atom by atom. To those of you who either whispered under your breath (or more likely squealed with delight), "Spaghettification," good for you!

It's unfortunate for your friend, however, as he will most certainly not survive the ordeal. There is a glimmer of good news, however. A friend and former professor of mine, Rich Gott did an interesting calculation in which he found that regardless of the size of the black hole, it would take approximately one tenth of a second between the moment when you first felt mildly uncomfortable to the time when you are ripped atom from atom. Incidentally, if you'd like to read more about what's in store when falling in, you should check out Neil Tyson's discussion of the subject or my own.

But let's forget about this unpleasantness. Even if your friend could somehow survive the process, you couldn't see him fall in because eventually his signal is going to disappear. If your clock is running slow, this means that everything you could possibly use to measure time — including the frequency of light, will also appear to run slow. Light emitted from your friend's ship, for example, becomes longer and longer in wavelength as he approaches the event horizon until you can't see him at all. Even if you were looking at him with a radio detector, eventually his signal would be too low a frequency for you to see him.

Of course, from his perspective, it happens the other way around. Photons (and other particles) coming from the outside will appear to have a much higher frequency and much higher energies than they would otherwise. Even if he could somehow have survived the spaghettification, the high energy particles would rip him apart. This is a common theme, and one of the big puzzles of black holes. After all, if everything — keys and chairs and friends and particles lose their identity when they fall in — where does that information go?

And there would be a lot of particles, too. After all, just as you see your friend running slow, he sees you running fast. Indeed, someone dangling near the edge of a black hole would see the rest of the universe infinitely sped up. He could literally see the entire future of the universe.

Sort of. This only works if we can dangle someone just outside the black hole without them falling in. Supposing they were actually falling, they'd cross the event horizon and barely notice it (except for the dying part). From the perspective of people (and particles) inside the black hole there is no paradox. Everything falls in in a perfectly reasonable amount of time.

How reasonable? Well, I suppose I'd better answer the original question. Let's see you dropped your friend into a black hole the mass of the sun, and let him go at the same distance the earth currently is from the sun. It takes a surprisingly long time to fall in (from his perspective), a bit over 2 months. Of course, except for the last second or so, this is pretty uneventful. In fact, up until the last minute or so, your friend isn't even traveling an appreciable fraction of the speed of light and is so far outside the event horizon that you two could have a perfectly normal, nearly time-synchronized, conversation.

But after your friend falls in, and he tries to tell you how long it took, he's just SOL. Remember, nothing can escape, not even light. But of course, you knew that already.

Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (follow us on facebook or twitter.) He is an Associate Professor of Physics at Drexel University. Feel free to send email to askaphysicist@io9.com with any questions about the universe.

By MG Siegler at TechCrunch:

Did you know you can buy uranium ore on Amazon? Well you can. It’s actually been on sale for a while — BoingBoing pointed it out back in 2007. But talk of it has recently started popping up around the Internet once again this past week. Our sister site CrunchGear did a quick post pointing it out last week. Since then, a whole new batch of great customer reviews have been flowing in, as Amazon CTO Werner Vogels points out today.

Some of the negative reviews note that uranium is “bad for you.” Another says that it killed a pet gorilla. But some positive reviews mark is as a “great gift for a hostile dictator.“

As Vogels points out, the best reviews are highlighted on reviews-only page. The most helpful positive review reads: “So glad I don’t have to buy this from Libyans in parking lots at the mall anymore.” Meanwhile, the most helpful negative review reads: ”I purchased this product 4.47 Billion Years ago and when I opened it today, it was half empty.”…

[continues at TechCrunch]

Last month rumors swirled that scientists at Fermilab's Tevatron particle accelerator found the Higgs Boson particle. Those reports were untrue, but we have made significant progress towards finding the elusive particle. Why is this such an important discovery?

What is the Higgs Boson?

The Higgs is one of the five bosonic elementary particles, each of which acts as a carrier of a fundamental property of nature. The other four bosons, known as the gauge bosons, are the carriers of the fundamental forces - photons carry electromagnetism, the W and Z bosons both carry the weak nuclear force, and gluons carry the strong nuclear force. (There's also another hypothetical gauge boson, the graviton, which unsurprisingly carries the gravitational force, but that one remains undiscovered.)

Now, the Higgs Boson is the carrier of mass in the universe. It does this by helping to form a Higgs field, a quantum structure through which all the other elementary particles pass. According to the Standard Model of physics, certain particles - such as the photon - pass through the field unaffected and remain massless, while others - such as the W and Z bosons - bring part of the field with them, giving them mass. This subatomic interaction with the Higgs field is what accounts for the existence of all the mass in the universe - at least, if the theory is correct. And the only way to confirm it is to find the Higgs Boson.

Why is the Higgs Boson so difficult to find?

It's a relatively massive subatomic particle, thought to be over a hundred times the mass of a proton. The problem is that the Higgs Boson is thought to exist at extremely high energy levels - so high that only the newly built Large Hadron Collider is thought to be capable of achieving them. And then it only survives for a few seconds before decaying into other particles. Then there's the fact that, despite some rather ingenious deductions of its nature using indirect evidence, we still don't really know exactly where we should be looking for the Higgs Boson.

Today we're getting closer to knowing the Higgs Boson mass

Scientists at Fermilab, home of the world's most powerful particle accelerator that isn't the LHC, have been able to significantly narrow down the possible masses of the Higgs.

Physicists use the unit of measurement GeV/c^2, or Gigaelectronvolts divided by the speed of light squared, to measure the mass of subatomic particles. An electronvolt is the amount of energy of, you guessed it, a single electron. Because of Einstein's iconic equation E=mc^2, dividing the electronvolt by the speed of light squared makes it a unit of mass. And because most subatomic particles are much, much bigger than the tiny electron, we have to bump up the unit of measurement we use from electronvolts to gigaelectronvolts, or a billion electronvolts. Protons have a mass of about one GeV/c^2.

Here's what the Fermilab scientists found - their experiments with the Tevatron particle accelerator have conclusively ruled out a Higgs Boson with a mass between 158 and 175 GeV/c^2. Since the previously known range extends from 114 to 185 GeV/c^2, that means nearly a quarter of the possible masses have been eliminated.

Those remaining higher masses may be soon to fall as well, says physicist Dmitri Denisov:

We are close to completely ruling out a Higgs boson with a large mass. Three years ago, we would not have thought that this would be possible. With more data coming in, our experiments are beginning to be sensitive to a low-mass Higgs boson.

But where is the Higgs Boson?

If the Higgs does exist, it's running out of possible hiding spots, says University of Manchester physicist Stefan Söldner-Rembold:

"Our latest result is based on about twice as much data as a year and a half ago. As we continue to collect and analyze data, the Tevatron experiments will either exclude the Standard Model Higgs boson in the entire allowed mass range or see first hints of its existence."

It was thought until recently that the Large Hadron Collider held the only practical hope of discovering the Higgs Boson, but now it looks as though Fermilab's Tevatron accelerator is back in the hunt as well. As Fermilab spokespeople point out, creating high energy environments may actually be less important than simply creating as huge an amount of collisions as possible. We haven't found the Higgs Boson yet, but we're fast approaching the moment of truth: either we will discover it and confirm the Standard Model in the process, or we will have to reluctantly head back to the drawing board and start building a Higgs-less universe.

[Fermilab]

Segmentation, the replication of anatomical structures throughout the body, is found in many animal species. It's also a huge reason why all those species succeeded, and it comes from a single common ancestor 600 million years ago.

Specifically, segmentation refers to instances where identical anatomical units are repeated on the axis running from the top to a bottom of an animal. (So the fact that we have eyes, ears, arms, and legs that are all identical doesn't count as segmentation.) Obviously segmented species include centipedes and millipedes, in which a single structure is repeated dozens, even hundreds, of times over, but they're hardly the only examples. Anything from earthworms to humans can possess segmented features.

Three of the most basic groups of animals - arthropods (insects, arachnids, and crustaceans), vertebrates (most animals that we're familiar with), and annelid worms (sea and earthworms, basically) - all heavily make use of segmentation throughout their individual species, and yet they're very distantly related groups. Recent evidence indicates that the genes controlling segmentation are essentially the same in anthropod and worm species, indicating that there was indeed a single common ancestor, probably a worm-like creature, some 600 million years ago that proved phenomenally successful because of its ability to segment.

So why does segmentation provide such a huge evolutionary advantage, and why has it helped bring about such fantastic animal diversity? The answer might be quite simple: segmentation creates ready-made spare parts, producing duplicated anatomical units that can be repurposed as needed. If a species is under heavy pressure to fit into a changing environmental niche, it may need to develop new structures that can deal with the altered conditions. In that instance, it would be much easier to modify an existing organ than build a whole new one. Segmentation would give species a better shot at quickly adapting to new environments, which would create more pronounced changes in the species and, thus, greater diversity.

In that case, segmentation is the ultimate example of what Stephen Jay Gould dubbed an exaptation, in which a trait becomes extremely useful for reasons unrelated to its initial development. If nothing else, there's a certain evolutionary irony in that the exact duplication of body parts is responsible for why animal species all look so wildly different.

via Science

CHICAGO — Most people find the palatability of in-flight entrees an oxymoron. But even frequent fliers seldom encounter more than a few such meals per week. Astronauts, in contrast, may have to survive months in orbit dining on a really limited menu of processed foods and reconstituted beverages served from oh-so-glamorous plastic pouches. Luckily, even the International Space Station can restock its pantry several times a year because these foods are relatively perishable. Which explains the problem NASA faces in planning for really long missions — like a trip to Mars.

Astronaut foods may appear indestructible, but many crew favorites don’t retain their nutrition or palatability for even a year, notes Michele Perchonok.

She should know. Perchonok manages not only NASA’s advanced food technology program, but also the development and preparation of foods for Shuttle astronauts. At the Institute of Food Technology annual meeting, on July 20, she described NASA’s limited larder.

Foods destined for Space Shuttle missions must have a shelf life of a year, and 18 months if they’ll be deployed on the International Space Station. Of the roughly 65 foods currently available for stocking spacecraft and deemed really palatable by NASA taste panels, 10 will lose their appeal within a year — turning off-color, mushy or tasteless, she reported. By the end of five years, Perchonok says, “we’re down to seven items.”

Servings of apples from a pouch packaged recently and sterilized with pressure-assisted technology (left) and from one 2 years ago (right) show the older serving doesn't pass muster.

Moreover, she adds, “studies have shown that if the acceptability or the sensory properties degrade, so does the [food's] nutrition.” Indeed, after one year, space food exhibits notable losses of vitamin A, folic acid (an important B vitamin) and thiamine (another B vitamin that plays a role in the body’s use of carbs and certain building blocks of proteins). And nutrient losses don’t end there, Perchonok says. “Basically, after one year, we are out of vitamin C.”

Sure, NASA could supply astronauts with multivitamin pills. But that’s no panacea, Perchonok observes, since preliminary studies by NASA have shown that the potency of vitamins diminishes faster in pills than it does in foods.

Clearly, she says, these food-nutrient losses are “pretty serious.” So if NASA wants to be able to stock a spacecraft for Mars travel, “we’ve got a problem.”

Using current propulsion systems, the space agency has to plan on it taking between six and eight months to travel each way to Mars, Perchonok explains. Since Mars and Earth come close to each other only once every other year, crews “will have to stay on the surface for 18 months” before returning home, she adds.

Because glitches may arise or crews may be asked to help stock a Martian pantry for followup visitors from Earth, NASA’s goal is the development of foods that will remain both safe and appetizing for at least five years.

Canned goods have a good shelf life, but can’t be heated in microwaves — and are considerably heavier than the pouches that astronaut food is dispensed in today. And weight is a big issue. It constitutes about 15 percent of the payload of food, which is expected to weigh 10.6 U.S. tons for a crew of six heading out to Mars.

To keep the weight down, foods are eaten in their packaging. And today’s foil-lined pouches tend to work well when conventionally heat sterilized — but can delaminate when their contents are subjected to “pressure assisted” thermal sterilization, a new technique being developed by C. Patrick Dunne of the Army’s Natick, Massachusetts, Soldier R&D Center and his colleagues. Dunne’s consumers are military troops sent into the field with rations packaged as meals ready to eat, or MREs. (Dunne’s innovative pressure-assisted sterilization helped him win selection as an IFT research fellow, last year.)

MREs must have a shelf life of three years at 79 degrees Fahrenheit, Dunne reported at the IFT meeting, last week. And though the experimental pressure-assisted sterilization system he reported on is slow (able to sterilize just 10 MREs per half hour), he expects the technology eventually can be scaled up to a continuous processing of 50 pouches per minute.

The new technology’s real benefit is taste, he noted. A salmon fillet in Alfredo sauce processed with the new pressure-assisted technology not only tastes yummy, he says, but also “looks like a salmon that was poached — not like cat food.”

Taste is a particularly pivotal issue for astronaut food: Crews need to eat every bite of what they open up. Crumbs from cookies or crackers can make a mess and eventually get into someone’s eyes, Perchonok says. Wet foods that aren’t completely gobbled up will eventually go bad and stink up a spacecraft. Which can become especially nasty since astronauts don’t take out the trash every day but bring it back home with them or hold it for months until they can pack it into a craft that is destined to travel toward Earth’s surface (but actually incinerate in the atmosphere).

Bottom line, Perchonok says: NASA needs new ideas for lightweight, very air-tight flexible food packaging that can seal in freshness and sterility for at least three years. Luckily, there should be ample time to find such alternatives since travel to Mars is still many years away, Perchonok says: “Probably 2035 at the earliest.”

Images: 1) No, this is not where astronauts buy food. This is a funny photo we found on Flickr. Flickr/gordbot. 2) NASA/Perchonok.

See Also:

• Barley + Space = Space Beer!
• Koreans Develop Space-Safe Kimchi For First Astronaut
• Japanese Propose Sustainable, Albeit Smelly, Martian Diet

This entire post is a spoiler. Stop reading if you have not seen Inception, because 1) I will reveal major plot points and 2) It will make no sense.

The literary critic Frank Kermode famously argued that all successful works of art have the ability to inspire multiple interpretations. We read the classics, he said, because we believe they say more than the author meant. In other words, it is the ambiguity of art  - this ability to inspire arguments and blog posts – that makes it so interesting.

Inception, of course, is all about the ambiguity. (Those who parse the wobbles of the spinning top in the final scene have missed the entire point of the scene.) This doesn’t mean the movie is a masterpiece – I personally thought it was a smart summer blockbuster but no Dark Knight. That said, I found this interpretation, by Devin Farci, to be mostly convincing:

Every single moment of Inception is a dream. I think that in a couple of years this will become the accepted reading of the film, and differing interpretations will have to be skillfully argued to be even remotely considered. The film makes this clear, and it never holds back the truth from audiences. Some find this idea to be narratively repugnant, since they think that a movie where everything is a dream is a movie without stakes, a movie where the audience is wasting their time.

Except that this is exactly what Nolan is arguing against. The film is a metaphor for the way that Nolan as a director works, and what he’s ultimately saying is that the catharsis found in a dream is as real as the catharsis found in a movie is as real as the catharsis found in life. Inception is about making movies, and cinema is the shared dream that truly interests the director.

I believe that Inception is a dream to the point where even the dream-sharing stuff is a dream. Dom Cobb isn’t an extractor. He can’t go into other people’s dreams. He isn’t on the run from the Cobol Corporation. At one point he tells himself this, through the voice of Mal, who is a projection of his own subconscious. She asks him how real he thinks his world is, where he’s being chased across the globe by faceless corporate goons.

What I like about this interpretation of Inception is that it also makes neurological sense. From the perspective of your brain, dreaming and movie-watching are strangely parallel experiences. In fact, one could argue that sitting in a darkened theater and staring at a thriller is the closest one can get to REM sleep with open eyes. Consider this study, led by Uri Hasson and Rafael Malach at Hebrew University. The experiment was simple: they showed subjects a vintage Clint Eastwood movie (“The Good, The Bad and the Ugly”) and watched what happened to the cortex in a scanner. The scientists found that when adults were watching the film their brains showed a peculiar pattern of activity, which was virtually universal. (The title of the study is “Intersubject Synchronization of Cortical Activity During Natural Vision”.) In particular, people showed a remarkable level of similarity when it came to the activation of areas including the visual cortex (no surprise there), fusiform gyrus (it was turned on when the camera zoomed in on a face), areas related to the processing of touch (they were activated during scenes involving physical contact) and so on. Here’s the nut graf from the paper:

This strong intersubject correlation shows that, despite the completely free viewing of dynamical, complex scenes, individual brains “tick together” in synchronized spatiotemporal patterns when exposed to the same visual environment.

But it’s also worth pointing out which brain areas didn’t “tick together” in the movie theater. The most notable of these “non-synchronous” regions is the prefrontal cortex, an area associated with logic, deliberative analysis, and self-awareness. Subsequent work by Malach and colleagues has found that, when we’re engaged in intense “sensorimotor processing” – and nothing is more intense for the senses than a big moving image and Dolby surround sound – we actually inhibit these prefrontal areas. The scientists argue that such “inactivation” allows us to lose ourself in the movie:

Our results show a clear segregation between regions engaged during self-related introspective processes and cortical regions involved in sensorimotor processing. Furthermore, self-related regions were inhibited during sensorimotor processing. Thus, the common idiom ”losing yourself in the act” receives here a clear neurophysiological underpinnings.

What these experiments reveal is the essential mental process of movie-watching. It’s a process in which your senses are hyperactive and yet your self-awareness is strangely diminished. Now here’s where things get interesting, at least for this interpretation of Inception. When we fall asleep, the brain undergoes a similar pattern of global activity, as the prefrontal cortex goes quiet and the visual cortex becomes even more active than usual. But this isn’t the usual excitement of reality: this activity is semirandom and unpredictable, unbound by the constraints of sensation. (This is usually blamed on those squirts of acetylcholine, an excitatory neurotransmitter, percolating upwards from the brain stem.) It’s as if our cortex is entertaining us with surreal cinema, filling our strange nighttime narratives with whatever spare details happen to be lying around. Furthermore, the dreaming state is accompanied by an increase in activation in a wide range of “limbic” areas, those chunks of the cortex associated with the production of emotion. This is why even the most absurd nightmares cause us to wake up in a cold sweat. We care about what happens in our dreams, even when what happens makes no sense.

I’d argue that Inception tries to collapse the already thin distinction between dreaming and movie-watching. It gives us a movie in which most of the major plot points are simultaneously nonsensical – Why are we suddenly watching a thriller set in the arctic? Why are all the subconscious mercenaries such bad shots? Why don’t Cobb’s kids ever age? – and strangely compelling, just like a dream. And so we bite our fingernails even though we “know” it’s just a silly movie. Thanks to the subdued activity of the frontal lobes and the excited visual cortex, we sit in our plush chairs munching on popcorn and confuse the fake with the real. We don’t question the non-sequiturs or complain about the imperfect special effects or the shallow characters. Instead, we just sit back and watch and lose track of the time together. It’s almost as if we’re being manipulated by Dom Cobb himself, as he effortlessly travels deep into our brain to plant an idea. But this Dom Cobb – we’ll call him Christopher Nolan – doesn’t need a specially formulated sedative. He just needs a big screen.

Image: Screengrab from the movie trailer.

Read more of this story at Slashdot.

The kind of drowning you see on T.V.—think thrashy, screamy—doesn't have much in common with what real drowning looks like, according to writer and Navy/Coast Guard veteran Mario Vittone. That's because of something called the Instinctive Drowning Response, a pattern of behavior that appears to be hard-wired into humans and pops up whenever somebody feels like they're suffocating in water.

Frank Pia, Ph.D., the psychologist and lifeguard to first described the Instinctive Drowning Response explains it this way:

1. 1. Except in rare circumstances, drowning people are physiologically unable to call out for help. The respiratory system was designed for breathing. Speech is the secondary or overlaid function. Breathing must be fulfilled, before speech occurs.

2. 2. Drowning people's mouths alternately sink below and reappear above the surface of the water. The mouths of drowning people are not above the surface of the water long enough for them to exhale, inhale, and call out for help. When the drowning people's mouths are above the surface, they exhale and inhale quickly as their mouths start to sink below the surface of the water.

3. 3. Drowning people cannot wave for help. Nature instinctively forces them to extend their arms laterally and press down on the water's surface. Pressing down on the surface of the water, permits drowning people to leverage their bodies so they can lift their mouths out of the water to breathe.

4. 4. Throughout the Instinctive Drowning Response, drowning people cannot voluntarily control their arm movements. Physiologically, drowning people who are struggling on the surface of the water cannot stop drowning and perform voluntary movements such as waving for help, moving toward a rescuer, or reaching out for a piece of rescue equipment.

5. 5. From beginning to end of the Instinctive Drowning Response people's bodies remain upright in the water, with no evidence of a supporting kick. Unless rescued by a trained lifeguard, these drowning people can only struggle on the surface of the water from 20 to 60 seconds before submersion occurs.

In real life, a drowning person will be a lot more still and silent than you expect.

I believe there is a good case to be made that having lots of cameras in the hands of citizens makes us more, rather than less, safe. Here's how bad it has gotten: Not long ago, an Amtrak representative did an interview with local TV station Fox 5 in Washington, D.C.'s Union Station to explain that you don't need a permit to take pictures there--only to be approached by a security guard who ordered them to stop filming without a permit.

Legally, it's pretty much always okay to take photos in a public place as long as you're not physically interfering with traffic or police operations. As Bert Krages, an attorney who specializes in photography-related legal problems and wrote Legal Handbook for Photographers, says, "The general rule is that if something is in a public place, you're entitled to photograph it." What's more, though national-security laws are often invoked when quashing photographers, Krages explains that "the Patriot Act does not restrict photography; neither does the Homeland Security Act." But this doesn't stop people from interfering with photographers, even in settings that don't seem much like national-security zones.

Time travel isn't just science fiction: Albert Einstein's general relativity suggests it could exist. And now we might have solved the tricky matter of time paradoxes. It's all just a question of adjusting probabilities.

A certain reading of Einstein's theories argue for the existence of closed timelike curves, which are strange paths of spacetime that take anything traveling on them into the past and then back to the future. First proposed by Kurt Gödel in 1949, CTCs could theoretically exist deep within black holes or other similarly chaotic corners of the universe. Since these CTCs, however difficult they might be to access, would apparently make time travel into the past a genuine possibility, the question then becomes how to deal with the potential time paradoxes.

As always, the Grandfather Paradox, in which a time traveler kills his or her grandfather before he fathered the traveler's parent, gets the most attention here. Various workarounds have been proposed over the years - Oxford physicist David Deutsch came up with an intriguing possibility in the early 90s when he suggested that it was impossible to kill your grandfather, but it was possible to remember killing your grandfather. In some weird way, the universe would forbid you from creating a paradox, even if your memories told you that you had.

This theory, like most others put forward, relies on liberal use of the word "somehow" and the phrase "for some reason" to explain how it works. As such, it's not an ideal explanation for paradox-free time travel, and that's where a new idea by MIT's Seth Lloyd comes into the picture. He says that paradoxes might be impossible, but extremely improbable things that prevent them from happening very definitely aren't.

Let's go back to the grandfather paradox to see what he means. Let's say you shoot your grandfather at point-blank range. This theory suggests that something will happen, such as the bullet being defective or the gun misfiring, to stop your temporal assassination. This can involve some very low-probability events - for instance, the manufacturer becomes incredibly more likely to make that specific bullet improperly than any other, for the sole reason that it will be later used to kill your grandfather. It might even come down to an ultra-low-probability quantum fluctuation, in which the bullet suddenly alters course for no apparent physical reason, in order to keep the paradox at bay.

Dubbed the post-selected model, Lloyd's theory is all about trading the impossible for the improbable, which admittedly can cause some very, very unlikely things to happen right around the specific moment where the paradox would otherwise occur. As Charles Bennett of IBM's Watson Research Center explains:

"If you make a slight change in the initial conditions, the paradoxical situation won't happen. That looks like a good thing, but what it means is that if you're very near the paradoxical condition, then slight differences will be extremely amplified."

Incredibly enough, Lloyd and his team say they actually have some experimental evidence of the theory. Though they obviously can't send anything, even a subatomic particle, back in time, they can at least create certain quantum conditions that would closely resemble those experienced by a time traveler. They placed photons in these temporal-like circumstances and then tried to push them towards what were essentially paradoxical situations. The closer they got, the more frequently the experiment failed, and they argue the universe as a whole could function in much the same way when it comes to stopping paradoxes. (This is probably one of those times when you're going to need to read the original paper to understand what they were up to here, because I'll readily admit this is a bit beyond my comprehension.)

In any event, other physicists have met the new theories with great enthusiasm. Todd Burn of the University of Southern California calls it "a nice, consistent loop" and "a really interesting body of work." However, he reminds us that, for now, these aren't much more than clever thought experiments:

"I don't expect these will be tested anytime soon. These are ideas. They're fun to play with."

Looks like we need to find a closed timelike curve. Who's up for a sightseeing trip to the nearest black hole?

[arXiv]

After David Mitchell's brilliant Cloud Atlas included science fiction in its genre mash-up, we were sad to hear his new book was straight-up litfic. But turns out it's the first volume of a trilogy... that will turn very science fictional.

Mitchell was in New York to promote his new novel, The Thousand Autumns of Jacob de Zoet, and someone asked about the novel's villain, an abbott who it's hinted has been alive for 600 years. And Mitchell shocked the crowd by sharing news he hadn't even told his publisher yet: Thousand Autumns will be followed by two more novels, which will deal with the theme of immortality. And these two volumes will venture much further into the realm of speculative fiction, and will be much closer to the experimental style of Mitchell's earlier work.

Asked to comment, a Random House spokesperson told Capital New York: "We have no official comment about David's plans for his upcoming novels. He simply likes dropping hints at readings." Anyway, fingers crossed! It sounds like a great idea for a trilogy, and I have a soft spot for book series that start out more or less realistic and then venture into science fiction, ever since I devoured Doris Lessing's Children Of Violence novels all in one go. [Capital New York]