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November 2012 Archives

The Earth Goes on Tilt

It's late November, and you find yourself driving (or walking, bicycling, subway-ing, Segway-ing...) home from work immersed in twilight. By the time you get home to see your loved ones, have dinner or do anything else, night has already fallen. We, in the northern hemisphere at least, are nearing the darkest time of the year. This is a bummer for many, and debilitating for some.

Why do the seasons change this way? It seems like a simple question, but do you really know for sure? To answer, we have to know a bit about how the earth's orbit works. You probably know that the Earth is tilted in regards to the sun. How does this produce seasons though?

First off, the Earth makes a complete circle around the Sun every 365.246 days. The path itself is elliptical, but it does repeat with high precision, despite gradually slowing down. The earth, on this path however, does not sit rotating exactly vertically relative to the sun. Instead, it is tilted by about 23.4 degrees from vertical, rotating like a top that is not standing up straight. The tilt of the Earth causes the sun to shine more directly on whichever part is angled towards the sun and to shine only weakly on the part tilted away:


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If the Earth were rotating vertically, we would have no seasons! The average day would be the same over the entire Earth at all times of the year, every year. It would be as though every day were late September or late March. This might not seem so dramatic, but consider that most of the plants on Earth are specifically adapted to have life cycles that vary with the time of year. Many animals, too, are adapted this way. Mammals hybernate and mate by the seasons; humans plant and grow and harvest by them.

Some planets are actually tilted at much stranger angles to the sun. Uranus rotates north to south, instead of east to west, and the planet has incredibly harsh 20 year long winters and summers! Venus rotates in the opposite direction to Earth, but almost vertically. Seasonal variation is nearly non-existent.

A few other facts about the Earth's orbit complicate slightly our seasons. First, they are shifted slightly by the elliptical nature of the earth's orbit. So close, though, is our orbit to a circle, that this only changes the amount of sunlight by about 7%, making northern seasons slightly milder than southern seasons.

Over the course of about 41,000 years, the tilt angle of the Earth goes through a cycle of changing by a few degrees. (The full range is roughly 21 to 25 degrees). Think of the top again; this is like the top wobbling. Warmer summers of longer days and colder winters of shorter days occur at one extreme and milder seasons with less variation in daylight hours at the other. This cycle is so gradual and small that it is almost undetectable.

The orbit of the earth is a very complicated thing. So is human brain chemistry. But, if you are feeling a bit cramped for time and down, the simple answer might be the former influencing the latter. Fortunately, in three weeks or so, the days lengthen once more!

November 2012 Archives

Getting Runner's High

Picture a dedicated distance runner. You probably wouldn't imagine him or her going behind the garage to get high. Runners, though, get high more than almost anyone else. They are like junkies, jonesing for a chemical rush. However, their fix is delivered not by huffing some illegal chemical, but by the body itself through exercise.

The idea of runner's high started many years ago. Just ask anyone who runs regularly and they will likely tell you how good they feel after a long jog. Studies, such as this one from 2008 making use of positron emission tomography (PET) scanning technology, have allowed scientists to conclude that this is likely a scientifically verifiable phenomenon.


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He might not have inhaled, but President Clinton certainly got high (AP)

The key is opioids. These chemicals take their name from the opium poppy, where they were first discovered. While this name conjures images of illegal narcotics, the body naturally produces these substances, which include such innocuous neurotransmitters as endorphins. The body also contains many receptors that mediate the effects of opioids. When an opioid receptor captures one of these chemicals, it can trigger reactions that include pain relief, sedation and even euphoria.

In this 2008 study, several runners were PET scanned before an endurance run (roughly 10 miles at a moderate pace). After the run, they were scanned again, and this time, opioid receptors in their brains were significantly less likely to be open for picking up an opioid chemical. This means that the receptor has already captured an opioid.

In the second part of the experiment, greater uptake at these receptors was correlated with reported feelings of "euphoria". Putting the pieces together: sustained running causes release of opioids in the brain, which, when captured by receptors, cause an increase in reported feelings of euphoria.

Next time you are feeling down, try going for a run. It has long been common sense that this will make you feel better. Now it is "scientific sense" too.

November 2012 Archives

String Theory Now on Life Support

There are plenty of reasons not to like string theory. Philosophical and logical arguments against the theory have long been apparent. Strong scientific evidence is increasingly joining them. The discovery of the Higgs boson exactly where the Standard Model says it should be last summer at the LHC was a first blow. Now, more evidence is coming in.

This week, LHCb (LHC-B), one of the many huge experiments along the LHC ring, reported a major result. The result itself is very technical, but its implications are general: big trouble for physics theories that involve supersymmetry (SUSY), string theory and many similar theories included. If SUSY is discarded, string theory goes right out with it.

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How is the new physics coming out of the LHC closing the window of validity for string theory?

When the LHC smashes its particles together at world record energies, a shower of debris (new particles) is created. Large detectors surround the circumference of the ring itself, like insulation built around a pipe. When a collision happens inside the pipe, the resulting particles get caught by the detector wrapped around it. The detector pours out a massive amount of data, telling where every particle goes.

The data itself is enough to fill a modern computer hard drive every second. To sift through all this information requires a tremendous amount of computer processing. What physicists ultimately want to know is the mass and trajectory of each particle that was created in the smash-up. They try to recreate the entire scene, from the collision through all the debris flying into the detector.

In this case, physicists were looking for a particular particle called the "strange B meson" (Bs) to break into two more particles, called mu particles (μ+ and μ-). These strange B mesons usually only live for roughly just less than one trillionth of a second before breaking apart (called decaying). Here's where the Standard Model (SM) vs. Supersymmetry (SUSY) argument comes in.

If the SM is correct, about once in every 280 million times the Bs decays, the two μ particles should be detected. The number found by the LHC? Roughly once every 310 million times, with some uncertainty. Very close agreement, especially for such a rare and hard to detect decay.

So what does this say about string theory? If supersymmetry is correct, then this decay should occur far more often. In fact, by establishing this number, nearly all reasonable string theory models have failed in a testable prediction. (Unfortunately this prediction is so technical that it would require its own entire essay to explain.)

SUSY supporters had put forth a number for this prediction. Then, as noted by Peter Woit, they changed it when experimental data ruled them out. Then they were shown wrong and changed it again. Now the third prediction has proven wrong. Soon, we will reach a point where further changes in prediction will leave SUSY, and by extension string theory, practically unobservable to us, thus moving them out of the realm of science. String theory is truly being backed into a corner.

Being a popular and respected field, theoretical SUSY and string research will continue on. If more news like this keeps coming out, however, funding may begin to wane in the coming years. Perhaps this will spawn a fresh theory, both more testable and more accountable.

Image: CERN

November 2012 Archives

Seeding Life: Asteroids Giveth and Taketh Away

Frequently, a formidably large chunk of space rock zooms past Earth, unseen until it is far too late. These random agents of chaos are the first thing that comes to mind when you think of asteroids. Surprisingly, however, most asteroids spend their time loafing around in a huge ring about the sun. 

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This ring, or belt, may actually be one of the key reasons that we currently exist, according to speculation published by astronomers this week.

When the solar system was young, individual planets formed out of something called an accretion disc. This is a flat circle of material surrounding a new star. Nearly all of the matter that will eventually form a solar system is sucked in to the center to form the star, but a little bit of it is left further away. Within this swirling mass of gas and dust, small chunks form. These chunks gradually accrete and begin to draw other chunks in through gravity, forming planets.

A particularly lucky chunk eventually grew into Jupiter. Jupiter is about two and a half times as massive as everything else in the solar system (except the sun) put together. This mass allows it to dominate the space around it. The part of the disc just inside of Jupiter tried to form planets too, but whenever Jupiter passed, its pull caused turbulence amongst the forming planets, smashing them together and shattering them into small pieces. Some of these pieces fly away into space never to be seen again, but many remain in orbit in a giant rotating cloud.

There are enough asteroids that all added together, they would weigh roughly 4% as much as the moon. However, they occupy such a large region of space that they are spread out very thinly. Spacecraft such as Pioneer and Voyager have easily traveled entirely through without hitting anything; the belt is not thick like a Star Wars movie depicts.

Having so many asteroids floating around should be bad for us living things, right? According to the speculation of those astronomers, this may not be the case. Some asteroids carry large amounts of water ice and organic compounds (from which life as we know it is primarily built). These asteroids are periodically tossed out of the belt by Jupiter's passing or colliding with one another. Many of these smash into a young, unsettled planet such as earth, depositing cargoes of water and chemicals which may help set the stage for life.

Further, occasional (we hope!) catastrophic asteroid collisions may help "shake things up", keeping one set of life forms from perpetually dominating and allowing evolution to stagnate. Bad for the dinosaurs, good for mammals like us.

Image: NASA

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