Posts Tagged ‘time’
"Time Cells" In the Brain Keep Track of Events, Firing As Time Goes By
In a new study involving rats, researchers at Boston University monitored neurons in the hippocampus, the center of memory and learning. Howard Eichenbaum and colleagues trained rats to perform a three-part task, which included a delay in the middle, . They learned to associate an object with an scent (a ball with oregano, for instance), and then they were presented with the object. The rats entered a separate chamber for 10 seconds, after which a doorway led them to a flowerpot full of scented sand. If the scent was the same as the object they’d been shown, the rats would dig for a food reward. The 10-second delay was at the heart of the study.
Eichenbaum et. al surgically implanted electrodes in the rats’ hippocampus, and monitored signals from 300 distinct neurons as the rats completed their work. During the delay, the researchers watched about a third of the cells continue to fire in a cascading pattern — suggesting the neurons were keeping track as time went by.
The hippocampus is known to have “place cells,” which keep track of locations and recalibrate when spatial cues are altered, the . In the same way, the time neurons continued to fire when the researchers lengthened the delay, “retiming” when temporal cues are altered. The hippocampus is considered the brain’s memory center, so it makes sense that there would be some mechanism for monitoring the variables that memory depends upon.
The neurons kept track of time in varying tests, but their firing patterns and the specific groups involved changed slightly, depending on which object was presented. This shows the neurons disambiguate different events, the researchers say: “(They) compose unique, temporally organized representations of specific experiences.” The research is reported in the journal Neuron.
So next time you say you’ve lost track of time, remember that you really haven’t — your biological clock has been ticking all the while.
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Space Boat: A Nautical Mission to an Alien Sea
In May, the TiME project received a $3-million development contract from NASA. If the space agency green-lights the mission, the capsule will lift off in 2016. By 2023, TiME will be about 800 million miles away in Titan’s north-polar region, home to its biggest lakes and seas. The capsule will take photographs, collect meteorological data, measure depth, and analyze samples. TiME will have no means of propulsion once it is on Titan, so it will float, carried by breezes across the sea’s surface. Then, by the mid-2020s, it will enter a decade-long winter of darkness as the moon’s orbit takes it to the dark side of Saturn, away from the sun and communication. It won’t have a line of sight to Earth to beam back more data until 2035.
Methane clouds drift in Titan’s smoggy orange skies, sometimes releasing hydrocarbon raindrops, which replenish the seas and sculpt the landscape the way water does on Earth. But Titan’s seas probably don’t contain life. “Life as we know it requires liquid water, and Titan’s surface is far too cold for this,” says Ralph Lorenz, a physicist at Johns Hopkins University who is working on TiME. “Its seas can tell us about how molecules organize and evolve, and how life may arise more generally.” TiME’s principal investigator, planetary geologist Ellen Stofan, wonders about the waves: “Are there hazes, sea spray? Is the liquid clear or cloudy? Is there scum floating on the surface? With Titan’s seas, there are endless questions.”
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Splash Landing on Titan's Sea
DROP IN
After a seven-year journey, including gravitational slingshots around Earth and Jupiter, the Titan Mare Explorer (TiME) passes through Titan’s thick nitrogen and methane atmosphere, protected by its heat shield.
DRIFT DOWN
At an altitude of about 100 miles, TiME deploys its parachute. During the remainder of its roughly two-hour descent, the probe’s camera snaps pictures while a thermometer and barometer record meteorological data.
FLOAT AROUND
After landing in the ethane and methane sea Ligeia Mare, TiME’s mass spectrometer collects a sample to analyze its chemical composition, while its sonar measures the sea’s depth. Without its own means of propulsion, the capsule will bob along in the gentle wind and currents. It might even experience alien rains or wash up on an extraterrestrial beach.
Cosmic Contenders
Next year, NASA will give full funding to one of three missions: a Mars probe to study how the planet formed, a “comet hopper” that will repeatedly land on a near-Earth comet, or a capsule to float on a sea on Titan, one of Saturn’s 53 known moons.
The Jet Propulsion Laboratory’s Geophysical Monitoring Station would study Mars’s interior structure and composition to better understand its geological history.
The Comet Hopper, conceived at NASA’s Goddard Space Flight Center, would land several times on a comet, studying how the icy body evolves as it warms up while approaching the sun.
New Strides Toward Better Clocks, Accurate to One Second in 32 Billion Years
Hyper-sensitive atomic clocks are used for GPS navigation, communications between space probes and Earth, and quantum computing studies, among other uses. Timekeepers keep building clocks that are , meaning it takes them longer and longer to “lose” a second through small uncertainties.
Right now, the most accurate atomic clock is an aluminum quantum-logic clock, which is based on atomic energy levels in a positively charged aluminum ion. Its uncertainty rate is such that it will be off by one second 3.7 billion years from now.
Atomic clocks define seconds based on the oscillations between the nucleus of an atom and its orbiting electrons. If anything affects those electrons, their oscillation rates could change, making the clock less accurate. Researchers at the Joint Quantum Institute found that heat radiation can do just this — even when atoms are completely isolated and protected, as is the case in atomic clocks.
Any object at any temperature releases some warmth, whether it’s the sun, yourself or a perfectly radiant object called a “black body.” Temperature is tied to the speed and distance at which electrons orbit the nuclei of atoms — in very general terms, colder objects move more slowly and warmer objects move faster. Black body radiation enlarges the size of the electron clouds of an atom, which affects their oscillations. This BBR effect is one part in a hundred trillion, but when you want something to stay the same for 32 billion years, that adds up to a lot.
Now that scientists have figured this out, they can calculate how much the aluminum ions’ energy levels will change because of black-body radiation.
Current clocks are actually more inaccurate than the changes induced by BBR effect, according to a news release from NIST. But the next generation of atomic clocks will have lower uncertainties, so knowledge of the BBR shift will make them even better.
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Where International Standard Units Come From, Part Three: The Candela
This week, the origin and continued preservation of five of our favorite standard units of measure
Most of the seven units were pretty consistent from the beginning. Scientists agreed on what each unit meant and were confident that people in different countries meant the same thing. The big exception to this consistency was the candela—the unit for the luminosity of light.
As the name implies, candelas were based on the burning of candles, and scientists tried as hard as they could to define a standard candle. The English standard, for instance, called for candles made of spermaceti (what sailors dug out of the skulls of whales in Moby Dick) weighing a sixth of a pound and burning at a precise rate per hour, presumably in a windless and otherwise pitch-black room. But the French had their own formula for candles, as did the Germans, and regardless, none of the candles satisfied scientists. Because, as anyone who ever tried to study by candlelight knows, the output is a little bipolar even in the best of circumstances.
Mr. Edison’s incandescent bulbs improved the situation, and by 1909, many countries had adopted a standard for luminescence based on a carbon filament. But as metrologists began to scrutinize filaments, they found them wanting, too, since even the output of a filament flickers too much for their liking. So they turned instead to blackbody radiation—the radiative heat emitted from all warm bodies. It’s the property that makes hot metal glow red or white, and it’s why infrared goggles can spy live bodies or illicit home-gardening operations through walls.
Every substance emits slightly different radiation at different temperatures, though, so scientists had to pick one element as the standard. And, showing rather refined tastes, they picked platinum, which gave a nice, steady glow. Of course, being metrologists, they had to specify a little bit more than that. The definition of a candela became the amount of light given off by a crucible of molten platinum as it froze (at 3,200° F) from liquid to solid and measured “in the perpendicular direction [from] a surface of 1/600,000 square meters ... under a pressure of 101,325 Newtons per square meter.”
Still not satisfied—it proved harder than expected to measure consistently the light emitted by molten platinum—scientists re-redefined the candela in 1979 and got rid of the platinum altogether. A candela has since become “the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.” Which is a heck of a lot more precise than trying to guess the output of a candle, but certainly lacks the charm.
Tune in tomorrow for the next installment of our exploration of the standards that make science tick. The series is written by Sam Kean, author of —a collection of funny and peculiar stories hidden throughout the periodic table.
Where International Standard Units Come From, Part Two: The Second
This week, the origin and continued preservation of five of our favorite standard units of measure
The definition of the second used to be 1/86,400th of one spin of the earth around its axis (less formally, the number of seconds in one day). But a few pesky facts made that standard inconvenient. The length of a day varies with every trip around the sun because of the sloshing of ocean tides, which drag and slow the earth’s rotation. And metrologists (measurement scientists) didn’t want to tie a supposedly universal unit of time to the transit of a small rock around a mediocre star.
To rectify this, scientists turned to the element cesium. More specifically, they turned to cesium’s lone electron. Like all the entries in its column on the periodic table, cesium has one more electron than the full set it really desires. This electron—which resides at a higher energy level than other electrons and is therefore more exposed—normally zooms around the cesium nucleus on a specific orbit. But if light strikes the electron, it can jump to an even higher orbit.
Now, depending on whether its “spin” (an inborn property) is up or down, an electron can jump to a slightly higher or lower orbit. If the original jump was like moving up an octave from G to G, this jump is from G to G-sharp or to G-flat. These slightly different levels are known as the fine structure. And if you measure things even more precisely and take even more factors into account (like the electron’s charge and nucleus’s magnetic field), you can observe an electron jumping between levels separated by even smaller amounts—like a musical difference not of a halftone but of a quarter-tone, or even an eighth-tone. This is known as the hyperfine structure.
Metrologists exploited those hyperfine differences to create the first atomic clocks with cesium-133. Inside these “beam clocks,” a gas of cesium atoms is gathered into a chamber with a pressure of about one-trillionth of normal atmospheric pressure and excited by an intense maser (a microwave laser). This strumming with the maser excites the cesium electrons and causes them to jump to a certain hyperfine level. The key point is that the electron cannot stay excited for long, so it soon drops back down to another hyperfine level. And when it does, it emits light. This cycle of jumping up and down repeats itself over and over, and each cycle is perfectly elastic and therefore takes the same amount of time. The precision of the maser ensures that all the cesium atoms are in synch, so the atomic clock can measure time simply by counting emitted photons.
Cesium proved convenient as the mainspring for atomic clocks because the solitude of its electron means that scientists don’t have to worry (as they might with other elements) about other electrons jumping up and down and shooting their own photons off. Cesium’s heavy, lumbering atoms are fat, easy targets for the maser as well. But even in plodding cesium, the outer electron is a quick bugger. Instead of a few dozen or few thousand times per second, it performs 9,192,631,770 back-and-forths every one-Mississippi.
Scientists picked that ungainly number instead of cutting themselves off at 9,192,631,769 or letting things drag on until 9,192,631,771 because it matched their best guess for a second back in 1955, when they built the first cesium clock. Regardless, 9,192,631,770 is now fixed as the definition. And nowadays, metrologists rely not on beam clocks but cesium “fountain clocks,” which operate on the same basic physics but at much lower temperatures, barely above absolute zero. Some of these clocks are accurate to within one second every 30 million years.
But while the cesium standard has profited science by ensuring precision and accuracy worldwide, humanity has undeniably lost something. Since before even the ancient Egyptians and Babylonians, human beings used the stars and seasons to track time and record their most important moments. Cesium severed that link with the heavens, effaced it just as surely as urban streetlamps blot out constellations. However fine an element, cesium certainly lacks the mythic feeling of the moon or sun.
Tune in tomorrow for the next installment of our exploration of the standards that make science tick. The series is written by Sam Kean, author of —a collection of funny and peculiar stories hidden throughout the periodic table.
The End of Time is Nigh (in a Cosmic Sense, Anyhow)
Why? Physics tells us that the universe has been expanding since the Big Bang some 13 billino years ago and that it is still expanding to this day (Did you just feel that? That was the universe expanding around you). But there’s a problem: if the universe expands infinitely, then every conceivable event – no matter how un-probabilistic – will occur. In an infinite universe, in fact, the most improbable event will happen an infinite number of times.
The idea of being unable to determine the probability of anything, as would be the case under such circumstances, pretty much pulls the rug from under modern physics, rendering them meaningless. In other words, even though physics tell us that the universe is eternally expanding, physics itself is untenable in such a universe.
So in order for physics to have meaning, time must end at some point. According to the mathematical crunching of UC Berkeley’s Raphael Bousso and colleagues, there’s a 50 percent chance of that happening in the next 3.7 billion years. That’s Biblical in a sense, if only because the Earth and Sun will likely still be around when the end of time comes. But, says Bousso, it’s unlikely we’ll see the end event coming before it dismantles life, the universe, and everything.
Of course, this entire analysis concerning the relevance of physics assumes physics are relevant, which is a philosophical rather than a scientific argument, as . Perhaps the laws of physics just work and we don’t need to understand – and possibly can’t even fathom – why they work they way they do. If that’s the case, the argument that time must end in the first place is flawed.
Put another way, live every day like time might cease some 3.7 billion years from now. Because maybe, just maybe, it will.
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German Scientists Measure How Fast an Electron Jumps, the Shortest Time Interval Ever Measured
During photoemission – the expulsion of electrons from an atom by bombarding them with high-energy light – it’s always been assumed that there is no delay between the photons’ impact and the breaking loose of the target electron. But a group of German researchers in collaboration with Greek, Austrian, and Saudi Arabian colleagues decided to challenge that assumption with extremely sensitive time measurement tech.
The team bombarded atoms of neon gas with near-infrared laser light in 10-15 second pulses and ultraviolet pulses of far shorter durations of just 180 attoseconds (remember, an attosecond is one billionth of one billionth of one second). The near-IR light served as an attosecond chronograph, measuring the time of UV impact and the time the electrons exited their orbits.
Their findings turned up two interesting results. For one, they found that electron ejection is not a “time zero” action as once presumed, but that excited electrons hesitate very, very briefly before leaving the atom. But perhaps more interesting, they found that electrons from different orbitals behaved differently, leaving the atom at slightly different times even though they were impacted simultaneously.
The researchers are not exactly sure why this is, but it likely has to do with some small, overlooked influence that electrons exert over one another that is different that the forces exerted on electrons by their nuclei. If that’s the case, the tiny time lag could have big consequences for physics, a discipline ruled by the interactions between atoms and the behavior of electrons. Until they figure all that out, they can at least take pride in their 20-attosecond record for the shortest time interval ever directly measured.
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