NASA bounces laser beams off of Lunar Reconnaissance Orbiter
After a decade of failed attempts, scientists successfully bounced photons off of a reflector aboard the Lunar Reconnaissance Orbiter, some 240,000 miles from Earth.
- Laser experiments can reveal precisely how far away an object is from Earth.
- For years scientists have been bouncing light off of reflectors on the lunar surface that were installed during the Apollo era, but these reflectors have become less efficient over time.
- The recent success could reveal the cause of the degradation, and also lead to new discoveries about the Moon's evolution.
NASA's Lunar Reconnaissance Orbiter (LRO) has been orbiting the Moon about a dozen times per day since 2009. The craft carries six high-tech instruments that have helped scientists create detailed maps of the moon, and learn more radiation and temperatures on the lunar surface.
But the craft also carries a rather simple instrument: a reflector the size of a paperback novel.
Over the past decade, scientists have been shooting laser beams from Earth to the LRO's reflector, some 240,000 miles away, hoping to catch a return signal. On Monday, NASA scientists and their French colleagues announced that they successfully received that return signal for the first time.
What can scientists learn by bouncing lasers off the LRO? For one, it reveals precisely how many seconds it takes for photons to travel there and back — an average of 2.5 seconds. Scientists can use this duration to tell exactly how far away an object is. Also, by measuring tiny fluctuations in the duration, they can study subtle movements of the Moon.
A close-up photograph of the laser reflecting panel deployed by Apollo 14 astronauts on the Moon in 1971.
The technology isn't quite new. During the Apollo era, astronauts installed on the lunar surface five reflecting panels, each containing at least 100 mirrors that reflect back to whichever direction it's coming from. By bouncing light off these panels, scientists have been able to learn, for example, that the Moon is drifting away from Earth at a rate of about 1.5 inches per year.
"Now that we've been collecting data for 50 years, we can see trends that we wouldn't have been able to see otherwise," Erwan Mazarico, a planetary scientist from NASA's Goddard Space Flight Center in Greenbelt, Maryland, said. "Laser-ranging science is a long game."
NASA's Lunar Reconnaissance Orbiter (LRO)
But the long game poses a problem: Over time, the panels on the Moon have become less efficient at bouncing light back to Earth. Some scientists suspect it's because dust, kicked up by micrometeorites, has settled on the surface of the panels, causing them to overheat. And if that's the case, scientists need to know for sure.
That's where the recent LRO laser experiment comes in. If scientists find discrepancies between the data sent back by the LRO reflector and those on the lunar surface, it could reveal what's causing the lunar reflectors to become less efficient. They could then account for these discrepancies in their models.
Studying the Moon's core
More precise laser experiments could also help scientists learn more about the moon's core. By measuring tiny wobbles as the Moon rotates, past laser experiments revealed that the satellite has a fluid core. But inside of that fluid could lie a solid core — one that might've helped to generate the Moon's now-extinct magnetic field.
However, confirming that hypothesis will require more precise measurements — and the continued success of laser experiments involving the LRO, or reflecting panels installed on the Moon during future missions.
"The precision of this one measurement has the potential to refine our understanding of gravity and the evolution of the solar system," Xiaoli Sun, a Goddard planetary scientist who helped design LRO's reflector, told NASA.
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The idea of 'absolute time' is an illusion. Physics and subjective experience reveal why.
- Since Einstein posited his theory of general relativity, we've understood that gravity has the power to warp space and time.
- This "time dilation" effect occurs even at small levels.
- Outside of physics, we experience distortions in how we perceive time — sometimes to a startling extent.
Physics without time<p>In his book "The Order of Time," Italian theoretical physicist Carlo Rovelli suggests that our perception of time — our sense that time is forever flowing forward — could be a highly subjective projection. After all, when you look at reality on the smallest scale (using equations of quantum gravity, at least), time vanishes.</p><p style="margin-left: 20px;">"If I observe the microscopic state of things," writes Rovelli, "then the difference between past and future vanishes … in the elementary grammar of things, there is no distinction between 'cause' and 'effect.'"</p><p>So, why do we perceive time as flowing <em>forward</em>? Rovelli notes that, although time disappears on extremely small scales, we still obviously perceive events occur sequentially in reality. In other words, we observe entropy: Order changing into disorder; an egg cracking and getting scrambled.</p><p>Rovelli says key aspects of time are described by the second law of thermodynamics, which states that heat always passes from hot to cold. This is a one-way street. For example, an ice cube melts into a hot cup of tea, never the reverse. Rovelli suggests a similar phenomenon might explain why we're only able to perceive the past and not the future.</p><p style="margin-left: 20px;">"Any time the future is definitely distinguishable from the past, there is something like heat involved," Rovelli wrote for the <a href="https://www.ft.com/content/ce6ef7b8-429a-11e8-93cf-67ac3a6482fd" target="_blank"><em>Financial Times</em></a>. "Thermodynamics traces the direction of time to something called the 'low entropy of the past', a still mysterious phenomenon on which discussions rage."</p>
The strange subjectivity of time<p>Time moves differently atop a mountain than it does on a beach. But you don't need to travel any distance at all to experience strange distortions in your perception of time. In moments of life-or-death fear, for example, your brain would release large amounts of adrenaline, which would speed up your internal clock, causing you to perceive the outside world as moving slowly.<br></p><p>Another common distortion occurs when we focus our attention in particular ways.</p><p style="margin-left: 20px;">"If you're thinking about how time is <em>currently</em> passing by, the biggest factor influencing your time perception is attention," Aaron Sackett, associate professor of marketing at the University of St. Thomas, told <em><a href="https://gizmodo.com/why-does-time-slow-down-and-speed-up-1840133782" target="_blank">Gizmodo</a></em>.<em> "</em>The more attention you give to the passage of time, the slower it tends to go. As you become distracted from time's passing—perhaps by something interesting happening nearby, or a good daydreaming session—you're more likely to lose track of time, giving you the feeling that it's slipping by more quickly than before. "Time flies when you're having fun," they say, but really, it's more like "time flies when you're thinking about other things." That's why time will also often fly by when you're definitely <em>not</em> having fun—like when you're having a heated argument or are terrified about an upcoming presentation."</p><p>One of the most mysterious ways people experience time-perception distortions is through psychedelic drugs. In an interview with <a href="https://www.theguardian.com/books/2018/apr/14/carlo-rovelli-exploding-commonsense-notions-order-of-time-interview" target="_blank"><em>The Guardian</em></a>, Rovelli described a time he experimented with LSD.</p><p style="margin-left: 20px;">"It was an extraordinarily strong experience that touched me also intellectually," he said. "Among the strange phenomena was the sense of time stopping. Things were happening in my mind but the clock was not going ahead; the flow of time was not passing any more. It was a total subversion of the structure of reality."<br></p><p>It seems few scientists or philosophers believe time is completely an illusion.</p><p style="margin-left: 20px;">"What we call <em>time</em> is a rich, stratified concept; it has many layers," Rovelli told <em><a href="https://physicstoday.scitation.org/do/10.1063/PT.6.4.20190219a/full/" target="_blank">Physics Today</a>.</em> "Some of time's layers apply only at limited scales within limited domains. This does not make them illusions."</p>What <em>is</em> an illusion is the idea that time flows at an absolute rate. The river of time might be flowing forever forward, but it moves at different speeds, between people, and even within your own mind.
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- The theory claims that natural selection produces both atoms and "observers".
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