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When are photons, photons?

You’ve probably heard the word “photon” before, as in “photon torpedoes” popularized in the original Star Trek. “Photons” are what physicists call “light” or electromagnetic radiation, when it displays it’s particle-like behavior.

Think of the light from the Sun. The Sun (~6000 K) and emits light over a large range of frequencies. In space, satellites measure x-ray emissions, on Earth our eyes are sensitive to optical radiation and a radio-telescope like SETI Institute’s ATA see’s the Sun as an extremely bright object — the Sun emits radio waves too. We don’t often think about radio waves or x-rays as being made of the same stuff as ordinary light, but that is all there is to it. And everything from x-rays to radio waves can be described as if it were made up of particle photons in the quantum theory of light.

Photons are very special particles. Elementary particles like electrons, protons, neutrons or composite quasi-particles like atoms, molecules, ball-bearings, planets, stars, etc. share one important feature; they have mass. Rest mass. That is, if you stop an electron and weigh it, you’ll discover it has a measurable mass.

Photons in vacuum, lets call them “pure” photons, have no rest mass. If you stop a photon and weigh it… wait, you can’t stop a photon. Pure photons always move at the speed of light (duh!). If you subtract kinetic energy from a pure photon in an attempt to slow it down, it does not slow down, it just oscillates more slowly.

This is all very interesting, but how often do we come across “pure” photons in our universe? NEVER! Why? Because nowhere in the universe is there a perfect vacuum. Matter is dispersed everywhere. In the outermost reaches of space even in the vast gaps between galaxies, there is a tiny density of Hydrogen gas, possibly less than 1 atom per cubic centimeter. Even this much material is enough to disturb the properties of “pure” photons.

When a photon interacts with matter, two things happen : 1) it picks up “rest mass” and 2) it slows down. This happens because regular matter is made up of charged particles like electrons and protons (one each in a Hydrogen atom). When the electromagnetic wave passes an atom, it causes the lighter electrons to “jiggle” around the heavier protons, jiggling with the same frequency as the incident light wave. Momentarily, some of the photon energy is bound up in electron motion, but after a short time the electron releases the energy once more at the same frequency but with a small time lag. Matter imposes a “drag” on the photons, slowing them down. The same is true if light is passing through the space between stars, Earth’s atmosphere, a glass lens, a copper wire, and so forth.

How can photons, or light as we know it, travel slower than the speed of light? This sounds like a paradox. The answer is that photons passing through matter are no longer (pure) photons. The photons pick up a little bit of the material properties and the material picks up a little bit of the photon properties. Physicists say that the photons and oscillating electrons form a “quasiparticle” that travels nearly at the speed of light and carries a tiny bit of rest mass.

Now for the fun part. First of all, we’ve already discovered that everyday light really does not travel at the speed of light.

It is not possible to transmit light waves of arbitrarily low frequency. Suppose you go to a spot halfway between the Earth and Alpha-Centauri. You set up a large antenna and connect a radio transmitter that generates frequencies of, say, 0.001 Hz. That is one oscillation every 15 minutes, but never mind, there’s nothing to stop you from trying. What happens? Well, no waves are emitted. How can this be?

Because of the small amount of gas, especially ionized gas, between stars in our galaxy, the quasiparticle photon rest mass is equal to that of a pure photon with frequency >0.001 Hz. In a sense, you can try to generate waves with lower frequencies, but the surrounding space will “reject” these photons and they eventually re-enter the transmitter, cancelling out your attempted radiation. Photons with such low frequencies do not propagate. If you turn up your transmitter to oscillate just fast enough to exceed the rest-mass threshold of photons, then you will observe those photons travel very slowly, much slower than the speed of light in vacuum.

We can even imagine, within the boundaries of real physics, the concept of “slow glass,” invented by science fiction writer Bob Shaw in a story in Analog (1966) called “The light of other days.” In this story, a special kind of glass is invented such that optical photons take a long time, perhaps 10 years, to travel through a 1″ sheet of glass. Science fiction? Yes! But slow glass is possible.

Nothing, not even the light that provides us with sight every day, can travel as fast or faster than the speed of light in vacuum. But anything, including light, can be made to travel as slow as we like. This is the flip side of Einstein’s speed limit and allows for some weird possibilities. Perhaps we’ll explore more of these possibilities in a later blog.

Swirly rocks

ESP_036436_2645_1.0A piece of Mars: Never mind the 4 m (13 ft) boulders that have fallen downslope, or the rippled sandy surfaces here. Look at those bright swirls in the ground. Those are the former interiors of sand dunes, which were trapped and incorporated into the bedrock (like dinosaur bones, but without so much rawr). The wind has been blowing sand around on Mars for a long, long time. (HiRISE ESP_036436_2645, NASA/JPL/Univ. of Arizona)

Whither the wind

ESP_036393_2560_1.0xA piece of Mars: Which way did the wind blow here? You can tell by looking at the dune and its ripples. The slip face (the avalanching slope of the dune) faces downwind, so the strongest wind here mainly blows toward the upper left. But that’s not the whole story, because, like on Earth, martian winds are always shifting. Recent avalanching and some ripples on the slip face show that the most recent wind blew toward the top of the frame. The dune is 267×110 m (876×361 ft). (HiRISE ESP_036393_2650, NASA/JPL/Univ. of Arizona)

Des mondes similaires au nôtre cachés dans des centaines d’exoplanètes ? SETI PR en Francais

Communiqué de presse de l’Institut SETI et de CASCA
Monday, June 09 2014 – 12:15pm, PDT

Mountain View, CA -
Cette année a été intense pour les chasseurs d’exoplanètes, ces planètes autour d’autres étoiles. Une équipe d’astronomes de l’Institut SETI et du centre de recherche de la NASA Ames a découvert 715 nouvelles exoplanètes enfouies dans les données du télescope spatial Kepler. Ces nouveaux mondes qui tournent autour de 305 étoiles différentes, constituent des systèmes planétaires multiples, similaires a notre système solaire, lui-même constitué de huit planètes. L’annonce de cette découverte a été suivie par une nouvelle encore plus importante dans le monde de l’astronomie : la même équipe a annoncé la découverte de Kepler 186f, une planète de la même taille que la Terre qui tourne autour de son étoile dans la zone dite habitable. Cette decouverte constitue une étape essentielle vers la détermination de l’existence de planètes de type Terre dans la Voie Lactée.

Une vue artistique décrivant les systèmes planétaires découverts par le télescope spatial Kepler. Crédit: NASA

Une vue artistique décrivant les systèmes planétaires découverts par le télescope spatial Kepler. Crédit: NASA


The always-changing landscape

ESP_035558_1830_0.831xA piece of Mars: Over time, windblown sand can wear down a surface. This isn’t so common on Earth, where water, ice, and life are more likely to change the landscape, but it’s typical of many places on Mars. Here, we see one moment in time, where neverending sand (blowing from bottom right to top left) creates a pattern on the surface and scours a hole around a resistant rock. (ESP_035558_1830, NASA/JPL/Univ. of Arizona)

First Observing Run

I recently returned from the third commissioning run for the Gemini Planet Imager. Up until now, I had never been observing. I had never even seen the Milky Way. And as far as firsts go, I hit the jackpot — my first observing run at Gemini South, commissioning GPI.

Pointing to Gemini

Up on the mountain for 6 days, sunset to sunrise we busily work to gather light from the sky into GPI. But everyone takes a few moments during the night to step outside and look at the sky with their own eyes — no one misses the opportunity.

I’ve always lived in a city where only a handful of stars are visible at best. While I was always fascinated with the universe (especially thanks to fantastic Hubble press releases) I guess astronomy never felt that accessible to an urbanite like me, for whatever reason. By some stroke of luck, this May I found myself surrounded by mountains and stars, sitting in the control room of a massive telescope filled with technologically impressive instruments and equally impressive brains grasping for answers from the sky. I am definitely fulfilling a childhood dream.

While taking data in the control room, the tone is mostly quiet concentration and focus. But when we get to see the 8-meter move, everyone watches in excitement and awe. Luckily, I hit record.

This has been one of the most interesting and exciting trips I’ve taken. I have an added appreciation for the Gemini Planet Imager and its operation after being on the front line. GPI is not only a platform for great science but an amazing resource and opportunity for the students that are part the commissioning workforce.


ESP_031944_1790_0.38xA piece of Mars: This is a bit of the flank of Arsia Mons, one of Mars’ great volcanoes. The big changes in topography are ancient relics of erosion by lava and great tectonic pulling. What I like is that the scene (1.58×1.18 km, or 0.98×0.74 mi) is covered in bright dust (looks a bit like snow here, doesn’t it?). And that dust has been eroded by wind channeled through the topography. So here we see signs of flow, both from ancient lava and from more recent wind. (HiRISE ESP_031944_1790, NASA/JPL/Univ. of Arizona)

54 years of space exploration: an updated map that you must see

National Geographic asked 5W Infographics to update its 50 Years of Exploration graphic, a classic that I use often in my talks to illustrate our space exploration program and its focus on the inner part of the solar system.

The updated version, renamed “Cosmic Journey“, is spectacular, better organized and easier to follow than its predecessor. It has been updated to include new missions sent over the past 4 years. The new color code includes the paths of failed, as well as successful, missions and also the nation that led them.

Cosmic Journey by Sean McNaughton, Samuel Velasco, 5W Infographics, Matthew Twombly and Jane Vessels, NGM staff, Amanda Hobbs. Source: NASA, Chris Gamble.

Cosmic Journey by Sean McNaughton, Samuel Velasco, 5W Infographics, Matthew Twombly and Jane Vessels, NGM staff, Amanda Hobbs.


Debunking Hoagland’s “Glass Worms” with HiRISE

ESP_035634_2160_1.0xA piece of Mars: Several years ago, a guy named Richard Hoagland claimed that some parallel linear features on Mars looked like the ridges of a transparent earthworm, calling these things “glass worms”. Phil Plait debunked it nicely, but Hoagland stood his ground. He hasn’t said much about them lately, has he? Here’s why. New images show that, as scientists originally thought, these are nothing more than windblown ripples in the floors of channels, just like the many thousands of ripples seen all over Mars. Go science! (HiRISE ESP_035634_2160, NASA/JPL/Univ. of Arizona)

SETI – Your Opinion Doesn’t Matter: part 3

This is part 3 and final installment in the Your Opinion Doesn’t Matter blog. Please read parts 1 and 2 for context.

In part 2 I divided spurious opinions regarding topics in SETI into 3 categories: science-free, anti-scientific, and convicted statements of the “self evident.” Here we exemplify the last category.

Convicted Opinions that are not so Self-Evident

Steven Hawking is the epitome of scientific hero. He overcame dramatic life challenges to become a peerless leader in the progress of general relativity theory of gravity and quantum mechanics. When it comes to theoretical gravitation, I’m not fit to tie his bootlaces. But he isn’t God. (I hear the jingling of sharpened spoons outside my window.) As far as I know! Speaking of intelligent life elsewhere, he said[4], “To my mathematical brain, the numbers alone make thinking about aliens perfectly rational.” This quote appears only because it supports my own point of view, ha ha! I’m about to say that Hawking isn’t the best source for information about ET.

Unlike many people, Hawking has an imagination: “I imagine they [aliens] might exist in massive ships, … become nomads, looking to conquer and colonise whatever planets they can reach.” Whoa! Where did that come from? Possibly… from a science fiction movie?  Proof that Steven Hawking is really a college undergraduate at heart.  (jingle…) Ahem, that was just my little joke, Mr. Dr. Professor Hawking. Sir.

It is plausible that Hawking’s comment inspired this excellent book[5], penned by scientists** who really are expert*** in SETI issues. For the greater part, experts suggest that aliens are more likely to be altruistic — willing to help us out with no expectation of immediate reward — than predatory. For example, members of a predatory species that have conquered their planet will have only each other to prey upon. This is a state of unstable equilibrium. So they had better lose their predatory instincts fast, or annihilation is inevitable in the long run. Hence, the aliens are not likely to be purely predatory.

**By sheer coincidence, this book is edited by none other than Doug Vakoch, the Director for Interstellar Message Construction at the SETI Institute, with an office a few doors down from mine.

*** At least, as expert as anyone could be.

So, what do you think? It doesn’t matter! “Do aliens exist?” is a question that will someday be answered by science. Provided we don’t give up on the search for them.