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Using “dispersion” to create honest to goodness photon topedos.

I’ve been thinking about how to make a powerful weapon based on a relatively weak light transmitter which can be tuned in frequency. This could be the basis of what, in Star Trek, they call photon torpedos. The physics principle behind this kind of weapon is called “dispersion.” We commonly talk about oil “dispersing” on the surface of water, or pollen dispersing in the wind, but in physics we use this word to mean something very special and also very interesting.

Try this thought experiment. Take everyone on Earth (7 billion) and have them all stand side by side on a long straight platform perpendicular to and orbiting Earth’s equator. Have everyone face in the direction of the orbital velocity, using a local coordinate system where all people are on the x-axis at z=0. Give each person a baseball, and have them throw their baseball directly forward (in the direction of increasing z coordinate) at the same time. The baseballs leave the hands of every person (z = 0) at exactly the same moment (t=0). We won’t worry about the x or y spatial coordinates, only how far the balls travel in the z direction as time goes on. For the moment, we shall not be concerned about the rebound of the platform due to the ejection of balls. Perhaps there is a small rocket which cancels the momentum of the expelled baseballs, keeping the platform on its original course.

Some balls are thrown faster than others, depending on each person’s ability. At t=0, all the balls are clumped together at z=0. Later at time t=t1, the fast balls have moved farther from the platform than slow balls. The balls are now spread out, or dispersed, along the z-axis. With increasing time, dispersion increases: the distance between the fastest ball and the slowest ball increases with time. Thanks to Kepler’s laws, each ball will return to its starting point after executing one orbit of the Earth, provided the balls don’t hit the Earth (too slow) or escape Earth’s gravity (too fast). If we register the time when the balls arrive, we will typically find a small number of fast balls (thrown by major league pitchers) will arrive first. This is followed by a large “hump” or high density of balls arriving later with time corresponding to the speed of an “average” thrower, since average humans are more numerous than those with special skills. Finally, a small number of balls come dribbling in at the end, thrown by unusually weak throwers such as small children. Plotting the number of balls returning versus the time it takes to circle the Earth, we will find a bell-shaped curve. This bell curve looks a little like the envelope of a “wave packet,” if you have heard of that expression in quantum mechanics.

Now turn the classical baseball experiment on its head. After the first experiment, the experimenter knows the time required for each person’s ball to make one orbit. Starting with the slowest thrower (longest orbital time), this person throws their ball first, which comes back to the platform after known time T. Then, we ask the second slowest thrower to throw next, at just the right moment so that the second ball arrives back at the platform at time T. Carrying on with the third slowest, who throws next at the appropriate moment, we continue through all 7 million people until we reach the fastest thrower, who waits until the very last moment such that her ball arrives back at the platform again at the time T.

For the sake of argument, we use our rocket to steer the platform such that when the baseballs return after one orbit, they strike the platform from behind (but don’t hit any people). If just one baseball hits the platform, we don’t expect much of an impact. Even if 7 million baseballs arrive one by one, over a period of a year, each impact is small, so the people on the platform may feel a rumble but not even enough to make them fall down, since the impact of each ball is absorbed separately.

But in our second thought experiment, all of the 7 million balls strike the platform at exactly the same time. The instantaneous transfer of momentum is huge. Not only are people likely to fall down, but the platform itself may be distorted beyond its elastic limits and break in two, or into a thousand pieces. This might seem somewhat surprising, and it arises from the fact that the impact felt by the platform depends not only on the amount of momentum that is transferred from the balls, but also the time period over which momentum is transferred. Newton’s law of action and reaction explains this concisely:

Reactive Force on Platform = (change in momentum caused by balls) / (period of impact), or
F = dp / dt

where F = force, p = momentum, and t = time.

How does this discussion lead to a powerful weapon? Suppose we build a machine that can throw one baseball at a time at a certain speed, v. We can build a destructive weapon merely by preparing 7 billion copies of this machine and causing them to throw at exactly the same moment. Since all balls have the same speed, they arrive at their destination in a giant clump, obliterating the target.  But this has two problems: 1) 7 billion are a lot of machines (expensive) and 2) the instantaneous power required to trigger all machines at the same time is extraordinary, and possibly so large that Earth technology cannot feasibly produce so much energy over such a short time.

So we build a different design with only one machine that is capable of throwing one ball after another with a small time delay dt, but each one having a different speed. The machine starts by throwing slow balls, and gradually increase the ball speed uniformly in proportion to (A n dt) where n refers to the nth ball thrown and A  is a constant chosen depending on the distance to the target. With suitable choice of A, we ensure that every baseball arrives at the same moment, transferring large momentum in a small time and obliterating the target.

What have we gained? 1) Instead of building 7 billion machines, we built only one that is slightly more complicated (cheap). 2) Over any time period Adt, only the energy required to throw one ball is required. This is a dramatically smaller power level, which is extended over a long period of time. In total, about the same amount of energy is required for either of the above weapons, but the latter is astronomically cheaper and more energetically feasible.

Don’t get me wrong, I’m not a big fan of weapons. But I can’t help myself describing this particular use of physical “dispersion” since it is so fascinating.

This blog is already much too long, so we’ll very quickly skip to the quantum case, in which situation we make photon torpedos.

As described in an earlier blog, the space between stars (interstellar medium) is filled with an extremely thin gas, mostly hydrogen, with approximate 1% of hydrogen atoms being ionized into free electrons and protons, called plasma. When light travels through plasma, it picks up a tiny bit of the properties of matter: photons combine with electron motions into quasiparticles that look almost like photons but have a teeny tiny bit of rest mass. This rest mass depends only on the plasma density and not on the photon energy or frequency. Any particle with rest mass suffers dispersion. Even though all “pure” photons passing through vacuum travel with the same speed, c = speed of light, quasiparticle photons with rest mass can travel with any speed v, where (0 <= vc), just like any other massive particle. This is what makes photon torpedos possible.

Set up a light generator, call it an idealized tunable laser that emits radiation into a region of the interstellar medium (ISM). Low frequency light, like radio waves, travel more slowly through the ISM because they carry less total energy, hence less kinetic energy as compared with their tiny rest mass. Optical light waves travel faster, since their kinetic energy >> rest mass. X-rays, and then gamma-rays travel even faster. As a reality check we note that astronomers can ignore the slow-down even in the optical frequency range since it is small. But the slow down is never zero, even for gamma rays.

Now we perform exactly the same process with light that we did with baseballs. We begin by emitting low-energy (low frequency) radio waves. These waves can travel much less the speed of light since their total energy is not much larger than their quasiparticle rest-mass.** A little later, the laser is tuned to a higher frequency with corresponding higher speed for photon travel. Later, higher and higher frequency waves are emitted. We adjust the time of emission of the different frequencies such that they all arrive at the target at exactly the same time, packing an astounding punch.  The result is in perfect analogy to the baseball experiment.

** In a typical region of the interstellar medium,
the quasiparticle photon rest mass is 4e-18 eV.  Oops! Did I just quote the mass in units of energy? Shame on my lazy physics habits. That should say 7e-54 kg. Despite being a small number, it is easily measured in astronomical observations.

Using only a single laser transmitter and by transmitting different frequencies at specific times, we can use a single machine to simulate the “impact” of a large number of identical machines shooting the same frequency at the same time. Also, the amount of power emitted by the laser is relatively small but carries on for a relatively long period of time. By using “dispersion” to our advantage, we cause all of that energy to arrive at the target in a short moment, packing a giant whallop far beyond the capability of a single-burst from the laser at one frequency.

So that is one way to make a photon torpedo. Or you can use electrons instead, or neutral H or He atoms, or even baseballs. All these weapons are always based on the same principle of dispersion, which is a common feature of every object that has rest mass. Which is everything.

I hope this stimulates some entertaining thoughts about dispersion.

In the lee

ESP_034084_1655_1.0xA piece of Mars: This crater (290 m or 950 ft across) is crawling with all sorts of ripples and dunes. The wind mainly blows from the top to the bottom of the frame, and it is responsible for the wonderful textures in the dark gray sand. It has also formed larger, cream-colored ripples. The creamy and dark gray sand have taken turns burying one another, like vines competing for sunlight. (HiRISE ESP_034084_1655 , NASA/JPL/Univ. of Arizona)

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

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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.

Flow

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.

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