Members of the GPI team recently attended the biennial SPIE Astronomical Telescopes and Instrumentation conference. This time it was in Montreal at the end of June, and you can check out #SPIEastro to find out more about the general topics covered at the conference.
After the conference, the presenters write manuscripts on their work and these are published in the Proceedings of SPIE. Last night we had a GPI “paper splash” of SPIE pre-prints at the Astro-ph ArXiv. There are 18 of them — that’s a lot of work from the GPI team! Thanks to Quinn for posting.
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A piece of Mars: Using dunes to interpret the winds can be a tricky business. Here’s one reason why: most of the dunes here go from the upper left to lower right. But the ones inside the funky oblong crater go from the upper right to the lower left. Why? One of two reasons. Either the rim of the crater rotates the winds that blow inside, or the rim blocks one wind but lets in another that is less effective at making dunes outside. (HiRISE ESP_036934_1915, NASA/JPL/Univ. of Arizona)
This article was originally published in Spanish in the website of Fundación madri+d. To access the original version, click here.The English translation was published in OpenMind, an interdisciplinary platform with bilingual articles in Spanish and English by te Fundación BBVA. The English version is here.
On the name of the satellites of Jupiter discovered by Galileo
Miguel de Cervantes died in 1616 a pauper. He is buried in the convent Trinitarians nuns in Madrid, where there is a search now underway for his tomb. As well as his monumental work Don Quixote, which he himself considered the first modern novel, his extensive literary production included poetry and theater. It also appears that his scientific culture must have been considerable, as he kept in touch with the advances that were made at the start of the 17th century following the invention of the telescope. It is even possible that he made a significant scientific contribution, naming the satellites of the planet Jupiter, which were identified when Galileo Galilei, the astronomer from Pisa, pointed the new instrument to the sky.
With the publication of “Sidereus Nuncius” (the Sidereal Messenger) in March 1610, Galileo began a real revolution, not only in astronomy but also in philosophy. He presented solid evidence overturning the interpretations of the world that had been firmly in place for centuries. In his work Galileo shows us an irregular and imperfect moon; he identifies a large number of new stars that are weaker than those seen with the naked eye; he reveals the complex nature of the Milky Way; and he discovers four bodies orbiting Jupiter, delivering a devastating blow to the Ptolemaic cosmology. In successive letters he continued his demolition of the static vision accepted by the Aristotelian orthodoxy. He observed the phases of Venus and the rings of Saturn, without identifying them as such; he also interpreted correctly that the sunspots are real features on the surface of the sun. In these and other discoveries, Galileo became immersed in major controversies that almost cost him his life when he faced the Roman Inquisition (censured in 1616 and condemned in 1633 to permanent house arrest). One of these disputes, limited to the academic arena and not resolved until the 20th century, involved the German astronomer Simon Marius (the Latin version of the German name Simon Mayr or Mayer), who claimed co-discovery of Jupiter’s satellites and was attacked roundly by Galileo as a result. The alleged plagiarism, accepted for 300 years, was disproved decades ago, although references to it can still be found in some texts. Let’s look at the sequence of events:
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A piece of Mars: Curiosity has been trolling around on Mars for one martian year, so I think it’s time I posted an update on where it is and what it’s seeing. Right now (late June 2014), the rover is rolling across meter-sized ripples, heading south toward Mt. Sharp. In the near future there will be even more impressive ripples, and then finally the terrain will start to grow more interesting. I will post more of these in the months to come. (HiRISE ESP_029034_1750, NASA/JPL/Univ. of Arizona)
Some news from Gemini Observatory,
Gemini Observatory has revealed the list of observing proposals scheduled in 2014B (the second half of 2014) that will use the GPI instrument. Those programs focused on the search for companions around nearby stars and also stars known to possess a disk and/or a planet by radial velocity. Other groups are using the quality of data provided by GPI to study planets already imaged with previous instruments, such as the HR8799 system and Beta Pic b. Their goal is the study the atmosphere of those planets and also to collect more astrometric positions to refine the orbit of the exoplanet.
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 first. The ball 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 Force (transfer of momentum) is huge. Not only are people likely to fall down, but the platform itself may be obliterated by the impact. This might seem somewhat surprising, and it arises from the fact that the “impact” or force felt by the platform depends on both the the amount of momentum that is transferred from the balls and 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 increases the ball speed uniformly in proportion to
(V_slowest_ball) ( ball num in throwing order) (time between throws), or
V0 n Dt
where V0 is the speed of the slowest ball, n refers to the nth ball thrown. With this choice, 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 ndt, 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. These quasiparticle photons, like any massive particle, suffer dispersion. Even though all “pure” photons in vacuum travel with the same speed, c = speed of light, quasiparticle photons with rest mass can travel with any speed v, where (0 <= v < c), 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.
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A 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)
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.
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A 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)
A 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)