Sunday, November 28, 2010

Habitability of the Gliese 581 system

The planets discovered in the Gliese 581 system (located only 20.3 light-years away) have generated much excitement in the scientific community as this system appears to have at least one habitable planet. Until recently, two planets were viewed as potential habitable planets as they orbited on the outskirts of the star's habitable zone, however now a new planet has been discovered which lies almost at the center of the habitable zone making it, officially, the first extrasolar Goldilocks planet.
Let's take a look at the Gliese 581 system and examine the possibility of life in that system. Gliese 581 is a red dwarf which, as the name suggests, is a small bright red star. Its mass and radius are both about 0.3 that of our Sun. This is a size and appearance comparison between the Sun and Gliese 581:

The star is also cooler and less bright than our own Sun, as its luminosity is just 1.3% that of the Sun. This means that the Goldilocks region is at a distance from the parent star which is much smaller than in our own solar system, since a planet that would have the same average temperature as Earth would need to be closer to the red dwarf. Fortunately, because Gliese 581 is so small, the planetary system has formed very close to the star.
So what are the planets of Gliese 581?
There are 6 known planets in that star system, we will talk about them in order of discovery.

The first planet to be found is Gliese 581 b, a planet comparable to Neptune in size and mass. This would indicate that it is a gas giant (like Jupiter, Saturn, Uranus and of course Neptune) and even though this is uncertain it is highly probable. It is very close to its parent star, orbiting at about 6 million kilometers and completing an orbit in about 5.4 days. By comparison, Mercury, the closest planet to the Sun in our own solar system, is at a distance of 58 million kilometers from the Sun and completes its orbit in 88 days. Gliese 581 b is the second closest planet to its parent star.

The second planet discovered is Gliese 581 c. This discovery led to significant media coverage of the event since this planet appeared to be a rocky super-Earth, with a mass 5.3 times that of Earth, orbiting inside the star's habitable zone. As mentioned in previous posts, the habitable zone is the area around a star where the conditions are such that liquid water can exist. It is also known as the Goldilocks zone, since it's not too hot and not too cold. Finding such a planet is a monumental discovery, as it is the best candidate for alien life. Unfortunately, although Gliese 581 c is in the habitable zone, it's at the inner edge, in the too hot area, and although the predicted surface temperature is Earth-like, it was calculated only based on radiation coming from the parent star. Taking into account the atmosphere, which could contain greenhouse gases (like carbon-dioxide) the temperature rises to the point where it becomes uninhabitable. Therefore, this planet is very similar to our own Venus since if we calculate the temperature on Venus based only on radiation from the Sun, we get a value of about 34.25 °C. However, the actual value is 463.85 °C, due to the planet's 96.5% carbon-dioxide atmosphere which has caused a runaway greenhouse effect (an extreme form of global warming). While the excitement of finding a potential habitable extrasolar planet was short lived, this discovery attracted astronomers' interest for the Gliese 581 system. Gliese 581 c is the third planet from it's star.

The next planet to be discovered was Gliese 581 d, the fifth planet in the star system which is also on the outskirts of the habitable zone, but unlike Gliese 581 c, this planet lies on the outer edge of the Goldilocks zone, in the too cold region, similar to our own Mars. It is 5.6 times more massive than Earth and so it is also a super-Earth planet. Regarding how the planet looks like, it is believed to be covered by a large and deep ocean. The planet is too far away from it's star to receive enough heat for liquid water to exists, however if it's atmosphere has greenhouse gases they could raise the temperature just enough to make liquid water possible.

Gliese 581 e is next on our list, the closest planet to it's star. This planet orbits further in than the habitable zone and is way too hot to support life. It has a minimum mass of 1.7 Earth masses, making it the smallest extrasolar planet discovered around a normal star, and the closest in mass to Earth. It is believed that the planet does not have an atmosphere, since it is so close to it's star that stellar winds would "blow" the atmosphere into space and it is also believed that it experiences intense volcanic activity as a result of tidal heating. Tidal heating is when an object is so close to a massive planet/star that the differences in the intense gravitational field generated by the massive body, create friction in the crust of the orbiting object, causing it to heat up. We have an example of this in our own solar system: Io, a moon of Jupiter is so close to Jupiter that it experiences tidal heating and as a result has massive volcanic eruptions.

Now comes the crown jewel of the Gliese system, the planet discovered on September 28th 2010, Gliese 581 g. Why has this planet generated so much excitement? Because it is well inside the habitable zone and almost at its center. In our own solar system, our home Earth lies between the orbits of Venus, the too hot planet, and Mars, the too cold planet. Well, in the Gliese 581 system, we have something very similar: Gliese 581 g lies between the too cold Gliese 581 c and the too hot Gliese 581 d making it the first Goldilocks planet ever discovered. The planet is tidally locked to its parent star, what this means is that one side of the planet always faces the red dwarf and one side is always dark. So there, a year (the time it takes a planet to orbit its star) is precisely equal to a day (the time it takes a planet to complete a rotation around its axis) and is about 36 Earth-days. It is believed to have a mass 3.1 to 4.3 times that of Earth, and a radius of 1.3 to 2.0 that of Earth. Assuming it is a rocky planet, it's surface gravity is expected to be between 1.1 and 1.7 times Earth's, meaning that walking on the planet's surface would be similar to walking on the surface of Earth, except you'd feel slightly heavier. It is also predicted that its atmosphere would be denser than Earth's.

What would life on such a planet look like?
It is believed that plants would be black in color. This is due to the fact that red dwarfs emit light mostly in the red and infrared part of the spectrum, so plants would need to absorb more of it in order to make photosynthesis. Therefore, they would have a black color, since black absorbs all light, making these plants more efficient at absorbing light than plants on Earth. These plants would probably exist only on the side of the planet facing the star, since the "dark side" would never receive any light and therefore plants undergoing photosynthesis would have no chance of surviving there. Even though the plants are more efficient, they would be less productive in the dim red sunlight, thus producing less oxygen than their counterparts on Earth. This means the atmosphere would have less oxygen and that potential animals that might exist would be smaller in size (it is believed that dinosaurs were so large because Earth's atmosphere had a higher concentration of oxygen back then). Also concerning animal life, it is possible that animals would be able to see in the red-infrared part of the spectrum since this is the most abundant form of radiation emitted by the star.
Weather models predict fierce winds blowing continually toward the night side and permanent torrential rain at the center of the dark side. Also, the red dwarf's variability could lead to serious climate changes. Red dwarfs are far more active than our own Sun, with solar flares and coronal mass ejections popping up fairly often. This can have a serious impact on life on the planet. It could prevent the existence of complex life, or complex life could develop forms of protection and dwell only in shielded places such as caves or deep under water.
Other problems include large periods of inactivity for the red dwarf, which could lead to ice ages. Many scientists believe that these variable conditions could render the planet uninhabitable. Still, the fact that the planet is within the habitable zone leaves the possibility open and we will only be able to know for sure when we send a probe there to investigate.
The last planet in the Gliese system is Gliese 581 f, a large planet (about 7 Earth masses) located at the edge of the solar system. This planet is probably a large terrestrial planet or a gaseous Neptune-like planet, and is far too cold to support the existence of liquid water on its surface.

Discovering such a complex star system like Gliese 581 so early in field of extrasolar planet finding, raises hopes regarding the discovery of other systems with the potential to support life. An abundance of systems like Gliese 581 doesn't necessarily imply intelligent extraterrestrial life and even complex life seems improbable, but even the existence of the most simple organisms would be a monumental discovery. Such an abundance would increase the chances of finding another intelligent species. Whether that has a positive or negative impact on our civilization remains to be seen.

Saturday, September 25, 2010

Time travel

Time travel is a concept used very often in science fiction. Basically it deals with questions like: what if we could travel to any point in time in either the past or the future? How could this be achieved? What would be the implications and consequences of such an endeavor?
As I've explained in the post dealing with sublight interstellar travel, Einstein's theory of relativity shows us that travelling close to the speed of light is time travel into the future. Time passes faster for everyone else except you. The theory of relativity has also shown us that an intense gravitational field produces the same effect. The closer you are to a gravity source (a massive body) the slower time will pass for you. For example, let's say we have a black hole with an event horizon at a distance R from it's center.

The event horizon is the surface beyond which light can no longer escape the intense gravity field of the black hole. If you go beyond it, there's no turning back, but as long as you keep your distance from the event horizon the black hole is just another massive object (it doesn't pull you in any more than any other object of the same mass). If you were at a distance of 2R from the black hole and stayed there for 7 years, you would notice that about 10 years have passed on Earth. As you can see, gravitational time dilation is a tiny effect.
So, we've solved the problem of time travel into the future. The effect has been measured, and current technologies like GPS satellites have to take time dilation into account so as to keep synchronized. So far we don't have the means to send a person into the future a meaningful length of time, but as technology advances and we develop faster propulsion techniques that moment will come some time in the near future.
Now, we deal with the really big question concerning time travel: is travel into the past possible?
For starters, let's tackle this from a purely logical manner. What would happen if you could go back in time? We can immediately see that many paradoxes can arise. For example, if you were to go back in time and kill your own grandfather before he met your grandmother, then you would cease to exist. But if that happens how can you go back in time and kill your grandfather?! This is known as the grandfather paradox. Another type of paradox that arises comes from conservation of mass/energy and information, the ontological paradox. If you go back in time to a point in which you already exist there will be two of you. And if both of you travel to a previous point, there will be three of you, and so on. Essentially you could fill the Earth with these "time clones" and it would appear that all of these versions of you are coming out of nowhere, a clear violation of conservation of mass/energy. There is also the problem of information: let's say that one day you find on your desk the plans for building a time-machine. So you build it and decide to send the plans back in time to your earlier self so that he can find them. Now comes the question: who invented the time-machine? You received the schematics from the future, built the time machine and then sent them back in time for you to discover. So no one actually wrote them. So where did they come from? These are examples of violations of causality, the basic notion that a cause produces an effect.
Another problem that arises is: if time travel into the past is possible, how come we haven't seen any time travelers from the future? Well, there are many ways to answer this question. Stephen Hawking postulated the chronology protection conjecture, which basically states that the laws of physics are such that they prevent time travel (well, except for submicroscopic systems which can't affect the timeline). Essentially, that would mean that travel to the past is impossible. Another answer would be that a time machine only allows you to go back in time to a point in which the device already exists, and since no one has built a time machine yet we don't have any visitors from the future. However, even if this is true, there are still the grandfather and ontological paradoxes to be solved. Fortunately, there are two solutions available both of which also solve the problem of why we haven't been visited by time travelers.
To begin with, we have the Novikov self-consistency principle which states that if time travel is possible then all time travel events are consistent with our current history. So basically, you could go back in time but you couldn't change anything from our current history. For example if you went back in time to kill your grandfather, you could be killed by some accident before you found him. It would also mean that you yourself are part of history, since your presence in the past is part of our current history. This solution to the paradoxes of time travel is somewhat unsatisfying as it implies a form of predetermination since your actions in the past seem to be restricted by history.
The second solution, which is my personal favorite, is the existence of parallel universes. The idea is that when you go back in time you are no longer in your own universe, but in another that is the product of your travel through time. So, in this case if you go back in time to kill your grandfather, it won't be your grandfather, it will be the one from that universe. In fact you can do whatever you want because, if let's say you are from the year 2050 and go back to the year 1950, in that universe the present is 1950, and the future of 2050 hasn't happened yet and that future will be determined by your actions in that universe. The existence of parallel universes is postulated in one of the interpretations of quantum mechanics, the many-worlds interpretation, but it makes the exact same predictions as the Copenhagen interpretation, which is currently used by the scientific community, making it difficult to assert the existence of such universes. At least it means it's plausible.
Now, let's tackle this from physics' perspective: how would a time machine work and how could we build one?
The simplest time machine can be constructed using a wormhole.

As explained in the previous post, a wormhole is a hypothetical tunnel through space-time which acts like a shortcut essentially uniting two separated regions. For example you could have one end of the wormhole on Earth and the other close to the Sun. Going from Earth to the Sun the classical route, takes a long time (even light takes about 8 minutes for this trip), but stepping through the wormhole will take you there instantly. Now, from what we've learned about time dilation we can take one end (A) of the wormhole and put it on Earth in the year 2010, and accelerate the other end (B) to near light speed or place it in a powerful gravitational field. Due to time dilation, time will pass slower at end B so that it can still be 2010 at that mouth and 2050 at mouth A. Now if we were to bring mouth B to Earth (where it is the year 2050), we have a wormhole that  takes you back to the year 2010. This is a type of time machine that can only send you back in time to a point in which it already exists. Since the wormhole didn't exist before 2010, you can't go to an earlier point in time. Unfortunately, as we know, creating a wormhole requires negative energy so this solution is impossible to implement for now. So what other options are there for time travel?
First of all, we need to find a proper definition for it. As we know from relativity, it is postulated that nothing that carries mass/energy or information can travel faster than the speed of light. Therefore we can construct the following space-time diagram (Minkowski diagram):

As we can see, points on the light cone are where x^2 = (ct)^2, which is equivalent to (x/t)^2 = c^2. Distance over time is velocity, so these are points that can be reached by travelling at the speed of light, hence the name light cone. Inside the cone are points where x^2 < (ct)^2 or (x/t)^2 < c^2 so these are points that can be reached by sublight travel and outside the cone we have the points which can only be reached at superluminal velocity. Keep in mind that these aren't points in space, they are points in space-time,  meaning that each point has an associated space and time coordinate and so travelling from one point to another is restricted by the lightspeed limit. For example, the origin can be (Earth, now); this is a space-time point, it defines the position on Earth and time as the present. A point which would be in the future light cone would be (Earth, now + 5 minutes), and a point in the past (Earth, now - 5 minutes). If we were to travel to the Sun, a point on our world line could be (Sun, now + 5 days), and the world line would be the curve between that point and (Earth, now). This would be inside the future light cone, as travelling from Earth to the Sun in 5 days can be achieved at sublight speed. What if I wanted to go to the point (Sun, now + 5 seconds)? That's not possible, since light takes 8 minutes to travel the distance so we can't go from Earth to the Sun in under that time, therefore the point (Sun, now + 5 seconds) is outside the light cone. Due to the  lightspeed restriction, observers are "trapped" inside the light cone, moving from past to future. The trajectory they follow is called a world line. For reasons beyond the scope of this post, points inside the cone are called "timelike" and points outside the cone are "spacelike". Now, if we could somehow make this world line loop back on itself it would essentially mean that the observer would be returning to a previous point in space-time and would have thus time traveled. Therefore time travel is defined by the existence of closed world lines, called closed timelike curves (CTC). They are called timelike, because they would be inside the light cone and so would not be in conflict with breaking the lightspeed barrier. An example of points which could be on a CTC are: (Earth, now), (Sun, now + 5 days), (Earth, now). Notice how we went from the present to 5 days in the future (which would have become the new present) and then 5 days back into the past.
How can we make CTCs?
The trick is to do something similar to FTL travel. If you recall, in that post, when I talked about the Alcubierre drive I explained how you couldn't pass the speed of light locally but you could globally. The situation is very similar. If the light cone only lets us go from past to future, then lets make a series of local light cones in which we're going from past to future but each light cone will be slightly tilted with respect to the previous so that in the global light cone we will have formed a closed curve like in the figure:

Pretty ingenious, but can it be done? Well, the good news is there is nothing in the laws of physics which prevents this from happening. The tilting of light cones can be achieved in curved space-time like the one we live in, and many theoretical models have been found. The bad news is none of those models can be physically constructed in our Universe. The very first such model was discovered by Kurt Gödel, one of the greatest mathematicians of the 20th century and a very close friend of Einstein. Gödel presented this model to Einstein as a gift for his birthday. However, his model only worked in a rotating universe, which isn't the case for the one we inhabit. After that, scientists began to search for alternatives in our own universe. One of them was the Tipler cylinder, a large rotating cylinder which would allow the existence of CTCs in its interior. Unfortunately, it was proven that this could only happen if the cylinder had infinite length, something which is physically impossible. All other models have similar problems, though in some cases it may only be a question of lack of technology, for example one model suggests that a rotating superconductor (electrical conductor with 0 resistance) would form CTCs provided the number of Cooper pairs (special groups of electrons) is several orders of magnitude greater than it is in current superconductors. This could be merely a question of developing better superconductors, but on the other hand it could be physically impossible.
Can we define time travel in some other way?
Yes, however these alternate definitions don't even have theoretical solutions. One way to define it is simply by "true" faster than light travel, that is, travelling locally faster than light. The theory of relativity predicts that this is equivalent to travelling backwards in time as seen by outside observers. There is even a funny limerick based on this definition:
There once was a girl named Blight,
Who could travel faster than light.
She took off one day,
In a relative way,
And came back the previous day.
The only true FTL travel method we know of is represented by tachyons, the theoretical particles which could only travel at superluminal speeds, mentioned in the previous post. But even if they do exist and we could detect them, it has been proven that they cannot be used to send information into the past and thus violate causality, since tachyons of the future are indistinguishable from tachyons of the present.
Another definition comes from quantum mechanics. I mentioned antimatter in a previous post, the opposite of matter which annihilates it. An interesting property of antimatter is that in quantum mechanics it is viewed as matter travelling backwards through time at the same rate that normal matter travels forward through time. So if you would reverse the time axis on matter you would get antimatter. It is unknown whether this could be used to engineer a time machine, it's debatable if this even qualifies as time travel. Since time passes at the same rate if you would travel through time using this method it could mean that to go back in time 5 years, you'd have to wait 5 years.
There is also an interesting insight from the Einstein-Cartan theory. This theory completes general relativity, by including spin into the theory. Spin is seen as dislocations in the fabric of space-time and one of the predictions made is that particles with spin, travelling on certain trajectories can translate into the past or the future by a tiny amount, but yet again there is no theoretical time machine based on this method.
Time travel and FTL travel are very similar, though it would seem that time travel has a better chance of being achieved. This is due to the fact that it only seems to be restricted by paradoxes not actual physical laws, while FTL travel faces a major obstacle: the lightspeed limit. Also, both concepts find ways to circumvent their restrictions but the solutions to time travel are somewhat less exotic, in my opinion, and many solutions to the problem of FTL travel would also result in time travel while the inverse is not always true. It may turn out that both are possible or neither. History has shown us that many challenges viewed as impossible by scientists of the time, were eventually proven to be possible and are currently all around us. Examples include: airplanes, supersonic flight, X-rays, space travel, atomic energy and others. Given enough time, man's understanding of physics advances, the number of possibilities increases, to quote the Russian writer Anton Chekhov from one of his plays: "The human race progresses, perfecting its powers. Everything that is unattainable now will some day be near at hand and comprehensible..."
If time travel is possible then it would be the ultimate irony, for the only obstacle we face in achieving it, is time itself.

Friday, September 10, 2010

Interstellar Travel (part 2)

Faster than light travel

Last post I mentioned one of the postulates of relativity: the speed of light in vacuum is constant for all observers and nothing that carries mass, energy or information can travel faster locally. You may be wondering how we could circumvent this. There are many ways to "attack" this postulate. For starters there are many phenomena in which the speed of light is exceeded, and yet all of them respect the postulate. Let's take some examples: if you were to point a laser at the Moon and then move the laser spot on the Moon very fast you could make the spot move faster than light-speed since the distance in which you move the laser is small, but the distance the spot travels is large. So how does this help you? It doesn't. This method doesn't allow you to transmit neither energy or information faster than light. The laser spot will reach the Moon at the speed of light, and assuming it travels between two Lunar bases faster than light, none of those bases can control how it moves, since the source is on Earth and communicating with Earth requires sending a signal which can only travel at the speed of light. Therefore, none of the bases can control the information the laser spot sends to one another. Another example is a wave's phase velocity. Phase velocity is the rate at which a wave's phase propagates through space, and mathematically it is the product between wavelength and frequency. An interesting property of this velocity is that for electromagnetic waves travelling in certain media, presenting anomalous dispersion, (like the ionosphere for example) it can exceed the speed of light. This however, again does not mean that you can transmit information, energy or matter faster than light. Information and energy are associated with a wave's group velocity, the speed of a wave packet. This velocity cannot exceed lightspeed. In quantum mechanics we have another intriguing phenomenon: quantum entanglement. It is a kind of connection between two objects which form a quantum system, in which by measuring a certain property of one object you instantaneously gain some information about the second. This link is independent of the space between the two bodies. For example if you have two entangled electrons and you separate them by whatever distance you want and you measure the spin of one of them, you will immediately know the spin of the other one, regardless of where it is. Thus, it would seem that the information regarding the second electron's spin, an intrinsic property of that particle, had traveled instantly. Einstein called this phenomenon "spooky action at a distance" and contributed to the formulation of the Einstein-Podolsky-Rosen paradox which deemed quantum mechanics an incomplete or incorrect theory as it seemed to allow instantaneous communication. The apparent paradox was solved by the no-communication theorem which states that information cannot be exchanged instantly, however this does not prohibit FTL (faster-than-light) communication. But how could it be achieved?
Let's tackle another aspect of the mentioned postulate of relativity: locality. What does it mean to travel faster than light locally? It's difficult to give a precise, non-mathematical definition, but locality essentially means with respect to the immediate surroundings. It's easier to define non-local faster than light travel, which isn't prohibited by relativity, with an interesting example: the Alcubierre drive (very similar to the Star Trek warp drive). You might be familiar with the concept of space-time. Basically it's the joining of our 3-dimensional space with time thus forming the 4-dimensional universe in which we live in. Einstein's general theory of relativity says that gravity is not a force but the geometry of space-time itself. Therefore anything that produces gravity will affect space-time. What produces gravity? Mass and/or energy. Thus, a certain configuration of mass-energy will create a certain configuration of space-time. A useful analogy is to think of empty space as a stretched-out blanket. When you place an object on the blanket, say a heavy ball, it curves and other objects placed near it tend to fall towards the ball. If you throw a smaller ball on the blanket, due to it's initial velocity, it will circle the large ball, thus orbiting it. This is reminiscent of our own solar-system in which the planets orbit the Sun. The relationship between mass-energy and the geometry of space-time is neatly expressed in Einstein's field equation, an elegant tensor equation that is the core of general relativity. All geometrizations of space time (so called metric tensors) must satisfy Einstein's field equation if they are to exist in our own Universe. One such solution is the metric for the Alcubierre drive. Going back to the blanket analogy, the Alcubierre drive works something like this: if I'm on the blanket and I want to move forward I contract the blanket in front of me, and expand it at the back. Essentially this is warping space-time around me. Assuming I could to this, there is no limit to how fast I could go because I'm not locally going faster than light. In my local space-time, which is the one inside my "warp bubble" I still can't go faster than light. But to an outside observer, that is non-local space, this ship is travelling faster than light. Voila! We have achieved FTL travel without breaking the laws of physics. So what's the catch? When you think about the blanket analogy you realize that all objects (objects with mass/energy) bend the blanket downwards (negative curvature), but in the Alcubierre drive when you contract the space in front of your ship, you bend it upwards (positive curvature). This is a problem, since creating positive curvatures would require negative mass or negative energy. There is no law of physics which prevents the existence of this but we've never encountered it and we don't know if it exists. Another problem is the enormous amount of normal mass/energy required (comparable to the Sun).
Other solutions to Einstein's equation exist in the form of wormholes, shortcuts through space-time. The name originates from an analogy: to get to the other side of an apple a worm doesn't have to travel on the surface, but can dig a hole through it. Wormholes are of many types and would theoretically allow travel between two points in space, two points in time, even between universes. Unfortunately, as with the Alcubierre drive, problems arise such as the necessity for negative energy density or exotic matter. It is theorized that you don't need any of that to create a wormhole but you do in order to stabilize it, more precisely the wormhole would collapse before anything could pass through it. It could be possible that natural wormholes exist somewhere in the Universe, for example it is believed that ring singularities (rotating black holes) could form wormholes. Even if wormholes exist somewhere, we can say with a fair amount of certainty that there aren't any in our solar system and it doesn't look like we'll be creating any in the near future.
The problem of negative energy density could be solved by a quantum physics phenomenon known as the Casimir effect. The experiment which led to its discovery is this: if you take two uncharged parallel plates and bring them extremely close to each other, a force will act upon them pushing them even closer. This can be explained through vacuum fluctuations. What we think of as vacuum is empty space, nothingness, but this isn't the physical vacuum which exists in the real world. Vacuum has an associated energy and is thought to be made of virtual particles. Why? It's a result of quantum mechanics which has shown us that a system can only occupy discrete energy levels (i.e. quantification) and that it is impossible to measure certain physical properties with absolute precision (Heisenberg's uncertainty principle). Therefore, the lowest possible energy level cannot be 0, as this would mean you could know energy with absolute precision, but is slightly above 0. This also explains why no physical system could ever reach absolute 0 temperature. Temperature is a measure of particle movement. The uncertainty principle tells us that you can't simultaneously know the position and momentum of a particle with infinite precision and if a system were at absolute 0, the particles's positions would be fixed and their momenta would be 0. Going back to the Casimir effect, vacuum has energy in the form of virtual particles. When you bring the two plates close to each other, the number of particles outside of the plates is greater than the number inside and therefore creates pressure, pushing the plates closer to each other. This system has an associated negative energy density, though it is unknown how it could be harnessed for stabilizing wormholes or constructing an Alcubierre drive. It is a problem of incorporating gravity into quantum mechanics (which for the moment are separated), something which can only be solved by a quantum theory of gravity.
The Casimir effect offers insight into another FTL phenomenon: considering that vacuum has an associated energy, this energy is thought to be responsible for the values of electric permittivity and magnetic permeability in vacuum. These are two very important constants. They are measures of the resistance of forming electric and magnetic fields in vacuum. Light is an electromagnetic wave and from Maxwell's equations it can be shown that the speed of light is inversely proportional to the square root of the product of these two constants. But what if the constants aren't constant? A vacuum with a lower associated energy would have lower constants and therefore a larger value for the speed of light. It is believed that due to the smaller density of virtual particles between the plates described in the Casimir effect experiment, there would be a lower vacuum energy and therefore light would travel faster. The difference would be very small, and this has made it difficult to measure such an effect. On a side note, it is also possible that we live in a false vacuum: a local area of space in which the associated vacuum energy is larger than everywhere else where we have the "true" vacuum. If something were to happen to cause even a tiny region of our space to tunnel to that lower energy level of true vacuum, it would cause a so called "vacuum metastability event", a doomsday scenario in which a bubble of true vacuum would expand at near light speed changing the very fabric of our space-time.
Another concept related to FTL travel is the tachyon. These are theoretical particles which can travel only above the speed of light and can never slow below it. They are predicted in string theory, though it is believed that even if they do exist they cannot be used to transmit information faster than light.
FTL travel is often associated with time travel as almost all FTL solutions would also permit time-travel. I will talk about time-travel in another post so I'm not going to go into details here, suffice to say that the paradoxes associated with time-travel would present a problem for developing FTL travel.
Other methods used in science fiction involve hyperspace. The idea is for the ship to go into another dimension where fundamental physical constants, like the speed of light, don't exist and a ship could travel infinitely fast. While the possibility of other dimensions is explored by string theory (which postulates the existence of between 10 and 26 dimensions) these other dimensions are theorized to be curled up at extremely small distances and could only be accessible to high-energy subatomic particles. There are many other science fiction FTL drives like: jump drives, slipstream drives etc but all of these achieve FTL travel by assuming that our current understanding of physics is either fundamentally wrong or largely incomplete. It is true that our current understanding of physics is incomplete as we have yet to find a theory of everything which will ultimately answer the question regarding the possibility of FTL travel.
Personally, I am unsure if FTL travel is possible but I am confident that a viable means of interstellar travel will be discovered some time in the future. It is something which I find inevitable, motivated not just by curiosity and ambition but by our need for survival. The method which will be used could be a variation of the ones discussed here or it could be something completely new. As someone once told me science fiction of today will shape the science of tomorrow.

Thursday, August 26, 2010

Interstellar Travel (part 1)

41 years ago, Neil Armstrong became the first man to set foot on the Moon and uttered his famous "It's one small step for [a] man, one giant leap for mankind". 518 years ago, Christopher Columbus set foot in the New World after his transatlantic journey, having thus undertaken an earlier "giant leap for mankind". History is filled with such milestones, motivated by political, military or technological reasons and driven by human ambition. Perhaps the next great milestone will be when humans first set foot on Mars, something which I hope will happen in my lifetime. But after we start expanding into the solar system the next giant leap will be to travel to the stars. Currently, we have no means of achieving interstellar travel in a reasonable amount of time, since the fastest man-made objects ever built would still take tens of thousands of years to reach the nearest star.
Interstellar travel can theoretically be of 2 types: slower than light or faster than light.
In this post I will only talk about slower than light interstellar travel.

Slower than light travel

One of the postulates of Einstein's theory of relativity is that the speed of light in vacuum is constant (exactly 299,792,458 m/s) and that nothing that carries mass, energy or information can travel faster locally. All experiments and observations carried out have obeyed this postulate. The theory of relativity has shown us this restriction, which is a major obstacle for interstellar travel. You might be thinking that the speed of light is absurdly great and is good enough for space travel. Assuming we could travel at that speed it would be convenient for travel inside our solar system, as it takes about 8 minutes for light from the Sun to reach Earth and depending on where you want to go it can take up to several hours to travel to other locations. But if we wanted to travel to our nearest neighbor, Proxima Centauri, it would take about 4.2 years, as it is located 4.2 light-years from our solar system. Travelling to other stars takes even longer, our galaxy, the Milky Way, is 100,000 light-years across. It would take more time to explore the galaxy, than the current age of the human species.
However, there is a loophole. I've been talking about time, but I haven't mentioned that in relativity time is ... relative. Different observers will measure different time intervals, and it can be shown that observers travelling at velocities close to the speed of light (starship reference frame) experience time dilation, with respect to the stationary observers (Earth reference frame). Time dilation means that less time passes inside the starship than on Earth. This is known as the twin paradox. I will not go into details as to why this happens (check out the links), the important thing is that it happens and it means that even if it takes one million years, in Earth's frame of reference, for the starship to explore the galaxy, in the ship itself only 50 years could have passed (the speed necessary to achieve this is 99.99999987% of the speed of light). In fact if you can go fast enough, you could explore the entire observable Universe within a lifetime, from your perspective, but it would also mean that everyone you knew back on Earth would have been dead for a very very long time. Travelling near the speed of light is basically time travel into the future. This isn't so much good news because it would still mean the people on Earth would have to wait years to receive information from the ship. Still, on short distances (relatively speaking this means under 40 light-years) it is practical to do this and explore many of our neighboring stars.
So what's keeping us from doing this?
The short answer is: lack of technology and financial support. We currently have no means to accelerate a ship anywhere near the speed of light but we do have some promising theories. To date, the fastest man-made objects, the Helios probes, achieved a speed of 252,792 km/h, which is just 0.023% light speed. All probes we've sent to explore the solar system have similar speeds and therefore it took years for them to reach their destinations. For example, the Voyager probes are the farthest of all probes, travelling on the outskirts of the solar system, but it took more than 3 decades for them go that far. Obviously we need to do better than that if we plan to send something light-years away, but we face many obstacles.
To begin with, the main problem is energy. An object travelling at relativistic speeds (close to the speed of light) will have a lot of kinetic energy and that energy has to come from somewhere. Let's say you wanted to accelerate a one hundred ton ship to one quarter light-speed. The energy required would be about 300 exajoules (10^18). Now take into account the fact that the total energy consumption in 2008 for the entire planet was 474 exajoules. As we can see, the energy requirements for achieving relativistic speeds are enormous. Even if we reduce the mass, and say one tenth light-speed is good enough, we still require tremendous amounts of energy. For this reason, no spacecraft can achieve those speeds through conventional means (that is through chemical engines, ion engines or any other propulsion method used by current spacecraft). Therefore, we must investigate the unconventional means of propulsion.
So far, the problem seems to be energy. Luckily we have an efficient means of releasing large amounts of energy, on the order of exajoules, in the blink of an eye: nuclear bombs. It's quite ironic that the weapons we feared for so long could destroy us, can actually save us by providing the means of accelerating ships to relativistic velocities, thus allowing us to reach other solar systems where potentially habitable planets exist. The project was called Orion and the idea is simple: the ship would be propelled by the shock waves produced by nuclear weapons, which would be detonated behind the ship. This is called nuclear pulse propulsion, and while it could solve the problem of achieving relativistic speeds (theoretically up to 10% light-speed could have been achieved) it is extremely problematic to implement. The problems that arise are of different natures: political problems since the Outer Space Treaty prohibits the placement of nuclear weapons or any other type of weapons of mass destruction, in space; environmental concerns due to the risk of nuclear fallout reaching Earth; financial problems, as the cost of constructing such a ship and transporting nuclear weapons (which are very heavy) in space is too great to make the project viable. The main advantage of project Orion is that it can be implemented with current technology.
Another idea was to use an inertial confinement fusion drive. The premise is to make a nuclear fusion reactor and propel the ship with the plasma and energy produced in the fusion reaction. This was known as project Daedalus, created by the British Interplanetary Society. The ship they designed could reach up to 12% light-speed and was supposed to travel to Barnard's star (almost 6 light-years away) which, at the time, was believed to have a planetary system (the initial claim proved to be false but there still may be a planetary system there and we just haven't detected it yet). The trip would have taken about 50 years, from Earth's perspective, which is reasonable as it is within a human being's lifetime. We can see that unlike project Orion, Daedalus can achieve a greater speed with no risk of nuclear fallout. However the problem with this idea, apart from the costs, is that fusion technology isn't advanced enough to construct the inertial confinement fusion drive, therefore this plan cannot be currently implemented and will only be possible in the near future.
As far as nuclear propulsion goes, all other ideas are variations of these 2 projects (you can also lookup project Longshot). So what other options are there?
Well, another method of solving the energy requirement problem is to use antimatter. Antimatter is the mirror "reflection" of matter and when they come into contact they annihilate each other releasing tremendous amounts of energy. Einstein's famous equation E=mc^2 (where c is the speed of light) shows the relationship between mass and energy as mass m can be converted into energy E and vice-versa. When matter of mass m encounters antimatter also of mass m the energy released from the reaction is exactly 2mc^2, as all matter annihilates all antimatter and everything is turned into energy. Comparatively, in nuclear fusion reactions only about 0.3% of matter is turned into energy (though it depends on the type of reaction), so matter-antimatter reactions are significantly more powerful. Where can we find antimatter? The apparent lack of antimatter in the observable Universe is a discussion for another post. For now what we need to know is that there are no known sources of antimatter in our solar system and while we can produce it in particle accelerators, by reversing the annihilation process and creating matter and antimatter from energy, the amounts produced are minuscule, on the order of hundreds of atoms. A CERN researcher said that all the antimatter produced at CERN would only be enough to power a light bulb for a few minutes.
We've explored techniques which rely on on-board fuel or energy source to propel the ship and while this is preferred since the ship has a larger autonomy we will also explore different propulsion methods. One such method is a solar sail. The idea is to attach the ship to a large, reflective sail. This sail would be propelled by radiation pressure and solar winds. We now know that energy and matter are different manifestations of the same thing and as such, photons or quanta of light have momentum and therefore light exerts pressure on matter. This would be the main driving force for the solar sail, radiation pressure coming either from the Sun or from high-powered lasers. Inside our solar system, the sail would also be pushed by solar winds and the combined action of these forces could accelerate it to a significant fraction of the speed of light though it would take a very long time to reach such a speed. During that time the sail could be damaged by dust and microasteroids. In fact another major problem for interstellar travel is that when travelling near the speed of light even a tiny grain of dust can destroy your ship if it hits you, since from your perspective dust is heading toward you at near light-speed and thus has an enormous kinetic energy.
A way to solve this problem is to use a Bussard ramjet, a ship with a giant scoop at its front to collect interstellar hydrogen, compress it until nuclear fusion is achieved and eject the exhaust from the back of the ship thus creating thrust. The scoop would also collect and eject interstellar dust in its path, however to gather sufficient quantities of hydrogen, it would have to be enormous (having a diameter of a few miles).
All theories presented here utilize only 2 of the 4 fundamental forces of nature to propel the ship: the strong nuclear force and the electromagnetic force. Why can't we use gravity or the weak nuclear force? Because they are the weakest interactions and using them in an intelligent manner to achieve our goal is extremely complicated. For example, a theory was proposed which makes use of gravity as the driving force: the idea is to create artificial black holes and propel the ship via Hawking radiation, though implementing this is impossible in the present and it's unlikely to be used even in the near future. As we will see in the next post, gravity plays an important role in faster-than-light travel.
Assuming a ship could be constructed that could travel to Proxima Centauri at sublight velocity, other problems that may arise are related to the passengers on board. They might not be able to endure such a long trip, even if from their perspective it's a shorter amount of time. Also, a large quantity of supplies would be required. Both of these problems could be solved if the people on board were put into cryogenic suspension for the entire trip. That means they would be put into a state similar to hibernation and not endure the effects of long-term space travel.
The conclusions we can draw are that there are many options for sublight interstellar travel as many solutions exists for the problems faced, but more research is needed before we can actually attempt a trip. As it stands no method for interstellar travel exists that is both technologically achievable in the present and economically feasible.

Sunday, August 22, 2010

Will we ever meet aliens? (part 2)

Intelligent alien life

In the last post I talked about primitive alien life and how it may turn out to be abundant throughout the Universe. But what about intelligent alien life? Again we are dealing with 2 different questions: 'what are the odds of intelligent life evolving someplace other than Earth?' and 'what are the odds of us encountering intelligent extraterrestrials?'.
To answer the first question we must, yet again, look at our own planet, and study the origin of intelligent life here. As mentioned in part 1, life exists on Earth for 3.5 billion years, but humans have been around for only a fraction of that time (200,000 years). From this statement alone we can see that even on a planet teaming with a large variety of organisms the emergence of sentience and intelligence is quite rare.
The conditions that led to the existence of humans are numerous, and if just one of these conditions were different we wouldn't have existed. Let's consider an example: for over 160 million years, dinosaurs were the dominant species on the planet. Had it not been for the cataclysm that wiped them out 65 million years ago it's entirely possible that mammals would have never taken over.
We know of no other species from Earth's history that achieved our level of intelligence. This also enforces the idea that it is a rare event, one might even call it an accident. Unfortunately this means that it isn't very likely to happen often.
In 1961, a scientist named Frank Drake, formulated an equation called 'the Drake equation' to estimate the number of potential extraterrestrial civilizations. The logic behind the equation is simple: life appears in solar systems (so we need to know the rate of star formation in our galaxy), not just any solar systems but the ones that have planets (fraction of planetary systems), out of these life can only appear on certain planets with the right conditions (number of potentially habitable planets), and will appear only on some of these planets (fraction of the previous on which life actually appears), and some lifeforms will achieve sentience and intelligence (fraction of the previous on which intelligent life develops), and some will evolve and create technology that is detectable from space (fraction of the previous which creates technology) but they will exist only for a certain amount of time (length of time a civilization is detectable). Sadly most of the terms in the equation are unknown and scientists can only estimate or guess their possible values. For this reason the result can vary a lot and until we actually meet another civilization or start exploring other solar systems we won't be able to make more realistic estimates.
Still, we could ask ourselves: why hasn't anyone contacted us? Why have we seen no signs of other intelligent beings out there?
It has been estimated that an advanced civilization could expand throughout the entire galaxy in a timescale on the order of millions of years. Considering that our galaxy has been around for about 13 billion years and our own planet has been present only the past 4.5 billion years there has been enough time for another civilization to conquer the galaxy. And yet, we see no signs of this whatsoever. You could say that maybe there are old civilizations out there but they just don't want to expand. This is highly unlikely since expansion is an imperative for any species for 2 very important reasons:
- first of all a condition necessary for the continued existence of a species is reproduction. As the civilization's population grows in number it will need more and more space
- any advanced civilization requires resources and energy and once it consumes the ones which exist in its local environment it will need to expand in search of other resources
This is actually quite concerning since it could mean that if aliens come here they might try to colonize our planet and take our solar system's resources. And since we would try to oppose them they would most likely destroy us. Well-known theoretical physicist Stephen Hawking has expressed his concern on this matter, saying "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans".
For now we have nothing to worry about. All the techniques we've employed to find signs of extraterrestrial intelligence have turned up nothing. This is referred to as 'the Fermi paradox', the contradiction between the theoretical high probability of the existence of alien civilizations and the lack of evidence. Although interstellar travel is very difficult (currently our fastest probes would still take thousands of years to reach the nearest star) there is still the question of why we haven't received any radio signals. There are many possible answers: maybe we haven't been listening at the right time and now broadcasting civilizations are using some other means of communication (or have ceased broadcasting in outer space), perhaps the distances are so great the signals are too weak for us, maybe we're not listening on the right frequencies, compressed data streams are impossible to distinguish from white noise, aliens could be using modulation techniques unknown to us etc.
Detecting extraterrestrial civilizations may prove so difficult it could not happen for another 1000 years. We currently employ the SETI (Search for Extraterrestrial Intelligence) program to detect radio signals which could be of alien origin. While it's a good idea, it has had little success (apart from the controversial 'Wow signal') and sadly it's possible that we will never detect any such signals. Our own planet is becoming more and more 'quiet'. With the advancement of communication technology, low-power directional-guided transmission is expanding, meaning less 'leakage' of radio signals into space thus making it harder for us to be discovered.
Of course, there are other search methods such as looking for Dyson spheres. Physicist Freeman Dyson hypothesized that a highly advanced civilization would try to harness most of the power radiated by a star and therefore construct a giant sphere around it. The sphere could be formed out of a swarm of satellites. In order to locate such a mega-structure, scientists have restricted their search to Sun-like stars and assumed that the satellites would be constructed out of heavy elements. If this is the case, it has been calculated that the sphere would reradiate energy absorbed from the star in the infrared part of the electromagnetic spectrum. Therefore SETI is searching for 'infrared heavy' spectra from Sun-like stars. Although some candidates were found, they could all be explained as natural phenomena.
New Scientist published an article once in which it talked about how the IceCube Neutrino Observatory may be able to detect neutrino emissions from alien nuclear reactors. Personally I think it's more likely to find a Dyson sphere.
As we can see, all attempts at detecting intelligent aliens have focused on finding civilizations more advanced than us. It's entirely possible that we are the most advanced species in our galaxy or that we are the only intelligent species. But, as mentioned in the movie Contact, if we are alone it would be a giant waste of space.
What would happen if we did actually make contact with an alien civilization or they came to visit us?
Our society would never be the same. It would probably be the single most important event in human history, affecting all aspects of our culture. It would stir a mixed reaction from the population, as many people would be extremely happy and many would be terrified. Astronomer Carl Sagan believed that regardless of the aliens' intent, the knowledge of their existence would unite the nations of the world, as they would realize that their internal conflicts are insignificant compared to the challenges of first contact. Assuming they're not hostile, one of these challenges would be communication. Unless they have observed us enough to decipher one of our languages, communication will probably be based on mathematics, something which is truly universal. All messages sent intentionally into space to be discovered by advanced civilizations have taken on a mathematical form relating to base 2 arithmetic, prime numbers and universal constants.
After we have established a means of communication we would need to carefully negotiate with the aliens, something which will be most difficult considering we would know nothing about their culture. This process must be handled with great care, people would have to cooperate and agree on what decisions to take in order to act in the benefit of all mankind. Our initial fear must not determine us to act irresponsibly but we must also be vigilant and make sure the aliens aren't harboring evil intentions.
All in all, first contact could prove to be humanity's greatest challenge, although it does depend on when it happens. Many believe we are unprepared for such an event until we solve the major problems we have here on Earth. I believe we are unprepared for contact with an advanced space-faring civilization until we ourselves develop the means for interstellar travel. As it stands, we are fragile, our existence is tied to this planet and this solar system and we can't control either of them. If we could travel to other planets and other stars our chances of survival, even in the face of an advanced hostile alien species, would be greatly increased. It could even mean that we might be the advanced civilization that initiates first contact by travelling to an inhabited alien solar system, in which case things will go along a lot easier for us.

Friday, August 20, 2010

Will we ever meet aliens? (part 1)

A question that has puzzled us humans for a very long time; but the real puzzling question is 'Are we alone in the Universe?'. You might be thinking 'What's the difference?'.
When people think of aliens, they tend to imagine what they've seen in SF movies, on the Internet, read in books etc, like little green humanoids, humanoids with various ridges on their heads, different colored skin etc. But aliens can be simple bacteria, microorganisms that just happened to evolve on another planet. Or they could be something completely different, something we've never encountered before. So extraterrestrials can take on many forms, but I believe that what people really want to know is if we will ever encounter intelligent extraterrestrial life.
In this first part I will not talk about intelligent alien life but about the possibility of finding simple alien life which may not be so spectacular but would still represent one of the greatest discoveries of all time.

Primitive alien life

Primitive alien life means unintelligent extraterrestrials. This type of alien life could be quite common throughout the Universe. As we've seen here on Earth, simple organisms can survive in even the most extreme environments (deep in the oceans, frozen in the ice, in the hot arid deserts, some bacteria even survived for a while in space when they were brought there by accident and it is well known that many insects can survive intense radiation which is fatal for other organisms). So primitive lifeforms are very resilient and adaptable but does that mean that we can find them on other planets? Not necessarily. While it is true that some organisms from Earth could survive on other planets in our solar system (Mars for example) you would still need some special conditions for life to appear there in the first place. Conditions like those found on our planet.
So how did life on Earth appear?
Earth is located in what is called 'the Goldilocks zone' or 'habitable zone' of our solar system. More precisely it is located at a certain distance from the Sun to allow the existence of liquid water on its surface. It is a consensus among scientists that water is a fundamental ingredient for life. All known forms of life depend on water. It is believed that comets, which contain ice, crashed into our planet during its early stages of formation and brought water here. So we know of water but what else is needed for life to appear? Well, living beings are composed of organic molecules which contain the elements carbon, hydrogen, oxygen and nitrogen. These are light elements which are produced in abundance by stars in the process of nuclear fusion, and were present on primordial Earth. But having the elements and putting them together in the desired order are two very different things. In this case, the 'desired order' would be that of amino acids.
Amino acids are also essential for life, since they form the building blocks for proteins and have many functions in metabolism. So how did amino acids appear? Unfortunately nobody knows for sure but it is believed that the conditions on primordial Earth were good enough for it to happen.
An experiment carried out in the 1950s called the Miller-Urey experiment simulated the conditions of the early Earth. The result was very promising as 22 amino acids were formed.
From this point on, evolution takes over and, over a timescale of billions of years (Earth has been around for about 4.5 billion years and it is thought that life appeared 1 billion years later), things become more and more complicated as lifeforms begin to emerge and develop.
If this process happened here then it is very likely that it will happen (or has happened) in places where the conditions are similar.
There are many who believe that life didn't originally start on Earth but someplace else (for example Mars again) and somehow ended up here (via asteroid maybe). This theory is called panspermia, but it's just shifting the problem to another place. Somewhere somehow the conditions were right for life to appear and that is what's important.
It's important because in a galaxy of 400 billion stars and who-knows how many planets, like our own Milky Way, it is very likely that the conditions necessary for the formation of life can exist in more than one place. I am still talking about primitive life. In the 3.5 billion years since life exists on our planet, humans have been around for only 200,000 years (less than 0.005% of this time). This is what's called the 'Rare Earth Hypothesis', the belief that simple life is abundant throughout the Universe but complex life is not.
Based on this reasoning, in order to find alien life we must search for planets displaying similar conditions to Earth (Earth-like planets). This is why modern telescopes, using techniques such as radial velocity measuring, transit timing variation, gravitational microlensing detection etc, are searching for extrasolar planets within a star's habitable zone. Although many extrasolar planets have been found (almost 500), very few of these are Earth analogs (a notable planet is Gliese 581 d).
Modern planet-finding telescopes like Kepler and others that will be launched soon, show promise and may find many habitable extrasolar planets; though answering the question of whether or not there is any life on those planets may prove difficult without actually sending a probe there.
I agree with the Rare Earth Hypothesis and I believe it is only a matter of time until we discover primitive alien life.