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.