DanielC

Giant cockroaches, and giant insects as you know them, are not physically possible. But the point you are really trying to make (that some life forms resist radiation better than others) is valid, but then again, even insects, and for that matter anything, can only tolerate certain amounts of radiation, simply because radiation breaks molecular bonds and that's bad for anything made of molecules. Btw, the reason giant insects are not physically possible is due to the fact that the weight of an animal goes with the cube of its size, while its physical strength, and the strength of the skin / shell / exoskeleton only goes with the square of their size. That's why insects can get away with the tiny legs they have, but larger animals like humans have much thicker limbs in comparison to their bodies, and the largest animals, like elephants, have really thick limbs to hold their weight.

I do not believe this is correct. Can you make a chemical argument to support your claim? As I understand it, the polar nature of water makes it better suited as a medium to transport ions, which in turn is going to be the basis of any kind of energy transport that is based on chemistry. I have seen some very compelling chemical arguments that polar liquids really are best. They are better for dissolving nutrients (whatever this life form considers a nutrient) and for exchanging ions (which any chemical life needs to do to transport energy). There are other useful properties of water that come from its polar nature. For example, the fact that ice floats is important to keep the oceans liquid. Using a liquid where ice sinks can very easily lead to the oceans freezing over. Water also has an amazing heat capacity, making it very useful as a way to smooth temperature changes. Water also has a fairly wide range of temperatures where it is liquid. Thus water will be liquid in a wider range of possible planets, and it will be more resistant to climate changes in the planet. There are other molecules that have some or nearly all of these properties, but they are quite rare in the universe. Remember my comment about elemental abundance, with CH4, NH3 and H2O being the most common molecules after molecular hydrogen. Daniel.

I am not a chemist (I'm a physicist), but my understanding is that water is a good medium in general due to its polar nature. But a chemist would be better able to explain this.

On a related note. I think we're spending too much time concentrating on Mars. I think we should drop a couple of baloons on the cloud tops of Venus. I said earlier that the Venusian surface is too hot, but the cloud tops (about 50km up) are actually the most Earth-like environment in the solar system, with a temperature and pressure about the same as the earth. There is microbial life in the clouds of earth. It is conceivable that life might have appeared on Venus before it got too hot, and that it might have survived in the clouds. I'm sure that's worth a single mission with a few baloons.

Saying that you are stuck with carbon is approximately true. Carbon is magnitudes better than any other atom. Silicon is probably #2, but it doesn't hold a candle to carbon. So, although I guess silicon-based life is permitted by the laws of physics, the probabilities are heavily stacked against it. When I say "you are stuck with carbon" I am speaking loosely, and the statement is to be an approximation to the truth. You are missing the forest for the trees. High pressure, temperature and radiation all break carbon bonds (btw, extremes in pH do too, I forgot about those). As you go toward higher pressure, temperature or radiation, the environment becomes less hospitable, and any life becomes more and more difficult. Extremophiles on earth have to come up with various creative ways to deal with these challenges, like running heat pumps to keep their internal temperature lower than the outside. Incidentally, my understanding is that hyperthermophiles actually thrive on temperatures up to about 100 degrees C, and not the 300 C that you get in the vents themselves. For comparison, the temperature on the surface of Venus is 460 C. Maybe there is life on the surface of Venus, but you have to understand that the conditions there really hare fundamentally inhospitable for carbon bonds. It is entirely reasonable to look at a chemistry textbook, find out what things break carbon bonds, and then say that environments with a lot of that are inhospitable for carbon bonds. Just so I know, are you a chemist? I am not a chemist, I am a physicist. If you are a chemist I will take your word for it, but my understanding of chemistry is that silicon really is a much poorer choice than carbon because the valence electrons are less bound to the atom, making it more difficult to build complex molecules without them falling apart. This argument is independent of what we see directly on earth. As a physicist, I can make another argument based on elemental abundance: Carbon is far more common in the universe than silicon. The most common atoms in the universe after H and He are C, N an O, and the most common molecules after molecular hydrogen are CH4, NH3 an H2O. Purely from elemental abundance, these molecules are logical choices for life, and their chemical properties together certainly gives them a lot of weight as the best candidates in our search for extraterrestrial life. As a scientist, I am more interested in what is probable than what is possible. A surprising number of things are technically possible. I don't think that silicon life is prohibited by the laws of physics. But saying that something is not necessarily impossible does not automatically make it likely or interesting. It is possible that there is an alien spaceship parked at the L3 point of Mercury, but looking for it is not a very intelligent way to search for alien life. We know the laws of physics, and we know quite a lot about chemistry. Physical laws are universal, and understanding them can go a long way in guiding us in the search for alien life. This is a strong statement to make. I'd like to know if you are a chemist. Since only a chemist should be able to make a blanket statement about whether we know enough chemistry or not to reach these conclusions. Earlier in this post I offered some data backing up the assumption of life based on C, N, O based on elemental abundance in the universe.

Always happy to help. The correct, but boring answer is that I cannot. And anyone who says they can is probably lying or delusional. We do not understand how the universe originated. Anyone who says that it was by a quantum fluctuation is just putting words together because quantum mechanics and general relativity are both useless in the realm of the origin of the universe. The first thing we need before we can answer your question is a quantum theory of gravity, and we do not have that. There are some research projects trying to do that. One is String Theory, and another is Quantum Loop Gravity. But both projects have a lot of issues yet to resolve, and neither one has made a single prediction that we've tested. I have no idea I know that's not what you want to hear, but on the other hand, this is why it's interesting. This is the process of discovery. You don't know what you are going to find out. But the #1 thing they hope to answer is whether the Higgs boson actually exists. It is predicted by quantum mechanics. If it doesn't exist, physicists will spend the next few years figuring out how to revise the theory of quantum mechanics. I am not a particle physicists (my background is astrophysics) so I cannot really answer questions about the LHC. If I tried to answer, I'd basically just be copying from Wikipedia.

The biggest problem with Venus is not the lack of water but the pressure and temperature. I'll explain: Start with the assumption that life is based on chemistry as opposed to some weird, novel physics that we have never heard of. Chemical life will almost certainly be based on carbon because carbon can make incredibly complex molecules and other elements cannot. We don't know what life is, but we know that it is something complex. And if you want complex chemistry, you are stuck with carbon. What does this mean? It means that any environment where carbon compounds are regularly destroyed will be inhospitable to life. And in general, anything that destroys chemical bonds will be inhospitable to life. So we can ask ourselves, what sort of things destroy chemical bonds? Well, some examples include high radiation, high temperature and high pressure. When you cook something, the "cooking" process consists of breaking carbon bonds (which is why cooked food is easier to digest). You can also cook things with pressure, because pressure breaks molecular bonds too. Anyway, long story short, Venus has both high pressure and high temperature, and that environment will break most chemical bonds, and in particular, it will break carbon-carbon bonds. Thus making Venus inhospitable to life, without having to invoke any argument about water. Cheers, Daniel.

+1 Indeed. Hawking has some hugely cited papers ("Particle creation by black holes" has 3088 cites on ADS which is incredible) but his popular work is old (1970s - 1980s). Let's compare Hawking with Kip Thorne, who is another top mind in that field, and has a similar age: * Thorne has more than twice as many published papers as Hawking. * Thorne has slightly more total citations than Hawking. * In the last 5 years Hawking has published 13 papers compared to Thorne's 200. * Hawking's recent work has been cited 219 times, while Thorne's has been cited 3570 times. While Hawking's contributions are indeed significant, does he really deserve to be so famous while Thorne is largely unknown?

I completely agree. I think that both Einstein and Stachel spoke poorly. Maybe Einstein was being asked a question orally and didn't have time to prepare a carefully thought-out answer, but Stachel was writing a book and he should have been more diligent in thinking about how the reader might interpret what he wrote and how he quoted.

I might have misunderstood, but I don't think what you said makes sense. The event horizon has a definitive size, and a definitive, non-zero area. If I may correct your English, the verb "read" is irregular and the past tense is written the same as the present, but it is pronounced differently: * "I read a book now" * "I read a book yesterday". Yes, English is a stupid language In the first example "read" sounds like /rid/ while in the past tense, "read" sounds like /red/. I wouldn't make this sort of logical jump without at least seeing the exact text that you read. The word "timelike" means that the time+distance between two events is such that something travelling at less than the speed of light could go between these events, and "spacelike" is the opposite. When you say that the time dimension becomes spacelike, what do you mean? When you say that the space dimensions become timelike, what do you mean?

The James Webb telescope is very interesting too. Although it can't image planets directly like the E-ELT, it can do important things that the E-ELT cannot do. The Webb is an infrared telescope. Infrared light is important because it can penetrate through dust clouds which normally block visible light. So, for example, it can look inside a planet-forming region which is all surrounded by dust. Learning how planets form would help us know how common they are in the universe. So this is related to how common Earth-like planets and life are in the universe. And the E-ELT will be completely useless for this kind of observation. Any telescope with a segmented mirror is useless for infrared, because the little cracks between the mirrors emit enough heat to completely ruin the observations. There are other exciting things happening soon. The Kepler mission is already in space and it seems to be finding planets by the week. The first planets it finds are the ones in very close orbits, but in a couple of years it should find a good number of Earth-like planets in Earth-like habitable zones. When that happens, we'll have an idea of how common Earth-like planets in the HZ are. And we'll have a catalog of interesting planets ready for the E-ELT to take a peek at when it comes online, so we can probe them for bio-signatures. By the end of the decade we might have found life outside of Earth (fingers crossed).

If you read that quote carefully, Einstein did not say "matter", he said "gravitational field". Look closely: "If we imagine the gravitational field, i.e. the functions gab to be removed, there does not remain a (flat) space, but absolutely nothing... there is no such thing as ... space without a (gravitational) field." This is absolutely correct, but notice that he didn't say "matter", he said "gravity". In GR, what we call "gravity" is actually just the curvature of space-time. You cannot separate space-time from its curvature. I can make a very good analogy: Think of a 2D surface: the surface of a sphere, a flat square, a cylinder, a saddle, etc. They all have a curvature, right? A sphere has positive curvature (lines that start parallel will converge), a saddle has negative curvature (lines that start parallel will diverge) and a flat square has a zero curvature (parallel lines remain parallel). Agreed? Now I imagine that I ask you to give me a 2D surface, but give it to me without any curvature... I want to add the curvature later. You'd say that that's not possible. Any 2D surface you can imagine has some curvature. It may be positive, negative, or zero, but it has something. Your reply to me might be: "If we imagine the curvature ... to be removed, there does not remain a (flat) 2D surface, but absolutely nothing... there is no such thing as ... a 2D surface without a curvature." The equations gab are the metric of space-time. The metric is the thing that tells you how space is curved. It is the function that lets you convert from your coordinate system to distance, and inherent in that formula is the curvature of your space. You are familiar with one metric: the metric of Eucledian 3D space. Imagine that I ask you to find the distance between two points (x0,y0,z0) and (x1,y1,z1). You would do this: distance = sqrt( (x0-x1)^2 + (y0-y1)^2 + (z0-z1)^2 ) This is a metric. If you were using polar coordinates, the equation would look different. This equation describes the curvature of the space (in this case, it is flat). The equations gab are the formulas that describe distance, an hence curvature, in our universe. Does this help? Thanks for highlighting that quote. I didn't read it carefully enough the first time. I can see how this can lead to confusion if you are not careful with language. In one context you might say that a flat square has no curvature, but what you really mean here is that it has a curvature of zero. Likewise, you might say that without matter there is no gravitational field, but in that case, what you mean is that you have a space-time curvature of zero. Then a week later, in a different context, you might say that every surface has to have a curvature, meaning that the curvature always has some value, even if that value is zero, just like you might say that you cannot have a universe without a gravitational field, because the universe always has some kind of curvature, even if it is zero. Then someone else reads these two quotes out of context and thinks that you are saying something that you didn't mean to say. Does that help?

Basically, yes. The idea is that the stars in Kepler's field of vision should be representative. So whatever it finds should be at least indicative of what you can expect around here as well. Which telescope are you referring to? The only one I know that will be able to do that is the E-ELT which will come around 2018. Is this what you are thinking of? I just want to make sure I know what you're talking about before I say anything. We came across the planet by the wobble method. The question about *human* life is really too specific for any meaningful answer right now. Look at my reply earlier, where I mentioned the work by Joshi (2003). His model suggests that actually the planet would be reasonably nice. If it has a good amount of ocean, or a thick atmosphere, the temperature range should not be too extreme and the planet should actually be fairly habitable. Basically, the more water it has the better. A thick atmosphere can also do a lot to equalize the temperature in a planet. Look at Venus. It has a ridiculously long day, but the temperature is actually quite uniform thanks to the thick atmosphere and strong winds.

Honestly, I don't know what he means. I am not sure that this is a published (aka refereed) article. I cannot find it in any refereed journal. I should mention that articles at arXiv.org are not refereed. Basically anyone with an account can upload whatever they want. This can be great because you can get very recent research, but you can also get complete garbage some times, and if you are not an expert it can be difficult to know which is which. I am not an expert on Big Bang cosmology, so I really cannot judge. But isn't it interesting that this article is 12 years old and I still can't find any referee articles that say "white hole" in the abstract? If you want to search for referred papers, you should try ADS: http://adsabs.harvard.edu/abstract_service.html If you scroll to the bottom of the page, under the section "FILTERS", there is an option to get only refereed articles. You should always pick that if you want to separate mainstream science from just whatever someone decided to upload. Hope that helps.

I would not advice it. I think ajb was just too polite to say it, but the idea that the Big Bang was a "white hole" or that it resulted from a black hole in another universe is basically a crackpot idea with no basis on modern science. Btw, none of this should be confused with the "Big Bounce" ideas from LQG. I just did a search on ADS. There are no published papers with the term "white hole" in the abstract.

Not only that, but we have tested our physical theories at great scales, from locally down here on earth all the way up to stars and galaxies. You don't have to invoke the cosmological principle as an a-priori assumption. The universal applicability of the laws of physics is something you can test empirically. Things like the CMBR, pulsars and binary pulsars all provide important testing grounds for both GR and quantum theory. For example, the masses of white dwarfs and neutron stars requires the application of both GR and quantum theory, and the observed masses match the predictions. Also, to make stars work you need quantum theory - not only for the fusion part, but because stars would not work without quantum tunneling. The energy output of stars thus provides a real test for the laws of quantum mechanics. It is wrong to think that scientists just dream up theories, try them out here on earth and assume that they work everywhere. This is not how science works. Astronomers regularly look for ways to test physical theories on the large scale. For example, one post-doc at my university is exploring ways that you could use observations of binary stars in the galactic center to test supersymmetric theories of dark matter.

Saying "a new way" is incorrect. Currently we have no way whatsoever. A theory of quantum gravity may or may not tell us something about the origin of the universe, and today we basically don't know anything. That's the most accurate way I can put it. There is an error here. We do not, and cannot, use GR to understand anything "before" the Big Bang, or, for that matter, a few Planck times after the Big Bang. This is a realm where quantum effects cannot be ignored (as they are in GR) and traditional GR is useless. I still think you are incorrect. I have studied GR, so I am speaking from personal knowledge. I believe that Stachel spoke poorly in some ways. Most, or nearly all of what he says is correct. But the way it is phrased, I understand how someone who is not familiar with the math behind GR might interpret things the way you did. I believe that the key point Stachel is making, which is novel for most people, is that matter does not travel through a fixed space background (as most people imagine), but that space itself is a function of mass, and that even the very notion of distance and time (aka, the metric) are flexible concepts which only realize their meaning in the context of the matter and energy that is actually present in the universe. In any case, it is perfectly possible to solve the Einstein field equations for a universe with no matter. These simply set the term T = 0 and they are known as "vacuum solutions". The simplest case is if the cosmological constant is also set to zero. In that case, the solution to the Einstein equations is just flat Minkowski space-time, which you may already be familiar with. Another interesting solution is the Kanser metric: http://en.wikipedia.org/wiki/Kasner_metric There is a Wikipedia article that talks about vacuum solutions: http://en.wikipedia.org/wiki/Vacuum_solution_(general_relativity). But I'm not sure how relevant it is for you because most of the solutions are not really about a universe that is devoid of matter, which is what you are interested in. Personally I would like to see a solution for a universe with no matter, but a non-zero cosmological constant. I could try to do it myself, but I haven't done GR in a while and solving the Einstein field equations requires *a* *lot* of tedious work. I might give it a try next week when I'm less busy. Hope this helps. I don't like the use of the word "theory" in this context. A scientific theory is something that makes specific predictions that can be tested. The sort of hypothesis that you are talking about do not lead to any testable predictions.

We do not know if wormholes can exist. The best current answer is "it looks unlikely". A black hole is not a wormhole and it is formed very differently. A black hole is just something that has so much mass and density that light cannot escape. For a wormhole to exist, the equations seem to suggest that you would need some material with "negative energy" and no such thing is known in the universe.

Don't listen to him. The truth is that we do not know what was happening in the early universe at around the Planck time (which is 10^(-44) sec). To understand the universe at this scale we need a theory of Quantum Gravity and we do not have one. So it is meaningless to make any claims about "before" the Big Bang or "what caused the Big Bang?". These are not questions that our current science can begin to address. One of the most intense research areas in physics today is quantum gravity, because we would really like to probe deeper and try to answer these kinds of questions.

Can you justify this claim? I see nothing about the Einstein field equations that justify such claim. Start with: G + lambda g = 8 pi T If there is no matter or energy, then T = 0 and you get a universe of constant space-time curvature. There is nothing magical or special about this solution. This is in fact the simplest exercise when learning GR. There is nothing here that implies "no space, no time, no spacetime" like you suggested. Sorry to say, but I don't think you know what you are talking about. There is a sense in which you can say that space-time "began" at the Big Bang, but it has nothing to do with what you suggested. It has *nothing* to do with "remove matter and you get no universe". The first thing you need to learn is that when we say that the universe is expanding, we mean that space itself is expanding / stretching. Run the clock backward and there was a time when the whole universe was the size of a tennis ball. This doesn't mean that there was a big empty universe and all matter was concentrated in a tiny space. No. The universe itself was that size. An ant crawling through that universe would have been able to walk on a "straight" line and come back to the point it started. Now continue running the clock backward and you find that at a time the entire universe was the size of an atom. This is the picture that you should have in mind when you hear that space-time began at the Big Bang. The Big Bang is the expansion of the universe itself at the very beginning. I hope some of what I've said made things clearer to you. If not, you could look for a GR textbook in your local library that can explain things a lot better than anything I can say in a forum post.

It was called an asteroid until the definition of "planet" was updated and the term "dwarf planet" was added. It is different from the normal asteroids because it has enough mass that gravity forces it into a round shape. That's the difference between a dwarf planet and, say, an asteroid. Objects in the asteroid belt come in many sizes. Most are tiny. Ceres is special. Ceres has 1/3 the mass of the entire asteroid belt.

Ceres is currently classified as a dwarf planet, rather than a planet. The reason for that is that it does not dominate its orbit. Ceres only has about 1/3 the mass of the asteroid belt. Contrast with all the proper planets which have (together with their moons) something like 99.999% of the mass of their orbit. In the current classification, a true planet dominates its orbit. Ceres does not dominate its orbit.

The origin of the universe is not currently understood. To probe it, we need to start by developing a coherent theory of quantum gravity. Current research paths include String Theory, Loop Quantum Gravity and the "Exceptionally Simple Theory of Everything" (which, btw, is not simple in the sense that you understand it - the name is a mathematical pun).

I think it surprised most scientists. I don't think any astronomer expected to find a planet like that so soon and so close. These guys really preempted the Kepler mission. We were expecting to get the first Earth-like planets in a couple of years, and we were expecting that they'd be very far away. We should still look forward to the Kepler results though. Kepler will give us a good idea of how common Earth-like planets are. Then we'll know, for example, if Gliese 581g is a freak accident or if there are probably more planets like it within 20 ly.

Just think about it for a minute. Why would you get big pieces? Before the supernova you have a star that is basically made of gas with a core of degenerate matter surrounded by a shell of plasma. Nothing here is solid in the sense that you and I understand the term "solid". When fusion stops at the core, you have a collapse. This collapse is driven by gravitational potential energy and it is so energetic that it actually runs fusion backward (photons have enough energy to break atomic nuclei into lighter elements). This energy tears apart the outer ~85% of the star leaving behind only a small ~15% which is the core of degenerate matter that we call a neutron star. Looking at this process, why should you expect there to be any kind of big solid pieces of any kind? At these energies you shouldn't even expect molecules. The energy that the particles have is many orders of magnitude greater than inter molecular forces. There should be no molecules at all, much less "big pieces".