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  Click here to go to the first staff post in this thread.   Thread: Ask me about astronomy

  1.   Click here to go to the next staff post in this thread.   #11
    Retired Staff Frank LeRenard's Avatar
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    Quote Originally Posted by Fay V View Post
    does physics tell us that photons are absent from time?

    So as you approach the speed of light time appears slower relative to you. In class today someone was making the argument that from the point of view of a photon there is no time, photons are out of time. Which seems weird. To us that makes sense, from our relative perspective time does not effect photons, but from the relative point of view of a photon wouldn't it still be effected by time?

    The claim was no, because when you hit the speed of light you divide by zero or get infinite, so math breaks. Personally I think this is a cope out, just because math breaks does not mean that the item itself is not effected by time, we do see photons can get from point A to point B. so wouldn't it have to have some effect from time? even if functionally from our perspective it is not effected by time.

    (this is not astronomy, but you know physics.)
    Hmmm.. special relativity. I'll do my best.

    So, how time passes depends on the frame of reference. If you are stationary and watching someone else move with respect to you, time will appear to tick more slowly for that moving person. However, from the moving person's perspective, he is stationary and you are the one moving, so now time is running normal speed for him and more slowly for you. Without some stationary background to measure against (say, the ground), this is the best you can do (because the only two points of reference are you and the other guy).

    The reason for this, though, is that there is sort of a universal rest frame; the speed of light. In any reference frame, the speed of light is constant. So if in your rest frame, you watch a light beam go from one side of the room to the other, it covers the distance of that straight line in the time D/c (D being distance, c being the speed of light). But if instead you're watching that same beam going straight across that same room, but from your point of view the room is moving, now the path the light appears to be making from your point of view is tilted; it starts at one side of the room, but by the time it gets to the other the room has moved (and therefore so has the part of the wall that the light beam was bound to hit). So now the light beam's path looks longer to you (D + dx, dx being the amount the room moved. And if you want to be a stickler here just pretend like there's a vector symbol over those variables).

    But from your point of view the light beam is still moving at the same speed. So now the beam of light, in your reference frame, is covering a longer distance than in the rest frame, but at the same speed. Which means it takes longer for the beam to travel across the room from your point of view. But you know that it's really going straight across the room in the moving person's frame, not at an angle. So the only way to rectify that is to say that time has slowed down in the moving person's frame.
    Incidentally, a similar line of reasoning leads one to realize that distances also appear foreshortened in the moving frame (that the speed of light is always the same).


    Does that make sense? No? Moving on.

    Now imagine the room itself is moving at the speed of light, away from you. Both the room and the light beam are traveling at the same speed, so from your point of view, the light beam has to travel an infinite extra distance to reach the other wall. So it looks like time has frozen in that moving room.

    So if the faster something moves relative to you, the slower time seems to tick, then once you reach that speed limit (the speed of light), time will appear to stop. So from your point of view, watching a photon, it doesn't seem to age (because it is moving relative to you at the speed of light). And from the point of view of the photon, the whole rest of the universe is moving at the speed of light, so now nothing in the universe appears to age from the photon's point of view. Also, all distances are infinitely foreshortened from the photon's point of view, so every photon comes into existence, travels 0 meters in 0 seconds, and then gets absorbed by some bit of matter. Every time. Hence photons are, in a way, outside of time.

    I don't know. Does that make sense? Relativity never fails to hurt my brain, but I do my best to understand it.
    Last edited by Frank LeRenard; 09-24-2014 at 10:10 PM.

  2.   Click here to go to the next staff post in this thread.   #12
    Didn't try, Succeeded Fay V's Avatar



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    I about half understand that, which is better than where I was. thank you. sorry for the hard question

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    Heretic! FlynnCoyote's Avatar


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    Hehe, nice. Picked up on the stuff I left out too.


    Okay, next: What affects a star's brightness compared to its mass? For example, among supergiants and hypergiants, there seem to be blue classed stars and red classed stars. Yet among smaller stars, blue seems to be rarer and yellow to white far more common. Why is this? (I'm using old info from a nine year old science textbook, so if this question is no longer relevant, feel free to correct me)

    What's the average lifespan of a sun sized or similarly small star, for example Sirius A. What's the lifespan of much larger stars, such as Rigel or the even larger Pistol Star? Why is it different?

    What causes smaller planets to form rocky and larger once to be mostly gas?
    * * *
    We'll find a reason, or else realize that we don't need one.

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    Senior Torrijos_sama's Avatar
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    What is the compatibility between a Virgo and a Sagittarius?

  5.   Click here to go to the next staff post in this thread.   #15
    Retired Staff Frank LeRenard's Avatar
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    Quote Originally Posted by FlynnCoyote View Post
    Hehe, nice. Picked up on the stuff I left out too.


    Okay, next: What affects a star's brightness compared to its mass? For example, among supergiants and hypergiants, there seem to be blue classed stars and red classed stars. Yet among smaller stars, blue seems to be rarer and yellow to white far more common. Why is this? (I'm using old info from a nine year old science textbook, so if this question is no longer relevant, feel free to correct me)

    What's the average lifespan of a sun sized or similarly small star, for example Sirius A. What's the lifespan of much larger stars, such as Rigel or the even larger Pistol Star? Why is it different?
    Since all the stellar ones are related, I'll answer them all kind of at the same time.
    So, mass is the main driver behind how a star lives out its life. A star is a star because it's massive enough that its own self gravity (the gravity that keeps it all as one body) is strong enough to create temperatures and pressures in its core that favor nuclear fusion. The outward pressure from the fusion reactions balances the inward pressure from gravity, and the star becomes stable (this is called 'hydrostatic equilibrium').
    But nuclear fusion reactions are extremely sensitive to temperature. For example, the proton-proton chain (the most common reaction in a sun-like star) has a fusion reaction rate that goes as ~T^4, so for an increase in temperature of 2 degrees, the nuclear fusion rate goes up by a factor of 2^4 = 16. Other fusion reactions have an even stronger dependence (the CNO cycle, for example, goes like ~T^20, so now your 2 degree increase increases the reaction rate by 2^20 = 1048576 times). The reaction rate going up means the fuel gets burned more quickly, which means the star's lifespan is shorter.
    So what changes the core temperature? Generically, the star's mass! If you remember your high school chemistry, there's a relationship between pressure, volume, and temperature known as the Ideal Gas Law (PV=nRT, or however you want to write it). The more massive the star, the higher the pressure on the core from all the weight bearing down on it; the volume doesn't change much because of the star being in hydrostatic equilibrium, so temperature must be higher. So more massive = higher core temperature = faster burning rate = shorter lifespan.
    Sunlike stars have an average lifespan of around 10 billion years (our sun is about half that old right now, so it has another 5 Gyr to go). Stars like Rigel (Rigel is about 20 solar masses) are around more like 10 million (with an M) years.


    Now, the shorter lifespan is why hot, massive, blue stars are rarer in the night sky than older, cooler yellow or red stars. Everything in space takes place over millions to billions of years (well... almost everything), so in the average human lifespan (or even in the span of all of human history), things look practically static. So what we're seeing in the night sky is more or less a snapshot of how things are at this stage in the universe's history. Now, the universe's history is about 14 billion years. So say you have a population of stars that's born sometime in the history of the universe (any time during that 14 billion years or so, after the point where stars can actually form). We don't know when, just sometime. As that population ages, the more massive stars die off and leave behind the less massive stars. So what are the chances that, when you look up right now, you'll see this population of stars at a time when all of its most massive stars are still alive?
    Well, to abuse the laws of probability, it's basically: Lifetime of stars / Age of universe = 10 million years / 14 billion years = 0.0000005 %
    So it's going to be rare that you'll see young stars precisely because the universe is so damned old. This is obviously mitigated by the actual number of stars that are out there (which is a lot), but you get the point.

    Now, as for what determines the brightness for a given mass, that has to do with something called luminosity. Luminosity is how much light is given off by the star per second (it's actually measured in Watts, like your lightbulbs). Obviously, the luminosity is going to be determined mainly by the nuclear reaction rate in the core (the more reactions you have per second, the more photons you get out), which means it's going to be determined by temperature, which we already discussed is determined by mass. But stars are also large physical bodies. They're spheres, in fact, and so the light that comes out of the core has to leave through some point on the outside of that sphere. If the star is small, there's not as much surface area for the light to leave from as there would be for a much bigger star. So actually, the star's brightness is also determined by how physically big it is.
    Luminosity is proportional to: surface area * temperature^4
    For a sphere, surface area is 4*pi*radius^2, so L ~ R^2 * T^4
    So for two stars with the same mass, the bigger star will be brighter.

    That said, the difference between things like blue supergiants or blue hypergiants or red giants or asymptotic giant branch stars or red clump stars or horizontal branch stars etc. etc. has more to do with the stage of evolution of the star, the mass of the star, the composition of the star (hydrogen + helium + what else?), and so on and so forth, rather than just, say, radius. But that's a whole different topic that would take six weeks to properly explain, so I'll stop here.

    What causes smaller planets to form rocky and larger once to be mostly gas?
    I admit, I'm not much of a planet guy, but I believe this has to do with both the mass of the planet and where in the debris disk the planet forms. A more massive planet is going to have stronger gravity, so it's going to be able to hold on to a bigger atmosphere (this is partly why Earth has a sizable atmosphere but Mars doesn't; Mars' atmosphere was able to be blown away over time because Mars wasn't strong enough to hold onto it). But also, planets that form farther out form where things are colder; colder things move slower, so lighter elements like hydrogen and helium won't be so ready to escape, and lots more of the heavier compounds will be able to form as well (things like methane, ammonia gas, carbon dioxide, etc.) because they won't be broken apart by high-speed collisions or bombardment by photons coming from the proto-sun. So if you have a more massive planet forming out in the cold part of the debris disk, it will probably collect a large, heavy atmosphere and hence end up looking more like Jupiter than like Earth.

    But again, I'm not a planet guy, so I'm sure I'm missing a lot of important details and controversy here.

    Quote Originally Posted by Torrijos_sama View Post
    What is the compatibility between a Virgo and a Sagittarius?
    47.46% +/- 30.11%
    Last edited by Frank LeRenard; 09-27-2014 at 10:36 AM.

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    Senior Torrijos_sama's Avatar
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    25 or 6 to 4?

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    Senior Rilvor's Avatar
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    Do you think the universe is expanding outward or merely expanding infinitely within itself? Or perhaps some other idea?

    What are your thoughts on String Theory?

    What happens if two Black Holes were to collide?

    Can you tell me more about Apparent Horizon?

  8.   Click here to go to the next staff post in this thread.   #18
    Retired Staff Frank LeRenard's Avatar
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    Been on hiatus for a while, folks, due to academic/other needs, so apologies for delayed responses. I'm going to go ahead and ignore Torrijos' question since I don't immediately know what that means.

    Quote Originally Posted by Rilvor View Post
    Do you think the universe is expanding outward or merely expanding infinitely within itself? Or perhaps some other idea?
    So... for those who don't know the background here, we've got some good evidence that the universe is, in fact, not static like everybody thought back in Einstein's day (Einstein invented the 'cosmological constant' to fix his equation so that a static universe was possible, because he really thought the universe was static; that was his apocryphal 'greatest blunder' you might have heard about).

    Now, we can measure how quickly a light-emitting astronomical object is moving either directly toward or directly away from us by looking at its spectrum. The spectrum is just the light split into its component wavelengths: a rainbow is basically a lot of water in the atmosphere splitting sunlight into a spectrum (so that you see all the colors that make up the 'white' sunlight). When stars emit light, though, there are things like hydrogen or iron or calcium or whatever present in their photospheres, which said emitted light has to pass through before it goes out into the universe. Those elements (because of quantum mechanics) absorb very specific wavelengths of light (they can also emit those same wavelengths under different conditions), and so the spectrum of a star will have all of these dark spaces in it -- colors in the rainbow that got eaten by the elements in the photosphere. We know where those dark spaces are supposed to be, but if the emitting object (the star) is moving with respect to you, the light it emits gets shifted either toward bluer colors (if it's moving toward you) or redder colors (if it's moving away from you). The amount of the shift is proportional to the speed at which the object is moving, which is handy.

    Basically it's the Doppler effect for light. Like, you know how when an ambulance blaring a siren is moving toward you, you hear a higher pitch? Higher pitch means higher frequency, so less space between wave peaks. In other words, shorter wavelengths, which when talking about light means bluer light.

    So the crazy thing is, if you look at a whole bunch of galaxies and you use that method (get their spectra) to figure out how they're moving with respect to us, almost every galaxy in the sky is moving AWAY from us. And the farther away the galaxy is, the faster it's moving away.

    So there's two interpretations, here: 1.) we're super special, and all other galaxies hate us and so are moving away from us, or 2.) the space between all galaxies is expanding. The latter is well-explained by the [in]famous raisin-cake analogy, and I found a cute little applet that explains it graphically: http://www.ucolick.org/~max/Astro18_...aisin_cake.htm

    Since we all like to believe in the Copernican principle (because it's reasonable), the second explanation is better. The universe is probably expanding. And of course recent observations of supernovae indicate that the expansion rate is increasing over time for some reason, but I won't get into that because it's not something anyone anywhere understands (but they call the thing that's making it speed up 'dark energy').

    Now... with regard to your question, Rilvor, the expansion seems to be isotropic and homogeneous, in other words, space itself (the metric) is getting bigger. Like, let's say you know of two perfectly stationary points in the universe, and you decide to measure their separation, with, like, a wood ruler (something that doesn't change its shape or size over time). And you know somehow that these two points are stationary, so you can assume that their separation is a good universal unit of distance. But if you were to wait a year (or a day, or a second, or anything), though, and then measure that distance again, and compare it to what you measured last year, you would find that your new unit of distance is now bigger. Even though neither of those points you were using to get the distance has moved (hence the 'perfectly stationary').

    So in that sense, saying things like 'expanding outward' doesn't have much meaning. Expanding 'outward' implies a universal frame of reference (like, a center of the universe, or something), but the way the universe is expanding implies that there is no such frame of reference. So it's not 'expanding outward', it's just... expanding.

    I admit it's a hard concept for me to wrap my brain around. And of course, it doesn't apply on scales where gravity takes over and keeps things together, hence why, for example, the Earth isn't getting bigger over time (and also why you're allowed to use a wood ruler to make the measurement; it's bound by its own inter-atomic forces and so doesn't change shape). Neither do individual galaxies, or even individual groups or individual clusters of galaxies. But different clusters of galaxies are moving away from each other with the expansion.

    Is this making any sense?

    What are your thoughts on String Theory?
    Well... I have some sparse ones. I really know very little about it. From what I've heard, it's apparently quite an elegant way to unify the fundamental forces (electromagnetism, gravity, and the strong and weak nuclear forces) and it can be used to explain a lot of the weirdness we see on crazy scales like what you find in black holes. But I guess the major complaint about it is that it's been really bad at making testable predictions, so it's essentially doomed to forever remain a cool idea that might be true. Personally, I'm loathe to really want to accept a theory where you have to apply 11 different spatial dimensions to solve your problems. That reeks of overparametrization to me; if you make something complicated enough, it will fit your data, but it won't be stable at all in terms of robust predictions. If you've ever heard the story of epicycles, you'll know what I'm talking about. Not that String Theory is that bad, but you know.

    What happens if two Black Holes were to collide?
    They form a more massive black hole with a larger Schwarzschild radius.
    I mean, essentially that's really what the end result is. The process itself is apparently quite a lot of fun and involves emission of gravitational waves and other highly energetic things, but that's what happens. Incidentally, there's a recent paper out showing what the gravitational lensing would look like during that process. A nice summary of it can be found here: http://astrobites.org/2014/11/04/wha...ger-look-like/

    Can you tell me more about Apparent Horizon?
    So, I just looked at the Wikipedia entry on that term, since I'd never heard it before, and after reading it, my answer is going to be 'no'. Or, at least, not without spending a hell of a lot more time trying to figure it out. Apologies, but my GR background is unfortunately severely lacking, and this appears to be another one of those horrible mind-bending concepts about null geodesics and curvature and light paths perpendicular to your spacetime metric and all that other shit that I just haven't taken the time to work out in my brain yet.

  9.   Click here to go to the next staff post in this thread.   #19
    Retired Staff Frank LeRenard's Avatar
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    Quote Originally Posted by XoPachi View Post
    What form of matter does light fall under if it's classified as any?
    'Matter' is generally defined as something that has mass, in which case, light is not a form of matter. But of course there's fluidity in this, because mass and energy are equivalent (for example, the faster something moves, the more kinetic energy it has and the more massive it is), so because you can readily convert mass into light and light into mass, it's a bit more ambiguous than that. But what is solid is that light does not have mass, and so generically it's more appropriate to call it 'energy' than to call it 'matter'.

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    Senior Littlerock's Avatar

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    What was the nomenclature behind the term 'spaghettification', who decided that it should be a real word, and where can I get some of that rediculous naming authority? I can think of a few things that need overly specific, utterly idiotic sounding names...

 

 

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