|
Post by Progenitor A on Sept 21, 2010 8:47:23 GMT 1
One hears so much nonsense about the velocity of light being the limit of velocity of any particle (but not phase velocity of course - we see phase velocities greater than the speed of light every day- well almost every day). What nonsense! Whole galaxies are moving faster than the speed of light!
How can I possibly say such things!?
Actually I am just parroting cosmologists that tell me this sort of thing
Here's the (cosmologists)reasoning: The age of the Universe is about 14 billion years That means of course that if a photon was emitted at the Big Bang, it would now be, travelling at c,about 14 billion light-years away
But, we are told, the diameter of the universe is about 93 billion light-years, that is outermost galaxies are some 46 billion light years away from the centre In other words, they are travelling at about three times the speed of light!
No getting away from that folks! You wil hear 'explanations' that the galaxies are not moving at that speed, but that the space in betweeen them is expanding at great speed Fair enough - if you can understand such concepts but as speed = d/t irrespective of how 'distance' is calcuated then the speed of galaxies is ~3c!
QED
|
|
|
Post by abacus9900 on Sept 21, 2010 10:03:49 GMT 1
I think this is the point, naymissus.
It is not the galaxies, per se, that are moving through space at faster than light speed, but the space itself that is doing so, therefore, no rules are broken. I may have this wrong, other people here know more than me. If we come back to the expanding balloon analogy it makes it a bit clearer. Ants, say, on the surface of a balloon can move about but only slowly, but the balloon is expanding at a greater rate than the ants can walk! Substitute galaxies for ants and it becomes clear (doesn't it?).
|
|
|
Post by Progenitor A on Sept 21, 2010 12:58:49 GMT 1
I think this is the point, naymissus. It is not the galaxies, per se, that are moving through space at faster than light speed, but the space itself that is doing so, therefore, no rules are broken. I may have this wrong, other people here know more than me. If we come back to the expanding balloon analogy it makes it a bit clearer. Ants, say, on the surface of a balloon can move about but only slowly, but the balloon is expanding at a greater rate than the ants can walk! Substitute galaxies for ants and it becomes clear (doesn't it?). Personally I do not thing the expanding balloon analogy is much good Abacus Think of a semi -inflated ballooon. Glue two sticks upright on the surface Wind a piece of cotton around a tiny reel so that the length of the cotton is 1m+ the distance between the two sticks. Now place the wound reel over one upright stick and tie the loose end to the other stick Now blow up the balloon and time how long it takes the cotton to unwind from the reel. It it takes 1 second then the sticks are moving apart at a speed of 1 m/s Doesn't matter if the space is expanding or no, the sticks are moving apart a that speed. Is it the analogy that is failing? I don't know really
|
|
|
Post by abacus9900 on Sept 21, 2010 13:45:38 GMT 1
Yes, the sticks are moving but they are moving with the space, not through it at faster than light speed. As long as the sticks are not trying to move faster than light independently across the balloon it's ok. The important thing is that nothing with mass can move faster than light through space. I suppose space can move faster than light because it does not contain mass, but I think we need an expert opinion here, naymissus.
|
|
|
Post by speakertoanimals on Sept 21, 2010 14:32:02 GMT 1
Nope, you're just misunderstanding popular science accounts and accounts from bad TV science programmes.
The point is that what we mean by moving AND what we mean by velocity (and distance and time) are no longer that straightforward once relativity comes in. So, if someone asks what speed a distant galaxy is moving at, that question is no longer simple -- because when we say velocity, we are asking about distances defined at two fixed, different times. And in terms of relativity, what now here is compared to NOW at alpha centauri (let alone a distant galaxy) depends on your point of view.
So, just as NOW depends on your point of view, so does distance, hence so does velocity. We CAN write things down so that in a particular frame of reference, the apparent speed of a distant galaxy is greater than lightspeed. But that doesn't contradict special relativity, because from the point of view of a local frame of reference at the distance galaxy, everything there is always described as travelling at less than lightspeed.
The usual simple explanation is that there is a difference to be drawn between something REALLY traveling at greater than lightspeed (i.e something overtakes light traveling in its vicinity, which surely is a totally physical, tortoise and hare definition of faster than or slower than), and something where the space between us is expanding so fast that light from the distant object can never reach us. In the first case, the light races the galaxy itself (and galaxy never goes faster than light), whereas in the second case, what the light is racing (and losing to) is the distance still to travel before it reaches us. It is the expanding distance that wins the race, NOT the galaxy overtaking the light.
|
|
|
Post by Progenitor A on Sept 21, 2010 16:46:08 GMT 1
The problem studiously avoided is this It does not matter what the mechanism separating the galaxies is. If the Classical law s=d/t holds, then the galaxies are moving at greater than the speed of light. The only way they cannot be doing so is if, in this case, time is dilated by a factor of 3, and even then the galaxies are moving at the speed of light. And why would the time factor for all the universe be dilated by this amount?
|
|
|
Post by speakertoanimals on Sept 21, 2010 17:06:42 GMT 1
If the Classical law s=d/t holds... There's your error right there! As I've already explained, distance no longer has such a simple meaning in relativity, once distance depends on your point of view (moving spaceships get shortened according to someone watching them rush past), and time does as well (moving clocks run slow). Given that, measuring speed using distance over time then allows a variety of speeds, depending on your point of view. Hence I present a SIMPLER definition. I and anyone else, will say that a galaxy is moving faster than light if it outdistances a beam of light moving along with it. By that definition, galaxies are not moving faster than lightspeed. What is 'moving faster' is in effect the space in between the galaxy and us, in that fast as the light moves, the distance still to go expands faster, so that the light can never reach us. I note that a red-shift of z=3 DOES NOT mean that the galaxy is receeding at 3 times lightspeed, because the velocity being used is only the normal Doppler recession speed that would give the same redshift (whereas actual cosmological redshift is caused by expansion of space, not recession of the source), and the result z=v/c is only APPROXIMATE anyway, and only holds good at low real velocities. Hence z=3 does not mean v=3c, either way you look at it. It is vitally important to all this to understand that there are TWO different Doppler effects, one caused by the source moving relative to us, the second caused by the space expanding as the light travels, which of course could have started happening after the light was emitted (hence source was stationary when light emitted) AND need not be constant over the trip the light makes to reach us. None of this is taken into account when we try to APPROXIMATE the second effect by the the first (apparent speed of recession stuff), which is why we SEEM to have galaxies receding faster than the speed of light.
|
|
|
Post by Progenitor A on Sept 21, 2010 18:14:07 GMT 1
If the Classical law s=d/t holds... There's your error right there! As I've already explained, distance no longer has such a simple meaning in relativity, once distance depends on your point of view (moving spaceships get shortened according to someone watching them rush past), and time does as well (moving clocks run slow). Given that, measuring speed using distance over time then allows a variety of speeds, depending on your point of view. My error indeed! It is not me that says the diameter of the universe is 46 billion years and its age 14 billion years, but cosmologistsWhy do they give figures if they do not really mean what they say? Is there dissension in your ranks? Why do you argue that the figures given by cosmologists are wrong? ;D
|
|
|
Post by speakertoanimals on Sept 21, 2010 18:30:48 GMT 1
Your error is taking that diameter and that age, and naively assuming (probably from some mistaken idea that all the galaxies fly out from some central point kind of thing) that you can just compute a speed from that, and label that speed as the average speed of the galaxies at the rim of the universe. Hence getting a speed for galaxies that is greater than lightspeed.
Even taking this simple picture as correct, it forgets that the distance that a galaxy (or photon) did travel has expanded since the photon traversed it. And the universe is continually expanding! So, suppose a photon starts travelling, and after a million years, has travelled a million lightyears, obvious yes? Yes, but since the photon started, the distance it had already traversed has expanded, so we can receive photons from objects that have been travelling for 14 billion years, yet now the objects from which these photons came are now 47 billion lightyears away, not because the objects (or the photons) have travelled faster than light, but because space is continually expanding.
This is a common misconception (or at best, question) that is answered on many cosmology FAQ lists. And the answer is the same -- expansion!
If you'd actually bothered to go and find out what cosmologists have to say about the matter, you'd realise that this is just total nonsense, and the only parrot is yourself.
|
|
|
Post by Progenitor A on Sept 21, 2010 18:52:29 GMT 1
Your error is taking that diameter and that age, and naively assuming (probably from some mistaken idea that all the galaxies fly out from some central point kind of thing) My error again! "probably from some mistaken idea that all the galaxies fly out from some central point kind of thing" But those bollocking cosmo-bollocking-ologists told me (the star-gazing bastards) that the Universe started with a BIG BANG from which everything exploded outward! Poor naive me! Imagining everything is flying outward from the BIG BANG Why do these cosmologists so mislead innocenti such as I?
|
|
|
Post by principled on Sept 21, 2010 19:51:21 GMT 1
"This is a quote from STA (how do I do a quote box BTW) Even taking this simple picture as correct, it forgets that the distance that a galaxy (or photon) did travel has expanded since the photon traversed it. And the universe is continually expanding! So, suppose a photon starts travelling, and after a million years, has travelled a million lightyears, obvious yes? Yes, but since the photon started, the distance it had already traversed has expanded, so we can receive photons from objects that have been travelling for 14 billion years, yet now the objects from which these photons came are now 47 billion lightyears away, not because the objects (or the photons) have travelled faster than light, but because space is continually expanding."
My issue is with the expanding space. A simple example: Galaxy "A" is moving to the right at 0.9c. Galaxy "B" is doing the same but in the opposite direction (This is the classic points on a balloon model). Neither galaxy breaks the speed of light rule, but the total space "distance" is increasing by 1.8c and in order for a photon to go from A to B it would have to traverse the expanding space at greater than c if is ever to arrive at the other galaxy (Space and time bending apart that is). If I'm not mistaken, then this is the point of the OP. P
|
|
|
Post by speakertoanimals on Sept 21, 2010 20:29:27 GMT 1
Yep. The naive view is everything flying outwards from a point, which is indeed just plain wrong. Instead, space is expanding, but there is no central point, and no outward. The big bang happened everywhere, just that everywhere was a lot closer together at that time!
Reason for the confusion -- because every TV science programme you see has galaxies exploding outwards type animation. Don't blame the cosmologists, blame the popular science writers and TV producers who keep using a cliched graphic, without bothering to explain that it is, in almost all aspects, wrong and misleading.
Well, except that moving to the right at c depends on who is doing the observing, and as long as you realise that moving isn't actually what it is doing, just that the space between the unnamed observer and the galaxy is increasing at that rate.
So, all this says is, if apparent recession speed increase with distance (which is another way of saying that is space doubles in size, a point twice as far away will appear to be receding twice as fast!), then at some point you will have galaxies whose apparent recession speed exceeds lightspeed (remembering all the complications that we have ignored here so far)...............
Okay, forget the figures -- if space expands (however it does it), then if you go far enough away, light from very distant objects will do the best it can to reach us, but if space keeps expanding faster than the light can cut down the distance, then light from those objects can NEVER reach us. We also have the different horizon, caused by the fact that ANY light has a finite time to travel. The first is the event horizon, the second is the particle horizon, which expands as the universe gets older.
And finally, don't forget that the universe referred to is actually just the size of the VISIBLE universe. The universe can still be infinite, since the entire universe need not have come from a point, although the visible universe would have (ie any finite chunk).
|
|
|
Post by StuartG on Sept 21, 2010 21:08:35 GMT 1
"This is a quote from STA (how do I do a quote box BTW) Even taking this simple picture as correct, it forgets that the distance that a galaxy (or photon) did travel has expanded since the photon traversed it. And the universe is continually expanding! So, suppose a photon starts travelling, and after a million years, has travelled a million lightyears, obvious yes? Yes, but since the photon started, the distance it had already traversed has expanded, so we can receive photons from objects that have been travelling for 14 billion years, yet now the objects from which these photons came are now 47 billion lightyears away, not because the objects (or the photons) have travelled faster than light, but because space is continually expanding." My issue is with the expanding space. A simple example: Galaxy "A" is moving to the right at 0.9c. Galaxy "B" is doing the same but in the opposite direction (This is the classic points on a balloon model). Neither galaxy breaks the speed of light rule, but the total space "distance" is increasing by 1.8c and in order for a photon to go from A to B it would have to traverse the expanding space at greater than c if is ever to arrive at the other galaxy (Space and time bending apart that is). If I'm not mistaken, then this is the point of the OP. P Top right of the box to quote from eg. StA's, that will take You to the full edit screen, at the top of the already inserted text something like this is seen ..... "This is a quote from STA (how do I do a quote box BTW) so if I want to quote P.... Nope, you're just misunderstanding popular science accounts and accounts from bad TV science programmes. The point is that what we mean by moving AND what we mean by velocity (and distance and time) are no longer that straightforward once relativity comes in. So, if someone asks what speed a distant galaxy is moving at, that question is no longer simple -- because when we say velocity, we are asking about distances defined at two fixed, different times. And in terms of relativity, what now here is compared to NOW at alpha centauri (let alone a distant galaxy) depends on your point of view. So, just as NOW depends on your point of view, so does distance, hence so does velocity. We CAN write things down so that in a particular frame of reference, the apparent speed of a distant galaxy is greater than lightspeed. But that doesn't contradict special relativity, because from the point of view of a local frame of reference at the distance galaxy, everything there is always described as travelling at less than lightspeed. The usual simple explanation is that there is a difference to be drawn between something REALLY traveling at greater than lightspeed (i.e something overtakes light traveling in its vicinity, which surely is a totally physical, tortoise and hare definition of faster than or slower than), and something where the space between us is expanding so fast that light from the distant object can never reach us. In the first case, the light races the galaxy itself (and galaxy never goes faster than light), whereas in the second case, what the light is racing (and losing to) is the distance still to travel before it reaches us. It is the expanding distance that wins the race, NOT the galaxy overtaking the light. then take a second screen on Your browser, find the thread, then the msg, and press the 'quote' button again on the other message for quoting, again the edit screen will appear, highlight all/some of the text to quote, right click, select 'copy', go back to first screen, paste in text, and so ad infinitum, it's possible to quote everybody, of all the threads on the board, or indeed to quote from another forum such as from next door at Marchesa's.... "Isaac Asimov distinguished between two types of scientific heretic: "Endoheretics are appropriately credentialed scientists. If the person is outside the scientific community or at least outside of his specialty, he is an exoheretic. If a person is an endoheretic, he will be considered as eccentric and incompetent, whereas if the person is an exoheretic, he will be regarded as a crackpot, charlatan, or fraud."" en.wikipedia.org/wiki/Heresy#Selected_quotations"I could get a little silly and start alluding to conspiracy theories along the lines of 'The Prisoner' but I'm not that way inclined." So why mention it? You 'go for broke' and mention everything You dislike, and then justify it with examples, a-n-d 'quelle horreur!' sign Your name. To quote another tv series, Perry Mason.... 'Incompetent, irrevalent and immaterial' Anon. Play around in the full edit screen and use the 'Reset' 'Preview' buttons to check that it 'looks right'. Any probs ask again, or use the internal email, by tapping on the name of the user to msg. radio4scienceboards.proboards.com/index.cgi?action=viewprofile&user=stuartg 'Send Personal Message' StuartG mod: remove 'Post Reply' add 'Reset'
|
|
|
Post by Progenitor A on Sept 22, 2010 7:31:17 GMT 1
Yep. The naive view is everything flying outwards from a point, which is indeed just plain wrong. Instead, space is expanding, but there is no central point, and no outward. The big bang happened everywhere, just that everywhere was a lot closer together at that time! Plain wrong ! Again! Will I never get anything right! Ah now if the Big Bang happened everywhere then there was no gross distortion of space-time that other cosmologists tell me is what happens when a singularity is formed? There could be no extreme curvature of space-time, because you say that the big bang - where all that matter was compressed into a tiny volume was everywhere - there was no outside! What then kept all that matter together? Gravity? But then cosmolgists tell me that gravity is really the curvature of space-time, but you say there was nothing outside the singularity therefor no space-time curvature therefore no gravitational forces! No wonder I sometimes (every time) get things wrong! And tell me was the Big bang isotropic or directional? [Reason for the confusion -- because every TV science programme you see has galaxies exploding outwards type animation. Don't blame the cosmologists, Ah , but if you are a cosmolgist then I do blame you. You just cannot explain things so that a resonably intelligent and inquisitive person can understand. [blame the popular science writers and TV producers who keep using a cliched graphic, without bothering to explain that it is, in almost all aspects, wrong and misleading. No thankyou, I will blame the cosmologists for not being able to explain themselves - not only that but in your case being quite dogmatic about it too
|
|
|
Post by speakertoanimals on Sept 22, 2010 12:34:40 GMT 1
First, you are mixing up two different kinds of singularities (which in maths terms just means some physical quantities become infinite). So a black hole singularity is a singular point within a pre-existing spacetime, where the curvature becomes infinite. Whereas in the big bang singularity, density was infinite.
Curvature doesn't mean there is an outside, spacetime doesn't need an outside in which to do the curving, it just can be intrinsically curved. This makes it different from 2d curved plates in 3d. So that the surface of a cylinder is extrinsically curved in 3D, but intrinsically (i.e. to an ant living on its surface) FLAT. Whereas the surface of a sphere is curved both extrinsically and intrinsically.
Why should it need anything to keep it together, where else was it going to go?
Wrong again , see above. And don't keep confusing expansion in terms of density, with curvature -- very small is not the same as all curved, it's not like screwing up a piece of paper.
All observations of the CMB say isotropic -- there was no centre! We just have space that expanded. Think of piece of squared paper, laid flat and filling the plane. As I shrink the paper, the squares get smaller (matter gets denser), but the piece is still flat, and still fills the plane. Take it all the way down, and you get infinite density, but still infinite and flat.
Why should you think that any reasonably intelligent and inquisitive person is capable of understanding almost anything? And again, you assume you are reasonably intelligent? AS I often find, when some people are unable to grasp a subject (or rather, unable to grasp their own inability to understand, or accurately gauge their own level of misunderstanding), then they often blame the teacher, rather than admit their own inability.
You are stuck because you keep assuming that your own commonsense is sufficient for the job -- it isn't. Much of what happens with curved spaces is totally outside our intuitive grasp, and unless you can admit that you'll never start to learn.
I keep explaining your simple errors, you're just not willing to listen. These errors aren't complicated, they are the everyday ones that almost everyone makes the first time they come across this stuff. But not everyone can get beyond those, and come to some grasp of the topic. It is hard, requires some ability to handle abstract mathematical concepts, and some ability to get beyond the level of analogies. And so far, you don't seem able to do that. Just as many seemingly intelligent people can't get even the simplest mathematical concepts beyond arithmetic.
|
|