Viewing 40 posts - 41 through 80 (of 97 total)
  • I have a question about spacetime
  • thols2
    Free Member

    The universe doesn’t have a centre.

    null

    thisisnotaspoon
    Full Member

    I think whwre I’ve been going wrong is it’s all about the point at which the light or sound wave was propagated, rather than the speed of the waves

    Pretty much, the sound (or waves on water) analogies only really start to fall apart when you approach the speed of those waves. e.g. a surfer paddling to catch a wave perceives such a long wavelength/low frequency that the wave stops. But that’s the Doppler effect (assuming they don’t catch it and the wave eventually passes them, then another and another) but it’s not the same as considering relativity as you approach the speed of light.

    jivehoneyjive
    Free Member

    So anyway, you know how the universe is expanding…

    If the space between everything is getting larger, does that mean in relative terms, everything is getting smaller?

    Or in time, will the vacuum effect of deep space stretch everything so the relative scale remains the same, just the overall volume increases…

    🤔

    kelvin
    Full Member

    The universe doesn’t have a centre.

    Define “centre”. I’m using it to mean that it’s where all other points have come from and all are increasingly distant from. I’m not saying it’s “in the middle” of something akin to a sphere or circle. My mum is the at the centre of our family… we’re not a volumetric space. [ I’m now feeling sorry for her. ] Call it the source, or start, if you want.

    kelvin
    Full Member

    Or in time, will the vacuum effect of deep space stretch everything so the relative scale remains the same, just the overall volume increases…

    One of many theories on how the universe “ends”. Entropy. Everything so spread out that it has no measurable/usable energy due to its massive volume.

    tjagain
    Full Member

    kelvin as I understand it every bit is moving away from every other bit so there is no centre everything moves away from

    Wherever you are seems like the centre as everything is moving away from it

    kelvin
    Full Member

    Well, we know that on the local scale (space and time) that isn’t true. Or we’d all be very alone. Everything is on average/eventually moving away from everything else, but there is a “centre” that everything is increasing distant from and nothing is moving towards, even temporarily or for short distance.

    seosamh77
    Full Member

    The speed of light is constant, but the speed that the fabric of the universe is expanding isn’t limited by the speed of light.

    That’s why the observable universe is 13.8 billion light years in diameter. I prefer to call it the time warped universe, the one we can see, where we look back in time.

    But there’s also what I’d call the real time universe, which is estimated to be 46 billion light years in diameter.

    Basically that point we can see 13.4 billion light years away, currently looks very different and is a different place after 13.4 billion years of time. It’s not frozen in time.

    Something like that anyhow.

    seosamh77
    Full Member

    The speed that the fabric of the universe is expanding isn’t constant either. It’s slower for closer objects and faster for further away objects. That’s why galaxies in the local cluster can still move towards us(and will eventually merge into one big galaxy with andromeda eating up the milky way and all the near by satelite galaxys(Of which there’s loads btw)), but further out ones will never get closer to us after point.

    The scales are mental in size to comprehend though.

    You go from galaxy to local cluster, to super cluster to your part the cosmic web and well beyond.

    I’m not even sure if galaxies in the super cluster can move towards us or not, or if they’ve reached the threshold of the expanding fabric of space? I’m guessing there’s a point where the expansion of space takes over and galaxies over a certain distance can never interact.

    Even out with the cosmic web, there’s vast areas of nothing that are dominated by dark matter/gravity/energy I think. And we haven’t a scooby what it is. I tend to think dark matter/gravity/energy is some other dimensional construct or something. It’s probably what is driving the expansion of the universe I guess. But maybe this expansion is just what it looks like to us, may be this expansion is as I say some kinda other dimensional construct, where time/space and speed don’t really matter? dunno, just havering out loud really. 😆

    thols2
    Free Member

    I’m guessing there’s a point where the expansion of space takes over and galaxies over a certain distance can never interact.

    Exactly. Some parts of the universe are so distant and expanding away from us so fast that light from there can never reach us. Eventually, the universe will expand so much that every single particle will be isolated from every other particle.

    ampthill
    Full Member

    Define “centre”. I’m using it to mean that it’s where all other points have come from and all are increasingly distant from

    Everything in the universe started in one very small point. Where is that point now? Every where, because everything was in that point.

    I teach A level physics. Here is a video I made about Doppler it starts with sound and moves onto binary stars

    This one’s the Big Bang and red shift

    The rest of my astrophysics stuff is here.

    PS most of these were made during lock down when despite what the press said I was quite busy

    avdave2
    Full Member

    Added to my watch later list ampthill

    greyspoke
    Free Member

    The universe doesn’t have a centre.

    That’s why the observable universe is 13.8 billion light years in diameter. I prefer to call it the time warped universe, the one we can see, where we look back in time.

    But there’s also what I’d call the real time universe, which is estimated to be 46 billion light years in diameter.

    I can’t see how both these statements can be true. If something has a diameter it has a centre* surely?

    *probably several different ones depending on how you define it see eg the centre of England/UK etc.

    seosamh77
    Full Member

    greyspoke
    Free Member
    The universe doesn’t have a centre.

    That’s why the observable universe is 13.8 billion light years in diameter. I prefer to call it the time warped universe, the one we can see, where we look back in time.

    But there’s also what I’d call the real time universe, which is estimated to be 46 billion light years in diameter.

    I can’t see how both these statements can be true. If something has a diameter it has a centre* surely?

    I actually meant radius. And we only extrapolate a 46 billion light year radius with our best guess(we don’t even know how fast the universe is expanding from our perspective), that’s not the end, only what we know existed in the past and likely exists today. It only has a centre in terms of our viewpoint. We are the centre in that perspective, but someone on a planet in a galaxy 5 billion light years away will see their own bubble. Doesn’t mean that’s where it started.

    As I say there’s definitely something else going on, cause time didn’t exist before the big bang, it’s just a consequence of the big bang, so what it all came out of isn’t necessarily something that plays by our laws of physics, imo. Probably some kinda of dimension where size doesn’t really mean much either I’d speculate.

    properbikeco
    Free Member

    look up doppler effect…

    slowoldman
    Full Member

    Probably some kinda of dimension where size doesn’t really mean much either I’d speculate.

    Maybe I should move there.

    molgrips
    Full Member

    I can’t see how both these statements can be true. If something has a diameter it has a centre* surely?

    Depends on how you are defining these terms. Traditional geometry that you learned in school is called Euclidian and it only applies to flat unbounded planes like infinitely large pieces of paper.

    It’s a lot easier for us to think in terms of two-dimensional analogies. Going back to the football again, the surface of the football has a finite size, and if you keep going in a straight line you’ll come back to where you started, after you’ve gone about 70cm or so. If you’re an ant crawling on the surface of the football then you’d think it was about 70cm in ‘size’ – now does that mean the circumference? I suppose so. Now, if it has a circumference then according to school geometry it has a radius and consequently a centre. But the ant can never conceive of it (it’s a two-dimensional ant). However in the case of the football, we as 3D beings know that there is a centre in the third dimension.

    A four-dimensional sphere is called a hypersphere. Is the universe hyperspherical? Well it might be, if it’s closed, but it can also have the same curvature and be open, which would be sort of saddle shaped (horse, not bike) if it were 3D, which it’s not.

    greyspoke
    Free Member

    So a better statement would be “we don’t know if the universe has a centre because we don’t know where the edge is, or if it has an edge, or what an edge even means for a universe”?

    thols2
    Free Member

    No. My understanding is that the universe is finite, but unbounded. The exact geometry will depend on the density. As above, it started as a singularity and expanded. Every point is effectively the center with the universe expanding about it.

    It doesn’t relate to our everyday experiences so it doesn’t seem to make sense. It can really only be understood mathematically, which is why I gave up studying physics, I realized that I would never really be able to understand it because I was too lazy to learn all the necessary maths.

    dyna-ti
    Full Member

    universe is expanding…

    And expanding.

    In all of the directions it can whizz…

    shermer75
    Free Member

    So is time effectively stopped at the centre of a super massive black hole? That’s going to get in the way of things when the universe is trying to end, isn’t it?

    thols2
    Free Member

    So is time effectively stopped at the centre of a super massive black hole? That’s going to get in the way of things when the universe is trying to end, isn’t it?

    If I understand it correctly, the black holes will slowly evaporate because of Hawking radiation. This will take an incredibly long time, something like 10^100 years.

    greyspoke
    Free Member

    Hmmm, so @thols2 would you agree with @molgrips that if we were sufficiently dimensionally well-endowed to step outside the universe, we would be able to see the whole ball, as it were, and find the centre? Or is that one of those “might be true but it might not be like that” things?

    kelvin
    Full Member

    Thanks ampthill, I could no doubt learn a lot from your videos… if I could apply myself. [ admits near total ignorance and walks away ]

    thols2
    Free Member

    I think the point is that the center doesn’t exist in our 3D space. The center of a ball doesn’t exist on the 2D surface and a 2D being cannot find it. From the 2D perspective, all points on the surface of the ball are effectively the center.

    kelvin
    Full Member

    That really helped thols2, thanks. Pretty much what TJ said as well. [ keeps walking towards a horizon he will never reach ]

    molgrips
    Full Member

    So is time effectively stopped at the centre of a super massive black hole?

    Not necessarily. Time is relative remember. You might observe time as being near stationary but the people inside wouldn’t. Time doesn’t run the same for everyone.

    Re the centre of the universe, a 4d ball would have a nice simple centre but the universe isn’t necessarily ball shaped. Plenty of weird geometries in our normal world, I mean where is the centre of a torus? It has no centre inside itself, or the centre is a circle within it. Or a Mobius strip with only one side but two edges. Where’s the centre of that?

    seosamh77
    Full Member

    greyspoke
    Free Member
    So a better statement would be “we don’t know if the universe has a centre because we don’t know where the edge is, or if it has an edge, or what an edge even means for a universe”?

    No, because measurements tell us that every point is expanding the same way. If there was a centre it would be easy to at least locate the direction of it, but in that sense the Universe is uniform, from what ever point you measure it.

    seosamh77
    Full Member

    The edge of the Universe is just an other unknown quantity. Tbh when discussing this stuff we are at the edge of physics and philosophy. So the edge can be speculatively discussed a number of ways.

    You know, what is a Universe?

    The largest structure we know of is the Cosmic Web, but even within that, due to expansion, it’s not even possible for some things to interact physically.

    So you could maybe define a Universe as matter that can potentially react gravitationally?(even that’s a blurred line) Should we draw the line at say the Super Cluster or our Local Group, or is everything we can see in our Universe. (and I guess over time, we’ll discover/define other structures with in the Cosmic Web that are larger than Super Clusters if we already haven’t)

    What about the stuff beyond the Cosmic Microwave Background Radiation that we can’t and likely never will see, is that in our Universe?

    tbh ‘our’, ‘a’ or ‘the’ Universe becomes about definition at some point.

    mariner
    Free Member

    God runs electromagnetics on Monday, Wednesday, and Friday by the wave theory, and the devil runs it by quantum theory on Tuesday, Thursday, and Saturday.

    shermer75
    Free Member

    Neil deGrasse Tyson subs in on Sundays to give everyone a break

    shermer75
    Free Member

    greyspoke
    Free Member

    No, because measurements tell us that every point is expanding the same way.

    So if something with a centre was expanding evenly, how would you find the direction to the centre?

    seosamh77
    Full Member

    greyspoke
    Free Member
    No, because measurements tell us that every point is expanding the same way.

    So if something with a centre was expanding evenly, how would you find the direction to the centre?

    Well if you are at the centre everything would be moving away from you uniformly. If you are one of the bits that isn’t at the centre it would be obvious that everything else isn’t moving away uniformly and you could calculate from the velocities of different points where the explosion began.

    The thing with the universe is that, at any point, everything is moving away uniformly. Therefore no centre or everywhere is the centre. So there’s weird shit afoot. 😆

    tjagain
    Full Member

    So there’s weird shit afoot.

    Its the gods having a laugh I tell you.

    molgrips
    Full Member

    No, because measurements tell us that every point is expanding the same way

    The key measurement is that (in general) the speed something is moving away, measuremed by the amount of red shift, is proportional to its distance from us. The only way this is possible is if everything is expanding.

    greyspoke
    Free Member

    Well if you are at the centre everything would be moving away from you uniformly. If you are one of the bits that isn’t at the centre it would be obvious that everything else isn’t moving away uniformly and you could calculate from the velocities of different points where the explosion began.

    I am not sure about explosions. I was thinking of the traditional analogy for this, which is a pudding of some description with currants in it, which is expanding. And not constrained by a bowl or plate, so maybe you are doing some cooking on the International Space Station and just leaving it to hover. Or maybe a loaf with seeds in it which you are leaving to prove on the ISS. Anyhow, thee dough/batter expands, the currants/seeds get farther sway from each other uniformly. The doughball/pudding has an eddge and a centre, but from a point say 1/4 the way across it, how do you tell where the centre is?

    seosamh77
    Full Member

    greyspoke
    Free Member

    I am not sure about explosions. I was thinking of the traditional analogy for this, which is a pudding of some description with currants in it, which is expanding. And not constrained by a bowl or plate, so maybe you are doing some cooking on the International Space Station and just leaving it to hover. Or maybe a loaf with seeds in it which you are leaving to prove on the ISS. Anyhow, thee dough/batter expands, the currants/seeds get farther sway from each other uniformly. The doughball/pudding has an eddge and a centre, but from a point say 1/4 the way across it, how do you tell where the centre is?

    the pudding anology doesn’t work, well on cause it’s opaque, so you can’t see the seeds, and secondly the explosion has finished by the time you see the cake, so you can’t look at their velocities and directions.

    We can see, stars, planets, galaxies etc, and they are still moving, so we can measure where they are going etc.

    But if you could see the currants and the seeds, and they were still in their expanding state, you could calculate where the original dough ball started.

    The universe doesn’t work like that though. We don’t understand the medium we are expanding into.

    molgrips
    Full Member

    With the pudding, you are using a 3d analogy (the raisins) in a 3d object. But when considering the universe you are thinking of a 4(at least)d object in 3 dimensions. This is why the polka dots on the balloon analogy is better IMO because by comparing dots on a balloon with dots on flat paper you can see the strange effects caused by curvature in a higher dimension.

    peaslaker
    Free Member

    if something with a centre was expanding evenly, how would you find the direction to the centre?

    We see homogeneity in the observable universe. No direction shows any difference in content. The CMB is as ancient in all directions.

    If there is a central origin in 3d space (4d spacetime) it isn’t within our observable universe.

    As far as the centre of the observable universe… It is wherever you are making your observations.

    Speculation on any structures or physics beyond the observable universe are moot. Not observable. Nothing can be observed. We can theorise but such theories are untestable. Our best theory about the larger universe is “more of the same”, but we have no justification for even drawing that conclusion which, on the face of it, is an extraordinary special case.

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