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Time/Expansion of Universe?

  1. Apr 12, 2009 #1
    I'm new here, basically just joined to get some intelligent input on this idea I've had. Was in an altered state of mind while formulating it but it still seems to make sense when sober.

    Assuming the following:
    - As we know, as one approaches the speed of light, one expands i.e. to a stationary observer you would appear stretched out but to someone travelling at the same velocity you would appear normal.

    - The same goes for time i.e. a stationary observer would experience 1 hour but if you went at near lightspeed you would experience say 5 minutes but end up at the same point in time as the observer: 1 hour after you left.

    - The universe itself is expanding i.e. no matter where you are it looks like everything is moving away from you, like drawing dots on an uninflated balloon then inflating it: all the dots look like they are 'the centre of the universe'. This presumably means we are expanding i.e. the atoms in our bodies etc. are moving apart, but everything is expanding at the same rate so we don't notice it.

    I reckon the way we experience time has something to do with the rate of expansion of the universe. See, if you travel at c for what you experience as e.g. 5 minutes, you will have been expanding at a greater rate than a stationary observer. When you stop you will have expanded to the 'size' you would have been had you stayed still for e.g. 1 hour. If an observer waits one hour he will have been expanding at the 'standard rate'. Both of you end up at the same 'point' in the expansion, just the person travelling at c has sort of taken a short cut.

    Similarly, this kind of rules out traveling backwards in time, as that would mean going back to a 'smaller' point in the expansion - more 'compressed' i.e. energy would need to be put in which does not exist. This bit is pretty hard to put into words btw so please bear with me. What I mean is obviously travelling forward in time is possible (meaning you experience a shorter length of time than an observer who stays at the 'default' expansion rate) simply by moving at high velocity. Going back in time however would imply experiencing a longer length of time than a stationary observer i.e. the observer 'sees' 1 hour but you 'see' 3 hours or whatever. Or more plainly, an observer experiences 10 years but you experience a lifetime, so when you come back to normal you'd be 80 years old in 2020 instead of 2060 or whatever. Although that isn't strictly speaking going back in time, more slowing time down. Going back would require you to experience a negative amount of time compared to an observer, not a fraction, which my idea seems to say would require contraction as opposed to expansion.

    There's more to it than that but I'll see what you think before making myself look any more of an idiot, ha ha. If this topic is covered elsewhere feel free to amend the situation.

    [I'm an electrical & mechanical engineer so this post may look like it was written by a novice (which I am in cosmology) but I understand *most* of the principles and terms.]
  2. jcsd
  3. Apr 12, 2009 #2


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    The red presumption is wrong.
    But the dots on the inflating balloon is indeed a good way to picture the increasing distances between isolated clusters and superclusters of galaxies.

    Distances inside individual galaxies do not participate in Hubble Law expansion. For the purposes of this kind of discussion, galaxies are "small scale". They are gravitationally bound structures, just like the atoms in a block of metal are held at a fixed separation by crystal lattice bonds. Also gravitationally bound clusters of galaxies do not participate.
    The Hubble Law pattern of increasing distance only involves "large scale" distances.

    Large scale distances (between objects which are at rest with respect to the CMB) currently increase at the rate of 1/140 of a percent per million years.

    If the distance is only a few hundred million lightyears, then that rate of increase is not very great. You can work it out.

    Since you are an engineer you might enjoy trying out Ned Wright's calculator (google "wright calculator"). It embodies the standard cosmological model so you can get some "hands-on" experience with the standard universe. Just type in some redshift z and press "general"---it will calculate some times and distances for you.

    Leave the default values of the matter density (0.27), the dark energy density (0.73) and the Hubble parameter (71) as they are and try different redshifts.
    If you want a similar calculator but one that gives you distance expansion rates (aka "recession speeds") then use the "uni.edu" link in my sig. That gets you Morgan's cosmos calculator. There you have to copy in the usual values of matter density, dark energy density, and Hubble parameter.
    So before starting a session you have to type in .27, .73, and 71. Then you are good to go and it will give the same results as Ned Wright's calculator but with "speeds" as well.

    If you don't have the link handy and want Morgan's cosmos calculator, just google "cosmos calculator". That should do it. These are widely used things, people like to play around with them.
    Last edited: Apr 12, 2009
  4. Apr 12, 2009 #3
    I see, of course as you change through different scales from micro to macroscopic, different rules apply. Like a fractal or basically a non-linear system, with different forces only applying on certain levels.

    I still think the way we perceive time i.e. what is the thing or process that we experience as time? has something to do with the general principle of the above post in that something way too big to be detected by humans is going on - affecting the whole universe - and it manifests itself at our level as time, kind of another dimension just beyond our reach.

    Imagine a 2D universe, simplified to a sheet of paper with a few stick men on it. They only see the world as a line, and each other as different colours on the line. If they have clothes on, we can see both inside and outside their clothes from a 3D perspective, but they cannot see both sides of each others clothes at once. If we were to poke our finger through the paper, they would see a finger-coloured line 'appear' in their line of vision - materialising from thin air, as it were. And when the finger was removed, a 'hole' would have been created in their universe.

    In the same vein, someone who was on the level where time is effective (i mean like gravity for example, only effective in a certain range i.e. close to earth it's strong but far away it's weak) could 'appear' at different points in time and we would see someone appearing out of thin air. I thought of this while I was in primary school and I've never figured out what a) we would actually observe should someone travel through time and stop at the point we are at? Of course we are already travelling through time so to talk to us they would need to 'slow down beside us', like 2 cars on the road. and b) what is it that we are observing, experiencing and measuring as time? Is it a dimension that we are literally moving through, or a process that is affecting us in some way? Because it can be altered by going at lightspeed.
  5. Apr 12, 2009 #4
    "As we know, as one approaches the speed of light, one expands i.e. to a stationary observer you would appear stretched out..." (stated in OP)

    Wouldn't a stationary observer see this person contract, not expand? (i.e. length contraction)

    If it's expand, then there is something wrong with my books on relativity. Either that, or I don't fully understand your example.
  6. Apr 12, 2009 #5
    Sorry, I didn't write that very well. What I meant was the formula L = L0 / [1/(v^2/c^2)]^0.5 which basically says your actual length (the length apparent to the human universe - what an observer sees) is a function of your speed. The closer to c you get the longer your length compared to L0, the length that you would measure. So a 30cm ruler would appear longer if an observer took a picture of it. The same formula works for time, the faster you go the longer the time an observer sees compared to your original time - what you would measure. If you measured 1 day going at 0.9c, an observer would measure 1 day, 2 hours and 40 minutes.
  7. Apr 12, 2009 #6
    I see what you're saying. You're comparing the length measured from the observer to the proper length, where as I was comparing the proper length to the length one would measure. This makes sense now.
  8. Apr 12, 2009 #7
    Interestingly, and something that introductory special relativity texts don't mention, but to a "stationary" observer (ie. a 'farmer' standing outside of the barn watching the other farm run into it), the relativistic 'mover' will actually look rotated, not just contracted. The degree of rotation is related to the speed. It's pretty interesting to think about, especially from an engineering point of view, except that it ruins a lot of nice SR thought experiments.
  9. Apr 13, 2009 #8


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    Right, but it is the length as measured in the rest frame of the object that remains constant. This means that if I'm in a space ship, and accelerate to some fantastically large speed, then the length I measure for the spaceship will be the same as when I was sitting on the ground. In special relativity, other observers can only ever measure this length as equal to or shorter than the length as measured by a person on the object in question. This is why the effect is considered length contraction, and not the other way around.
  10. Apr 13, 2009 #9


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    Er, the Lorentz transformations don't include any mixing with the coordinates orthogonal to the axis of movement. So this statement doesn't make any sense to me.
  11. Apr 13, 2009 #10


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    He's speaking of how an object will "look"; its appearance. This is a property of how light reaches the eye, and involves more than merely the Lorentz transformations.

    The appearance of a rotation is called "Penrose-Terrell Rotation". It's explained by Chris Hillman at Can You See the Lorentz-Fitzgerald Contraction? Or: Penrose-Terrell Rotation. This is part of the internet physics and relativity FAQ; a handy resource.

    Cheers -- Sylas
    Last edited: Apr 13, 2009
  12. Apr 13, 2009 #11
    Right. I don't have any education in this at all, this is mostly stuff I figured out ages ago as a kid or was told in physics at school. From then on it was all about functional stuff as opposed to theoretical stuff so applying principles is second nature to me, but obviously I need to understand them first, ha ha. Leaving aside the length contraction (which is much clearer now, cheers) what I was essentially trying to get to the bottom of was how we perceive time.

    Evidently time, speed, length etc. are all related in some way, just we do not yet understand enough to be able to express it in any meaningful way which would be directly applicable to, say, the development of time travel or reversing a person's perception of the direction of time. The human brain is extraordinarily good at linearising a system about an operating point e.g. we can distinguish EM radiation between 500nm and 700nm extremely well but outside of that very small bandwidth the human brain is completely useless. At different wavelengths the response is totally different, some radiation causes cancer, some we detect as heat, some passes straight through us with no effect. The same applies for all our other senses - the only way information can enter the brain. So once you leave the operating point of the brain (the stable conditions on earth, basically) there will be processes going on that we literally have no way of imagining, let alone experiencing. Surely time is something like this?

    Of course a person can no more construct a mental image of a coordinate system with 4 dimensions than a 2D person could with a 3D system. It would appear that the dimensions we experience, 2 or 3D, can stay as they are, with us moving around in them, yet they can move as a whole through the 'extra' dimension. For example, with the 2D universe on a sheet of paper, it would seem to the 2D folks that they were not changing at all but their universe has changed position in the 3rd dimension. Same with our 3 dimensions, and time, the extra dimension, which is exactly what we observe i.e. we can move freely in our 3D world, but the entire lot moves as a whole through time.

    In the same way that we could see both inside and outside a 2D person's clothes at the same time, perhaps this is analogous to why instantaneous communication is impossible in 3 dimensions. Anyone can use a phone and say, look, it IS instant, but obviously it's not. Nobody can receive information as it is being transmitted, simultaneously - it needs to travel through time before anyone can receive it, in the same way that the light from inside the 2D clothes needs to travel through the 3rd dimension and back into the 2D plane if one 2D guy wants to see under a 2D girl's dress while she's wearing it.

    Weird. If anyone knows much about the rotation thing, fire in, quite intrigued by that. Thanks for all the replies so far btw, its pretty much impossible to find anyone who understands this stuff in Glasgow.
  13. Apr 13, 2009 #12


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    The experience of time would be tied to the density of relationships possible. So rich variety vs improverished simplicity.

    If you travel at c like a photon, you would experience no time. Your history - future and past - would be as simple as possible in terms of your relationships with the universe in general.

    But as you slow down - approach absolute rest - the space of possible relationships opens up. You have more things that can potentially happen. And so you inhabit a more richly structured world.

    So at c, the world is c/c. You, as a locale, are going "as fast" as your global context.

    But at rest, you lag behind the going speed of relationship forming. This opens a yawning gap that can be filled with experience.

    So your world becomes r..................................c. There is you at rest and then all the scales of motion and interaction up to c.

    So time experience is maximised for people/objects at rest. And minimised for those that close the gap to travel at c. As the rocketship travellers discover.

    The physical story of time experience and the psychological one are not necessarily the same thing of course. But time does fly when you are busy and drag when you are idle.

    Time experience also freezes in situations like car crashes where we are helpless to react. Instead of going with the moment, closing the c/c gap so to speak, we are mentally at absolute rest and are maximising the r..........c gap.

    One final point to throw in the mix, even at rest, you are spreading your presence at c. So a mass just sitting there is in communication with the world, radiating charge and gravity at c rate. Doing nothing is still actively doing something.

    And time experience is richest when the gap has been stretched to its fullest possible. When you are most located (absolute rest being ruled by planck scale considerations) and the rest of the world (including the spreading aspects of your own located existence) are strung out towards the most distant possible, c rate, event horizon.
  14. Apr 13, 2009 #13


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    On the contrary! We do know how they are related, and relativity (general relativity in the more general case) spells it out. It's perfectly meaningful, and lots of people here can explain it for you. A student may take a while to learn, but the knowledge is there and unambiguous and meaningful.

    From these well known relationships you can show that time travel is not possible.

    Weird exceptions in twisted spaces are ignored; they don't show up just by taking some unusual path in normal spacetime. They are fun to think about, but unfortunately for intending time travelers, superman's trick in DC comics, of using high speeds to travel time, won't work. We know it won't work, because we know enough about how time and space are related to be sure it won't work.

    Science is able to study and explain very nicely all kinds of conditions far removed from the stable conditions on Earth, and we can imagine all kinds of things we can't experience. Unrestrained imagination is a poor foundation for anything; but science is a good foundation for letting your imagination stay grounded in reality -- even reality far beyond your personal experience!

    Well, you can develop mental images of 4D co-ordinates. Or not. Some folks work better by having a kind of mental image. Whether someone's mental image is sensible or not depends on whether it conforms with the maths.

    Perhaps that's a better way to put it... the mental images we do have are never definitive. To stay grounded, mental images must always be evaluated and developed in the light of the formal mathematical account. People do have mental images to help them think about things, and how can you judge whether a mental image is appropriate? You use the maths.

    When things go bad is when someone elevates a personal image above the maths, and rejects the maths when it fails to match their image. You should do it ther other way around. There's nothing in principle to prevent you having a mental image that fits the maths, but you get there by adjusting the image, not the maths.

    It's the same thing as before. Mental images and imagination are an integral part of how we think, and they are not simply omitted when we are using science and maths.

    You're trying to imagine things now, and that's good. You're also trying to learn, and that's good. They two go together, and you can develop your imagination as a result to remain grounded in what you learn.

    That's a mental image, which might work in some cases. For example, you can think of our 3D world changing over time. Or you can think of a 4D world with a time dimension. When you say the "entire lot moves as a whole" you are effectively taking a successive slices through the 4D space, and treating it as a motion along the time dimension. That mental image might break down for you and need revising somewhat when you run into issues with simultaneity. Effectively, you can cut the "slice" in different ways.

    The concept of a "worldline" in 4D space for a single observer is a handy mental image with a good mathematical foundation.

    Ah. Apparently there are other ways imagination can get you into trouble. :wink:

    Cheers -- Sylas
  15. Apr 15, 2009 #14


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    Ah, okay, that makes sense. I was thrown by the fact that he said it throws a wrench into a lot of SR thought experiments, because it really doesn't. The rotation is just an optical illusion, while the contraction and time dilation appear to be real.
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