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A thought experiment regarding the nature of space expansion

  1. Jan 16, 2012 #1
    I devised this using a setup that I found easiest to imagine, while trying to be clear on how modern physics considers space to expand.

    We have a nifty device that can cause spatial distortion (no, you cannot has one!). Before we use it, we lay down two meter rulers, end to end. They both look the same, so we get down to business. Turning on our device, we expand a cubic meter of space to twice its original size. I have three basic questions. I'll provide my current answers, based on how I understand space to work. Number 2 is my most hazy. I treat the cut off between normal and expanded space as being sharp rather than smooth, for convenience.

    1) What has become of the one meter ruler where our space has been expanded?
    2) What happens if we pick up the unaffected ruler and place it halfway into the expanded space?
    3) What happens when we shoot a beam of light across the expanded space?

    I say "we" to make you partly culpable if the experiment breaks the universe.

    My answers are:

    1) To the external observer the ruler appears twice as large.
    2) The part we put in the expanded space will become larger, and then shrink to a percieved normal once removed.
    3) This seems to be a nice example of relativity. Since the external observe would measure the external dimensions of the space as 2m, light apparently moves twice as fast. So we must assume that light, at least, behaves as if the expanded space were two meters across. However, an 'internal' observer inside the space now would see light moving at half speed. And so we must conclude apparent time dilation.

    I was interested to note that the external observe can use the internal observers clock and ruler to get the correct speed of light, instead of his own, but he can't use his ruler and the internal observers clock, or vice versa.

    So there's my take. One question about it is the use of relativity here. Part of me sees that the light can clearly be seen by both observers to travel at the correct speed, since the external observer can see the ruler and understand that the light isn't actually moving faster, just that space is bigger. Is the issue then that light will naturally take twice as long to travel across the expanded space, therefore requiring relativity to keep c constant? In a sense, light is not affect by expanded space in the same way the ruler is (assuming I even answered question 2 correctly?

  2. jcsd
  3. Jan 17, 2012 #2


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    Assuming that your machine works the same way as normal expansion:

    1. Your ruler is still the same size. Gravity, the EM force, and the Strong Force are all much much stronger than the expansion of space.

    2. This cannot happen, so I don't know what to tell you.

    3. Nothing. The space is no longer expanding and acts exactly like normal space. Normal expansion is continual and will stretch the wavelength of the light as it travels.
  4. Jan 17, 2012 #3
    So, when expanding space is discussed, the idea is that there is more space coming into existence? I want to be really clear on this.
  5. Jan 17, 2012 #4


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    Either that or space is simply expanding. Either way works fine as far as I know.
  6. Jan 17, 2012 #5
    We have to be careful about saying what an outside observer can see. There is no way to be an observer outside the universe and no way to make a machine in the universe that can expand space. We can visualize scenarios; but what you're doing in this case is breaking the laws of physics so we can observe the laws of physics in action. We can't create a separate chunk of universe one cubic meter in size and expand it. We don't know exactly why the real universe began to expand, though we have theories. These are fun games; but don't expect real answers to questions about impossible physical experiments.
  7. Jan 18, 2012 #6
    Yet he did provide real answers. Well, as real as they can be, since we lack the ability to fly into the voids between clusters and see what's going on. In order to begin answering the question, we need to have an idea of what we think that answer might be. He provided a framework in which to consider a variety of issues that might lead to a practical hypothesis that might also be testable. He particularly highlighted a flaw in my thinking, reminding me that the inherent gravitational field of the ruler would counter alot of the affect.

    But I still feel like I'm not on top of the accepted physics of spatial expansion. Growing, expanding and stretching are three very different words that I hear used interchangeably.

    Clearly from my own answers to the thought experiment I considered space to stretch, such that each point of spacetime was pulled further apart from every other point. An overly literal quantisation of spacetime? It naturally begs the question of what I think is occupying the spaces between the spaces. Obviously not crystal aliens. :P

    If energy only exists in discrete amounts, why not spacetime? /shrug. It's only half a thought at the moment. For now, I'm interested in understanding what is considered the mainstream description of expanding space.
  8. Jan 18, 2012 #7
    Your idea of spacetime existing in the equivalent of quanta in matter and energy is pretty much what I am thnking, as well as others. See the latest Scientific American, Feb. 2011. The article on pg. 30 is entitled Is Space Digital? by Michael Moyer. Re: growing, expanding and stretching, growing is more or less colloquial. Expanding (of space) takes matter with it and simply separates matter without expanding it. The reason it appears that energy is expanding is that the matter further from us that sends light is traveling faster and faster with distance from us, as you well know. This is simply doppler shift. Talking about the expansion of the universe stretching energy is just confusing the issue. Think doppler. Rulers don't expand, though, if we could see rulers lightyears away, they would appear shorter in the direction of their motion away from us. Stretching, in my experience, is simply an easy word to express the doppler shifting of light frequencies. In fact, nothing actually stretches. It is just that, as the electromagnetic radiation was being generated, the generating object was moving away at a speed that made the "pitch" lower. I know most of this is simplified and I apologize if any of it makes you feel I am talking down to you. I have simply found that simpler is usually best for visualization as long as it's not so simple as to be wrong.
    I'm not sure the "expansion" of space (or spacetime) is not more than a geometrical construct. If we get to the point that we know what a gravitational field is, we will probably find out. I have postulated that gravity is the time gradient generated by a massive object; but I don't know exactly how this gradient (or field) is generated. Physicists talk about gravity traveling at the speed of light; but gravity is always just there. It doesn't have to travel like a light beam to get to the object. It affects any object instantly at any point. So, spacetime really is gravity, or the intermix of time gradients throughout the universe. And, I think it's digital. You can imagine time "stretching" or dilating. Things speed up in the absence of gravity (no time dragging). In space far from any massive body, time clicks at its fastest rate. The expansion of space dilutes gravity (time gradients) by the simple inverse square law. Michael Greene has stated that objects tend to migrate toward regions where they age slower; but even he has not actually equated the time gradient with gravity. He has said that people think of warped space; but they can't visualize warped time, which is what is really taking place around massive bodies. That's close to my theory of gravitational impetus; but not there yet. His book, The Fabric of Space, is nevertheless the way to get the best "mainstream description of expanding space." Putting mathematics to words is fraught with danger, as was indicated by your questioning of the words growing, expanding and stretching. There's no such confusion in the mathematics, though every solution seems to have two explanations for the same end result. Does the airplane get lift by the differential between pressures above the wing and below, or is it because the mass of air diverted downward by the wing equal the mass of the airplane? Both. It's like that.
    Last edited: Jan 18, 2012
  9. Jan 18, 2012 #8


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    Impetus, I don't believe that the increasing redshift we see when looking at far away galaxies can be explained by doppler shift due to velocity through space.
  10. Jan 18, 2012 #9
    If space is getting... 'more' ... then I have questions about conservation of energy.

    When I talk about stretched, I mean pulled apart in all directions, so that each point of spacetime is seperated from every other point. It naturally creates a void between these points - rather, straightforward quantisation does. I'm not happy with that - a series of bubbles sperated by utter nothingness. Seems redundant. I am currently thinking more in terms of a quilt, where, in 2D, squares of spacetime are side by side and distort as space is literally stretched out.

    I was thinking in terms of stretching like this as it seemed a logical opposition to the compression of spacetime as it is curved by mass. I've never heard it ever suggested there is less space when curvature occurs, so it seems odd to then hear it said there is more space in the absence of matter. I have been thinking alot lately of space being elastic and highly malleable. I put less emphasis on time, but doing so tends to lead me... further astray ;)
  11. Jan 18, 2012 #10


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    Conservation of energy does not apply at the cosmological scale, only locally. A perfect example is the redshift of light due to expansion. Where has the energy gone??

    Also, just ignore quantization of space. Unless you are on the cutting edge of theoretical physics and understand the math used, you don't have a chance of really finding out anything. For now just realize that if two points in space are seperated by expansion, there is still space in between those original points.
  12. Jan 18, 2012 #11
    I think it would be fairer to say I have little chance of proving anything. Quantisation of space is a fairly natural conclusion to reach given the direction physics has generally been heading. But since it isn't mainstream, let's set it aside, and discuss your opening statements.

    Conservation of energy. I know the virtual particales also don't obey the conservation, but do not exist long enough to matter, but it all sounds an awful lot like playing games. The same goes for notions of new space. If new space is created then it must contain new vacuum energy. On what basis has the law of conservation of energy been put aside on the cosmological scale? Is it no longer consider so that the universe contains no more energy than when it began?

    I can't actually respond to your question in context of the accepted expansion - but the wording implies that energy is lost by the increased wavelength and goes nowhere. I could answer it vaguely using the subject I agreed to set aside, but that's not going to be helpful to you, who won't be interested, or me, who actually wants to understand the current model.
  13. Jan 20, 2012 #12
    Just a quick reply about where the energy goes when the expansion of spacetime "stretches" the frequency of light: The light had a higher frequency over a shorter distance before expansion. After expansion it has a lower frequency over a longer distance. Higher frequency, greater energy; lower frequency, lesser energy. But, the same number of light cycles represents the same amount of total energy per cone of light volume. In general terms, a lot of red light has the same energy as a little bit of violet light.
  14. Jan 20, 2012 #13


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    Impetus, the key is that single photons are being redshifted. They are not being turned into more lower energy photons. I've also never heard of this "cone of light volume".

    Salvestrom, I wish I could elaborate on the conservation of energy relation to Cosmology, but I am not educated enough on the subject. All I know is that, to my knowledge, energy is not conserved on a universal scale. Of course the fact that I've repeatedly seen the statement "energy is not well defined in GR" could have something to do with this.
  15. Jan 20, 2012 #14
    Drakkith, The definition of doppler shift is the changing of frequency due to the approach or recession of the object producing the vibration, whether sound or light, to or from the observer. In the case of sound, the air between doesn't expand. In space, we say that spacetime expands, but that doesn't preclude it still being a doppler shift. In the case of spacetime, it seems that there is nothing required to "carry" electromagnetic waves. They are self-carrying; but doppler effects still occur.
  16. Jan 20, 2012 #15
    Drakkith, You are right. More, lower energy photons are not being produced by the expansion of space; but every photon is a wave that travels through space creating, in effect, the "appearance" of a long wave passage, a long string of individual photons. The emitting object can be considered as moving away from us, or we can be considered as moving away from the emitting object. Either way, that chain of waves appears to have a lowered frequency and less energy. Here's a thought experiment: If you get in your space ship and travel toward the light source, it will appear to blue shift, even though it was red shifted due to the expansion of the universe. If you go fast enough, you will observe blue, violet, ultraviolet and on up to x-rays just because you are heading into the oncoming frequency and increasing its effect on you. The light didn't change; but your observation sees it as x-rays; and they will go through your body, even though an observer on the earth you left would eventually see the same chain of waves as, say, yellow.
  17. Jan 20, 2012 #16
    Re: conservation of energy universally -- Gravity is generally considered negative energy. Matter and energy are equivalent; so we will call them both energy for the purposes of this comment. Energy in the form of matter and the energy associated with it are considered positive energy. The total of Gravity and energy is zero and this is conserved. If you think about it, where could energy go. It can be exchanged with gravity; but neither gravity nor energy has anyplace to go since, for us, there is no "outside" to the universe.
  18. Jan 20, 2012 #17


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    I'm not sure whether you have changed your mind about the cosmological redshift. Are you still treating it as a doppler effect? I had a response to that but if you now agree with Drakkith about it I won't bother.
  19. Jan 20, 2012 #18
    Marcus, The speed of the expansion of the universe at the time of the CMB emissions was extreme. It was, as I recall, just after the time of inflation. (Correct me if I am wrong; I am not taking the time to look it up.) You don't have to sum every photon in a string of photons to get the red shift. All you have to do is know the recession speed of a galaxy. It doesn't take big math to calculate it. I won't belabor the point; but, whether or not you accept that doppler shifting takes place, you can use it to determine the frequency of light. If you want to also consider the expansion of spacetime, it can be treated as a separate problem. Physicists that I know haven't yet even agreed that the expansion of spacetime (or its warping around a gravitating body) is more than a geometric construct. We do know that the universe is expanding; but there's not universal agreement that spacetime is. For example, the space between atoms is huge compared to the nucleus and the electrons of the atom. That space is not expanding. And the space between the molecules of our body is not expanding. At what point can we say space IS expanding?
    At some point out at the edge of the universe we can (theoretically) observe, galaxies are receding at the speed of light. At that point, the red shift of all light reaches a frequency of zero. The visible edge of the universe is as if it were a black hole. No light can come to us from beyond it. Forgive me. I'm introducing much to much.
  20. Jan 21, 2012 #19


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    Actually, the speed of expansion when CMB photons were being emitted was rather pedestrian, inflation was a distant memory. I take it you don't know many physicists.
  21. Jan 21, 2012 #20
    I'm actually having some trouble determining how Doppler Redshift and Relativistic Redshift are actually told apart. I understand the Redshift concept: a light spectrum contains black lines that indicate the elements the light has passed through, shifted toward the red end of the spectrum. The amount of shift indicates the speed of recession. But what is it that makes relativistic redshift 'stand out'? Is it an extreme shifting, indicating FTL speeds, which requires explanation? Is there anything else coming into play?

    Also, is a photon able to radiate heat? 13 billion years in absolute zero? They aren't strictly massless either, right, it's just considered negligable, kinda leading me toward the idea that 13 billion years of even a minute gravitational affect must surely add up to something meaningful? Again, though, I'd be interested to hear more on just what defines a redshift as relativistic over doppler.

    Oh, also, just to test my own knowledge, growing at pedestrian speed: the CMB photons were emitted prior to the inflationary period, which is at least part of the reason we see them entirely around us and it has taken them so long to get here.
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