At the end of last week, I wrote a post explaining how the Universe is so big (93 billion light years across) when it’s only 13.7 billion years old. The key visualization is to think of space as being the surface of an expanding balloon, while all the things in the Universe (stars, galaxies, etc.) are like ants on that surface.

Now, I explained to you that two ants will appear to move apart from one another due to the expansion of the Universe, even though neither ant is moving relative to the surface of the balloon. If I use light to measure whether this other ant is moving away from me, I find that she is; the light coming off from her is redshifted, in the same way that a fast-moving automobile speeding away from you has a lower pitched sound.

And yet, last week, I said that:

It’s only that space (i.e., the balloon) is expanding; there’s no matter that’s moving. So, in principle, space can expand as quickly as it wants, even faster than the speed of light, because there’s nothing moving.

This statement is confusing, and I realize this. After all, you measure how quickly that other ant is moving relative to you, you find that she’s moving away from you, and yet here I am telling you that neither one of you is moving. This is a very deep problem. Here’s how to help make sense of it.

When you think of the word “motion” or of something “moving”, you probably think of something being at one place at one point in time and then being at a different place at a different point in time. After all, that’s how we conventionally use the word. The act of going from one time and place to another is what we think of as moving.

But when we think about this, we make an implicit assumption. We assume that we can do something like put a grid down — or in general terms, put a system of coordinates down — and measure our changes in position relative to those coordinates.

Seems like a reasonable assumption, doesn’t it? After all, that’s what we do on Earth. In fact, that’s what we do within our entire galaxy, and we never have a problem with it.

But beyond our galaxy, there is a problem with that. The reason? The “grid” that we would put down to measure our Universe is not constant in time.

Want an analogy? Imagine the following hypothetical situation: imagine that the Earth is expanding. Let’s even pretend it’s expanding rapidly; by this time next year, the Earth will have double the radius that it has now.

Image credit: Marc Reiter.

From New York, Los Angeles appears to be 2,400 miles (3,900 km) away, and vice versa. But a year later, they measure the distance again, and find the distance between them is 4,800 miles (7,800 km). In New York, they accuse Los Angeles of moving away from them. In Los Angeles, they accuse New York of moving away from them. Yet both cities insist they’ve been stationary the entire time.

Things get worse when they start measuring velocities. In New York, they use light to measure how quickly Los Angeles is moving away from New York, and find that Los Angeles is moving away at 0.27 miles per hour (0.44 km per hour). In Los Angeles, they make the same measurement of New York, and find that New York, too, appears to be moving away from them at 0.27 miles per hour (0.44 km per hour).

But, in truth, neither one is moving. What’s going on is that the world that these cities live on is growing, and it makes them perceive that they’re all moving away from each other. It gets worse for things that are farther away; Shanghai, China, is presently 7,400 miles (11,900 km) away from New York. But a year from now, if the Earth is expanding, it will be 14,800 miles (23,800 km) away, and so appears to be moving away (in our expanding Earth example) at 0.84 miles per hour (1.36 km per hour). Yet Shanghai, Los Angeles, and New York are not moving at all. They simply appear to be moving relative to one another.

So it is with the Universe, with an expanding Universe instead of the expanding Earth, and with unchanging galaxies instead of unchanging cities. And that’s — probably — the last tricky part: the expansion is restricted to the Universe on the largest scales. Galaxies, stars, planets, cells, and atoms do not expand as the Universe does; the expansion of the Universe is far too weak to affect any of the dynamics of these relatively small objects. (Yes, compared to the expanding Universe, even a galaxy is too small to be of consequence.)

So does that clear things up? Or does that make them worse? Something as fundamental and seemingly simple as distances, positions, and velocities are — even among professional cosmologists — one of the most confusing (and poorly understood) things that we can talk about. It isn’t intuitive at all, because it runs so contrary to our everyday experience. And that’s part of what makes it so interesting.

Comments

  1. #1 Rob Knop
    August 3, 2009

    I agree that when you’re talking about the largest scales in the Universe, the concept of distance doesn’t fully make sense. It’s safest just to talk about “redshift”, but that’s jargonistic; it makes sense if you do cosmology and are used to it, and indeed is the thing that makes the most sense, but it doesn’t necessarily make sense when trying to communicate this to somebody outside cosmology.

    I like to refer to “lookback time”, because that has a clean definition. It’s not a relativistically invariant definition, but there is one obvious interpretation: “age of the Universe as measured by a comoving observer at the event of detection of light minus age of the Universe as measured by a comoving observer at the event of emission of light.” I don’t usually say it out at that length, but that’s the precise definition.

    To be sure there *are* things that could be defined as distance, but they tend to be more confusing for all the reasons you say. For instance, there is “proper distance now at fixed FRW t”, “proper distance at time of emssion at fixed FRW t”.

    The concept that *really* doesn’t make sense is velocity. Strictly speaking, velocity is a 4-vector. As such, it is defined at one point. Now, yes, you can parallel transport anything anywhere, but when you try to put a number on velocity, generally you are implicitly parallel transporting it through flat spacetime– that is, you’re working in a local SR reference frame. That approximation is fine for the closer galaxies (hence the “galaxies flying apart” description is fine for nearby stuff), but you need to be more careful for more distant things. Again, you can write down things that have the units of velocity — for instance, change in FRW-constant-t proper distance per change in FRW-t — but they aren’t always going to be the same, or even close to, components of the 4-vector velocity.

    Cook and Burns wrote an astro-ph paper (http://arxiv.org/abs/0803.2701) that investigates the whole “galaxies flying apart” vs. “space itself expands” ways of describing the expanding Universe. I’m in the “space itself expands” camp myself, but they really go through it carefully and point out that either way of describing it is ultimately something of a frame-dependent description.

  2. #2 Mu
    August 3, 2009

    I still have an issue understanding the “there’s no center of the universe” in regards to the expansion. In the ant on the balloon example of expansion, the expansion is in the radius r, a dimension that is not perceived by the two dimensional ant. Which is the dimension the universe expands in which we do not perceive, and which we therefor can’t trace back (via analysis of “expansion movement vectors”) to find the “center” of the expansion?

  3. #3 Sili
    August 3, 2009

    Great. Way to feed the Expanding Earthers.

    <wink goes here>

  4. #4 healthphysicist
    August 3, 2009

    I also have difficulty understanding the expansion relative to the fact that the universe is considered to be flat.

    The expansion examples (balloon & Earth) assume a sphere.

    How does expansion in all directions work with a flat universe?

  5. #5 dave w.
    August 3, 2009

    I’m still waiting for someone to figure out how to use this “expansion” to travel through space at the speed of light.

  6. #6 Gray Gaffer
    August 3, 2009

    Last night I was idly watching a Science Channel show with a segment on the LIGO experiments. This is a quest to measure Gravity Waves. But I think the subject of this post, coupled with some accidentally related material elsewhere in the show, may explain why they have been unsuccessful.\

    The crux of the experiment is based on the hypothesis that the lengths of the optical paths, being at 90 degrees to each other, will not vary in unison as a graviton passes by, and this should be detectable by interferometry. Until yesterday I thought this was a reasonable hypothesis – gravity changes anisotropically, lengths change, laser range-finders will show this.

    But this time a description of the space-time ripples that are gravity waves made a different sense to me. It is not that the local gravitation field wobbles. It is that local space-time flexes. And it occurred to me that that would also affect photon propagation, probably in such a way as to entirely cancel out the relativistic length variations.

    IOW, the ruler changes shape along with the ruled.

    Now the brains working LIGO are far more involved than am I, at a much higher e=mc^2 level of functioning. So what am I missing?

  7. #7 Austin
    August 3, 2009

    One aspect of expanding space I’ve always found fun is a little hard weird.

    So, space is expanding in every direction. We, however, aren’t (aside from a rise in obesity). This is troubling, because most matter is actually made of space, and matter is inherently linked to space.

    In the ant/balloon example, we wouldn’t expect the ants to remain in close proximity because they’re not (significantly, anyway) acting on each other. Replace the ants with atoms, however, and they apparently do. What this means is that the forces that hold matter together – weak and strong, electomagnetic, gravitational – are actually *stronger* than we measure, because a portion of their force is expended countering the expansion of the universe (otherwise we’d all fly apart).

    I don’t know that anyone’s ever tried to account for that extra “unseen” force anywhere.

  8. #8 Nick
    August 3, 2009

    Because space only expands on a really huge scale wouldnt that mean that some of the force might be expended on countering expansion, but on short scales the force required to counter expansion would be really really small

  9. #9 Question?
    August 3, 2009

    One thing few people ever consider is that the red-shifting of distant light and the expanding universe may just be an optical illusion explained away if we really understood what electromagnetic radiation was. It cannot be a particle called the photon because the energy and momentum contained in a photon is directly linked to the length of a man-made unit of time, the second.

  10. #10 Tom
    August 3, 2009

    I’ve seen recent references to the idea of time slowing down.

    While I understand that no better than space expanding, somehow it seems a more comfortable explanation. Light from long ago is redshifted because time was faster then.

    Or maybe I’m hallucinating

  11. #11 MadScientist
    August 3, 2009

    I’m still confused about the expansion of space – what bounds space and why should we imagine it’s bounded?

    @Question: It’s silly to say things like “if we really understood what electromagnetic radiation was”. It’s an observable phenomenon and we can measure its effects; the question of “what is it *really*?” is nonsensical. And why can’t the concept of ‘photon’ describe quantities of electromagnetic radiation simply because it’s related to the concept of time?

  12. #12 José
    August 3, 2009

    @Tom
    Time dilation is one of the key, crazy, experimentally observed confirmations of General Relativity, and General Relativity predicts either an expanding or contracting universe, which redshift (among other things) confirms.

  13. #13 Question?
    August 3, 2009

    The reason a photon cannot be a basic unit or particle in nature is because the value all basic units in nature must be independent of all man-made units of measurements. A photon’s value must remain constant regardless as to the length of a second. For example: if a second was 1/2 its present length a photon would contain 1/2 the number of waves, therefore 1/2 the energy and momentum it now has. This is just not permitted in physics. Now there is another alternative and that is the value of Planck’s constant is not a fundamental unit in nature. But is must be either Planck’s constant or the photon that is not a basic unit in nature, both cannot be. Change the length of a second and do the math, you will see this is true.

  14. #14 José
    August 3, 2009

    @Mu
    In the ant on the balloon example of expansion, the expansion is in the radius r, a dimension that is not perceived by the two dimensional ant.

    I touched on this way down in the comments on the last post as this used to confuse me as well. The answer is simply that for the analogy to work, you have to accept that the surface of the balloon is all there is. Once you start imagining things off the surface of the balloon, you’ve broken the analogy. In the real world, there is nothing that corresponds to anything off the balloon.

    @healthphysicist
    The expansion examples (balloon & Earth) assume a sphere.

    It doesn’t assume a sphere. It just uses a sphere to describe something in a way that humans can wrap their brains around.

  15. #15 Derek
    August 3, 2009

    @Question
    Are you joking?

    The reason a photon cannot be a basic unit or particle in nature is because the value all basic units in nature must be independent of all man-made units of measurements. A photon’s value must remain constant regardless as to the length of a second. For example: if a second was 1/2 its present length a photon would contain 1/2 the number of waves, therefore 1/2 the energy and momentum it now has. This is just not permitted in physics.

    I’m sorry, but that’s not even wrong. This is like saying an impact has less energy if the projectile mass is expressed in kilograms instead of pounds. Calculators now have buttons that can solve such intractable paradoxes. ;-)

  16. #16 Laborum
    August 4, 2009

    Do you have a public facebook page Ethan? I’d like to share some of your studies with friends, but some of them don’t like reading blogs for some reason.

  17. #17 piratebrido
    August 4, 2009

    I can’t help but think that Los Angeles and New York will also double in size, thus tearing apart all buildings and razing the cities to the ground! What a terrible world you describe!

    Also, how many balloons has this blog been through in the last month?

  18. #18 Naked Bunny with a Whip
    August 4, 2009

    @piratebrido: So that’s what’ll cause the end of the world in 2012: planetary inflation!

  19. #19 Question?
    August 4, 2009

    “Now there is another alternative and that is the value of Planck’s constant is not a fundamental unit in nature. But is must be either Planck’s constant or the photon that is not a basic unit in nature, both cannot be. ”

    No, I’m not joking. A second is a man-made unit of time.
    If the second were only 1/2 its present length there would only be 1/2 the number of waves in a photon of any given frequency. Now tell me how this photon could possibly contain the same amount of energy and momentum?

    This is not like a simple conversion from kilograms to pounds. If we changed the value of a kilogram, would that change the mass of a proton? No. Then why would changing the length of a second change the fundamental value of a photon?

  20. #20 Bjoern
    August 4, 2009

    @Question?:
    What you are apparently missing is that if we defined a second as being of only 1/2 its present length, we’d also have to change the numeric expressions (not the values!) of the light speed, Planck’s constant etc. accordingly.

    I.e. with the usual length of a second, we have h = 6.626 * 10^(-34) Js. With a second of only half the usual length, you’d have to use h = 13.252 * 10^(-34) Js.

    And no, this doesn’t mean that the value of Planck’s constant depends on how we define a second. This only means that the numeric expression of the constant is different – the actual value remains the same.

    As Derek already pointed out: this is quite similar to the difference between kilogram and pound…

  21. #21 Bjoern
    August 4, 2009

    @Question?:
    Another analogy: if we defined a second to have only half of its usual length, a minute would suddenly have 120 seconds instead of only 60. Does that mean that the length of a minute has changed? No, that only means that our measure of its length has changed. Previously, we had “one minute per 60 seconds”; then we have “one minute per 120 seconds”.

    It’s the same with h. If you have the usual second, h is 6.626 * 10^(-34) Js. If you have the changed second, h is 3.252 * 10^(-34) Js. The actual photon energy stays the same – only our measure of it changes.

  22. #22 Bjoern
    August 4, 2009

    @Mu:
    “Which is the dimension the universe expands in which we do not perceive, and which we therefor can’t trace back (via analysis of “expansion movement vectors”) to find the “center” of the expansion?”

    We don’t know if such a dimension exists, and, as hard as this is to swallow, such a dimension does not even need to exist. The math of General Relativity (differential geometry) talks only about the internal properties of space – no reference to an external “dimension” is necessary.

  23. #23 Bjoern
    August 4, 2009

    @Gray Gaffer: What you are missing is that “local space-time flexes” is identical to the “relativistic length variations”. These are not two different effects which could cancel out each other – these are one and the same effect.

  24. #24 Bjoern
    August 4, 2009

    @MadScientist:
    “I’m still confused about the expansion of space – what bounds space and why should we imagine it’s bounded?”

    If space is closed (sphere-like), it bounds itself, it *is* the boundary. If space isn’t closed (i. e., it is flat or hyperbolic), it isn’t bound at all, but infinitely large. (well, there are also more exotic possibilities like space being donut-shaped, but let’s leave that for further discussion… ;-) )

  25. #25 Naked Bunny with a Whip
    August 4, 2009

    Space donuts. Yummies.

  26. #26 Question?
    August 4, 2009

    Bjoern, I agree the numerical values are what must change. Planck’s constant is the same regardless as to changes in its numerical value or the length of a second. I think we both agree on that. When the length of a second is changed the value of h does not change but the f (frequency) does change and therefore the value of e (energy of photon) changes in the formula e=hf.

    In order to keep the value of a photon we must add or subtract from the value of h or f, if the length of a second is changed.

    It would appear to me that the photon is just a man-made unit used to measure electromagnetic energy, not a fundamental particle in nature like electrons or protons.

  27. #27 Bjoern
    August 4, 2009

    @Question:
    “Bjoern, I agree the numerical values are what must change.”

    That’s not what I said. Read again.

    “When the length of a second is changed the value of h does not change …”

    Right – but as I pointed out, its numeric expression changes. You seem to have missed that. Read again.

    “… but the f (frequency) does change …”

    Right. E. g. if we have an electromagnetic wave with a frequency of 1 Hz and change our definition of second so that the “new second” has only half of the length of the old one, we’ll have an electromagnetic wave with a frequency of 0.5 Hz then.

    “… and therefore the value of e (energy of photon) changes in the formula e=hf.”

    Here it is were you go wrong. As I pointed out, the numerical expression of h has to be changed also (keep in mind that this doesn’t change the actual value of h, merely its numeric expression!).

    Using the “old second”:
    E = 6.626 * 10^(-34) Js * 1 Hz = 6.626 * 10^(-34) J
    Using the “new second”:
    E = 13.252 * 10^(-34) Js * 0,5 Hz = 6.626 * 10^(-34) J

    Same energy using both definitions of “second”.

    “In order to keep the value of a photon…”

    What is this supposed to mean? Do you mean the energy of the photon?

    “…we must add or subtract from the value of h or f, if the length of a second is changed.”

    Actually, we have to multiply or divide the numeric expression of h – as explained several times now.

    “It would appear to me that the photon is just a man-made unit used to measure electromagnetic energy, not a fundamental particle in nature like electrons or protons. ”

    That appears wrong to you. Look up the Compton effect and try to explain that without photons.

  28. #28 Question?
    August 4, 2009

    “That appears wrong to you. Look up the Compton effect and try to explain that without photons.

    Posted by: Bjoern | August 4, 2009 7:40 AM”

    The Compton effect can be explain quite easily using only Classical physics. It is explained in “An Alternative to the Standard Model of Physics”, bottom of pg 12 – top of pg 13.

    http://www.scribd.com/doc/16251160/An-Alternative-to-the-Standard-Model-of-Physics

  29. #29 Mu
    August 4, 2009

    Thanks a lot for the attempts to make me understand this; but I’d like to come back to detecting the direction of expansion. If we can detect expansion by measuring the movement of distant bodies, would we in time (and this might be beyond human lifespan, I’m talking theoretical, not practical) end up with a series of vectors pointing away from a “center” of expansion? Or is it observer dependent, and every observer only sees himself as the center from which everything else is expanding away, with no detectable difference (like north moves at double the speed of south, indicating we’re part of a southern movement)?

  30. #30 Bjoern
    August 4, 2009

    @Mu: It is indeed observer dependent (or rather, independent!): every observer sees himself at the “center” from which everything else moves away. That’s easy to visualize using the balloon analogy.

  31. #31 Naked Bunny with a Whip
    August 4, 2009

    @Mu: Every observer sees himself as the center of expansion. There’s no fixed, universal centerpoint to spacetime that everything is moving away from.

  32. #32 Bjoern
    August 4, 2009

    @Question?
    The document you cite does not provide an explanation, but simply a story made up from thin air – with no evidence to support it, with no quantitative analysis of the phenomenon, with no attempt to derive the formula which gives the energy shift experienced in the Compton effect, with no attempt to explain why the Compton effect depends on the scattering angle etc. etc. etc. This “explanation” is about on the level of the claim that the Compton effect is caused by fairies which sit on the atoms and juggle the x-rays around.

  33. #33 Naked Bunny with a Whip
    August 4, 2009

    @Bjoern: Unfortunately, it’s only easy to visualize with the balloon analogy once you grasp that the latex — and only the latex — is spacetime and that the “inside” and “outside” of the balloon are merely a limit of the analogy with no physical significance.

  34. #34 Bjoern
    August 4, 2009

    @Question:
    To be more precise, go to the Wikipedia article on “Compton scattering”. What this “alternative model” should (at least!) be able to explain is the “Compton scattering equation” shown there. That equation can be quite easily derived using the idea of photons (and some basic relativistic calculations) – and that equation has been shown to hold experimentally to very great precision. As long as this “altetrnative model” can’t reproduce this equation, you can’t claim that it explains the Compton effect.

    If it has managed to do that, it can go on to the next step: using Quantum Electrodynamics (which also is based on the idea of photons), one can calculate the scattering cross section for the Compton effect (see the article on the Klein–Nishina formula). That’s another formula which has been shown to hold experimentally to very great precision. And hence another formula which this alternative model has to reproduce before you can claim that it explains the Compton effect.

    But a quick glace at the document you cited shows that it apparently does not contain even one formula! And nevertheless, it claims to be “an alternative to the Standard Model of Physics”… *sigh* (of, BTW, what the author obviously means is the Standard Model of *Particle* Physics… but why should he bother being correct?)

  35. #35 bjoern
    August 4, 2009

    @Question?
    I’ve just read the first half page of the document you cited, and noticed two things:
    1) It contains the same argument that a second is “man-made”, and hence the energy of a photon would change if one changed the definition of a second (an argument which I showed to be wrong above – you ignored that explanation); hence I conclude that you are the author of that document. Why didn’t you say so right away?
    2) Even in these few lines, there are so many basic errors and downright nonsensical statement that it is quite clear that you don’t know much about physics. And your behaviour in this thread shows that you aren’t willing to learn.

    I conclude that further discussion with you makes little sense – because you obviously have already made up your mind that you know everything one needs to know (whereas in reality you know almost nothing), and hence won’t listen. Bye.

  36. #36 Question?
    August 4, 2009

    “The kinetic energy gained by the electron in the Compton effect is independent of the change in frequency of the reflected light.”

    Source: Encyclopedia Britannica Online 24 Oct. 2003

    http://www..search.eb.com/eb/article?eu=61313

    Bjoern: How does the photon theory of light explain that?

  37. #37 Bjoern
    August 4, 2009

    @Question?
    *sigh* I said “bye”, but you apparently won’t go away… hence one more try (although you obviously don’t listen – you have ignored pretty much all of what I wrote!):

    “The kinetic energy gained by the electron in the Compton effect is independent of the change in frequency of the reflected light.”

    That statement is wrong, plain and simple. The kinetic energy gained by the electron is equal to the change in frequency of the reflected light (or rather, x-rays) times Planck’s constant, minus the binding energy (which usually is negligible) – not independent of that change.

    I suggest that you consult actual physics books instead of an Encyclopedia… If you don’t believe me, you could e. g. go to the Wikipedia article which I already mentioned. The formulas shown there, especially the conservation of energy, support what I said.

    Oh, and if you insist on discussing this further: we are off-topic on this thread, so you perhaps want to mail me. My mail address is my name (as given here)at feuerbacher minus web point de.

  38. #38 Ethan Siegel
    August 4, 2009

    There have been enough Q&As in this thread about how redshift and distance works in the expanding Universe that I will write about it on Wednesday.

    Continue the Compton discussion!

  39. #39 Naked Bunny with a Whip
    August 4, 2009

    Compton! Compton! He’s our man!
    If light can’t scatter, no photon can!

    /me doesn’t quit his day job

  40. #40 the backpacker
    August 4, 2009

    Bjoern-

    Question may not be learning anything but I am. I just have one question, the Wikipedia article on Compton Scattering states that the phenomenon proves that light can’t be explained solely as a wave, how does it show that? I am fuzzy on wave particle duality so use small words.

  41. #41 Mu
    August 4, 2009

    30 and 31, thanks, that makes me feel better. I’m still an idiot who can’t grasp time-space, but at least I’m the center of the universe whenever I look (channeling Zaphod B. here).

  42. #42 Bjoern
    August 4, 2009

    @the backpacker:
    In science, one shouldn’t say that something is “proven”. A better way of stating this is: using only classical electrodynamics, where light (and x-rays) is described as a wave, one can’t explain the Compton effect. Classically, one would expect that the amplitude of the wave (the brightness of the light) changes when its energy is transmitted to electrons, whereas what one observes is that the frequency of the wave (the color of the light) changes.

    On the other hand, using the concept of photons, the effect is explained quite easily (the Wikipedia article shows all the relevant calculations). The crucial point there is that light consists of “energy packages” (the photons), and the energy “content” of a photon is directly related to the frequency of the wave.

  43. #43 Question?
    August 4, 2009

    Oh well, one more try on this side also.

    Bjoern: I got a question for you, has anyone actually counted the number of photons used the conduct a Compton Effect experiment? Has anyone ever counted the red-shifted photons exiting the experiment? How would these two numbers compare? Until they conduct such a test, it hasn’t been proved that energy and momentum are conserved in the Compton Effect in the manner you and the photon theory of light claim. Why? Because there may be more red-shifted photons exiting than there were photons used to conduct (entering) the experiment. The Alternative claims the actual number of waves entering and exiting the experiment is close to the same and that the kinetic energy gained by the electrons comes from the heat energy these electrons contain. (A few waves will be lost or absorbed by the electrons and atoms in the experiment.)
    The photon theory claims the number of waves is reduced after the x-rays collide with the electrons but the number of photons does not change. A test should be conducted to find out which is correct.

    There is another potential error in the photon theory of light regarding the Compton Effect. Has anyone ever measured the temperature of the electrons before and after the experiment? The Alternative claims the temperature of the electrons’ are reduced in direct proportion to the kinetic energy they gain, plus any radiation these electrons may absorb. The photon theory does not even consider this potential energy well.

    Just because one can make things work out mathematically is not proof the theory correctly explains what is happening.

  44. #44 Gray Gaffer
    August 4, 2009

    @ Bjoern: I was thinking not that they cancel each other out, but that the effect on the light path would be the same as the effect on the tube length. In other words, the ruler (the light beam’s space-time path) is distorted by the same amount in the same directions as what it is to measure so it continues to measure the same distance according to its gradations. I did not get from your answer why the light would not be affected while the tube is. Unless its propagation time results in slight phase differences (relative to the passage of the gravity wave) by the time it gets back to the detector? I can see that effect being observable but tiny. I do not see a static picture being observable at all.

    Thanks.

  45. #45 Naked Bunny with a Whip
    August 4, 2009

    @Gray Gaffer: According to the LIGO factsheet:

    When [gravity waves] enter the LIGO detector they will decrease the distance between the test masses in one arm of the L, while increasing it in the other…. The tiny changes in test-mass distances throw the two arms’ laser beams out of phase with each other, thereby disturbing their interference and revealing the form of the passing gravitational wave.

  46. #46 João Carlos
    August 4, 2009

    Just for the record: I’ve commited another translation into Brazilian-Portuguese at O Temanho do Universo (…)Parte 2.

    The questions I’ve got are pretty much the same kind: it’s really non-ituitive to grasp the real size (?) of the Universe.

  47. #47 Bjoern
    August 5, 2009

    @GrayGaffer: The light is indeed affected, and what is measured are indeed phase differences, and these are indeed tiny.

  48. #48 Bjoern
    August 5, 2009

    @Question?
    “I got a question for you, …”

    Well, what about answering all my questions and arguments first…?

    And what about my suggestion to switch to e-mail?

    “..has anyone actually counted the number of photons used the conduct a Compton Effect experiment?”

    This sentence does not even make sense grammatically; I don’t understand what exactly you want to know, sorry. Judging from what you write afterwards, you perhaps ask if anyone has measured the number of photons which “enter” a Compton effect experiment. If yes: first, see answer below; second, “counting” a photon by detecting it effectively destroys it, so counting the number of photons entering such an experiment would make the experiment impossible, I’d say. What you could do is measure the intensity of the X-rays and then calculate the number of photons from that, but I doubt that you would be satisfied with that…

    “Has anyone ever counted the red-shifted photons exiting the experiment?”

    Since there would be a really *very* huge number of them, I doubt it. But again, a calculation is possible.

    “How would these two numbers compare?”

    If you do the calculation, the two numbers come out equal.

    “Until they conduct such a test, it hasn’t been proved that energy and momentum are conserved in the Compton Effect in the manner you and the photon theory of light claim.”

    Nonsense.

    “Why? Because there may be more red-shifted photons exiting than there were photons used to conduct (entering) the experiment. ”

    Even if there were, that would in no way violate conservation of energy and momentum.

    “The Alternative claims …”

    I already pointed out that you don’t have an “alternative” – what you have are wild, unsupported stories.

    “…the actual number of waves entering and exiting the experiment is close to the same…”

    “number of waves” makes little sense in this context. What on earth do you mean? Are you perhaps talking about wave packages?

    “… and that the kinetic energy gained by the electrons comes from the heat energy these electrons contain.”

    Well, first, it is unclear how electrons could contain heat – since heat is essentially the unordered movement of constituent particles of a body, and electrons are elementary particles, as far as we know. Second, how do you explain then that the kinetic energy gained by the electrons is always equal to h times the frequency change of the x-rays? Third, have you ever heard of the second law of thermodynamics? It states (in one of its various forms) that it is impossible to convert heat totally to kinetic energy – making your suggestion impossible.

    “(A few waves will be lost or absorbed by the electrons and atoms in the experiment.)”

    It is unclear what “a few waves” is supposed to mean. There is only *one* incoming wave in such an experiment – the incoming x-rays.

    “The photon theory claims the number of waves is reduced …”

    Err, no, the photon “theory” (thanks for showing that you don’t know what that word means) doesn’t claim anything like that, because physicists don’t use the term “number of waves” (because it makes little sense here, as I already pointed out). Judging from context, you apparently mean the frequency of the x-rays. Why don’t you use that word instead of the meaningless “number of waves”?

    “A test should be conducted to find out which is correct.”

    If you accept that each photon carries the energy E = hf, one can do the calculation, as I already pointed out – and finds that the photon number indeed stays the same.

    “Has anyone ever measured the temperature of the electrons before and after the experiment?”

    Since the electrons are in thermal equilibrium with the object which contains them, they obviously have the same temperature as that body. And since the electrons gain kinetic energy due to the Compton effect, which is then dissipated to the object (provided the electrons don’t escape the object), the temperature of that object will increase during the experiment. However, I don’t know if this has ever been measured.

    “The Alternative claims the temperature of the electrons’ are reduced in direct proportion to the kinetic energy they gain, plus any radiation these electrons may absorb. The photon theory does not even consider this potential energy well.”

    I don’t understand what you want to say here. You apparently speak of the temperature of *single* electrons, whereas the concept of temperature makes only sense if one considers a whole huge ensemble of particles at once. You apparently don’t know what “temperature” actually means in physics.

    “Just because one can make things work out mathematically is not proof the theory correctly explains what is happening.”

    1) As I already pointed out, one usually doesn’t talk about proving a theory in physics.
    2) If a so-called “alternative” can’t make *anything* come out right mathematically, whereas the usual model can, with great precision, it is quite clear which one wins…

  49. #49 Question?
    August 5, 2009

    Bjoern: Sorry but I do not answer most of your questions because I cannot even understand them. But I have another question, where would the energy from solar sailing using the sun’s light come from? It cannot come from the change in the frequency of the reflected light because at the start of such a journey the reflected light is essentially the same frequency as the light striking the solar sail. So little if any energy could come from this change in frequency. Assuming one uses solar light to accelerate a spaceship to 1/2 c, now the reflected light would be red-shifted to 1/2 its source frequency. It would now appear that 1/2 the photon’s energy would be used to accelerate the spaceship. Wrong, it would actually 1/2 of next to the nothing you got at the start of such a solar sailing adventure. Why? Because the light striking the solar sail is actually red-shifted BEFORE it strikes the solar sail when the spaceship is moving away from the sun at 1/2 c.

    The exact same thing happens in the Compton Effect. Each succeeding wave striking the electron is red-shifter BEFORE it strike the electon. We just cannot visualize it with the scale of electrons.

    The conservation of energy and momentum cannot be conserved in solar sailing using the photon concept of light. This is all explained in greater detail in the Alternative on pg. 10 at the link below.

    http://www.scribd.com/doc/16251160/An-Alternative-to-the-Standard-Model-of-Physics

  50. #50 Bjoern
    August 5, 2009

    @Question:
    “Sorry but I do not answer most of your questions because I cannot even understand them.”

    Then I’d suggest that you first go learning some basic physics before making up your own “alternative model”…

    “But I have another question, where would the energy from solar sailing using the sun’s light come from? It cannot come from the change in the frequency of the reflected light because at the start of such a journey the reflected light is essentially the same frequency as the light striking the solar sail. So little if any energy could come from this change in frequency.”

    1) Even if the change in frequency is only tiny, due to the huge amount of photons, you’d still have an appreciable energy change of the sail.
    2) You neglect that some of the photons are also absorbed, not all are reflected.

    You are right that the sunlight is redshifted even before it hits the sail (from the viewpoint of the sail) – but how do you conclude from this that the energy transfer is “next to nothing”?

    “The exact same thing happens in the Compton Effect. Each succeeding wave striking the electron is red-shifter BEFORE it strike the electon.”

    Err, the electrons are (in the mean) resting with respect to the x-ray source, so where should the redshift come from?

    And I yet again notice that you have only a wild story, but no supporting evidence for that, no way to derive the well-known formulas (which describe the experimental results with great precision) from that idea etc.

    “The conservation of energy and momentum cannot be conserved in solar sailing using the photon concept of light.”

    Nonsense. Oh, and BTW, this is yet another sentence from you which does not even make sense grammatically. But why bother writing consistently…?

    “This is all explained in greater detail in the Alternative on pg. 10 at the link below.”

    You are still confusing wild, unsupported claims with an explanation, I see. Oh, and actually this is on page 11 (only the headline is on page 10); apparently you don’t even know your own document…

    You write there:
    “How can this push come from a change in frequency when the reflected photon, at the start of this solar sailing adventure, has the same frequency as the photon striking the sail?”
    The answer is quite simple: the reflected photon does not have the same frequency, but a lower one. End of story.

  51. #51 Bjoern
    August 5, 2009

    @Question?
    What you are neglecting in your “solar sail” scenario is how light reflection actually works on the quantum level: photons are absorved and the re-emitted (in the opposite direction). When a photon is absorbed, its energy and momentum is transferred to the sail – hence the sail begins moving, even if it was at rest (with respect to the sun) in the beginning. Only afterwards the photon is emitted again, backwards; in the frame of reference, with the same energy as the absorbed photon, but in the frame of reference of an outside observer (at rest with respect to the sun), that photon has now a smaller energy, and the remaining energy is contained in the sail now. (Looking at this from the viewpoint of electromagnetic waves, this corresponds to the Doppler shift.)

  52. #52 Question?
    August 5, 2009

    Bjoern: I will try to respond to a few of your answers.

    [(Bjoern) 1) Even if the change in frequency is only tiny, due to the huge amount of photons, you'd still have an appreciable energy change of the sail.
    2) You neglect that some of the photons are also absorbed, not all are reflected.

    You are right that the sunlight is redshifted even before it hits the sail (from the viewpoint of the sail) - but how do you conclude from this that the energy transfer is "next to nothing"?]

    (Question?) The change in frequency is so small I defy anyone to even measure it. You are correct that a few photons are absorbed but this actually makes the solar sail less efficient than if this same light had been reflected with NO change in frequency.

    [(Question?) "The exact same thing happens in the Compton Effect. Each succeeding wave striking the electron is red-shifter BEFORE it strike the electron."

    (Bjoern) Err, the electrons are (in the mean) resting with respect to the x-ray source, so where should the redshift come from?]

    (Question?) You bring up a valid point here that the photon theory does not even deal with in the Compton effect. First the electron must be ionized from the atom. This takes energy absorbed from the source x-rays. Step two, the electron is heated from the energy it absorbs. Step three, once ionized it zips off at its near top speed. (Why near top speed? Because that is the speed it is vibrating at from the heat energy it got from the x-rays.) Any light waves striking it from here on are red-shifted before being reflected off this electron. You should read a few pages back from the solar sailing section in the Alternative, it is explain there.

    By the way where is the energy needed to ionize the electron figured in the equation below? Or are they using free electrons, I am confused on this one?

    The final result gives us the Compton scattering equation:
    (equation at wiki)

    [(Question?) "The conservation of energy and momentum cannot be conserved in solar sailing using the photon concept of light."

    (Bjoern) Nonsense. Oh, and BTW, this is yet another sentence from you which does not even make sense grammatically. But why bother writing consistently...?]

    (Question?) I cannot understand your answer. The reason the photon theory does not conserve energy (it actually creates it out of nothing) is because at the beginning of any solar sailing adventure the energy gained by the spacecraft would be at its maximum with essentially NO change in frequency of the reflected light.

    (Question?) On posting 51, you state the photon is absorbed and reemitted in the opposite direction. How can that be when the material the solar sail in made of cannot absorb that frequency? That is why the light is reflected.

    This will be my final posting unless you want a specific answer to a question.

  53. #53 Bjoern
    August 5, 2009

    @Question?
    “The change in frequency is so small I defy anyone to even measure it.”

    You still ignore the fact that there is a huge amount of such tiny energy transfers. A hint: a very small number times a very large number can easily give a noticeable result. Hey, why don’t you simply do the math? Put some numbers into the equations, then you’ll see that it all works out nicely.

    But apparently you don’t think that you actually need quantitative arguments, right? You prefer vague, qualitative claims – without supporting them in the least.

    “You bring up a valid point here that the photon theory does not even deal with in the Compton effect. First the electron must be ionized from the atom. This takes energy absorbed from the source x-rays.”

    Usual treatments of the Compton effect in textbooks take that into account. Look it up. (as I already pointed out, the binding energy is usually negligible).

    “Step two, the electron is heated from the energy it absorbs.”

    Yet another wild, nonsensical claim without any shred of supporting evidence… I already explained why talking about heat contained in electrons makes no sense.

    “Step three, once ionized it zips off at its near top
    speed.”

    What is “top speed” supposed to mean?

    “Why near top speed? Because that is the speed it is vibrating at from the heat energy it got from the x-rays.”

    This makes no sense at all. Please try to rephrase this so that other people can understand what you mean.

    “You should read a few pages back from the solar sailing section in the Alternative, it is explain there.”

    I already told you that I looked at the first half page, and even that short section was full of stuff which made no sense at all.

    “By the way where is the energy needed to ionize the electron figured in the equation below? Or are they using free electrons, I am confused on this one?”

    Again: the binding energy is usually negligble – as you would already now if you had bothered to actually research this a bit, instead of simply going on talking about things you don’t understand.

    “The final result gives us the Compton scattering equation:”

    You still have not shown how that equation follows from your model. Show your calculations, please. Or finally admit that your model can’t explain the Compton effect.

    “The reason the photon theory does not conserve energy (it actually creates it out of nothing)…”

    Huh? Nowhere does the photon “theory” create energy out of nothing.

    “…is because at the beginning of any solar sailing adventure the energy gained by the spacecraft would be at its maximum with essentially NO change in frequency of the reflected light.”

    *sigh* I explained that in great detail.

    “On posting 51, you state the photon is absorbed and reemitted in the opposite direction. How can that be when the material the solar sail in made of cannot absorb that frequency?”

    The material can not absorb the light *permanently*. But essentially *every* interaction of light with matter works by the matter first (usually temporarily) absorbing photons, and then emitting them again. Look e. g. at the Feynman diagram for the Compton effect – there this is shown quite nicely. Or you could also look at the Feynman diagram for bremsstrahlung.

    “This will be my final posting unless you want a specific answer to a question.”

    Yes, I want a specific answer to a question:
    Do you finally admit now that what you said about a second being a man-made unit and therefore the energy of a photon would depend on its length was wrong?

  54. #54 Naked Bunny with a Whip
    August 5, 2009

    @Bjoern: I know it’s been frustrating, but I wanted you to know that there people learning from your comments. (I can’t possibly be the only one.)

  55. #55 TBRP
    August 5, 2009

    Bjoern, I just wanted to second Naked Bunny’s comment. I’ve found your exchange (as one sided as it seems) very enlightening (slight pun intended).

  56. #56 Andrew
    August 5, 2009

    Indeed. Having read the whole thing, it seems that Question? is teetering on crackpot territory; he’s latched on to a single fact he doesn’t understand, invented an elaborate “common-sense” explanation for it, and has decided his original thinking overturns all other established knowledge.

    It’s particularly illuminating that halfway through a fairly technical (but not obscure) physics discussion, he admits to not understanding the questions at all, but this does not shake his faith that he has the right of it.

  57. #57 DOF
    August 6, 2009

    @Mu: #2 & #29
    “In the ant on the balloon example of expansion, the expansion is in the radius r, a dimension that is not perceived by the two dimensional ant. Which is the dimension the universe expands in which we do not perceive, Or is it observer dependent, and every observer only sees himself as the center from which everything else is expanding away, with no detectable difference”

    Mu I think you are on the right track questioning the dimensionality of the balloon analogy. According to the standard cosmological model we are in fact experiencing existence inside the ‘balloon’ (sometimes referred to as the bulk) which represents our observable universe. It has at least 3 spatial dimensions verses the 2 on the balloon’s surface. So keeping the balloon purely for familiarity sake not shape, let’s try this….

    Imagine our ant crawls inside a balloon lying on the ground. Once inside he notices a spider dangling in the absolute center (this is the spider’s universe) with his web already weaved. Caught in his web are a lady bug and a worm with the worm being twice the distance of the Lady Bug from center. Our ant gets one of his feet stuck in the web and now he is trapped, though luckily at the furthest possible point away from the spider, along the inner circumference! 11 year old Alec comes walking along and sees the balloon, picks it up and exhales into it, rapidly expanding its volume.

    Take a moment to imagine what happens inside the balloon? The result is somewhat different from the original analogy? The sides of the balloon all move away from the spider at the same rate in all directions. But the insects appear to be moving away in proportion to their initial distance from the spider. The spider notices his lunch and dinner are moving away and quickly detaches himself and starts heading for them.

    Meanwhile Alec continues exhaling further increasing the volume in the balloon. After taken a couple steps the spider stops to check his progress and notices that the Lady Bug is further away now then when he started and the worm is further-further away! A couple more steps and now the Lady Bug is further-further away and the worm is further-further-further-further away and hardly visible. Light from our ant (breakfast) can no longer be seen through the mass of webbing and distance. More on him in a moment. Realizing he cannot travel fast enough to reach his meals the spider gives up the futile chase and returns to the center of his universe. By the time he is repositioned the worm is a distant memory and the Lady Bug’s light is slightly visable though she too, in reality, has vanished. In time, there will be nothing to be seen in any direction.

    Now replace the inner walls of the balloon with the edge of our observable universe, the spider with us, the Lady Bug and worm with distant neighboring galaxies and our ant friend with the most distant galaxy on our universal horizon. That should do it.

    Want more mind numbing antics? Combine the original analogy with mine. Now add ‘copies’ of the other insects to the ‘copy’ of the ant on the exterior of the balloon. This conceptually describes the Holographic Principle which postulates that physics of the interior of a 3 dimensional volume of space (the bulk) can be described by physics on the exterior (it’s boundary) of a 2 dimensional surface. This concept can be challenging to grasp and while it has yet to gather sufficient supporting evidence, the physics and math are well motivated.

  58. #58 Bjoern
    August 6, 2009

    @Bunny, TBRP, Andrew: Thanks for the support. :-)

    I’ve encountered the mindset of “Question” quite frequently… A nice demonstration of the Dunning-Kruger-effect! (see Wikipedia if you don’t know that effect – Darwin originally expressed it very nicely: “ignorance more frequently begets confidence than does knowledge”)

  59. #59 Question?
    August 6, 2009

    Bjoern: “Yes, I want a specific answer to a question:
    Do you finally admit now that what you said about a second being a man-made unit and therefore the energy of a photon would depend on its length was wrong?”

    No: A photon’s energy and momentum are dependent on the length of the second, a man-made unit of time, by way of its frequency.
    I do not know of any way around that, energy of photon=hf.

  60. #60 crstnj
    August 6, 2009

    Hi Ethan
    Your car sound analogy doesn’t hold, does it ?
    Sound waves are affected by the doppler effect because the speed depends on the speed of the emitter and receiver. But light always travels at light speed and cannot be affected by the doppler effect. So what is happening here ?

  61. #61 Ethan Siegel
    August 6, 2009

    crstnj,
    The speed of sound doesn’t change, either. A police car, 50 meters away from you, will have its siren’s sound reach you in the same amount of time regardless of whether it’s moving towards you, away from you, or not at all.

    But the pitch will change; that’s what the doppler shift says. Same deal with light. Exactly the same deal.

  62. #62 crstnj
    August 6, 2009

    Ahh ,of course, i’ve always had problems with this analogy – but not anymore.
    Thanks

  63. #63 lowerleavell
    August 7, 2009

    Forgive me for butting into this discussion, but this question is directed towards Ethan. I am a YEC who has been questioning the Big Bang for quite some time, and though I am not a scientist, many things do not ring true for me. I have been conversing with Ed Darrel on timpanogos.wordpress for about two years about the Big Bang and evolution and he directed me to your web-site for further information.

    I recently became aware of “Abell 1835 IR1916″ which is estimated at “13.2 billion light years away” obviously meaning that the light from “Abell” has taken 13.2 billion light years to reach earth. According to wikipedia (I’m a simple googler, I understand that) the comoving distance between us and this galaxy now stands at roughly 31 light years away. While I do think I understand what you’re saying about the balloon illustration (it was very helpful, thank you!) the problem that I am facing is that if the universe is expanding faster than the speed of light, how is it that we are seeing light from “Abell”? Wouldn’t the light not be able to match the rate of expansion?

    I have a couple other questions that I doubt I can formulate well, and may be elementary to you, but here goes. My question is in the balloon illustration, as the balloon expands, wouldn’t you expect matter (i.e. the ant) to be moving away from where the point of expansion was (i.e. the “Bang”)? If that is the case, would we not expect to see a LOT of empty space with no matter with at least billions of light years across coming from the point of origin (Big Bang) before objects began slowing down from the impact of the explosion (if it is indeed slowing down rather than expanding faster)? Is what we obvserve what we should expect from a Big Bang?

    Lastly, how was/is the speed of the expansion determined? Not to ask for a calculation of how you got your figures, but what made the universe expand faster than light? If it is possible for the universe to move faster than the speed of light, would it also not stand to reason that the rate of speed could possibly be infinite? What is it that limited the expansion to a particular rate, especially if the matter of the universe is proportionately inconsequencial? What is governing the rate of expansion if not the speed of light?

    I probably put these questions out badly, but thank you for your time in reading them and responding to them.

    Lowerleavell

  64. #64 Ethan Siegel
    August 7, 2009

    Lower Leavell,

    Abell 1835 wasn’t always moving away from us that quickly. In the past, it was closer to us, and therefore the expansion rate was slower.

    If your expansion rate is a constant, say, 1 m/s/m, and something starts 1 meter away from you, then it moves away at 1 m/s. After 1 second, it’s 2 meters away. But, now that it’s 2 meters away, it’s moving away from you at 2 m/s. So after 2 seconds, it’s 4 meters away. And then, since it’s 4 meters away, it’s moving away from you at 4 m/s. So after 3 seconds, it’ll be 8 meters away, and will be moving away at 8 m/s.

    Do you see how this builds up to easily be faster than the speed of light after a few seconds? This means that if this thing was once close enough to us, we’ll be able to see the light from it. But at some point in the future, the light it emitted won’t be able to reach us anymore, and it will appear to wink out of existence. This is a phenomenon that cosmologists call “redding out”, and it happens in any Universe with dark energy in it.

    And you might be surprised to hear me say this, but it’s okay to be skeptical of the big bang! Just make sure you get all the evidence in before you decide what you believe!

  65. #65 Mike
    August 11, 2009

    Are the farthest things away from us on the verge of redding out? Have we ever witnessed this happen?

  66. #66 Sriram
    August 11, 2009

    This universe expansion is so frustratingly beautiful :). You get what the author is trying to say one sentence but the next sentence you go, that’s just not right!!! :).. Amazing article man..

  67. #67 lowerleavell
    August 12, 2009

    Ethan,

    Thanks for your gracious reply. I very much appreciate your time.

    If the build up to the speed of light is pretty quick, my question would still stand, how is it that we’re able to see light from 13.2 billion light years away? Especially if our galaxy is going a different or opposite direction that surpasses the speed of light, wouldn’t the speed of expansion be so fast that perhaps light from Abel 1835 would have stopped reaching us ages and ages ago?

    If what you say is true about expansion, then wouldn’t we be able to somehow observe where the Big Bang occurred because all galaxies would be moving away from that point and there would be no matter there because the universe is expanding? Wouldn’t there be a location in space where we observe simply…empty space?

    What then would be the rate of expansion that would determine the approximate speed of our galaxy? I would assume that the original rate of speed coming from the Big Bang would have been pretty fast considering the force of the explosion, am I right? How long then would it have taken for us to go faster than the speed of light? After 13.5-7 billion years of expansion, just how fast are we moving? From my limited viewpoint, after 13.5 billion years of expanding at such high rate of speeds as you describe, especially if there is no cap to the rate of speed, it seems to me that the universe should be so expanded that light from Abel 1835 would not be reaching us anymore.

    One other thing that doesn’t make sense about your illustration about the balloon and the ant is that while you’ve described how things expand on the outside of the balloon and how the balloon expands, the universe’s galaxies are on the inside of the universe, not the outside. Galaxies are then traveling at a different rate of speed than the exterior of the universe. If they were not different, then all matter would be at the edge of the universe, not at various locations and distances within the universe. How then do you determine that the galaxies are traveling faster than the speed of light, or what rate of speed they travel since their rate of speed is not tied to the rate of expansion of the universe?

    Thank you again for your time with us novices. :-) I wish I had the scientific terms and stuff for this to help, but hopefully you get what I’m saying.

  68. #68 Finally!
    August 12, 2009

    So it is with the Universe, with an expanding Universe instead of the expanding Earth, and with unchanging galaxies instead of unchanging cities. And that’s — probably — the last tricky part: the expansion is restricted to the Universe on the largest scales. Galaxies, stars, planets, cells, and atoms do not expand as the Universe does; the expansion of the Universe is far too weak to affect any of the dynamics of these relatively small objects. (Yes, compared to the expanding Universe, even a galaxy is too small to be of consequence.)

    FINALLY! Someone finally partially explained the thing that didn’t make sense about expansion. So, it ONLY affects galaxies relative to each other (or rather, the space in between them). Unfortunately, it raises another question. As I understand it (and I may well be wrong), the Plank constant relates to very small things/times. Is there such a thing as the “smallest unit of spacetime” that matter can occupy? I.e. just like light is “quantized”, is space quantized – could I draw a grid on it? And, after expansion of a certain amount, are the grids now larger? Are there more grids of the same small size?

    Or, do grids near enough matter (galaxies, planets, atoms, etc) stay the same size due to the forces of matter, and only those grids where there is relatively little matter stretch?

    As I understand it, matter “bends” spacetime. So, does matter effectively shrink the grids to some “normal” size when there’s enough there, and even expanded grids become normal size again? If we could somehow plop a huge amount of matter in between two galaxies that are expanding away from each other, would this bring them “closer” together, or cause the space between them to expand more, while the space the blob of matter occupies contracts?

    Sorry if I’m way off the deep end…

  69. #69 Donn
    August 14, 2009

    #68 – You read my mind there.
    It’s:
    A. I had the thought of a ship moving away from our ‘bunch’ into open ‘space’ — would the space between all the atoms not begin to ‘open’ up and make the ship cleave to the local space-size? If so, it should get to the destination much faster than thought: If meters are stretching, and the ship is also stretching, then it’s functionally the same as neither of them stretching.

    Contrasted against:
    B. The ship actually shrinks the grid — space gets smaller than the surrounds — where the ship is. I picture the atoms of the ship kind of sucking-in the space. Now the ship will have a longer journey because it is literally making its road longer as it goes.

    Add all that to Xeno’s paradoxes of movement and I don’t know what to think really! Ow, time to go ask Zeus for a new puppy.

    With this amazing blog post (came from Pharyngula) I am given a chance to wonder aloud at the nature of space on the interface between matter and ‘space’. What does it look like?

    Not to mention the nature of a stuff that can grow larger but seemingly without supply from anywhere — I mean are we talking ‘spacicles’ or ‘spaceoms’ here?

    What are the chances of the entire spreading of space being an illusion brought about by our ways of measuring stuff? If space is stretching, why does it swell uniformly, why not fold again and again like a blanket being stuffed into a barrel?

    Oi, I wish I had a better brain :)

  70. #70 Torbjörn Larsson, OM
    August 15, 2009

    FWIW:

    @ lowerleavell:

    You are reasking questions that already been answered in the thread:

    Wouldn’t there be a location in space where we observe simply…empty space?

    No, asked of Mu and answered earlier: the expansion is everywhere. So no place would be empty.

    But we _do_ see the matter rarefy – the number of observable galaxies decrease as the age of the universe increase.

    What then would be the rate of expansion that would determine the approximate speed of our galaxy?

    See comment #1.

    One other thing that doesn’t make sense about your illustration about the balloon and the ant is that

    See comment #14.

  71. #71 Torbjörn Larsson, OM
    August 15, 2009

    FWIW:

    @ Finally!, Donn:

    You are asking deep questions on the nature of spacetime. This is my take:

    Spacetime is known to be an emergent phenomena. (Ie, is based on more fundamental theory, which we know is needed.) In that sense it is indeed “an illusion”, and have a “smallest unit of spacetime”, or rather observable volume, that makes sense. But it is also eminently observable, and it has no corresponding smallest relative distance unit, because relativity applies on every scale.

    And why should that be quantized anyway? Spacetime is an object consisting of many fields and observables, not a single quantized field (with observables). AFAIU there is no theory how to quantize composite objects, nor any need to. (Likely no way to, either.)

    [One can quantize gravity FWIW, but that is an "effective field theory" on gravity's action, not spacetime itself.]

  72. #72 Finally!
    August 17, 2009

    #71
    If I understand you correctly, what you’re basically saying: ok, it looks like space/spacetime is expanding at certain points, but we really don’t know enough to say that’s the case, because we have no real clue what spacetime actually is.

    In the meantime, all the equations, redshift calculations, etc are a decent substitute, because they make meaningful predictions/let us work out this stuff. But, don’t get too attached to the idea of some tiny piece of space expanding relative to the atom inside it, because there’s no way we can say one way or the other what’s really going on

    Is that about right?

  73. #73 Mike
    August 20, 2009

    Galaxies all moving away from each other. The further away they are, the faster they are moving away. (I know they are not moving space is expanding). We can only see the ones that are within ? because after they get this far away they disappear over the “horizon”. How can anyone really really know how big the universe is? My guess, the universe is infinite. Matter may be finite but, when the matter stops there is still the empty void of the universe. How can we even guess how old the universe is? It is all based on what we can actually observe, science admits (I think) that there is more here than we can observe. Couldn’t the universe be 2, 3, 4 or more times larger than thought? If so, couldn’t it be many times older as well? Explain the missing mass of the universe, maybe it is not missing maybe it is just over the horizon. I don’t know, I may just be to stupid to understand all this stuff, but I have a feeling I am not the only one.

  74. #74 Russ Redrick
    September 19, 2009

    All supposition is well and good concerning the expanding universe, but it is still speculation. For all intents and purposes the part of the universe that we can see could be expanding toward the center of the universe. What we observe could be from a body exploding and we are on the side toward the center of the universe. The truth is that as far as our knowledge extends the Earth is the center of our knowledge therefore the center of the known universe.
    We are setting on a small planet with the ability to view approximately 15 billion light years and I have read that some of the deep field pictures is 47 billion light years. Still no center of the universe, if we have no ideal where the center is how can we make these suppositions?