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Speed of light relative to universe expansion?

  1. Aug 7, 2007 #1
    Hi I am only 16 and some people may find this very naive but I always like to ask physics questions.
    Is the speed of light relative to the speed of the universe expansion?
    It seems to me perfectly viable to me as it would act as a barrier from escaping the universe.
    However this would cause the speed of light to at some point be 0 (and eventually negative) or to be ever increasing depending on the mass of the universe. But I recently read that somebody had discovered that the speed of light was infact decending???
    Could somebody please correct me and tell me I'm wasting my brain cells if it's the case.

    Thanks, Tom
  2. jcsd
  3. Aug 8, 2007 #2
    I’m no physicist, but I have taken a class on modern physics so I will attempt your question.

    From what I understand, the speed of light is fixed. It has nothing to do with the size of the Universe. This is true no matter where you are or what speed you are going. If you are traveling 90% of the speed of light as compared to the Earth and light goes by, you will measure its speed as the same speed as the person sitting on the Earth. Weird no?

    Keep asking questions and welcome to PF!!!
  4. Aug 8, 2007 #3


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    The speed of light is always 'c' (using local clocks and rulers), and this fact has nothing at all to do to do with universal expansion as other posters have already remarked.

    It is important how you measure and define the speed of light - the speed of light is equal to 'c' only when you measure it over short distances using local clocks and rulers. Some other ratios of distance to time in cosmological situation might not be equal to c, but these discrepancies can usually be attributed to the curvature of space-time. Curvature effects are more complicated to talk about and describe, and are dealt with in General relativity.

    However, it is necessary to understand SR well before one can move on to GR. In a small area of space-time, curvature effects can be ignored, similar to the way that the Earth is a round ball, but appears flat from the perspective of someone who does not travel a great distance over its surface. This means that over small distances, well away from strong gravity fields, one can use special relativity. And in special relativity, the speed of light is always equal to 'c'.

    The independence of the speed of light from motion of the source has been measured in a number of ways, one of the most direct was to measure the speed of light emitted from a moving source. There are both cosmological sources that move with high velocities to us, and terrestrial sources that move with a high velocity and emit electromagnetic radiation, such as pi mesons (which move at a high fraction of 'c'). The speed of light from pi0 mesons has been measured to be equal to 'c' within 400 parts per million, in spite of the large velocity (.99975 c) of the mesons. See for instance http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html for details of this and other experiments.

    The lack of dependence on the speed of light shows up in experiments like the Michelson Morley experiment. The Earth is constantly changing it's speed due to the fact that it is orbiting the sun, but we've never seen any indication that this affects measurements of the speed of light.

    Some of the experimental tests of relativity are discussed at http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html
  5. Aug 8, 2007 #4
    Thanks for your replies wildman and pervect.
    Does the fact that we are spinning on an axis, orbitting the sun, orbitting the black hole in the center of the milky way and the universe is expanding not effect our measurements of light?
    Also as we are all expanding with the universe, how can we be sure that our methods of measuring havn't been affected and that c is infact changing but as we are all changing with it we can't tell. Or in simpler words; yes the speed of light is fixed, but maybe our rulers and clocks are changing.
  6. Aug 8, 2007 #5


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    There have been those that have claimed that a variable speed of light explains some of what is observed about the Universe as an alternative to how we currently understand things, however the evidence for this is questionable and it is not a generally held view in the Cosmology community. You may well have seen an article on one of these theories at some point as it does tend to generate media interest, but it is by no means 'proven'.
  7. Aug 8, 2007 #6
    If you consider the Hubble sphere in combination with the cosmological principle, any observer will believe he is at the center of expansion - this would be the case if we lived on the surface of a 2 sphere universe - an inflating balloon - all the galaxies would be seen as moving away from us irrspective of where we were located - in such a closed cosmos, the velocity of expansion could be c -

    perhaps the local velocity of light as experimentally measured is determined by the global properties of our universe rather than vice versa
  8. Aug 9, 2007 #7


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    None of these matters when the speed of light is measured over short distances. The trickiest points arise with respect to rotation. It would be the least confusing to say that the speed should be measured in a non-rotating frame, but if you want the gory details of how to deal with rotation in special relativity you could read about the "sagnac effect", the effect that makes laser gyroscopes possible.

    The sagnac effect is well enough understood by physicists that they don't actually use gyrostabalized non-rotating platforms when they make their speed measurements, rather they just design their experiment so that the sagnac effect does not affect their results (by insuring that the area of the path over which the speed is being measured is zero). This care would not be needed if the Earth were not rotating.

    The motion of the Earth around the sun does not affect the measurement of the speed of light at all (except insofar as it affects the rotation of the Earth, by making the sidereal day a bit shorter than the solar day). Neither does the motion of the sun around the galaxy, or the motion of the galaxy relative to whatever else you might chose as a reference.

    Gravitational fields will have only a second order effect on speed measurements - thus if the beam of light was vertical, one might see such a second order effects on the speed measurement if the vertical distance were long enough. Taking the limit as the distance approaches zero yields a speed equal to 'c' however, even for a vertical beam of light. There is no effect whatsoever due to gravity if the source and receiver are at the same altitude. To get some idea of the effect of gravity on time, look at the harvard tower experiment:


    To talk about measuring the speed of light at all, one is implicitly using the old distance standard of that platinum alloy bar that they keep in Paris, and some standard of time (the modern standard is based on a cesium clock).

    Using these as a standard, the speed of light does not vary with time in any way that we have been able to detect. It is a prediction of relativity that we will never see such a variation - if we do see any such variation, relativity would be falsified.

    Obviously one needs to define some standards of distance and time before one can measure 'c' - nowadays we actually use 'c' to define our standard of distance, but back in the old days, when the meter bar in Paris was the distance standard, 'c' was an experimentally measuarable quantity based on this distance standard. There has been some evolution of time standards as well, our modern cesium clocks are much better at measuring time than the older astronomical based standards were.
  9. Aug 9, 2007 #8
    every reply seems to have focused on the fact that the speed of light is the same and always propagates at c relative to all observers in spacetime

    but can the expansion of the universe - the rate spacetime itself expands - have an effect on the speed of light? if we're asking why light travels at c, an answer could be because it matches the rate the universe expands. any faster and a beam of light would go outside the universe, any slower and it would be left behind. I hope this makes sense?
  10. Aug 9, 2007 #9
    Well - the constancy of the speed of light seems to be the case for local measurements as pervect pointed out - but there are situations involving sagnac type experiments where the speed of light depends on the motion of the receiver in a coordinate system attached to the sender. For example. in GPS, correction for the one-way sagnaic effect is required - even thoug it really has nothing to do with rotation per se.

    IMO, the expansion rate determines the velocity of light propagation. To arrive at this result however, it is necessary that space be considered as having the properties of a medium - call it what you want.
  11. Aug 9, 2007 #10
    Although I appreciate everyone's very informative replies, I still don't think that my question has been answered. (How we measure the speed of light may be interesting but not entirely relavent (that may sound stupid but it's not).)
    My (relatively small) knowledge of theoretical physics has enabled me to let loose on concepts that just don't make sense to me. For instance how can the speed of light effect both space and time without spacetime (not space and time) affecting the speed of light. By this I mean a distortion, or expansion, of spacetime. The problem I think is that many replies are in a dimension to small. By this I mean that most of the replies only consider the bend of spacetime caused by mass, rather than the stretch caused by the big bang. Thinking inside rather than outside the box (universe)

    By picturing the 2d model of spacetime it is impossible to imagine it stretching without the speed of light changing with it. And it is stretching.
    Basically that:

    spacetime α c
    (but individually space and time are not)

    By the way if anyone got offended by "think outside the box" or "replies are in a dimension to small" it wasn't an attack at your cognitive capacity, they are very literal metaphors (that's a nice juxtaposition).

    "Whether you can observe a thing or not depends on the theory which you use. It is the theory which decides what can be observed." - Albert Einstein

    "Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, giving birth to evolution. " - Albert Einstein
  12. Aug 10, 2007 #11


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    If you'll think a bit further, I hope you'll see that how we define and measure distances, and the speed of light, is very relevant.

    Science is about testable consequences. If all rulers, everywhere, uniformly changed size, we'd never be able to tell that this happened by any measurement that we could make. (If you think differently, please explain how you think we could tell).

    This puts this question of "changing rulers" outside the realm of things that can be tested empirically, i.e. outside the realm of science.
    You ask:

    I'm sorry, but this question doesn't make any sense at all to me. Note that we don't discuss personal theories here (which this appears to be) - what we attempt to do is to present the mainstream scientific views and answer questions about them.
  13. Aug 10, 2007 #12
    No it's not

    At least I can't be proved wrong then.

    Most theory when it is presented is not testable, has anybody tried to make a wormhole yet?
    Surely the challenge of science is find a test for the questions. Anyway I'm sure one day someone will discover something that makes me right :rofl:

    Okay I didn't know that sorry
  14. Aug 10, 2007 #13


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    This is patently false, unless you think that physics is monopolized by subject matters such as String Theory and exotic cosmology. If you do, then get rid of that thought and go look at the various field of studies in physics, and which one is the largest (hint: it is the field of study that produces results that you use directly in your modern electronics).

    So no, MOST theories in physics, when presented, are definitely testable.

    And since it appears that you may not have read it, please review the PF Guidelines before proceeding any further.

    And BTW, Imagination without Knowledge is Ignorance waiting to happen. You yourself has just proven this.

    Last edited: Aug 10, 2007
  15. Aug 10, 2007 #14
    This is patently irrelevant, I did not suggest that physics was monopolized by anything

    Theories that are testable aren't theories, they're true or false.

    It seems you lack the imagination to even structure an influential reply, you are just putting words in my mouth and claiming that I stick by them.
  16. Aug 10, 2007 #15


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    Er... then I bet you are using the pedestrian definition of a "theory". Please check how such a word is used in physics. BCS theory of superconductivity is definitely testable and have been tested. There's no such thing as "true" or "false" since physical theories can only be verified, and there is also a degree of certainty. General Relativity has many verification, but no one would claim it is "true" or "false" because it makes many predictions that haven't been tested. Nothing in physics is like that.

    A theory (as opposed to an experiment) means a logical and mathematical description of an idea. It isn't a "guess" the way the word is used in ordinary conversation. So Newton's laws can be considered as a theory.

    "an influential reply"? I don't believe I was trying to influence anything.

  17. Aug 10, 2007 #16
    You've gone slightly off topic and are picking little definitions and tiny mistakes to try to argue something, okay my use of the word theory was wrong.
  18. Aug 10, 2007 #17


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    I may be able to help you with your initial question, but you are going to need to soften your attitude if you want to have any hope of learning anything. While you admit (and have aptly demonstrated) that you know virtually nothing of physics, you are combative toward new information from people trying to teach you what you wish to learn. This makes no sense.

    Anyway, consider this: The observable universe has a horizon beyond which we cannot see due to the fact that at that distance, the expansion of the universe is preventing light from reaching us. As the light from an object on the other side of the horizon moves towards us, the expansion causes it to just keep getting further and further away. What does this say about the expansion's effect on the speed of light? Nothing at all. The expansion has nothing at all to do with the speed of light. The fact that light on the other side of our horizon is moving towards us but getting further away does not imply that it has a negative speed.
    Last edited: Aug 10, 2007
  19. Aug 10, 2007 #18


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    I fully admit that I'm nitpicking. But I'm sure you can see how that led to your post. Furthermore, such usage is causing a lot of grief in the confrontation between science/evolution and creationism. This is because one often hears the argument that "Evolution is only a theory" being uttered by people who have no clue how the word "theory" is being used in science.

    So yes, it is a small issue that actually has large consequences. You have also misapplied that often-bastardized Einstein quote because the intention of such quote is often missing.

    One of the things we try to do here on PF is to make sure that people go deeper into some of the things that are prevalently used and not perpetuate the misunderstanding/misconception. This means that sources must be cited clearly, and terminologies that are used must be clearly understood. Since this is a physics forum, the words being used must be checked against how they are defined in physics. If not, there will be utter confusion and misunderstanding, because most of us here will assume that the physics definition is being used.

    And finally, as a "cultural" reference, unlike other public forums that you may have encountered, PF has many regulars who are professional physicists, engineers, biologists, chemists, etc., and even many others who are either undergraduate or graduate students in these areas. In other words, many of the members here are not "amateurs" in these field of studies. This means that in many cases, we have to be on our toes when we write something and that anything nonsensical will be challenged. Everyone here had that at one point or another.

  20. Aug 10, 2007 #19


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    I don't see how you can possibly say that the sagnac effect has nothing to do with rotation.

    Look at the wikipedia defintion, for starters:

    So, what elicits the sagnac effect? Rotation.
  21. Aug 10, 2007 #20


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    I think you may be talking about the idea of "stretchy space"? This thread appears to be a good (bad?) example of the sort of misconceptions that this idea can cause. To clarify, I'm talking about the following idea:

    I've stolen this from Chronon's webpage (which I agree with partially, not wholly): I hope he doesn't mind:

    It is important to realize that relativity predicts that there is no measurable effect due to the "motion" or "flow" of space on the speed of light, or on any other sort of measuring instrument, for that matter.

    If any such effect was detected, it would be an indication that relativity was wrong.
  22. Aug 10, 2007 #21


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    A belief that cannot, even in principle, be "proven wrong" is not a scientific belief. In order for something to be science, it must be possible for it to be falsified. This gets into rather basic ideas which are related to the philosophy of science, not just relativity. Karl Popper would be a good source for information on this, unfortunately I don't have any specific quotes. But I would definitely suggest that you do some more reading about such issues, you seem to have several ideas that are not at all mainstream.

    It may be comforting to you to believe that you can "never be proven wrong", but it's basically a sign that there isn't much point in talking with you, that you've made up your mind and are not interested in examining the facts. (I hope I'm wrong about this, and that you'll calm down a bit once you get past the shock of the existence of people who disagree with you).

    You also write:
    You seem to be very passionate, but you also seem to be very much "set" in your own ideas. I'm hoping we can get beyond some of the "knee-jerk" defnesiveness I'm sensing, where you automatically react to any challenge of your ideas as some sort of attack, and get you to think and read a little bit more about what science is, and what it is about.

    To address your example:

    Wormholes" are not regarded as any sort of scientific fact. In fact, they are regarded as a rather extreme test of general relativity, one that we are not able to perform. Note that they are not a theory, they are a prediction from a theory.

    There is a difference between not being able to perform a test because it's impractical, and not being able to perform a test because the theory itself can never, even in principle, be tested.

    In the later case, the theory isn't of any real, practical use.

    While practical considerations do not motivate all scientists, one of the reasons science has succeded so well is that its focus on the practical makes useful things possible - like television, the internet on which we are communicating, GPS, electricity, you name it.
  23. Aug 10, 2007 #22
    With the model of spacetime (considering wormholes) why is it bent over in a horshoe shape?
  24. Aug 10, 2007 #23


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    Does relativity assume or predict an expanding universe?
  25. Aug 10, 2007 #24


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    General relativity is certainly compatible with an expanding universe. Historically, though, GR did not actually predict an expanding universe before it was observed by Hubble.

    By introducing a "cosmological constant", Einstein was able to come up with a cosmology that was static, i.e one that did not expand or contract. Einstein later wrote that this was one of his greatest blunders, that he should not have introduced such a constant - i.e. Einstein felt that he should have predicted an an expanding universe after the fact, but he did not actually make such a prediction in advance of the observations by Hubble.

    Interestingly enough, though, the cosmological constant is back in current cosmological models, so perhaps it wasn't such a "blunder". The cosmological constant today is sometimes known as quintessence, or "dark energy", and is thought to be responsible for the accelerated expansion of the universe. Without dark energy or quintessence, the expansion of the universe would slow down as a function of time - with it, the expansion actually accelerates. This is quite a different use for the idea than the one Einstein had (Einstein's use was to eliminate expansion entirely), but the idea is currently back in favor.

    This experimental basis for the acceleration of the expansion is fairly recent observations using supernovae as "standard candles". As always, future progress may change our ideas, but currently this is the standard idea. (There isn't much question that the universe is expanding ever since Hubble's observations, but there is still some small question about whether or not the expansion is accelerating, deaccelerating, or coasting, though the mainstream consensus view now is that the expansion is accelerating).

    One difficulty is that while we have some cosmological evidence for quintessence, we don't currently have any laboratory evidence of its existence.

    GR is currently the reference standard theory of gravity for cosmology - so far all of its predictions have been consistent with observation, some of them with very high accuracy. Standard current cosmological models of the expanding universe use GR as the theory of gravity and are known as FRW cosmologies (or Friedman Robertson Walker cosmologies).
  26. Aug 10, 2007 #25
    I hate to nitpick (OK not really) but since this seems to be the nitpickers' thread, I was under the impression that "quintessence" and Einstein's "cosmological constant" (Λ) are two distinctly different things. Namely that quintessence varies in space and time, whereas Λ is well, constant. So quintessence and Λ are both possible, but distinct, candidates for the explanation of dark energy. Am I wrong? If so, please forgive me. I'm just a mortal.

    [EDIT: Of course, the observational question of how do we distinguish quintessence from a cosmological constant is another story.]
    Last edited: Aug 10, 2007
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