Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Staff Emeritus
    Science Advisor

    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


    User Avatar
    Science Advisor

    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


    User Avatar
    Staff Emeritus
    Science Advisor

    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


    User Avatar
    Staff Emeritus
    Science Advisor

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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


    User Avatar

    Staff: Mentor

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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


    User Avatar
    Staff Emeritus
    Science Advisor

    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


    User Avatar
    Staff Emeritus
    Science Advisor

    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?