5 ms·

I have a question as a non physics/math person: We assume that light speed is a constant, but we also know that speed is measured as distance/time. We know tha

• M_Grey 10y ago
  It's constant within a given frame of reference.
  • abakker 10y ago
    So, really it isn't a universal constant, then? I guess the makes sense. Sub question - if photons are light particles, moving at the speed of light, then do they actually experience time dilation themselves? i.e. from the frame of reference of the photon, is the local time for the photon ticking away in one frame, while the outside world appears to move much much faster? Or, relative to us is time on the photon moving very slowly?
    • akeruu 10y ago
      Actually for a photon time is so dilated, it does not exists anymore. From its perspective everything "happens" at the same "time". There are quite a few answers for this : https://www.quora.com/Does-a-photon-experience-time https://www.quora.com/Does-a-photon-experience-time https://www.reddit.com/r/askscience/comments/2slx5n/how_is_it_that_photons_dont_experience_time https://www.reddit.com/r/askscience/comments/2slx5n/how_is_i... https://www.reddit.com/r/askscience/comments/2t8qr1/do_light_photons_experience_time_dilation https://www.reddit.com/r/askscience/comments/2t8qr1/do_light...
    • andrepd 10y ago
      It is a universal constant. See above. The photon travels at the speed of light c. If you look at the expression for the time dilation, you will see that it tends to infinity as v approaches c. Remember that this relates the time experienced between two frames moving relative to each other at a constant velocity v. However the fact is that the expression fails when v=c, since you have a division by zero. This reflects the fact that it is impossible to do a Lorentz boost that relates two frames moving relative to each other at c. This in turn means concept of time dilation in an object moving at the speed of light, such as a photon, is meaningless. There is no such thing as the time experienced from the point of view of the photon. I realise I probably wasn't very clear, but it's difficult to lay it all out in a small paragraph.
      • abakker 10y ago
        That is actually a great explanation, thanks. So, photons don't experience time? Does that mean that they don't experience entropy, either? I.e. They don't lose any energy in transit, since to them "transit" is instantaneous. Another question this raises for me is that if photons move in a timeless state between 2 points, what accounts for the period of their oscillation in the particle/wave duality? If they are tracing the fastest path through spacetime, yet to them, the time dimension is non existent, how do they have wave characteristics?
        • agentgt 10y ago
          I'm on mobile and not a physicist but the key is the observer. Photons don't observe anything. We observe them since we observe time and they do have energy changes that we observe (I believe this is how colors and red shift work).
    • spuz 10y ago
      The speed of light does not change for each given reference frame, i.e. it is constant. However, space and time are observed differently depending on a given reference frame. With regards to your second question, photons experience no time whatsoever. From the photon's perspective, they travel the universe from the point of emission to the point of absorption instantly. If you want to understand this, I highly suggest watching this video explaining reference frames: https://www.youtube.com/watch?v=aRDOqiqBUQY https://www.youtube.com/watch?v=aRDOqiqBUQY And this playlist of the excellent PBS Space Time series: https://www.youtube.com/watch?v=YycAzdtUIko&list=PLsPUh22kYmNAmjsHke4pd8S9z6m_hVRur&index=1 https://www.youtube.com/watch?v=YycAzdtUIko&list=PLsPUh22kYm...
  • andrepd 10y ago
    No, the speed of light is the same in ALL inertial frames of reference. This is the crucial postulate from which all special relativity comes as a consequence.
    • kurthr 10y ago
      Although I think your comment is clearer... I think he's saying "within any given frame of reference", which is the same. As you say: The speed of light measured within ALL inertial (non-accelerated eg by GRAVITY!) frames of reference.
    • trhway 10y ago
      SR is the model of fixed continuous space-time (and the "inertial frames of reference" are defined only in such model). Thus the SR is applicable to the reality only as much as the real space-time can be approximated by such a model. That approximation is pretty good usually, except at the very large scales (the non-fixed, expanding, nature of the real space-time starts to show more than negligible deviation from the SR model) and at the very low scales where the non-continuous nature of the real space-time starts to play a role. >“anti-de Sitter” (AdS) space. In these warped worlds, spatial increments get shorter and shorter as you move out from the center. and that sounds like the metric is p-adic there. While our typical Cartesian coordinates express the metric naturally produced by the real numbers, at large scale the world may have different metric. Our 46B ly radius visible Universe's Schwarzschild radius is about 10B ly, so if we go about 160B ly beyond the visible Universe border - we "can" do it as otherwise the Earth would be the "center of Creation" - we will have the 200B ly radius Universe which is inside its own Schwarzschild radius equal to those 200B ly - so our visible 46B ly radius Universe is basically inside that large black hole and at that scale the space seems to be AdS.
  • mark_edward 10y ago
    This is exactly wrong. Light speed is constant in all frames
    • jcoffland 10y ago
      What is the speed of light in a photon's frame of reference?
      • Retric 10y ago
        Same as any other, however. A photon has a very distorted view of distance. Play around with this: http://gamelab.mit.edu/games/a-slower-speed-of-light/ http://gamelab.mit.edu/games/a-slower-speed-of-light/
• jperras 10y ago
  > We assume that light speed is a constant, but we also know that speed is measured as distance/time. Correct. However, the crucial part that is missing from your thought process is that: 1) light can only travel in a "straight" line, and 2) "straight" lines are defined by the spacetime metric. As a lower-dimensional analogy, think of what a "straight" line means on the surface of a sphere[1]. The shortest path between two points is, by definition, a "straight" line, but if the space itself that the path is embedded in is curved, then when viewed from a higher dimension (i.e. on the surface of the sphere it's straight for you, but if you were in space) it would appear curved. The analogy fails when you attempt to think about it in higher dimensions, since humans have a very difficult time perceiving anything greater than the standard 3+1 spatial & time dimensions. But the math still holds. Light will always travel in a straight line, it just so happens that gravity redefines what "straight" actually means[2]. 1. http://mathworld.wolfram.com/GreatCircle.html http://mathworld.wolfram.com/GreatCircle.html 2. Explanation simplified for clarity.
  • highd 10y ago
    Said another way - light follows the shortest path between two points. In flat spacetime, that is a line. In curved spacetime, it may be some other path - like a path traced by an ant walking on a sphere. These paths are called geodesics: https://en.wikipedia.org/wiki/Geodesics_in_general_relativity https://en.wikipedia.org/wiki/Geodesics_in_general_relativit...
  • whoopdedo 10y ago
    Gravity can stretch spacetime to make the distance light has to travel longer than in an unaltered "flat" universe. But can it also shrink, causing a photon to arrive at a destination in a shorter time? And I don't mean a wormhole, which requires tearing in a topological sense.
• flaviojuvenal 10y ago
  I'm also not a physicist, but as far as I understood special relativity and also according to this [1] Quora answer, the speed of light is same in every frame of reference, so the universe compensates variations in speed by dilating time. This is also consistent with Maxwell's equations [2]. [1] https://www.quora.com/Why-does-time-dilation-occur-And-why-do-gravity-and-speed-both-affect-time https://www.quora.com/Why-does-time-dilation-occur-And-why-d... [2] https://van.physics.illinois.edu/qa/listing.php?id=2605 https://van.physics.illinois.edu/qa/listing.php?id=2605
• raattgift 10y ago
  There is a little bit of geometry and group theory jargon here in my description of 'c' in General Relativity, but no equations: The plane tangent to the surface of the ball in this diagram is a tangent space. https://upload.wikimedia.org/wikipedia/commons/thumb/6/66/Image_Tangent-plane.svg/640px-Image_Tangent-plane.svg.png https://upload.wikimedia.org/wikipedia/commons/thumb/6/66/Im... A metric on a space determines lengths (both spatial lengths and time durations). The geometry of spacetime induces the Minkowski metric on tangent spaces. At every point in a Minkowski space lengths are invariant under the Poincaré isometry group, which means that lengths stay the same under any or all of three spatial translations along orthogonal axes, three rotations about these axes, three Lorentz boosts, and one time translation. There is one free parameter in the Poincaré group, which is "c", which physically corresponds to a particle with zero invariant mass. Photons are expected to have zero invariant mass, and experiment sets a very very very small upper bound on their mass. Observers in the same tangent space as a photon will measure it as massless and moving at 'c'. Minkowski space is flat spacetime, and in flat spacetime each tangent space covers the whole spacetime; they overlap completely. As you add curvature^2 to the geometry of spacetime, the tangent spaces shrink and are oriented to each other at different angles. Observers in one tangent space measuring a photon in another tangent space may not agree on its speed. In fact, in General Relativity, it is mostly forbidden to talk about the relative velocities of objects in other tangent spaces -- that's a dramatic departure from Special Relativity, but then SR is the theory of Minkowski space, where all tangent spaces cover the same region of spacetime, and so they are indistinguishable. Spacetime curvature around us is pretty gentle, so within a small laboratory test apparatus everything is in the same tangent space or so close to it that differences aren't detectable. We have excellent and readily reproduced data on the speed of light in such setups. The Poincaré isometry group is a subgroup of the group theory of the Standard Model. All the scatterings allowed under the Standard Model are in a region of spacetime where curvature doesn't matter, and we have many exabytes of evidence supporting the Standard Model in terrestrial experiments, in experimental systems elsewhere in the solar system, and in astronomical observations. Indeed, we can use the repeatability of the Standard Model and its built-in Poincaré invariance to measure spacetime curvature, and we do so with GNSS systems like GPS. A violation of Poincaré invariance would produce different behaviours in microscopic systems, and so far they have not been detected. The Lorentz group is a subgroup of the Poincaré group, and there have been many tests specifically probing for violations of Lorentz invariance near us and in distant parts of the observable universe, and as of today none have been found after a century of trying. So unless there is a low energy breakdown of the Standard Model, we can be pretty confident that the photon is a massless particle and moves at 'c' for all observers close to the photon.
• walrus1066 10y ago
  'Does the distance moved change relative to the proximity to mass' Yes, length contraction happens alongside time dilation. If I travel towards you, the distance between us shrinks in my frame of reference.