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Comment by M_Grey | Hacker News Reader

It's constant within a given frame of reference.

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.

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.

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.

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?

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.reddit.com/r/askscience/comments/2slx5n/how_is_i...

https://www.reddit.com/r/askscience/comments/2t8qr1/do_light...

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.

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?

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).

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

And this playlist of the excellent PBS Space Time series: https://www.youtube.com/watch?v=YycAzdtUIko&list=PLsPUh22kYm...

This is exactly wrong. Light speed is constant in all frames

What is the speed of light in a photon's frame of reference?

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/

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