Do we feel spacetime only because we are not moving at the speed of light?

Question

In classical physics, nothing is more fundamental than space and time. But in modern physics, is it true that spacetime are just some "derived" quantities relying on other things more fundamental.

For example, I always wonder how the Universe appears to a photon. Will there be no spacetime?

I always wonder whether spacetime are just illusions because we are in an "un-natural" state of not moving in the speed of light. While things in the Universe in a "natural" state should be moving with light-speed like a photon, and there will be no spacetime?

For example, instead of considering light-cones as something we constructed in spacetime, is it possible that light-cones are something more fundamental that we can derive spacetime from it? Say, lightcones are "something" more fundamental and spacetime emerges when an object moves across different lightcones?

Answer 1

Yes, in classical physics, space and time were fundamental, but they were separate.

Now, we know that the two are inextricably tangled. We cannot see spacetime, but we can feel the curvature of spacetime in the form of gravity, and we can see the effects of spacetime. Think about wind. We cannot see it, but we can see and feel its effects. Spacetime is sort of the same. We can predict what it will do when we plop mass into the middle of it, or we accelerate to near light speed within it; we can see how the Earth orbits around the sun and the Moon around the Earth; we can "feel" gravity pulling us down onto Earth, but we cannot see spacetime.

Photons, and light, are just as affected by spacetime as we are. A photon can be sucked into a black hole, and bent by the gravity of a massive object - this has been proven. A photon feels the effects of spacetime. Spacetime is also not an illusion. Its effects have been measured time and time again. General relativity is a rock solid theory that has predicted many things correctly. We may come up with a better theory that explains more phenomena, but the phenomena general relativity does explain, it explains well.

Light cones are the possible tracks light can follow - through spacetime. Light cones are not more fundamental than spacetime. Now, for your question about derived quantities and whether or not spacetime is fundamental (thanks for the clarifying comment). Take a diagram of a light cone.

The point in the middle is a specific event. Let's say it's you pressing the button to start the microwave. That very moment you press the button, that's the event. Now, the photons that were present at that event have a limited number of choices where they could go that first second after the event. A light cone is kind of like a graph. One second after the event is up a little bit after the event. And it isn't that wide, because there aren't to many choices. But, if you fast forward a year, say, those photons are probably pretty far away, or they could be at the same spot - the light cone is much wider. If you go a second before the event, there are a limited number of spots the photons could be to reach the event properly. A year before the event, there are many places those photons could be. That's why it looks like an hourglass.

Now, the light cone is located within spacetime. And there is spacetime outside of the light cone, and there are many light cones (one for every event ever). So there is light outside of light cones. A photon can travel between light cones if the light cones overlap, and therefore end up a ways away from the original light cone, if that makes sense. Spacetime is more fundamental than a light cone, because light cones make sense only within spacetime.

Hope this helps.

Answer 2

For example, I always wonder how the Universe appears to a photon. Will there be no spacetime?

Movements along lightcones are lightlike movements, and as such their spacetime interval (and their proper time) equals zero. For massless particles such as photons this means that from their - hypothetical - point of view, only the place of emission and the place of absorption do exist, but both are at the same place and between there is nothing, no photon, just a mere momentum. From the hypothetical point of view of a massless particle, it does not exist, there are only the emitting and the absorbing particle which are adjacent.

In contrast, observers are observing the space interval between both places, and as there is a momentum transmitted between them, they observe a photon particle as a sort of "placeholder" at the place of the space interval, where the spacetime interval is zero.

I always wonder whether spacetime are just illusions because we are in an "un-natural" state of not moving in the speed of light.

Physically, you are talking about rest energy which is not "un-natural" but a characteristic of mass particles. Rest energy permits a particle to move below speed of light.

For example, instead of considering light-cones as something we constructed in spacetime, is it possible that light-cones are something more fundamental that we can derive spacetime from it? Say, lightcones are "something" more fundamental and spacetime emerges when an object move across different lightcones?

Light cones are a characteristic of spacetime, not inversely. They represent the speed limit of the second postulate of special relativity for all particles within spacetime.