# QuestionSeveral, really!

#### jackglewis

1. We have been told that the speed of light is the fastest speed possible. Is that only because we have NOT detected, for lack of tools, anything faster?

2. A beam of light is directed through a vessel filled with liquid. As it enters that liquid, it slows down. When it emerges from the liquid does that beam again speed up to normal and if so what energy is expended to cause the increase?

3. If the universe started from a singularity and expanded to present volume (13.8 billion LY?, why is some of the light just reaching us today from that distance?

#### Helio

1. We have been told that the speed of light is the fastest speed possible. Is that only because we have NOT detected, for lack of tools, anything faster?
The speed of light is something that requires a measurement, so it may help to think of a max. speed that can be measured for light, even if one were to think of it as going faster. The time it takes light to travel from any point A to point B is zero seconds from it's own clock, surprisingly. Mighty strange but Einstein is famous for a reason.

A beam of light is directed through a vessel filled with liquid. As it enters that liquid, it slows down. When it emerges from the liquid does that beam again speed up to normal and if so what energy is expended to cause the increase?
The light that passes through the liquid is "scattering" from particle to particle, so the denser the particles the longer it takes for us looking at it on the outside. I think this is a reasonably correct answer, but I'm not a physicist. [ Some light (photons) will scatter inelastically and lose energy, and some will be absorbed.]

3. If the universe started from a singularity and expanded to present volume (13.8 billion LY?, why is some of the light just reaching us today from that distance?
If it were an explosion of mass and energy into space, that question would be fine. But space-time "exploded" with the energy, which is why most prefer expansion. It's the light from the far side of the expansion that is just now reaching us -- the light from closer regions has already passed us by -- but this light comes from the light scattering event (ie Recombination) about 380,000 years after the initial burst, when electrons combined with nuclei to form atoms, which allowed light to take-off in all directions for the first time. Prior to that, light was scattering constantly and never really go anywyere.

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