# I Physics in a universe with instant communication

1. May 21, 2016

### EnumaElish

Faster-than-light communication is not allowed in this universe. Imagine a sister universe that looks much the same as ours but allows instant communication between any two points. In that universe, simultaneity can be established using instantaneous communication. How would that affect our* description of the universe through laws of physics? In such a universe, would speed of light still be constant? Would there be time dilation? Would objects elongate with velocity?
———
*Assuming human-like species would have evolved in such a universe.

2. May 21, 2016

### robphy

That's the Galilean space-time of PHY101.

3. May 21, 2016

### Staff: Mentor

What he said.

It's a perfectly consistent physics, basically the state of the art around the middle of the 19th century. There's Galilean relativity instead of Einstein's relativity, so no relativistic effects such as time dilation or length contraction. The speed of light is not constant because the law of addition of velocitiies is the Galilean $w=u+v$ instead of the relativistic $w=(u+v)/(1+uv/c^2)$ - the latter form doesn't even make sense if the speed of light is not constant.

As for what the universe would be like? We really don't know, because all of our theories about the behavior of subatomic particles are relativistic (as well as quantum mechanical). Thus that mid-19th century physics provides no basis for chemistry and many of the physical properties of matter.

Last edited: May 21, 2016
4. May 21, 2016

### Flatland

In short if instantaneous communication is allowed then the Universe cannot exist because pretty much everything will happen all at once.

5. May 21, 2016

### robphy

One should distinguish
"allowing instantaneous communication"
from
"requiring that all communication is instantaneous".