As Borek says, the physics of the show is whatever the writer wants. Typically a writer will violate only a few laws of physics to make the story interesting.
This is done for two reasons:
- one is that viewers can accept a few violations but if everything can be changed then there's no fun for the viewer to try to predict the action.
- two given that you can't change everything to match some story you must choose wisely to not paint your self into a corner storywise.
As an example, the show Star Trek:
- faster than light travel because we know that planets orbiting other stars are lightyears away and since they wanted to tell stories about them without the crew aging out they needed a boost of warp speed.
- used the transporter device because it took too long to get the crew on/off the planet (45 min show) and cost too much to build the necessary set props (although later they had the shuttlecraft)
In contrast, the Stargate SG-1 show:
- used the stargate (a kind of dialable intergalactic wormhole network to travel from planet to planet)
- later used FTL spaceships and other time travel plot extensions to develop new stories
Here's an article on the Flash and superhero physics:
http://www.techinsider.io/the-flash-superhero-physics-2016-4