Special and General relativity rely on the possibility of having clocks and propagating light rays for defining (locally) time and length. My question is about how to define such things inside matter. Inside a metal block or inside a neutron star, light will not propagate that far, or not at all. Maybe the electromagnetic wave can be made to propagate when going to higher frequencies, but again, one could imagine very dense materials in which this would not be possible. The same question about clocks : How should one imagine a clock sitting inside a dense material ? And who could be there to observe it ? So, it seems that all the nice thought experiments used in special relativity would not apply inside matter. Is this an indication that relativity in general is not strictly valid inside matter (since time and length can not adequately defined there) ?