Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum question about radar and relativity

Hawking in 'a brief history of time' wrote
'Each observer could use radar to say where and when an event took place by sending out a pulse of light or radio waves. Part of the pulse is reflected back at the event and the observer measures the time at which he receives the echo. The time of the event is then said to be the time halfway between when the pulse was sent and the time when the reflection was received back: the distance of the event is half the time taken for this round trip, multiplied by the speed of light. (An event, in this sense, is something that takes place at a single point in space, at a specified point in time.)Using this procedure, observers who are moving relative to each other will assign different times and positions to the same event.'

my question is,how can a moving observer receive the reflected light pulse?

the speed of light it set, it does not change. the pulse would be doplered to one way or another depending on his direction in relation to the opject reflecting the wave.

Simply put the reflection is not a perfect reflection back to the source but rather is spread out in space and thus receivable over a wide area.

The above explanation also explains how "light clocks" work when they are in motion. Light will have a zig-zag path without "missing" the mirrors at the two ends of the "light clock"