Consequences of the Two Postulates
So the speed of light is a universal constant not relative to the movement of the observer. In fact it must be the same for all observers irrespective of their reference frame.
If this is true (which experiments have virtually proven it to be) what are the consequences? Consider the situation below.
| This is a space station in orbit around the Earth. In the middle of the
craft is a device which emits pulses of light in opposite directions. At
either end of the craft are mirrors which reflect the pulse back to the
centre. An observer inside the craft would see the pulses reach each mirror at the same time then arrive back at the same time. Whether the craft is moving in a certain direction or not does not affect the speed of the light pulse. |
But what would an observer on Earth observe?
| The speed of the light pulses in either direction must be the same
relative to this observer too. But in this case the pulses do not hit the
mirrors at the same time. Time for this observer must be running at a different rate for the two observers. |
When things are moving faster time runs slower
When things are moving faster distances are contracted
OK, so we don't notice these effects. We shall see later that one doesn't unless one is travelling very fast. The clock on an aeroplane runs 1.0000000000005 times slower than one on Earth. (Experiments have been done using very accurate atomic clocks on airliners travelling in opposite directions. The results show a predictable difference)
Some things do, however, travel very fast. Particles in a particle accelerator or some of the particles entering our atmosphere from space travel at speeds approaching the speed of light.
| A famous thought experiment is the twin paradox. One of
two identical twins gets on a spaceship the tours the Universe at incredible
speed leaving their sibling behind. When the travelling twin returns he/she is
considerably younger than the twin that was left on Earth.
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