- Motion is relative:
- if you are asked 'how fast is such-and-such moving?' what is left out of the question?
- Simultaneity is relative (= Einstein's 'relativity'):
- if you are asked 'did this and that occur at the same time?' what is left out of the question?
- Length is relative:
- if you are asked 'is such-and-such 1m long?' what is left out of the question?
2. Relativity of Simultaneity ≠ Seeing Things as Simultaneous
- When you see some event depends on when light from that event reaches you, and that depends only on the distance (relative to you) between you and where it happened – even if two events happen simultaneously relative to you, you won't see them simultaneously if they take place at different distances from you.
- E.g., consider the explosion of two super novas as seen by four
different people, pictured in spacetime.
You can't see something until a photon – travelling at the speed of light – has enough time to travel the distance from it to you. This diagram shows the worldlines of four people and two photons produced by two super nova explosions: it thus shows at exactly what moments the various observers can see the supernovas.
-
Earth
X
A
B
Simultaneous?
Sees alpha at
Sees omega at
Sees simultaneously?
- Both the order in which events occur and the order in which they are seen to occur are relative – but not the same thing:
- The supernovas are not simultaneous relative to B, but she sees them in simultaneously.
- They are simulataneous relative to X, but he sees them at different times.
- And omega occurs before alpha relative to A, but A sees alpha before omega!
- Relativity of seen-order arises because the speed of light is
finite – relativity of simultaneity because it is constant.