Despite being a physicist (albeit in the field of fibre science rather than cosmology), I still find Einstein’s theories difficult to get my head around. One aspect that bothers me is what is often referred to as “Lorentz contraction”: the notion that a rapidly-moving object, as viewed by a “stationary” observer, will be foreshortened in the direction of travel.
Now I can quite accept that the object’s length will appear to be altered relative to what it would be if it were not moving; but what I just can’t seem to sort out is why it should always appear shorter - irrespective of whether it is travelling towards or away from the observer.
The only way I can rationalise an apparent length-change is by using this (admittedly naïve) argument:
Imagine an immensely long “cosmic ruler”, as it were, consisting of two lamps joined by a rigid rod of length Lo. (And let’s make it a bit more interesting by making the lamps green). Now suppose the two bulbs are almost aligned as viewed by us – a long distance L1, say, from the nearer light. Thus:
For the moment this “dumbbell” is not moving; we therefore judge the distance between the two bulbs to be Lo (which could be an astronomical distance, to make the process easier to accept).
Then, at some point, suppose the whole object begins to move at high speed towards us. (Ignore the fact that this is actually impossible without finite acceleration!). We’ll see no movement at this instant, but after a certain period of time – determined by the speed of light and the distance L1 - the nearer light will appear to start travelling towards us. (It’s going pretty fast, so maybe it’ll be slightly blue-shifted, too!). However, because the other light is so much further away, it appears to stay where it is for the time-being - the signal indicating its movement not having reached us yet. It also remains green:
So what we would presumably see, to begin with, is the nearer light travelling towards us while the rod gets progressively stretched out.
Eventually we will see the second light begin to move - and it will then also become blue-shifted:
So we now see the whole thing moving towards us at constant speed. Its length will appear to be constant again but – crucially – longer than it was to begin with, not shorter!
Applying similar logic, but supposing the object to be moving away from us, we should find that it will indeed appear shortened (and also red-shifted)
Thus, according to my logic, we should have either contraction or expansion, depending on which direction things are moving.
I believe this to be incorrect though, so I'm clearly missing out something vital.
Help - can someone possibly tell me where I've gone wrong?
Oh yes ... Happy New Year, by the way!
Cheers
Mike
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