Claus Schertel asked:
Thanks for a great show, I just came across the show a couple of months ago and listen every time since.
Since the expansion started in one point with the big bang there must be galaxies that fly away from us in the exact opposite direction of our own galaxy. If these galaxies and our own accelerate there should be cases where the speed relative to each other in the opposite directions is so large that the light can´t reach us anymore (e.g. if both fly away from the big bang center with half the speed of light in opposite direction). If we catch a galaxy that just accelerates and crosses the threshold it should go dark. Are there any cases were such an effect was observed, or are they still too slow, to cause such an effect?
Thanks and keep up the good work.
Nice question but so hard to answer. It's the inflation that's supposed to make galaxies move away from each other. That the 'space' between them is 'stretching out', getting 'bigger'. So it isn't the 'relative motion' between two galaxies that is expected to raise, if you think of it in form of a 'momentum' gained through that relative motion. There should be no more 'momentum' than without that 'expansion' of space. Eh, and as 'space' actually accelerates, expanding between galaxies, we also might assume more 'gravity' as a result if that was a 'real' acceleration (as an acceleration is the equivalence to a gravity) but I never heard of any such observations?
Einstein's laws of relativity state that light moves at a set speed relative to the observer and independant of the speed of the object that emits it. Consider 2 spaceships both travelling at c/2 away from eachother. One shines a torch at the other from the rear window - would it be seen? Since the light from that torch moves at the speed of light as soon as the photon is emitted, even though the torch is moving away at c/2. The light, travelling at c will catch up with the other spaceship moving the opposite direction. Sprool, Fri, 16th Mar 2012
Yep, and the reason is that 'c' is a constant in relativity. Nothing can 'move faster' than 'c' if you want relativity to survive. Tachyons isn't about relativity, they are descriptions of something 'outside' of SpaceTime. So I don't really get people thinking that you can have 'real speeds' inside SpaceTime 'faster' that 'c'. It has to do with the conceptual 'box' we exist in, the 'closed SpaceTime' described in symmetries and constants, as well as the conservation laws. yor_on, Fri, 16th Mar 2012
Try this for size, from an applied physics major at Cornell University:
But when it comes to why galaxies can 'disappear FTL' it have nothing to do with the constant 'c'. That has to do with 'Space' later expanding (as some slower inflation).
Excuse me please but I have a misconception about the big bang and I think I now understand what is meant by the expanding Universe. I had always assumed that the big bang was like a massive explosion throwing particles out from an infinitely small point, a bit like looking at an explosion in slow motion where you can see the shock wave expanding outwards.
Yeah, been thinking the same AT :)
Could the fifth dimension be the source of the inflation? An analogy would be the source of air for the balloon.
To get back to the original question.