Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: geordief on 20/03/2017 11:00:08

Title: Can movement appear to be faster than light?
Post by: geordief on 20/03/2017 11:00:08

It is not  ruled out by Relativity ( quite the contrary).


I am just referring to the  instance when we shine a laser on the moon from a distance such as the Earth. The  line drawn by the passage on the surface of the Moon  can be seen to move at any arbitrary speed  only limited by the speed of rotation of the source and  the distance between the source (the Earth) and the Moon (in this instance)


If an observer in situ (on the surface of the Moon) were to examine  this line ,perhaps by means of some photosensitive material   ,he or she would find  no causal relationship between adjacent points on the line ,even though both points would share a common source whereas  the two corresponding emissions (from the laser on the Earth)  would in fact be causally connected (in a standard  less-than -the-speed -of-light way)


Can any more be said about this  circumstance or is it as simple as that?   

Title: Re: Apparent Faster than Light movement.
Post by: yor_on on 20/03/2017 13:09:56

Maybe this. Using the argument we also can propose that by turning my head (and possibly oculars) I now can 'see' faster than light :) Depending on the distance of what you're looking at. I'll leave it to others to define wether this can be considered me having a 'flt vision'.

Title: Re: Apparent Faster than Light movement.
Post by: Kryptid on 20/03/2017 20:38:03

Observationally "faster-than-light" events like this are allowed so long as they cannot be used to transmit information faster than light (and therefore violate causality). I'd say your analysis sums it up pretty well.

Title: Re: Can movement appear to be faster than light?
Post by: Colin2B on 20/03/2017 23:52:29

Also, the light is not actually moving across the face of the moon, it is just that the photons are hitting at different points. The photons from the light are still travelling at c towards the moon (ignoring GR effects :) )

Title: Re: Can movement appear to be faster than light?
Post by: evan_au on 22/03/2017 10:47:17

There are some other scenarios, see: https://en.wikipedia.org/wiki/Superluminal_motion

Title: Re: Can movement appear to be faster than light?
Post by: jeffreyH on 22/03/2017 18:44:54

I am personally not satisfied by the optical illusion explanation.

Title: Re: Can movement appear to be faster than light?
Post by: yor_on on 24/03/2017 18:35:53

I am :)
Then again, I still have to see a proof for ftl

Title: Re: Can movement appear to be faster than light?
Post by: yor_on on 24/03/2017 18:49:10

What I can say Jeffrey is that when you consider it a ' continuous wave' It creates questions. If you consider it 'wave packets' or 'photons' it seems more correct. I've always been partial to the idea of 'photons' myself. To me it seems to fit Quantum mechanics better, then again :) I also think of 'time' as a continuous process. Maybe the simplest answer is the one is where one is weaved into the other?

but yea, it's a good question.

Title: Re: Can movement appear to be faster than light?
Post by: yor_on on 24/03/2017 18:53:48

You could of course combine the idea of time as discontinuous with an idea of 'wave packets' or 'photons'. And so reach a conclusion in where we would have no possibility to measure if it was continuous or not.