It is currently Wed Oct 23, 2019 9:30 pm


Post new topic Reply to topic  [ 7 posts ] 
Author Message
 Post subject: Light Speed
PostPosted: Wed Jul 21, 2010 11:00 am 
Offline

Joined: Sat Dec 11, 2004 8:18 pm
Posts: 6502
Location: Manchester
Dear al(L)
I have happily taken it for granted (for longer that I want to remind myself) that the speed of light is approximately 300,000 kps ie roughly 186,000 mps in a vacuum.
However, in recent times I have felt a bit puzzled because from what I gather space isn't a true vacuum.
So does anyone really know what the speed of light might really be in a true vacuum?
Best wishes from Cliff


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Wed Jul 21, 2010 11:21 am 
Offline

Joined: Fri Dec 03, 2004 2:41 pm
Posts: 1450
Location: 55° 57'N: 03° 08'W
Yes, the velocity of light in a vacuum is 299,792.458 km/sec.


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Wed Jul 21, 2010 11:31 am 
Offline
Site Admin

Joined: Fri Dec 03, 2004 11:24 am
Posts: 4375
Location: Greenwich, London
I wonder what the differences are between a vacuum (in the laboratory, for example) and a true vacuum and whether those differences are anywhere near enough to have an effect on light speed? Maybe over vast distances, if anything.

_________________
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Wed Jul 21, 2010 3:52 pm 
Offline

Joined: Sat Jan 24, 2009 10:22 pm
Posts: 37
Location: Devon : UK
OK so if the speed of light is different between travelling in air and in a vacuum it makes me wonder what is happening when propagating through air or in something close to a vacuum.
Presumably in any fluid or solid for that matter it is a full vacuum between the particles so do the photons travel at full vacuum speed between particles and slow down on encountering one and then speed back up on passing (?) the particle. Where does the energy for the acceleration come from?
Makes you think doesn't it, well it does me anyway !?

Cheers


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Wed Jul 21, 2010 4:09 pm 
Offline
Site Admin

Joined: Fri Dec 03, 2004 11:24 am
Posts: 4375
Location: Greenwich, London
Steve Young wrote:
do the photons travel at full vacuum speed between particles and slow down on encountering one and then speed back up on passing (?) the particle.

A photon is absorbed by the particle and then re-emitted. There is no deceleration/acceleration, as far as I'm aware, although, I think I may have read somewhere about "particle recoil" when a photon is emitted.

_________________
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Wed Jul 21, 2010 8:52 pm 
Offline

Joined: Sat Jan 24, 2009 10:22 pm
Posts: 37
Location: Devon : UK
So if there is no deceleration/acceleration involved then why is the speed in different materials different?. The particles of the material may as well not be there if they don't affect the speed of the photon.
An opaque material stops all photons, (does it get warmer or heavier), a transparent material passes all? Photons but slows them down, but then they speed up again after passing through?.
Sorry for the rhetorical questions, I regularly read this forum with great interest, lots of questions but no answers!
Cheers


Top
 Profile Send private message  
 
 Post subject:
PostPosted: Wed Jul 21, 2010 11:28 pm 
Offline

Joined: Sat Dec 11, 2004 8:18 pm
Posts: 6502
Location: Manchester
Dear Stella, Steve and Joe
First, thanks Stella for stating an accurate speed for light in a vacuum.
That said in my current state of bloody mindedness, I'm inclined to say that I'm not convinced that stated speed is true. My current state of bloody mindedness is telling me that astrophysicists and cosmologists are digging themselves into a sort of hole.
Could the speed of light in a real vacuum actually be infinitely fast - although I cann't help thinking that would cause other compllications
Steve and Joe, my knowledge of astro-physics, cosmology and the associated mathematics is very limited. That said ???
I like to think I have, or at least though I did have a reasonable grasp of newtonian mechanics in my younger days and could and indeed still can accept many aspects of relativity theories (I only say many aspects because there are probably numerous other things about relativity I don't have a clue about.
However, in recent years I have become more sceptical about various aspects of modern astro-physics and cosmology.
My one time faith in mathematics has taken a battering (though not completely demolished because of my own limitations preventing me demolishing it.
However, for quite a long time I have thought it possible that the supposed constants of physics aren't really constant at all.
Although being limited in mathematical ability, which makes me feel inadequate to suggest it, I'm inclined to think that the Universe is not mathematically knowable. I might be inclined to suggest that the universe is 50% mathematically knowable and 50% otherwise knowable (but not using mathematics), or not knowable. However, that would suggest the 50% unknowable bit has some sort of mathematical function ie even if that is only in being 50%.
So I'll say that the universe is 50% mathematical and some otherwise indefinable amount non-mathematical.
However, if that is the case then arguably if the non-mathematical part isn't exactly 50% then the mathematical part cann't be 50%. If that is the case then I might argue the indefinable mathematical part isn't truly mathematical either.
I suppose one way out of my silly conundrum might be to introduce the idea of three parts (instead of only two) let's say, a mathematical part accounting for say a third of the the whole and an ill defined second part making up the weird difference between my rigidly mathematical part (which I arbitarily opted to be exactly a third of the whole) and the non-mathematical part.
I hope you'll excuse me talking the above nonsense, it's just my crude way of trying to justify what might be unjustifiable ie my thought that maths cann't analyse everything and going even further that we'll never know the answer to everything.
As well as that my other current thinking is that good old relativity is still pretty good but possbly needing modifying with a touch of MOND (modified newtonian dynamics).
Now I may well be wrong but it seems a nice simplistic way of semi-understading our universe, whilst accepting we'll never understand everything\anything properly.
Just one other thing at the smaller extremes, we'll never know whether the ultimate bits are particles, strings or waves or whatever but they'll always be nothing better understood than "ultimate bits".
Best wishes from Cliff
PS Who needs Dark Matter Dark Energy and who knows what else - they're just a pain in the bum perhaps.
PPS - 30% mathematical, ?% otherwise knowable and ?% non-knowable.
The two ? marks adding up to 70% of course ! but neither ? being ever known.


Top
 Profile Send private message  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 7 posts ] 

All times are UTC


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group