Speed of light in a vacuum? (1 Viewer)

[stag_j]

I've been going over my notes for the space topic, and i noticed that very often, when the speed of light is given it's stated as being in a vacuum. If the speed of light is constant, why is this so?

 

[underthesun]

isn't it because light bounces off the atom lattices of glass, or crystal hence slowing it down, etc?

 

[PoLaRbEaR]

isn't it because light bounces off the atom lattices of glass, or crystal hence slowing it down, etc?

Light travels at a constant velocity so it doesnt slow down..
I guess it's because space is a vacuum so it just mentions it??

 

[stag_j]

I think I have a vague recollection of talking about this in class.
I'm sure there must be a reason for it always being stated in this way.

Underthesun - I think you might be right. While the speed of light remains the same, because it is bounced around off the atoms within the matter it will have a greater distance to travel. To an observer, i guess this would look as if it had been slowed down... Does that seem right or am I way off???

 

[wogboy]

This is where everyone gets confused about the Special Theory of Relativity, the speed of light is *not* constant if you change the medium through which it propogates.

If you measure the speed of light out in space (in vacuum) it will be a bit faster than if you measure it here on Earth (in the atmosphere). Light travels the fastest in vacuum.

The speed of light is however constant regardless of the relative motion between the light source and observer. Imagine if someone was moving very fast towards you (say at a speed v), shining a torch at you. You would naturally expect that the speed of light that you would measure would be c+v. This is wrong, since you can't just add the velocities as vectors anymore (in relativistic physics). Instead of measuring c+v, you would measure c. No matter what relative velocity you move with respect to the light source, no matter which inertial frame of reference you're in, you will always measure the speed of light in vacuum to be c

This doesn't alter the fact that light travels more slowly in media other than vacuum (e.g. air, water).

 

[yurbel]

Light travels at a constant velocity so it doesnt slow down..
I guess it's because space is a vacuum so it just mentions it?? [/QUOTE]

Refraction involves the bending of light due to a change in its velocity.

 

[PoLaRbEaR]

If light remains the same then it cant 'seem' to slow down...the observer would just see the light at a later time...remember that time is a variable in space-time...

 

[Giant Lobster]

nar its simple really... you cant compare things across different frames of reference. this is no exception for spd of light. For the statement of spd being constant to be true u have to be measuring spd of light from the medium in which it is traveling.

In vacuo its the fastest, @ approx 3e8m/s but it slows down in denser mediums. you cant say spd isnt constant by observing light slowing down as it changes medium. Thats like comparing two functions on different axes (stretching the analogy abit here )

I dunno exactly why light slows down, the conceptual explanation is surely beyond the scope of hsc physics, but i think it has somefin to do with disruption of electromagnetism of EMR by nearby particles [i.e. atoms] in a dense medium.

 

[inasero]

u can travel to the past but ont the future if the theory that light is constant for all travellers is constant

 

[Giant Lobster]

actually u got it backwards. u can travel to the future, not the past

for past time travel to work, u need faster than the spd of light and approach the asymptote [@c=3e8m/s] from the opposite side

just a thought.

 

[inasero]

back to tha future!

 

[underthesun]

btw, light slowing down on different mediums are prelim stuff, and I've never seen them on the HSC course. So relax, and just concentrate on relativity

 

[inasero]

no way man u can travel to the past...
consider the gedanken...what we pweceive as reality is what is observable right?
so if why can somwhoe catch up to the lght informatio which has long sinve relected off the original surfaces into deep space, by going faster than the speed of light then we can catch up with this light information and effectively view events as they happened in the past..agree...haha so YOU"RE the one thats wrong lol admit it!

 

[inasero]

btw this discussion is becoming increasingly more philospohical lol

 

[Mathematician]

...

No u can travel to future but not past(according to relativity).

According to some relativity and gravity unification theory u can travel to past and u will think irrationaly(uni work, i heard this from someone)

 

[underthesun]

What if what you think is real is not real? What if Einstein was not there, and all these theories of relativities are not real? What if, it's just a rule established by The Matrix to make sure no-one goes at a speed too fast inside the "matrix world" ?