Lowest speed of light.

(I may be wrong!)

c is defined as the speed of light in a vacuum.

Diffraction is caused by light being slowed in one of the media (glass or water, for example).

So, the speed of light in glass can be stated as xc, where 0 < x <1.

What is the smallest theoretical value of x? (strictly > 0!)

Could we manufacture a substance where the speed of light was 1 metre per second, for example?

the speed of light is not xc really. it is when measured acrossed the whole distance acrossed the medium, but that light is still moving at c as it moves from atom to atom in the medium. At least that is how it makes sence to me. I was actually wondering about this a couple days ago, they talk about being able to slow down light, but are they really slowing it down? It seems to me that all that is happening is that the light is being absorbed by the atoms of a material and being held there for a period of time, keeping all of the same properties that it had when it entered the atom. So i see it not so much as light being slowed down, as it is “memorizing” the light.


I understand your argument. What you are saying is that the light is bouncing around at c within the medium, and then finally emerging.

A bit like if you ran the 100 metres at the Olympics, at 10 m/s, but not in a straight line. You’d cross the line 50 seconds after the start, having said hello to all your friends at track side!

What’s your speed?!

Scientists have actually stopped light all together. The speed of light is only constant in a vaccum.

p.s I don’t know what exact substance was used though.

black holes


Yeah true, although I meant that they actually did it here on earth using some exotic substance. Meaning we don’t have to rip the fabric of space time in order to completly stop light.

I meant that the “natural” thing that slows and stops light is gravity…

we probably used a lead wall or something


The speed of light is a theoretical upper limit. The actual distance over time we can observe is controlled by several factors. Imp mentioned gravity as one variable. Another would be ‘dust’ that forces the light source to ‘go around’ minutely changing the apparent or measurable speed. The theoretical speed is a constant, but outside the theory…


Outside what theory ? Relativity? The constant speed of light has been confirmed through a pluthera of experiments. In relativity the speed of light is actually the rate we are traveling at when we are at rest. We travel through time at the speed of light. When we move through space some of our energy is diverted to the spatial dimensions, and hence time slows down, and so no object can go faster then the speed of light.

So the photons in light moving through a vaccum never ages.

Dear all,

Can we get back on track?

OK, so light can have velocity = 0. A brick wall, for example, will stop light.

But what about a medium that slows it doen, without stopping it?


Well, you said it yourself. c is the speed of light in a vacuum. Light passing through a medium of any density would slow down to an external observer. That’s why light bends in glass and in water.
But keep in mind DietCoke’s post. He’s right on. Rounder’s last one too. At least as far as I understand it…
As for light stopping in a black hole: while relative to an outside observer, this appears to be what happening (thus, black). However, inside the black hole, the light still travels at c, just time in the black hole moves infinitly slow relative to time of the observer… if that makes sense. I could be wrong about this though…

I don’t see why not… Out of curiousity, why do you ask?


Just asking!

I’ve been unable to find an answer, and it may be interesting to see a light beam enter my substance and emerge only a few seconds later.

According to our current understanding, light wouldn’t be standing still in a black hole. Rather it’s just that the strength of the gravity of an object so massive warps space around upon itself, effectively removing it from the universe, so to speak. This is called a singularity. Light is moving thru space but space is curved & closed off.

I’ve never entered the event horizon of a singularity to see if this is true, though. :wink:

… and I can’t see myself creating a black hole at home to observe the phenomenon.

the problem is that the speed of light itself isn’t changing…

light itself is still moving at 299,792,458 m/s

the only thing a filter does is make it travel further so it appears to slow down…



At last, an answer I can understand and that addresses the question!

So, what happens is that the light bounces around within the medium and then exits.

Sounds plausible. It leads to the secondary question:

Is there a medium that makes the light bounce around so much that the light will exit the medium, 1 metre thick say, 1 second after entering?

Tertiary question!

Why does the light exit consistently from the same point? If it bounces around randomly, would you not expect it to be diffracted completely, in all directions?

Yeah thats right. See light cannot escape the event horizon of a black hole, but it stays there, and we would percieve it as being stopped. So the entire light speed is required to remain at the event horizon, and not be completely sucked in. So all its energy is still being exerted in the spatial dimensions. So when I said before that photons don’t age where light is moving through a vaccum is wrong. Its that the photons don’t age at all.

In effect we could most likely create a substance where light traveled, or appeared to travel, at 1 metre per second, but the energy exerted by light is still the same, its still going to be within the spatial dimensions. So we could produce such a material, and effect our perception of light, but we can’t in theory change the amount of energy light exerts in the spatial dimensions.

as a side note I recently read a short article about how scientists were able to speed up light by sending it through some gas?? Anyone know about this? I was going to follow up on it myself, but I ran into the article when I was on vacation, so in effect it managed to slip my mind untill now. Anyways I think its time to hit up google for some info :smiley:

you sure you dont mean slow down? i dont think there have been any experiments where light has been speeded up… einstein would freak.