# Science Questions

## Where matter slows light, is spacetime affected?

Fri, 25th Feb 2011

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### Question

Hi Naked Scientists,

A little knowledge is a dangerous thing.

I have been reading Dr Brian Cox's excellent book Why E=MC^2, which introduces spacetime and special relativity, with my 11 year old son.

So far I have been able to answer most questions, and I have certainly learnt many things by reading it with him and having to explain things in a simpler way.

One thing has me stumped about spacetime.

I understand the time dimension has a constant c introduced to make the units similar to spacial dimensions, and this constant ultimately becomes the speed of massless particles (light). This got me wondering what happens to spacetime in a situation where light is slowed down, for example when it goes through glass or water. Is spacetime warped in an analogous way to mass warping spacetime?

Thanks

Martin

#### Make a comment

Martin Fennell asked the Naked Scientists: Hi Naked Scientists, A little knowledge is a dangerous thing. I have been reading Dr Brian Cox's excellent book Why E=MC^2, which introduces spacetime and special relativity, with my 11 year old son. So far I have been able to answer most questions, and I have certainly learnt many things by reading it with him and having to explain things in a simpler way. One thing has me stumped about spacetime. I understand the time dimension has a constant c introduced to make the units similar to spacial dimensions, and this constant ultimately becomes the speed of massless particles (light). This got me wondering what happens to spacetime in a situation where light is slowed down, for example when it goes through glass or water. Is spacetime warped in an analogous way to mass warping spacetime? Thanks Martin What do you think? Martin Fennell, Mon, 7th Feb 2011

It's the other way around I think :)

Einstein found that as light had the same speed 'everywhere' something had to be done to time to 'fit light in' with time and those other dimensions we have. But it is a weird thing, to make a imprint on a super-cooled gas with a boson, and then when the gas heats up, see a boson leaving it. You might assume that it is the 'energy'  that somehow gets stored, pushing the quantum states of the condensate maybe? But I don't think we twist the room geometry by 'freezing light'? We keep on debating this idea here, that light itself can twist the 'room geometry, some models say that it does, but only when having light moving in different directions, and then looking at that as a 'system'. yor_on, Tue, 8th Feb 2011

The short answer is that nothing special happens to space-time just because light is moving slower in matter than in a vacuum.

The slowness of light in matter is due to the light interacting with the particles that make up the matter.  In between these particles, light is still moving at c, the speed of light in a vacuum.  You can somewhat think of it that each molecule in the matter is catching and releasing photons of light.  When they're released, they move at c.  When they've been caught, they're stopped, so that the overall speed is slower than c.  Since the vacuum speed is still the same, nothing funny has happened to space-time. jpetruccelli, Tue, 8th Feb 2011

Sorry, read you wrong there Martin. JP explains it perfectly, same as I understands it. Now it's only one small thing I'm wondering ah, about. When it's a wave instead of photons we see propagating. Where and how does it 'slow down' in that matter?

Snell's law anyone?
yor_on, Wed, 9th Feb 2011

The usual way of treating light-matter interactions classically is to treat the matter as having charges which can move in response to incident light.  In most matter, the electrons stay stuck to their molecules, but their orbits can change slightly in response to the incident light. The effect of these electrons moving slightly is the creation of another light field, which interferes with the incident light in such a way that it slows down. jpetruccelli, Wed, 9th Feb 2011