Studying the speed of light on the Discworld.

[Crossroads Inc.]

In the story series "Discworld" by Terry Pratchett, a common theme is "Light travels slowly in the High Magical field of the Disc." It is a device used in many of the books mostly for alliterative purposes. When the sun comes up, it does not instantly wash over the surface, but instead moves 'slowly' from on end to the other. It is said in several books that a person at the top of a very high mountain would be able to physically watch light "move" across the surface from the sun.

While it is never stated in the books just what the speed of light it is, it is often sighted on many Discworld websites to be roughly the speed of sound, or around 600mph.

So the question follows. If you are a scientist on a world where the speed of light is a mere 600mph, what sort of research or studies would you do? How might this be used to explore science?

[Samuel]

Can you trap light in a box? What happens to an object that moves faster than light? Do you get a wave of images all hitting a spot similtaneously? Do all the properties move at the same speed or is just the visible spectrum slowed down?

[Parallax]

Yes, it's mentioned numerous times that light slows down as it hits the magical field that envelops the Disc. When it encounters a mountain, for example, it tends to pool up and gather before washing over it.

[Vehrec]

Can you trap light in a box? What happens to an object that moves faster than light? Do you get a wave of images all hitting a spot similtaneously? Do all the properties move at the same speed or is just the visible spectrum slowed down?

This may or may not be revealing, but in the book THUD, Sam Vimes gets on a magically enhanced carriage and travels at nearly a hundred miles an hour. At that speed, on the discworld, light is visibly blue and red shifted looking ahead and behind the direction of travel.

[Surlethe]

This is going to Fantasy.

[Batman]

Doesn't one of the books state you can use dams to store light in a reservoir like we do with water in the real world?

[Gil Hamilton]

It's not going to be easy, because there is Light (golden radiating stuff that seems to travel slowly) and light (which behaves as normal). People in Discworld see just fine without Speed of Light problems, but only when light seems to move like a physical entity does it move slowly. The speed of light, therefore, is context sensitive.

The real science comes in in determining the context in which light behaves magically. The problem is that a sort of quantum error happanes that the act of measuring and dissecting it scientifically may make the light behave differently. A sort of uncertainty principle sets in that you can only observe the magical effects of light when it is breathtaking. Clearly, a very careful experiment must be done in which wonder is conserved.

[Molyneux]

If I remember correctly, someone in one of the books did measure the speed of light, and found it to be roughly on par with the speed of sound. Light on the Disc is some slow stuff. Then again, the world does run on narrativium...

[Crossroads Inc.]

If I remember correctly, someone in one of the books did measure the speed of light, and found it to be roughly on par with the speed of sound. Light on the Disc is some slow stuff. Then again, the world does run on narrativium...

You know the "Discworld" operates under its own funky laws of physics, but it still exists within a "Real" universe of some-sort. I was talking about this with a friend who mentioned that the "real" speed of light is probably exactly the same as ours, universe wise, it just slows down in the Discworld. In theory you could travel indeed "faster" then light on the Disc and it shouldn't hurt you. It might LOOK funky but it shouldn't shatter any laws of physics.

[Sarevok]

However the speed of light is a constant that appears in many physical equations. Can you really have a universe where humans could exist if you change the value of c ?

[Werrf]

However the speed of light is a constant that appears in many physical equations. Can you really have a universe where humans could exist if you change the value of c ?

There are actually theorised to be two forms of light, the regular kind which moves at about the speed of sound, an another type of light that allows you to see the first kind of light. However, nobody has found a use for the second kind of light, because it moves too fast.

Secondarily, Pterry has specifically stated that studying the Discworld scientifically is "er...dumb." The Disc doesn't run on physical laws of the kind we're used to, it runs on narrative imperative. People don't exist because the value of c is just right, people exist because that's what people do.

[Crazedwraith]

However the speed of light is a constant that appears in many physical equations. Can you really have a universe where humans could exist if you change the value of c ?

Yes, As long as you have magic.

The slow light doesn't affect anything because that would distract from the story. And on Discworld stories are everything. One of their basic elements is Narrativium after all.

[Molyneux]

If I remember correctly, someone in one of the books did measure the speed of light, and found it to be roughly on par with the speed of sound. Light on the Disc is some slow stuff. Then again, the world does run on narrativium...

You know the "Discworld" operates under its own funky laws of physics, but it still exists within a "Real" universe of some-sort. I was talking about this with a friend who mentioned that the "real" speed of light is probably exactly the same as ours, universe wise, it just slows down in the Discworld. In theory you could travel indeed "faster" then light on the Disc and it shouldn't hurt you. It might LOOK funky but it shouldn't shatter any laws of physics.

Most likely that's correct. The measurements were taken, remember, by people living on the Disc - inside the heavy-magic field.

[Bedlam]

However the speed of light is a constant that appears in many physical equations. Can you really have a universe where humans could exist if you change the value of c ?

Remember that c is the speed of light in a vacuum, in the real world you can slow down light under certain situations it doesn't change the constant.

[Sarevok]

However the speed of light is a constant that appears in many physical equations. Can you really have a universe where humans could exist if you change the value of c ?

Remember that c is the speed of light in a vacuum, in the real world you can slow down light under certain situations it doesn't change the constant.

Light can have different speeds in different mediums. But 1 ) it does not vary by several orders of magnitudes 2 ) if the air in discworld had such properties it would be not be air as we know it.

[Gil Hamilton]

Remember that c is the speed of light in a vacuum, in the real world you can slow down light under certain situations it doesn't change the constant.

Only sort of. That's a complicated subject.

What interests me is that if magic slows the speed of light, the presence of highly magically things should cause refraction as light is bent by increasingly high refractive indexes as you approach the object. This should make magically things have a shimmery aura around them, for exactly the same reason a hot road does when you look at it at a narrow angle; that is, an inferior mirage.

[Crossroads Inc.]

Remember that c is the speed of light in a vacuum, in the real world you can slow down light under certain situations it doesn't change the constant.

Only sort of. That's a complicated subject.

What interests me is that if magic slows the speed of light, the presence of highly magically things should cause refraction as light is bent by increasingly high refractive indexes as you approach the object. This should make magically things have a shimmery aura around them, for exactly the same reason a hot road does when you look at it at a narrow angle; that is, an inferior mirage.

Im pretty sure such things are mentioned often in Discworld books. Specifically very magical htings, such as the 'color' Octraine, being 'invisible' which may be it distorting light all together.

[Werrf]

Light can have different speeds in different mediums. But 1 ) it does not vary by several orders of magnitudes 2 ) if the air in discworld had such properties it would be not be air as we know it.

It's not the Disc's air that slows the light down, it's the intense magical field.
[/quote="Gil Hamilton"]
What interests me is that if magic slows the speed of light, the presence of highly magically things should cause refraction as light is bent by increasingly high refractive indexes as you approach the object. This should make magically things have a shimmery aura around them, for exactly the same reason a hot road does when you look at it at a narrow angle; that is, an inferior mirage.[/quote]
Not necessarily - we do have two types of light, after all. Slow light provides illumination, but fast light is what people see with. Of course, fast light is way too fast to be any use.

[Gil Hamilton]

Not necessarily - we do have two types of light, after all. Slow light provides illumination, but fast light is what people see with. Of course, fast light is way too fast to be any use.

I believe I mentioned that very thing above. However, slow light is still subject to refraction, so if magic changes the refractive index of some volume and the higher degrees of magic change it more, then there is no reason magic shouldn't cause an inferior or superior mirage if viewed from the right angle.

[Kuroneko]

It just doesn't square with physics at all. A million-fold slowdown in phase velocity would distort images so completely that from refraction, there wouldn't be anything visually recognizable as a "Sun" in Discworld. Trying to make this a group velocity doesn't improve things by much. The visible range has an angular frequency half-width Δω = 1.25 PHz around c/ω = 80 nm, which would correspond to the average change in the index of refraction of Δn ~ 3E5, which, while smaller than the ~1E6 factor above, is not exactly reasonable either.

However Discworld light behaves, using ordinary physics to try to predict its behavior seems futile.

[LadyTevar]

As recall in "ReaperMan" Death uses the 'slow light' of sunrise to sharpen his scythe.

[SirNitram]

The Speed Of Light is only one thing on the Disc. However, it's rather slow; in Hubland plains, it's around mach 1. In some deserts, it's so slow it moves over dunes like molasses. This Is Perfectly Fine.

The SPeed Of Dark, however, is presumably the usual constant, associated with the Uncertain Whatchacallit, a type of lepard that goes so fast it can't see their mates, only their afterimages. Thus, it is going extinct. The Unseen University has studied one. It appears to be three inches thick, five feet in diameter, charred-black, and with an odor like cooked meat.

[Molyneux]

It just doesn't square with physics at all. A million-fold slowdown in phase velocity would distort images so completely that from refraction, there wouldn't be anything visually recognizable as a "Sun" in Discworld. Trying to make this a group velocity doesn't improve things by much. The visible range has an angular frequency half-width Δω = 1.25 PHz around c/ω = 80 nm, which would correspond to the average change in the index of refraction of Δn ~ 3E5, which, while smaller than the ~1E6 factor above, is not exactly reasonable either.

However Discworld light behaves, using ordinary physics to try to predict its behavior seems futile.

It gets worse. The sun in Discworld moves at a fairly good clip, keeping up a complicated dance around the Disc and between the elephants' various legs; it is technically a faster-than-light particle from the perspective of anyone on the Disc.

[Rossum]

I suspect that the inhabitants of the Discworld have adapted to whatever changes the magic field has on the refractive properties of light. Wizards have specialized structures in their eyes called Octagons (I think) which allows them to see objects in the magical spectrum. It could be that even 'normal' humans in the Discworld have something structurally different about their eyes or the way the process visual information that lets them see things comfortably on the Disc.

A normal human like ourselves visiting Discworld would run into all sorts of problems and might become functionally blind in the new environment. Or not... I suppose Narrativium could provide some protection from the alteration if one one has convenient access to a source of plot hooks.

Though I wonder if one could test the speed of Light exactly by stretching out a really long length of string several miles long starting a point A and ending at point B. End B would have a bell attached to it and the person at end A could pull it to (theoretically) ring the bell instantly on the other side (since the kinetic energy would be intensity transferred throughout the whole length). If person A were to pull the string while simultaneous shining a light, then from their perspective the light would shine at the same moment the bell rings, while the person at end B would hear the bell ring and could time how long it takes for the light to reach them.

Thus, my String Theory is that the Speed of Light on the discworld may or not be faster than the Speed of Sound, but neither of them are as fast as the Speed of String (which can transfer information between any two points instantly provided the two points are properly entangled). If the experiment fails it is clearly because the string was improperly entangled on something in the intervening space between the two points.

[Molyneux]

I suspect that the inhabitants of the Discworld have adapted to whatever changes the magic field has on the refractive properties of light. Wizards have specialized structures in their eyes called Octagons (I think) which allows them to see objects in the magical spectrum. It could be that even 'normal' humans in the Discworld have something structurally different about their eyes or the way the process visual information that lets them see things comfortably on the Disc.

A normal human like ourselves visiting Discworld would run into all sorts of problems and might become functionally blind in the new environment. Or not... I suppose Narrativium could provide some protection from the alteration if one one has convenient access to a source of plot hooks.

Though I wonder if one could test the speed of Light exactly by stretching out a really long length of string several miles long starting a point A and ending at point B. End B would have a bell attached to it and the person at end A could pull it to (theoretically) ring the bell instantly on the other side (since the kinetic energy would be intensity transferred throughout the whole length). If person A were to pull the string while simultaneous shining a light, then from their perspective the light would shine at the same moment the bell rings, while the person at end B would hear the bell ring and could time how long it takes for the light to reach them.

Thus, my String Theory is that the Speed of Light on the discworld may or not be faster than the Speed of Sound, but neither of them are as fast as the Speed of String (which can transfer information between any two points instantly provided the two points are properly entangled). If the experiment fails it is clearly because the string was improperly entangled on something in the intervening space between the two points.

Wouldn't the Speed of String be limited to the (real-world) speed of light, though? The pull still has to propagate down the length of the string.