The Effect of Light moving at speed of sound?

[Crossroads Inc.]

In the "Discworld" universe, it is said that "Light moves slowly through a strong magical field"
Since the whole of the Great A'Tuin the star turtle, is surrounded by a "Strong Magical Field" it basically means that Light, in the Discworld, travels at the speed of sound.
A few weird things... In one Discworld book, it was mentioned that for the tiny Discworld sun to move around the Disc once per day, it was traveling several thousands of miles per hour, IE, faster than the light it produced...

So... In a world where "Magic" makes light no faster than the speed of sound... What bizarre consequences would this have for science in general? The study of Astronomy? Physics? Particle Physics?

[Darth Tanner]

I believe there was also a line where the wizards thought there must be another sort of light that moves much faster so that the could see the slow moving light. I always thought the 'slow moving' light was something completely different than what we would call light - or their simply describing dawn breaking or just something magical that we would have no comprehension of?

It doesn't seem to play any part in any of the plots, even when the clacks is rolled out and you would expect some lag between towers. But then if it is their order of the day its likely not remarkable.

If light genuinely is only moving at the speed of sound though it doesn't make that huge a difference to a medieval level of society - the fact it only slows down on a local level still means astronomy will work reasonably normally and on a local basis it won't make that much difference to things like ballistics until ranges have massively expanded.

Establishing things like telephones and the internet will pose problems however as a cross Atlantic cable would have 4 hour lag! Although I guess the speed of electricity would actually stay as light speed?

The first supersonic jet would be fun though. Picard manoeuvre here we come!

[Simon_Jester]

Atomic physics might be complicated more than you think; the speed of light is a physical constant that crops up a lot in electromagnetic interactions... like the relationship between nuclei and electrons. Change the rules of that interaction and you change the outcome.

[Crossroads Inc.]

Went back and re-read through a couple of parts from books last night...
Am feeling I want to amend the original post by stating that the "Cosmic" speed of light still seems to be in effect for basically all particles EXCEPT Photons. So electrons, protons, neutrons, Etc, all still move at the "cosmic speed limit" while it seems "just light" IE photons move at the speed of sound.

The part about the Wizards talking about "Another faster form of light" seems to referencing other forces such as radio waves or electricity.

Of course the question still arises, what would it be like riding in a supersonic jet in such a field?
In one book, someone traveling at almost 100mph (The fastest anyone has ever gone in a book) Noticed a visible shift in colors. Objects in front of him looked dark red, while objects behind looked faint blue

[Sea Skimmer]

At that point I suspect atmospheric refraction might become radically more effective and it would become impossible to have a useful long distance line of sight. It'd be crazy mirages everywhere. I maybe wrong in assuming that happens, but the air can already bend visual wavelengths to a pretty crazy extent. Giving it six million times more time to try has to do something! Oh and the photons might no longer have enough energy to penetrate the atmosphere at all.

[Eternal_Freedom]

Atomic physics might be complicated more than you think; the speed of light is a physical constant that crops up a lot in electromagnetic interactions... like the relationship between nuclei and electrons. Change the rules of that interaction and you change the outcome.

AS I understnad it, Crossroads was only talking about changing the speed of light in a certain medium, not in a vacuum. In which case, all those equations still work, but plenty of weird shit can happen in a "strong magical field." Hell, it's an actual magical field. Weird shit should be expected.

[Crossroads Inc.]

Something else to consider…

Light obviously travels at it's 'Normal' speed Outside of the Discworld, and then slows down as it enters…
Wouldn't this cause a massive "Backup" of photons around the edge of the magical field?
What would the Discworld look like to someone from the outside? Light from the stars would be 'impacting' the field constantly, but the light has to slow down by millions of times before it can actually pass through that area.

Also… something to screw things up even MORE is the fact that the huge Star Turtle is moving in space. In some books given the distance it travels, there is evidence that the turtle ITSELF can travel beyond the speed of normal light!
(in the first book it travels to a star and then back to deep space in the course of a year or less.)

[Lord Relvenous]

Something else to consider…

Light obviously travels at it's 'Normal' speed Outside of the Discworld, and then slows down as it enters…
Wouldn't this cause a massive "Backup" of photons around the edge of the magical field?

No, the spacing between photons would just shrink within the magical field. There's no barrier only letting in a portion of the light, the light just slows at a boundary. The timing between the photons stays the same, however.

.____.____.____.|__.__.__.__.

[Crossroads Inc.]

AH…. Interesting!
That is one mystery cleared up at least!

[Simon_Jester]

Something else to consider…

Light obviously travels at it's 'Normal' speed Outside of the Discworld, and then slows down as it enters…
Wouldn't this cause a massive "Backup" of photons around the edge of the magical field?

Is there a backup of photons at the interface between air and a pane of glass? Light travels in glass at about 2/3 the speed it does in air.

Each individual photon simply moves slower in the region in question. There's no backlog because photons don't need time or distance to decelerate. And an arbitrary number of them can take up the same space at the same time because they follow Bose-Einstein statistics.

So photons don't "slam on the brakes" and create a pileup the way that, say, cars or people would.

What would the Discworld look like to someone from the outside? Light from the stars would be 'impacting' the field constantly, but the light has to slow down by millions of times before it can actually pass through that area.

The region around the Discworld would distort light very, very much. Like looking at the world through the other side of a glass ball, except many orders of magnitude worse, such that it would be effectively impossible to figure out what you were really looking at.

Also… something to screw things up even MORE is the fact that the huge Star Turtle is moving in space. In some books given the distance it travels, there is evidence that the turtle ITSELF can travel beyond the speed of normal light!
(in the first book it travels to a star and then back to deep space in the course of a year or less.)

On the other hand, there is considerable evidence that it took months of time to approach this star, and the turtle hasn't made any other close passes of stars in living memory. I'd actually figure on A'Tuin moving at sublight speeds.

Atomic physics might be complicated more than you think; the speed of light is a physical constant that crops up a lot in electromagnetic interactions... like the relationship between nuclei and electrons. Change the rules of that interaction and you change the outcome.

AS I understnad it, Crossroads was only talking about changing the speed of light in a certain medium, not in a vacuum. In which case, all those equations still work, but plenty of weird shit can happen in a "strong magical field." Hell, it's an actual magical field. Weird shit should be expected.

The catch is that this implies that magic interacts with all electromagnetic fields, in which case there would still be very noticeable effects on anything that depends on electromagnetic forces.

Like, say, interatomic chemical interactions.

[Eternal_Freedom]

Atomic physics might be complicated more than you think; the speed of light is a physical constant that crops up a lot in electromagnetic interactions... like the relationship between nuclei and electrons. Change the rules of that interaction and you change the outcome.

AS I understnad it, Crossroads was only talking about changing the speed of light in a certain medium, not in a vacuum. In which case, all those equations still work, but plenty of weird shit can happen in a "strong magical field." Hell, it's an actual magical field. Weird shit should be expected.

The catch is that this implies that magic interacts with all electromagnetic fields, in which case there would still be very noticeable effects on anything that depends on electromagnetic forces.

Like, say, interatomic chemical interactions.

Like I said, it's a "magical" field. Weird shit is to be expected. Hell, I'd be suspicious of a "magical" field that didn't cause weird shit to happen.

[Batman]

Um-all this implies is same said magic field does is interfere with visible wavelength light. There's no mention of it interfering with UV, IR, radio waves, microwaves...
That to me either implies that a) the magical field only affects visible light or b) it affects all EM but since all the side effects we should see from photons moving at the speed of sound are somehow absent Discworld physics are so wonky it's pointless to apply real world physics to a lowered speed of light.

I'd also like to mention that apparently, Discworld light has weight and can be stockpiled.

[Simon_Jester]

I'm going to go with Batman's (b).

I don't think we can use physics to explain or analyze something like this, because we can't make basic assumptions like "the electric fields that make up light are following the same physical laws as other electric fields in the same place."

[madd0ct0r]

in one of the first books a knife is thrown so fast it gets shorter.

[Simon_Jester]

Now see, that is consistent with the physical constant c, not just the speed light waves happen to travel at, changing. Which is, again, fine- but the point here is that I don't think the idea of "Discworld light" is physically consistent enough to permit analysis under the existing laws of physics.

We can talk about "what if c were 100 m/s?" And we can talk about "what if c were still normal, but for some weird reason light, specifically moved at 100 m/s, rather than moving at c?"

But we can't talk about "what if light were a liquid that flows across the landscape at low speed but there's another kind of light that allows the first kind of light to be seen, and relativistic effects consistent with c being around 300 m/s occur but not consistently?" Or rather, we CAN talk about such a situation, but not in rigorous physical terms where we use known facts about reality to predict the consequences of an unknown effect.

Because the unknown effect is too... unknown... to be predictable that way.

[Crossroads Inc.]

Personally I think I should have posted this under the science forum

I think a more "refined" question would simply be "the effect of photons moving at the speed of sound, within an atmosphere"

As far as discworld goes, there are a LOT of things said in the earlier books that you end up tossing out. The "heavy light" is one of those things that is quietly never mentioned again in later books

[Simon_Jester]

I think Pratchett had more 'throwaway wonders' in the early novels, a lot of "I'm parodying other fantasy! But look at how weird and cool this imaginary world is, with things nobody else ever thought of!"

Then in later novels he settled down and started writing serious stories about individual protagonist characters, and the focus began to center on the people rather than the places they traveled through. This is probably why he stopped writing Rincewind stories nearly so often. Rincewind is a perfect travelogue character because he flees from danger and is constantly being brought into new places against his will. But he doesn't get a lot of character development out of his experiences.

By contrast, someone like Sam Vimes does not see lots of exciting new places, and when he does, he carries with him so much essential Vimesness that the story remains a Vimes story. Likewise a Granny Weatherwax story.

[Ziggy Stardust]

We can talk about "what if c were 100 m/s?" And we can talk about "what if c were still normal, but for some weird reason light, specifically moved at 100 m/s, rather than moving at c?"

Out of curiosity, what would be the general consequences of these two scenarios?

[Simon_Jester]

Basically, if light specifically moves at 100 m/s, rather than moving at c, you get all sorts of (to our mind) weird optical effects, but they are purely optical in nature.

I suspect that nearly everything at room temperature would then create Cherenkov radiation constantly, which would tend to sharply reduce the velocity of moving particles and atoms while flooding the world with stray electromagnetic radiation. Since this would exert a 'drag' even on the thermal motion of molecules in the air, it might have complicated effects on the temperature of matter as average molecular velocity approached c.


If c itself is 100 m/s, then anything moving at much more than forty miles an hour will experience relativistic effects detectable to unaided humans. Even aside from the effects on a human being who moves fast, or who has parts of his body move fast (such as when winding up for a throw)... this has too many side effects to count.

For example, the kinetic energy of air molecules at room temperature is enough to make them highly relativistic. As a result, the relationship between momentum and energy changes, and without doing any algebra to confirm it, I'm pretty sure the relationship between temperature and pressure becomes nonlinear. The dynamics of a relativistic gas are well-understood or at least well-understandable... but they're not the ones we're casually familiar with for air in real life.

Sound waves in dense materials, like water or stone, are sharply affected by relativistic processes- because the speed of sound in water or stone is a lot higher than 100 m/s. I don't know enough to speculate on exact consequences, but I'm damn sure there will be some, and they'll impact the dynamics of those substances.

Particle physics might be very, very changed- again, I can't figure out the details off the top of my head. But they would be sufficient that atoms and molecules would work differently... or not work at all, in which case the earlier statements are irrelevant, because there'd be no such thing as water or stone as we know them.

[Ziggy Stardust]

Interesting. I wonder if it would be possible to mathematically figure out some speed limit of c (for a sci-fi setting or something) such that it would still be physically possible for our world to exist as is, but c is still slow enough that relativistic wonkery becomes more apparent due to things more readily approaching that limit. From what you say, and my limited understanding of the relevant physics, it does seem like the speed of sound is a bit too "slow," in that if c were that low it seems unlikely that particles and atoms would work similarly enough for our universe as currently constituted would exist.