ask a physicist

[Umbran]

I don't think this has anything to do with speed of light though.

In reality, we don't think of the electron as a point object going around the protons in a classical orbit at all! And the photon isn't a discrete object in this either - it would be virtual. So, I was really being descritive - the quantum mechanical description doesn't make *intuitive* sense, after all.

Right, there IS no 'surface of the fireball' at the big bang, we're INSIDE IT still, and always forever. Its just getting bigger and cooling off. No outside, no inside, just everywhere.

Exactly. The universe may well have been created as infinite in size, it is only the *observable* universe that has an edge, and that's not a physical boundary at all.

 

[AbdulAlhazred]

In reality, we don't think of the electron as a point object going around the protons in a classical orbit at all! And the photon isn't a discrete object in this either - it would be virtual. So, I was really being descritive - the quantum mechanical description doesn't make *intuitive* sense, after all.

Well, yes, this is just ONE of several ways in which the concept of 'locality' no longer makes sense. Even in the purely classical system of SR/GR the idea of 'here' and 'there' don't actually add up. At best there are 'relations' between things, which provide some localized basis from which to build a coordinate system, but in GR you certainly can't extend it. QM of course just reduces the whole concept to no more than just some state variables that tell you what might happen. Or in the case of relativistic quantum field theory one can wonder if they really tell you much of anything at all, since somehow that photon interacts with that electron no mater what. I believe this was the original impetus for the 'pilot wave' class of QM interpretations, which generally involve fun things like entities which travel backwards in time.

 

[Umbran]

Well, yes, this is just ONE of several ways in which the concept of 'locality' no longer makes sense.

I think you are using that term "locality" in a different way than I would normally use it.

Even in the purely classical system of SR/GR the idea of 'here' and 'there' don't actually add up.

Well, that is probably only because "don't add up" is not a well-defined phrase, so that nobody outside your head knows what you mean.

If to "add up" they must be objective absolutes, then yes, they don't add up. But your experience of the color blue is not an objective absolute, either, but you and others can talk about it quite sensibly.

At best there are 'relations' between things, which provide some localized basis from which to build a coordinate system, but in GR you certainly can't extend it.

Extend it in what sense? And "can't" in what sense?

I mean, consider that even without GR, there are more coordinate systems that are problematic than not. Geocentric coordinates were the first we ever used, even before Newton, and those coordinates *sucked* for just about everything other than talking about the Moon. You'd not want to extend that to infinity and try to describe movements in the universe in it, even if spacetime didn't curve around masses. It isn't so much "can't" as "Nobody sane would want to."

Or in the case of relativistic quantum field theory one can wonder if they really tell you much of anything at all, since somehow that photon interacts with that electron no mater what.

It is perhaps more accurate to say that time and space are still fine, the problem is that the photon never actually exists, but we insist on using it to describe the interaction anyway. That is not spacetime's fault.

 

[freyar]

It is perhaps more accurate to say that time and space are still fine, the problem is that the photon never actually exists, but we insist on using it to describe the interaction anyway. That is not spacetime's fault.

It's not really the photon's fault, either. Really, the problem with the language is saying that "virtual photons" create/carry the electromagnetic potential/force (or really just the use of "virtual particle" more generally). It's fine as a shorthand in jargon when people know the technical meaning, but it unfortunately describes in plain English something very different than what the physics and mathematics of quantum field theory describe. Unfortunately, "off-shell field excitation" or "Green function" or "2-point function" doesn't do it for most people.

 

[Bedrockgames]

I am trying to put a liquid mercury lake and river (that cycles on a square pallet chain pump) in a dungeon. What are some plausibility issues with this and how would liquid mercury behave when players do things like plunge into the lake or try to ride across it on make shift boogie boards?

 

[Nagol]

Anything dropped on the surface will remain on top unless it is gold or platinum -- their densities are high enough that items will in fact sink. Steel, lead. silver, copper will bob on the surface.

Plunging into the lake is going to hurt. At about 13 times the density of water (and almost twice that of steel), people would probably be able to walk getting only their toes submerged if it offered any friction -- which it pretty much doesn't. Its viscosity isn't much more than water (somewhere between fresh water and milk).

Simple way across would be to sit down and pole/oar with a sword.

Most metals will likely be destroyed over time (gold would go pretty quickly) through amalgamation though iron and platinum are pretty much immune.

 

[Bedrockgames]

Plunging into the lake is going to hurt. At about 13 times the density of water (and almost twice that of steel), people would probably be able to walk getting only their toes submerged if it offered any friction -- which it pretty much doesn't. Its viscosity isn't much more than water (somewhere between fresh water and milk).
.

Thanks for the information. That is quite helpful. I hadn't considered that the higher density would potentially mean injury if someone feel into it from a ledge or something.

How hurt do think people would get? Is it equivalent to falling onto solid ground in terms of injury?

 

[Nagol]

There will be some give but not a lot. A 6' person somehow standing will stabilise with about 6 inches of leg under the mercury-line and the viscosity is low enough the liquid will flow away from the impact point. I'd probably be nice and drop a d6 off the damage.

 

[Displacer Kitten]

Could Sigil (a small torus-shaped universe with gravity pulling toward the "walls" of the universe) exist with those physical laws? Can it be modeled mathematically?

 

[Umbran]

Could Sigil (a small torus-shaped universe with gravity pulling toward the "walls" of the universe) exist with those physical laws? Can it be modeled mathematically?

We can't say if it can exist. It would have to have a set of laws and/or distribution of materials (matter, dark matter, and dark energy) drastically different from our own.