General Physics Thread

Related: What would the universe look like if light traveled instantly? Would the night sky still be dark? I think so. How would you measure frequency? No longer in wavelength. Oh, that would completely make radio antennas not work.

If there was no speed limit, then I assume gravity would travel instantly instead of at the speed of light. This changes the orbital mechanics of really fast systems like two neutron stars in close proximity.

I am not sure black holes would be possible without relativity.

Holy -bleep-. It would make infinitely fast computers possible.

edited 25th Apr '16 9:37:04 PM by war877

I don't think the night sky would still be dark. If the universe is a) isotropic and b) without boundaries (two very reasonable assumptions) then there's no line of sight that doesn't go through a star at some point. If light travels instantaneously then we could see each of these star. Unless I'm mistaken, that's equivalent to being inside a hollow sphere made of star surface.

This would make life (and possibly even the existence of planets) impossible. Stars would have to work differently too, since they would receive energy comparable to (sometimes even superior) their own output. Which calls into question whether stars could exist in the first place.

edited 26th Apr '16 4:16:01 AM by Aetol

Worldbuilding is fun, writing is a chore

You are not entirely mistaken. There is a lot of dust in space. But if there were no obstructing dust, clouds, or planets, then it would be as you say, if the universe is infinite, and of consistent density at some scale.

edited 26th Apr '16 4:32:03 AM by war877

It doesn't even need to be infinite, as long as you can travel indefinitely in any direction. A finite universe can have this property (the surface of a sphere is a 2-dimensional example).

Regarding dust and gas clouds : since they are subjected to the "bright sky" themselves, they'll reach thermodynamic equilibrium with the rest of the universe and become just as bright as the average. However, since stars only make up some 10% of the universe's baryonic matter, that average would indeed be lower than expected.

EDIT : the non-star matter actually does jack shit to reduce the sky's temperature. If the average temperature of the gas clouds was lower than the average temperature of the stars, then the average sky temperature would be higher than that of the gas (if only so slightly) and would not be at thermodynamic equilibrium. The equilibrium is reached when everything is as bright as the star average. How quickly it can be reached is another problem.

However, this raises a new question : this only work if stars with lower-than average temperature stars still exist, and they obviously can't (see: even inert gas is white hot, above). The solution is that the thermodynamic equilibrium is never actually reached. When heat (including ER radiations) is created through gravitational collapse or nuclear fusion it is redistributed instantly throughout the universe and raises its temperature a bit (this also works with an infinite universe, since it would have an infinity of energy sources). So, the end result is basically the same as in our universe, thanks to its isotropy. There's a lot of matter and only so much energy to heat it.

Conclusion : interstellar gas would stay cold and stop most of the distant stars' light (when looking in a random direction, your line of sight is much more likely to first meet an isolated hydrogen atom than a star, by several orders of magnitude). So, no "bright sky" after all.

edited 26th Apr '16 8:32:38 AM by Aetol

Worldbuilding is fun, writing is a chore

Hi, I have a new question. I'm trying to analyse the dynamics of a slinky. The model I'm using is :

• The slinky's relaxed length is l₀, its uniformly distributed mass is m, and its uniformly distributed stiffness is k.
• Each point of the length of the slinky is identified by the parameter x : x = 0 at the top of the slinky and x = 1 at the bottom.
• Each fraction dx of the slinky is a linear spring of length dl (when relaxed dl = l₀.dx), of mass dm = m.dx, and of stiffness k/dx.

The static problem (slinky suspended by its top end) is easy to solve. If we consider the portion [x, x+dx] of the slinky :

FT = (k/dx) * (dl - l0.dx) = k * (dl/dx - l0)    // tension of the spring  
FG = (1-x) * mg                         // weight supported by the spring  
FT = FG => dl/dx = l0 + (1-x) * mg/k 

dl/dx - l₀ measures the local stretching of the slinky : as expected it is maximal at the top and null at the bottom.

However, I don't know how to go about the dynamic analysis. If I considered the acceleration of an infinitesimal element, I'd compare an infinitesimal rate of change of momentum (since the mass is dm) to forces that are not infinitesimal, as seen above. I'm stumped here.

So, any idea?

(To be clear : I do not want a solution, as I'm doing this for fun. I just need a lead.)

edited 9th May '16 5:34:09 AM by Aetol

Worldbuilding is fun, writing is a chore

So basically you want to know the behaviour of a slinky bouncing up and down?

Well, ignoring the standard equations of a spring, I can tell you that force=mass*acceleration. Therefore, on a piece of the spring with an infinitesimal mass and finite acceleration, the force must also be infinitesimal.

My goal is to find the behavior of the slinky in free fall, and in particular to confirm that its bottom end remains stationary at first.

Regarding the equation, well, you've summed up my problem. Maybe I could use dF_T and dF_G? It would be mathematically sound, but I'm not sure it makes sense physically speaking. I'm having a hard time picturing the forces that are exerted on that infinitesimal spring.

EDIT : well, it seems to work. Still not entirely sure how.

dm/dx * a = dFG/dx - dFT/dx  
m.a = -m.g - k * d2l/dx2 

In the static case, we have a = 0, so :

0 = -m.g - k * d2l/dx2  
=> d2l/dx2 = -mg/k  
=> dl/dx = K - x * mg/k  
dl/dx(x=1) = l0 = K = mg/k => K = l0 + mg/k  
dl/dx = l0 + (1-x) * mg/k 

Which is the expected result.

edited 9th May '16 8:26:11 AM by Aetol

Worldbuilding is fun, writing is a chore

Modelling vibrations is tricky. You are either going to end up with differential equations or a computer simulation.

Vibrations have a propagation speed. How to properly model this without going to a computer simulation, I am not sure.

Well, in the dynamic case (both oscillations and freefalling) I end up with a wave equation.

Worldbuilding is fun, writing is a chore

A real world slinky will collapse, with the top half travelling down at roughly double the acceleration of gravity when released. It will then remain in a collapsed state for the remainder of the freefall. This is assuming no weight is hanging off the bottom of the slinky.

I could try a discrete model : a chain of N masses linked by N weightless springs. Solve, then take the limit when N tends to infinity. Probably beats solving differential equations.

edited 9th May '16 11:02:01 AM by Aetol

Worldbuilding is fun, writing is a chore

That sounds like a plan that would work for sure. I would bet that a differential equation is what you would end up with, but because of the overall elegance of this system, I think it might have a simpler solution.

... So this spring-based thing is called a "slinky"? I didn't know Toy Story's Slinky was named after the actual name of this thing (which his body is admittedly based on).

Fiat iustitia, et pereat mundus.

Question for a story.

I got a character who's a physicist. Throughout the course of the story, he keeps getting asked to build a machines. He eventually gets fed up and say: He's a physicist. His job is to discover or explain. If they want to get something designed or built, that's an engineer's job.

Would that be an appropriate response from a physicist?

I'm a (socialist) professional writer serializing a WWII alternate history webnovel.

"I'm a physicist, [expletive], go get an engineer." Is exactly something I would expect a physicist to say. Explaining why as you did may be something they would do if they were not truly completely frustrated yet.

Other responses include: "I do lasers. You want a car mechanic." "If you want to calculate the odds of a spacecraft surviving a close pass to a black hole, or the size of the debris cloud from the impact of a meteor one mile across, I'm your girl. If you want me to build more than a Turkey Club, you're out of luck."

Crossposting from the Biology Thread due to the possibility that the issue pertains more to physics than biology.

Been reading about compound eyes on Wikipedia and elsewhere. It says that due to the physics involved, compound eyes have an inherent limitation on how good the resolution of the images they get can achieve (maximum of 1°... whatever the "°" symbol means), unless the many lenses in such eyes actually operate as "phased arrays", which is (apparently) very unlikely. What is meant by that last bit? What does it mean for a eye's lens to work as a "phased array", and is it possible for an animal eye, simple or compound, to work that way?

Fiat iustitia, et pereat mundus.

In this case, it basically means each compound eye tracks not only the incoming light from various sources, but also the phase of the light, then compares the phase against the phase recorded by a different eye...

Yeah, not happening I don't think.

What does "phase" mean when it comes to light as an EM wave, though? I know what the term means when it comes to oscillating waveforms such as sine waves◊, but my academic studies on the subject never touched on how that manifests outside the mathematical world.

Why not?

edited 22nd Sep '16 5:32:54 AM by MarqFJA

Fiat iustitia, et pereat mundus.

Phase means exactly the same thing in physics as in mathematics.

Biological photoreceptors are capable of detecting the intensity of light only. To get a group of photoreceptors to detect a phase, I actually don't know how to do that with light waves.

... Let me rephrase: If two devices emitted non-invisible laser beams at a wall with identical properties (same wavelength, frequency, etc.), and then I had one of the devices change the phase of its beam by a significant margin, what difference would I detect as I look upon the beams again?

Well, how do we do it with machines? Or are you telling me that we haven't gotten around to doing that yet?

Fiat iustitia, et pereat mundus.

If both rays of light have the same phase, then their waveforms will add up (constructive interference) and the spot on the wall will be brighter. If you then shift the phases, the intensity will decrease until the waveforms are in opposition and cancel each other out (destructive interference), then increase again, over and over cyclically.

Note that you can't actually control the phase of a light source. The only way to create interferences is to make the light from a single source take different paths: for example by shining a light through two pinholes in a wall, or by using mirrors to split the beam.

Worldbuilding is fun, writing is a chore

I believe you might be able to control the phase of light, the same as you can control the phase of low frequency electromagnetic radiation.

If you had such a device, you could create an identical device and have the path to the wall of one device be exactly one half wavelength farther away than the other device to get destructive interference.

It is over my head.

edited 22nd Sep '16 6:42:03 PM by war877

Science Question: If you were to take a small drone with an extremely large battery and put him on a flat field and make him hover indefinitely in the air, would the earth rotate underneath him?

If you run in a circle clockwise, the earth will rotate as a consequence. But if the copter is not spinning, it is not producing this effect.

The blades are. The earth will rotate due to the borrowed angular momentum of the blades.

edited 5th Oct '16 4:43:26 PM by war877

I'm not sure if I explained well enough. I meant if you put a drone on one end of a football field and made it hover their, would it eventually be over the other side of the football field because the earth rotated underneath it.