Why would'nt this work?
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I don't think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels.
I don’t know how you would measure gravitational waves without measuring gravitational attraction.
It's not light that is "communicating" that attraction.
Nobody said it was. The “speed of light” isn’t about “light”. Gravity propagates at the same speed, aka “c.”
This Reddit discussion on r/AskPhysics might help clear up your misconceptions. Notably:
Just to clarify: when people talk about the speed of gravity, they mean the speed at which changes propagate. It's the answer to questions like: if I take the Sun and wiggle it around, how long does it take for the Earth to feel the varitation in the force of gravity? And the answer is that changes in gravity travel at the speed of light.
But that's not what you're asking about. Whenever you're close to the Earth, gravity is always acting on you: it's not waiting until you step off a cliff, like in the Coyote and the Roadrunner. The very instant your foot is no longer on the ground, gravity will start to move it downwards. The only detail is that it takes some time for it to build up an appreciable speed, and this is what allows us to do stuff like jump over pits: if you're fast enough, gravity won't be able to accelerate you enough - but gravity is still there.
I get the sense that you’re thinking about the second scenario when objecting to the concept that gravity travels at the speed of light.
I was definitely talking about the first scenario, as is mostly everyone else. I know not everyone admits gravity (gravitational attraction) might travel faster than light as in the "sun moving" thought experiment.
I'm not confused, I'm discussing like everybody else.
You linked an article about gravitational waves which must transmit through some sort of gravitational field and they might transmit at approximately c as predicted in general relativity.
What I believe is that gravitational attraction, so the general effect of the field will be felt as if it acts almost instantly, and that does not contradict anything about the waves in that field. Because the waves in that field are not responsible for the attraction. This is similar to how photons do not mediate the magnetic attraction in magnets even though they are electromagnetic waves.
The current theories (which you are pulling from) manage to mathematically explain that in our moving sun thought experiment, the gravitational force coming from the sun appears to "update" instantly as if it's acting from it's actual position without the lag, because of (to my understanding) the curvature of space-time. So I personally can't fight that on mathematical grounds because that's above my understanding. But in the end it doesn't change much of anything to our discussion, because the force of gravity still updates "as if" it was mostly instantaneous and that's the standard model. Meanwhile, gravitational waves do travel at c but are kind of unrelated to the continous force. They are merely fluctuations in that force.
Please keep poking and challenging me at that, I'm still wrapping my head around it and will need better and better sources while I'm hyper focusing on it until I move on lol -
It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
The motion of the stick will actually only propagate to the other end at the speed of sound in the material the stick is made of.
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For anyone looking for other cool ideas or videos about speed of light etc
What Is The Speed of Dark? - Vsauce (13m:31s)
- Cool older vsauce video going over shadows and light speed etc
The Faster-Than-Light Guillotine - Because Science (w/ Kyle Hill) (14m:19s)
- Basically goes over the "FTL Scissor action" that a lot of people have covered but he does a good segment covering it.
Here's a video that actually kinda answers the question:
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If your stick is unbreakable and unavoidable you have already broken laws of physics anyway
If your stick is unbreakable and unavoidable you have already broken laws of physics anyway
You have it backwards: if your stick is unavoidable, NOT HAVING IT is the impossible thing.
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The motion of the stick will actually only propagate to the other end at the speed of sound in the material the stick is made of.
So when you pull on the stick and it doesnt immediately get pulled back on the other side, you are, at that instant, creating more stick?
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So when you pull on the stick and it doesnt immediately get pulled back on the other side, you are, at that instant, creating more stick?
It would stretch like a rubber band stretches just a lot less. Wood, metal, whatever is slightly flexible. The stick would either get slightly thinner or slightly less dense as you pulled it. Also, you won't be able to pull it much because there's so much stick.
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As an object becomes "closer" to a perfectly rigid object it becomes denser, would such an object eventually collapse onto itself and become a black hole? Or is there another limit to how dense/rigid an object can be?
Seems likely. The most rigid materially known, (or at least theorized) is nuclear pasta.. Nuclear pasta only forms inside neutron stars, stellar objects that are the last stage of matter before matter gives up entirely and collapses into a black hole.
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You're pushing the atoms on your end, which in turn push the next atoms, which push the next ones and so on up to the atoms at the end of the rod which push the hand of your friend on the moon.
As it so happens the way the atoms push each other is electromagnetism, in other words sending photons (same thing light is made of) to each other but these photons are not at visible wavelengths so you don't see them as light.
So pushing the rod is just sending a wave down the rod of atoms pushing each other which the gaps between atoms being bridged using photons, so it will never be faster than the speed at which photons can travel in vacuum (it's actually slower because there's some delay since part of the movement of that wave is actual atoms moving and atoms have mass so they can't travel as fast as the speed of light).
In normal day to day life the rods are far to short for us to notice the delay between the pushing the rod on one end and the rod pushing something on the other end.
Very well put.
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So when you pull on the stick and it doesnt immediately get pulled back on the other side, you are, at that instant, creating more stick?
You're not creating more stick, but you're making the stick longer. The pressure wave in the stick will travel at the speed of sound in the stick which will be faster than sound in air, but orders of magnitude slower than light.
Everything has some elasticity. Rigidity is an illusion . Things that feel rigid to us are rigid in human terms only.
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Photons don't have mass, but they do have momentum.
How does that work?
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You're not creating more stick, but you're making the stick longer. The pressure wave in the stick will travel at the speed of sound in the stick which will be faster than sound in air, but orders of magnitude slower than light.
Everything has some elasticity. Rigidity is an illusion . Things that feel rigid to us are rigid in human terms only.
I get it. Elasticity isn't something you think about in the every day so it all seems rigid.
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So folks have already explained the stick, but you're actually somewhat close to one of the ways you can sort of bend the rules of FTL, at least when it comes to a group of photons.
Instead of a stick, imagine a laser on earth pointed at one edge of the moon. Now suddenly shift the laser to the other side of the moon. What happens to the laser point on the moon's surface?
Well, it still takes light speed (1.3 seconds to the moon) for the movement to take effect, but once it starts, the "point" will "travel" to the other side faster than light. It's not the same photons; and if you could trace the path of the laser, you'd find that the photons space out so much that there are gaps like a dotted line; but if you had a set of sensors on each side of the moon set up to detect the laser, they would find that the time between the first and second sensor detecting the beam would be faster than what light speed would typically allow.
It's not exactly practical, and such an edge case that I doubt we can find a good way to use it, but yeah; FTL through arc lengths can kind of be a thing.
This is hard to truly eli5, so I'll have a go too, in case the others haven't cleared it up for you.
The spot on the moon that moves isn't a real thing, it's the effect of photons hitting the left side, then other photons hitting the right side. The 'reason' or 'cause' for those photons comes from earth very much at light speed. But the left side of the moon can't cause an effect in the right side, that fast. It just experiences a thing right before the right side experiences something similar.
Like if two cars drive from London to Manchester and Liverpool, arriving within seconds of each other. It doesn't mean you can drive from Manchester to Liverpool in seconds.
There's an SMBC I love on this: "The shadows of reality go as fast as they like." https://www.smbc-comics.com/comic/superluminal
Bonus: IIRC, any two events that are too close in time for light to travel from one to the other, can be viewed from a different "inertial reference frame" (someone else moving fast and analysing things with the same physics) as being the other way round. I.e. the right observer could see the right hand side of the moon get lit up before the left hand side. But the chap on earth wiggling the laser pointer is still wiggling it slower than the speed of light, so this observer would still see the laser pointer move from left to right. How does that work?
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If your stick is unbreakable and unavoidable you have already broken laws of physics anyway
You have it backwards: if your stick is unavoidable, NOT HAVING IT is the impossible thing.
Autocorrected from unfoldable. This is what I get for occasionally browsing on a shitty Amazon tablet. At least it was cheap to the point of being almost free.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Even if the stick were made of the hardest known material, the information would take about 7 hours to travel from Earth to the Moon, according to the equation relating Young's modulus and the material's density.
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Even if the stick were made of the hardest known material, the information would take about 7 hours to travel from Earth to the Moon, according to the equation relating Young's modulus and the material's density.
Also, even if you could somehow pull the stick, Newton’s Second Law (F = ma) tells us that the force required to move it depends on its mass and desired acceleration. If the stick were made of steel with a 1 cm radius, it would have a mass of approximately 754x10^6kg due to its enormous length. Now, if you tried to give it just a tiny acceleration of 0.01 m/s² (barely noticeable movement), the required force would be:
F = (754×10^6) × (0.01) = 7.54×10^6 N
That’s 7.54 MN, equivalent to the thrust of a Saturn V rocket, just to make it move at all! And that’s not even considering internal stresses, gravity differences, or the fact that the force wouldn’t propagate instantly through the stick.
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You're pushing the atoms on your end, which in turn push the next atoms, which push the next ones and so on up to the atoms at the end of the rod which push the hand of your friend on the moon.
As it so happens the way the atoms push each other is electromagnetism, in other words sending photons (same thing light is made of) to each other but these photons are not at visible wavelengths so you don't see them as light.
So pushing the rod is just sending a wave down the rod of atoms pushing each other which the gaps between atoms being bridged using photons, so it will never be faster than the speed at which photons can travel in vacuum (it's actually slower because there's some delay since part of the movement of that wave is actual atoms moving and atoms have mass so they can't travel as fast as the speed of light).
In normal day to day life the rods are far to short for us to notice the delay between the pushing the rod on one end and the rod pushing something on the other end.
As it so happens the way the atoms push each other is electromagnetism, in other words sending photons (same thing light is made of) to each other but these photons are not at visible wavelengths so you don't see them as light.
Wat? I strongly believe you are not correct. Which is to say, I think you are talking out of your arse entirely. If you push on a thing you peturb the electron structure of the material. These peturbations propagate as vibratory modes modeled as phonons.
While technically some of this energy is emitted as thermal radiation that is not primarily where it goes.
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I don't get it. Care to explain?
There are multiple forces at work in a rocket nozzle:
- The exhaust is pushed outward, giving the rocket thrust
- The exhaust hits the wall of the nozzle as it gets thinner, braking the rocket
These two effectively cancel out, which is why the actual effect of making the nozzle thinner/converge is that it increases the back pressure within the engine (constricted space, smaller hole), essentially (idk how) increasing the efficiency of the fuel burning.
However, when the nozzle gets too thin, the exhaust becomes faster than its speed of sound. Since the pressure travels at the speed of sound, it can now not actually get back into the engine anymore. So that's the limit of how thin you can make the nozzle. The pressure has to get back into the engine to have its effect, so you can't make the exhaust travel faster than its speed of sound.
If any of this sounds wrong to anyone, let me know, I'm not an expert in this.
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It can look dumb, but I always had this question as a kid, what physical principles would prevent this?
Nah, I prefer using quantum spookiness for that. Send a steady stream of entangled particles to the other person on the moon first. Any time you do something to the particles on Earth, the ones on the Moon are affected also. The catch is that this disentangles them, so you have only a few limited uses. This is why you want a constant stream of them being entangled.
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As it so happens the way the atoms push each other is electromagnetism, in other words sending photons (same thing light is made of) to each other but these photons are not at visible wavelengths so you don't see them as light.
Wat? I strongly believe you are not correct. Which is to say, I think you are talking out of your arse entirely. If you push on a thing you peturb the electron structure of the material. These peturbations propagate as vibratory modes modeled as phonons.
While technically some of this energy is emitted as thermal radiation that is not primarily where it goes.
And how do you think the information that an electrically charged particle is moving reaches other electrically charged particles...
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And how do you think the information that an electrically charged particle is moving reaches other electrically charged particles...
My mistake, that's why sound travels at the speed of light.
It's just not useful to talk about this at the level of the standard model. We are interested in the bulk behaviour of condensed matter, the fact of the matter is that you will not be able to tell that the other end of the stick has been touched until the pressure wave reaches the end. It doesn't matter if individual force carriers are moving at the speed of light because they are not moving in a single straight line. You are interest in the net velocity.
