Space In General

[Famine]

So what if you bashed one end of the rod with a hammer, and listened on the other end?

That would be sound and would be governed by the speed of sound in that solid.

Sound is vibration, not just a single movement in one direction. Sound requires individual atoms to move back and forth in a specific manner and it takes time for each subsequent atom to adopt the specific vibration of its predecessor. A single, unidirectional shunt doesn't.

I remember having this discussion a few years ago, and one guy, an electrical engineering post-Doc, seemed to think that if you were to move the rod at one end, the other end wouldn't move at all. I didn't really understand his explanation at the time, but it was something to do with the other end being inobservable from the pusher's perspective. Or something. I don't really get this relativity lark. Practically, I can see this arising from the interaction of elasticity and inertia, though.

Yes - you'd need to imagine out materials science for this one. For a start, we've made a frickin' honking great space tube six trillion miles long. It's going to be heavy as hell - heavy enough to have its own planet-sized gravitational field - it needs to be attached on one end to a body that has an orbit 190 million miles across, attached on the other end to a body that probably also has an orbit of some variety, while, I should add, the Sun we're orbiting is moving at a million miles a day around the galaxy (which is also moving) and it'll be bombarded with spacecrap and asteroids (and Kang and Kodos) giving false positives.

It's ri-god-damn-diculous. But assuming we could make an impossibly long, impossibly rigid rod and push it in one direction, we could manage a phase velocity of greater than c for "messages" sent with it. Probably.

 

[Stevisiov]

In both instances Famine you are talking about infinitely rigid bodies, a carbon nanotube is exceptionally rigid, but is not infinitely. A carbon nanotube could not behave in the way you describe. A perfectly rigid body would not allow sound to propagate within it and it would of course, behave in the manner you describe, a shove would cause the whole body to move uniformly at exactly the same moment in time.

The Asker was talking about carbon tubes, which do not behave in such a fashion, although if you gave the internal electrons a shove instead with an electrical charge...

[edit]

A perhaps more interesting question is - How do quantum entangled particles react to each other at quicker than the speed of light (no information carried). You could almost say they are connected by an invisible, infinitely rigid beam.

 

[Griffith500]

That would be sound and would be governed by the speed of sound in that solid.

Sound is vibration, not just a single movement in one direction. Sound requires individual atoms to move back and forth in a specific manner and it takes time for each subsequent atom to adopt the specific vibration of its predecessor. A single, unidirectional shunt doesn't.

I was just thinking aloud there. Although, sonic booms are also sound. They're pretty much unidirectional shunts, leading practically to discontinuities in a supposedly continuous medium. Which is where it all falls down, there is no continuous medium except a vacuum. (Even then, who really knows?)

Basically, I don't think it's right to assume that both ends of an object begin moving at the exact same time when pushed from one end.

Yes - you'd need to imagine out materials science for this one. For a start, we've made a frickin' honking great space tube six trillion miles long. It's going to be heavy as hell - heavy enough to have its own planet-sized gravitational field - it needs to be attached on one end to a body that has an orbit 190 million miles across, attached on the other end to a body that probably also has an orbit of some variety, while, I should add, the Sun we're orbiting is moving at a million miles a day around the galaxy (which is also moving) and it'll be bombarded with spacecrap and asteroids (and Kang and Kodos) giving false positives.

It's ri-god-damn-diculous. But assuming we could make an impossibly long, impossibly rigid rod and push it in one direction, we could manage a phase velocity of greater than c for "messages" sent with it. Probably.

Assuming all of that, you would only need to move it very slowly (i.e. non-relativistic) to, say, flick a switch at the other end. Again, there is no guarantee that when you begin moving the rod, the other end moves at precisely the same time. I would wager that the force from one end takes a finite time to propagate all the way through the rod to the other end. That time is likely to be on the same scale as the time taken for a "sound wave" to travel that same distance, given the same physical phenomena that hold the thing together are used in transmitting the sound. Specifically: inter-atomic forces. You push one end, and the group of atoms you happen to be "holding" are pushed into the ones further up the rod (non-continuous medium) which then push the ones in front of them, etc. So, at what speed is kinetic energy exchanged?

EDIT: ^ That is a much more interesting question. Although I don't suppose anybody can really answer it. I was thinking of any material at those scales; although, really, the scale is irrelevant, since there will still be a delay in any real material. I missed the "impossibly rigid" part, though. Shame on me.

 

[Famine]

In both instances Famine you are talking about infinitely rigid bodies ... The Asker was talking about carbon tubes

I translated, presuming he was asking hypothetically but using the most rigid material he could think of. If we're doing a flat-out thought-experiment, we need to abandon the idea of what the tube is made of, or we end up shearing it off before we even get to a few miles off the deck.

A perhaps more interesting question is - How do quantum entangled particles react to each other at quicker than the speed of light (no information carried). You could almost say they are connected by an invisible, infinitely rigid beam.

Superluminal speeds without information exchange are pretty normal in this universe - X-rays in glass have a phase velocity well in excess of c.

An even better question is... how fast is gravity?

 

[Danoff]

The Dawn spacecraft has started science data collection at the asteroid Vesta. You can find daily images here:

http://dawn.jpl.nasa.gov/

 

[Keef]

The old space theories I grew up on made a lot of sense. So much sense, in fact, that you could easily explain the concepts to 5th graders and they would nod their heads in agreement.

But...how fast is gravity? What the hell kind of question is that, seriously. That'll get you nothing but funny looks from little kids.

 

[Stevisiov]

Superluminal speeds without information exchange are pretty normal in this universe - X-rays in glass have a phase velocity well in excess of c.

Indeed, hearing that the speed of light description for the speed limit of the universe does not sit well with me, it my eyes it should more commonly be referred to as the universal speed limit for information.

An even better question is... how fast is gravity?

Indeed, we all know what Einstein would say...

...and most theoretical calculations appear to agree with him. As far as I am aware however all attempts to practically prove this by measuring gravity waves (in fact to even register a gravity wave at all) have been unsuccessful, let alone any attempts to measure their speed.

The old space theories I grew up on made a lot of sense. So much sense, in fact, that you could easily explain the concepts to 5th graders and they would nod their heads in agreement.

But...how fast is gravity? What the hell kind of question is that, seriously. That'll get you nothing but funny looks from little kids.

I'm not sure what your point is.

Are you suggesting that modern physics should be simplified to the point it is wrong, so that it makes 5th graders feel blissful in their ignorance?

To a point, all science is simplified so that people can understand it easier, but we should be aiming to make people understand more complex concepts rather than simplifying the concepts too much, so that people find them easier to understand.

 

[BobK]

Let's try another thought experiment here. Let's attach a speaker cone to our 1 light year long rod. Let's play a tone at let's say 1 KHz in the speaker. The sound is going to travel through the rod at a given speed. Let's decrease the frequency to 100 Hz. It's still going to travel at the same speed. Okay, let's crank up the amplitude so the cone moves twice as far. Still no change in the speed of the sound through the rod. Okay, now let's really crank up the amplitude and lower the frequency so that the motion is indistinguishable from the nudge that famine wants to give it. The speed of the sound, or nudge if you will, is still the same as it always was.

If we're talking about an infinitely stiff rod here, by the way, the speed of sound through it would be infinite. So yeah, the experiment would work.

 

[Famine]

If we're talking about an infinitely stiff rod here, by the way, the speed of sound through it would be

Zero.

If the atoms cannot vibrate in relation to one another, they cannot transmit sound to each other.

 

[Griffith500]

Zero.

If the atoms cannot vibrate in relation to one another, they cannot transmit sound to each other.

Precisely, however we wouldn't need an infinitely rigid beam to reach the sun. Just a really, really rigid one.

Anyway, the "nudge" we're talking about here (via the loudspeaker) is at the particle velocity, not the phase velocity of the sound. The particle velocity is dependent on the amplitude and frequency of the displacement of the speaker cone, and so is far from constant. If the particle velocity gets close to the phase velocity, they are trans-sonic, and weird stuff starts to happen. Supersonic stuff is even more strange, and we now have non-linearities and discontinuities in the wave caused by the discrete nature of matter.

If the rod were made of one homogeneous block of "stuff" instead of discrete atoms it would be a different matter (literally...), any "signal" on one end would pass straight through to the other immediately. The two ends are essentially the same thing, though, so its volume has little meaning. In fact, "particles" are sometimes treated in this manner for the purposes of sound propagation anyway.

 

[Alex.]

Brilliant read so far guys.

 

[Dennisch]

An even better question is... how fast is gravity?

Decipher the E8 Lie Group, go find the Graviton particle, do the math and tell us.

Seriously, the way you spit out facts about anything, you are up there with Einstein and Newton.

 

[PeterJB]

The Dawn spacecraft has started science data collection at the asteroid Vesta. You can find daily images here:

http://dawn.jpl.nasa.gov/

I would argue from this image that Vesta be considered a Dwarf Planet. It has a roughly round shape certainly, far from being spherical though, plus the enormous crater in it's southern pole dosen't help. Although, it spins in less than 6 hours, so there would be a fairly hefty equatorial bulge as well which could also contribute to it's rather squashed appearence..

 

[Danoff]

It's a proto-planet, and it is enormous (for an asteroid). Part of the squashed appearance is most likely due to an impact in the southern hemisphere that took part of Vesta with it.

 

[Jawsers]

An even better question is... how fast is gravity?

I do believe Brian Greene brings this up in one of his books that I have. I might be able to find you the solution he presents.

 

[axletramp]

I have to admit, I had no idea my initial - and rather dubious - question would end up in such an interesting discussion. You guy's really know your stuff! 👍

Can't help feeling I created a bit of a monster...

 

[Vince_Fiero]

Decipher the E8 Lie Group, go find the Graviton particle, do the math and tell us.

Seriously, the way you spit out facts about anything, you are up there with Einstein and Newton.

I believe this is the goal of discussion.

Teach people Newton: some will have sufficient with this, might even forget that level.

Teach people Einstein: for those up to understanding something of it.

Teach people the questions that remain and get them to push the limits, even ask more questions, this will stimulate progress.

For people in doubt, most recent space theories have come first from maths and then were confirmed by experiments. Take the time to study (if that is your calling), even things that seems weird, it will help human kind sometime.

And remember sometimes it is the one that is lucky not the smartest that helps most.


=====================

I translated, presuming he was asking hypothetically but using the most rigid material he could think of. If we're doing a flat-out thought-experiment, we need to abandon the idea of what the tube is made of, or we end up shearing it off before we even get to a few miles off the deck.

In this hypothetical case: I see the material as one thin but super long atom.
The communication would be faster then light. Since you move a rigid object over a lightyears distance.
Just see 2 membranes connected with this rod acting as loudspeakers.

However the movement is not faster then light since you do not move that object faster then light.

Now all we need to do is create super long atoms without them turning into black holes.

Maybe a new question came up here: How do thin but super (several lightyears) long atoms behave in the special relativity theory?

Some other hypothesis needed: the distance stays the same or the atom can adapt to the changes. This brought me back to the input of Famine, if the planets have a difference reference from each other there must be some serious distortion on the line due to relativity of time.



I stick with the wormhole solution.

===========

An even better question is... how fast is gravity?

Did anyone see theories on how gravity moves? I believe Einstein tried to explain with curved space = gravity does not move, we just use the wrong set of reference.

 

[Dennisch]

^ I was sticking a feather up Famine's bum.

But seriously, are we gonna find the answer in math? With today's computer power, we should have found god, destroyed the fact that god excists, and find the true answer.

 

[Griffith500]

I dunno, I think there's a reason that many accomplished and knowledgeable mathematicians and physicists are deeply spiritual people.

Or maybe that's just due to the mind altering substances they need to abuse to come up with the maths and theories in the first place

 

[Danoff]

I dunno, I think there's a reason that many accomplished and knowledgeable mathematicians and physicists are deeply spiritual people.

Or maybe that's just due to the mind altering substances they need to abuse to come up with the maths and theories in the first place

That hasn't been my experience. I studied under many of these people and I work alongside them now. I know a ton of these people, and most of them are atheists - so much so that I assume that new people I meet at work are atheist.

 

[Dotini]

An even better question is... how fast is gravity?

I've wondered about this a bit in the past, and found two basic choices: It's the speed of light, or it's infinite.

Here's an explanation for why it might be infinite:

Newton’s gravity propagates at INFINITE speed, this is universally accepted as the basis for his theory, and it’s also the gravitational theory we used to calculate orbits and trajectories for the Apollo moon missions.

Take the simple observation of the Earth in orbit around the Sun. If gravity was delayed to the speed of light, the Earth would fly off its orbit after a mere 1200 years.

As viewed from the Earth’s frame, light from the Sun has aberration. Light requires about 8.3 minutes to arrive from the Sun, during which time the Sun seems to move through an angle of 20 arc seconds. The arriving sunlight shows us where the Sun was 8.3 minutes ago. The true, instantaneous position of the Sun is about 20 arc seconds east of its visible position, and we will see the Sun in its true present position about 8.3 minutes into the future. In the same way, star positions are displaced from their yearly average position by up to 20 arc seconds, depending on the relative direction of the Earth’s motion around the Sun. This well-known phenomenon is classical aberration, and was discovered by the astronomer Bradley in 1728.
…
If gravity were a simple force that propagated outward from the Sun at the speed of light, as radiation pressure does, its mostly radial effect would also have a small transverse component because of the motion of the target. … the net effect of such a force would be to double the Earth’s distance from the Sun in 1200 years. There can be no doubt from astronomical observations that no such force is acting. The computation using the instantaneous positions of Sun and Earth is the correct one. The computation using retarded positions is in conflict with observations.

There can be no doubt that gravity does indeed propagate at a speed faster than that of light.

Respectfully submitted,
Steve

 

[Vince_Fiero]

That hasn't been my experience. I studied under many of these people and I work alongside them now. I know a ton of these people, and most of them are atheists - so much so that I assume that new people I meet at work are atheist.

I believe that you get the point wrong.

My quantumphysics professor (at that time 64 years old) explained that he was choosing between fundamental physics and philosophy at the start of his career and that he thought fundamental physics was the future of philosophy at the time and that he never did regret his choice.

These people might be atheists, but that does not exclude that they are spiritual. Actually they believe in their maths and the incredible things these maths show, even if human kind never saw any prove of some of the things those maths have revealed.

If you say a black hole (space in general) is a creation of god or a singularity of differential equation does not matter that much, it is quite spiritual to talk about these phenomena that will probably never fundamentally influence our lives.

 

[Famine]

In this hypothetical case: I see the material as one thin but super long atom.

There'd be an issue with that. Quite a big one.

The atom would be six trillion miles long but one proton/neutron wide. Or about 2 femtometres (2 x 10^-15m) by 10 Petametres (1 x 10^16). It would be composed of approximately 5 x 10 ^30 protons and neutrons and weigh approximately 8.4 tonnes. The heaviest known atom is ununhexium-293 which weighs approximately 0.4 zeptograms - and that decays in 60ms. The heavist known stable atom is lead-208* which weighs approximately 0.3 zeptograms. You'd need to construct an atom that is 32 orders of magnitude heavier than the heaviest, stable atom known to date. And you'd need to do it in a supercollider that has a collision chamber that is in excess of a light year long - I'd be hazarding a guess that the collider would be in the order of tens of thousands of light years around, so each attempt would take more than tens of thousands of years.

And of course even atoms aren't infinitely rigid, or we couldn't pull them apart so easily. So you'd need to step down a level to quarks. At this point all hell breaks loose.

^ I was sticking a feather up Famine's bum.

But seriously, are we gonna find the answer in math? With today's computer power, we should have found god, destroyed the fact that god excists, and find the true answer.

God is nonfalsifiable.

I've wondered about this a bit in the past, and found two basic choices: It's the speed of light, or it's infinite.

Here's an explanation for why it might be infinite:

Newton’s gravity propagates at INFINITE speed, this is universally accepted as the basis for his theory, and it’s also the gravitational theory we used to calculate orbits and trajectories for the Apollo moon missions.

Take the simple observation of the Earth in orbit around the Sun. If gravity was delayed to the speed of light, the Earth would fly off its orbit after a mere 1200 years.

As viewed from the Earth’s frame, light from the Sun has aberration. Light requires about 8.3 minutes to arrive from the Sun, during which time the Sun seems to move through an angle of 20 arc seconds. The arriving sunlight shows us where the Sun was 8.3 minutes ago. The true, instantaneous position of the Sun is about 20 arc seconds east of its visible position, and we will see the Sun in its true present position about 8.3 minutes into the future. In the same way, star positions are displaced from their yearly average position by up to 20 arc seconds, depending on the relative direction of the Earth’s motion around the Sun. This well-known phenomenon is classical aberration, and was discovered by the astronomer Bradley in 1728.
…
If gravity were a simple force that propagated outward from the Sun at the speed of light, as radiation pressure does, its mostly radial effect would also have a small transverse component because of the motion of the target. … the net effect of such a force would be to double the Earth’s distance from the Sun in 1200 years. There can be no doubt from astronomical observations that no such force is acting. The computation using the instantaneous positions of Sun and Earth is the correct one. The computation using retarded positions is in conflict with observations.

There can be no doubt that gravity does indeed propagate at a speed faster than that of light.

The big problem with that assessment is the assumption made at the end.

"If gravity were a simple force that propagated outward from the Sun"

Even classical mechanics of gravity disagree with this assumption - it's a simple force which propagates outwards from any source of mass (see later). The Earth isn't affected by the Sun's gravity, the Earth-Sun system is a result of the gravity of both the Earth and the Sun - and in fact larger than that, the Solar System is a result of the gravity of every body within it, but that's a story for a more complex day.

However, classical, Newtonian gravity agrees that gravity moves at infinite speed. If we were to pull the Sun out of existence right now, nothing would continue orbiting it, but head straight off in the direction it was going last. No mass = nothing to cause gravity.


General relativity disagrees. For a start, it's not just mass but the velocity of the mass that causes gravity - and in fact more than that, it's energy (mass times velocity = kinetic energy). For a follow-up, it doesn't affect the bodies in the system, but space-time itself.

Moving on to the lovely "rubber-sheet" method of imagining space-time and gravitational influence upon it, if you dunk a large, energetic mass like... oooooh, the Sun onto the rubber sheet, it bends under it. If you wink out that mass in an instant, the rubber sheet snaps back into place. But if you watch it on a high speed camera, you'll see that it takes time to do it, oscillates around the plane of the sheet (sometimes the sun-point will be higher than the plane, sometimes lower) and sends out ripples across it, like dunking a rock into a pond. And that speed is constrained to... guess what?

And that's where the second problem with the quote comes in:

"its mostly radial effect would also have a small transverse component because of the motion of the target"

It would also have an equal and opposite small transverse component, which would cancel that out, because of the motion of the origin! Gravity pulls the Earth towards the Sun, not in the position the Sun is now but the position it was 8.3 minutes ago, while gravity also pulls the Sun towards the Earth, not in the position the Earth is now but the position it was 8.3 minutes ago.

It turns out that the speed of gravity is more or less the speed of light - it might actually be the speed of light but it's very hard to say for certain unless it turns out that the graviton has a rest-mass (then it won't be).


*Fun fact - one atom of lead-208 will kill a star in a few seconds. That's kill a star in a few seconds with one atom.

 

[Danoff]

I believe that you get the point wrong.

My quantumphysics professor (at that time 64 years old) explained that he was choosing between fundamental physics and philosophy at the start of his career and that he thought fundamental physics was the future of philosophy at the time and that he never did regret his choice.

These people might be atheists, but that does not exclude that they are spiritual. Actually they believe in their maths and the incredible things these maths show, even if human kind never saw any prove of some of the things those maths have revealed.

If you say a black hole (space in general) is a creation of god or a singularity of differential equation does not matter that much, it is quite spiritual to talk about these phenomena that will probably never fundamentally influence our lives.

Accepting math or physics is not spiritual. It's logical.

 

[Griffith500]

Accepting math or physics is not spiritual. It's logical.

And that's the problem with some of the scientific community, in my experience. They can seem a little bit giddy on the supposed power of science, and forget that it is a totally human invention. Not that I mean to discredit science or scientists, I mean the very nature of their work is often so tightly constrained that they rarely get the opportunity for a bigger-picture philosophy. "Accepting" is also an interesting choice of wording, there.

To think that all of mathematics is conceived by people, and yet it can describe so many things in the universe, is that not more than just "logic"?
What is the logic behind infinitesimals, other than a means to an end? Isn't there debate as to their validity, and yet calculus pretty much kick-started the industrial revolution by influencing the "classical" studies of thermodynamics, among other things. Why should something so tenuous be so effective?

 

[Danoff]

And that's the problem with some of the scientific community, in my experience. They can seem a little bit giddy on the supposed power of science, and forget that it is a totally human invention. Not that I mean to discredit science or scientists, I mean the very nature of their work is often so tightly constrained that they rarely get the opportunity for a bigger-picture philosophy. "Accepting" is also an interesting choice of wording, there.

To think that all of mathematics is conceived by people, and yet it can describe so many things in the universe, is that not more than just "logic"?
What is the logic behind infinitesimals, other than a means to an end? Isn't there debate as to their validity, and yet calculus pretty much kick-started the industrial revolution by influencing the "classical" studies of thermodynamics, among other things. Why should something so tenuous be so effective?

Fundamentally, mathematics is neither tenuous nor invented by man. It's a language that describes reality (logic). The logic exists independent of humanity. If we met an advanced alien civilization tomorrow, they would understand mathematics too. They'd have different symbols, different conventions, etc. but they'd understand the fundamentals.

 

[Famine]

Indeed. Stargate SG-1 touched on this with the universal translator of the Four Great Races - a holographic projection of the chemical elements forming a common language throughout the universe.

 

[Griffith500]

Fundamentally, mathematics is neither tenuous nor invented by man. It's a language that describes reality (logic). The logic exists independent of humanity. If we met an advanced alien civilization tomorrow, they would understand mathematics too. They'd have different symbols, different conventions, etc. but they'd understand the fundamentals.

Except that everything we do is subject to our interpretations, our so-called "sentience" and awareness - are these things really absolutes? Even then, would the existence of such a "universal language" not perturb you somewhat? It would me. Besides, how would you know the aliens wouldn't take that Boron atom the wrong way?

Logic, though, is a human invention - it is about reasoning, a human process, and is not only constrained to established mathematics, but extends to philosophy in general. What may seem "logical" may not at all be "reality", and vice versa. Sure, you can make up new rules to fit these discrepancies, and all will be well.

 

[Danoff]

Except that everything we do is subject to our interpretations, our so-called "sentience" and awareness - are these things really absolutes? Even then, would the existence of such a "universal language" not perturb you somewhat? It would me. Besides, how would you know the aliens wouldn't take that Boron atom the wrong way?

Logic, though, is a human invention - it is about reasoning, a human process, and is not only constrained to established mathematics, but extends to philosophy in general. What may seem "logical" may not at all be "reality", and vice versa. Sure, you can make up new rules to fit these discrepancies, and all will be well.

The field of logic (not the same as the colloquial use of the word "logical") just like the field of mathematics, is a recognition of the fundamental behavior of the universe. It is subject to our understanding, but it is not subject to our interpretation.

The logical construct modus tollens is not a human invention - it's a description of the behavior of the universe, just like the mathematical concept of addition is not a human invention but a description of the universal concept of grouping objects.

 

[Griffith500]

So what you're really saying is that, subject to an array of sub-sets of different rules and definitions, mathematics describes the universe, partition by partition?

Modus tollens is pretty robust, I won't argue with that. It's definitely worked for us so far.

I am right in saying that "our" understanding of reality is still "incomplete", though, aren't I?