Understanding the Speed of Force: Instantaneous vs. Speed of Light

[Pierre007080]

Gravity is instantaneous in Newtonian physics. In general relativity it is not.


That is a common argument used against general relativity by those who do not understand it. If that is all there was to general relativity that argument would be correct. However, there is more (a whole lot more) to general relativity than a finite propagation time for gravity. Other terms such as frame dragging nearly cancel the effects of frame dragging. For example, for planets far from the Sun and moving at relatively slow speeds compared to light speed it looks very much like gravity does propagate instantaneously. This is not the case for Mercury. That general relativity explained a known defect with the Newtonian explanation of Mercury's orbit was one of the reasons general relativity won the day over Newtonian mechanics.


Please explain which aspect of General Relativity has to do with gravity being intstantaneous. Frame dragging has other explanations, has it not?

 

[D H]

Please explain which aspect of General Relativity has to do with gravity being intstantaneous. Frame dragging has other explanations, has it not?

I never said it did. Gravity has a non-zero propagation time in general relativity. Specifically, the speed of light.

I did make a mistake in my previous post. I said "However, there is more (a whole lot more) to general relativity than a finite propagation time for gravity. Other terms such as frame dragging nearly cancel the effects of frame dragging."

That should read "However, there is more (a whole lot more) to general relativity than a finite propagation speed for gravity. Other terms such as frame dragging nearly cancel the effects of this finite propagation speed."

Sorry for any confusion.


There are competing non-crackpot theories of gravity. They all have a finite propagation speed for gravity. They all have something analogous to frame dragging. Most are a lot more complex than general relativity and have lots of magic numbers (tuning parameters). General relativity has but one magic number, G. Until some experimental result shows that general relativity is wrong on some account, it wins by virtue of being the simplest theory that matches known facts.

 

[A.T.]

I do not think that electric and gravitational fields should be used as a comparison

I did not say that they are the same. I just gave you an example of a field which propagates at a finite speed and yet pulls towards the current position of a inertially moving source, and not the position where it was emitted. Just because an attractive field was emitted at a certain position, it does not necessarily mean that the it still pulls towards that position at a certain distance.

 

[VincentJL]

No it isn't. It is a human word used to describe a natural phenomena. Time exists whether we are here to observe it or not.

This is not a scientific claim. Though you may hold this belief yourself, it is no more supported by evidence than the existence of an Intelligent Designer. There is a fine line between science and philosophy, and one should tread lightly when close to it

 

[russ_watters]

It is a starting assumption upon which science must be based and all of our theories depend.

 

[D H]

This is not a scientific claim.

I disagree. Russ' claim, "Time exists whether we are here to observe it or not," is very much a scientific claim. It touches upon the core motivation for science: That there exists a material, physical world out there that operates by some set of rules independent of us. Scientists are not solipsists.

 

[VincentJL]

Science is not a religion, and is very much based on the notion that all models are wrong, but some are usefull. You could state simply that science does not claim that which it cannot prove.

To argue that the world IS indeed in one way or another is, no matter which way you would argue, a philosophical discussion.
It's true, that science wouldn't be of much use if indeed all the models where wrong enough to be unusefull, but this does not change the fact, that simply because a model is usefull, it should be considered true. Which would also seem a very stupid notion, given the history of science, where one "True" theory after another have fallen pray to falsification when new phenomena was discovered.

Whether the statement that "Time exists whether we are here to observe it or not" is true or false is not something any scientist should spend his time trying to answer, the more important question is, if we assume this as a model, what are the consequences, and are these such that we can falsify the model. If we cannot falsify the model, then we examine what usage we can find with model. Of cause in the end, if the model is without uses, and cannot be falsified, then it is simply rubbish (even if true).

But of cause this debate has already digressed, I simply though I would add to it, that the subject is of cause philosophy and not science, and debating philosophy often brings very little reward with it.

 

[D H]

Science is not a religion, and is very much based on the notion that all models are wrong, but some are usefull.

I disagree. I'll keep the discussion to physics, but the same discussion applies to other sciences.

Up until the turn of the 20^th century, physicists thought their models were absolutely correct. The statement that "science is very much based on the notion that all models are wrong" is historically false. Except for the few physicists who aspire to the most austere interpretation of "shut up and calculate," it is not true today. For one thing, that would make theoretical physicists little more than glorified engineers. For another, physicists would not spend time debating which is the correct interpretation of quantum mechanics if "science is very much based on the notion that all models are wrong". Finally, even if that statement were true, that does not mean that the truth is not out there. The fact is that the vast majority of physicists still believe the truth is out there -- and it is their job to find it, or at least a better approximation of the truth.

The view that "all models are wrong" is a mischaracterization of how science works. Popper's philosophy of science is a bit naive. That physicists and engineers still use Newtonian mechanics to this day falsifies Popper's philosophy of science. Newtonian mechanics has not been falsified; it has instead been shown to be of limited (but very useful) domain. Suppose some physicist or team of physicists come up with a Theory of Everything. This will not falsify quantum mechanics and general relativity. It will instead show that QM and GR are themselves of limited domain.

 

[QuantumPion]

...For one thing, that would make theoretical physicists little more than glorified engineers.

Hey...what you you have against engineers?

 

[D H]

Me personally? Nothing. I've been working as an engineer for the last thirty years. Scientists apparently do; I was more-or-less parroting words from the recent Copenhagen interpretation thread.

 

[Winter Flower]

Because the messenger particles that mediate the force only travel at c.

Hi. Can someone explain more about the theories that say forces are
mediated by particles? So is it that the type of particle determines
the speed of propigation?

For electromagnitism the particles are photons, which move at c and elecromagnetic waves = light which also propigates at c?

 

[russ_watters]

It is a starting assumption upon which science must be based and all of our theories depend.

While what I and DH said is correct here, it is actually too weak and incomplete.

The idea that the laws of the universe are consistent throughout the universe isn't just a starting postulate for the whole of science, it is a testable theory as well. It is contained explicitly in Special Relativity as a postulate, but by testing the theory, we test the postulate. Put another way: Every time a new experiment is performed or a new observation is made in space, this fundamental postulate is tested and the theory that the laws of the universe are consistent throughout the universe gains a higher level of proof. At this point, the possible room for this postulate to not be true is very, very small.

 

[russ_watters]

Just to add to what DH said:

Which would also seem a very stupid notion, given the history of science, where one "True" theory after another have fallen pray to falsification when new phenomena was discovered.

Science is the endless pursuit of the "True" laws of the universe. That individual scientists have been confident that their theory is that True law is not a failure of science - it isn't even a failure of human nature. It is the personal belief/confidence by a scientist that they are capable of finding the True laws that drives them to look. And the self correcting nature of the scientific process encourages - even requires - scientists to make errors for the purpose of learning from them.

Note that you are no longer arguing that True laws don't exist, but only that we haven't found them yet or aren't capable of finding them. This is, of course, irrelevant to whether or not they exist and you are now providing an argument that requires that they exist.

Whether the statement that "Time exists whether we are here to observe it or not" is true or false is not something any scientist should spend his time trying to answer...

Indeed any scientist who investigates the nature of time is investigating precisely that question.

But of cause this debate has already digressed, I simply though I would add to it, that the subject is of cause philosophy and not science, and debating philosophy often brings very little reward with it.

Though it has digressed, sometimes we find (particularly with the subject of time) that people don't have a clear understanding of what the point of all this is. So it is good to remind them sometimes.

 

[pgardn]

It is also fairly obvious to me that humans can only understand and perceive a certain amount of what might be present in the universe. We are limited by our evolutionary history. We are not infallible machines. We use metaphor and analogy to try and model some things that don't lend themselves to our senses built on everyday experience. We use machines to turn events we cannot perceive into light and sound... we are highly visual animals that might be missing a heck of a lot. The tremendous invention of logic and math help us predict and further more lead us to look for things we might have never have expected to even exist. In fact our "common sense" based on the world we sense directly (which play a major role in the proper development of our brain) can do us a diservice in some cases. I don't mean to sound like Plato, that observation creates confusion, but I have to be humbled by how limited we are. And amazed how clever some people can be.

And I still don't understand how some particles separated can apparently "inform" one another of the state they are in simultaneously. I don't get this at all. I may be misinterpreting what is a very difficult idea... Its the Bell stuff and entangled particles. The experiments done with this stuff seem to have consequences that are apparently consistent with quantum mechanics but seem to violate some rules I may have wrongly attributed to the idea of simultaneous events. Maybe this is entirely the wrong section to bring this up, but the original post and responses thereafter got me thinking about this again. I am led to the belief we have to be careful how we think about time based on some of the experiments done. Or maybe I am just entirely missing something.

 

[Gear300]

It is also fairly obvious to me that humans can only understand and perceive a certain amount of what might be present in the universe. We are limited by our evolutionary history. We are not infallible machines. We use metaphor and analogy to try and model some things that don't lend themselves to our senses built on everyday experience. We use machines to turn events we cannot perceive into light and sound... we are highly visual animals that might be missing a heck of a lot. The tremendous invention of logic and math help us predict and further more lead us to look for things we might have never have expected to even exist. In fact our "common sense" based on the world we sense directly (which play a major role in the proper development of our brain) can do us a diservice in some cases. I don't mean to sound like Plato, that observation creates confusion, but I have to be humbled by how limited we are. And amazed how clever some people can be.

And I still don't understand how some particles separated can apparently "inform" one another of the state they are in simultaneously. I don't get this at all. I may be misinterpreting what is a very difficult idea... Its the Bell stuff and entangled particles. The experiments done with this stuff seem to have consequences that are apparently consistent with quantum mechanics but seem to violate some rules I may have wrongly attributed to the idea of simultaneous events. Maybe this is entirely the wrong section to bring this up, but the original post and responses thereafter got me thinking about this again. I am led to the belief we have to be careful how we think about time based on some of the experiments done. Or maybe I am just entirely missing something.

Common sense is not necessarily truth, whereas even truth is not necessarily truth...though I guess we do have to be more careful when thinking scientifically.

 

[pgardn]

Common sense is not necessarily truth, whereas even truth is not necessarily truth...though I guess we do have to be more careful when thinking scientifically.

This really does not get us anywhere. We have to have some set of conventions we agree upon in Science even though we should be careful in examining their potential of becoming inadequate based on experimental evidence. I personally am having difficulty with the whole action at a distance idea and time because of the findings in some of these entagled particle experiments.

 

[russ_watters]

It is also fairly obvious to me that humans can only understand and perceive a certain amount of what might be present in the universe. We are limited by our evolutionary history. We are not infallible machines.

Some of those statements are true, but the first does not follow from them. You are assuming it requires an intelligence beyond what humans posess in order to understand everything. Maybe it does and maybe it doesn't, but the only piece of evidence we have to go on about that is that we are still learning - so if that point exists, we aren't there yet.

 

[Red_CCF]

Assuming you are applying a force to something, then the effects of that force travel through the object at the speed of sound in that object. Much slower then the speed of light.

How do we actually know that?

 

[pgardn]

Some of those statements are true, but the first does not follow from them. You are assuming it requires an intelligence beyond what humans posess in order to understand everything. Maybe it does and maybe it doesn't, but the only piece of evidence we have to go on about that is that we are still learning - so if that point exists, we aren't there yet.

I don't know what an intelligence beyond really means. Maybe a different sort of intelligence that arises due to a very diff way of actually being able to assess the world around us through senses which might lead to the evolution of a brain that views the world differently due to the input it received while going through development. We are clearly slaves to our evolutionary history. A species of dolphin that lives in hopelessly turbid water clicks and receives echoed signals that help it get a "picture" of the world around it. I don't know what smooth hard rock vs. an algae covered rock "looks" like to this animal. And I am very sure it will not be doing science. But the way this animal's brain develops to help it survive and survey its world must be incredibly different than ours. So its perception might lead the animal to, if it had our inquisitive brain that recognizes and ponders its own existence and what the future holds, might lead to a very diff. sort of thought process. (crazy example, but I am very attuned to how sensory input affects the development of a nervous system, especially complex nervous systems that are not totally involuntary) I also realize this dolphin is not typing on a Physics forum.

Imo we will always be learning and models will always require modification as experimental methods become refined or advances in technology allow us to look at phenomena in a new light. It seems to me that sometimes particular generations get very full of themselves and seem convinced they are at the pinnacle of understanding. I just see shifts in models and new ways of looking at problems. Imo we will never understand everything. Maybe my view is way off. Sorry for running on so.

Anyways... still perplexed about what seems to be a simultaneous relay of information.

 

[russ_watters]

How do we actually know that?

The implications of that are vast and as such the evidence is as we'll. One large field dedicated to the concept is mechanical vibrations.

 

[russ_watters]

I don't know what an intelligence beyond really means.

Well that's the point: you are postulating the existence of a beyond when we have yet to reach a limit. We don't know if there is a beyond!

Anyways... still perplexed about what seems to be a simultaneous relay of information.

Maybe we should get back to that. Here's how the guy with the laser pointer thing works:

Person 1 points a laser pointer at a person 2 and turns it on. A finite amount of time later, person 2 sees it go on. The information of the lighting of the laser pointer is transferred from person 1 to person 2 at the speed of light. Let's say, that took 1 second. Then, person 1 sweeps the laser from person 1 to person 2. It takes 1 second for person 1 to see the laser start to move away from him, then an arbitrarily small time later, person 3 sees the laser. He's also 1 light second from person 1. So, did information get transferred from person 2 to person 3 in a very short time? No, the information is the disposition of the laser. It gets transferred from person 1 to both person 2 and person 3, not from person 2 to person 3. When person 3 sees the laser, he sees it because 1 second before, person 1 started to move it.

Understand?

 

[pgardn]

Well that's the point: you are postulating the existence of a beyond when we have yet to reach a limit. We don't know if there is a beyond! Maybe we should get back to that. Here's how the guy with the laser pointer thing works:

Person 1 points a laser pointer at a person 2 and turns it on. A finite amount of time later, person 2 sees it go on. The information of the lighting of the laser pointer is transferred from person 1 to person 2 at the speed of light. Let's say, that took 1 second. Then, person 1 sweeps the laser from person 1 to person 2. It takes 1 second for person 1 to see the laser start to move away from him, then an arbitrarily small time later, person 3 sees the laser. He's also 1 light second from person 1. So, did information get transferred from person 2 to person 3 in a very short time? No, the information is the disposition of the laser. It gets transferred from person 1 to both person 2 and person 3, not from person 2 to person 3. When person 3 sees the laser, he sees it because 1 second before, person 1 started to move it.

Understand?

With light... yes.

So now explain this.
Two entangled particles are separated. The spin on one is changed and the spin on the other one changes simultaneously to mimic its partner. There is no "time" delay. It is considered instantaneous by experimentation. The information has been transferred instantaneously. You can continue... help... So this is all just an observer/ frame of reference problem? It has not been presented that way.

 

[pgardn]

Well that's the point: you are postulating the existence of a beyond when we have yet to reach a limit. We don't know if there is a beyond! Maybe we should get back to that.

I don't see this as a beyond. Different is better for me. But I see your point. Maybe I should add that I think the program sending out radio signals and trying to pick up radio signals in order to communicate with something else out there was/is rather silly. Sending pi via EM waves... I found this terribly strange for anyone who understands the history of life on earth, that somehow human-like intelligence is inevitable via evolution. Maybe this statement helps explain where my thoughts above come from.