There's another question regarding momentum conservation.Imagine a distant galaxy on the edges of the visible universe. The matter in the galaxy possesses momentum which contributes to the absolute momentum of the visible universe. If space expansion between two points gets greater than the speed of light, then the galaxy finally leaves our personal universal event horizon - and with it its momentum. From our POV, the momentum of the system "universe" seems to change (i.e. part of it is seemingly lost), because space expansion becomes greater than the speed with which information can be transferred (speed of light). One moment we know the "countable" momentum is X, the next moment it becomes X-N."What can't be measured, does not exist" (I hope this phrasing is correct). What are the consequences for momentum conservation, going by this example? Are there any?Cheers,Xpl0rer
Quote from: Xpl0rer on 03/24/2013 02:24 pmImagine a distant galaxy on the edges of the visible universe. The matter in the galaxy possesses momentum which contributes to the absolute momentum of the visible universe. If space expansion between two points gets greater than the speed of light, then the galaxy finally leaves our personal universal event horizon - and with it its momentum. From our POV, the momentum of the system "universe" seems to change (i.e. part of it is seemingly lost), because space expansion becomes greater than the speed with which information can be transferred (speed of light). One moment we know the "countable" momentum is X, the next moment it becomes X-N.This is the heart of the scenario I posted a few weeks ago. We have one universe which started expanding from one big bang. At first, everything, all mass, was in the same light cone. Then some of it expanded out of the light cone. It's as if a significant part of the universe is disappearing with each successive second.The phrase, "I sense a great disturbance in the force" comes to mind.Also, where the heck would this boundary be? There's a star or galaxy on one side of the boundary, which we can no longer see. But is that boundary also expanding faster than the speed of lite? Then we could never see the boundary.So when these theoretical physicists say that the inertia here is caused by mass there, how can that be? There is literally no "there" there; the boundary is receding too fast. How can that distant mass affect local inertia instantaneously?
Imagine a distant galaxy on the edges of the visible universe. The matter in the galaxy possesses momentum which contributes to the absolute momentum of the visible universe. If space expansion between two points gets greater than the speed of light, then the galaxy finally leaves our personal universal event horizon - and with it its momentum. From our POV, the momentum of the system "universe" seems to change (i.e. part of it is seemingly lost), because space expansion becomes greater than the speed with which information can be transferred (speed of light). One moment we know the "countable" momentum is X, the next moment it becomes X-N.
There are no consequences for momentum conservation.Momentum conservation applies whether the parts of the system are visible to a particular observer or not.In your example, there are two systems, one that includes the distance galaxy and one that does not. These two systems have different momenta. In each system, the momentum is conserved. At the point in time where the distance galaxy passes out of the region where you could ever receive information about it again, those two systems still exist, and each still has exactly the same momentum it did before. The only difference is that the "visible universe" from your point of view changed from one of the two systems to the other. Each system still independently conserves its own momentum.Conservation of momentum doesn't care a bit about visibility. It keeps right on going independently of whether any particular observer can ever receive information from all the parts of the system, even if no observer exists that can receive information from all parts of the system.
Quote from: ChrisWilson68 on 04/03/2013 05:13 amThere are no consequences for momentum conservation.Momentum conservation applies whether the parts of the system are visible to a particular observer or not.In your example, there are two systems, one that includes the distance galaxy and one that does not. These two systems have different momenta. In each system, the momentum is conserved. At the point in time where the distance galaxy passes out of the region where you could ever receive information about it again, those two systems still exist, and each still has exactly the same momentum it did before. The only difference is that the "visible universe" from your point of view changed from one of the two systems to the other. Each system still independently conserves its own momentum.Conservation of momentum doesn't care a bit about visibility. It keeps right on going independently of whether any particular observer can ever receive information from all the parts of the system, even if no observer exists that can receive information from all parts of the system.I agree that "visibility" should have no consequences for whether momentum is conserved or not, omniversally speaking. I just see a problem arising in the form that there could be a situation in which some distant matter of the universe and its momentum gets separated by superluminal spacial expansion and thus leaves "our" space-time domain that's defined by "our" light cone.
Quote from: ChrisWilson68 on 04/03/2013 05:13 amThere are no consequences for momentum conservation.Momentum conservation applies whether the parts of the system are visible to a particular observer or not.In your example, there are two systems, one that includes the distance galaxy and one that does not. These two systems have different momenta. In each system, the momentum is conserved. At the point in time where the distance galaxy passes out of the region where you could ever receive information about it again, those two systems still exist, and each still has exactly the same momentum it did before. The only difference is that the "visible universe" from your point of view changed from one of the two systems to the other. Each system still independently conserves its own momentum.Conservation of momentum doesn't care a bit about visibility. It keeps right on going independently of whether any particular observer can ever receive information from all the parts of the system, even if no observer exists that can receive information from all parts of the system.It should be clear that the momentum within an inertial system is constant and the vectors annihilate time averaged. However, when some distant matter leaves "our" light cone, the momentum vectors in "our" system don't add up to Zero anymore. A factual imbalance has occurred.
Quote from: ChrisWilson68 on 04/03/2013 05:13 amThere are no consequences for momentum conservation.Momentum conservation applies whether the parts of the system are visible to a particular observer or not.In your example, there are two systems, one that includes the distance galaxy and one that does not. These two systems have different momenta. In each system, the momentum is conserved. At the point in time where the distance galaxy passes out of the region where you could ever receive information about it again, those two systems still exist, and each still has exactly the same momentum it did before. The only difference is that the "visible universe" from your point of view changed from one of the two systems to the other. Each system still independently conserves its own momentum.Conservation of momentum doesn't care a bit about visibility. It keeps right on going independently of whether any particular observer can ever receive information from all the parts of the system, even if no observer exists that can receive information from all parts of the system.Concerning Woodward's effect, there's something bothering me. ...
Hello,there's a thought experiment concerning propulsion I'd like to present. Imagine two objects with different mass in free space, being apart 1ly. They are connected by an ideal rope of negligible mass. As is known, space is expanding at a rate of about (21.25km/s)/Mly or (2.125cm/s)/ly. Since the amount of space between the objects is increasing and the rope prevents the objects from moving apart, they experience a net diametral force, each pulling on the other object.Is my assumption correct, that the object with more mass pulls stronger on the object with less mass (since both objects are being moved with their local space-time section) and thus the less mass-rich object moves away from a nearby free observer? Wouldn't this also be a propellantless propulsion, or am I missing something?Best regards
By using this "100 feet away" picture, I think you ignore a relevant qualitative difference. In your example, the ejected propellant is still within a volume that is connected to the rocket via light speed (i.e. rocket and propellant are within a causally connected volume) - even if the propellant is outside your arbitrary set system limits.
I think that what you don't quite realize is that a superluminal spacial expanison between very distant objects irrecoverably cuts off the causal connectivity between those objects.
This is not trivial and can't be ignored by simply saying "the vanished matter still retains its impulse". It is scientifically impossible to state what happens or not outside our light cone.
Our causal reality is defined by the things we can interact with. And that's exactly the stuff that's connected by causality (i.e. light speed).I agree that, purely logically, the momentum of the vanished matter should be the same in itself even after the vanishing outside our light cone. But I see a problem here, too. Momentum is a vector, which is coordinate system dependent. But if the matter leaves "our" coordinate system via spacial expansion, are those systems still equivalent? Can this be falsified in any way? I doubt it.
I just want to say with all of that that reality isn't that simple, even if some would like to have it that way.
I don't know if Woodward's math or explanations have anything to do with how reality works. Math is IMO just a tool that (sometimes) allows for good enough approximations of what reality does. Until the models need improvement.
I personally find the idea of waves traveling back in time from distant matter to justify the Mach effect hair-raising, to say the least. But hey, what do I know.
Also, where the heck would this boundary be? There's a star or galaxy on one side of the boundary, which we can no longer see. But is that boundary also expanding faster than the speed of lite? Then we could never see the boundary.
So when these theoretical physicists say that the inertia here is caused by mass there, how can that be? There is literally no "there" there; the boundary is receding too fast. How can that distant mass affect local inertia instantaneously?
Thanks, QuantumG :) .I'd like to expand on that thought experiment and change it slightly. Imagine the two objects being identical viewing platforms. Two astronauts of equal mass are placed on them, their heads facing towards each other. Since there is a constant spacial displacement going on, the astronauts are being "pushed" against their respective viewing platforms. I think this should have the same effect as gravity would have for them, as long as they are standing on the platforms which are connected via the aforementioned rope. What do you think?
There are no consequences for momentum conservation.Momentum conservation applies whether the parts of the system are visible to a particular observer or not.... At the point in time where the distant galaxy passes out of the region where you could ever receive information about it again, those two systems still exist, and each still has exactly the same momentum it did before. The only difference is that the "visible universe" from your point of view changed from one of the two systems to the other. Each system still independently conserves its own momentum.Conservation of momentum doesn't care a bit about visibility. It keeps right on going independently of whether any particular observer can ever receive information from all the parts of the system, even if no observer exists that can receive information from all parts of the system.
I think you have a fundamental misunderstanding of the idea of conservation of momentum.Conservation of momentum does not say that distant masses instantaneously affect local masses. What it does say is that whenever two masses do affect each other through a force, the effects on one of the masses will be equal and opposite to the effects on the other mass.As long as there are no external forces, the combined mass of the entire system will be constant. This does not require any instantaneous action at a distance. It doesn't require any action at a distance at all. If two masses interact while close and then move a great distance away and stop having a force between them, mass will continue to be conserved for the whole system simply by the fact that each will continue to have the same momentum from one moment to the next as they fly away from each other.
It should be clear that the momentum within an inertial system is constant and the vectors annihilate time averaged. However, when some distant matter leaves "our" light cone, the momentum vectors in "our" system don't add up to Zero anymore. A factual imbalance has occurred.
The system of "all the mass currently in my light cone" is *not* a closed system. Mass is being removed from it over time. You can't expect momentum to be the same before and after you remove some of the mass from that system.
Here's an analogy (an imperfect analogy, but one that might give some intuitive feel for what is going on): Imagine a flat merry-go round. Place several rocks on one side and several others on the other side. Tie one of the rocks from one side to another rock on the other side with a different mass.
But causal connectivity has nothing to do with conservation of momentum. Momentum continues to be conserved in a system even if two parts of that system can no longer have any effect on one another.
It doesn't leave our coordinate system. It only leaves our light cone. It's still in our coordinate system.
@ JohnFornaroI don't think I understand what you mean. Why should something happening "an eternity away" affect any local momentum? It sounds more like magic than physics to me.