Apparent Inconsistency in General Relativity

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I know that this is probably me not knowing enough about the theory but I have noticed a seeming inconsistency in the theory. It follows thus:

1. Nothing can move faster than light.

2. Mass distorts the curvature of spacetime.

3. A change in an object's position changes the distortion.

4. This distortion would either have to travel, causing strange effects with the gravity of the universe and moving high-mass objects, or be instantaneous, thus in violation of one of the theory's postulates.

Can anybody help me with this? I'm kinda stuck on this one.

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Gravity waves travel at c.

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Small changes in an object course small changes in the gravitational field that travel at the speed of light, as ydoaPs has stated.

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3. A change in an object's position changes the distortion.

The object isn't moving in its own frame, unless it is accelerating. The distortion travels with the object, and is static in that frame. If there is an acceleration, then you get gravity waves.

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• 3 months later...

I know that this is probably me not knowing enough about the theory but I have noticed a seeming inconsistency in the theory. It follows thus:

1. Nothing can move faster than light.

2. Mass distorts the curvature of spacetime.

3. A change in an object's position changes the distortion.

4. This distortion would either have to travel, causing strange effects with the gravity of the universe and moving high-mass objects, or be instantaneous, thus in violation of one of the theory's postulates.

Can anybody help me with this? I'm kinda stuck on this one.

I can help with some of it:<br style="">

1) Improperly stated. There are particles that travel faster than the speed of light (tachyons, neutrinos, etc). An object traveling slower than the speed of light cannot be "ACCELERATED" beyond the speed of light. A particle can be “BORN” faster than the speed of light. Generally these are considered MASSLESS particles but even that is up in the air. It is a subtle but very important difference.

2) Correct, but no one understands how it works YET. Further more, if you believe there are more unobservable dimensions (I do), then a particle can appear to travel vastly faster than the speed of light although it probably just took a shortcut/path through a dimension (space-time) that you cannot observe. For instance, the quantum mechanical circumstance coined by Einstein as "Spooky action at a distance" appears to us as if two particles with vast amounts of distance between them seem to both react simultaneously to a stimulus applied to only one of them. These particles are paired by some yet unknown force/dimension. It is likely that the particles are actually the same particle connected via multi dimensional space. No one truly understands these phenomena YET. But they are real.

~MUTANT

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Hi MUTANT, welcome to sfn. There's a few points that are wrong in your post and I hope you don't mind me correcting them.

There are particles that travel faster than the speed of light (tachyons, neutrinos, etc). An object traveling slower than the speed of light cannot be "ACCELERATED" beyond the speed of light. A particle can be “BORN” faster than the speed of light. Generally these are considered MASSLESS particles but even that is up in the air.

- Neutrinos are not believed to travel faster than lightspeed.

- The term "tachyon" simply means a particle with the property to travel faster than lightspeed so saying they do travel faster than the speed is somewhat redundant. There is no tachyon known to actually exist. In other words: It's a bit like saying that white horses with a single horn on the center of their head exist: unicorns.

- Massless particles do travel at exactly lightspeed, not faster as you seem to assume.

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• 2 weeks later...

I know that this is probably me not knowing enough about the theory but I have noticed a seeming inconsistency in the theory. It follows thus:

1. Nothing can move faster than light.

2. Mass distorts the curvature of spacetime.

3. A change in an object's position changes the distortion.

4. This distortion would either have to travel, causing strange effects with the gravity of the universe and moving high-mass objects.

Can anybody help me with this? I'm kinda stuck on this one.

No. 4, as it appears here, is the correct one. Indeed changes in any quantity that would influence a body's gravitational field would propagate. In the form of a gravitational wave, with speed equal to $c$. Theoretically, such a wave resmbles an electromagnetic wave in many aspects. And that would indeed cause effects on those bodies it would encounter. For example, as the vibration of such a wave would be an increase of the gravitational field followed by a subsequent decrease, followed by another increase - and so on... - the molecules of an object encountered by that wave would be attracted harder among them, causing the body to shrink, then expand, then shrink again...Not in a visually detecteble way, to be sure! We're talking about angstroms! (Remember gravity is a very weak interraction.) And that is probably the reason why gravitational waves have not yet been detected - although some experimenters claim they have.

Experiments towards detection of gravitational waves have been set up and carried out by many researchers, the most famous of whom is Joseph Weber. More about it here: GWD and here: Weber Bar

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So-called spacetime curvature is transmitted at the same speed as light. The mass/energy of the Sun, for example, curves (distorts) spacetime just above its surface. This spacetime curvature distorts spacetime just a little further out. And so on. Eventually, this distortion reaches where the Earth is, and holds the Earth in orbit around the Sun. This step-by-step distortion takes time, alittle over 8 minutes travelling at the speed as light from the Sun to the Earth. (REF: John Archibald Wheeler, A Journey into Gravity and Spacetime, p. 84)

As an analogy, think of a trampoline which is perfectly flat. Then a jumper strikes the center of the trampoline. The fabric at the center where the jumper's feet hit is distorted. This distortion then affects the trampoline fabric a little further out. This in turn distorts the fabric even more further out. And so on. Soon the entire fabric is distorted. But this distortion takes time to propagate from the center to the edges of the trampoline.

Similarly, the Sun is continually distorting spacetime (the fabric of the universe). Imagine that the immortal Q from Star Trek made our Sun suddenly disappear. What would happen? Spacetime just adjacent to the Sun would become undistorted (flat). This change would propagate out step-by-step at the speed of light till it reached the Earth. Then and only then would the Earth fly out of its orbit around the Sun. So if the Sun were to suddenly disappear, the Earth would continue to orbit around where the Sun was for a little over 8 minutes; because this is how long it would take (at the speed of light) for the change in spacetime from curved to flat to travel the approximately 93 million miles from the Sun to the Earth.

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General relativity is connected to what had been formerly known as gravity and the gravitational force. Here is a scenario. Say a cloud of stella gas was forming a new star due to gravity. As time goes on, the mass of the star is increasing at an accelerating rate. This means the gravitational force of the star is accelerating. Does the acceleration of the acceleration of the gravitational force, due to the mass of the star accelerating, impact the results of GR? Does GR break down when we have an acceleration of a gravity acceleration? Would we need to do an iteration method to approximate this, or do the GR equations make provisions for this reality event? I am not being difficult, just curious.

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• 1 month later...

So-called spacetime curvature is transmitted at the same speed as light. The mass/energy of the Sun, for example, curves (distorts) spacetime just above its surface. This spacetime curvature distorts spacetime just a little further out. And so on. Eventually, this distortion reaches where the Earth is, and holds the Earth in orbit around the Sun. This step-by-step distortion takes time, alittle over 8 minutes travelling at the speed as light from the Sun to the Earth. (REF: John Archibald Wheeler, A Journey into Gravity and Spacetime, p. 84)

As an analogy, think of a trampoline which is perfectly flat. Then a jumper strikes the center of the trampoline. The fabric at the center where the jumper's feet hit is distorted. This distortion then affects the trampoline fabric a little further out. This in turn distorts the fabric even more further out. And so on. Soon the entire fabric is distorted. But this distortion takes time to propagate from the center to the edges of the trampoline.

Alright, I have seen examples like this given to explain the distortion of spacetime. In all examples with the trampoline or stretched rubber what is required is gravity and a material that has it's edges secured so they will not move. Also in these examples an object is actually in contact with some material. Spacetime has no scientific data that states that it is a physical thing existing in a location. On and on the dialogue goes regarding spacetime as though this thing actually exists, yet no one has ever discribed spacetime as a physical thing.

Spacetime is a mathematical construct. No mention anywhere that spacetime is a physical thing. Everyone who speaks of it in terms of being a thing that exists in a location has not actually looked to see if this thing is an actual entity. Anyone willing to do some research and find out for yourself?

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Alright, I have seen examples like this given to explain the distortion of spacetime. In all examples with the trampoline or stretched rubber what is required is gravity and a material that has it's edges secured so they will not move. Also in these examples an object is actually in contact with some material. Spacetime has no scientific data that states that it is a physical thing existing in a location. On and on the dialogue goes regarding spacetime as though this thing actually exists, yet no one has ever discribed spacetime as a physical thing.

It's called an analogy — a comparison because there are similarities. It's not meant to be taken as exactly the same.

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Alright, I have seen examples like this given to explain the distortion of spacetime. In all examples with the trampoline or stretched rubber what is required is gravity and a material that has it's edges secured so they will not move. Also in these examples an object is actually in contact with some material.

These are all analogies. That is things from our common experience that can illustrate or demonstrate deeper ideas. No-one should take the trampoline analogy too far.

Spacetime has no scientific data that states that it is a physical thing existing in a location. On and on the dialogue goes regarding spacetime as though this thing actually exists, yet no one has ever discribed spacetime as a physical thing.

Space-time itself is not really considered a "physical entity", however fields on it are. This includes the metric which describes the local geometry. Often, incorrectly space-time and it's local geometry are used synonymously.

Spacetime is a mathematical construct. No mention anywhere that spacetime is a physical thing. Everyone who speaks of it in terms of being a thing that exists in a location has not actually looked to see if this thing is an actual entity.

Of course this is all a mathematical construct to describe our natural world. This is shared with all physical theories and should not really present a philosophical stumbling block.

It maybe poor language, but in my opinion theoretical physicists are not always very clear on what is "purely mathematical" and what is "physically observable". Usually, this is not a problem for those working in or near theoretical physics, it is usually obvious from the context. So, people will talk about many mathematical constructs as if they really do exist in nature.

My personal attitude (not to be taken to heart or as gospel) is that "the only physical things are that which we can measure".

Anyone willing to do some research and find out for yourself?

Personally, I think very mathematically about physics. Most people here would not consider my lines of investigation physics at all!

To my mind we have several stages of understanding a concept. One starts in nature and what ones sees about us. We have a "physical concept". That is then taken to build a "mathematical concept". Calculations and constructions here then modify our original understanding. So we go back to a "physical concept" which may be quite far from our original. This then keeps on "looping".

In physical theories there are always plenty of unobservable elements that are fundamental. These are required by the mathematical formulation and are necessary for consistency. They may also be fundamental in preforming calculations of things that are observable.

In short, I would not worry too much about the separation of observables and mathematics when describing the theories of physics. Though this is very important when it comes to making the connection with experiments and observations, so at least it should always be in the back of your mind.

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