Warping of space and special relativity?

[SUROJL]

I saw couple of documentaries and read some books in general relativity. I find the theory quiet impressive. But I am confused with the notion that heavy and big objects warp space more than light objects. Why should heavy objects warp space more. Space is not like we feel like in earth. Why should heavy object warp space and time more than light objects? any ideas

Please don't say silly question(I also feel like that before but now I am quiet doubtful...it might have serious consequences in physics if we find reason), please post your views.

 

[George Jones]

Why does an apply fall?

 

[SUROJL]

What does apply fall mean?

 

[Fredrik]

What does "quiet impressive" mean? (He almost certainly meant "apple").

General relativity doesn't explain why the relationship between the matter in spacetime and the metric of spacetime is the way it is. It simply states that the relationship is given by Einstein's equation.

The only thing that can tell us why, would be a better theory of gravity, but we don't have one. And if we did, you'd be asking about its assumptions instead.

 

[Passionflower]

Why should heavy objects warp space more. Space is not like we feel like in earth. Why should heavy object warp space and time more than light objects? any ideas

What determines if an object is heavy or light depends on how much it warps spacetime.
Thus if all objects would warp spacetime the same level we obviously would not be able to distinguish them as light or heavy. Hence since we can distinguish light and heavy objects it must imply that they warp spacetime differently.

Note than an object warps spacetime only locally (Ricci curvature) as there is no action at a distance in GR. However since warped spacetime also warps neighboring spacetime (but with a certain 'damping factor') the effect gets propagated by the speed of light (Weyl curvature).

Hence not only matter tells spacetime how to curve, also spacetime tells surrounding spacetime how to curve.

 

[Elroch]

I saw couple of documentaries and read some books in general relativity. I find the theory quiet impressive. But I am confused with the notion that heavy and big objects warp space more than light objects. Why should heavy objects warp space more. Space is not like we feel like in earth. Why should heavy object warp space and time more than light objects? any ideas

Please don't say silly question(I also feel like that before but now I am quiet doubtful...it might have serious consequences in physics if we find reason), please post your views.

Suppose you have an apple and it warps space. Then you break it into two halves and separate them by a fraction. The warping should not change much, one would expect. How much of the warping is due to each half of the apple?

[This is not meant to be rigorous, just a rough intuitive picture of an additive effect]

 

[Naty1]

Why should heavy objects warp space more.

You probably mean...warp spacetime...that is space and time...

Why should pressure, energy,stress and energy warp spacetime??

That's just the way THIS universe works. If you subscribe to a many worlds interpretation, then some universes may not have such relationships...we just can't be there to observe that insofar as is currently known.

The next step beyond quantum mechanics and relativity might be quantum gravity, so far an incomplete theory...stay tuned!

 

[elevin]

Wow you guys are making this complicated. Heavy objects have more mass, more mass = more gravity, gravity warps spacetime only in that an object (be it photon or basketball) traveling in an inert "straight" path will inevitably follow the easiest route. If the obect's (photon, basketball, beer can) path goes near an apple it won't really be diverted, half an apple, half the diversion, but as it travels by black hole, it will certainly appear to alter it's direction / speed. I'm not much for the quanta, but do enjoy 'dem bosons. Also interesting, check out Erik Verlinde's "Entropic Gravity", Self-Creation Cosmos, or Conformal Gravity.

 

[zonde]

And yet the best theory we have about space, time and motion (general relativity) is built up around that idea.

By what criterions do you judge GR as the best?
I think that SR is a way better if we speak about space, time and motion.
And Newtonian gravity is better theory for gravity.

warping of space time is just an attempt to explain the phenomenon...by space I mean clear vacuum & time exist only in our mind so what's out there to warp..time is constant, time never slow down or speed up, speed and gravity have no effect on time.

Now you're just rambling on with personal speculation. That's something that no one who came here to learn about the established theories wants to see, and it's something that no one who likes to teach the established theories wants to talk about either. It's also against the forum rules, precisely for that reason.

There is something to talk about. Idea that time and space are unphysical is common understanding. Relativity on the other hand redefined time and space as clocks and rulers. And certainly clocks and rulers have physical properties and so have relativistic space and time.
And so if someone is using space and time as commonly understood it's up to you to explain that within context of relativity space and time has slightly different meaning.

 

[PAllen]

By what criterions do you judge GR as the best?
I think that SR is a way better if we speak about space, time and motion.
And Newtonian gravity is better theory for gravity.

So all the tests described in the following, showing agreement with GR and disagreement with other theories of gravity don't count? Correspondence with observation is not relevant?

http://relativity.livingreviews.org/Articles/lrr-2006-3/

 

[Fredrik]

By what criterions do you judge GR as the best?

I judge all theories by their ability to make predictions about results of experiments. A better theory makes more accurate predictions about results of experiments.

I think that SR is a way better if we speak about space, time and motion.
And Newtonian gravity is better theory for gravity.

I would agree that those theories are easier to work with, and also make pretty accurate predictions about most experiments. You could argue that they're more practical, but I wouldn't call them "better" when GR makes more accurate predictions about everything.

 

[Fredrik]

There is something to talk about.

In the first half of the part I quoted, yes. In the second half, no.

Idea that time and space are unphysical is common understanding. Relativity on the other hand redefined time and space as clocks and rulers. And certainly clocks and rulers have physical properties and so have relativistic space and time.
And so if someone is using space and time as commonly understood it's up to you to explain that within context of relativity space and time has slightly different meaning.

These are good points, but I have explained such things too many times to still feel like answering every person who makes that mistake. When people make claims like "matter warping spacetime is nonsense" (effectively saying that GR is nonsense), I don't particularly feel like helping them at all. People who really want to learn come here to ask questions, not to claim that the theories we are willing to explain to them are nonsense.

 

[alexg]

I wonder how an instructor of physics 101 at a community college must feel, year after year, the same questions, the same answers and the same blank looks.

 

[zonde]

So all the tests described in the following, showing agreement with GR and disagreement with other theories of gravity don't count? Correspondence with observation is not relevant?

http://relativity.livingreviews.org/Articles/lrr-2006-3/

It should be possible to check:
1) consistency of theory
2) how predictions are made (they are consistent, they uniquely follow from proposed theory)
3) interpretation of observations intended to test the theory should be independent from proposed theory

And if I can check it myself then I would consider theory reliable.

 

[Dale]

3) interpretation of observations intended to test the theory should be independent from proposed theory

For this, what is needed is a test theory. I think that the best one we currently have is the parameterized post Newtonian theory: http://en.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism

It is not completely general because it doesn't test for deviations from the equivalence principle, and it only tests the weak field limit, but it does admit a wide class of parameters that would falsify GR.

 

[zonde]

I judge all theories by their ability to make predictions about results of experiments. A better theory makes more accurate predictions about results of experiments.

If domains where two theories can adequately make predictions overlap this might work. What if they don't?

I would agree that those theories are easier to work with, and also make pretty accurate predictions about most experiments. You could argue that they're more practical, but I wouldn't call them "better" when GR makes more accurate predictions about everything.

That "everything" part is not quite clear for me. Is it really so?
I believe there are rather few exact solutions in GR. And if you can't get exact solution then predictions are not exactly uniquely determined.
Another thing is that GR is not extension to Newtonian gravity but rather alternative approach to gravity. This makes it harder to compare two theories. And it rises a question what you would get if you would continue with approach taken by Newtonian gravity?

These are good points, but I have explained such things too many times to still feel like answering every person who makes that mistake. When people make claims like "matter warping spacetime is nonsense" (effectively saying that GR is nonsense), I don't particularly feel like helping them at all. People who really want to learn come here to ask questions, not to claim that the theories we are willing to explain to them are nonsense.

Sure, such claims are not the best starting point but slipping in new concept under common name that means something different is not very good starting point either. Another example being "'surface' of last scattering", grr.
Say why isn't spacetime of relativity called something like "Einstein's aether"?
Do you think you would still see claims like "matter warping Einstein's aether is nonsense"?

 

[Dale]

I believe there are rather few exact solutions in GR. And if you can't get exact solution then predictions are not exactly uniquely determined.

This is incorrect. A solution may be numerical but still unique.

Another thing is that GR is not extension to Newtonian gravity but rather alternative approach to gravity. This makes it harder to compare two theories. And it rises a question what you would get if you would continue with approach taken by Newtonian gravity?

That is why you need a test theory, like PPN, which encompasses both. PPN reduces to Newtonian gravity and to GR with suitable settings of the PPN parameters. So experiments which measure the value of one of the PPN parameters where GR and Newtonian gravity differ can legitimately compare the two theories. So far, all such tests have agreed with the GR values.

 

[zonde]

For this, what is needed is a test theory.

No, you need rival theory for point 2) i.e. what predictions are worth testing.

But you express prediction in terms that you don't question.
So for point 3) you need previously established theory that you don't question. Like you don't question how your telescope is working (optics).

In real life straw-mans are used for point 2) and you would assume that theory is true when interpreting observations in point 3). But that of course makes theory less reliable.

I think that the best one we currently have is the parameterized post Newtonian theory: http://en.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism

It is not completely general because it doesn't test for deviations from the equivalence principle, but it does admit a wide class of parameters that would falsify GR.

(Weak) equivalence principle is prerequisite for Newtonian gravity and GR. We can test it but it is not prediction. If equivalence principle holds then predictions are valid according to theory. If equivalence principle does not hold the theory is silent (not wrong).
But certainly we take principles as reasonable and usually we would have to test them only when we can't make sense assuming they are true.

 

[Dale]

No, you need rival theory for point 2) i.e. what predictions are worth testing.

No, you need a test theory. A theory which has some parameter which leads to one theory in one limit and a large class of rival theories in other limits.

Your point 2 can be determined purely mathematically and doesn't require any experimental testing at all, nor does it require reference to any other theories.

 

[zonde]

No, you need a test theory. A theory which has some parameter which leads to one theory in one limit and a large class of rival theories in other limits.

It seems I misunderstood what you meant by test theory. But still why proposed theory should follow from other theory as parametrization of that theory? Maybe with parameters you mean the same thing as measurements allowed by that theory?

 

[Fredrik]

If domains where two theories can adequately make predictions overlap this might work. What if they don't?

That can be a problem, but I don't think it is a problem in any other comparison between theories than the one between GR and the standard model. It's hard to say which one of them is the "best theory of physics" precisely for that reason.

In the comparison between GR and Newton's theory of gravity, I can't think of anything that Newton's theory does better. GR does a better job of predicting the orbits of planets, and it also predicts time dilation, length contraction, that clocks on different floors of the same building have different ticking rates (gravitational time dilation), the rate at which the orbital period of a binary pulsar slows down, the existence of black holes, the expansion of the universe, etc. All of these are things that Newton's theory incorrectly says won't ever happen.

 

[zonde]

In the comparison between GR and Newton's theory of gravity, I can't think of anything that Newton's theory does better.

Superposition works in Newton's gravity and doesn't in GR. So I suppose that Newton's gravity should be better for complex systems consisting of many gravitation sources.

Another thing is global coordinate system. There is no problem with that in Newton's gravity but as I understand in GR this is rather controversial question. But you need that to check consistency of theory. Related to this - I simply do not understand how you can model interior of gravitating body if you don't have single coordinate system that spans that area. Maybe I'm missing something here.

GR does a better job of predicting the orbits of planets, and it also predicts time dilation, length contraction, that clocks on different floors of the same building have different ticking rates (gravitational time dilation), the rate at which the orbital period of a binary pulsar slows down, the existence of black holes, the expansion of the universe, etc.

If you mean perihelion precession of Mercury then the word is explanation not prediction.
Yes, GR predicted gravitational time dilation. And orbital decay is very interesting topic.

All of these are things that Newton's theory incorrectly says won't ever happen.

This is quite dubious statement.
It is well known limitation of Newton's gravity that it approximates speed of gravitational interaction as infinite. But this is not prediction this is prerequisite and if it does not hold you have to update theory.

On the other hand it would be nice to find out from GR why Newtonian gravity gives wrong prediction. Is it because of that speed of interaction or because moving body "sees" gravitation field differently or something else?

And if we speak about existence of black holes, the expansion of the universe I would say that this is benefit of Newton's gravity that it does not predict anything like that.

 

[Fredrik]

Superposition works in Newton's gravity and doesn't in GR. So I suppose that Newton's gravity should be better for complex systems consisting of many gravitation sources.

Here I take it that "better" means "easier to calculate with"? There's no reason to think that Newton's theory makes better predictions.

Another thing is global coordinate system.

That can probably complicate calculations too. Other than that, I don't see a problem.

If you mean perihelion precession of Mercury then the word is explanation not prediction.

It's a prediction by my definitions. What I call predictions are just the statements the theory makes about results of experiments. It doesn't matter to me if the experimental discovery or the theory came first.

This is quite dubious statement.

In what way? It's only wrong if Newton's theory predicts time dilation, length contraction, gravitational time dilation, orbital decay of binary pulsars, black holes, or that a homogeneous and isotropic universe must be expanding. Do you think it predicts one or more of those?

And if we speak about existence of black holes, the expansion of the universe I would say that this is benefit of Newton's gravity that it does not predict anything like that.

So GR is wrong about those things? How do you know this?

 

[Dale]

Maybe with parameters you mean the same thing as measurements allowed by that theory?

No, parameters are constants or functions which are not determined by the theory, but must be determined by experiment. For example, the cosmological constant is the only parameter in GR.

Suppose that you want to test between GR and Newtonian gravity, in order to do that you need some theory (called a test theory) that has an undetermined parameter which causes the theory to reduce to Newtonian gravity if it takes one value and reduce to GR if it takes another value. Then, you use the test theory to design an experiment to measure the undetermined parameter and see if it agrees with the value predicted by GR or Newtonian gravity or neither.

That is essentially how you test any theory.

 

[zonde]

Here I take it that "better" means "easier to calculate with"? There's no reason to think that Newton's theory makes better predictions.

That can probably complicate calculations too. Other than that, I don't see a problem.

Well, you think that complexity of calculations affects only the effort you have to make to get result but it does not affect quality of result. I tend to think the same way ... but only so far.

In math values do not have uncertainties. In physics uncertainties are always non zero. Mathematical manipulations on general principle increase uncertainties and if you are doing very long chain of mathematical manipulations involving values with uncertainties you can arrive at useless result.

When math is very complex people ten to take less rigorous "shortcuts" making assumptions about what contributes to result significantly and what doesn't. That of course introduces some uncertainty about result. Another thing is that if you discover that prediction is not good it might be hard to track down the place where it deviates from reality most significantly (it's hard to fix it because of too many assumptions).

And of course more complex math is more open for errors. And people make errors.

In what way? It's only wrong if Newton's theory predicts time dilation, length contraction, gravitational time dilation, orbital decay of binary pulsars, black holes, or that a homogeneous and isotropic universe must be expanding. Do you think it predicts one or more of those?

It's only right when
- Newton's theory explicitly predicts:
NO time dilation AND NO length contraction AND NO gravitational time dilation AND NO orbital decay of binary pulsars AND NO black holes AND that a homogeneous and isotropic universe must NOT be expanding
- AND these predictions are correct:
time dilation AND length contraction AND gravitational time dilation AND orbital decay of binary pulsars AND black holes AND that given homogeneous and isotropic universe it is expanding.

Too many ANDs. And NO predictions are dubious by themselves - Newton's theory is not some no go theorem.

So GR is wrong about those things? How do you know this?

Or maybe GR has no saying in these things? Say it is stretched too far using some highly speculative assumptions.

 

[Fredrik]

It's only right when
- Newton's theory explicitly predicts:
NO time dilation AND NO length contraction AND NO gravitational time dilation AND NO orbital decay of binary pulsars AND NO black holes AND that a homogeneous and isotropic universe must NOT be expanding

I'd say that the negation of GR's prediction of expansion is that a homogeneous and isotropic universe that doesn't expand at any point in its history is a possibility. Disregarding that detail (we could nitpick the details for a long time), I would say that Newtonian physics very clearly does say all those things.

- AND these predictions are correct:
time dilation AND length contraction AND gravitational time dilation AND orbital decay of binary pulsars AND black holes AND that given homogeneous and isotropic universe it is expanding.

There's experimental/observational support for all of those.

Or maybe GR has no saying in these things? Say it is stretched too far using some highly speculative assumptions.

It's very hard to doubt that the universe is expanding, given what's been observed. I think there's also strong observational evidence for black holes, but I'm not familiar with the details.

 

[pervect]

There is something to talk about. Idea that time and space are unphysical is common understanding. Relativity on the other hand redefined time and space as clocks and rulers. And certainly clocks and rulers have physical properties and so have relativistic space and time.
And so if someone is using space and time as commonly understood it's up to you to explain that within context of relativity space and time has slightly different meaning.

How exactly, do we do this explaining?

It certainly is common for people to have some understanding of space and time in a manner that is pre-relativistic. And we can try and point this out to the people, but the usual result is they don't understand what we mean when we say this. It's especially a problem if they're asking some question about GR without the understanding of SR to back up the explanation.

If someone does understand and/or is able to listen to an explanation that the distance between two events is not an invariant in special relativity the way it is in pre-relativistic theories, that what is invariant is not the distance between the events, but the Lorentz interval between the events, the conversation is progressing in a useful manner.

Mainly because the person, in order to understand this, has already done enough reading to have grasped much of the point on their own. Otherwise, they'll tend to not be familiar with the Lorentz interval or understand why it was mentioned at all - and while they are (in my opinion at least) perfectly capable of understanding the words that "the distance depends on the reference frame and hence is not solely a property of the events", they won't actually ACCEPT it.

More typically, if we point out the pre-relativity space and time is different than post-relativity space and time, the original poster doesn't even see the relevance of pointing this out. After all, on the surface at least, they were asking about something that was completely different.

It is often rather difficult to get the points about space, time, and their interrelationship in SR, even if they are specifically interested in this issue, for that mater. Textbooks, realistically, probably do a better job of explaining than a post on a forum can do.

My position, after a lot of experience, is that not only is it "not up to me" to explain things in a way people can accept, it's probably not even generally possible. You can lead a horse to water, but you can't make it drink.

That said, I am interested in ways to improve the success rate of "getting the horse to drink", i.e. getting people to understand some of the basic ideas of relativistic space-time, and/or ore advanced topics, and how to present it clearly at the most elementary level possible in a way that allows it to be accepted and assimilated.

 

[zonde]

It's very hard to doubt that the universe is expanding, given what's been observed.

To talk about this it would be nice to find out what does it means for universe to be expanding.
As I argued earlier space in relativity acquires it's physical properties because it is defined using physical rulers. Now I would like to argue that expanding space can be viewed as rescaling of rulers (we have to change either time scale or speed of light as well).
So the picture I get is matter appearing at very large scale over very long period of time and then rapidly (comparatively) shrinking to smaller scale.
For me this picture makes expansion possible but not plausible.

If we speak about observations.
GR was used as explanation of redshift. No cosmological constant - no tunable parameters, very neat indeed.
But with discovery of "accelerated expansion" this neat version was falsified.

So what would be alternative. Some unknown process that absorbs light and stretches very long SN light pulse even longer? Ok, rather contrived but possible. But that won't work with continuous stream of repeated pulses.
But wonder what? It seems that quasars do not show time dilation: On time dilation in quasar light curves

 

[zonde]

How exactly, do we do this explaining?

It certainly is common for people to have some understanding of space and time in a manner that is pre-relativistic. And we can try and point this out to the people, but the usual result is they don't understand what we mean when we say this. It's especially a problem if they're asking some question about GR without the understanding of SR to back up the explanation.

If someone does understand and/or is able to listen to an explanation that the distance between two events is not an invariant in special relativity the way it is in pre-relativistic theories, that what is invariant is not the distance between the events, but the Lorentz interval between the events, the conversation is progressing in a useful manner.

Mainly because the person, in order to understand this, has already done enough reading to have grasped much of the point on their own. Otherwise, they'll tend to not be familiar with the Lorentz interval or understand why it was mentioned at all - and while they are (in my opinion at least) perfectly capable of understanding the words that "the distance depends on the reference frame and hence is not solely a property of the events", they won't actually ACCEPT it.

More typically, if we point out the pre-relativity space and time is different than post-relativity space and time, the original poster doesn't even see the relevance of pointing this out. After all, on the surface at least, they were asking about something that was completely different.

It is often rather difficult to get the points about space, time, and their interrelationship in SR, even if they are specifically interested in this issue, for that mater. Textbooks, realistically, probably do a better job of explaining than a post on a forum can do.

My position, after a lot of experience, is that not only is it "not up to me" to explain things in a way people can accept, it's probably not even generally possible. You can lead a horse to water, but you can't make it drink.

That said, I am interested in ways to improve the success rate of "getting the horse to drink", i.e. getting people to understand some of the basic ideas of relativistic space-time, and/or ore advanced topics, and how to present it clearly at the most elementary level possible in a way that allows it to be accepted and assimilated.

As I see, the problem is that at first sight it looks like relativity is promoting many alternative realities i.e. it is not realistic.
To check that person can do this - pick one most complete viewpoint as baseline and at any point when he has suspicions that some alternative viewpoint is not realistic he relates it back to baseline and checks that it still makes sense. We can call this baseline "reality" and this process - "crap filter".

Now it seems like "space" and "time" are kind of "hardcoded" terms used for "crap filter" and they are not available for redefinition (from outside). If you are trying to redefine them it might be perceived as attempt to sneak past "crap filter".

And of course you can sell nothing to a person before you have passed "crap filter" so there is no use to talk about symmetry of LT or invariance of spacetime intervals at the very start.

Does this sound sensible?



And if we speak about GR it is even claimed that there is no single global reference frame so it appears like there is no way how GR could pass "crap filter". However, I am certain that GR is realistic at least as Einstein conceived it.

 

[elfmotat]

Does this sound sensible?

Not even a little bit. To me, it sounds like a sophisticated way of plugging your ears and going "blah blah blah I can't hear you!" The test of a good theory is whether or not it makes accurate predictions, and not whether it does or doesn't conform to your naive preconceived notions.

Edit: nevermind, it appears I misinterpreted what you were saying. My apologies.