# Warping of space and special relativity?

Fredrik
Staff Emeritus
Gold Member
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
Staff Emeritus
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
Gold Member
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.

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
Gold Member
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.

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.

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Dale
Mentor
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.
Then you misunderstand relativity.

zonde
Gold Member
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.
I think this part still holds even if you misinterpreted my post.

And I would like to comment that it is not enough to have confirmed predictions.

You see the thing is that predictions are not made by theory but by people. If practically everyone will come up with the same prediction when using some theory there is no use from hair splitting and we indeed can say that "theory makes predictions".

But if this is not so (different people come up with different predictions) can we say that we have confirmed prediction of a theory?

Let's say that theory is recipe how to come up with prediction. We have person who has made good predictions (we have confirmed them) and here is the recipe from that person how to do that. From skeptical stance we can say that - fine, predictions are good but how do you know that the recipe describes the way how predictions where made. Maybe it has nothing to do with predictions (recipe is red herring), or maybe it does but it is missing some important steps i.e. it is incomplete.

PAllen
2019 Award
I think this part still holds even if you misinterpreted my post.

And I would like to comment that it is not enough to have confirmed predictions.

You see the thing is that predictions are not made by theory but by people. If practically everyone will come up with the same prediction when using some theory there is no use from hair splitting and we indeed can say that "theory makes predictions".

But if this is not so (different people come up with different predictions) can we say that we have confirmed prediction of a theory?

Let's say that theory is recipe how to come up with prediction. We have person who has made good predictions (we have confirmed them) and here is the recipe from that person how to do that. From skeptical stance we can say that - fine, predictions are good but how do you know that the recipe describes the way how predictions where made. Maybe it has nothing to do with predictions (recipe is red herring), or maybe it does but it is missing some important steps i.e. it is incomplete.
Let's clarify the difference between subjective or ambiguous predictions versus incorrect predictions.

One can state that GR prediction of singularities is incorrect. One might argue that GR cosmological predictions are dubious because they require dark matter and cosmological constant to match observation (though dark matter is gathering ever more direct observational support; and the cosmological constant is analogous to a constant of integration - it is incorrect to assume it is zero without some motivation).

An ambiguous prediction would be experts differing on what GR says about some real world or well defined hypothetical situation. Can you point to a any instances of this?

zonde
Gold Member
Then you misunderstand relativity.
I misunderstood relativity when I first heard about symmetrical length contraction and time dilation and it was quite some time until I found out that there is third part - "relativity of simultaneity". But I suppose that it was just a brain teaser for me.

I understand about parameters. I do not understand why test theory is required (and maybe what it is). If you want to make your point maybe you can give example? What would be test theory in case of Shapiro delay experiment?

pervect
Staff Emeritus
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.
It's a problem, because of Bell's theorem. Though upon re-reading, I'm not sure if that's really what you're trying to say, you didn't mention the theorem explicitly.

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".

"one most complete viewpoint as baseline" ?? I've no idea what this means.

It sounds like you've already done a lot of processing of what I'd call "raw sensory data" to come up with "one complete viewpoint", though.

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".
If you try to redefine space and time, you're probalby going to wind up with humpty-dumptyism, alas, which is where words mean what you say what they mean, not what they do mean.

So if you want to talk about something that's not space and time, you'll need to come up with your own terms. Unfortunately, whatever abstract philosophical concept you're trying to convey via "crap filter" isn't terribly clear. I was originally thinking "process of perception", but as I try to read more, I'm getting a different interpretation.

I suspect that you've skipped over the process entirely of how to assemble "raw sensory data" into "one complete viewpoint" via some "personal philosophy", and that this is where the problem is.

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?
I now think what you're saying is that people come up with a personal philosophy, (i.e. "crap filter" means personal philosophy), and if the personal philosphy that they come up with is incompatible with relativity, they reject relativity, rather than think about whether or not their personal philosophy has problems.

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.
I think that reference frames are confusing at best. A simpler but more flexible concept is needed - a concept of a map. The map represents reality, but isn't reality. And there is some process from which you can get out of the map things that you actually can measure, like distances (if you look at nearby points), or perhaps radar signal propgation delays.

And I'd suggest that seriously interested people look at how you can use flat maps to represent the curved surfaces of the Earth, which should be a concrete and practical idea to get familiar with the general process of what I'm calling "maps", and also illustrate how and why you need a metric to convert "map distances" into "measured distances".

http://arxiv.org/abs/gr-qc/9508043

has a very rough overview of this process and the associate philosphy.

zonde
Gold Member
I now think what you're saying is that people come up with a personal philosophy, (i.e. "crap filter" means personal philosophy), and if the personal philosphy that they come up with is incompatible with relativity, they reject relativity, rather than think about whether or not their personal philosophy has problems.
With "crap filter" I was meaning some quality control. Imagine that the person is asking you: "Why should I trust you?", and you are saying: "Ok, close your eyes and I will tell you why." This probably won't work because this is suspicious and not very clear why it is needed.

Likewise about relativity if you want to redefine important concepts before you have proved that you can be trusted (your viewpoint is realistic) it rises suspicions. So if you want to make a point, make it without redefining space and time. If you need some coordinate dependent quantities similar to pre-relativistic space and time call them differently, for example, "clock rate" and "ruler length".

Dale
Mentor
I understand about parameters. I do not understand why test theory is required (and maybe what it is). If you want to make your point maybe you can give example? What would be test theory in case of Shapiro delay experiment?
For Shapiro delay the test theory would be PPN which I mentioned above. In the PPN formalism the Shapiro delay is proportional to the PPN parameter γ, meaning that a measurement of the Shapiro delay gives a measurement of the PPN parameter γ:
http://en.wikipedia.org/wiki/Shapiro_delay#Time_delay_due_to_light_traveling_around_a_single_mass

If that value is 0 (to within experimental error) then the test supports Newtonian gravity, if the value is 1 (to within experimental error) then the test supports GR.Currently, the experimental data places γ at 1±2.3x10^-5. This confirms GR and falsifies Newtonian gravity:
http://www.nature.com/nature/journal/v425/n6956/full/nature01997.html

With "crap filter" I was meaning some quality control.
The ultimate crap filter in science is experimental evidence. That is how you prove that your viewpoint is realistic and that your redefinitions are worthwhile.

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Fredrik
Staff Emeritus
Gold Member
Likewise about relativity if you want to redefine important concepts before you have proved that you can be trusted (your viewpoint is realistic) it rises suspicions. So if you want to make a point, make it without redefining space and time.
But a person who gets suspicious about that hasn't understood what science is. So it would make a lot more sense to explain that first and then make the necessary definitions, than to pretend that his views are valid.

zonde
Gold Member
For Shapiro delay the test theory would be PPN which I mentioned above. In the PPN formalism the Shapiro delay is proportional to the PPN parameter γ, meaning that a measurement of the Shapiro delay gives a measurement of the PPN parameter γ:
http://en.wikipedia.org/wiki/Shapiro_delay#Time_delay_due_to_light_traveling_around_a_single_mass
According to wikipedia Shapiro delay is proportional to (1+γ) not γ. But certainly it is enough to have $\Delta t$ to test GR.

If that value is 0 (to within experimental error) then the test supports Newtonian gravity, if the value is 1 (to within experimental error) then the test supports GR.Currently, the experimental data places γ at 1±2.3x10^-5. This confirms GR and falsifies Newtonian gravity:
http://www.nature.com/nature/journal/v425/n6956/full/nature01997.html
Interesting experiment.

The ultimate crap filter in science is experimental evidence. That is how you prove that your viewpoint is realistic and that your redefinitions are worthwhile.
Experiment can not prove that your viewpoint is realistic. Haven't you heard about particle wave duality or quantum entanglement? And how can experiment prove that we have to use particular word in our definition?

I agree that experiment is the ultimate crap filter in science. But unfortunately it can not prove anything, only disprove something.

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zonde
Gold Member
But a person who gets suspicious about that hasn't understood what science is. So it would make a lot more sense to explain that first and then make the necessary definitions, than to pretend that his views are valid.
Take math. If we have one variable at two different places and I am not sure it's the same it should be OK to use two different variable names and check that they come together somewhere along derivation as clearly the same variable. There is nothing unscientific about that. The same with definitions. Where is the problem in giving different name for your defined concept and checking that at the end it looks the same as the old one?

Fredrik
Staff Emeritus
Gold Member
Take math. If we have one variable at two different places and I am not sure it's the same it should be OK to use two different variable names and check that they come together somewhere along derivation as clearly the same variable. There is nothing unscientific about that. The same with definitions. Where is the problem in giving different name for your defined concept and checking that at the end it looks the same as the old one?
If you would prefer to invent a new and non-suggestive term like "flurpiness" for ##(\gamma-1)m## instead of calling it "kinetic energy" when you're trying to explain SR to someone, then I don't have a problem with it. I would however prefer to just explain that it's very common that two theories assign different meanings to the same term.

Dale
Mentor
According to wikipedia Shapiro delay is proportional to (1+γ) not γ. But certainly it is enough to have $\Delta t$ to test GR.
Oops, you are correct. Thanks.

Experiment can not prove that your viewpoint is realistic.
Sure it does. How else would you prove that it is realistic?

Haven't you heard about particle wave duality or quantum entanglement?
Yes, the related experiments clearly demonstrate that reality is weird.

pervect
Staff Emeritus
With "crap filter" I was meaning some quality control. Imagine that the person is asking you: "Why should I trust you?", and you are saying: "Ok, close your eyes and I will tell you why." This probably won't work because this is suspicious and not very clear why it is needed.

Likewise about relativity if you want to redefine important concepts before you have proved that you can be trusted (your viewpoint is realistic) it rises suspicions. So if you want to make a point, make it without redefining space and time. If you need some coordinate dependent quantities similar to pre-relativistic space and time call them differently, for example, "clock rate" and "ruler length".
I guess my general comment is that if someone's personal "crap filter" is falsely filtering out relativity, their "crap filter" needs a little bit of adjustment. A little bit like someone's mailbox filter putting important messages in the spam container.

Since the only coordinate independent quantity in relativity is the Loretnz interval, any breaking up of the Lorentz interval into a "time difference' and a "space difference" is going to be coordinate dependent. So it's not that we need coordinate dependent quantities for time and distance, it's we can't avoid getting rid of the concept of "universal time", a time that's coordinate independent, because it no longer exists.

A plea to "explain relativity in terms of some "universal time" just like you used to believe in is just not going to work. Sorry.

As far as any obligation to "prove realism", I can't really agree. It appears to me to be the all-too-common case of some personal philosophy (here said philosohpy is being called realism, and may or may not be related to what a philosopher would call realism) is inhibiting the understanding of relativity. It's sad, but I don't think there's much that can be done about it.

Dale
Mentor
As far as any obligation to "prove realism", I can't really agree. It appears to me to be the all-too-common case of some personal philosophy (here said philosohpy is being called realism, and may or may not be related to what a philosopher would call realism) is inhibiting the understanding of relativity. It's sad, but I don't think there's much that can be done about it.
Well said. That is why experiment is the final arbiter of realism.

pervect
Staff Emeritus
One other comment I have to make. That is the tricky matter - it's possible with enough detachment to work with a theory, to understand it, without totally believing it as an absolute, to think of it as "if this is true, then the rest follows".

It's sort of a hard way to proceed, to believe something enough to see where it leads while keeping one's mind open about the truth of the precondtions.

It's certainly preferrable way to look at relativity this way. In fact that's probably the scientific ideal - to always keep in mind that the premises might be wrong. Just rejecting it out of hand because it doesn't match one's preconceptions really isn't right, though it seems there is a sort of counter-culture that encourages it (why, I don't know). If some of the preconceptions are very strong, it can be hard to overcome them :-(.

zonde
Gold Member
One other comment I have to make. That is the tricky matter - it's possible with enough detachment to work with a theory, to understand it, without totally believing it as an absolute, to think of it as "if this is true, then the rest follows".

It's sort of a hard way to proceed, to believe something enough to see where it leads while keeping one's mind open about the truth of the precondtions.

It's certainly preferrable way to look at relativity this way. In fact that's probably the scientific ideal - to always keep in mind that the premises might be wrong. Just rejecting it out of hand because it doesn't match one's preconceptions really isn't right, though it seems there is a sort of counter-culture that encourages it (why, I don't know). If some of the preconceptions are very strong, it can be hard to overcome them :-(.
I would like to continue discussion from here. You have formulated it much better and more up to the point.

So if the person you are trying to teach about relativity is going the hard way it might be that at some point he can't maintain that detachment and he has to stop to sort things out. It does not mean that he is rejecting relativity, he just can't go at the pace you are pushing him.

But you can go faster (I guess) if you can present your point already in that detached form so that the person has to do less "translation".