question4477

Is Space-Time a Physical Entity or a Mathematical Model?

Recommended Posts

But there is no problem of time as time is inherent in mathematics, right? ;)

 

 

In a sense I think you are right. Physics deals with mathematical models and matching them with nature. Physics does not really say much about what 'exists', what is 'real' and so on. Physics is about what we can calculate and measure - and if we can measure it then it is 'real'. But this definition I am sure will fall flat on its face if we think about it too hard!

+1 for good memory and sense of humor.

 

However my point about mathematics (that nobody agreed with) is exactly the contrary.

Share this post


Link to post
Share on other sites

I can imagine that as theoretical physics advances some metaphysical questions will evaporate - but only to be replaced with others!

  • Upvote 1

Share this post


Link to post
Share on other sites

" ... In my opinion the answer to this question is, briefly, this: as far as the propositions of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality. ... "

Albert Einstein, "Geometry and Experience", 1921

plus one for Einstein :)

Share this post


Link to post
Share on other sites

" ... In my opinion the answer to this question is, briefly, this: as far as the propositions of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality. ... "

Albert Einstein, "Geometry and Experience", 1921

I personally find this contradictory considering that he also seemed to subscribe to the block universe theory which implies substantivalism as opposed to relationism when he stated - "The distinction between the past, present and future is only a stubbornly persistent illusion." or am I missing something here.

 

Someone like Brian Greene for example in his book 'Fabric Of the Cosmos' makes it specifically clear that he regards space-time as a physical entity and not a mathematical abstraction which I guess would make him a substantivalist, in a recent documentary he expressed his belief that Gravity Probe B which measured an effect on space-time curvature provided evidence to support this view, how plausible is this claim?

Edited by question4477

Share this post


Link to post
Share on other sites

... in a recent documentary he expressed his belief that Gravity Probe B which measured an effect on space-time curvature provided evidence to support this view, how plausible is this claim?

Einstein proposed it then Gravity Probe B measured it and confirmed it. How real do you want it?

Share this post


Link to post
Share on other sites

I personally find this contradictory...

I don't think that Einstein was greatly interested in metaphysics and so I would not be surprised if you can find differing quotes.

 

I know for sure that he was not impressed by some philosophical views on time.

 

Someone like Brian Greene for example in his book 'Fabric Of the Cosmos' makes it specifically clear that he regards space-time as a physical entity and not a mathematical abstraction which I guess would make him a substantivalist...

He is free to hold that view. The problem is that as our understanding of space-time changes, and it must do at some level when we take quantum effects into account, will the classical view of space-time still be 'physical'?

 

I am not sure, other than mathematically we should have a good understanding of how the classical notion comes from the quantum one - otherwise we will be at a complete loss as to why classical general relativity has worked so well so far.

 

...in a recent documentary he expressed his belief that Gravity Probe B which measured an effect on space-time curvature provided evidence to support this view, how plausible is this claim?

All the tests of general relativity tell us that the physical theory matches nature very very very well. I don't think one can really say much more.

Share this post


Link to post
Share on other sites

I don't think that Einstein was greatly interested in metaphysics and so I would not be surprised if you can find differing quotes.

 

I know for sure that he was not impressed by some philosophical views on time.

 

 

He is free to hold that view. The problem is that as our understanding of space-time changes, and it must do at some level when we take quantum effects into account, will the classical view of space-time still be 'physical'?

 

I am not sure, other than mathematically we should have a good understanding of how the classical notion comes from the quantum one - otherwise we will be at a complete loss as to why classical general relativity has worked so well so far.

 

 

All the tests of general relativity tell us that the physical theory matches nature very very very well. I don't think one can really say much more.

By the tests of general relativity matching 'Physical Theory' do you mean the substantivalist view of Space-Time?

Share this post


Link to post
Share on other sites

By the tests of general relativity matching 'Physical Theory' do you mean the substantivalist view of Space-Time?

 

 

It just means that the predictions made by the theory match the measurements we make.

Share this post


Link to post
Share on other sites

By the tests of general relativity matching 'Physical Theory' do you mean the substantivalist view of Space-Time?

I am not sure we can decide what metaphysical view of space-time is correct. All we can really do is put our models to the test - and with that general relativity works. I don't think one can say a lot more..

Share this post


Link to post
Share on other sites

He is free to hold that view. The problem is that as our understanding of space-time changes, and it must do at some level when we take quantum effects into account, will the classical view of space-time still be 'physical'?

 

Probably not. The cost of being a realist is that you have to keep adjusting your view of what reality is when theories change.

Share this post


Link to post
Share on other sites

 

 

It just means that the predictions made by the theory match the measurements we make.

Please expand further - By that I presume you mean experiments such as Gravity Probe B lend support to the substantivalist (Space-Time as a Physical Entity) view as opposed to the relationist view?

Share this post


Link to post
Share on other sites

Please expand further - By that I presume you mean experiments such as Gravity Probe B lend support to the substantivalist (Space-Time as a Physical Entity) view as opposed to the relationist view?

You take a peice of paper and a pen, and do some calculation that say I will measure some value of something to be X. I then do some experiment and I see that I do, near enough measure the value of that thing to be X.

 

We can claim no more that this.

Edited by ajb

Share this post


Link to post
Share on other sites

Please expand further - By that I presume you mean experiments such as Gravity Probe B lend support to the substantivalist (Space-Time as a Physical Entity) view as opposed to the relationist view?

 

 

No. They are philosophical views so the evidence doesn't distinguish between them.

 

All I mean is that the mathematics of the theory makes predictions. Experiments make measurement which are compared with those predictions. If the measurements match the predictions then the theory is working.

 

If you are a substantialist (not a word I have ever come across before) or a relationist (whatever that is) or an idealist or a realist or an orthodox pastafarian doesn't make any difference as to whether the measurements match the theory.

Share this post


Link to post
Share on other sites

Which shows that if physics progress enough to explain the problem of time, the metaphysical issue may vanish.

Maybe, yeah, depending on how you view it. This is metaphysics indexed to physical theory. It's not reasoning what is real full stop; it's reasoning what is real if GR is true*.

For this particular problem to go away, GR would have to be abandoned in all but name, as it's a matter that arises from the theory itself and is formalism invariant. It's not just an artifact of the EFEs. It is present in all equivalent formulations. Anything that we can accurately call GR has the problem that we can start with two identical metrics at the exact same point, take each along a different path to identical ending points, and have different metrics when you get there.

 

So, if you mean that progress of physics removes the problem from the metaphysics of time, then sure. If you mean it removes the problem from GR, I'm not so sure, since it will mean GR will have changed to the point that it is unrecognizable from what it is now.

 

 

IOW that the metaphysical arises only because there is no affordable explanation from the physicists.

Sort of. Some physicists still do metaphysics (like my intellectual mancrush, Carlo Rovelli), but over time, more physicists have abandoned going further from the model to what options it implies for reality. The problem with that is that sometimes the metaphysics is really important in coming up with new models (like in quantum gravity).

 

 

I can imagine that as theoretical physics advances some metaphysical questions will evaporate - but only to be replaced with others!

I'd agree with that. And it's possible that it will go the other way for some issues, too. For example, if Rovelli's metaphysics of time ends up solving the problem of time for quantum gravity. Often, in metaphysics of physics, the line between the physicist and the metaphysicist is blurred (and sometimes physicists do metaphysics and sometimes philosophers do physics [i think it was John Earman that wrote a fantastic paper about whether or not one could survive time travel]).

 

 

Einstein proposed it then Gravity Probe B measured it and confirmed it. How real do you want it?

What is "it" that Gravity Probe B measured?

Share this post


Link to post
Share on other sites

What is "it" that Gravity Probe B measured?

Variations in distance over time relative to a reference. These are measurable properties of the spacetime model. 'It' is nothing more than is measured. Don't go all ontological/philosophical/metaphysical on me. :P

Edited by StringJunky
  • Upvote 1

Share this post


Link to post
Share on other sites

Very interesting post ( and subject ), ydoaPs.

It seems like Loop Quantum Gravity, as pushed by your 'man-crush' ( Rovelli ), has built-in answers to a lot of the problems.

The fact that, in the classical limit, it seems to reduce to Einstein-Cartan theory and not GR does however present a problem.

It seems that the torsion tensor being non-zero in ECT, causes the Dirac equation to become non-linear, and causes problems with quantization.

 

But I'm very much out of my depth here.

Maybe one of the other members with a deeper understanding of GR and ECT can break it down for us

  • Upvote 1

Share this post


Link to post
Share on other sites

By the way ,is it entirely apparent that we cannot hit spacetime with a hammer (a very big one) ? Did the BBH achieve that ? (did it "shiver our timbers" as its waves passed through our bodies?)

 

 

Yes, though not an aether-like medium, we attribute all sorts of things to this thing called "spacetime": gravitational waves, the ability to "stretch and effect EM wave frequency," the possible need for it to contain energy in order to exist, the notion that spacetime has a shape (the disc-like universe) the ability to inflate at a faster than light "speed," etc.

 

Space may or may not have mass, but it may intrinsically require the presence of stress-energy. Even the quantum vacuum of space implies density and the presence of certain particles, but the most important factor is dark energy:

 

if there’s an intrinsic energy to space, and it’s expanding (and therefore creating more space), aren’t we violating the conservation of energy? The answer is no, because dark energy doesn’t only have an energy density: it also has a negative pressure with very specific properties. As that negative pressure pushes outwards on space, it does negative work on the Universe, and the work it does is exactly equal to the increased mass/energy of whatever patch of space you’re looking at...If the Universe either had dark energy and no acceleration, or had acceleration with no dark energy, then we’d have a problem. But we have one because we have the other, and in fact that’s how we know we have dark energy. http://scienceblogs.com/startswithabang/2012/07/26/empty-space-has-more-energy-than-everything-in-the-universe-combined/

In short, it seems fatuous to try to attribute physical characteristics to an abstract, mathematical notion of spacetime itself; however, it seems more feasible that spacetime is an attribute of stress energy, and that stress energy is connected with mass.

Edited by disarray

Share this post


Link to post
Share on other sites

In short, it seems fatuous to try to attribute physical characteristics to an abstract, mathematical notion of spacetime itself; however, it seems more feasible that spacetime is an attribute of stress energy, and that stress energy is connected with mass.

If you were fluent in the maths you would see what is meant when these physical terms are used and what aspects they are describing. Why not just learn the language and concepts as they are understood and described instead of criticising it or offering alternatives ? I don't know myself but I know there are ideas beyond what I know at the moment; no point in arguing from ignorance... is there? :)

Edited by StringJunky

Share this post


Link to post
Share on other sites

Strange quoted someone at the begining of this thread:

"On another forum, someone has defined "real" as meaning "you can hit it with a hammer". By this criterion, space-time is not real, even if the mathematical model accurately describes something that exists!"

 

You cannot hit radio waves with a hammer and yet they are very real. If you hit space-time with gravity instead of a hammer you will be able to see that it reacts, Einstein did a pretty good job explaining that. Spacetime is as real to me as the phone Im holding in my hands right now. Ofcourse spacetime is a mathematical model too, I dont see why it couldn't be both.

Share this post


Link to post
Share on other sites

You cannot hit radio waves with a hammer and yet they are very real.

 

 

Are they? They might just be a mathematical abstraction that does a good job of describing reality.

Share this post


Link to post
Share on other sites

 

 

Are they? They might just be a mathematical abstraction that does a good job of describing reality.

 

Please do prove that notion ;)

Share this post


Link to post
Share on other sites

Strange quoted someone at the begining of this thread:

"On another forum, someone has defined "real" as meaning "you can hit it with a hammer". By this criterion, space-time is not real, even if the mathematical model accurately describes something that exists!"

You cannot hit radio waves with a hammer and yet they are very real. If you hit space-time with gravity instead of a hammer you will be able to see that it reacts, Einstein did a pretty good job explaining that. Spacetime is as real to me as the phone Im holding in my hands right now. Ofcourse spacetime is a mathematical model too, I dont see why it couldn't be both.

The resulting effect is real, but how do you show that the radio waves are real?

Share this post


Link to post
Share on other sites

The resulting effect is real, but how do you show that the radio waves are real?

Correct me if I'm wrong but following the above logic might get us to the point where nothing can be proven real...even a nail being hit by a hammer.

Share this post


Link to post
Share on other sites

Correct me if I'm wrong but following the above logic might get us to the point where nothing can be proven real...even a nail being hit by a hammer.

Yes, reality, as we know it, is a macro phenomenon and radio waves are not a macro phonomenon; the ontology of them is a mathematical.one. Any thing fundamental, like photons, are made up of measurable parameters but what they are as an entity is subject to modelling, which can change with new information. What scientists know about fundamental phenomena is always an abstraction; they are outside of our physical sensibilities to detect without apparatus.

Edited by StringJunky

Share this post


Link to post
Share on other sites

Correct me if I'm wrong but following the above logic might get us to the point where nothing can be proven real...even a nail being hit by a hammer.

 

 

Well, the effect of the nail hitting the hammer can ultimately be described in terms of the electromagnetic forces between the outmost surface electrons in the hammer and the nail. These are quantum effects that behave in very strange and counter-intuitive ways. So I wonder if this really is a good test of "reality". :)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now