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

[question4477]

As far as I know there are some Physicists who consider Space-Time to be an actual physical thing whereas others regard it as merely an abstract concept - Brian Greene expressed his belief that the results from Gravity Probe B confirm this. What is the general consensus of this?

[ajb]

In physics one builds mathematical models and see if they match nature well. As the mathematical notion of space-time works so well - and Gravity Probe B is part of the evidence for this - people often speak as it space-time is real.

 

But I am not fully sure how much is just philosophy and how much is physically real. I mean, any calculation and experiment needs to be conducted and then interpreted within a physical theory. Really separating what one 'sees' from mathematics is not so clear.

 

Anyway, the usual thinking is that space-time is a mathematical construction - but how different is this to other notion in physics?

[StringJunky]

In physics one builds mathematical models and see if they match nature well. As the mathematical notion of space-time works so well - and Gravity Probe B is part of the evidence for this - people often speak as it space-time is real.

 

But I am not fully sure how much is just philosophy and how much is physically real. I mean, any calculation and experiment needs to be conducted and then interpreted within a physical theory. Really separating what one 'sees' from mathematics is not so clear.

 

Anyway, the usual thinking is that space-time is a mathematical construction - but how different is this to other notion in physics?

Would you say spacetime is as 'real' as, say, the magnetic field? It's real in the sense that there are parameters to it that can be measured?

Edited August 5, 2016 by StringJunky

[ajb]

Would you say spacetime is as 'real' as, say, the magnetic field? It's real in the sense that there are parameters to it that can be measured?

Basically yes - I am not sure what a deeper meaning of 'real' is. This starts to get into metaphsyics.

 

But again yes, the electromagnetic field is just as 'real' as space-time or the curvature thereof.

[StringJunky]

Basically yes - I am not sure what a deeper meaning of 'real' is. This starts to get into metaphsyics.

 

But again yes, the electromagnetic field is just as 'real' as space-time or the curvature thereof.

Thanks. I know the notion of 'real' is really a macroworld concept, usually related to whether something is sensorily tangible to us.

Edited August 5, 2016 by StringJunky

[Strange]

Thanks. I know the notion of 'real' is really a macroworld concept, usually related to whether something is sensorily tangible to us.

 

 

I would say it is more of a philosophical concept. By which I just mean that one has to define (precisely) what one means by the word "real" before the conversation can actually be useful.

 

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!

[StringJunky]

I would say it is more of a philosophical concept. By which I just mean that one has to define (precisely) what one means by the word "real" before the conversation can actually be useful.

 

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!

Rightly or wrongly, I think a practical definition of it is that it is something that can be objectively measured i.e. it is not a manifestation detected or perceived by just one person. Other people can do the same procedure and get the same result. The notion of something being real must have more than one observer to confirm its existence.

Edited August 5, 2016 by StringJunky

[Strange]

Rightly or wrongly, I think a practical definition of it is that it is something that can be objectively measured i.e. it is not a manifestation detected or perceived by just one person.

 

The trouble is, it isn't always clear what is being measured. For example, it can be argues that the Pound-Rebka directly measures the effects of the curvature of space-time. Or, does it just measure "something" that we describe as the curvature of space-time. (Which is back to the question in the OP).

[StringJunky]

The trouble is, it isn't always clear what is being measured. For example, it can be argues that the Pound-Rebka directly measures the effects of the curvature of space-time. Or, does it just measure "something" that we describe as the curvature of space-time. (Which is back to the question in the OP).

I see what you mean but if I put a voltmeter probe to a wire, is it measuring that which we call 'electricity', or is it something else? What something 'is', is ultimately agreed by consensus and may, of course, change with further experiments. As you know, science is always in a state of evolving flux.

Edited August 5, 2016 by StringJunky

[swansont]

I see what you mean but if I put a voltmeter probe to a wire, is it measuring that which we call 'electricity', or is it something else? What something 'is', is ultimately agreed by consensus and may, of course, change with further experiments. As you know, science is always in a state of evolving flux.

 

 

But is that flux real?

[Strange]

I see what you mean but if I put a voltmeter probe to a wire, is it measuring that which we call 'electricity', or is it something else?

 

It is measuring something we call electricity. I have no idea what that is, though. It might be something to do with electrons, if they exist rather than just being a useful model ...

[StringJunky]

But is that flux real?

Flux is not a thing though; it is a descriptive describing something that is not static in terms of change.

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It is measuring something we call electricity. I have no idea what that is, though. It might be something to do with electrons, if they exist rather than just being a useful model ...

I'm not arguing that things are only models in science. I agree. The point is that spacetime is a useful model that has measurements to support it. Now, take string theory as a contrast... that is a figment of mathematical imagination.. so far.

Edited August 5, 2016 by StringJunky

[ajb]

We all see that it becomes confusing very quickly, this is the nature of metaphysics

[Strange]

What is the general consensus of this?

 

I don't think there is one. I am a "naive realist" by inclination so I tend to think that things like electrons and spacetime are real things that are described by our theories.

 

But I know that opinion is silly and completely irrational.

[StringJunky]

We all see that it becomes confusing very quickly, this is the nature of metaphysics

Yes.

[MigL]

You can hit your thumb with a hammer.

We know our thumbs are real. And we can see the atoms that make up our thumb.

One could make the case that we can even 'see' the electron orbitals of those atoms, and so, they must be real.

 

According to our best model they are manifestations of quantum fields.

But ( and I think Swansont has brought this up in a thread a few yrs back ) there are many quantum fields which have associated quantum particles. Do particles like phonons, inflatons, etc. actually exist ? will we ever be able to see them ? Or are they just mathematical constructs which make calculations easier ?

 

As has been mentioned already; it all depends on your subjective definition of 'reality'.

Can there evenbe an objective definition ?

[StringJunky]

Can there evenbe an objective definition ?

Can't something be defined as the 'sum of its measurable parameters? If it's not measurable it's not a scientific reality.

[geordief]

How about this.? Space-time is a mathematical model (and is real as such but perhaps subjectively -or collectively** subjectively- real) ).

 

But Spacetime is also a name that we give to the thing that we think the forenamed model describes (this is also "real"- perhaps objectively real)

 

So there are two spacetimes which share the same name but they should have different names.

 

Approaching it from another angle or aspect, we have the measurer (the spacetime model) and the measured (the "reality" it is attempting to model)

 

Now the measurer and the measured may be a symbiotic collective entity and so each is as real as the other and neither can stand on its own.

 

** I am imagining a group who broadly agree upon a scientific consensus.

 

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?)

Edited August 5, 2016 by geordief

[StringJunky]

collectively** subjectively- real) ).

 

 

** I am imagining a group who broadly agree upon a scientific consensus.

In other words: Intersubjectively real

 

I go along with this. They do it anyway; acknowledged or not.

Edited August 5, 2016 by StringJunky

[Strange]

Do particles like phonons, inflatons, etc. actually exist ? will we ever be able to see them ? Or are they just mathematical constructs which make calculations easier ?

 

 

In our industry we make use of holes as charge carriers. It is quite hard to argue that holes "exist"

[ydoaPs]

The modern form of this question has been around since (and discussed by) Einstein in the form of the Hole Argument. TL;DR: The mathematical formalism leaves us with two options. Either GR is indeterministic, or spacetime points are merely mathematical convenience rather than anything ontologically real.

 

This problem goes from blurring-the-line-between-philosophy-and-physics to a big effing deal for physicists when it comes to quantum gravity theories, because it is the direct reason behind The Problem of Time.

[michel123456]

The modern form of this question has been around since (and discussed by) Einstein in the form of the Hole Argument. TL;DR: The mathematical formalism leaves us with two options. Either GR is indeterministic, or spacetime points are merely mathematical convenience rather than anything ontologically real.

 

This problem goes from blurring-the-line-between-philosophy-and-physics to a big effing deal for physicists when it comes to quantum gravity theories, because it is the direct reason behind The Problem of Time.

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

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

Edited August 6, 2016 by michel123456

[ajb]

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

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

 

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

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!

[question4477]

The modern form of this question has been around since (and discussed by) Einstein in the form of the Hole Argument. TL;DR: The mathematical formalism leaves us with two options. Either GR is indeterministic, or spacetime points are merely mathematical convenience rather than anything ontologically real.

 

This problem goes from blurring-the-line-between-philosophy-and-physics to a big effing deal for physicists when it comes to quantum gravity theories, because it is the direct reason behind The Problem of Time.

Very interesting, so did Einstein himself have a substantivalist or relationist view of Space-Time?

[ajb]

Very interesting, so did Einstein himself have a substantivalist or relationist view of Space-Time?

" ... 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