Spacetime curvature: Real or Not?

[dm4b]

Hi,

I'm new to this forum so maybe this topic has been addressed ad nauseum at some point before, so I apologize if so.

But, as the title suggests, do you feel the spacetime curvature is a reality, or is it just a mathematical convenience for making predictions?

dm4b

 

[DaveC426913]

do you feel the spacetime curvature is a reality, or is it just a mathematical convenience for making predictions?

What's the difference? Our model describes what we observe.

Curvature is just a word - an analogy - that helps us grasp the model and put it in more human-conceivable terms.

 

[dm4b]

What's the difference? Our model describes what we observe.

If you're only in the business to make predictions, I guess there is no difference.

But, if you want to really understand reality, there is a huge difference in my mind.

All depends on your mindset, I guess.

 

[DaveC426913]

But, if you want to really understand reality, there is a huge difference in my mind.

But what constitutes "really" understanding it? What constitutes "reality" beyond what we observe?

Our mathematical model is our current best fit for our observations. How do you propose to have some deeper insight than observations and a model to explain those observations?

I suppose the best answer to your question is: all we have is a model that so far seems to do a good job at describing what we see. There is nothing to suggest that our observations tomorrow will not render it obsolete. So, no our model canot be said to describe reality, it can only describe the aspects we know of so far.

 

[DaveC426913]

More succinctly:

If you're only in the business to make predictions, I guess there is no difference.

But, if you want to really understand reality, there is a huge difference in my mind.

If you're not satisfied with predictions and want to understand something deeper, then you're a philospoher, not a scientist.

 

[LAHLH]

This is a subject for Philosophers rather than Physicists (you may think the two should be one and the same like the good old days, but there you go..), google things like positivism and scientific realism for various takes on the matter. The issues are not confined to complex developments like spacetime, but say "Is an electron really 'real'?" or is it just a component of a mathematical model that makes predictions in line with observations? The classic example of this is phlogiston...

 

[Dale]

if you want to really understand reality

Do you have a scientific definition of "reality"? I.e. what experiment could you do to determine spacetime curvature's "realness"? If you cannot come up with a scientific definition of "reality" then you are asking a philosophical or religious question which is inappropriate for this forum.

 

[DaveC426913]

...you are asking a philosophical ... question which is inappropriate for this forum.

But certainly appropriate for the Philosophy Forum.

 

[dm4b]

Although I agree that philosophy and physics are two separate subjects, I don't think this question leaves the realm of physics.

Is physics about physically describing the nature of the Universe or not? Or, is it just about developing a mathematical model that makes accurate predictions? I personally feel both are a requirement! Also, if you can pick the mathematical model that also accurately describes the physical nature of the Universe, it's much more likely that model will have increased longevity compared to another.

Jump back a few hundred years. Ptolemy's epicycles made increasingly accurate predictions about the solar system, by adding more and more terms. Today, the fact that we actually send probes to other planets shows the necessity for another model that physically models the solar system correctly. But, on the logic I am hearing here, perhaps insisting that the sun was at the center of the solar system back then wasn't all that noble after all? After all, why should they have cared back then. Ptolemy's model did accurately predict what we observed as Earth bound citizens back in that day.

In retrospect and with hindsight at our side, we can easily look back and see the desire to get rid of Ptolemy's model and accurately represent the physical nature of the solar system. Why do we not have that same desire about spacetime curvature today.

Or, is it because we can see no "practical" value on determining whether or not spacetime curvature exists, we willing put the blinders back on? In the future, will humanity look back and regard us not worrying about questions like these as the most prudent thing we can do.

Or, is it something else ... this sort of reminds me of the teacher we had for our GR class back in school. He insisted that spacetime curvature does not physically exist. We assumed he must have had a good reason being an intelligent guy, so we excitedly asked him why, to hear what he had to say. He said he thought spacetime curvature was unreal, because he could not picture it in his mind. So I guess the Universe is constrained by what he could or could not imagine. We thought that was sort of a disappointing answer, to say the least.

But, what I am hearing here is to not even worry about it at all. Even less inspiring.

 

[dm4b]

Do you have a scientific definition of "reality"? I.e. what experiment could you do to determine spacetime curvature's "realness"? If you cannot come up with a scientific definition of "reality" then you are asking a philosophical or religious question which is inappropriate for this forum.

Why don't we just look up reality in the dictionary instead.

reality: a real thing or fact.

So is spacetime curvature a real thing, a fact? Or isn't it?

Or, do we let the philosophers beat us to the answer?

 

[DaveC426913]

Why don't we just look up reality in the dictionary instead.

reality: a real thing or fact.

So is spacetime curvature a real thing, a fact? Or isn't it?

Or, do we let the philosophers beat us to the answer?

Spacetime curvature is a name we use to label a mathematical model that does a good job of describing our observations of the interaction of massive bodies.

Please, again, what is the difference between "a model that describes what we observe", and "real reality"?

 

[LAHLH]

It's not that you should not worry about these things, I certainly like to think about them too, but the nature of these questions deems them to fall under the remit of Philosophy of Science. Sometimes physicists can treat Philosophy as a dirty word; I think this tradition may have become fashionable with the era of Physicists such as Feynman and it reminds me of a nice quote by marcus on the beyond the standard model boards

he withdrawal of philosophy into a "professional" shell of its own has had disastrous consequences. The younger generation of physicists, the Feynmans, the Schwingers, etc., may be very bright; they may be more intelligent than their predecessors, than Bohr, Einstein, Schrödinger, Boltzmann, Mach and so on. But they are uncivilized savages, they lack in philosophical depth – and this is the fault of the very same idea of professionalism which you are now defending.

It is within the subject of Philosophy of Science that these ideas about what it means for something to be 'really real' actually mean, and if there is something more we should seek than just models that agree with observational data. Note that if epicycles worked then they worked (I can't remember if there are some other problems with epicycles) but for the sake of argument we only stopped using that mathematical model because we went to space and had a look...then going to space and having a look at the way "things really are" counts as observational data that our models from then on need to fit to just as much...without such observation the epicycles model is just as real to the thinkers of the time as the Copernican model etc

What if you have multiple theories that all fit your data? then typically Occams razor or parsimony is invoked, and we choose the one that makes the simplest assumptions and still gets the right answers. We have no other measure of 'reality'.

 

[DaveC426913]

Or, do we let the philosophers beat us to the answer?

That's the thing. Philosophers will not get to an answer. They cannot conclude what really "is". But they can talk about it all day long, knowing that there is no wrong answer.

Scientists see that we can make tangible progress whether or not we know "what really is". That's not uninspiring at all.

 

[dm4b]

More succinctly:


If you're not satisfied with predictions and want to understand something deeper, then you're a philospoher, not a scientist.

So you're saying science is not about gaining a deep understanding of the Universe?

My goal with science was to gain the deepest understanding of the Universe I possibly could. Maybe I was misguided ...

 

[DaveC426913]

So you're saying science is not about gaining a deep understanding of the Universe?

No. It is. It's just about ensuring that your deep understanding is empirical, not philosophical.

And empirical is reality, philosophy is not reality.

Likewise: the formulae describe the actual behaviour, whereas the word "curvature" does not.

 

[Dale]

Why don't we just look up reality in the dictionary instead.

Dictionary definitions are next to useless in physics, e.g. just look up the dictionary definitions of work, force, and energy.

If you are speaking scientifically then you need a scientific definition. Scientific definitions consist of a mathematical relationship with other defined quantities and/or an experimental process for measuring it. I have yet to see a good scientific definition of reality.

reality: a real thing or fact.

Not only is this definition non-scientific, it is also circular.

 

[Dale]

Although I agree that philosophy and physics are two separate subjects, I don't think this question leaves the realm of physics.

Physics is a science, and therefore it answers questions using the scientific method. So the key difference between philosophy and physics is experimentation. If you cannot, even in principle, perform an experiment that would answer the question then the question cannot be addressed by the scientific method and is therefore not a scientific question.

So, if the question "is spacetime curvature real" could have an associated experiment (e.g. the bending of light) to answer it, then it is a scientific question. If however in your mind no experiment could possibly answer the question then it is philosophy, not physics.

Is physics about physically describing the nature of the Universe or not? Or, is it just about developing a mathematical model that makes accurate predictions? I personally feel both are a requirement!

I personally feel they are both the same. If your mathematical model can accurately predict the behavior of the universe then in what sense would your model not be a physical description of the nature of the universe?

 

[DaveC426913]

I personally feel they are both the same. If your mathematical model can accurately predict the behavior of the universe then in what sense would your model not be a physical description of the nature of the universe?

I agree.

dm4b, it's not that we're shutting you down, it's that we don't what distinction you're making.

 

[dm4b]

No. It is. It's just about ensuring that your deep understanding is empirical, not philosophical.
And empirical is reality, philosophy is not reality.

I disagree with "And empirical is reality" and here's why (in a long winded sort of way)

First, I know us modern scientific types like to think physics and "philosophy" are totally separate, but they are not (totally). Like above when it was recommended that I Google positivism, or when you mentioned empiricism here, or when we think of other terms like materialism and reductionism - well, these are all philosophical viewpoints. And physics and science in general, always works under the current paradigm, which is essentially a philosophical world view. Today, science operates under a mixture of reductionistic, materialistic, positivistic paradigms amongst others, to differing extents. So, keeping that in mind ...

You say empiricism is reality? Empiricism essentially means derived, proved, verified, by experience or experiment. If you say that IS reality, you're saying reality is constrained by what we can observe or experience and, in turn, experiment upon. (Remember my GR teacher from above?) Well, the Universe and reality got along fine without human beings and our experiments and observations for billions of years. So, I say empiricism IS NOT reality, it's what you, as a human being, are capable of experiencing and seeing of reality, within your current mindset or paradigm you operate under. Because, once you get the results of your experiments, your interpretations of the results will be "colored" by or subjected to your paradigmatical thinking, to at least some extent.

Now, like all paradigms and philosophical mindsets, they evolve, leaving currently held ones eventually obsolete. What does the future hold for empiricism? Well, hard to say, but here's what I see.

String Theory is already positing realms that may be inaccessible to direct experiment (in any reasonable timeframe). String Theory allows extra compactified dimensions of a certain size, that we may be able to directly probe, but it does not guarantee that. It may be that we will never be able to empirically verify the existence of these higher spatial dimensions. So, under the logic you mentioned, that leaves us with two options - if the extra dimensions are large enough to be empirically observed/verified, then they are real. But, if they are too small to be empirically observed then they are unreal. Now, maybe in the distant future, we will be able to probe them, and by doing so, will we then have made them real at that very instant? No, I don't think so. I don't think empiricism IS reality.

What will the theory AFTER String Theory be like. How difficult will it be to bring under empirical, experimental validation? Something to think about.

Likewise: the formulae describe the actual behaviour, whereas the word "curvature" does not.

Yes, formulae do describe behavior. But, that is a human invented tool. Gravity worked before humans and their tools and concepts existed, right? And, if gravity IS spacetime curvature, than curvature has more to do with reality than the formulae.

So, once again, is spacetime curvature real? Or, isn't it?

 

[DaveC426913]

You say empiricism is reality? Empiricism essentially means derived, proved, verified, by experience or experiment.

No, it means what we actually see as opposed to what is derived.

That the planets orbit in ellipses is empirical. That it is due to the "force" of gravity or the curvature or spacetime is the model htat we've derived to explain it.

What is empirical is what is.

 

[dm4b]

I agree.

dm4b, it's not that we're shutting you down, it's that we don't what distinction you're making.

Thanks, I'll have to try and answer this and Dale's post later.

My fingers are tired from my typing up my last post, lol

In the meantime, anybody else got a viewpoint on the spacetime curvature, per the OP ;-)

 

[PeterDonis]

Well, the Universe and reality got along fine without human beings and our experiments and observations for billions of years.

How do you know this, if not from human beings' empirical observations (and models that were constructed based on them)?

 

[dm4b]

No, it means what we actually see as opposed to what is derived.

Read what I said again and notice the word "by" nestled in between derive and experience/experiment, and you'll see we are saying the same thing.

 

[jtbell]

This is philosophy, not physics. Any further discussion along these lines should take place in the Philosophy forum here, after taking due regard of the specific guidelines for that forum.