# Space and time

In physics space and time (or spacetime) are talked of as real entities that can be stretched and bent, but is there a justification for thinking in this way? I see space and time as relations between objects or events: you need at least two points to have a distance between them and two events to have a time elapsed. So is spacetime really a "thing" that can be curved or merely a relational concept?

The only justification necessary for a physical theory is that it meets the criteria.
In General Relativity (GR), the geometry of spacetime is be effected by matter.
Similarly, the motion of matter is effected by the geometry of spacetime.
This deep and unexpected insight made by Einstein has been verified by experiment.
If you accept that GR is a correct theory, then spacetime has geometry.
Should this fit your definition of a "thing" then yes, spacetime is a "thing".

The justification is that a photon (i.e. light) despite having no mass can have its path bent by gravity (This is how GR was originally proved, by observing that the ligth from stars as seen behind the sun during a solar eclipse (because you can actually make out these stars during a solar eclipse where they're drowned out otherwise) were 'bent' as the passed near the sun. (there are of course many many other results but that's a quick and easy one).

No, "space" and "time" are not the names of any things. They are nouns because this is forced on us by the grammar of our language, but not all nouns are simply a person, place, or thing.

When a physicist says that spacetime stretches and bends, he just means that gravitational motion can be explained using a geometric theory wherein masses always move in straight lines but the coordinates we use to measure that become distorted. Instead of "the bending of spacetime" we could talk about "the distortion of spacetime coordinates."

I think that the geometry is a mathematical construct and isn't the same thing as the physical reality. We may know empirically that objects will follow a curved path but we do not know that this "spacetime" is something there that is curved. I think that you need at least two (pointlike) objects or events for the concepts of space and time to make sense. Without anything in the universe would it make sense to say that space or time existed? There would be nothing to have a distance or time between. So what I am asking is whether you think spacetime exists independently of the objects within it or only comes into existence as a relation between them.

Except relativistic time distortion is seen all the time in experiments.

No, "space" and "time" are not the names of any things. They are nouns because this is forced on us by the grammar of our language, but not all nouns are simply a person, place, or thing.

When a physicist says that spacetime stretches and bends, he just means that gravitational motion can be explained using a geometric theory wherein masses always move in straight lines but the coordinates we use to measure that become distorted. Instead of "the bending of spacetime" we could talk about "the distortion of spacetime coordinates."

true but is an apple really an apple or do we just call it that?

I don't think that existential philosophical questions should be discussed in the context of physics.
We are best to avoid bickering philosophy and semantics.

Have you read a Brian Greene book? Popular physics is all about metaphysical ramblings.

Have you read a Brian Greene book? Popular physics is all about metaphysical ramblings.

... which is why I hate popular physics.
Anyone interested in learning something solid should check out Lee Smolin.

Leonard Euler said:

"Although to penetrate into the intimate mysteries of nature
and thence to learn the true causes of phenomena is not allowed
to us, nevertheless it can happen that a certain fictive
hypothesis may suffice for explaining many phenomena."

I don't think that existential philosophical questions should be discussed in the context of physics. We are best to avoid bickering philosophy and semantics.

It sounds like you are afraid of what might be true, and you would prefer to keep your head in the sand. I also think we should avoid bickering, but what is "bickering philosophy?" In case you think philosophy is a waste of time, then I would ask what isn't a waste of time?

true but is an apple really an apple or do we just call it that?

That's silly. Only if we had convincing fake apples would we ever wonder if a particular apple was really an apple. And if we had convincing fake apples strewn about then the question "is that a real apple?" is perfectly sensible.

Anyway, "apple" is a simple noun that refers to something. What about "pain"? If you think that "pain" refers to a private sensation that only one person is experiencing, then how could you have learned the correct use of the word "pain" at your mother's knee?

Brian Greene

Abomination.

What does the fact that time distortion is seen in experiments have to do with anything? All this means is that the relation between two events is different in different depending on the velocity of the observer. The very fact that it is called a "distortion" is what is misleading you into thinking that time is something tangible, rather than a relation between two events.
I also don't see why I can't discuss existential topics in the context of physics - this is the philosophy forum, and it's in a physics forum. Lastly, I don't see how this is any more a metaphysical rambling than the standard interpretation of spacetime as something which can be curved and distorted, has anyone actually seen this spacetime?

One can get a fair amount of metaphysical mileage out of all things not GUT (or TOE). Lee Smolin is just a proponent of another type of GUT.

What does the fact that time distortion is seen in experiments have to do with anything? All this means is that the relation between two events is different in different depending on the velocity of the observer. The very fact that it is called a "distortion" is what is misleading you into thinking that time is something tangible, rather than a relation between two events.
I also don't see why I can't discuss existential topics in the context of physics - this is the philosophy forum, and it's in a physics forum. Lastly, I don't see how this is any more a metaphysical rambling than the standard interpretation of spacetime as something which can be curved and distorted, has anyone actually seen this spacetime?

Well no one is thinking there is some 'tangible' space-time which can physically be 'stretched'. Rather we say that empirically we believe that the correct predictive physical model of events is one where mass causes a change in the geometry of the mathematical space-time construct through which we abstract our real-world variables. You can take from that what you will

The very fact that it is called a "distortion" is what is misleading you into thinking that time is something tangible, rather than a relation between two events.

I totally agree with everything madness says (uh oh), and I want to point out that the word reify does not exist for no reason.

Reify: To regard or treat an abstraction as if it had concrete or material existence.

Also I take offence at your recommendation of Brian Greene and popular physics for its "metaphysical ramblings", I am nearly finished my mathematical physics degree and have no need for popular physics.

It sounds like you are afraid of what might be true, and you would prefer to keep your head in the sand. I also think we should avoid bickering, but what is "bickering philosophy?" In case you think philosophy is a waste of time, then I would ask what isn't a waste of time?

Oh, come on, that's a personal attack and totally uncalled for.
I didn't notice that this thread is under the General Discussion > Philosophy heading.
I take it back, this is the perfect place to bicker philosophy.

You should read some of Feynman's lectures. He was not a fan of these kinds of discussions. He takes some reasonably humorous jabs at them.

Well no one is thinking there is some 'tangible' space-time which can physically be 'stretched'.

Then why didn't you answer the question "no", instead of saying "the justification is..."

I didn't notice that this thread is under the General Discussion Philosophy heading.
I take it back, this is the perfect place to bicker philosophy.

Fair enough, I apologize.

You should read some of Feynman's lectures. He was not a fan of these kinds of discussions. He takes some reasonably humorous jabs at them.

I tried to read Feynman's lectures, but he dodges all the good questions. Very cookie-cutter...

I did find him humorous in high school, and discovering his books back then made me major in physics, but now I find him to be full of hot air.

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And whether you're talking about the 'space-time distortion' of a mathematically abstract frame work intended to model the real-world or a real-world distortion doesn't make a lick of difference in terms of how we understand the universe.

You should read some of Feynman's lectures. He was not a fan of these kinds of discussions. He takes some reasonably humorous jabs at them.

Yes, I feel the same way, which is why I was trying to avoid the philosophy.
Instead, I should just stay out of it.

The one thing I wish to contribute is a reminder that physical laws do not translate well into philosophy.
At least, not well from the physicists point of view.
LOL

Well Feynman would say stuff like "And then we might ask what is the nature of the coulomb force and physicists will say it of the nature kqq/r^2 and then philosophers might ask questions like what is a force, and what is a coulomb and what is "the", meanwhile physicist have built a longer last light bulb".

Whether the distortion is of an abstract geometry or a real spacetime certainly makes a difference to how I understand the universe, maybe not you. I think my original question was very similar to Newton's bucket vs Mach's principle.

But the problem is you're not going to get an answer. I mean ultimately you can just back yourself into a Cartesian corner with Descartes cogito ergo sum (I think therefore I am) and say that you have no proof that the universe exists at all (only that you, as a consciousness exist) and until you've determine whether the universe exists or whether you're a brain in some jar in an evil genius' lab there's no point in trying to understand said universe. Philosophically you are correct but it just gets you nowhere.

I am not looking for an answer I just wanted a discussion. I don't think I'm heading in the sollipsist direction at all, I'm just questioning the physicist's conception of spacetime as a physical thing. I see space and time as relations, but I am not denying their existence completely.

I approve of such discussion and my point is merely that you can only really say that you can predict the position and time of events (and the notion of the 'time' of an event itself carries some metaphysical baggage) through treating them as a mathematically abstract 4 dimensional system which is non-euclidian. Past that there is no real way to 'improve' ones intuition about the goings on or relate that in a more 'physical' way to reality.

For example, consider the foundation of number theory. In the 1800's their was a sort of crisis of mathematical rigour where people felt they needed to put all of math on an absolutely solid mathematical ground. And one of the considerations was 'what is a number' what is the intrinsic 'oneness' of something that makes it 'one'. The problem is you really can't answer that question, all you can really say is that if we define a number as something like the set of all sets with that amount of elements and go on to dedekind cuts and such we can recover/create the mathematical framework we had/desire and move on. You're just not going to make any more headway then that even if it seems completely unlike reality to define 'one' in terms of set theory.

There's just no answer you're ever going to hear in your life that's going to make you go 'oh I see, that makes perfect sense'.

I agree, but when people talk of spacetime as some fabric which is expanding and distorting I feel as though they are talking complete jibberish. Look at the title of Brian Greene's book "the fabric of the cosmos". This often seems to be the standard scientific viewpoint.

But the problem is you're not going to get an answer. I mean ultimately you can just back yourself into a Cartesian corner with Descartes cogito ergo sum (I think therefore I am) and say that you have no proof that the universe exists at all (only that you, as a consciousness exist) and until you've determine whether the universe exists or whether you're a brain in some jar in an evil genius' lab there's no point in trying to understand said universe. Philosophically you are correct but it just gets you nowhere.

Wow, what a bad attitude towards philosophy. Have you read Descartes writings or thought about the context in which they were written? Think of all the counterarguments his solipsism spawned!

And whether you're talking about the 'space-time distortion' of a mathematically abstract frame work intended to model the real-world or a real-world distortion doesn't make a lick of difference in terms of how we understand the universe.

It does not make any pragmatic difference, but it does make an aesthetic difference. For me the study of physics and mathematics is more about aesthetics then pragmatics.

"And then we might ask what is the nature of the coulomb force and physicists will say it of the nature kqq/r^2 and then philosophers might ask questions like what is a force, and what is a coulomb and what is "the", meanwhile physicist have built a longer last light bulb"

Let's all bow before the false idol of pragmatic value, clutch our lightbulbs, and forget about the questions that we all wanted answers to in the first place.

Well the whole 'fabric' thing is jsut science popularizing stuff. I've always hated that as well. My personal favorite (read most hated) is when entropy is explained as 'the amount of disorder'. And then people think like hmm.... well I guess I just cleaned my room by putting my dirty clothes in the drawers and putting dirty dishes in the sink so I guess i've lowered its entropy or some such silliness.