I don't understand the Michelson–Morley experiment

[Dale]

You cant detect something that dont exist, frame/ space dont exist

Again, I can measure amounts of space, so space clearly does exist.

 

[Ibix]

You cant detect something that dont exist, frame/ space dont exist, nothing by definition cant exist.

You're the only one saying space is nothing, so that would seem to be a problem for you, not us. At least two of our members have pointed out that space is whatever it is that rulers are measuring.

 

[gen x]

What is definition of space in physics?

google search

• Classical Physics: Space is an absolute, static container or background independent of the objects inside it, defined by three linear dimensions (width, height, and depth).
• Modern Physics: Space is inseparable from time, forming a dynamic four-dimensional continuum known as spacetime as described by Einstein's theory of relativity, which can be warped by mass and energy.
• Quantum View: Space is not an absolute nothingness or empty void, but a complex fabric that holds quantum fields and energy fluctuations even in the absence of matter.

 

[Ibix]

What is definition of space in physics?

The operational definition is that space is "whatever it is rulers measure", and time is "whatever it is clocks measure". Taken together they appear to behave as a 4d manifold which we call spacetime. What that is, we don't know. Perhaps a future theory will tell us, or perhaps it will use a different model from which spacetime will be an emergent phenomenon.

 

[Dale]

What is definition of space in physics?

I would also use the operational definition. Those are the most important definitions in physics, in my opinion. So formally, "space is the measurand of a ruler", or less formally "space is what a ruler measures"

 

[Ibix]

Those are the most important definitions in physics, in my opinion.

Agree. They have the absolute minimum of assumptions. Rulers measure something (that we call space) and you can't have a theory that doesn't include that. You might build different theories that explain in different ways how that measurement comes to be (a quantum foam, spacetime, whatever), but on some level they have to say "when I Iay a ruler between these two objects it will read X".

 

[gen x]

The operational definition is that space is "whatever it is rulers measure", and time is "whatever it is clocks measure". Taken together they appear to behave as a 4d manifold which we call spacetime. What that is, we don't know. Perhaps a future theory will tell us, or perhaps it will use a different model from which spacetime will be an emergent phenomenon.

I would also use the operational definition. Those are the most important definitions in physics, in my opinion. So formally, "space is the measurand of a ruler", or less formally "space is what a ruler measures"

I think deeply we go into physics, we end up in philosophy, what is time, space, energy ...

Imagine you have very very long straight ruler, and put it close to huge planet(strong gravity), how will this ruler measure curved space if it is straight?

 

[Ibix]

Imagine you have very very long straight ruler, and put it close to huge planet(strong gravity), how will this ruler measure curved space if it is straight?

Ideally it would measure distance along a spacelike geodesic. I would be cautious calling that a straight line.

In general you will find that if you lay out three rulers (or equivalent devices) forming a triangle the corner angles will not quite sum to 180° and the area is not quite half base times height. The deviation is very small, but is visible in analysis of GPS signals, IIRC.

 

[PeterDonis]

how will this ruler measure curved space

You can't measure curvature with a single ruler. Curvature is geodesic deviation. You need two or more geodesics to compare in order to see curvature.

 

[PeterDonis]

distance along a spacelike geodesic. I would be cautious calling that a straight line.

"Geodsic" is the generalization to curved manifolds of "straight line", so I don't see a problem with such a phrasing. But of course if there is any possibility of confusion with "straight lines" in a flat manifold, yes, "geodesic" is the better term.

 

[Ibix]

"Geodsic" is the generalization to curved manifolds of "straight line", so I don't see a problem with such a phrasing. But of course if there is any possibility of confusion with "straight lines" in a flat manifold, yes, "geodesic" is the better term.

I personally prefer to reserve "straight line" for geodesics in flat spaces/spacetimes, but I agree there isn't anything else it could mean in curved spaces. In this thread and in light of the OP's instinctive rejection of non-Euclidean geometry I wanted to be clear that a ruler doesn't measure Euclidean distance if it is in a curved spacetime. There is no Euclidean distance for it to measure.

 

[A.T.]

Imagine you have very very long straight ruler, and put it close to huge planet(strong gravity), how will this ruler measure curved space if it is straight?

If you build the ruler in flat space (far away from any masses), and then near the mass you support it, such that it doesn't deform anywhere (as measured with local strain gauges), then it will be laid out along a spatial geodesic (shortest path in curved space, just like a straight line in flat space).

From 3 such rulers you can build a triangle, and measure its internal angles, to detect spatial curvature.

From: https://missionastro.org/geometry-on-a-cosmic-scale-the-surprising-flatness-of-space/

 

[gen x]

If you build the ruler in flat space (far away from any masses), and then near the mass you support it, such that it doesn't deform anywhere (as measured with local strain gauges), then it will be laid out along a spatial geodesic (shortest path in curved space, just like a straight line in flat space).

From 3 such rulers you can build a triangle, and measure its internal angles, to detect spatial curvature.

View attachment 373918

From: https://missionastro.org/geometry-on-a-cosmic-scale-the-surprising-flatness-of-space/

You mean my long ruler will be deformed due to gravity?

 

[Ibix]

You mean my long ruler will be deformed due to gravity?

No, he specifically said to provide supports so it isn't deformed. Spacetime, however, is not flat so the behaviour of triangles made of non-deformed rulers is different from the behaviour of triangles made of non-deformed rulers in flat spacetime.

 

[A.T.]

You mean my long ruler will be deformed due to gravity?

No. I wrote the exact opposite:

... you support it, such that it doesn't deform anywhere (as measured with local strain gauges)

 

[Andrew Mason]

Is this relative time ,space just math abstraction that somehow produce correct results or reality?
Does theory need to fit reality or only need to fit measurments results?

So long as a theory does not conflict with what is accurately observed by careful measurement/experiment, the theory is valid. But it is valid only within the limits of that non-conflict. So, Newton's laws are valid on a macroscopic scale with baryonic matter and interactions at relative speeds << c. Whether a theory reflects "reality" is something that is beyond science. Science does not distinguish between "reality" and a falsifiable theory that fits all experimental testing.

Why Tesla didnt accept relative time,space ?

I am not sure Tesla understood relativity or the Michelson-Morley result. He was a clever engineer, but not a scientist. He never stopped believing in the ether concept - that the universe is filled with ether, a fluid that is behind to all phenomena: light, electro-magnetism, gravity, matter.

 

[gen x]

No. I wrote the exact opposite:

if strain gauge measure zero when I build ruler in no gravity place and then I put ruler near heavy planet, so ruler will bend to follow "spacetime", then gauge must show non zero, because something bend material?

 

[gen x]

So long as a theory does not conflict with what is accurately observed by careful measurement/experiment, the theory is valid. But it is valid only within the limits of that non-conflict. So, Newton's laws are valid on a macroscopic scale with baryonic matter and interactions at relative speeds << c. Whether a theory reflects "reality" is something that is beyond science. Science does not distinguish between "reality" and a falsifiable theory that fits all experimental testing.

We can have more theories that deliver correct results (same as experiment), so that mean theories are just math models nor reality.

Why we think that only one theory from gravity will get correct result?

 

[Ibix]

if strain gauge measure zero when I build ruler in no gravity place and then I put ruler near heavy planet, so ruler will bend to follow "spacetime", then gauge must show non zero, because something bend material?

No. There is no distortion of the ruler. It is just embedded in a spacetime whose geometry does not follow Euclid's rules, so unstressed and undistorted objects do not behave as they would in a Euclidean space.

I suspect you are thinking of spacetime as embedded in a higher dimensional Euclidean space, just as the surface of the Earth is a 2d spherical surface embedded in a 3d Euclidean (well, nearly Euclidean) space. You would have to bend a straight ruler to get it to follow the Earth's surface, yes, but that isn't what is happening in @A.T.'s example. He isn't taking the ruler and trying to force it to conform to a curved surface embedded in spacetime - it's already in spacetime and it's the spacetime that's curved, not the ruler.

 

[Ibix]

We can have more theories that deliver correct results (same as experiment), so that mean theories are just math models nor reality.

Why we think that only one theory from gravity will get correct result?

Terminology isn't entirely consistent here, but typically two models that have the same maths but different "what does the maths mean" are called interpretations while models with different maths are different theories. The Lorentz Ether model, for example, has the same maths as SR, so would typically be thought of as an interpretation (although it's usually called Lorentz Ether Theory - like I say, terminology isn't perfect) while something like Brans-Dicke gravity (which makes different predictions from GR) would be a separate theory.

In that sense there is only one theory that can match experiment in all circumstances, but there can be multiple ways of interpreting the maths. In relativity the curved spacetime interpretation is overwhelmingly the most popular (probably because the Equivalence Principle has a trivial explanation in those terms). I believe it is possible to construct it as a gauge field theory on a flat background spacetime if you really want, although I'm not sure how well that fits with topologically non-trivial solutions of the field equations. I think we have an Insight article on that somewhere (edit: here), although you'll need a lot of maths to follow it.

 

[A.T.]

if strain gauge measure zero when I build ruler in no gravity place and then I put ruler near heavy planet, so ruler will bend to follow "spacetime", then gauge must show non zero, because something bend material?

No, read what I wrote:

you support it, such that it doesn't deform anywhere

If you support it by properly distributed forces, you can avoid the bending, and keep the local material stains at zero. So your ruler is not bent, but a triangle from 3 such rulers will still have a triangle angle sum different from 180°.

 

[Ibix]

if strain gauge measure zero when I build ruler in no gravity place and then I put ruler near heavy planet, so ruler will bend to follow "spacetime", then gauge must show non zero, because something bend material?

Adding to what I said above, it's probably worth noting that seen as a four dimensional object the ruler is curved (it has non-zero proper acceleration). It is, however, unstressed and therefore undistorted so remains straight in space (to the extent "straight" is the right word in curved spacetime).

Trying to treat GR as a theory of space confuses a lot of things. It's a 4d theory, not a 3d one.

 

[Herman Trivilino]

In short, I just ask what is wrong with Newton absolute frame. I dont understand why his choice/assumption is wrong.

Newton's absolute frame included an absolute time. We know from observation and experiment that that is demonstrably false. If engineers used that theory when synchronizing the atomic clocks aboard GPS satellites the whole system would fail.

 

[PeterDonis]

Newton's absolute frame included an absolute time. We know from observation and experiment that that is demonstrably false. If engineers used that theory when synchronizing the atomic clocks aboard GPS satellites the whole system would fail.

More precisely, we know that Newton's belief, which got baked into Newtonian physics, that all clocks tick at the same rate relative to each other, regardless of their state of motion, is demonstrably false. That's the belief that GPS falsifies.

"Absolute time", to Newton, kinda sorta meant the same thing, but over time that term accumulated other meanings (for example, that yes, clocks can tick at different rates relative to each other, but that's not because there is no absolute time, but only because clocks don't tick absolute time unless they're at rest relative to the unobservable ether), which are inherently unfalsifiable (part of the definition of the ether, at least after the Michelson Morley experiment, became that it was inherently unobservable which inertial frame was the absolute frame in which the ether is at rest).

Part of the issue the OP appears to be having is to focus on particular terms like "absolute time", "absolute space", etc., without really considering what they mean as a matter of physics.

 

[Herman Trivilino]

theories are just math models

The construction of a theory is a modeling process. It creates models.

 

[Dale]

I think deeply we go into physics, we end up in philosophy, what is time, space, energy .

I prefer the operational definitions precisely because they minimize the philosophical dead ends. It also gives our theories an explicit connection to experiment. That is what makes a theory different from just math. The theory has an explicit mapping between elements of the theory (time) and experimental results (what a clock measures).

Imagine you have very very long straight ruler

How do you know that it is straight? (This turns out to be important, it isn’t impossible but it requires careful consideration.)

As others already mentioned, curvature comes from the combination of multiple rulers. Suppose that you have 12 rulers, each of which is straight and all of which are equal length. Assemble them into a hexagon with 6 spokes all going to the center.

If the space is flat then opposite spokes will line up into a straight line. If the space has positive curvature then there will be an angle between opposite spokes at the center. If the space is negatively curved then it will not be possible to gather all of the spokes together at the center without stretching them.

This formulation doesn’t require you to measure angles, but is conceptually the same as the examples given by others above where the interior angles of a triangle don’t sum to 180.