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Is Spacetime Real?

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Re: Is Spacetime Real?

#51

Post by jodrellblank »

thisiscorrect wrote: post #19
jakelazaroff wrote: post #11

Earlier quoted context omitted.

Atoms aren’t just something we’ve modeled — they’re a physical thing , i.e. matter. We can sense them directly, not just infer their existence by measuring related phenomena. I realize when you get into theoretical physics the “thingness” of concepts becomes more and more abstract, but atoms don’t fall into that bucket.

You sense spacetime directly too. It's what holds you do this planet we inhabit. I think atomic phenomena may feel somehow more familiar because it takes far less energy to make them dynamic than it does for gravitational phenomena. If we were somehow huge beings made of binary stars or something, gravity would probably be the very familiar force and atomic phenomena almost inconceivably small to you.

Atomic phenomena are almost inconcievably small to me. There's around a hundred trillion atoms in a human body cell, and around a hundred trillion cells in a human body, plus or minus an order of magnitude or two. It's astonishing to me that people in the early 1900s could measure and calculate the charge on an electron, or that Marie Curie had to process tons of pitchblende to purify micrograms of Radium - and could, and did. Or that Rutherford could say where most of the mass inside an atom is, and split the nucleus of Nitrogen atoms - 1/1000th of the size of the whole atom - and tell what was happening down at that scale - using the technology of 1917.

Re: Is Spacetime Real?

#52

Post by sailingparrot »

AnIdiotOnTheNet wrote: post #47
sailingparrot wrote: post #43

Earlier quoted context omitted.

So then the Aether was real for a while and stopped being real 200 years ago? If we discover a new model that makes better prediction than our current spacetime based model, just like relativity was to Newton's theory, will spacetime stop being real? The article was clearly using "real" with the definition we attach to it in our everyday language, not in deep philosphical sense.

> So then the Aether was real for a while and stopped being real 200 years ago? Yeah, pretty much. > If we discover a new model that makes better prediction than our current spacetime based model, just like relativity was to Newton's theory, will spacetime stop being real? In so far as Newtonian gravitation is also not-real. We still use that model for a lot of things of course. I don't know much about Aether theory…

I do agree with your point, but I think we are seeing the limit of language: when we don't attach the exact same definition to words, we can misunderstand each others while trying to convey the same thing.

The way you use "real" is the way I would use "valid": A theory is valid until proven otherwise.

And the way I use "real" (and I think it is the same way the article used it), is whether something has any grounding in our universe (some pattern as you said before).

For example, wormholes are possible following the theory of general relativity, but we don't know if they are "real", e.g. can there actually be a wormhole in our universe. If we do end up finding one, then they are real. If we however find conclusive evidence that they cannot exist in our universe, then they are not real, but only an artifact of our (incomplete) theory of general relativity.

Re: Is Spacetime Real?

#53

Post by AnIdiotOnTheNet »

jodrellblank wrote: post #50
AnIdiotOnTheNet wrote: post #37

Earlier quoted context omitted.

I'm saying the differentiation is meaningless. We have a model of space time that makes accurate and measurable predictions and that is as real as real gets.

We have a photo of a building that makes accurate and measurable predictions of things like the roof being above the door, and how far, and which parts are transparent (windows) and which parts opaque (walls), and how many people could fit inside it. Is that picture as real as buildings get?

Is your claim that the picture is the building? Because that's the equivalent of claiming that the a Penrose diagram is spacetime, instead of a representation of the model.

The photo of the building is an accurate diagram of the model of that one individual building. It describes the building, it is not actually the building. However, your model (roof height, walls, location, etc) is the building. If that building is destroyed in a fire and remade to those exact specs, do we not say it is still a building?

Re: Is Spacetime Real?

#54

Post by f154hfds »

thisiscorrect wrote: post #5

“What exactly is spacetime? Is it a real thing like an atom, or just a mathematical construct that is used to describe how mass 'generates' gravity?” This is a false dichotomy. Atoms too are "just" a mathematical construct used to describe how certain observable, physical systems behave. It's even called the atomic _theory_, and like all other good theories it knows its limits. At low enough energies, it's too fine a…

I think you and the author are talking about different concepts. To your credit, for the author's 'real thing like an atom' here, the term real is ambiguous.

The question the author is talking about isn't whether a particular theory is correct, or even whether a theory contains explanatory power. I believe the author is assuming general relativity for the sake of argument, and discussing the concept of existence within the general relativity theory's framework.

For example, take the three-body problem [1]: does the _theory_ of general relativity require us to add initial conditions to the three body problem? In other words, are there more fundamental initial conditions not derivable that must be inserted as inputs into the equations when we use general relativity to solve the problem instead of say Newtonian mechanics? The answer is No. While the equations will change, the initial conditions (the degrees of freedom) do not.

What does that mean? It demonstrates indivisible properties of the universe given a set of theories. With general relativity now 'energy' and 'mass' are interchangeable, but the existence of mass/energy isn't a fictitious force, it can't be explained away by coordinate transformations. Gravity can't either, but a gravity well points to an underlying reality of mass/energy, not the other way around. If it were the other way around, we would say gravity is the 'real' thing, and mass/energy are resulting effects.

[1]: https://en.wikipedia.org/wiki/Three-body_problem

Re: Is Spacetime Real?

#55

Post by jodrellblank »

Always casually wonder that as you approach the speed of light, Lorentz contraction shrinks distance in the direction of travel, and slows time. Photons travelling at the speed of light experience no time in the direction of travel. Therefore they experience no distance in the direction of travel.

In some frame of reference, a photon leaving the Sun and hitting your retina, is a 0-distance, instantaneous connection between the Sun and your retina. And in another frame of reference, the photon "should" be stretched a hundred million miles long - or, not be a discrete corpuscular thing with a front and a back which travels through the intervening space.

It would be simpler to imagine if there was a nice Aether already connecting the Sun and your eye, for the photon to be a peturbation of, and then the emission of a photon wouldn't be a generation of a new thing de novo, but an electron dropping down an orbital level and its electric charge yanking on the electromagnetic field Aether like shoving a compression wave down a slinky. :/

Re: Is Spacetime Real?

#56

Post by taylodl »

Is spacetime real? Does mass & energy "curve" spacetime? Is gravity an actual force or is it a virtual force emerging from "curved" spacetime? Like all forces gravity can be modeled by geometry. Where gravity is different from the other forces is it applies to everything - regardless of charge, regardless of mass, regardless of energy. Therefore the geometry, and gravity's metric, applies to everything in the universe. Does spacetime exist? Is it curved? We simply can't tell - spacetime is indistinguishable from gravity's metric.

I think a more interesting question then is whether spacetime arises as a result of the presence mass & energy?

Re: Is Spacetime Real?

#57

Post by iand »

randallsquared wrote: post #41
thisiscorrect wrote: post #28

Earlier quoted context omitted.

"The same applies to atoms, we all agree on what they are, and we can show pictures of them etc." This is where you bury the lede. It's clear if you read many of the comments here that there is no such agreement. Some people _think_ of an atom as something you can reach out and touch, or grok in some deep sense distinct from performing specific experiments. Others take an approach that's more in line with theory in t…

> Some people _think_ of an atom as something you can reach out and touch, or grok in some deep sense distinct from performing specific experiments. Right, with an atomic force microscope. ...but it almost sounds like you're suggesting that atoms and molecules are NOT something you could individually pick up with an AFM?

The parent was referring to the perceived physicality of atoms which you cannot touch or understand except in terms of the forces between your instrument and the bundle of quanta we call an atom.

Re: Is Spacetime Real?

#58

Post by iand »

mmcconnell1618 wrote: post #24

Does anyone else see a logical flaw in explaining the distortion of spacetime with examples like a weight on a rubber sheet or too many actors on a stage causing it to warp? It seems that the easiest way to explain how gravity is created by spacetime is to use a self referential example of gravity itself.

Those aren't explanations, they are analogies to help the human mind picture the effect.

Re: Is Spacetime Real?

#59

Post by Balgair »

enkid wrote: post #16
jakelazaroff wrote: post #11

Earlier quoted context omitted.

Atoms aren’t just something we’ve modeled — they’re a physical thing , i.e. matter. We can sense them directly, not just infer their existence by measuring related phenomena. I realize when you get into theoretical physics the “thingness” of concepts becomes more and more abstract, but atoms don’t fall into that bucket.

I don't see the distinction you're making between "sensing" and "measuring related phenomena." What's different from using a specialized microscope to take a picture of an atom and using a telescope to take a picture of black hole?

The amount of grant funding! ;P
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