General relativity

RPaulB, your summary is mainly good. But it is highly useful to think of things passing through space without interacting with it, it's probably best to think of this as everything cancelling out.

For example, the space around a charge is different to that far from one, because the non-zero electric field leads to an inhomogeneity in the virtual particles. This results in charges increasing as you get near them due to charge shielding. Don't let me give the false impression I fully understand that, but I can understand that this can arise from a Feynman diagram calculation, and can find some details in Griffiths or elsewhere.

Ok, so here we now have some discussion on dark matter and clusters with mergers, in relation to modified gravity theories. Some discussion on Gravitational Waves, with statements that they do not make sense. Some discussion on how you do science, with statements that theories like GR, even though they explain a huge amount of experiments, and make successful predictions, are still wrong on some conceptual basis (although at the beginning there were attempts at probing GR wrong on some observational basis, so that makes it even more confusing). Then there is some general point (somehow related to the anti-GR arguments above) that space-time is a nonsense concept.

RPaulB, I disagree with everything you say, about all of the issues above, but I no longer know where to start from. Please let me know what you want to discuss first.

To be fair, this one post wasn't all bad. (Characteristically) dumb to say "gravity doesn't exist", horrendously imprecise and unclear in places, but not _all_ bad!

Indeed, I do not disagree with everything in that post either (I do disagree on 3. and 6. I do not understand what 5 and 7 means exactly, I am ok with the rest). I also think that some questions that were asked on GW are clever.  My point is however that there are 10 issues raised, and it would be better to decide what is the main problem here: GR, dark matter, space time, GW, the scientific method... If the answer is "all of the above", then it is still better to set priorities and discuss topics one by one, in order of importance. 

The (naive?) hope behind all this is that at some point it will be possible to discuss science and astronomy in notes and forums, without always ending into "space particles", "preons", "GR is wrong", "black holes do not exist" etc

I concur. But I feel RPaulB provides inpenetrable barriers to good discussion!

I think that the essential problem here is one of method, independently on whether we discuss GR, Quantum Mechanics, or anything else. 

When you ask "Is there a first particle?" I never saw Elroch saying "no", he says "this question does not make sense". And I agree.

Some questions cannot be answered to because they are logically flawed.

"Is there a first particle" assumes

1. Some notion of time ("first"), which you do not specify (since you are talking about the beginning of the universe, this is non-trivial)

2. Especially, some notion of particle.

3. Some mathematical/logical/scientific framework which tells me that this is an essential question in order to explain actual observations and data related to the origin of the Universe. 

When I say 1. and 2., by "notion" I mean 

a) An empirical description, related to the description of a set of measurements, which tells us how to assign numerical values to the properties of this object (position, speed, energy...)

b) A mathematical object, with some properties that can be associated to each of the measurements above. This mathematical model must allow me to do correct quantitative predictions about the measurements.

If you agree with a) and b) I can tell you what, in the sense I outlined above, "is" space. And I think we should work at a purely classical, macroscopic level, ignoring subtleties of GR, Quantum Mechanics and Quantum Field Theory, which are irrelevant to this methodological discussion. Let me know if you like the program.

A lot of people, possibly including RPaulB, don't fully realise that most of the most intuitive concepts have been demolished by 20th century physics. What we are left with is the idea (well-described by Michele78) of predictive models of reality. Any question that cannot be associated with even a hypothetical experiment has been rejected as not a question of science.

One corollary of that there is no guarantee that correct models of reality are unique. Rather, correct models of reality must agree in their predictions for every feasible experiment. Otherwise, there is an experiment that could feasibly be performed to reject at least one of the competing models.

In quantum mechanics, there are many models which look quite different to each other which have the property of having identical predictions for all possible experiements. There can never be a scientific answer to the question of which of these models is right.

In large scale physics, one model that has been falsified is GR with zero cosmological constant. The simplest successful model is GR with a non-zero cosmological constant. There are also other competing models which have been falsified by evidence (and others where the evidence probably falsifies them, but it is not certain). In addition, there are more complex models which agree with GR on all tested predictions, but which would disagree with it in some untested scenarios. Until some experiment falsifies the simplest model (GR with cosmological constant), the more complex models have to wait in the wings, as Occam's razor advises.

The particles of the SM live in 4D, Minkowskian spacetime, and they are associated to wavefunctions (vectors in a Hilbert space; more precisely, in a set of Hilbert spaces called Fock space). The fields of the SM are operators acting on these wavefunctions. The eigenvalues of different operators give the results of possible measurements.  

In classical Physics, particles are point masses, the positions of which are defined by vectors, "(x,y,z)", in an Euclidean 3D space. Their velocities are the derivatives of these vectors, and so on.

In special relativity, particles are point masses defined by 4-vectors (t,x,y,z) in Minkowskian spacetime. You can define a 4-velocity as a derivative of this with respect to proper time.

You reject the idea of space and spacetime, as it is used in all of the definitions above, and all the maths that goes with it, so your "particles", whatever they are, are none of the objects above. As far as I understand, you do not even want to assign coordinates to define their positions. Whatever I think a particle is (and note that what I call particle changes depending on the energy scales and distances I am looking at), it requires a mathematical construction to define space and time in which my particle live. You reject all this. Hence I do not know how to interpret them from the start. When you say "particle", this clearly is not related to any notion of particle I can have in mind, and to any of the maths I know. On the other hand you do not say what else your particle is, or how to measure its position, speed, mass, energy...

Also, we are back discussing QFT and so on... I really would like to know whether you agree on my methdological points a) and b) in the previous post.

Finally, summarizing quickly one of your points: "before the BB there are 0 particles, so you have to go to 1, 2, 3..."

It is not a good starting point, to me

1. Before the BB it is not even clear how we define time, so it is not even obvious what one means with "before". Moreover there are no experimental tests of anyhting before the BB, and discussing "before the BB" is already close to being a non-scientifical question (note: I am not saying an unintersting question, I am saying an untestable, hence non-scientific, question, see Elroch's post above).

2. The most accredited Early Universe theory is Inflation. In that case, the vacuum energy of a (scalar) field decays at once into the ocean of particles you see today after certain processes happen. It is not particle 1, then 2, then 3... It is absolutely normal to have multi-particle production, so I do not see the logic of one particle at the time. 

I think I was as clear as I could in saying why the "space is a particle statement", or "was there a first particle"statement do not make sense to me.

As a first thing, I need to know which mathematical object you use to define the position and velocity of a particle. Just for a start. Since you do not want to use (x,y,z) coordinates and define velocity as a derivative, even just in a classical sense, I am already lost there.

Then, let me take another sentence, at random, in what you write

"Time started at T=0. The next time is 10^-85 sec. At t=0 there is a first particle."

How do you test this? What is the experiment/observation that will make me believe this?  

"The three particles are in a straight line and are about 100 billion years apart"

Independently on the usual issue of how to experimentally backing up of a sentence of this kind, you say three things are aligned and they are 100 billion years apart. So you use geometry., But you reject it at the same time

"There is no space"

etc

This is why all this just does not make sense to me form the start.

Please tell which mathematical object is associated tou your "particle" and how do you measure distances, positions, and straight lines without some standard geometrical description of space.