Markus Hanke

Fair enough...I actually missed this one 😄

No. The period between when you go to sleep at night, and when you wake up in the morning, subjectively feels like only a moment has passed, whereas the clock on your bedside table will have recorded (e.g.) 8 hours.

So far as I can make it out, every single sentence in this post contains an incorrect claim of some kind or another - that’s quite an achievement in its own right.

Have a look here: https://einsteinrelativelyeasy.com/index.php/dictionary/25-christoffel-symbol Essentially, the Christoffel symbols (of the 2nd kind) tell you how the components of your basis vectors in a given coordinate system change as you move around a manifold. If you want to define a concept of a covariant derivative - which by definition needs to be consistent everywhere on the manifold -, you need to compensate for this change, which is why the Christoffel symbols appear in the definition of the covariant derivative.

No. It depends a lot on what exactly you mean by the qualifier "pretty much", though. I for example am on the autism spectrum (I have Asperger's), which I consider to be a form of neurodivergence, rather than a disability - I'm differently abled, not disabled. I have noticed all my life that my approach to solving certain problems can be very different from the way a neurotypical person would approach the same problem, so clearly my built-in framework is subtly different compared to that of others. Sometimes that's an advantage, other times it's a hindrance, depending on circumstance.

You can't use a Tipler cylinder to travel back in time, as that would violate the weak energy condition.

It is not possible to capture all the necessary degrees of freedom for gravity using vectors fields - irrespective of whether there is just one vector field (=Newtonian gravity), or several. For fundamental reasons, you need at least a rank-2 tensor field - not least because the source term is itself a rank-2 tensor field. To put it simply - gravity is not a force.

Yes, that’s my understanding. That also seems to be consistent with what is stated on Wiki. Not really, this is something I have never looked into in any detail.

I don’t know where you are getting this idea from (YouTube is not a valid source of scientific information), but the Planck mission ‘discovered’ no such thing. The universe doesn’t have a topology that allows for the existence of an axis of any sort. Even if such a thing as an axis existed, and the Earth was on it, this would still be a non-sequitur.

Ok, Penrose didn’t actually say this in the article you linked. He said that eventually, all matter will decay - that’s not quite the same. I wrote that post before I saw the article, so I only guessed that it was referring to the “Big Rip” conjecture. I’ve never actually looked at this in any detail, but my understanding is that it postulates a changing Hubble constant, in which case the effective scale on which expansion is “felt” decreases over time, until even subatomic scales will be affected. This would happen in a finite time. Or am I getting this wrong?

Well, it isn’t a common term that you’d come across frequently in the physics literature - it really belongs more to philosophy. The advantage would be precisely to contrast it with simultaneity (which has a very specific meaning in physics) - events can coexist without being simultaneous; especially since there are scenarios where it is difficult or even impossible to even define a meaningful concept of simultaneity.

We have powerful enough particle accelerators to be able to look for them. Basically what you do is accelerate some particle (usually electrons) to very high velocities, and then shoot them at protons and neutrons, being the composite particles that make up atomic nuclei. You then observe what happens, which allows you to deduce the internal structure of the protons and neutrons. This is called “deep inelastic scattering”. By observing how they interact with their environments. We already know the laws that govern the dynamics of all hitherto known particles, so what you do is observe how these particles interact and behave. For example, you can collide particles at high velocities in particle accelerators; since their dynamics are subject to a number of (known) conservation laws, you can - by observing the outcome of such collisions - deduce whether or not the particles are elementary or composite, because generally speaking the known laws of physics place very stringent constraints on what is possible and what isn’t. A small correction - classical gravity is deterministic, but not necessarily predictable. These are two different things. You’re right, I actually overlooked the ‘classical’ bit. Technically speaking gravity on atomic scales is no longer purely classical, since the constituents of the atom are quantum systems, and subject to the laws of quantum mechanics. This would need to be accounted for. Nonetheless, gravity (quantum or classical) plays no role in the structure of the atom itself, since its effects are so weak as to be negligible.

Yes, and I think that’s a really important part of the process. Novel ideas need to be scrutinised, tested and evaluated in great detail, since that is the only way to establish whether they are of any scientific value or not. This being said, this process needs to happen within reason - sometimes peer reviews read like personal vendettas, and to me that’s not ok. But that’s just human nature. I disagree. Every single physicist I know wants to see new physics being discovered...simply because that’s where the excitement and gratification comes in. As scientists we are driven by one basic motivation - curiosity, a desire to know and discover. Discovering new physics would be the pinnacle of every scientist’s career, so no one feels “threatened” by this. For example, my own area of expertise is General Relativity; I am perfectly aware that the model has limitations, and that it breaks down if you extend it too far. It’s evidently an effective theory that is the limit of some more fundamental model. I would love nothing more than to know what that underlying level of reality is...but that does not mean I would just blindly accept any old idea that someone puts forward to extend GR. I am very sceptical towards all new ideas, until such time when they have been thoroughly investigated and tested. That’s how science works. People usually only see the one model that has been accepted into mainstream science, but they can’t see the 9999 other models that were also proposed, but ultimately turned out not to work.

Yes...but on these scales the effects of gravity are so small as to be negligible. Electromagnetism, the strong interaction, and the general laws of quantum mechanics. No. The strong interaction has a very short range, it is only effective within the nucleus. No, they are entirely different phenomena. The strong interaction is not a force in the Newtonian sense, rather, it denotes the dynamics of how colour charges are exchanged between quarks, by way of gluons. Its precise details are very complicated.

Thinking out of the box means you acknowledge that the box is there, and then you look for meaningful ways to extend its boundaries outwards. It does not mean you discard the box and replace it with some other random shape...that would be more like taking a shot in the dark, which is rarely successful, and sometimes disastrous.

It always depends which wider context these terms are used in. You are right that in colloquial speech, they are largely interchangeable; however, the OP specifically asked about the situation in physics, and here the context is always spacetime. There are in fact many such terms that have specific meanings in physics, which differ from how these terms are used in normal everyday speech.

! Moderator Note Moved to Speculations.

I would just like to add a remark here, perhaps some readers may find it helpful. I am a regular and committed meditator - I practice several hours of formal meditation every day, and have done so for some years. Many of the perceptions described here are common and well known phenomena that naturally arise when the mind settles and becomes concentrated; in the Pāli language they are called nimittā. This can be anything from a slight tickling sensation somewhere, to pins and needles, to a sensation of something moving as a current through the body, to various pains, to full blown auditory and/or visual hallucinations, among other things. A sensation of electrical currents in parts of the body is especially common, from what I have seen. A had a period a few years back when I used to get this regularly, and the sensation of electricity sometimes got strong enough to cause me considerable discomfort, and gave me twitches and involuntary muscle spams during meditation sits. I have heard of people for whom this becomes so strong that they suffer intense pain, muscle cramps, and involuntary movements - they literally “jump” on their meditation cushions. Some people need to temporarily stop sitting because of this. As described here, with a little practice it is easy to induce these sensation at will, and control them to some extent - one can move them around the body, make them stronger or weaker, change their qualities etc etc. I cannot speculate what the underlying mechanisms are, as the human body is not my area of scientific expertise. What is clear though is that body and mind are not separate things, they are intimately connected, so it isn’t surprising that such things may occur. These phenomena are quite natural, and very common among meditators; there are specific ways and methods to address these things, in the context of an ongoing meditation practice. The general advice is to not pay too much attention to them, since directing the focus of attention towards these phenomena will strengthen them and make them occur over and over again. In many specific practice frameworks the occurrence of such phenomena is in fact taken as a sign of progress, since they naturally develop when concentration and single-pointedness become stronger. They can also become a hindrance though, because they can distract from practicing the main technique, and some people become infatuated with these sensations, as they also can feel very pleasant at times. So most of what has been described here is natural and quite well known, and not a cause for concern. This, however, is not: As someone with experience as a volunteer in the emergency services, this would have me concerned; anisocoria isn’t normal (unless you were born with it, which does happen), so I would strongly advise a precautionary trip to your doctor, to rule out other underlying issues.

Muons are elementary particles, they have no internal parts or mechanisms. The decay times of muons is a probability distribution, with a peak (most probable) around ~2.2 μs.

Just a quick to add to the other (excellent) answers here - the "gravitational field" is described by tensor quantities, so you are free to choose whichever coordinate system is most convenient, since such quantities do not depend on your choice of coordinates. To put it differently - events and their relationships are physically real and everyone agrees on them, but what you call those events is largely arbitrary. In the same manner you could replace all the street names in your town or city, without affecting the physical layout of that town in any way. People might get confused, but life would go on as usual

This is not what you originally described in your first post - you were talking about a supermassive electron-positron pair at relativistic velocities. Again, you were originally talking about a pair of supermassive particles 'orbiting' one another at relativistic velocities, which is most definitely not a possibility prior to 10^-35s, which is why the concept doesn't make sense. My comments were based on that opening post. Also, even at or after 10^-35s, such a concept is still not a possibility, since it is not compatible with the known Standard Model. So I'm not sure what is really being suggested here. If you wish to instead discuss a unified quantum field prior to 10^-35s, then you need to suggest what kind of symmetry group such a field would have, and how that symmetry group would be broken to yield the known Standard Model. Such a mechanism is conceivably possible, and the idea of a unified field theory has been pursued since the Standard Model first came to be developed. But without any specifics, we can't really discuss this any further. It's perfectly fine to suggest an extension to the Standard Model that would cover the period prior to 10^-35s - however, such an extension must yield the already known Standard Model at the appropriate energies, and must of course be mathematically self-consistent. Note that it is up to you to present such a model (a general hand-wavey idea is not a model), and show that it indeed does what you say it does; because I certainly can't see how you obtain a SU(3)xSU(2)xU(1) quantum field theory with the required properties from an electron-positron pair. That's why at present it doesn't make sense to me.

One of the more counterintuitive things about General Relativity is that there is no law of energy conservation for regions of spacetime with non-zero curvature. It is possible to formulate other, more general conservation laws by taking into account curvature itself, but they are not quite the same as what we traditionally understand by 'energy conservation'. It doesn't actually go anywhere, it just changes form. What Penrose likely means (you didn't link the article) is that the end result of an accelerating expansion is a universe that expands so fast that it eventually 'rips apart' atomic structures, and even composite particles. What you end up with is just vacuum and a thermal bath of particles that is incapable of doing work.

Coexistence and simultaneity are not interchangeable, in my opinion. Example - consider two muons, which are unstable and short-lived particles. Suppose we have one muon around the time when the Earth begins to form, and one muon right now, here in your living room. Both of these come into existence, live for a short time, then decay. It is reasonable to now say that these muons coexist in spacetime, since spacetime is by definition the set of all events; however, there is no convention by which one can say that these events are simultaneous, because no physical clock can be constructed that shows them to be simultaneous - you can only make those events appear to happen in very quick succession, given the right setup, but they will never appear simultaneous. So these concepts aren't the same.

No, the source of gravity is energy-momentum, which includes many more things other than just mass. For example, and electromagnetic field (in otherwise empty space) would also be a source of gravity, as would be stresses and strains in the interior of a planet (e.g.). It needs to be spacetime, not just space. It is not meaningful, in the context of gravity, to separate space from time, and vice versa. As to what spacetime is - it is quite simply the set of all events, meaning the set of all spatial locations at all instances in time. It is thus a mathematical model. No. When any test particle - irrespective of whether it has mass or not - is affected by gravity (and only gravity, for simplicity), then that means that its world line in spacetime is a geodesic of that spacetime. It is a purely a geometric phenomenon. When space is expanding, that means that the separation between any two points within that space increases over time. However, locally those points remain at rest - you can attribute a relative velocity to these specific points, but not to space (that would be meaningless). Relative motion is not a source of gravity, so it does not 'warp' spacetime. What GR does is model the motion of test particles in the presence of sources of energy-momentum; as such, its predictions are quite physical indeed. This is just what Newtonian gravity does, and such a model works quite well in the low-velocity, weak field domain. However, once you venture further into the strong field regime, the predictions of Newtonian gravity are no longer accurate. And even in the everyday low energy domain - consider putting an accelerometer into free fall (drop it off a tower etc). It will read exactly zero at all times while it is falling - and zero acceleration means no force is present. And yet, the falling accelerometer is very clearly still affected by gravity. So gravity cannot be a force in the Newtonian sense. There are also deeper, more technical reasons why gravity cannot accurately be modelled by a vector field. You cannot accelerate a massless test particle. You have every right to be, because the way GR is generally presented does indeed make it confusing, once you give it more than just a passing glance. Spacetime is not a mechanical medium, so curvature is not any kind of mechanical 'bending'. As mentioned above, spacetime is simply the set of all events, and the geometry of spacetime can be thought of as how these events are related to one another. If the geometry is flat, then that means the relationship between any pair of neighbouring events will be the same, regardless of where/when in spacetime you are (like on a flat sheet of paper). If spacetime is curved, then this is no longer true - the relationship between a given pair of neighbouring events depends on where that pair of events is located in space and time. That's the meaning of curvature - a change in the relationship between events. It's a geometric property, not a mechanical action. This is analogous to the longitudinal lines on a globe - at the equator, they are spaced apart by a specific distance, but as you go north (or south), that distance will change, even though these lines are perfectly straight within the surface. That's because the relationship between points on those lines changes depending on where you are, since the surface has intrinsic curvature. Spacetime is the same, just in two more dimensions.