Do you have more context, explanation or source for this? This is the first time I’ve heard of this being the case and would love to learn more about it.

No it wouldn't. Our fundamental unit of distance (the meter) is defined in terms of the speed of light, so the radius will stay exactly the same, in meters.

The https://en.m.wikipedia.org/wiki/Bullet_Cluster is pretty interesting.

These terms are mostly placeholders for things we don't understand.

I don't know whether the next breakthrough in physics will be quite as relevant in our lives as quantum and relativistic physics. It would be nice if we could link gravity and E/M like we did with the strong and weak forces. Who knows what we could do if we knew how those two go together.

I'm sure someone has proposed this is due to physical constants changing over time, rather than the expansion of space-time, and I'm sure someone else has explained why this is wrong.

Also, the second law is only applicable to closed systems. The universe may not be a closed system in the way we normally think of it.

That there is no time before the big bang (possibly with some qualifiers to define the big bang, start of the universe, etc.) is the overwhelmingly prevailing view of modern cosmologists, from how I understand things.

I suppose these are equivalent, but one feels like a historical distinction while the other feels like a thermodynamic one and I think it's thermodynamics that contrasts the theories better.

Does that theory come with a testable hypothesis?

For a while there was also the theory that the universe is cyclical: it eventually collapses and from that compressed state a new big bang is born. That seems very unlikely with what we know right now though.

Then there are various forms of the multiverse theory where are kind of spontaneously created in a continuous process. Each universe experiences a big bang in the moment it is created, so talking about "before the big bang" only makes sense outside the universe

But I don't think anything rules out a universe laying dormant and then something triggering the big bang either. Changing fundamental constants might well be that something. They don't even have to change continuously or frequently for this to work

As you might have guessed, testing any of these is really difficult. Not necessarily impossible, but really really difficult

We can have several very large bangs, but there can be only one Big Bang™, and nothing comes before it. This is for the same reason that Harry Potter is a wizard, it's not about evidence, it's just defined that way.

The isotopes produced during the natural nuclear reactor 2 billion years ago were produced in certain ratios because of the relative sizes of their nuclear cross sections, which depend on the fine structure constant.

The isotopes used in radio dating are produced by spontaneous transmutation over time, which is governed by entirely different processes.

Which is why a synthetic clock is needed here. That will have a known inception date and the changes if any can be compared.

The problem with both is they're not exactly fully closed systems anyway so there will be some margin of error ever with the length of the operation.

And during the test, we might just find out something completely unaccounted for in current physics... That isn't a universal constant related at all.

That is the problem with any argument for some new physics - it might exist, but it can't have much effect or we would detect it. Generally I only see people arguing for new physics because they really want faster than light travel (typically also without all the weird time effects, but a small minority would accept it with time effects)

Since photons move at c, they experience zero time between creation and destruction.

An object at full rest is according to its wave/path equation literally everywhere at all times.

However superconductivity has a bunch of truck sized holes for this. Specifically we don't quite understand Bose-Einstein condensate completely. Funky entities like time crystals appear in the mathematics, etc.

why would you think that neutrinos can magically ignore the curvature of spacetime? completely wrong.

What you seem to be trying to remember is that certain types of extragalactic supernovae produce a tremendous number of neutrinos, and those can be detected on Earth before the associated light does. The reason is that both are produced deep within the dying star, and while the star's outer layers are largely transparent to the neutrinos (not completely: it's neutrino pressure that makes the star explode[1]), the deep-in-the-star supernova photons bounce around inside. That's very much not empty space.

https://www.astronomy.com/science/in-a-supernova-why-do-we-d...

We can detect type SN1a supernovae out to about z=4 (redshift of about four in light-years is about twelve billion years of light travel time from the supernova to us). That's not really enough for the delayed pulse of light to catch up to the neutrinos produced also produced in the dying star's interior at the same time. Also, not all of the emitted photons are likely to scatter off gas and dust in any interstellar medium along the way, so the relative delay at Earth of the bright electromagnetic flash is dominated by the dying star's outer layers.

(There are more complicated electromagnetic signals like light echos <https://en.wikipedia.org/wiki/Light_echo> that can follow on much later; there aren't any neutrino equivalents really).

> photons aren't affected by gravity directly because massless but their path, their limit of causality, is affected

Not sure what you are trying to say here, but photons certainly both feel and source spacetime curvature. In empty space, photons always travel along null geodesics. The distribution of all matter, energy and momentum, the expanding background of inter-galaxy-cluster space, and the collapsing background of galaxy clusters (and galaxies, and components of galaxies like their central black holes and stars) picks out what geodesics fill spacetime. Some are null, and massless things can find themselves on ("couple to") them. Some are timelike, and massive things can find themselves on them.

Geodesics are free-fall trajectories, so are inertial as in "things in motion tend to stay in motion", and barring any further accelerations a photon coupled to a null geodesic will stay on that null geodesic and a neutrino coupled to a timelike geodesic will stay on that timelike geodesic.

Some neutrinos and photons start on parallel geodesics within the supernova's exploding core. Each neutrino stays coupled to its timelike geodesic all the way to detection on Earth. The photons are all forced off their initial null geodesic by mostly scattering off nuclear matter near the star's core, find themselves on a second null geodesic, more nuclear scattering, possibly some scattering off ions in outer layers, and each ultimately might end up on a geodesic that it stays on until it reaches Earth.

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[1] on neutrinos driving the explosions and how alot of them stick around as they are captured into heavier chemical elements and isotopes https://www.mpg.de/11368641/neutrinos-supernovae

The so-called Planck system of units where Newton's constant is set to 1 is an interesting mathematical curiosity, because in it all the physical quantities become dimensionless.

Nevertheless, when Newton's constant is set to 1, the number of fundamental constants is not reduced, but another constant that was 1 in other systems of natural units becomes a fundamental constant that must be measured experimentally, for instance the elementary charge.

Besides not having any advantage, because the number of fundamental constants in non-nuclear physics remains 2, the system where Newton's constant is set to 1 cannot be used in practice.

The reason is that the experimental measurement of Newton's constant has huge uncertainties. If its value is forced to be the exact "1", then those uncertainties are transferred to the absolute values of all other physical quantities. In such a system of units the only values that would be known precisely would be the ratios of two quantities of the same kind, e.g. the ratios of 2 lengths or of 2 masses. Any absolute value, such as the value of a length or the value of a mass, would be affected by huge uncertainties.

So the use of such a system of units is completely impossible, even if it is mentioned from time to time by naive people who know nothing about metrology. The choice of units for the physical quantities cannot be completely arbitrary, only units that ensure very low uncertainties for the experimental measurements are eligible.

Currently and in the foreseeable future, that means that one of the units that are chosen must be a frequency. For now that is the frequency corresponding to a transition in the spectrum of the cesium atom, which is likely to be changed in a few years to a frequency in the visible range or perhaps in the ultraviolet range. In a more distant future it might be changed to a frequency in a nuclear spectrum, like this frequency that has just been measured for Th229, if it would become possible to make better nuclear clocks than the current optical atomic clocks, which use either trapped ions or lattices of neutral atoms.

Some of the parameters of the "standard model" are fundamental constants associated to the strong and weak interactions. It is debatable whether it makes sense to call as fundamental constants the rest of the parameters, which are specific properties of certain objects, i.e. leptons and quarks.

They are the properties of those particles. There are such properties for leptons, for hadrons, for nuclei, for atoms, for molecules, for chemical substances, for humans and so on.

Any object, either as small as an electron or as big as the Sun is characterized by various numeric properties, such as mass.

The fundamental constants are not specific to any particular object. As I have said, after eliminating the fundamental constants that are determined by conventional choices of the system of units, the only fundamental constants that remain are those that characterize the strength of each fundamental interaction, as expressed in a natural system of units.

Because most objects are composed of smaller subobjects, it should have been possible to compute their properties from the properties of their components. Starting from the properties of leptons and quarks, it should have been possible to compute the properties of hadrons, nuclei, atoms, molecules and so on.

Unfortunately we do not have any theory that can compute the desired properties with enough precision and in most cases even approximate values are impossible to compute. So almost all properties of particles, nuclei, atoms or molecules must be measured experimentally.

Besides the question whether the fundamental constants can change in time, one can put a separate question whether the properties of leptons and quarks can vary in time.

Some of the properties of leptons and quarks are constrained by symmetry rules, but there remain a few that could vary, for instance the mass ratio between muon and electron. It is likely that a future theory might discover that this mass ratio is not an arbitrary parameter, but the muon is a kind of excited state of the electron, in which case this mass ratio could be computed as a function of the fundamental constants, so the question whether it can vary would be reduced to the question about the variation of the fundamental constants.