Wikipedia:Reference desk/Archives/Science/2024 July 11

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July 11

Why is the universe not fractal?

The laws of gravity are presumably the same throughout the universe: the force is proportional to the product of the masses and inversely proportional to the square of the distances. So why do we observe very different structures at different scales? Solar systems involve a few discrete objects orbiting a sun, galaxies have various shapes but often spirals, and then over larger distances the distribution of galaxies is like a 3-D network of filaments. I believe that simulation models can produce all these different structures, but I am hoping for some intuitive explanation of why the different outcomes. One possibility might be that things happen relatively faster over small distances, and that the universe would also develop into something like a giant solar system given more time. Another possibility is that some processes happening only at the local scale (e.g. nuclear fusion in stars) interfere with what would happen if gravity alone were operating. Or is it something else entirely? Thanks. JMCHutchinson (talk) 12:01, 11 July 2024 (UTC)

The small-scale stuctures, such as the discrete objects in solar systems and the spiral structure of galaxies (even the disk itself) arise due to non-gravitational processes. When a cloud of ordinary (baryonic) gas collapses its density and temperature increase and it gives off an increasing amount of electromagnetic radiation, this leads to a loss of energy (radiative cooling) that speeds up the collapse and leads to the formation of small-scale structures. Dark matter, which is only subject to gravity, does not do that and there is no comparable small-scale structure in the dark-matter distribution. There are purely gravitational cooling mechanisms such as violent relaxation but they are much less effective than radiative cooling and operate on larger time scales. The time since the Big Band has been sufficient for galaxies and clusters of galaxies to form (less massive objects form first, more massive objects later), but not yet for objects on larger mass scales (superclusters exist but they are not bound objects yet). This is the reason why matter on the largest scales is organised in filaments but not in bound, more or less spherical objects. Finally, the presence of dark energy and the consequent accelerating expansion of the Universe set an upper limit to the mass of bound objects that will ever form — I don't know what that limit is but I guess it is in the supercluster range. --Wrongfilter (talk) 12:27, 11 July 2024 (UTC)

This is a very clear answer and exactly what I wanted. Thanks! JMCHutchinson (talk) 17:20, 11 July 2024 (UTC)

Is nociplastic pain same as neuroplastic pain?

Hi. I've noticed that the term "neuroplastic pain" has 700 thousand hits on Bing search, but there is no article or redirect for it on Wikipedia. However, it seems similar to nociplastic pain, but I'm not completely sure. Is here anyone with medical background who could confirm/decline this? --Pek (talk) 16:35, 11 July 2024 (UTC)

It seems to be an incipient medical term, with 107 Google Scholar results. Just glancing down the squibs Google provides shows that it is listed separately, for instance, "Nociplastic pain is hypothesized to differ from nociceptive and neuroplastic pain...". Neuroplastic pain appears to be a sequela of neuropathic pain. The results from the Google Scholar and regular searches show a lot of scammy stuff, and I worry that creating a redirect (to neuropathic pain) may be a bad idea. Conversely, without good sourcing, an article may be impossible to create at this time. I would take this to WT:WikiProject Medicine. Abductive (reasoning) 21:36, 12 July 2024 (UTC)

Does the velocity of an electromagnetic wavefront depend on the medium?

Our article Velocity factor states: The velocity factor...is the ratio, of the speed at which a wavefront (of an electromagnetic signal)...passes through the medium, to the speed of light in vacuum. So it seems that the velocity of an electromagnetic wavefront does depend on the medium.

However, our article Front velocity states: The earliest appearance of the front of an electromagnetic disturbance (the precursor) travels at the front velocity, which is c, no matter what the medium. Similarly, our article Wavefront states: Wavefronts travel with the speed of light in all directions in an isotropic medium. So it seems that the velocity of an electromagnetic wavefront does not depend on the medium.

I wonder whether this is a contradiction between our articles (if so then how should they be corrected?), or - probably - I miss something here (if so then what do I miss?)... HOTmag (talk) 21:00, 11 July 2024 (UTC)

I have corrected the sentence in wavefront — the article talks about waves in general, not specifically about electromagnetic waves (in which case it should have been the speed of light in the medium). The "front" discussed in front velocity is not the same thing as a wavefront (= a surface of constant phase) but, as in your quote, the earliest appearance of an electromagnetic disturbance (again one could talk about non-em disturbances but we won't). For instance, some switches on a lightbulb a distance $s$ from me. The question is "At what time can I know the lightbulb is on?" and the answer is $s/c_{\mathrm {vac} }$. The reasoning is that because this is a discontinuous signal (the mathematical formulation involves the Heaviside function) the spectrum of the signal includes all frequencies, in particular very high ones. In most (any?) media the refractive index approaches 1 for very high frequencies, i.e. the propagation speed approaches the vacuum speed of light for high frequencies. Therefore, the high-frequency part of the signal arrives first, and the bulk of the signal later. Due to this dispersion, the temporal/spatial shape of the wave signal changes as it propagates (think of what sound does on a frozen lake). --Wrongfilter (talk) 05:52, 12 July 2024 (UTC)

Thank you for correcting our article Wave front.

Regarding our article Front velocity: As opposed to other readers (including me), you've understood that it refers to a beam of light containing the whole spectrum. However, if the beam of light contains, not the whole spectrum, but e.g. two types of waves only, e.g. red and blue, then the front velocity of this beam of light does depend on the medium, right?

Here is the full quote, from our article Front velocity: the wave discontinuity, called the front, propagates at a speed less than or equal to the speed of light c in any medium. In fact, the earliest appearance of the front of an electromagnetic disturbance (the precursor) travels at the front velocity, which is c, no matter what the medium.

Is the claim in this quote correct, as far as my red-blue example mentioned above is concerned? In my example, "the earliest appearance of the front" of this electromagnetic disturbance is the appearance of the blue wave, right? If it is, then shouldn't the paragraph be corrected, for all readers to understand that it only refers to a beam of light containing the whole spectrum? HOTmag (talk) 08:55, 12 July 2024 (UTC)

The spectrum is the Fourier transform of the disturbance. A discontinuous disturbance has signal at all frequencies, not just "red" and "blue". These things are not independent. --Wrongfilter (talk) 09:16, 12 July 2024 (UTC)

Got it, thank you.

So, If I want the value of the speed of light to be independent of medium, I must refer to the front velocity of a beam of light containing the highest frequencies possible, right?

If your answer is positive, then how can the formula $p=\gamma mv$ be justified, when applied to a red light moving in water? In this case, $v<c,$ so $\gamma$ is a finite number, while $m=0,$ so according to this formula, we get $p=0,$ which is false...

I'm pretty confused now. Unless no mass is allowed to be attributed to the light, not even a zero-mass, so my last question will vanish. Am I right? HOTmag (talk) 10:14, 12 July 2024 (UTC)

I'm not letting myself get drawn into the mass debate again. Light is complicated, light in matter is even more complicated. If you want the momentum of a photon, use $\hbar k$. --Wrongfilter (talk) 10:25, 12 July 2024 (UTC)

Thank you. What about my only question still left, in my second sentence? "If I want the value of the speed of light to be independent of medium, I must refer to the front velocity of a beam of light containing the highest frequencies possible, right?" HOTmag (talk) 10:53, 12 July 2024 (UTC)

I don't see the point of that sentence. Why would you "want the value of the speed of light to be independent of medium"? The front velocity looks like an interesting theoretical curiosity with little actual physical relevance. --Wrongfilter (talk) 11:13, 12 July 2024 (UTC)

I've only wanted to know if, the only way to define the well known speed of light - usually denoted by c - must rely on the vacuum, or the speed of light can also be defined without the concept of vacuum? Assuming we don't want to define it numerically (299,792,458 m/s), nor by ratios between other physical constants.

So according to your previous clarifications, I thought that perhaps the speed of light could be defined as the front velocity of a beam of light containing the highest frequencies possible... HOTmag (talk) 12:39, 12 July 2024 (UTC)

You are perhaps falling into the trap of thinking that the speed of light in a vacuum, c, is determined by a property of light (in the sense of e-m radiation). It is not: it is (as a limit) a fundamental property of Spacetime, to which light and everything else (though not spacetime itself) has to conform, including other massless 'particles' which must perforce travel at it in a vacuum. It just so happens that it is easiest to observe (and measure) using light, and was thus discovered and named. Or so I understand the matter. {The poster formerly known as 87.81.230.195} 94.6.82.201 (talk) 07:37, 13 July 2024 (UTC)

Oh, of course I've always knwon that c is just a limit being a fundamental property of Spacetime. But instead of using the complicated expression: "a limit being a fundamental property of Spacetime", I used the common term "speed of light", as most people do (including you - as I guess), just for the sake of convenience and simplicity. That said, I only asked if this limit - which I called "the speed of light" for the sake of convenience and simplicity, could be defined - without the concept of vacuum - as the front velocity of a beam of light containing the highest frequencies possible. HOTmag (talk) 22:33, 13 July 2024 (UTC)