Wikipedia:Reference desk/Archives/Science/2024 August 6

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August 6

Is there any natural process (i.e. without human intervention), conserving the kinetic energy, without conserving the speed (i.e. the absolute value of velocity)?

By conserving the KE without conserving the speed, I mean the following:

Let a system, carry (at the initial moment) an initial mass $m_{1}$ and an initial velocity ${\vec {v}}_{1}$ and an initial kinetic energy $E_{K1},$ and carry (at the final moment) a final mass $m_{2}$ and a final velocity ${\vec {v}}_{2}$ and a final kinetic energy $E_{K2}.$

Is it natural (i.e. without human contact), that $E_{K1}=E_{K2},$ but $|{\vec {v}}_{1}|\neq |{\vec {v}}_{2}|$ [for every reference frame]? while the reference frame remains the same - during the whole process?

Mathematically, it's possible of course, but I wonder if it's also physically possible in any natural process.

Note, that I'm only asking about a change in speed, rather than about a change in velocity, because no question would arise, had I replaced speed by velocity, in which case there would be natural processes conserving the KE without conserving the velocity, e.g. when the system is any given body that elastically collides with a wall sharing a reference frame with an observer who measures the body's kinetic energy. HOTmag (talk) 07:10, 6 August 2024 (UTC)

This can only happen when the mass changes inversely proportional to the square of the speed. It can be done naturally through clever accounting. Suppose a rock changes speed and breaks into four equal parts; a natural process. We can choose to define our system such that three of the parts of the rock leave the system, reducing the mass to one quarter. We can also choose a frame of reference in which the speed of the remaining fragment doubles compared to the speed of the original rock. Then your condition is met. Obviously, this has no physical relevance. PiusImpavidus (talk) 09:04, 6 August 2024 (UTC)

Oh, I forgot to add a crucial condition: the reference frame must remain the same, during the whole process. Thanks to your exmaple, I've just added this crucial condition I'd forgotten (See above). Sorry for the confusion. HOTmag (talk) 12:35, 6 August 2024 (UTC)

I don't change the reference frame during the process; I just conveniently choose it beforehand. If both speed and mass change, I can always find a reference frame where the kinetic energy is conserved and I can also find one where it isn't. And as the physics cannot depend on the chosen reference frame (principle of relativity), this conservation of kinetic energy can't have physical relevance. PiusImpavidus (talk) 20:24, 6 August 2024 (UTC)

Ok, my question wants this process to be independent of the reference frame. I've just added this new condition [in brackets] to my original post. Again, I apologize for the confusion.

Let's put my original question this way: Some natural forces (e.g. the elastic force exerted by a spring), conserve the kinetic energy for the long term (with respect to the "initial moment" defined as such), for every reference frame. Is there any combination of natural forces - that conserves the kinetic energy (in the above sense), but at some moments - at which the kinetic energy turns out to have been conserved (with respect to the "initial moment" defined as such) under that force for every reference frame - the speed (i.e. the absolute value of velocity) does not? HOTmag (talk) 07:43, 7 August 2024 (UTC)

I take the rest frame of the system before the process happens. The speed of the system is 0, and so is the kinetic energy. Now the process happens and the speed changes. My reference frame is no longer the rest frame of the system, so the kinetic energy can no longer be 0. Which proves that I can always find a reference frame in which your condition is violated. This means that there is no process that satisfies your condition in every reference frame. PiusImpavidus (talk) 09:58, 8 August 2024 (UTC)

Your consideration can also be used to prove, that if the kinetic energy is conserved for every reference frame, then not only the speed - but also the velocity - is conserved for every reference frame, right? HOTmag (talk) 11:30, 8 August 2024 (UTC)

Yes, if the kinetic energy is conserved in every reference frame, the velocity must be conserved too. The mass too. In other words, not much can have happened. (Note: I've assumed we're dealing with inertial reference frames. I think you assumed that too.) PiusImpavidus (talk) 15:54, 9 August 2024 (UTC)

Thank you. HOTmag (talk) 17:34, 10 August 2024 (UTC)

Metastasis of glioblastoma

From Glioblastoma#Surgery:

GBM cells are widely infiltrative through the brain at diagnosis, and despite a "total resection" of all obvious tumor, most people with GBM later develop recurrent tumors either near the original site or at more distant locations within the brain.

Why is this cancer so widely infiltrative? Nyttend (talk) 22:03, 6 August 2024 (UTC)

The glial cells thought to be involved are astrocytes, which are like neurons in many ways except they don't carry electrical signals. They reach out along the same pathways as neurons and if they become cancerous are predisposed to reach into distant areas. Here is a review article. Combine that with the fact that the somatic immune system is barred from the brain, and they cannot be stopped or contained. Abductive (reasoning) 23:02, 6 August 2024 (UTC)

Abductive, could you add something to the article explaining this? Proper understanding of the review article demands a higher-than-I-possess understanding of human biology. Even the bits that I can understand are hard to interpret in context, e.g. I understand "Glioblastoma cells generally invade as single cells", but I don't know if it's at all relevant to the "why" question, and I don't want to go dumping content into the article and accidentally cause it to imply something not in the source. Nyttend (talk) 19:45, 7 August 2024 (UTC)

I gave it a shot. Neurobio is the most difficult biology field, along with immunobio. Abductive (reasoning) 20:15, 7 August 2024 (UTC)

It may not be especially infiltrative as compared to other tumors. The problem is that in case of brain the resection of tumors with sufficiently wide margins is impossible for obvious reasons. Ruslik_Zero 19:16, 7 August 2024 (UTC)

You might consider reading the review article I linked above. Abductive (reasoning) 19:47, 7 August 2024 (UTC)

Sound effects for Jeopardy dollar amounts heard on 1960s phones

I have this memory of using a telephone in the early 1960s and if I couldn't hear anything else, I could hear a sound (very faint) similar to the old sound effects of dollar amounts appearing on Jeopardy!. a sound effect used in the opening of Jeopardy! Masters. Any idea what I was hearing?— Vchimpanzee • talk • contributions • 22:10, 6 August 2024 (UTC)

• Old relays. Here is a Youtube video with the sound amplified. Abductive (reasoning) 23:08, 6 August 2024 (UTC)

That doesn't seem to be it. It was much more of an electronic or robotic sound. Maybe it was a change in the technology from the old relays to something new.— Vchimpanzee • talk • contributions • 15:55, 8 August 2024 (UTC)

I recall those rapid electronic sound effects when Jeopardy revealed the dollar amounts (here is a Youtube clip [1] of them)... and on the telephone system they were called Touch-Tones. From Push-button telephone: "In 1963, the Bell System introduced to the public dual-tone multi-frequency (DTMF) technology under the name Touch-Tone, which was a trademark in the U.S. until 1984. The Touch-Tone system used push-button telephones. In the decades after 1963, rotary dials were gradually phased out on new telephone models in favor of keypads and the primary dialing method to the central office became touchtone dialing." In fact, DTMF touch-tones are still in use with landlines, but have become less noticeable with speed-dialing. In addition, in the U.S.landline phone use is way down, with 73% of households only having cell phones. [2] Modocc (talk) 17:55, 8 August 2024 (UTC)

I'm guessing what I was hearing was what replaced relays on the newer systems that could handle touch tone telephones.— Vchimpanzee • talk • contributions • 21:21, 8 August 2024 (UTC)

Most likely, although in those early days I rarely used the telephone and when I did it was only to answer an incoming call. :-) Modocc (talk) 23:05, 8 August 2024 (UTC)

That sound effect reminds me of DTMF touch-tone dialing. But, like automatic dialing like a computer might do.

Did you have a phone with an auto-dial feature? You might be remembering the sound that made, but only when it was actually dialing. For that matter, you might be thinking of the sound it makes when you dial manually.

Another possibility is that your phone was getting interference from something. Either from something in your house (In which case we'll never guess what it was), or some kind of cross-talk with other phone wires. ApLundell (talk) 06:50, 17 August 2024 (UTC)