Wikipedia:Reference desk/Archives/Science/2023 October 14

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October 14

Potentially riskable?

May it be harmful for the visual perception to directly look at sunlight through a video or TV? I mean, can the data transmitted by those medias include ultraviolet like it does include visible wavelengths of light? --2001:999:404:8155:D267:90D1:4F35:7D6 (talk) 18:48, 14 October 2023 (UTC)

Nope, similarly as watching someone shooting a gun at the camera would not kill you through the TV or cinema screen. --CiaPan (talk) 19:00, 14 October 2023 (UTC)

Whatever potential damage should have been filtered out during the recording process. If not, it would likely wreck the camera. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:44, 14 October 2023 (UTC)

Cameras do exist to record UV light that would be harmful to human vision. The issue is not in the camera but in the display device. For obvious reasons, display devices intended for human viewing don't emit harmful UV radiation, although it would certainly be possible to build one that does. CodeTalker (talk) 20:54, 14 October 2023 (UTC)

That's the right answer, but just to be clear, a direct look at the Sun can deliver dangerous levels of infrared and visible light as well as ultraviolet. A display device for human viewing will not be able to produce those either. --142.112.221.114 (talk) 23:52, 14 October 2023 (UTC)

Also, the first formulation of the question posed does not make sense, To look at an image of the Sun on a video or TV screen cannot be called "to directly look at sunlight".  --Lambiam 08:15, 16 October 2023 (UTC)

Any non-magical video display has an inherent maximum brightness, which is far less than that of sunlight. —Tamfang (talk) 18:41, 21 October 2023 (UTC)

The connection between: relativistic mass, rest mass, and velocity. Am I right?

Introduction: The first to prove the connection between relativistic mass, rest mass, and velocity: $m_{0}=m_{rel}{\sqrt {1-v^{2}/c^{2}}}$ (without relying on the mass-energy equivalence), was Richard Chace Tolman, and he did it twice:

1. For the first time, in a common article with Gilbert Newton Lewis (1909): http://en.wikisource.org/wiki/The_Principle_of_Relativity,_and_Non-Newtonian_Mechanics , at the beginning of the chapter: "Non-Newtonian Mechanics".

2. The second time, by Tolman alone (1912): http://en.wikisource.org/wiki/The_Mass_of_a_Moving_Body, in the chapter: "Longitudinal Collision".

There are four fundamental differences between the two proofs:

A. The first proof describes two observers, each of whom: 1. Sees the other one moving. 2. Sees a ball launched from him and moving perpendicularly to the direction of motion of the second observer. The second proof describes almost the same scenario, but this time each ball is observed moving in the same line of the relative motion between both observers.

B. Unlike the first proof, the second proof relies on the law of conservation of relativistic mass, so that during the whole process - the sum of the relativistic masses of both balls - is conserved.

C. Indeed, both proofs rely on the law of conservation of momentum: $p=vm_{rel}.$ However, the first proof uses the law of conservation of momentum - in order to be allowed to assume, that the momentum of the first ball observed by the first observer, is the momentum of the second ball - as calculated by him - to be the momentum (of this ball) observed by the second observer. On the other hand, the second proof uses the law of conservation of momentum - in order to be allowed to assume, that the sum of - the momentum of the first ball - and the momentum of the second ball, is the momentum of the whole system containing both balls,

D. Unlike the second proof, the first proof is based on the implicit assumption that the distance traveled by a given ball until it collides with the other ball is the same distance - whether it is observed by a given observer - or it is calculated by him to be a distance observed by the other observer. The first proof uses this implicit assumption - in order to logically conclude, that each of the two observers concludes that the ratio between the velocity of a given ball before the collision and the velocity of this ball after the collision - is different from the ratio between the velocity of the other ball before the collision and the velocity of this other ball after the collision - when this second ratio is calculated by him to be a ratio observed by the other observer.

In my opinion, the first proof contains two mistakes: The first mistake is the assumption indicated in Section C on behalf of this proof. The second mistake in this proof is the implicit assumption indicated in Section D on behalf of this proof: For if the ball were a photon - whose velocity is of course calculated by both observers to be observed as constant by both observers, then both ratios discussed in Section D would be identical to 1, unlike the conclusion of Section D on behalf of this proof.

Question: Assuming that paragraphs A to D describe correctly the differences between both proofs, is my opinion indicated in the previous paragraph correct?

2A06:C701:7455:C600:5169:C2D6:AB07:4DE9 (talk) 21:11, 14 October 2023 (UTC)

Yes Jellprism (talk) 02:45, 15 October 2023 (UTC)

Probably deserves a prize for the highest question/answer length ratio on the Desk to date :-). {The poster formerly known as 87.81.230.195} 5.71.208.84 (talk) 06:49, 15 October 2023 (UTC)

The answer should be discarded because this user is engaging in borderline vandalism. The answer I believe is no, because for starters a photon has no mass, and therefore $p=mv$ does not apply and one must appeal to the general energy-momentum relation.--Jasper Deng (talk) 09:49, 15 October 2023 (UTC)

A photon does not have a non-zero rest mass $m_{0}$, yet it does have a non-zero relativistic mass $m_{rel}$, for which the formula $p=vm_{rel}$ does apply, as does the connection $m_{0}=m_{rel}{\sqrt {1-v^{2}/c^{2}}}.$

As for the general energy-momentum relation: I have already pointed out that Tolman was the first to prove the connection $m_{0}=m_{rel}{\sqrt {1-v^{2}/c^{2}}}$, without relying on the mass-energy equivalence, hence without using the general energy-momentum relation, which was actually found by Minkowsky, who would not have invented this formula - nor Minkowsky's space - if he had not been aware of Einstein's mass-energy equivalence in advance, beacsue Minkowsky's space - as well as his energy-momentum relation - are a way less intuitive than Einstein's mass-energy equivalence.

Next time please refrain from supporting a banned user whose vandalisms have all been reverted, and who has been warned three times on his talk page for his vandalisms on Wiki. His unconstructive repsonse to my thread made some users respond to him, thus making other users think my question has altready been answered correctly, thus preventing them from answering correctly to it. Anyway, thank you for not being misled by this banned user and for responding to my question by a response different from the banned user's response. I will be more grateful, though, if you also revert his unconstructive response.

2A06:C701:7455:C600:5169:C2D6:AB07:4DE9 (talk) 11:01, 15 October 2023 (UTC)

$p=\gamma mv$ is not meaningful at all at v = c (which all massless particles travel at) due to a zero denominator so you are incorrect about that part; the formula was never meant to be valid for massless particles. I might respond more fully later, but please note WP:DAW: "Wiki" is not a correct abbreviation of "Wikipedia". The unconstructive response will remain because it has already been replied to, per WP:TPG, which requires context to be preserved.--Jasper Deng (talk) 11:21, 15 October 2023 (UTC)

Please notice I have never used the formula $p=\gamma mv$ you've indicated, but rather the formula: $p=vm_{rel}$, as well as the formula $m_{0}=m_{rel}{\sqrt {1-v^{2}/c^{2}}}.$ Both formulas I've used are universally correct, and are applicable to photons as well. As for the first formula, $p=vm_{rel},$ it lets you calculate a photon's relativistic mass $m_{rel}$, once you are given the photon's wavelength - hence its momentum P.

Per Wilkipedia guidelines, a banned user's responses should be deleted even if they have already been reponded to. Anyway, you decided to prevent me from being more grateful to you. It was your choice, not mine. 2A06:C701:7455:C600:5169:C2D6:AB07:4DE9 (talk) 11:33, 15 October 2023 (UTC)

The user is not blocked or banned and there is no requirement to blanket revert banned users' edits. I'm an experienced editor explaining why we do certain things here and you are not in a position to lecture me on what is policy or not. Your right hand side $m_{\text{rel}}=\gamma m_{0}$ isn't meaningful at v = c either. You are therefore incorrect that this is universal, because it is inapplicable for massless particles. --Jasper Deng (talk) 11:41, 15 October 2023 (UTC)

Again, I've never used the formula $m_{\text{rel}}=\gamma m_{0}$. It seems like you confuse the term ${\sqrt {1-v^{2}/c^{2}}},$ I've used, with the term $\gamma$, i.e. with the term $1 \over {\sqrt {1-v^{2}/c^{2}}}$, I've never used.

I'm too an experienced editor (actually since 2006), and as I have already pointed out, it was your choice to prevent me from being more grateful to you. 2A06:C701:7455:C600:5169:C2D6:AB07:4DE9 (talk) 11:50, 15 October 2023 (UTC)

No you're not, because you fail to understand some basic Wikipedia policies like the meaning of being banned. I also frankly don't care whether you are grateful or not. Cut it out and stick to the topic at hand.

You also are ignoring that relativistic mass (not the same as invariant mass, so you cannot try to get around this using the energy-momentum relation) is defined in the form I gave, and you cannot derive a new identity from an existing one by multiplying both sides by something equal to zero (the square root, for a massless particle), or I could argue that $1/(x-1)-1/(x+1)=2/(x^{2}-1)$ even for x = -1, 1 by clearing denominators. Therefore your attempted extension is a howler. Note how both proofs give the relation in the form I gave, and is therefore not claiming it is valid for massless particles.--Jasper Deng (talk) 12:04, 15 October 2023 (UTC)

I don't ignore anything. I've always been careful to avoid using the well known formula $m_{rel}={\frac {m_{0}}{\sqrt {1-v^{2}/c^{2}}}}$, which is not applicable to photons. Instead, I've only used the formula $m_{0}=m_{rel}{\sqrt {1-v^{2}/c^{2}}}$, which is proved to be valid to photons as well, by substituting $m_{0}=0,v=c.$ How can you ignore fundamental algebra?

Of course, as you have pointed out, the first proof uses the formula $m_{rel}={\frac {m_{0}}{\sqrt {1-v^{2}/c^{2}}}}$, which is not applicable to photons. But my question about the first proof, was not about this formula - which is not applicable to photons, nor about the formula $m_{0}=m_{rel}{\sqrt {1-v^{2}/c^{2}}}$ I did use - which is applicable to photons as well. I've only asked about whether the argument contained in this proof is correct, even though it does not refer to photons, because if this argument had been correct, it would (logically) be valid to photons as well, thus leading to a contradiction, regardless of the formulas mentioned above - which were not what I was asking about. Once you see again my section D (in my original question), you will understand soon, why this argument would have been valid to photons as well, if it had been valid at all.

As for your advice to stick to the topic, please notice that my comments about the disruptive response, have always been placed in small scripts, and always at the end of my main response, as opposed to what other editors in this thread have done, so I'm not the person who your advice should refer to. As for the issue about who is more exprerienced: This issue is not about the topic, and that's why my comment about the year 2006 when I began to edit in Wikipedia, was placed in small scripts. As for your comment that you don't care about whether I'm grateful: Again it's your choice. Anyway, I do thank you, though, for not being misled by the disruptive response (which should be deleted in my opinion, just to prevent others from being misled by it).

2A06:C701:7455:C600:5169:C2D6:AB07:4DE9 (talk) 13:16, 15 October 2023 (UTC)