Wikipedia:Reference desk/Archives/Science/2020 March 22

= March 22 =

Request for comment: The rest frame of the Cosmic Microwave Background (CMB) radiation might be the kinematic time-dilation minimising velocity and the key to understanding the twins "paradox"
Unfortunately the following comment was deleted from the talk page of the twins paradox article before anyone had a chance to reply.

Could the twins-paradox be explained by considering that kinematic time-dilation might be a function of velocity relative to the rest-frame of the CMB?

In a sense, observers who are "stationary" here on Earth, ARE the travelling-twin from the twins-paradox, because we're moving relative to the CMB, we're moving due to the rotation of the earth, the orbit of the sun, the galactic orbit, and due to inter-galactic attraction. Acceleration in the opposite direction (up to a point), could be thought of as a return to "stationary", and might result in a speed-up of time relative to Earth-time, just like when the travelling-twin in the twins-paradox returns to Earth.

In theory we could test this experimentally, perhaps by using a linear accelerator and taking two or three sets of measurements 6 or 12-hours hours apart when the linear accelerator is pointing in a different direction due to the rotation of the Earth. In theory, the accelerated particles should be going ever so slightly slower relative to the Earth for a given input power when the linear accelerator is pointed in the direction of our motion through the CMB. Alternatively, at a given particle speed relative to Earth, their half-life should be greater when the linear accelerator is pointed in the direction of our motion through the CMB compared to when it is pointed in the opposite direction. Alternatively, there might be some other non-zero kinematic time-dilation minimising velocity. — Preceding unsigned comment added by MathewMunro (talk • contribs) 00:16, 22 March 2020 (UTC)


 * The Twin Paradox is adequately explained by general relativity. The twin on earth is stationary, the travelling twin necessarily has to accelerate multiple times. If the CMB rest frame was a privileged frame of reference, that would be easily detected in e.g. Michelson–Morley style experiments, and we should also see different life times for cosmic-ray-induced muons on different sides of the Earth. Since GR is about the most thoroughly tested theory in history, either of these would be well-known. So: No. ;-) --Stephan Schulz (talk) 04:42, 22 March 2020 (UTC)
 * Firstly, on a somewhat semantic matter, the twins paradox concerns special relativity, not general relativity.


 * Secondly, Michelson–Morley experiments aimed to detect variations in the speed of light. I am not proposing a variable speed of light. I am proposing that there may be a non-zero kinematic time-dilation minimising velocity with respect to observers on the surface of the Earth, and that it may be the opposite of our velocity through the CMB.


 * Regarding why it may not have yet been detected: The speed we are travelling through the CMB is a fairly small portion of the speed of light (0.0012c), so the effect would be very subtle, (a Lorentz factor of 1.0006 vs the rest frame of the CMB presuming the rest frame of the CMB is the absolute minimum time-dilation velocity). Also, because of the way relativistic speeds are added together non-linearly, the effect on the Lorentz factor and half-life of particles moving at relativistic speeds would be much smaller than that - for example, if a muon were travelling at 0.9997c relative to the CMB, the Earth's motion through the CMB at 0.0012c would only increase the muon's velocity relative to us to 0.9997007c, and that would only increase the Lorentz factor from 40.8279 to 40.8756, with the difference being even less noticeable if the direction of travel of the muon were not aligned to or opposite the direction of Earth's motion through the CMB, and there is significant variation in natural phenomenon like observed half-lives and Muon flux, so it could be easily written-off as "background noise". Also, in most studies of kinematic time-dilation, the effects are entangled with gravitational time-dilation. Also, the CMB dipole was only discovered very recently, and before its discovery no one would have known which axis to look for a difference in the magnitude of time-dilation for a given speed relative to an observer on Earth's surface. And lastly, the fact that there have been so many tests of relativity already makes it harder to get support for another test.


 * Interestingly, I just located an article that claims: "... the duration of Muon decay, which should be a constant, definitely appears to shorten gradually from 1946 to 2017 from very roughly 2.330 microseconds (1946) to very roughly 2.202 microseconds (1963) to very roughly 2.078 microseconds in 2017" - http://www.gravwave.com/docs/V9%20Analyses%20of%20Muon%20Decay%20to%20Journal%20SS&T.pdf


 * Another study found: "The data showed a small but generally constant increase in muon generation during daylight hours when compared to night hours" - https://www.i2u2.org/elab/cosmic/posters/display.jsp?name=fluxvstimeofday.data


 * Both those observations could be explained by changes in the velocity of the observer with respect to the CMB if we throw out what we think we know about relativity and time-dilation.


 * In any event, the study of Muon decay would be a poor test of my postulate, compared to a linear-accelerator based test, because there appears to be numerous factors affecting Muon flux, such as solar wind speed & density & magnetic field fluctuations, see http://taurus.unicamp.br/bitstream/REPOSIP/73227/1/WOS000310911300066.pdf — Preceding unsigned comment added by MathewMunro (talk • contribs) 06:10, 22 March 2020 (UTC)

Is it gravitational potential energy, or the strength of gravity that determines time dilation?
Unfortunately the following comment was deleted from the talk page of the Gravitational time dilation article before anyone had a chance to reply:

It seems implausible to me that time dilation would be based on the distance from the centre of a massive object, including when inside the bounds of the object where gravity is less than on the surface, rather than being based on the strength of gravity. I expect that the gravitational time-dilation at the centre of the Earth would be zero, because gravitational forces are zero, yet the article suggests that's where gravitational time-dilation is locally maximised.

Has anyone ever tried taking an atomic-clock to the bottom of a very deep mine shaft, where gravity is lower than on the surface of the Earth, and comparing it to an atomic clock that is on the surface, ideally at a different latitude, an equal distance from the earth's axis, so that it is travelling at the same speed? If not, someone ought to, because it is hard to be certain of anything if we don't try to experimentally isolate the effects of kinematic time-dilation and gravitational time-dilation. — Preceding unsigned comment added by MathewMunro (talk • contribs) 00:19, 22 March 2020 (UTC)


 * Normally we think of the gravitational potential as a positive quantity that increases with the distance to a massive body. However, this makes the gravitational potential of a point in space dependent on the specific body (such as Earth or the Sun) relative to which that distance is taken. In this context we need a quantity that is independent of any specific body. We must use a position infinitely (or very) far away from all masses as the fixed reference location with respect to which the potential energy is defined. At that reference location, there is no gravitational time dilation. As an observer approaches a massive body, the time dilation increases. Suppose they now enter a very very deep mine shaft. Do you think it plausible the time dilation will now decrease? I don’t know if your proposed experiment has been performed in that form, but those that have would have revealed that the formulas are off if "the strength of gravity" was the determining factor. --Lambiam 04:31, 22 March 2020 (UTC)


 * Why not put it to the test, it would be a very cheap and easy experiment to conduct, and it could almost completely isolate gravitational time-dilation from kinematic time-dilation. — Preceding unsigned comment added by MathewMunro (talk • contribs) 07:16, 22 March 2020 (UTC)
 * If it is a very cheap and easy experiment to conduct, why not conduct it yourself? Just borrow two atomic clocks and head to a mine shaft. Do you think putting clock #1 at ground level and clock #2 at a distance d deeper will reveal anything not revealed by putting clock #1 at a height of d above ground and clock #2 at ground level? Since the experiments conducted already confirm the existing theory (within experimental error) and thereby disprove your theory, I'm afraid that if you cannot conduct the experiment yourself, you'll have to bribe or blackmail a team of experimental physicists, since otherwise they have no incentive for doing this futile exercise. --Lambiam 08:42, 22 March 2020 (UTC)


 * You wrote "Do you think putting clock #1 at ground level and clock #2 at a distance d deeper will reveal anything not revealed by putting clock #1 at a height of d above ground and clock #2 at ground level?"


 * That is not what I proposed. I proposed moving clock #2 to a different latitude, so that it is the same distance from the Earth's axis, and therefore travelling at the same speed as clock #1, but at a different gravity.


 * Simply raising clock #1 above clock #2 would not achieve the same result for two reasons, firstly the speed that the two clocks are travelling at due to the Earth's rotation would be different, so you would face the problem of muddled special and general relativistic effects. Secondly, it would not reveal whether or not it is the strength of gravity or the distance from the centre of mass of the Earth that is the determining factor in the degree of time dilation as it would in the experiment I propose.


 * Of course, the act of moving the clocks into position itself would introduce some kinematic time-dilation, however, it could be done arbitrarily slowly and the clocks could be left in position arbitrarily long, so that the effect was negligible.— — Preceding unsigned comment added by MathewMunro (talk • contribs) 14:26, 22 March 2020 (UTC)


 * I'm sorry if I misunderstood you, but I assumed your use of "ideally" indicated that that aspect was not essential. I can assure you that the physicists who conducted experiments verifying time dilation in which kinematic and gravitational time dilation were both significant, such as the Hafele–Keating experiment and Gravity Probe A, took both into account; if the theory was erroneously based on gravitational potential, that would have come out – as it would have by the correction applied for GPS satellites not working properly. --Lambiam 16:32, 22 March 2020 (UTC)


 * Regarding "Gravity Probe A" - clearly, a space-based measurement of time-dilation can not settle whether or not time-dilation is less or more underground than on the surface of the Earth!MathewMunro (talk) 01:48, 23 March 2020 (UTC)


 * It seems relative density becomes a factor and a estimate by Feynman of younger by "a couple days" was quite a bit off. fiveby(zero) 17:00, 22 March 2020 (UTC)


 * A preliminary scan of "The young centre of the Earth" paper suggests that it was a purely calculational paper, not supported by any measurements of time-dilation deep underground. They used the phrase "proof by ethos" to explain why the great Richard Feynman's gross mis-estimation was not picked-up for decades - well I suggest that the rejection of my criticism and my suggested way of experimentally testing it has been rejected by Wikipedians due to my lack of ethos! — Preceding unsigned comment added by MathewMunro (talk • contribs) 23:55, 22 March 2020 (UTC)


 * EarthGravityPREM.jpg I really have no idea, but found a neat graph showing acceleration due to gravity is actually larger within the mantle. equivalence principle and the integration $$T_d(h) = \exp\left[\frac{1}{c^2}\int_0^h g(h') dh'\right]$$ in Gravitational_time_dilation may help. $$g(h')$$ is the g-force. fiveby(zero) 16:19, 23 March 2020 (UTC)
 * This graph shows that (in the model used to compute it) there is a discontinuity in the density at the boundary between the core and the mantle. The section title "Definition" in the article is IMO a misnomer. A theoretical formula cannot be the definition of something that can be measured in an experiment. This is like calling using Kepler's Third Law formula $$T = \frac{2\pi}{\sqrt{GM}} r^{3/2}$$ a definition of "orbital period". In fact, the article is in want of a definition. --Lambiam 17:51, 23 March 2020 (UTC)


 * That's an interesting model of Earth's gravity below the surface, however it's in conflict with the observation that the gravity at the bottom of the Mariana Trench is 99.83% as strong as it is at the surface - MathewMunro (talk) 00:12, 24 March 2020 (UTC)


 * Gravity_anomalies_on_Earth.jpg Earth is lumpy. fiveby(zero) 01:12, 24 March 2020 (UTC)


 * "As you go down below the Earth's surface, in a mine shaft for example, the force of gravity lessens." - Dr Nicole Bell, Chief Investigator at the ARC Centre of Excellence for Particle Physics at the Terascale (CoEPP), and School of Physics at The University of Melbourne. Source https://www.abc.net.au/science/articles/2012/11/21/3636714.htm - they may have been wrong, but regardless, underground time-flow-rate measurements, combined with gravity measurements, combined with those same measurements made an equal distance from the Earth's axis at a different gravity could be used to both isolate general-relativity/gravitational time-dilation, and to provide experimental verification or rejection of the theory that time dilation is a function of distance from the centre of a massive object rather than being a function of the strength of gravity. MathewMunro (talk) 05:53, 24 March 2020 (UTC)


 * My understanding is that the relative time dilation between two locations is not a function of distance from the center of a massive object. It is a function of the "strength of gravity", but you'll need someone with a physics background to explain properly and correctly. Was hoping "The Young Centre of the Earth" and the graph of gravitational acceleration would lead to a good explanation. It seems you may be making a mistake in thinking about time dilation at a location (time-dilation at the centre of the Earth would be zero), it's time dilation relative to a reference location. When the time dilation of GPS sattelites are discussed it's relative to the surface of the Earth, not the center of the Earth. You can simplify for sattelites using the classical shell theorem and law of universal gravitation to make it look like a "function of distance from the centre", but as "The Young Centre of the Earth" shows and you correctly point out, that simplification does not hold when a location is beneath the surface of the Earth. fiveby(zero) 13:22, 24 March 2020 (UTC)


 * Time runs faster in orbit where gravity is lower. Logic would dictate that it also runs faster at the centre of the Earth, but that is not current dogma, ever since Feynman declared that the Earth's core would be a few seconds younger than the surface. It also has not been experimentally verified by checking the rate clocks run at underground near the equator compared to how fast they run on the surface further from the equator, an equal distance from the Earth's axis. MathewMunro (talk) 00:46, 25 March 2020 (UTC)


 * Is that because of gravity, or because of speed relative to earth? ←Baseball Bugs What's up, Doc? carrots→ 01:44, 25 March 2020 (UTC)


 * I shouldn't have introduced orbital objects, as I've just opened another can of worms :) There are special relativity effects, however below a certain orbit level somewhat higher than the orbit of the moon, objects in orbit will be travelling faster than they do on the surface of the Earth at the equator, which would only slow down the rate that time passes for them even more according to current scientific consensus. "Does time go faster at the top of a building compared to the bottom? - Yes, time goes faster the farther away you are from the earth's surface compared to the time on the surface of the earth. This effect is known as "gravitational time dilation"..." - Assistant Professor of Physics at West Texas A&M University. Source: https://wtamu.edu/~cbaird/sq/2013/06/24/does-time-go-faster-at-the-top-of-a-building-compared-to-the-bottom/ MathewMunro (talk) 04:24, 25 March 2020 (UTC)


 * I was expecting to see the elevator and rocket ship passing by clocks explanation similar to this. Also thought using the term 'g-force' instead of acceleration was confusing when talking equivalence principle. Anyway, integrate the acceleration of the rocket ship over the distance between clocks to find relative velocity and how much slower the clock is running (special relativity), integrate the acceleration due to gravity in free fall and general relativity tells you there is no difference is my limited understanding. I did not see the word 'acceleration' in any of the responses The graph is from Preliminary reference Earth model, here's the source From a practical viewpoint, such a modular construction is very convenien. It is much easier to perturb a particular feature of a model when it is separated from the remaining ones by clearly defined discontinuities than to alter a model that by definition is continuous.. Thought the graph was illustrative, but i am sure they could do better this century vs. 1981. fiveby(zero) 18:54, 23 March 2020 (UTC)


 * MathewMunro, do you have a source for that observation? The only obviously related thing that crops up in search results for Mariana Trench and 99.83% is this also calculational (and pretty dubious) discussion on Reddit. 89.172.17.213 (talk) 04:39, 24 March 2020 (UTC)


 * You're right, it was an estimation, by a reddit user, based on an incorrect assumption of uniform density. Gravity likely does increase a tiny bit going down a mine shaft vs the gravity immediately above, however, unless there are some pretty exceptional local density anomolies, the gravity at the bottom of a deep mineshaft anywhere near the tropics would be less than the gravity on the surface at an equal distance from the Earth's axis, further from the equator, (due to the distance from the centre of the Earth being less than it is at the surface of the mine shaft - due to the Earth's bulge at the equator, and due to the fact that in the mine shaft, some of the Earth's mass is acting against the gravity from the Earth's centre of mass), and so it would still be a great way to detect gravity-based time-dilation without any differences being due to special relativity, and it would also either confirm or refute the young centre of the Earth theory. MathewMunro (talk) 06:55, 24 March 2020 (UTC)

Why in normal there are amylase and lipase in blood?
Amylase and lipase are enzymes that considered exocrinic (rather than endocrinic). My question is why do we see them in blood (they have a normal range there) while they should allegedly to be in the GI only as exocrinic. I didn't find the explanation for their presence in blood as exocrine enzymes. ThePupil (talk) 00:20, 22 March 2020 (UTC)


 * High amylase levels in blood plasma are associated with trauma to the glands producing amylase. Apparently, a (very) low level of leakage is normal. As to the lipases, lipoprotein lipase is supposed to function in the bloodstream, where it hydrolyzes triglycerides into fatty acids for cell consumption, and hepatic lipase can function in the bloodstream to keep HDL inactive. Rather than "exocrine", the term "paracrine" may be more appropriate for glandular excretions that are internal but not endocrine. --Lambiam 04:00, 22 March 2020 (UTC)
 * Thank you. In all normal health people there is a level of amylase. What's its function in the blood? (I'm not asking about pathological levels of it, but about its presence in healthy people and its function then). You explained well the function of lipase in healthy people, thank you! ThePupil (talk) 13:29, 22 March 2020 (UTC)
 * Don't take my word for it as I'm not an expert, but my understanding is that its presence, although normal (abnormally low levels may also indicate morbidity), is accidental rather than functional, as I attemped to suggest above by using the term "leakage". Once in the blood, amylase also breaks down polysaccharides in the blood serum, but that is not a significant thing. This source states explicitly that pancreatic amylase enters the blood through "an unknown pathway", but Chempedia states that amylase enters the blood largely via the lymphatics. That may also hold for salivary amylase. --Lambiam 15:51, 22 March 2020 (UTC)