1

Physics, Astronomy & Cosmology / Re: Is the inverse square law only approximately correct in general relativity?

« on: 26/06/2022 02:58:44 »

Suppose it was something else like Beta particles being emitted isotropically by the source.   Why would that not follow the 1/r2 law for the bombardment intensity received on the surface of a sphere held at a constant metric distance (a radius) r from the source?

Presuming you didn't do anything funny like put detector/source at different potentials, the inverse square law would work given this constant r (say both held at opposite ends of a stick).  Space expansion would make no difference. Dark energy probably would, but that counts as 'something funny' just like gravity does. Dark energy would put tension on the stick. Space expansion would not.

If we assume that the particles are traveling at (say) c/10, then there will be an event horizon beyond which these particles will not pass, because space will be expanding faster than c/10 by the time they got there.

OK, but if distant detector is held at constant distance from this emitter, it will cross over that 'event horizon' (towards us) and the particles will get to it.

What you're talking about isn't the event horizon, it's the Hubble radius, the distance where Hubble's law yields c. The event horizon is a little further away from that, and it has to do with acceleration, and is not a function of the current expansion rate like the Hubble radius is.  So a beta particle moving at 0.1c would get at most a 10th of the way to the Hubble radius, and would take an infinite time to do so.

This event horizon will be much smaller than the event horizon for light (which defines the limits of our observable universe).

The light event horizon is about 16 BLY away. Current radius of the visible universe is about thrice that, so they're very different things. The latter is all the material in the universe which at some past time might have had a causal impact on a given event (Earth, here, now).  The event horizon is the comoving distance of the nearest current event from which light can never reach here in any amount of time.

After all, the size of our observable universe is not at a fixed distance - it expands at the speed of c.

The Hubble sphere expands at c (by definition). The visible universe expands at somewhat over 3c, which is why we can see galaxies that are currently about 32 BLY away (comoving distance).  The event horizon is barely expanding at all.

so (in principle) there are distant galaxies that people on Earth could see today, but
 which will not be visible in 10 billion years

Hate to disagree, but new galaxies become visible over time. The most distant ones were not visible several billion years ago, even if one used the best telescopes. Yes, the galaxies cross beyond the event horizon, but that doesn't mean we can't see them any more than we stop seeing somebody falling into a black hole.

The inverse square law is about the intensity received at a distance, r, from the source.   That is a physical distance, so it is determined by the metric.   It is not determined by reference to a difference in the values assigned to locations in the co-ordinate system we commonly use to describe an expanding universe.

Just so, yes.

The usual co-ordinates used in an expanding universe are the called the co-moving co-ordinates.  Galaxy 1 can have fixed co-moving co-ordinates and it's tempting to say it has a fixed position.   Galaxy 2 can also have fixed co-ordinates and we can be tempted to say it has a fixed position.

Right. The rate that a given galaxy changes its coordinates is called peculiar velocity, and the peculiar velocity of almost all objects is quite low, a few percent of c at best.

If you posit some particle that travelled at c/10 (and didn't slow down)

In an expanding metric, the paricle will slow down without some force maintaining its peculiar velocity. Newton's laws only work in a static metric.

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Physics, Astronomy & Cosmology / Re: Is the inverse square law only approximately correct in general relativity?

« on: 25/06/2022 14:33:45 »

Hi.

     I'm always very grateful for anyones time spent in conversation.  So thank you very much @evan_au.
However, I can't always agree with everything and that is the nature of discussion after all.   What you've said isn't wrong, it's just misleading.   Although, equally, what I've said before could be considered misleading.
    We need to establish a few things and then hopefully we will be seeing and understanding the same things.

I was not thinking about a horizon at a fixed distance.

   OK  -  BUT you should be.    The inverse square law is about the intensity received at a distance, r, from the source.   That is a physical distance, so it is determined by the metric.   It is not determined by reference to a difference in the values assigned to locations in the co-ordinate system we commonly use to describe an expanding universe.
   The usual co-ordinates used in an expanding universe are the called the co-moving co-ordinates.  Galaxy 1 can have fixed co-moving co-ordinates and it's tempting to say it has a fixed position.   Galaxy 2 can also have fixed co-ordinates and we can be tempted to say it has a fixed position.    The co-ordinate differences between the galaxies never changes, so that seems fine.   However, the physical distance between the galaxies is increasing with time,  in that respect it doesn't look like the galaxies are fixed in position in any ordinary sense of the word.   To avoid the confusion, it's best if you just don't say or imagine that either of the galaxies have a fixed position.   Instead, just say that the galaxies are "co-moving with the universe",  or  that they are "co-moving" for short.

But space can expand faster than c, so (in principle) there are distant galaxies that people on Earth could see today, but
 which will not be visible in 10 billion years, because the expansion of space has carried them outside our visible universe.

      This is correct.   However, the light emitted by the distant galaxy will travel an infinite physical distance.   It's just that how far it can go in 1 unit of time will not be sufficient to match the expansion of space that is occurring between the distant galaxy and earth.
      We would place our sphere, where the intensity is being determined, at a fixed physical distance from the distant galaxy and not at a fixed co-moving co-ordinate separation from the distant galaxy.   The light from that distant galaxy will reach the surface of that sphere.    Just to get the image and understanding right,  the surface of that sphere might have started at time t=0 precisely where planet earth was.   However, the surface of that sphere will be a long way from earth at a later time t=1 unit,  when the photons from the distant galaxy finally cross over that sphere.
     We can place a sphere with a fixed physical radius, r, around the distant galaxy (it doesn't matter how large r is)  the photons from that galaxy will (eventually) reach the surface of that sphere.   In no way does this contradict the idea that the photon won't reach earth - the surface of the sphere is nowhere near planet earth when the photons cross over the sphere.

If you posit some particle that travelled at c/10 (and didn't slow down), there would be regions of our visible universe that could never detect these particles, because the space in between is/will be expanding faster than c/10.

   Yes, total agreement.   However, that has nothing to do with the particles crossing over a sphere at some fixed and pre-determined physical distance from the source.

Best Wishes.

3

Technology / Re: What Is The Benefit Of Cryogenically Treating Cables ?

« on: 16/06/2022 13:53:48 »

You have to get copper down to 35K before it becomes superconducting

I'm fairly sure that copper doesn't superconduct  even at much lower temperatures than that.

4

Physics, Astronomy & Cosmology / Re: Does The Gravity Of A Black Hole Travel Faster Than The Speed Of Light ?

« on: 15/06/2022 02:02:07 »

Hi.
   Thanks for the extra information, @evan_au .  There's stuff I didn't know there and I may look into Supernova again to see what the latest is.   
   The rest of this post may look like a criticism, sorry.  It isn't meant to be.

Time dilation is so extreme within millimeters of the event horizon that the image of an infalling rock would be very quickly red-shifted into oblivion.

    I think we need to establish where the observer is and assume they are using a frame of reference where they are at rest.   The distant observer = An observer at a fixed radial co-ordinate, r >> Schwarzschild radius.      The rock = the rock or anyone close to the rock and falling in with it.

      We all agree that the rock doesn't spend long outside the event horizon, it just falls in within a finite amount of time as far as its concerned.   However, for the distant observer the rock takes an infinite amount of time to reach the event horizon   (well, certainly if the rock is treated as such a small mass that it doesn't affect the spacetime geometry - a previous post discussed that and mentioned my uncertainty about it).
     As far as the distant observer is concerned, light from the rock is progressively red-shifted and total luminosity from it reduces.   This isn't a quick process.

So: Very few photons, severely red-shifted: The rock would not "float" near the event horizon, it would just disappear.

   Bits of this are OK.   However, it doesn't "just disappear", it fades away.  It's like having a studio engineer with the slowest hands in the world, turning the fader knob so slowly that the universe will end before the stage actually goes dark. 

...forming a black hole, with almost the same mass as the star before it imploded...

    Is that right?  I know stellar collapse varies quite a lot and I'm not aware of the latest ideas for the typical behaviour.   Old texts used to suggest that typically there is a supernova explosion where the outer layers of the star making up about 20% of the original mass of the star is blown away.    Some sources put the amount of matter ejected far higher than that...
About 75% of the mass of the star is ejected into space in the supernova.   

https://imagine.gsfc.nasa.gov/science/objects/stars1.html

Best Wishes.

5

Plant Sciences, Zoology & Evolution / Re: Have animal reactions to mirrors and windows been studied?

« on: 28/05/2022 00:50:28 »

https://twitter.com/buitengebieden/status/1523065975654195201
https://twitter.com/buitengebieden/status/1527524346872332289
https://twitter.com/philipnolan1/status/1515260865892732928
https://twitter.com/Yoda4ever/status/1519881660744962049

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Plant Sciences, Zoology & Evolution / Re: Have animal reactions to mirrors and windows been studied?

« on: 27/05/2022 04:19:41 »

Well, this cat certainly appears to perceive motion in this static picture:

Perhaps just like we do:

rotsnake.jpg (613.75 kB . 1024x768 - viewed 1201 times)

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Technology / Re: How can I achieve even heating with induction cooking?

« on: 08/05/2022 22:56:59 »

Of more concern is that the bundle of wires appear to be plastic-insulated.

The wires appear to be standard enamel insulation.  Also, the plastic tie-wraps are not showing any damage so far.

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The Environment / Re: (What's) the optimal temperature for life?

« on: 01/05/2022 16:11:28 »

in a sterile desert, more life will grow if the temperature falls

No. In a sterile desert more life will grow if there is more water available at the surface. Of course - more water available at the surface will also cause surface temperatures to fall, due to increased evaporative cooling. Temperature is an issue in many hot arid areas - such as the Sahara - due to lack of water. The equator gets more sunlight above it, but also has more clouds, and more water (at the surface) to cool it. So water is the issue in arid areas - not temperature.

BTW: In some arid regions of China day-time, growing season, temperatures fell by over 6C when mass irrigation was introduced. See: Yang/Huang/Tang, 2019

9

Physics, Astronomy & Cosmology / Re: Why don't gamma rays have a higher speed?

« on: 21/02/2022 13:20:51 »

Do any Europeans care about cricket?

Yes.
Notably quite a lot of Europeans who live in England care about Cricket.
Ask a geography teacher for further clarification.
(Don't ask a politician; they don't seem to understand).

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Physics, Astronomy & Cosmology / Re: Why don't gamma rays have a higher speed?

« on: 20/02/2022 15:35:57 »

Hi again.

....and the idea of a massless particle transferring momentum really does confuse people.

     I'm definitely with you, or at least sympathetic to, a lot of what you've said alancalverd.    It connects with something I was trying to get together and tidy up to make a post.   I'm just going to post it now, scruffy as it is...

    I would like to target something that @evan_au  mentioned first but lots of other people have also added to.

Massless particles (eg photons or the hypothetical graviton) which carry energy can only travel at "c"
(ES adjusted the spelling of the word "the")

    Go for it, evan_au, or anyone else reading this.  Present your proof or provide links to a reference etc.   

I've seen a lot of partial proofs and plausibility arguments but nothing that I'm completely happy with.   It doesn't mean it's wrong, I don't know everything - I'm just old enough and stubborn enough to question every little detail these days.

    A lot of arguments will start from this equation   

E²  =  m²c⁴ + p²c²       

[Eqn.1]

(with the usual meanings,   p = 3-momentum,  m = rest mass   etc.)

    ....because I would stop you there and ask exactly how you derived that equation to start with.    For massive particles that equation is easily obtained by considering the 4-momentum, P^μ, of a particle.    It's simple enough since a 4-velocity, U^μ = is well defined for particles with timelike paths and then we can define P^μ = m U^μ.   

      However, for particles that follow null paths we can't use the same tricks:  The 4-velocity is not defined since =  0   on a null path.   It seems common to just introduce a new definition for the 4-momentum of a photon,

P^μ  =   ( E/c   ,   V_xE/c² ,  V_yE/c² ,  V_zE/c² )      with  V_x = x-component of velocity (w.r.t. co-ordinate time) for the photon etc.
   ... but the question is how did you come to decide on putting those particular components into the 4-momentum? 

   The only "proof" or plasuibility argument I've ever seen for that definition of the 4-momentum of a photon is based on using some additional infomation about the behaviour of photons.  Something that you could observe in experiments on photons but is not otherwise obviously true for all massless particles.
    For example, assuming straight away that the momentum and energy are related  by  p = E/c.   Alternatively, you can derive that form for the 4-momentum under the assumption that the 3-momentum  in any spatial direction will be proportional to the velocity (w.r.t. co-ordinate time t and not proper time Tau) of the particle in that direction.   Another "proof" I've seen utilises a 4-wave vector for the photon (which assumed the photon had wave-like properties and propagated at speed c to begin with).   Anyway, all of these things are extra pieces of information you could obtain by experiment on light but I don't see why they're necessarily true for arbitary massless particles.

Best Wishes.

LATE EDITING:   This article,  https://www.wtamu.edu/~cbaird/sq/2014/04/01/light-has-no-mass-so-it-also-has-no-energy-according-to-einstein-but-how-can-sunlight-warm-the-earth-without-energy/
is quite a good one, it's easily readable by non-specialists, if anyone wants a general guide for why massless particles travel at the speed c.   It's typical of many Popular Science articles where E² = m²c⁴ + p²c²  [Eqn 1]  is the basis or starting point for the  argument.

11

Physiology & Medicine / Re: Can we combine axolotl DNA with human DNA?

« on: 10/02/2022 05:17:57 »

So until there are some major breakthroughs, I suggest that researching limb growth is perhaps a thousand to a million times harder than producing a vaccine against one variant of COVID.

If we think linearly, the obstacles are seemingly impossible to overcome. But technological progress is exponential. If the effectiveness of our technology improve by twice a year, we will get a million fold improvement in just 2 decades.

12

Physics, Astronomy & Cosmology / Re: Would string theory enable very long distance communication?

« on: 07/02/2022 17:46:06 »

I guess it's just me...
https://en.wikipedia.org/wiki/Tin_can_telephone

13

Physics, Astronomy & Cosmology / Re: What would your reaction be to a sudden volcanic eruption burst on the moon?

« on: 17/01/2022 19:30:44 »

What would the scientific community's reaction be to a sudden volcanic eruption from a moonquake(s)?

For about the first second, nothing at all.

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Just Chat! / Re: Should we relegalize dueling?

« on: 07/10/2021 19:40:40 »

In countries with a public health system (eg not the USA), the health system consumes a large fraction of the taxation income.
- Dueling will only increase load on the health system, since it your taxes that pay to patch up the people who partially survive the dual.
- If there is no real public health system (eg the USA), gun violence is rife, and people are encouraged to get more guns to protect themselves from the idiots with guns. There is no incentive for the government to do much about it, because repairing the damage doesn't really come out of their pockets.

It was ironic to hear Donald Trump criticizing the mayor of London on a night when 3 people were killed in stabbings, when the homicide rate in New York was twice as high (you are much more likely to die from a gun attack than a knife attack).
See: https://www.bbc.com/news/uk-england-london-48651131

There is a branch of mathematics called Galois Theory, applied in 4G mobile phones and cable TV, among many others. Galois documented this in a letter at age 20. He had foolishly challenged a soldier to a duel, and died of gunshot wounds received the next morning.
See: https://en.wikipedia.org/wiki/%C3%89variste_Galois

The relevant difference between the US and the UK is that neither of them allow duelling.

15

Physics, Astronomy & Cosmology / Re: Why do objects travel through space-time and not space and time?

« on: 23/08/2021 03:07:44 »

The twin paradox comes about because the "stay at home" twin has a constant time coordinate

Actually, he has a constant spatial coordinate (at least in the Earth frame) and the time coordinate is the only one that varies (by a year).

and so has a "shorter" worldline through spacetime than the "leave home" twin

E-S has the gist of it. Minkowskian spacetime isn't Euclidean. In Euclidean spacetime, 4D distances would be computed as √(t²+x²+y²+z²) but spacetime intervals subtract the xyz from the time: s² = t²-(x²+y²+z²). So the straight line (Earth result in the longest interval, and the shortest interval (that followed by light) is where t² equals distance². So I'd say the travelling twin has the shorter worldline, as measured as an interval.
A spacelike worldline (where spatial separation is greater than temporal separation) results in a negative interval s², meaning s would be imaginary. Nothing can 'follow' such a worldline.

16

Physics, Astronomy & Cosmology / Re: Can you handle old pipe X-ray radiation shielding?

« on: 30/06/2021 00:15:29 »

I think the question relates to shielding materials used during radiographic weld and pipe integrity inspection.  Personnel shielding around the pipe is most likely to be lead, usually sandwiched in plastic or plywood, but the radiation source  is often a radionuclide pellet that is carried in a "torch" with tungsten or uranium filling.

Depleted uranium isn't much less radioactive than the natural stuff, which contains less than 1% of U235, so most of the U235 emission is actually absorbed by the bulk U238. The problem is its reactivity and alpha emission - it can deliver very large doses to e.g.lung tissue if the dust is inhaled.

17

General Science / Re: How does one Quantify the Colors of a Quark?

« on: 14/06/2021 13:58:21 »

I don't have a direct link to the story behind why they named it 'colors'.

But           " As mentioned and shown in Figure 1, quarks carry another quantum number, which we call color. Of course, it is not the color we sense with visible light, but its properties are analogous to those of three primary and three secondary colors. Specifically, a quark can have one of three color values we call red (R), green (G), and blue (B) in analogy to those primary visible colors. Antiquarks have three values we call antired or cyan (¯R), antigreen or magenta (¯G), and antiblue or yellow (¯B) in analogy to those secondary visible colors. The reason for these names is that when certain visual colors are combined, the eye sees white.

The analogy of the colors combining to white is used to explain why baryons are made of three quarks, why mesons are a quark and an antiquark, and why we cannot isolate a single quark. The force between the quarks is such that their combined colors produce white. This is illustrated in Figure 5. A baryon must have one of each primary color or RGB, which produces white. A meson must have a primary color and its anticolor, also producing white. "

https://courses.lumenlearning.com/physics/chapter/33-5-quarks-is-that-all-there-is/

Does it pretty well.  I used to have a more 'original' and rather fun link to how they came up with it but it's unfortunately gone.

18

Geek Speak / Re: Why the median does not change?

« on: 14/06/2021 11:54:22 »

A common example where the median is preferred over the mean is wealth, because the mean gets distorted by a tiny number of mega-rich people.

19

Physics, Astronomy & Cosmology / Re: what would happen if gravitational mass were different than inertial mass?

« on: 12/05/2021 07:13:30 »

just like the rotation of the planets in the Solar System prevented the Solar System from collapsing into the Sun.

The correct term is revolution of planets, instead of rotation.

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Physics, Astronomy & Cosmology / Re: Why Didn't Einstein FULLY Address Simultaneity-at-a-Distance?

« on: 25/04/2021 15:53:05 »

This Veritassium video suggests that Einstein did address simultaneity at a distance, by saying it is impossible to measure, and defining the convention that the speed of light is identical in every direction.
[...]

At about the 15 minute point in the video, the author says that it's possible that in some directions, we're seeing extremely distant galaxies as they were in the distant past, whereas in other directions we're seeing extremely distant galaxies as they are now.  It seems to me that that possibility can be discounted because of the fact that what we see in all directions is amazingly uniform, and that seems to contradict our understanding that the universe has greatly changed since the big bang.