1

Physics, Astronomy & Cosmology / Re: Can Light Experience 'Time'

« on: 14/05/2023 15:57:06 »

The key here is that it is "c" which is the important thing, not the light itself.  As far as we know, light in a vacuum travels at c.  But even if this turned out not to be the case, it would not change the importance of the speed c as an invariant speed. It would just mean that the photon is not the truly "massless" particle we now consider it to be. 

2

Physics, Astronomy & Cosmology / Re: Forgotten vocabulary - a thing being pulled apart by tidal forces.

« on: 07/04/2023 17:01:49 »

Hi.

    Well I was recently reading about the Large (and also the Small) Magellanic Cloud in our Milky Way.
As you may know, this object is thought to be a dwarf galaxy in its own right that has started to merge with our galaxy.   Anyway, it's falling in toward the centre of our galaxy due to what is called "friction", although its more like gravitational braking rather than anything like air resistance.    Very roughly speaking - as the cloud moves through diffuse gas at the outer edge of our galaxy, the gravity of the Large Magellanic Cloud (LMC) deflects gas particles from our Galaxy toward itself.   This will end up causing some increased density in the wake of where the cloud has been.   This over-dense region behind the LMC is then a source of gravity acting on the LMC and will effectively create a net braking force on it.    Finally, since the LMC is experiencing this net torque applied to it, it's losing angular momentum, the orbit is decaying and the LMC is sinking down to the centre of our galaxy.

    Now, the bit that was relevant is that while the LMC is sinking, it's also being stripped of its own gas.   So there won't be much of the LMC left as one big blob by the time it's near the centre of galaxy anyway.   I was just suspicious that the stripping process might be much the same as hitting the Roche Limit  - but it doesn't seem to be.
    The gas stripping is happening at a distance from the centre of our galaxy which is much larger than the Roche limit.  I'm not sure why, except that the  Small and Large magellanic clouds are very gassy (i.e. not at all solid or rigid) and thus easily shredded.   They just do seem to be breaking up in a significantly different way.   Unlike the thing described by @Halc  just a moment ago,  the LMC and SMC aren't breaking up in such a sudden and dramatic way.  I suppose they are just being stretched and deformed progressively.   Anyway, that's what was troubling me a bit yesterday - why aren't they just doing what we know about the Roche limit, presumably it is just that the objects aren't at all rigid.   The Roche limit never gets mentioned when discussing the stripping of gas from these things but it just does seem that they should be related.
   

gas trails.jpg (86.3 kB . 957x503 - viewed 1747 times)

   Image based on this source:   https://esahubble.org/news/heic1314/

Best Wishes.

This action seems to be a "Roche Lobe" effect : https://en.wikipedia.org/wiki/Roche_lobe

You get the same type of thing in some binary systems, where a white dwarf will siphon mass from its companion.  When enough mass accretes, you get a type Ia supernova.

3

Physics, Astronomy & Cosmology / Re: Will tidal power cause the moon to crash into us?

« on: 08/02/2023 16:48:59 »

Eventually, you'd expect Earth's rotation to slow down until a day was as long as a lunar orbit.

I think it is more likely that the earth would become tidally locked with the sun.

The tidal effect of the Moon on the Earth is roughly twice that of the Sun's, Tidal forces fall off by the cube of the distance, so even with its great mass disadvantage, the much closer distance to the Moon more than compensates.

4

Physics, Astronomy & Cosmology / Re: Why does this twin paradox thought experiment fail for me?

« on: 26/01/2023 15:57:12 »

instantaneous velocity change

I must take issue with anyone who uses this phrase!
For any body with nonzero mass, an instantaneous velocity change requires the input of a finite quantity of energy (½m(Δv)²) in zero time, i.e infinite power. This implies that the laws of physics have been suspended, including those of relativity and time dilation. The subsequent analysis is therefore invalid.
 

The issue with considering "instantaneous velocity change"   is that the magnitude of the acceleration becomes undefined.
v =at, so to find acceleration needed for a known change of v over a known period of t, you use a= v/t. but if t is zero, this is division by zero which is undefined. (not infinite).

5

General Science / Re: Why does Earth's atmosphere have a whiter color near the horizon?

« on: 12/12/2022 16:15:45 »

But with this explanation, the sky near the horizon would be supposed to be orange, red like at sunset and sunrise instead of white, right?

No,  You see orange during sunrises and sunset, because that is the direct light from the Sun which is scattered the least by passing through more atmosphere.
When you are looking at the sky near the horizon when the sun is high in the sky, you are seeing the scattered light.  All the colors are scattered to some degree, blue just the most. So when you are looking near the horizon, you are seeing light that has been scattered enough for you to see a more full range of frequencies.

6

Physics, Astronomy & Cosmology / Re: Orbiting or descending into the black hole at the centre of the Milky Way?

« on: 28/11/2022 16:30:16 »

Mainly the MW is in orbit around itself.

And the mass of the Milky way is dominated by the (so far) invisible Dark Matter halo.

Though a great bit of that halo extends beyond the Galactic disk, and thus, due to the shell theorem, has no effect on the stars' orbits around the Sun.  Only that DM at a lesser distance from the center than the star is question would.
 So for the Sun, DM has an effect on its orbit, but doesn't dominate, as the DM closer to the center than we are, while a significant fraction of the total mass affecting our orbit, it isn't the majority of it.
If you look at the predicted and observed rotation curves here:
https://www.astronomy.ohio-state.edu/thompson.1847/1101/RotCurve2.gif
Note that close in to the center, you see little to no difference, as the mass of visible matter heavily dominates there. You don't see the deviation until you get out of the central bulge region and into the disk.

7

Physics, Astronomy & Cosmology / Re: Orbiting or descending into the black hole at the centre of the Milky Way?

« on: 27/11/2022 16:52:02 »

To build on what Halc said about the spiral arms.  They are produced by compression waves moving through the galaxy. This induces a greater rate of new star formation within them.  So they tend to have a higher percentage of young stars.  The brighter the star, the faster it goes through its fuel, and the shorter its lifetime.  By the time a newly born really bright star leaves the spiral arm, it has passed its lifetime as a main sequence star and is a husk of its former self.  The stars that tend to live long enough to leave the spiral arm it was born in are the medium to dim stars.
In other words, the spiral arms look bright not because they are made up of a significantly higher concentration of stars or total material, but because they contain a higher ratio of young, bright stars.

8

Physics, Astronomy & Cosmology / Re: Nuclear fussion and anti-nuclear technology

« on: 12/11/2022 15:57:10 »

Thanks very much for getting back to the topic.
The anti-nuclear technology, as name suggests, is technology capable of tackling radioactive material and atomic reactions related to it. I think being able to sustain fusion reaction in a very unique and specific way also gives possibility to produce anti-nuclear technology. Such technology is the best chance for this civilization to recover from nuclear weapons and nuclear power plants.

Both Fission (present power plants) and Fusion are "nuclear" technologies.  One taps energy by making lighter nuclei from heavy ones, and the other by making heavier nuclei from light ones.

And a fusion power plant would still produce radioactive waste.( Due to the fact that the fast neutrons produced would react with the walls of the containment vessel.) The difference being that fusion waste would have a shorter half-life than fission waste.  The waste storage issue is lessened, but not eliminated.

9

Physics, Astronomy & Cosmology / Re: How do photons relay information across distances?

« on: 18/10/2022 16:20:49 »

Daniel is looking for answers to this question.

"It is my understanding that photons are massless and can act as either particles or waves. None of this information helps me understand how a photon relays information across various distances. How is the color red stored and transmitted from a stop sign, across a void, and into my eyeball?"

Discuss your scientific findings in the comments below...

Light comes in various wavelengths. For visible light this ranges from 380 to 700 nanometers.  This encompasses the colors from violet to red.
Typical "white" light is really just a mix of light made up of all these wavelengths.  When white light strikes the stop sign, most of these wavelengths are absorbed by the paint, with only those wavelengths in the red region reflected back.
When that light enters your eye passes through the lens, and on to the retina.  The retina is finely covered with receptors; rods, which are just sensitive to intensity and cones which come in three types: Ones sensitive to the red wavelenths, ones most sensitive to Blue wavelengths, one  most sensitive to red ones, and ones most sensitive to Green wavelengths
With the stop sign, only the red-sensitive cones will detect anything. and it will be signaled to the brain that you see red.
For other colors,  the three types combine their signals to reproduce the color detected.

If you want to think of it in terms of photons, the wavelength determines the energy of each photon (red photons are less energetic than violet one)

10

Cells, Microbes & Viruses / Re: Does the fittest always survive?

« on: 06/10/2021 16:00:34 »

"Survival of the Fittest", means the "best fit" for the present environment.  So, the newer strain could have just been better adapted to a wider range of conditions, if the petri dish was an environment that suited the original strain, there would no reason for the new strain to have an advantage over it since both could flourish under those conditions.
However, if you changed the conditions so that was more to the liking of one and less to the other, then one strain would dominate.

11

Physics, Astronomy & Cosmology / Re: If the earth stopped orbiting the sun?

« on: 25/09/2021 22:28:54 »

This works out to be about 64 days.

Thanks Janus. Have you taken into account that the earth is starting from a stand still it will take some time to get going?

Yes.  It, like anything else, will start "falling" immediately, and at the same acceleration.  However, that is not to say that the mass of the Earth is completely irrelevant.  It can have an effect on impact time, just in the opposite manner that you seemed to imply.  Increasing the mass of the Earth would decrease the time to impact.  The reason for this is that not only is the Earth falling towards the Sun, but the Sun would fall towards the Earth due to Earth's gravity.  Since the Sun is some 333,000 times more massive than the Earth, the amount it would "fall" towards the Earth is pretty insignificant, and we can safely ignore it.
However, if the Earth had a mass that was a much larger fraction of the Sun's, you would have to account for it. 

12

Physics, Astronomy & Cosmology / Re: If the earth stopped orbiting the sun?

« on: 25/09/2021 22:08:21 »

    Anyway, other articles consider the journey toward the sun to be over in about 65 days but that seems to be because they reckon the tidal forces from suns gravity would rip the planet apart before we even reach the outer edges of the sun.

Tidal forces wouldn't effect the answer significantly.  The Roche limit( as measured in multiples of the primary's radius) is dependent on the relative densities of the bodies.
The Earth is some 4 times denser than than the Sun.  If we treat the Earth as a fluid body, this puts the Roche limit at 1.55 Sun radii from the center of the Sun our  just over 1/2 Sun radius above its surface.
Treated as a rigid body, the Roche limit ends up at ~0.8 the Sun's radius, or under the Sun's surface.  The actual Roche limit for the Earth will likely fall somewhere between.
But even if we put it at 1/2 the Sun's radius away, the Earth is just ~350,000 km from the surface, and will be moving close to its final speed of over 616 km/sec.  In other words, it is only around 10 min from surface impact.

13

Physics, Astronomy & Cosmology / Re: If the earth stopped orbiting the sun?

« on: 25/09/2021 21:34:42 »

If the earth stopped orbiting the sun. This is a hypothetical question and may have an interesting answer. Can anyone do the math and reveal the answer as to the arrival velocity and time period for the earth as it makes contact with the sun. The earth has great mass so I'm sure it will be slow getting started on its journey to the sun. I haven't calculated the speed or time for this event as I have no understanding of the equations involved. I hope someone can give the answer?

The quick and dirty method of computing the fall time is to assume the Earth's orbit is an extremely eccentric ellipse with the Earth's present distance being at perihelion.  This puts the Semi major axis at half the perihelion distance, and you can just calculate half the period of such an orbit.  This works out to be about 64 days.
A more accurate answer which will give you the time to reach the Sun's surface can be found by using the equation at the bottom of this article:
https://en.wikipedia.org/wiki/Equations_for_a_falling_body
Just use the Sun's radius for x and the Earth's orbital radius for r.
Impact speed can be arrived at by taking the difference between the gravitational potential energy at the suns surface and that at Earth orbital distance, and solving for the velocity needed for the Earth to have a equal kinetic energy.
I get ~616.64 km/sec
The fall time from Jupiter distance is ~ 2 years.

14

Physics, Astronomy & Cosmology / Re: How does Time Dilation happen on on the Atomic Scale?

« on: 22/08/2021 18:38:53 »

Hi @Janus
   Your reply looks good.   Just check these numbers:

and fast for 1.8 month.
Since the respective rates are 1/3 and 3, he would see it tick off 3.6 mo while ticking slow, and then another 3.6 mo while ticking fast,

I think that  1.8   should be edited to  1.2,      everything else seems OK   3 x 1.2 = 3.6   etc.

Yep, thanks for catching that. I went back and edited it.

15

Physics, Astronomy & Cosmology / Re: How does Time Dilation happen on on the Atomic Scale?

« on: 21/08/2021 22:59:03 »

So eventually...

Why is Time Dilating then?

If the Earth Twin eats 100 mangoes in 1 Earth Year

The Space Twin would have eaten 100 mangoes in it's 1 Space Year Journey.

If both video recordings are Compared, there would be 100 mangoes in each.

So like the Space Twin's video would be Shorter?
6months?

So then if that SpaceTwin video is played at 0.5x speed(half) then it would resemble Earth Twin's video length?

But then that would show SpaceTwin in Slow Motion Right?

Jeez!
🥴
Mind goes Mangoes understanding all this...wat do u intelligent folks eat?

ps - does anyone feel Utube songs are sloww n play them at 1.25x
Dis has nothin 2 do wit da pond n everythin 2 do wit a DiscoToad!
🤭
(Solly)

Assume The space twin travels at 0.8 c both out and back.
The round trip takes 1 Earth year, meaning he got 0.4 ly from earth when he turned back ( as measured from the Earth)
Our earth observer watching him will see him eat mangoes 1/3 as fact as he himself does (a part of this is due to the time delay caused by the increasing distance between them.)*
After 1/2 year, the space twin reverse direction back towards Earth. however since this occurs when he is 0.4 light years from Earth, the Earth twin doesn't see this occur for another 0.4 years or 0.9 years after the space twin left. During this whole time, he sees the Space twin eating mangoes 1/3 as fast. Since he has eaten 90 mangoes by this time, he will have seen the Space twin eat 30 mangoes. Once he sees the space twin reverse direction, he will see him eating mangoes 3 times faster than himself.**
But by the time he actually sees the space twin eat 30 mangoes and reverse direction, the space twin has already traveled most of the way back to him, and arrives back at Earth just 1/10 of a year later (having been gone a total of 1 year) during this time the Earth time eats another 10 Mangoes, while watching the space twin eat 30 mangoes.
Thus he eats a total of 100 while the space twin eats just 60.

If we look at things from the space twin's perspective, we get this:
The space twin recedes at 0.8c from the Earth, during which time he sees the Earth twin eat mangoes 1/3 as fast. After he has eaten 30 mangoes he reverses direction to head back to Earth ( the fact that he has eaten 30 mangoes by the time he turns around is something that he and the Earth twin must agree on.)
during this time he sees the Earth twin eat 10 Mangoes.
Now here comes a major difference between him and the Earth twin. The Earth twin had to wait to see the effects of the Space twin turning around because that event took place 4/10 of a light year away. But for the space twin, his reversing direction happens right there, so he has no delay. He immediately sees The Earth twin start eating mangoes 3 times faster than himself.  He eats the same number of mangoes for the return trip as on the outbound trip, so he eats 30 more mangoes, for a total of 60. During the return leg he sees the Earth twin eat three times as much as himself, or 90 mangoes, plus the 10 he'd seen him eat during the outbound leg gives 100 for a total.  This is the same end result the Earth twin arrived at.
* what he sees is the combination of this increasing lag and time dilation. If he were to factor out the light delay, he would conclude that the space twin was eating mangoes at  rate 6/10 his rate.
** again a combination of a ever decreasing time lag and time dilation, factoring out the light delay has him concluding that the space twin once again is eating mangoes 6/10 as fast during the return leg.

16

Physics, Astronomy & Cosmology / Re: What Propels Light ?

« on: 05/08/2021 21:49:16 »

Hi again.

Did this one ever get answered?

Thank ewe Halcy. So what is the speed of light in my room ? I'm not in a vacuum !

     Light in air is 1.0003 times slower than light in a vacuum, which slows it all the way down from 299,792,458 meters per second to 299,702,547 meters per second.

[taken from forbes.com]

     I hate air.  I know everyone else likes the stuff but I'm deeply suspicious of it.  Air is a mixture of gases and so it is deeply sinister stuff.  None the less, it's considered a (single) medium by most people.
    On a similar note, light never travels at the speed of light in a vacuum - because there are no vacuums found in the universe.  We believe that there is interstellar gas, neutrinos (CNB) and other photons (CMB) flying around in every part of space. 
     I think most of us accept that matter seems to slow down light but it's also quite likely that any form of energy does.  For example, other photons may interact with and hinder the passage of photons.   NOTE:  I've not seen any hard and reliable references to back that up.  It just seems likely.  I'd be grateful for any good references that support or refute that.

Light does not interact with neutrinos, and no, other photons have no effect on its speed.  As far as interstellar gas goes, we are talking about a few atoms per cubic meter.  Since the spacing between the particles is so much larger than the wavelength of the photons, they only interact by scattering the light which only occurs if a photon strikes a particle.
But, because the particles are so few and spread out, this occurs rarely. Put another way, the mean free path of a photon( The average distance it travels before interacting with one of those particles) in interstellar space is pretty long.

17

Physics, Astronomy & Cosmology / Re: What Propels Light ?

« on: 05/08/2021 15:19:08 »

how does light on the right hand side of a flame know to go one way and light on the other side know to go the other way ?

A sensible sheep will not try to walk through a fire.

But photons are fearless, and will travel in every possible direction.

Some photons do make it through a candle flame from the far side .
- Outside the candle, air is cool and clear, so light from the near side has an unobstructed path to your eye
- But a photon from the far side has to pass through a hot maelstrom of unstable reaction products, which are more likely to absorb it, and then reradiate the energy from a closer point
- Some photons do make it all the way through the candle flame - but they are hard to see against the brightness of the nearer side

PS: Is there such a thing as a sensible sheep?

Thank ewe Evan. I guess this is similar to the way that light takes a gazillion years to finally leave the sun, but (confewesed sheep)...I thought as soon as light interacts with something it gets absorbed or converted into something else. So how does it survive all those collisions in the sun before it finally makes a run for it ?


sensible sheep ? guffaw chortle  !!

The photons that leave the surface are not the same photons that started at the core.  If a photon interacts with an atom, it adds energy to that atom. That atom will end up shedding all or part of that energy (depending on the type of interaction) by emitting another photon. That photon won't necessarily be emitted in the same direction as the original photon.
So while "a" photon eventually leaves the surface, it removed by a chain of many, many, of these types of reactions from the photon that started at the core.

18

Question of the Week / Re: QotW - 21.07.06 - Why is it cold up mountains?

« on: 06/07/2021 15:46:57 »

Air temperature in the Troposphere( the lowest atmospheric layer), is driven by being heated by the ground, which was, in turn heated by the Sun.   Higher altitudes, being further from the ground are heated less by this effect. Now while it is true that air near a mountain is close to the ground of the Mountain, that air is always being mixed with air that comes from the much larger regions where air at that altitude is much further from the ground.
As far as heat rising goes. Yes, warmer air, in general, will rise in colder air, But in doing so, it has to climb against gravity, and it can't do this for free, it has to give up some of its heat energy in doing so. So even warm air rising from the surface will not be as warm at altitude as it was near sea level.

19

Chemistry / Re: Can elements contain multiple isotopes within one sample?

« on: 24/06/2021 14:51:51 »

Any random sample of an element can contain a number of isotopes.  For example, Uranium in its natural form is made up of both U-238 and U-235.  Since only U-235 is fissile, "raw" Uranium has to be enriched (The percentage of U-235 increased) in order for it to be useful for power plants or nuclear weapons(Nuclear weapons need a much higher enrichment than reactor fuel needs)

20

Physics, Astronomy & Cosmology / Re: Another Relativity paradox

« on: 08/06/2021 16:31:31 »

Hi all.

   Does anyone have some knowledge or insight about this "paradox" in the theory of Relativity?

   Imagine a submarine underwater.
Initially:
The submarine is at rest relative to the fluid and has adjusted it's tanks so that it has equal density with the fluid and remains at a depth of 100 metres.  (No thrust required from the engines, it just has neutral buoyancy).
 
Subsequently
    The submarine accelerates rapidly to reach relativistic speeds (let's say 0.9 c) relative to the fluid and then sustains a constant velocity.  This is intended to be a horizontal motion, the fins, bow planes etc. were not set to drive the submarine up or down.

Finally
     As is usual for these sorts of paradoxes,  we have two observers in two different frames of reference.

The submarine commander is at rest inside the submarine.  She should observe length contraction for the fluid in her rest frame and a corresponding increase in density of the fluid.  The submarine retains it's rest characteristics, including density in her frame.

A mermaid is at rest on the ocean floor.  In her rest frame, the density of the fluid has not changed,  however the submarine has undergone length contraction in her frame and it's density has increased.

Question
   Will the submarine rise or sink due to buoyancy?

Background info:   You may like to read the Wikipedia article about Supplee's paradox.
https://en.wikipedia.org/wiki/Supplee%27s_paradox
There is also a similar discussion about a Helium balloon moving through air on another forum.  (I'm not sure I should put links to another forum).

   I do not know the answer.  I can see references to articles in that Wikipedia entry but they seem to demand some application of General Relativity and a complete re-write of the Archimedian principle.  I was wondering if there is a resolution based only on Special Relativity - but I'll take any insight or discussion I can get.

As with most apparent "paradoxes" in Relativity, this one is likely due to only focusing on one aspect of the theory and ignoring the rest.
For example, here, you say that the Water appears more dense due to length contraction from the Sub's frame of reference, and thus should be more buoyant. However, what causes buoyancy?  First you need gravity ( this in of itself means you need GR to properly address the issue).   This in turn causes a difference in the water pressure between the top of bottom of the sub.  It is this difference in pressure ( water pushing up on the bottom more than it pushes down on the top) that produces the net upwards force.
Now you need to consider the effect that water flowing past the sub has on the static pressure of the water. Bernoulli's principle states that a moving fluid's static pressure decreases with fluid velocity.   That suggests that if the sub starts with neutral buoyancy at rest, it will not remain so once it begins to move relative to the water.  Thus if it wants to stay at constant depth, it will need to adjust its bow planes to a new angle.* 
But now, because of length contraction, the Mermaid will measure that angle as being different, changing the effect they produce...

The point is that just considering how length contraction acts on the water or sub as seen by sub or mermaid doesn't give you the full picture as to what is going on.  SR has been proven to be entirely self-consistent. Which means that no such thought experiments can never produce a real paradox, as long as you properly apply it.**

*and this doesn't even go into how Relativity would factor in to calculating the resultant effect.
** Now this, in of itself, does not prove the correctness of SR, just that thought experiment alone is not enough to disprove it.  You need a real life experiment or observation that doesn't match what Relativity predicts for that.