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Physics, Astronomy & Cosmology / Re: The early Sun at the time just after Theia's impact? What colour was it?

« on: 10/06/2022 17:21:42 »

Hi.

From outside the atmosphere, would the young cooler Sun be yellow like it is today, or dimmer, red or orange?

   and this question....

if you could see the Sun, even from above the atmosphere, would it have looked cooler than today's view, maybe it was orange or red? Not bright yellow?

Seems to have been overlooked.

    The sun is basically a fairly standard star right in the middle of the main sequence on the  Hertzsprung-Russell diagram.    It hasn't changed it's brightness, temperature, colour or anything very much for a few billion years.    This is because its mass (which isn't thought to have changed very much) basically determines the hydrostatic equilibrium that will be achieved, which determines the temperature which in turn determines everything else (colour and brightness) because as @Bored chemist  has just mentioned,    the sun is almost exactly a black-body.

    The only complication is that the Theia impact could have been a very, very long time ago.   4.5 billion years by one source  ( https://en.wikipedia.org/wiki/Theia_(planet) ).   That does put the sun right at the earliest moments of its evolution and where it might have been just slightly off the main sequence.

      Using this source of information,  "The Suns evolution" - Northwestern University, U.S.  ( https://faculty.wcas.northwestern.edu/infocom/The%20Website/evolution.html  ) , it is suggested that the sun could have been about 30% dimmer 4.5 billion years ago.
      Using the Stefan-Boltzman law for blackbodies   we have     (Luminosity)  α  (Temperature)⁴,   so that the temperature would have been about  91.4%  of what it is now,    i.e. almost exactly the same.    This means the spectrum of radiation from the sun would have been almost exactly the same.    Using Wien's displacement law,  the peak wavelength would have been just slightly higher.   To say that another way,  the sun's light would have been slightly redder.    How red?   I'm not much of an artist,  I'd say it was slightly orangey.

    There are online calculators you can use to determine the peak wavelength and show that on the spectrum of visible light  BUT  it's better if you use something that considers all the wavelengths that are emitted and attempts to show you the colour you perceive with human eyes on the screen.    I don't know how reliable this website is but this one attempts to do exactly that:    https://sciencedemos.org.uk/color_blackbody.php?temperature=5300  .   In my opinion what that website shows looks a bit pink at times to my eyes ....  I'm really not sure how reliable it is or how good my eyes are but it illustrates the point if nothing else.   Let's just say the early sun was a bit more orange, if you squint and stand on one leg. 

Put the temperature in at 5800 k  (which should be the sun today) and you get this overall colour and brightness:

5800k.JPG (9.96 kB . 262x120 - viewed 515 times)
Put the temperature in at 91.4% of 5800  =  5301k  and you get this colour and brightness:

5301k.JPG (9.87 kB . 232x122 - viewed 516 times)
So you would see something like that if you were above the atmosphere of the planet earth and looking at the sun.   I hope that helps.

Best Wishes.

If I go into Blender, make a couple of spheres, and give them an emission shader using black body color selection. (there is and option to choose color by directly entering the black body temperature) , this is the result I get.

blackbody.png (320.01 kB . 960x540 - viewed 444 times)

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Physics, Astronomy & Cosmology / Re: Can we hitchhike on asteroids to travel through space?

« on: 08/06/2022 16:23:23 »

Here's the thing, to "hitch a ride" with an asteroid, you first have to match velocity with it.  But once you do that, your probe is already traveling on the same path the asteroid is on, and would remain so, whether the asteroid was there or not.
So, unless you needed the physical body of the asteroid for some reason (as a platform to build a large structure?), there is no point. 

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Question of the Week / Re: QotW - 22.03.21 - Does relativity make rocks on the moon older than Earth rocks?

« on: 24/03/2022 14:40:26 »

Hi all,

Thank you Janus for that response, it sits better with me than the previous analogy of a gravity field not following the inverse square law.

If I may borrow part of your explanation to respond, and explain why/how I understand your statement is correct :

Janus
 

First off, the time dilation is not due to a difference in the strength of gravity, but to a difference in gravitational potential.

So firstly if you calculate the strength of g and escape velocity and time dilation at the Earths surface gives

                                    g  = 9.82 m/s^2
             escape velocity  =  11.19 x10^3 m/s
                  time dilation  =  + 6.97 x10^-10 sec/sec

Then to borrow your example:
 "Now double the Earth's radius and quadruple its mass"

Lets call this plant B gives the following equivalent values.

                                g  = 9.82 m/s^2
             escape velocity  =  15.82 x10^3 m/s
                  time dilation  =  + 1.39 x10^-9 sec/sec

As can be seen the identical value of g for Earth and planet B at their surface, but a different value for escape velocity and therefore gravitational time dilation.

 

Because escape velocity is a function of gravitational potential. Specifically, the difference between the gravitational potential at the point you start and a point an infinite distance away.
If we take the planet Uranus for example, its "surface" gravity is 0.889g, less than Earth's. Yet the escape velocity from that same surface is 21.3 km/s which is nearly double that of the Earth's.  Thus, just considering the gravity of each planet alone( factoring out the effect of their different orbits around the Sun), a clock on the surface of Uranus would run slower than one on the Earth, despite the fact that the surface gravity of Uranus is weaker.

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Question of the Week / Re: QotW - 22.03.21 - Does relativity make rocks on the moon older than Earth rocks?

« on: 23/03/2022 15:24:50 »

HI all
Janus

First off, the time dilation is not due to a difference in the strength of gravity, but to a difference in gravitational potential.
To illustrate the difference, imagine you had a uniform gravity field (one that did not change in strength with altitude). If you place two clock at different heights in this field, the higher clock runs faster even though it feels exactly the same gravitational force as the lower clock.

Has this been shown experimentally, if so could you provide information please.

It is demonstrated every time we measure gravitational time dilation and the results match those predicted by Relativity.
So, for instance on a planet like the Earth, we can calculate both the gravitational force and the predicted gravitational time dilation between two points.  Gravitational force changes by the inverse of the square of the distance from the center of the Earth, and gravitational potential changes by the simple inverse of the distance.
So, by measuring time dilation differences between various points, we can determine which of these two matters. And we always get the answer that it is the gravitational potential difference, not the gravitational force difference.

As far as the uniform field goes:  Once we have determined that the accepted gravitational time dilation formula give the correct results, we can do the following thought experiment.
Start with the Earth and two clocks, one at sea level, and the other at 1000 km above it.  Work out both the gravitational force difference between the two and the gravitational time dilation.
Now double the Earth's radius and quadruple its mass.  The sea level clock feels exactly the same gravitational force as it did before.  But, since 1000 km is a smaller percentage of the total new Earth radius, the difference in gravitational force between the two clocks is less than what it was before.  However, the time dilation difference(as per the previously confirmed formula), increases.
keep doubling the radius and quadrupling the mass.  At each step the gravitational force difference between the clocks decreases, and the gravitational time dilation difference increases. And as the gravitational force difference approaches 0, the gravitational time dilation approaches a finite maximum.

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Question of the Week / Re: QotW - 22.03.21 - Does relativity make rocks on the moon older than Earth rocks?

« on: 21/03/2022 15:15:17 »

Ranjit has been lost in this lunar conundrum for a while so he submitted it to The Naked Scientists...

"So, gravity and time relate. Greater the gravity slower time. Time runs faster on moon than on earth. So the question-  When the moonrocks were brought back- they must have looked older than the comparable rocks left behind on earth early on when the moon 'departed' earth? Maybe not by much given the margin of error in measurement. However if that is right (and I hesitate to ask this....) will there come a period when the rocks originated from the earth....BEFORE the earth was formed?"

Do you have any answers that can make things relatively simple? Leave them in the comments below...

First off, the time dilation is not due to a difference in the strength of gravity, but to a difference in gravitational potential.
To illustrate the difference, imagine you had a uniform gravity field (one that did not change in strength with altitude). If you place two clock at different heights in this field, the higher clock runs faster even though it feels exactly the same gravitational force as the lower clock.

Now, to the other point.  Say you have a rock on Earth, before the Moon was formed. it is broken in half and one half is blasted off and becomes part of the newly formed Moon. Billions of years later, that rock is collected and returned to Earth and put next to the other half.
Both rocks share the same "end points" They both agree that they were once part of the same rock and that they were separated at the same moment.  They agree that they were reunited at the same moment. They just disagree on how much time elapsed for them between those moments.
Using the racer analogy from the other post.  The racers start at the same starting line and end at the same finish line, but one took a different route and ended up running a further distance.

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Physiology & Medicine / Re: Can we train our eyes to prevent needing glasses?

« on: 05/03/2022 16:42:09 »

Another factor to consider is that as we age, the lens gets stiffer and/or muscles get weaker, so we have a reduced ability to focus over a wide range. That's why older people often end up with bifocals.

Or, like in my case, since I was already near-sighted, start looking over the top of your glasses when looking at things close up.( since my glasses were shifting the focus point further away, not using them meant it reverted to its "natural" closer point.
I did that for awhile, before finally getting bifocals, and I still do it if I have to shift back and forth between something on my desk and the computer screen. Glasses for desk, over the top of them for computer screen. (it beats constantly tipping my head up and down)

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Physics, Astronomy & Cosmology / Re: How did this rocket stage gain the energy to get to the moon?

« on: 05/03/2022 16:28:57 »

The stage separation adds nothing to the energy of the craft.  It is merely done to shed excess mass(empty fuel tanks and superstructure) that will just get in the way during later maneuvers.
For example, by the time the third Apollo stage separated from the rest of the craft the whole assembly had already achieved the velocity needed to reach the Moon.  The stage is then  jettisoned so that its mass isn't a factor during the Lunar Orbital Insertion burn needed once the craft arrived at the Moon.
The jettisoned third stage still has enough velocity to continue on the the Moon after separation(just on an slightly altered course than the rest of the craft.)

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Physiology & Medicine / Re: Can we train our eyes to prevent needing glasses?

« on: 04/03/2022 16:19:53 »

Nearsightness is most commonly caused by one of two things: Too thick a cornea or an elongated eyeball. The result of either is to alter the "rest" focal point of the eye with respect to the retina. 
The eye focuses on objects by distorting the shape of the lens from its rest state, but there is a limit to the lens' flexibility, and the shift in the rest point, puts focusing on distant objects outside the lens' flexibility range.
This not something you can "train" your eye to overcome.
On top of that, you can have astigmatism.  This is caused by a mismatched curve in either the cornea or lens.  This prevents the eye from having a sharp focus point and, again, is not something you can train your eye to correct.

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Geology, Palaeontology & Archaeology / Re: How does erosion stop?

« on: 02/03/2022 16:19:06 »

As alancalverd stated above, erosion can't be stopped.  It's rate isn't constant either.  It depends on the material it is eroding.
For example, I live not too far from Haystack Rock in OR.  It is a rock monolith on the coast.  Formed from a magma uplift, it was exposed when the surrounding softer material eroded away. 
But it is not immune to erosion itself. Just last summer while visiting the coast, I was fairly near it when I heard a noise, looked towards it, and saw a small section of its side slide off. Erosion in action.

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COVID-19 / Re: What is the science behind a booster shot?

« on: 23/02/2022 16:12:21 »

I guess the best thing to do is have him ask his doctor.  They would be the ones most likely to have the right answer.

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Physics, Astronomy & Cosmology / Re: What's the point of crashing a satellite into an asteroid?

« on: 27/01/2022 16:54:21 »

Is this in regards to the DART mission?  DART is a 'test of concept' mission to see how feasible the "deflection by collision" method is. 
As far as the different variables go, sure they'd effect exactly how precisely you'd deflect the asteroid, But when it comes to deflecting a Earth-impact asteroid, precision isn't really the issue.  It wouldn't matter if it missed the Earth by 1,000 km or 10,000 km, as long as it missed.  So any real attempt to deflect an asteroid would want to err on the generous side, so that these variables aren't an significant factor.

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Question of the Week / Re: QotW - 22.01.24 - Why do stars twinkle?

« on: 25/01/2022 16:26:14 »

The twinkling you see in stars when you look up at them from Earth is caused by atmospheric interference. (the reason the Hubble was able to get such good pictures, even though being smaller than many Earth-bound telescopes was that it was above the atmosphere.
The color of the stars tells us their spectral class, and thus their temperature.  Red stars are the coolest, then orange, yellow, yellow-white, white, and blue-white.

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Physics, Astronomy & Cosmology / Re: How long have black holes existed for?

« on: 19/01/2022 16:38:29 »

Some stars will become black holes, but only if they were massive enough to begin with.  But you don't necessarily need for there to be a star first for a black hole to form.  When the universe was young, it was much more compact, under such conditions, clumps of material could have collapsed into what are called "primordial black holes".  One of these could have formed the "seed" around which the galaxy first formed.  Alternatively, the collection of material that condensed and eventually formed the galaxy could have just become so compact at the center that the material there collapsed into a black hole under its own gravitational attraction.  We don't know for sure which was the case. 

We do know however, that even after the black hole and galaxy have formed, the black hole continues to "feed" on material in the central part of the galaxy for some time.  This is what produces quasars in very distant galaxies(The distance also means we are seeing them when they were young.)  These strong sources of radiation are caused by the "death throes" of material spiraling into the central black hole.( Eventually, the vast majority of material in the vicinity is cleared out and the quasar quiets down.)

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Physics, Astronomy & Cosmology / Re: Is the universe being observed 'in the past'?

« on: 13/01/2022 16:03:19 »

To expand a bit on the previous post.
Yes, when we look out at the universe, we are looking into the past. However, how far "into the past" we are looking depends on how far we are looking away.  We see a galaxy 1billion ly away as it was 1 billion yrs ago, and one 5 billion ly away as it was 5 billion years ago.  So,for example, the fact that quasars show up in very distant galaxies, and not nearby ones, indicates that they are a feature of the early universe.

In terms of the universal expansion, we see that red-shift increases with distance, this, in of itself, indicates that the universe is expanding.  Going further than that, exactly how the red-shift changes with distance indicates how the rate has changed over time.  If you plotted distance over red-shift and it resulted in a straight-line plot, it would mean that the universe expanded at a constant rate over time.  If the plot curves away from the straight-line in one direction, it shows that the universe was expanding faster in the past than it is now, and that the expansion is slowing over time. If it curves away in the other direction, it shows that it expanded slower in the past, and that the rate has increased over time.
In the 1990's a study was done which produced the last result, indicating that the universe has increased its rate of expansion over time (This was contrary to what they expected to find.  They had assumed that the expansion would have slowed over time, and were trying to determine by how much.)

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Technology / Re: Would minirockets have any military value?

« on: 29/12/2021 00:11:11 »

https://en.wikipedia.org/wiki/Gyrojet

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Physics, Astronomy & Cosmology / Re: How does time dilation work?

« on: 02/12/2021 01:54:01 »

True, and a good argument as I think. Then we have that classical muon thought experiment in where the reason it can reach so far into earths atmosphere is due to the complementary of time dilation and LorentzFitzGerald contraction, depending on what frame of reference you use. Earth defining it as a 'slower clock' for the muon while the muon defines it as a length contraction of the distance.

But I've had serious arguments with people wanting it to belong solely to accelerations. And there the 'twin experiment' is a often used argument, referring to the 'turnabout' as the reason for a time dilation.

People tend to confuse "time dilation" with "total difference in elapsed time"
Time dilation is what you measure happening to a clock that is either moving relative to you or at a different gravitational potential.  Total difference in elapsed time is what you are considering after you bring the clocks back together in twin experiment.
For the "stay at home" twin which never undergoes acceleration, the time difference is entirely due to time dilation caused by the relative motion of their twin, the only effect their twin's acceleration has is to cause the time dilation the stay at home twin measures for their twin to change moment by moment while they accelerate.
For the twin that does undergoes acceleration, things are a bit different. During the outbound and inbound legs, he simply measures* time dilation for his twin based on  their relative velocity. But when he is accelerating he would measure an additional effect.  Clocks in the direction of his acceleration run faster, and the further away, the faster they run. Clocks in the opposite direction run slow (this effect is on top of the time dilation due to relative motion.)

So when he accelerates to come to a rest with his twin and then accelerates back up to speed towards him, he will measure his twin's clock as running very fast. So fast that it more than makes up for the outbound and inbound legs where his measures it as running slow.
When he returns to his twin he will end up arriving at the same answer for the total difference in their elapsed times as his twin does, he'll just conclude that it came about  for different reasons.
And that's the crux of it. While both twins agree at the end about the difference in total elapsed time experience by either, they don't have to agree upon what "caused" the end result, and neither twin's interpretation of events is any more of the "truth" than the other's.

* when I use "measure" here, I don't mean what he directly observes, but what he would determine as what is happening to the other clock after he accounts for light propagation delay.

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Physics, Astronomy & Cosmology / Re: The equivalence principle and experiments concerning it?

« on: 26/11/2021 05:01:46 »

Exceeding the speed of light relative to what?

Relative to the frame when the experiment began.

If you read the rest of my post, that's exactly what I addressed.

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Physics, Astronomy & Cosmology / Re: The equivalence principle and experiments concerning it?

« on: 25/11/2021 16:05:22 »

Equivalence principle according to wikipedia.

The equivalence principle was properly introduced by Albert Einstein in 1907, when he observed that the acceleration of bodies towards the center of the Earth at a rate of 1g (g = 9.81 m/s2 being a standard reference of gravitational acceleration at the Earth's surface) is equivalent to the acceleration of an inertially moving body that would be observed on a rocket in free space being accelerated at a rate of 1g. Einstein stated it thus:

we ... assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.

— Einstein, 1907

So if we are inside a rocket and feel the pushing force of 1 g by the floor, we can't distinguish if we are stationary under gravitational acceleration, or accelerating in free space without looking outside. But if we wait long enough experiencing constant 1 g, can't we be sure that we are not linearly accelerating in free space because we would exceed the speed of light?

Exceeding the speed of light relative to what?.  There is no absolute reference to judge your speed against.  So, you must first choose one.   The easiest one to work with in this case is the inertial frame you were at rest with respect to before you began your acceleration.
So let's say that before firing your engines, you drop off a space buoy.  This gives you something to act as a marker to measure your progress.  After accelerating for some time, you find yourself moving at 0.1c relative to your first buoy.  You release a 2nd buoy which now maintains a speed of 0.1c relative to the first buoy.  You continue to accelerate, and after the same time interval, you are moving at 0.1c relative to the 2nd buoy. wat do you measure your speed as being relative to the first buoy?  Newtonian physics would say 0.1c + 0.1c = 0.2c.  But velocities really don't add up that way in our Relativistic universe, but instead like this: (0.1c+0.1c)/(1+0.1c(0.1c)/c^2) = ~0.198 c, just a tad under 0.2c.
You can do this again,dropping a 3rd buoy and accelerating to 0.1c relative to it, and you will now measure your speed relative to the first buoy as being ~0.292 c
You can keep doing this as many times as you want, and each time your speed relative to the first buoy will increase, but by a smaller amount each time, and no matter how long you do this, it will never equal c, let alone exceed it.

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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.

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

« on: 25/09/2021 22:40:35 »

Hi again.

Tidal forces wouldn't effect the answer significantly.

   You may very well be right but earth isn't a rigid body.  It's a dangerously loose collection of techtonic plates.   The possibility of earth breaking apart was an idea presented here:
https://www.wired.com/2014/12/empzeal-earthfall/
   I certainly haven't done anything with the tidal forces,  it was only mentoned because their answer was less than 91 days but they make it clear that earth never reaches the sun. 

Best Wishes.

That's why I used the purely "fluid body" Roche limit to get the 10 min from impact answer.  That is the furthest point at which the Sun could begin to pull the Earth apart.