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Physics, Astronomy & Cosmology / Re: Why is light bent whent it passes a massive body in the space?

« on: 15/02/2021 06:10:56 »

I don't think the mathematical model was "lucky" or "fudged" - it just followed the evidence
- The part where Einstein did find out he had fudged the result was in picking a value for the cosmological constant to fit in with the common view at the time (stretching all the way back to Newton, and probably to Greek philosophers) that the universe as a whole (outside the Earth & Planets) was static and unchanging.

A couple of points.  Einstein tended to approach things a bit differently that other theorists.  While they would start from the experimental evidence and work back to a theory to explain it, he tended to start from basic concepts and assumptions, follow them to their logical conclusion and see if it matched the evidence.  Pretty much the opposite of "fudging".

The other thing to consider with his cosmological constant was that, at the time, he developed GR, it still had not been established that the universe extended past the Milky way.  So not only did he not know of  the expansion, but he was considering a much smaller "universe".

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Physics, Astronomy & Cosmology / Re: The explanation of the darkness of the sky to the space between Earth and Moon

« on: 10/02/2021 04:01:40 »

Explanations of this as due to refractive effects in the Earth's atmosphere, don't seem entirely convincing.

I wasn't aware anyone would have tried. The blood moon effect is due to  scattering.

Isn't "scattering" another word for "refraction"?

No.  Refraction is caused by light passing form crossing through a surface that defines two regions with different light propagation speeds ( like when passes from air to glass and back to air again.
Scattering is caused by the light interacting with individual particles as it passes though a medium.

Here are 4 cubes, two large and two small.
The left ones show refraction and the left ones scattering.

scatter.png (345.97 kB . 940x529 - viewed 286 times)
 
With the refraction cubes, the size doesn't effect the refraction, just the angle the light passes through the faces.
With the scattering cubes, the volume effects the scatter, the larger cube scatters more light because the light has to pass through a larger volume.

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Physics, Astronomy & Cosmology / Re: The explanation of the darkness of the sky to the space between Earth and Moon

« on: 09/02/2021 21:06:59 »

How to apply the red-shifting-of-light explanation of the darkness of the sky to the space between the Earth and the Moon: we see this area dark though the Sun's light travels through it?

Could it be due to "Dark Matter".  Current theory suggests that most of the Universe consists of this "Dark Matter".

If that is true, the Solar System must be full of DM.  Mightn't its presence account for some anomalies in our observations of the Moon?

For example, why the Moon sometimes has a reddish colour during a lunar eclipse, instead of disappearing from view entirely.

Explanations of this as due to refractive effects in the Earth's atmosphere, don't seem entirely convincing.

Suppose instead,  there are varying amounts of DM between the Earth and Moon.  One might expect such variations, and they might cause colour changes in transmitted light between the two bodies.

The total estimated amount of dark matter contained within the Solar system is the equivalent to the mass of a single small asteroid.
Do not be misled by the fact that dark matter is expected to make up most of the matter in the universe as a whole.  It is a matter of relative densities.   As empty as it seems to us, the solar system is extremely more compact and dense that the visible part of galaxy as a whole,( if you took all the mass in the solar system, spreading out evenly over its volume, and then did the same with a volume of 100 light years in radius in a representative part of the galaxy as the solar system is in, taking into account all the stars, etc in that volume, and compared the density of the two, the first volume would be many, many many times more dense than the second).  In addition, the majority of dark matter for our galaxy is in that part of a spherical volume that lay outside of the visible disk of the galaxy.   
So, if you take the total galactic dark matter, spread out over its spherical volume, the amount expected to be found within the volume of the solar system is quite small.

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That CAN'T be true! / Re: Final evidence of a rigged election in 2020?

« on: 07/02/2021 19:46:55 »

Although this group confess to intentionally changing laws like act 77 in Pennsylvania which was changed the year before the election happened,  to allow mail in ballots, to remove requirements for signature verification,  and for ballots to be accepted days after the election had happened.

Act 77 passed with heavy Republican support( even more so than Democratic support), as a compromise deal with the Democrats.  In exchange for exapnded vote by mail, they got rid of straight ticket voting (Where you could go into a booth and click one lever that voted for all candidates of a given party.)   The Republicans felt that this would help pick up some seats in the state legislature (which it ended up doing).
Off course, the instant this compromise hurt them in the 2020 presidential election, and their compromise no longer benefited them,  they immediately had buyer's remorse.

Pennsylvania officials tried to have these law changes over turned as they were unconstitutional,  yet were stuck down on technicalities not on merits.

Those  "technicalities" were that they had waited until over a year and 2 elections had passed. ( Oh, and by the way, one of those elections was were they picked up a couple of seats in the state legislature.  You didn't hear them complaining that those results should be overturned).
If they had filed their complaint before the election was run, things might have been different. But they didn't.  This just tells me it wasn't the law they were really objecting to, but just an election result they didn't like.

The voters that voted by mail did so while the law was in effect and did so with the assurance that they were voting legally, and you can't just go back and disenfranchise those voters just because it was later decided that the law shouldn't have been passed.

Let's put it this way:  Supposed a town passes a law that allows drivers in their town to make a right turn at a red light without first coming to a full stop.  The law stays in effect for a year.  But then someone points out that it is conflict with state laws.
Can you now go back, using traffic camera footage, and issue tickets to everyone that made a right turn without coming to a stop during that year?

As far a mail-in voting goes: My state has had mail-in voting exclusively for decades, and in that time, the cases of voter fraud has been negligible. 
Republicans are not against vote by mail due to concerns over fraud, but over concerns that it will increase voter turn out, and historically, Republicans don't fare as well when voter turnout is high.

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Physics, Astronomy & Cosmology / Re: Sun-synchronous orbit spacecrafts

« on: 05/02/2021 20:52:10 »

He seems to be referring to a Solar-synchronous orbit.   This is a polar orbit which precesses at a rate equal to a Solar day.

Ah, I was picturing something that hovered over one point on the sun, for some unfathomable reason. This is something orbiting Earth, not the sun.

How do they get a polar orbit to precess all the way around once per year? Expend energy?
Seems like it would be at geosync altitude on average, but different orbital axis.
It need not be a circular orbit at all, and would benefit from being furthest from Earth in the day to give more time to measure stuff. Such an eccentric orbit also keeps it out of the high-contention geosync path.

They fine tune the inclination of the orbit according to its altitude ( in other words, it is not a perfectly polar orbit).  That way, the Earth's own equatorial bulge produces the desired precession.  Solar synchronous orbits are not put at geo-sync altitudes.  They are much lower.  They can orbit the Earth multiple times a day. (for weather and surveillance satellites this means you can image multiple points on the Earth's surface)  You just want consistent lighting for any given region on every pass.

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That CAN'T be true! / Re: How can a question be fake news?

« on: 21/01/2021 17:01:53 »

The answer is "possibly, but no evidence to date".

Next question, please.

That doesn't answer the question. That reply relates to the question could the Covid vaccine damage fertility?
And it doesn't answer it, it leaves it as UNKNOWN.

The question was how can a question be fake news?

When it is just a ploy to push an agenda.   

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Question of the Week / Re: QotW 21.01.19 - Could Earth get so big that rockets couldn't go to space?

« on: 21/01/2021 16:51:48 »

As mentioned by evan_au, ISP is an important factor for rockets. Another is the thrust to weight ratio, especially when dealing with launch vehicles.
ION engines have very high ISPs, but because they have thrust to weight ratios of less than 1, are useless as launch vehicles.

The RS-68A, which is an engine used for the Delta rocket, has an ISP of 360.5 sec (3.5 km/s exhaust V)*, and a thrust to weight ratio of 47.4   

Thus, if we consider an "ideal" rocket which just consists of rocket engine and fuel (zero mass allotment for fuel tanks, superstructure, etc.) then if the fuel massed 46.4 times the rocket engine, the rocket would just be able to hover above the ground.
With the given ISP, it requires a mass ratio of ~24.4 for our ideal rocket to reach escape velocity. In other words, you need 23.4 times the engine mass in fuel.  This is well below the 47.4 thrust to mass ratio.
So what kind of increase in the Earth's mass would it take in order for the mass ratio to approach or exceed the thrust to mass ratio?
First off, we'll assume this mass increase occurs without increasing the Earth's size*
Increasing the Earth's mass has two effects:
It increases the value of g at the surface, thus decreasing the effective thrust to mass ratio.
it increases the escape velocity, which increases the required mass ratio.
With a 25% increase in Earth mass, the mass ratio increases to 35.6 and the effective thrust to mass ratio decreases to 27.92.
While the thrust to weight ratio still is larger, it is just barely so. A rocket under these conditions trying to launch vertically would still be using most of its thrust just to hold up it own weight and likely wouldn't get very far before exhausting its fuel and falling back.
So, we could safely say that a 25% increase in Earth mass would effectively prevent a Delta rocket from achieving escape velocity**
Real life practical issues would drive this maximum mass increase down.



*At sea level conditions
** This assumes a single stage rocket, and the Delta is a multistage launch system.  However, our single stage "ideal" rocket would out perform a practical multistage rocket.

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Physics, Astronomy & Cosmology / Re: What was the outcome of the New Horizons probe's journey to Pluto?

« on: 20/01/2021 18:15:48 »

Exactly 15 years ago an uncrewed NASA spacecraft by the Name of New Horizons blasted off on a 3 billion-mile journey to Pluto from Cape Canaveral Air Force Station at a speed 36,400 mph (16.26 km/s) Of that time it was the fastest human made object.

It did not break speed records.  Before then, both Helios, Pioneers 10 & 11, Galileo, and one manhole cover achieved higher speeds.

The 16.26 km/sec was measured relative to the Earth. It was a record in terms of velocity achieved relative to the Earth via the launch vehicle alone.

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General Science / Re: The journey to the red planet: does humanity need Mars colonisation?

« on: 30/12/2020 20:03:40 »

Yes, I know that, but they most probably need to cary large amounts of oxygen and hydrogen to create water using a special machine. This has many logistical problems but will multiply in complexity as they are travelling the longest distance yet.

The water would be "mined" from Mars surface. They might need some digging/drilling and purification.

Yeah, definitely this water is not acceptable for drinking. If we have there a high level of radiation, the water is 100% radioactive, and I don`t think that such a purification system exists to clean water from radiation.

While radiation levels on Mars are higher than those on the Earth ( an average of 8 rad/yr vs 0.62 rad/yr) and would be unhealthy over long term exposure, we are not talking about sitting next to an unshielded nuclear pile levels.
Besides, that isn't how radiation works.  Just being exposed to radiation doesn't make something "radioactive". 
For water on Mars to be unsafe due to radiation, it would have to be contaminated with a radioactive element*. (which like any other contaminant be filtered out.)

*or having a higher than normal H3 vs H1 content.  But since H3 only makes up a trace amount of  all Hydrogen, and any process energetic enough to convert H1 in a water molecule to H3 would rip apart the molecule, this is not likely.

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That CAN'T be true! / Re: I don’t understand physics: does anyone understand physics these days?

« on: 30/12/2020 01:35:12 »

Appealing to statistics is the last refuge of incompetent Physics

This is beginning to more and more like a simple "sour grapes" attitude.  Modern physics has grown beyond your understanding, so you you respond by claiming that modern physics isn't worth understanding.

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That CAN'T be true! / Re: I don’t understand physics: does anyone understand physics these days?

« on: 29/12/2020 21:16:12 »

Much the same as rolling a pair of dice.  You cannot predict what number the dice will add up to, at the instant they come to rest...
How can there be any "laws" governing "randomness"?

There are laws of statistics.

If you roll a pair of dice, the total score can be anywhere from 2 (snake eyes) up to 12 (double 6).
- If you roll the die once, you could get any of these
- But if you roll it many times (eg 100 times, or more) you are almost certain to see a 7 more than a 2 or 12
- This is because out of the 36 possible combinations of two dice:
- Only 1 combination gives a 2 = 1+1
- Only 1 combination gives a 12 = 6+6
- But 6 combinations gives a 7 = 6+1=5+2=4+3=3+4=2+5=1+6
- So a 7 is six times more likely than 2 or 12
- So you can predict the outcome of rolling a pair of dice, if you are predicting the results over many experiments.

Just like you can predict the bright and dark bands in the 2-slit experiment, if you run the test over very many photons.

Or another example that deals with atoms:  There is no way to tell when any given atom of Radon 222 will decay. It could be in the next sec or a thousand years from now.  However if you have a  significant number of Radon 222 atoms ( say even a gram's worth),  you can be certain that after 3.82 days, half of them will have decayed.

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Plant Sciences, Zoology & Evolution / Re: Why do mosquitoes avoid me?

« on: 02/12/2020 23:16:21 »

Mosquitoes are attracted by smells, so it could be that they are just not all that interested in your smell.

Speaking for myself, while I don't notice that they ignore me, I have no reaction to them after being bitten. No welt, no itching.
This wasn't always the case.  Growing up in Northern MN, I got my share of bites and resultant itching.  I think all those bites in my youth caused me to build up a tolerance to them.

Don't mosquitoes "want" to bite as many people as they can?  Wouldn't this goal would be better achieved, if mosquitoes evolved to make their bites, not irritating and unpleasant to people, but nice, and enjoyable.

Suppose the mosquitoes did that.  Made their delicate skin- bites pleasant to humans.  Then wouldn't humans welcome mosquitoes as a source of pleasure.  Instead of swatting them, trying to ward them off by putting up mosquito nets, and spraying cans of insecticide at them.

 I mean, looking at it from a mosquito's point of view, surely you don't want to annoy your hosts, ie humans.

Rather, shouldn't you ingratiate yourself with your human hosts.  And so win their co-operation in multiplying your mosquito genes?

The reason the bites welt and itch is that the mosquitoes have to inject a serum to thin the blood enough for them to draw it.  Your body detects this as a foreign substance and sends it defenses to action.    The reason the reaction (as it did in my case), lessens over time is that your body, begins to learn to ignore this foreign substance, as it is not  any real threat to the body.

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Chemistry / Re: Can reactions that use oxygen also use Nitrogen?

« on: 07/11/2020 00:02:02 »

It might be a good idea to explain just why Nitrogen is as non-reactive as it is.  Like oxygen, gaseous Nitrogen forms diatomic molecules, where two nitrogen atoms bond together.   Each Nitrogen atom has 5 electrons in its outer shell, so in order for each atom to feel like it has a full 8 electrons in its outer shell, each atom share 3 electrons with the other. ( 6 total are shared between the two atoms)
Oxygen does the same type of sharing, but since each atom has 6 electrons in its outer shell the atoms only share 2 electrons each ( four total).
Each pair of electrons makes up a bond. 
So the Oxygen molecule has two bonds and the Nitrogen molecule 3 bonds.
In order for either to participate in a reaction with another element, these bonds have to be broken first.
Three bonds are stronger than two, so it is harder to pull the nitrogen atoms apart in order to take part in a reaction.
This makes the Nitrogen molecule more stable and be less reactive.

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Physics, Astronomy & Cosmology / Re: Can you see anything if you travel faster than the speed of light?

« on: 21/09/2020 16:02:33 »

This question falls under the category of  " What would physics predict if you ignored the laws of physics"

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Physics, Astronomy & Cosmology / Re: Are light and sound similar?

« on: 12/05/2020 17:09:33 »

Yeah BC, that's the one I was thinking of, no space suit.
=

But it is molecules particles 'vibrating', right? Gaining energy by me shouting, exiting them?

Imagine you had a lung full of air, You scream. The air is forced past your vocal chords which vibrate, modulating the air leaving your mouth and producing alternating waves of air compressions.  These compressions are further propagated by the air outside your body.
Now, what happens if there were no air outside of your body. 
The air inside your lungs is made up of molecules which are themselves traveling at something like 340 m/sec.  The air molecules are close enough together that they can't travel very far before bumping into another molecule (the mean free path).  This distance is very small compared to the wavelength of a sound wave. which is why air at standard pressure can carry sound waves for long distances.
So when the air which has been modulated by your vocal chords leaves your mouth, there are no air molecules out there to "contain" them.  Instead of bumping into other molecules and creating sound waves, they scatter and spread out a 340 m/sec.
So the "energy"  from the sound is rapidly being spread out thinner and thinner over this expanding volume.
Not only that, but the mean free path of the molecules increase;  they individually travel further and further before encountering another molecule. Thus the mean free path becomes long compared to the wavelength of the "sound", eventually surpassing it. 
The upshot is that the "sound energy" becomes more and more just a matter of the average kinetic energy of the molecules which is randomly distributed and no longer takes the form of discernible sound waves.

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Physics, Astronomy & Cosmology / Re: What do you know about supernovae?

« on: 11/04/2020 00:13:14 »

I'm not sure how to go about doing the math, but it should be pointed out that the majority of stars are not massive enough to create supernovae. About 75% of the stars in the Milky Way are red dwarfs, for example: https://www.bbc.com/news/science-environment-21350899

While this the state of the galaxy today, this not what we expect the early universe to be like. The main sequence stars you refer to are Population I stars, typically metal rich.    There are also Population II stars,  These are metal poor stars, they are also the oldest stars and were formed during the Early age of the universe.  Their lack of metal could be due to their being more likely the progeny of Type II supernovae, while Type IA supernovae became more prevalent later.

Population III stars (Or generation I stars) are the hypothetical earliest stars, they would have been massive, almost entirely Hydrogen and Helium, and due to their masses, short-lived, ending their lives as supernovae, which seeded the galaxy with elements used in the formation of later generations of stars

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Physics, Astronomy & Cosmology / Re: Can you help me understand "speed"?

« on: 04/04/2020 16:54:07 »

Both Alice and Bob agree that the reflected Laser hits Alice first.

Is there any reference frame which sees that reflected light hits Bob first due to relativity of simultaneity?

No.  Relativity of simultaneity cannot reverse the causality of events.  In this case, Alice is always between the mirror and Bob. and the the reflected light always leaves Bob, hits the mirror and returns to Bob, in that order. Ergo, the light must pass alice first on its way back to Bob.
Everyone in all  frames agree that the light leaves Bob as Alice passes,  both Bob's and Alice's clocks read 0, that Alice's clock reads 1 hr when the reflected light hits it and Bob's clock reads 2 hrs when the light reaches it.
All frames will also agree that Alice's clock reads 1 hr before Bob's clock reads 2 hr.

Below shows the Space-time diagrams for these events in as measured from different frame of reference.

Image11.gif (23.41 kB . 1536x1106 - viewed 3899 times)
Top left is "Bob(blue line) and mirror(red line) at rest", with Alice (green line) moving at 0.6c  The yellow lines are the light moving at c.
In these diagrams simultaneous events are directly horizontal to each other.
as we see, the returning light hits Alice before Bob.
Next over is "Alice at rest", with Bob and The mirror moving at 0.6 c .
Again, the reflected Laser hits Alice first on its way to Bob.

Next is a frame of reference has a relative velocity of 0.6c with respect to Bob and the mirror, but in the opposite direction from Alice.  Note while the vertical separation (representing time difference) is less for the period between the light hitting Alice and hitting Bob, it still hits Alice first.
Lastly, we have a frame with a relative velocity of ~0.882 c relative to Bob, in the same direction as the last frame.
The time gap between The light hitting Alice and Bob has shrunk even more, but it still hits Alice first.
we can keep increasing the velocity of this frame relative to Bob closer and closer and closer to c, and while the time gap between the light hitting Alice and hitting Bob will continue to decrease, it will reverse the order of these events (or even shrink the time gap to zero).

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Physics, Astronomy & Cosmology / Re: Can you help me understand "speed"?

« on: 03/04/2020 17:38:41 »

A mirror is placed 1 light hour away from a powerful laser pointer. Right when the laser is turned on, Alice move from laser pointer towards the mirror at 1 m/s. Bob stays where the laser pointer is.
Who will see the reflected laser first?
What's the speed of light before and after reflection when measured by Alice? Is it still the same as distance divided by travel time?

To make this simple, we will assume Alice is already moving towards the mirror at 1 m/s relative to the pointer as she passes it and it is turned on.
Because she is moving at 1m/sec relative to both pointer and mirror, she would not measure the distance between them as being 1 light hr, but rather 0.99999999999999999443674971973191 light hr.  as measured relative to herself, the laser will travel at c, towards the the mirror, while the mirror moves towards her at 1m/sec.  It will take 0.99999999666435905358172976595532 hrs for the laser and mirror to meet.  During which time the distance between Alice and the mirror would decrease by 3599.9999879916925928942271574392 meters.  The laser will then return at c from this distance to Alice.  At this point, I'm going to quit giving numbers, because it is just too cumbersome to keep working with all these decimal places.
Instead, I'm going to change the scenario to use a velocity for Alice that is easier to work with.
Thus Alice now moves at 0.6c. relative to laser pointer and mirror.
Now:
As Alice passes the pointer, she will measure the distance to the mirror as being 0.8 light hrs, and the mirror will be approaching at 0.6c, while the light from the laser will be speeding towards it at c relative to Alice.  Thus, according to Alice, the light will take (0.8 lh/(1c+.6c) = 0.5 light hr to meet up with the mirror.  During which time, the distance between Alice and the Mirror will have decrease by 0.5 hr * 0.6c = 0.3 lh to 0.5 light hr.   The refelcted light, traveling at c will take 0.5 hr to get back to Alice.  Total time for Alice for round trip of laser  1 light hr.
Now, if we work it out from Bob's view:
The light travels at c towards the Mirror, arrives at the mirror in one hour and returns 1 hr later, total trip time 2 hr.
Alice, is moving towards the mirror at 0.6 c, so, in the time it take for the light to reach the Mirror, Alice will have moved to be 0.6 light hrs closer to the mirror to be 0.4 lh away from it, and will meet with the returning light in another (0.4 lh/(1c+0.c) = 0.25 hr,  total time between firing of laser and it meeting up again with Alice is 1.25 hrs.  However, since Alice is moving at 0.6 c relative to Bob, He will measure her clock as time dilated by a factor of 0.8 and only tick off 1.25hr * 0.8 = 1 hr.  The same amount of time that Alice says here clock ticked off.
If we go back to Alice's view, we can also work out how much time she would say ticks off on Bob's clock between Firing the laser and the light returning to Bob.
As noted above, Alice measures 1 hr for the light to meet up with her. During which time, she would measure Bob's clock as being time dilated and ticking off 0.8 hr.  Now in that hr, Bob, with a relative velocity of 0.6c, has moved 0.6 lh away from Alice.  The light passing Alice on it way to him has to "chase after" the receding Bob at c.  This takes 0.6 lh (1-0.6c) = 1.5 hours by Alice's clock. During which time Bob's clock accumulates 1.5 hr *.8 = 1.2 hr.  Added to the 0.8 hr already accumulated equals 2 hrs total time accumulated by Bob's clock according to Alice.  The same as what Bob's recorded.
Both Alice and Bob agree that the reflected Laser hits Alice first. ( though Alice would say that when the Light hits her, Bob's clock read 0.8 hr, and Bob would say that when the light hit Alice his clock read 1.25 hrs, And Alice would say that when the light returns to Bob, her clock reads 2.5 hrs, while Bob would say that her clock reads 1.6 hrs.
 
So the answer to what the speed of the light is according to Alice both before and after refection, it is c ( relative to Alice.) just like it is c relative to Bob as measured by Bob's.   It is also distance divided by travel time, keeping in mind that Bob and Alice do not measure either distance or time the same.

19

General Science / Re: Manoeuvrability in space travel?

« on: 02/04/2020 18:12:11 »

It propels against the mass of it's own fuel! Brilliant!

It is simply the law of action and reaction.  In order to eject the fuel at high speed out of the nozzle, the rocket has to exert a force on it in that direction.  This, in turn, also results in an equal force pushing the rocket in the other direction.
There are a couple of misconceptions going around about rockets.  One is that they need something Air or something outside of the rocket to "Push against" in order work, which is not true.  If you were floating in the vacuum of space, holding a brick, and threw the brick away from yourself, you would move start moving in the opposite direction to the brick.  It's a matter of conservation of momentum.  Both you and the brick together start with zero momentum total.  You toss the brick to the right, giving it a non-zero momentum to the right.  In order for the combination of you and brick to maintain a total momentum of 0, you need to move at some speed to the left so your momentum to the left cancels out the brick's momentum to the right.
In reality, it is a disadvantage to have an atmosphere outside of the rocket. The outside air pushing in on the rocket nozzle decreases the speed at which the gasses can escape, and reduce the effective thrust of the rocket.

The other misconception is that a rocket's top speed is limited to the to its exhaust speed.   As long as a rocket is firing and expelling exhaust, it will continue to accelerate.  The only thing that limits its final velocity ( other than the Relativistic limit of the speed of light), is how much fuel it carries.

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Physics, Astronomy & Cosmology / Re: Which twin is older when they meet again?

« on: 31/03/2020 17:17:53 »

One thing about this so-called paradox that is worth a little thought touches on the very basics of how we think about time travel. 

Dan and Emily are twins, and Dan makes the obligatory journey, after which he will be 20  yrs younger than his sister.  Fast forward to the point of return in (say) the year 2150.  Dan has aged 20 yrs less than Emily; he is at the stage she would have been at 20 yrs earlier.  It is easy to think of this as his having travelled 20 yrs into his sister’s past, but is that the wrong way of looking at it?  Both are at the same point in time – 2150.  In what sense has Dan travelled into the past? 

In no sense has he traveled into Emily's past.  He just experienced 20 yrs less time than she did between their separation and rejoining.  Relativity never claims that Dan travels into the past, only that the time interval he measured between the two events was shorter.  Time passed differently for the two.  If Dan left Emily when both their calendars read 2100, then when they met up again, Emily's calendar would read 2150, while Dan's would read 2130.
When They meet up again, it is 50 yrs into Emily's "future" and 30 yrs into Dan's future.
When you state that they are are the same point in time, 2150.  you are implying some type of absolute universal nature to time, and that 2150 is the "real" time.  It is only 2150 by the Earth clocks and this no more the "real time" than the  2130 according Dan's clocks.  It is just that when they meet up again, Dan agrees that by the Earth clock it is 2150 (twenty more years passed for Earth than did for him.)
There is the time as measured by Emily, and there is the time as measured by Dan. and there is no more meaning to time than that in this scenario. 

Consider another scenario.  This time there is no journey into space, but in 2150 Dan uses a time machine to travel back 20 yrs.  He meets Emily.  She is as she was in 2130.  He has travelled 20 yrs into her past and she is younger, not older, as is the case in the former scenario. 

Again, the SR twin scenario does not claim any travel "into the past".

Dan comes back from space younger than Emily.  Pop Sci books and on-line discussions provide the "hitch-hiker" with abundant explanations for that.  However, if, instead of the high-speed journey, Dan had been placed in some sort of “stasis chamber” in which his development and ageing had been halted for 20 yrs, the result in 2150 would be the same as in the first scenario.  He would appear to be 20 yrs younger than Emily.  So, would we claim that he had time-travelled?   

Once again.  SR does not imply "time travel" other than the time travel we all experience everyday from this moment to the next.  What SR does say is the there is no universal meaning to "the passage of time".  Every inertial reference frame measures time by its own independent standard, and that is the only meaning to "time" there is.
It's like the notions of left and right.  Everyone's "left" and "right" is unique to them.  If we are standing next to each and not facing the same direction, my "left" will not be the same as your "left'.   And there is no "universal" concept of "leftness".

Putting Dan in a stasis chamber, while he stays at rest with respect to Emily might, for Dan, "seem" the same personally, but only If Dan were not allowed to measure what was happening outside his chamber.  It might "mimic" the end result of Relativistic effects, but it wouldn't be the same.