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You are correct. Given an ideal laboratory scale, an object will register an increase in mass as its temperature is raised. Note well: An ideal scale is needed. This increase in mass is immeasurably small in any terrestrial application. However, you need to keep the target audience in mind. One does not need to be a graduate student in physics to understand special relativity; some aspects of special relativity are now taught at the high school level. The original poster is someone who is confused by the mass m in E=mc2, so it is best to assume a high school or a freshman level understanding of physics. Relativistic mass is an unneeded complication at this level of understanding, as is the general relativistic concept of invariant mass.

Pretty much everything. It would be rather nice to be able to throw an object into the past. I would use that ability to throw a message to the 2004 version of myself: "Buy Google stock as soon as it goes IPO. Oh, and buy some Apple stock too. Buy as much as you possibly can." Because you are still seeing the object after you threw it. You are not throwing into the past.

This post is a reply to three posts, the original post, post #2, and post #7. False, and it doesn't answer your homework question. You are talking about the calculated response of the Earth to two recent large earthquakes, the 2004 Indian Ocean earthquake and the 2010 Chile earthquake. The small external torques on the Earth can be ignored over the short time span of an earthquake. Those earthquakes changed the Earth's inertial tensor by a slight bit, and thus the Earth's angular velocity had to change by a corresponding tiny bit to conserve angular momentum. (Note: It is angular momentum, not angular velocity, that is conserved in the absence of external torques.) These effects are tiny, tiny, tiny. They were not observed because they were below our ability to measure them. Even if they were observed, this has nothing to do with your homework question. This was a one-time shift in orientation, not a precession or a nutation. Your homework question is more than a couple months old, so I have no qualms answering it. The principle behavior of the Earth's rotation is that of torque induced precession. It's called the lunisolar precession. Because the Earth is not a perfect sphere, the inverse square gravity of the Moon and the Sun create external torques on the Earth. Because the Earth is rotating, these external torques manifest as a precession. There are many, many terms (2,000+ terms!) in the modern description of the Earth's orientation and rotation. The largest of these is the lunisolar precession. This dominant term is huge compared to all of the other terms. So the first answer, and second, and 2,000+ answer is that it the Earth undergoes a torque induced precession. That article talks about polar wander. Polar wander is the difference between the Earth's orientation as calculated by the SOFA (Standards of Fundamental Astronomy) model and the observed orientation. The article misses the mark of answering the OP's question. It doesn't talk about lunisolar precession, or the smaller nutations that the SOFA model describes quite well. The SOFA rotation model is semi-analytic. There are some small effects that the model currently does not capture. They instead have to be captured after the fact, based on observation. The largest of these unmodeled effects is the Chandler wobble, which is a torque-free precession. It can't be modeled analytically (yet) because the Earth is not a rigid body. The Chandler wobble exhibits some weird behaviors because the Earth is instead an elastoplastic body. This entire post was one line of nonsense followed by another. The lunisolar precession is not illusionary. It is quite real, and it is very well explained by physics. Newton was the first to give an explanation for why this well-known precession occurs.

Longitudinal and transverse mass are old, defunct concepts. There is no reason to use them whatsoever.

Perhaps because it is utter nonsense? Remember crop circles? It was a prank. The goofy alien crowd continued to hang on to their delusion that crop circles were done by aliens -- even after videos showed exactly how it was done. There's lots of money to be made off a gullible public, and some people have absolutely no qualms about doing so. People dress up in fake sasquatch costumes, pander snake oil concoctions, and make false claims regard alien visitors just to make a few legal but rather unethical bucks. There is no credible evidence of ancient aliens.

When you write [math]E=mc^2[/math] the mass to which you are referring is called relativistic mass. Relativistic mass is a concept that most physicists don't think is necessary. All that dragging in the concept of relativistic mass does is confuse the student. What [math]E=mc^2[/math] is saying is that relativistic mass is a synonym for energy -- so why not just use energy? Better is to use rest mass, or intrinsic mass -- the mass an object appears to have to an observer at rest with respect to the object. In terms of intrinsic mass, the mass-energy equivalence formula becomes [math]E^2 = (m_0c^2)^2 + (pc)^2[/math] where [math]p=m_0v/\sqrt{1-(v/c)^2}[/math] is relativistic momentum. Now this is saying something: Energy is a function of rest mass and relativistic momentum. Another way to write this latter expression for energy is [math]E^2 = \left(m_0c^2\right)^2\left(1 + \frac{(v/c)^2}{1-(v/c)^2}\right) = \left(m_0c^2\right)^2 \frac{1}{1-(v/c)^2}[/math] Now look what happens to the rightmost term as v approaches c. It gets ever larger and larger because the denominator [math]1-(v/c)^2\to0\ \text{as}\ v\to c[/math]. The relativistic momentum grows without bounds, and thus so does the energy. Another way to look at it is that it takes an unbounded amount of energy to make something with non-zero mass approach the speed of light. Edit Fixed the numerator and simplified.

Utter nonsense, and no, I am not the one who gave you the negative rep. Is general relativity absolutely correct? Of course not. It doesn't jibe with quantum mechanics, and it admits singularities. Are Einstein's days numbered? Of course not. For an analogy, look to Newtonian mechanics. Relativity and quantum mechanics collectively demonstrated that Newtonian mechanics was in a way fundamentally flawed. However, they also showed that Newtonian mechanics was fundamentally correct (but in a limited domain). They had to! There was a huge accumulation of confirming evidence during the 200 years between Newton's publication of his Principia and those late 19th century experiments that showed problems with Newtonian mechanics. Newtonian mechanics is still taught because it is still approximately valid in the limited domain of our everyday world. Any theory that overturns relativity must similarly show that relativity is fundamentally correct in the domain where relativity has been well tested. Good luck with that. Or should I say "bad luck with that?" Unfortunately, the public gobbles up non-science. Barbour is first and foremost a relativistic physicist. If he Barbour is right he will not show that Einstein was wrong. Barbour's principal focus is Mach's principle, a concept that strongly motivated Einstein. "Mass tells spacetime how to curve, curved spacetime tells mass how to move" is a concept straight out of Mach's principle. There are some aspects of general relativity that are at odds with Mach's principle. Barbour is trying to find a slight modification to general relativity that makes it fully compliant with Mach's principle. He is not seeking a complete negation of general relativity. Your website (I found it, but I will not provide a link; I don't link to such nonsense websites), on the other hand, appears to reject every aspect of general relativity. Your nonsense includes ... "Where are the flying cars?" This has nothing to do with relativity. It has everything to do with the trash widely published in science popularization magazines in the 1950s. Just because we don't have flying cards, furniture that one cleans with a hose, cheap plastic dishes that dissolve in water doesn't mean that science has abandoned us. It just means that those 1950s magazines were peddling crap because crap science sells. "Why is Einstein considered the genius of our time yet no one knows anything about him or his theory?" This website has multiple members who know quite a bit about relativity. So, bzzt, wrong. "Why don't we understand anything theoretical and astrophysicists say yet we give them billions of dollars to build experiments that don't better mankind?" This is a non sequitur, and it is wrong. Some people do understand what those theoreticians say, and at a very deep level. Others understand it to some extent at a lay level. Just because you don't understand it or like it does not mean it is wrong. As for not benefiting mankind, says who? You? Theoretical physics has benefited mankind quite a bit. Astrophysics, too. You are ignoring that it can take quite a while for science to move from theory to application. More importantly, you are ignoring the huge benefit just in the increase in knowledge. Knowledge is the ultimate wealth in this new millennium.

There are some problems here. - You used time to define time. This is a huge problem in and of itself. - You are conflating a process in which time is the independent variable with time. The process is not time. - This is a very Newtonian view of time. Time is anything but simple in relativity and in quantum mechanics. Correct. Motion is not time. Motion is a process in which time is the independent variable. Big difference. I would say "or not." That a particle can have the same position in space at two different times begs the question, "what is time", and also ignores that what you are talking about is observer dependent. There are several mysteries of time. - Why does time have an arrow? This is partially, but not completely, answered by thermodynamics. - Are there quantum violations of CPT symmetry? There are some signs that time has an arrow even at the quantum level. - Is time fundamental or is it an emergent property? This is a big open question. - What exactly is "time"? This is an even bigger open question.

Yeah, the name for it is an oblate spheroid. I already mentioned this in post #13. By a small amount. The tidal flex is somewhere between 1 meter and 30 meters. The smaller value results if Europa is completely solid (solid ice over a solid core); the larger if Europa has a significant liquid ocean under its ice surface. Note that the Earth, too, changes shape due to the gravity of the Moon and the Sun. There are tides in both the oceans and in the Earth itself. However, Europa is still quite "round", as is the Earth. The Earth's equatorial bulge, the Earth tides, and the tidal flexing of Europa are tiny compared to the overall size of the Earth and Europa. Saying that these features mean that the Earth (or Europa) isn't "round" is, in the words of Isaac Asimov, "wronger than wrong."

Gravitons, if they exist, are massless. You cannot use Newtonian gravity in general relativity, let alone some quantum gravity model. Gravitons, if they exist, will always be moving at the speed of light. Your logic is flawed.

What inverse square law? The only thing that makes sense here is that gravitons, if they exist, would indeed have the minimum possible mass. In other words, the would be massless. Two gravitons would not accelerate toward one another. How could they? Being massless, they could only travel at the speed of light. Nobody yet has a working theory of quantum gravity. Getting a consistent theory that yields both general relativity and quantum mechanics is a rather difficult task. It certainly won't be achieved by looking at things from a Newtonian perspective, which is how you appear to be looking at it.

That is exactly what TAI is. Well, almost. TAI is what a bunch of clocks measure, about 300 or so atomic clocks around the world. That is not what atomic clocks measure. If it was, there would be a fundamental problem in the definition of a meter. That the speed of light is constant means that it can be used as a basis for defining either time or distance, but not both. The second is defined in terms of the frequency of the electromagnetic radiation that results from a specific transition. The frequency distribution of the radiation that results from this transition has an incredibly high Q (incredibly narrow bell curve). This high Q means that this frequency can be used as to drive a clock with incredibly accuracy.

Hubble expansion, observed. Cosmic microwave background radiation, observed. Stellar fusion, not only observed, but also reproduced. Nucleosynthesis, not only observed, but also reproduced. Did you read the link provided in posts #75 and #77? Read it. Now you are starting to play a god of the gaps game. That is a very dangerous game to play because scientists have this uncanny ability to fill in those gaps. You do realize that the big bang theory was originally proposed by Georges Lemaître, a Belgian priest? He had no problems with the idea, nor did the Catholic Church. The only religions that have a problem with it are a bit goofy.

The problem is that - You put words in my mouth. Some of what that which is attributed to me in post #22 are not my words. - You make it nigh impossible to respond to what you wrote. Do not embed your responses inside the quote tags! Update You were right with the double negative. The Harrison chronometer, grandfather clock, telephone, Computer GPS -- none of them are adjusted as you "described above." (And other than an allusion to "simple Astrodynamics 101", you didn't describe anything.) So, Modern Timekeeping 101. The standard reference for all other times is International Atomic Time (TAI, for Temps Atomique International). This is an average of the time as measured by several atomic clocks around the world. The individual clocks are adjusted for their calculated tick rate and then averaged. The adjustment for the local tick rate is a relativistic correction, but it is largely an altitude correction. There are also tidal effects in this correction, mostly from the Earth tides (which cause slight changes in elevation). There are also tidal effects in that a clock ticks a tiny, tiny bit faster at midnight than it does at noon. Lunar effects are twice as big as are those from the Sun. Note well: There is no correction in TAI for the distance between the Earth and the Sun. The corrections are made only to get all of the clocks marching to the same beat. A whole bunch of time standards are offset from TAI by a fixed amount. These include Loran time, Terrestrial Time (TT), and GPS time. Our civilian time, UTC, is also a fixed offset from TAI, but that fixed offset is incremented or decremented by one second on occasion. This offset plus the occasional leap second keeps UTC to within one second of UT1, which is essentially mean solar time. Once again, there is no correction for the distance between the Earth and the Sun for these time scales. It would in fact be erroneous to make such a correction. It is only when one looks outside the Earth-Moon system and does so from a fully relativistic perspective that one needs a time standard that does account for the varying distance between the Earth and the Sun. JPL, for example, uses a home-brewed time standard, Teph. JPL's Teph on average ticks at the same rate as do our Earthbound clocks, but the exact rate varies ever so slightly as the Earth goes from perihelion to aphelion and back again. The difference between JPL's Teph and TT is never more than a few microseconds.

You live in the Netherlands, so you probably don't know. That you picked summertime rather than wintertime is a big clue that (a) you don't know why people do need humidifiers, and (b) you don't need one yourself. Here's a simple test: Shuffle your feet as you walk across a carpeted room toward the door. Reach for the doorknob. Describe the spark that results. It was several centimeters / several inches long! And it hurt! You need a humidifier, big time. It was tiny, but it was there. You might or might not need a humidifier. What spark? What are you talking about? You need a humidifier about as much as do I. The problem of low humidity is most acute in wintertime. If it is very, very cold outside it doesn't matter if the outdoor relative humidity is 100% or 0%. The absolute humidity is very, very low in either case. The air in your house will have a very low relativity humidity because there is essentially no moisture in the air. In a dry climate, it doesn't have to get all that cold to have a low relative humidity problem indoors. As to why it's a health problem, the incidence of the common cold and influenza peak sometime in December or January -- precisely when absolute humidity reaches its minimum. http://www.pnas.org/content/106/9/3243.long.

There are a number of things wrong with your post. First off, this thread was dead for more than six years. Let old threads die a graceful death. Don't necromance them back to life! Secondly, there's that silly font and color business. Don't do that. It is almost always best to tust go with the defaults. Thirdly, there's the technical content. Not since 1956. any more. See below. Not since 1656, when Christiaan Huygens invented the pendulum clock. See below. That's called a year, not a day. This makes no sense. Not since 1582, when the Julian calendar was replaced by the Gregorian calendar. Humans have developed three basic techniques for measuring time: Sundials, the motions of the planets, and mechanical devices. Sundials are the oldest and also the worst of these techniques. Sundials measure what is called apparent solar time. Apparent solar time varies markedly over the course of a year and also varies with latitude. A solar day on February 11 is considerably shorter(half an hour shorter!) than a solar day on November 3, and the length of a apparent solar day today for you probably is not the length of a solar day for me. This discrepancy is called the equation of time. Sundials are a rather lousy timing device. People began moving away from time as measured by a sundial to time as measured by a mechanical device (i.e., a clock) as soon as good clocks were widely available. Although the equation of time was known to the ancients, it was more or less a scientific curiosity until Huygen's invention of the pendulum clock. Those mechanical clocks measure what is called mean solar time. Mean solar time stays in synch with apparent solar time on average, but removes that 30+ minute variation from February 11 to November 3. Even after removing these huge swings, there's yet another problem with time based on the Earth's daily rotation: A day now is about 2 milliseconds longer than a day in 1800. The international standards committee switched from time based on the Earth's daily rotation in the 1950s to time based on the Earth's yearly orbit, and then in the 1960s switched again to using atomic clocks (a quantum mechanical device). The motions of the planets provides yet another way to measure time. Measuring time was the obvious solution to solving the longitude problem. The problem was how to measure time. The issue of clocks versus celestial observations until recently was not fully resolved. A problem with clocks on the Earth is that they vary by a slight amount as the Earth moves from perihelion to aphelion and back again. The best approach is to combine the concepts of clocks and celestial observations, and this is exactly what the various organizations that model the motions of the planets do.

Yes. Well, to be blunt, you're wrong. Theories in physics have to follow three simple metarules. The theory has to have some logically consistent underpinning. This can range from a simple ad hoc equation whose only logically consistent underpinning is that the equation is a mathematically well-formed formula to a much deeper set of precepts from which one can derive mathematical predictions. More important: It has to describe some testable behavior. No matter how internally consistent it might be, a "just-so" story that is not testable is not a physical theory. Most important: It has to agree with observable reality. Your concept is wrong for the simple reason that it does not agree with observable reality. The Hafele–Keating experiment, for example. You appear to be misconstruing the concept of covariance in general relativity, which says that all reference frames are equally valid, to say something contrary to relativity theory. You are ignoring that no matter which explanation one uses / which point of view one takes in the twin paradox, the answers are always the same. The traveling twin ages less. To argue against general relativity you need to construct some experiment and show how relativity fails that test. General relativity, at least so far, is consistent with every experiment constructed to test it.

That's a narrow view of disease. Diseases can be caused by pathogens (e.g., the flu), improper nutrition (e.g., beriberi), physiology (e.g., some cancers), and also heredity. Some hereditary diseases such as sickle cell anemia are defenses against other diseases. Aging is apparently another such hereditary disease that is a defense against another disease, in this case, cancer. Human cells can only reproduce a finite number of times thanks to telomere shortening. This finite limit prevents a lot of cancers from running amok, but it has the side effect of death by getting old. To answer the original question, "would the world governments tell us if the biggest killer disease could be cured," suppose such a cure was found. How could our governments possibly suppress such a discovery? Someone would tell. There are no big, big secrets anymore, and this would be the biggest of them all.

No. I. Special Relativity As a starting point, the speed of light is constant because that is exactly what falls out of Maxwell's equations. Neither the velocity of the emitter nor that of the receiver appears in the calculation of the velocity of Maxwell's electromagnetic waves. There were two big problems here: Every wave phenomenon known to the physicists of that time required some kind of medium as a transport mechanism for the waves, and This was very much at odds with Newtonian mechanics. This conflict between electromagnetism and Newtonian mechanics was one of the biggest problems of latter 19th century physics. There were two ways around this. One is to say that there is some special frame, the ether frame, in which the one-way speed of light is indeed c. We can't measure the one-way speed of light. All experiments are ultimately measurements of the two-way (round trip) speed of light. We can't see this ether frame because the Lorentz transformation act to hide it from us. This is Lorentz Ether Theory, the main proponents of which were Poincare and Lorentz. Another way around this dilemma is to simply accept Maxwell's equations at face value: The speed of light is the same to all inertial observers. This requires some fundamental changes in how one views space and time. This was Einstein's insight. These two theories, Lorentz Ether Theory and Special Relativity, are experimentally indistinguishable from one another. So why choose one over the other? Physicists obviously have done so; Lorentz Ether Theory nowadays lives on only at crackpot websites. The reasons are many. Lorentz Ether Theory has some rather goofy axioms. Time dilation and length contraction are axiomatic in Lorentz Ether Theory (watch me pull this rabbit out of a hat!). The existence of an ether frame (also axiomatic) is even worse. This ether frame that lies at the heart of Lorentz Ether Theory is an untestable axiom. It's metaphysics, not physics. Compare to the axioms of special relativity: (1) The speed of light is the same to all inertial observers, which is bizarre but is consistent with theory and experimental results, and (2) The laws of physics are the same to all inertial observers, which is anything but bizarre. This axiom goes back to Galileo (Dialogue Concerning the Two Chief World Systems). The final nail in the coffin for Lorentz Ether Theory was quantum mechanics. There is no need for this metaphysical ether frame that contains some ether that acts as the transport mechanism for electromagnetic phenomena. Electromagnetic phenomena do not need a medium. Electromagnetism is carried by photons. So one answer to your question, "why is C constant?" is that it is axiomatic. If you pester your parents or teachers with "why" question after "why" question, you will inevitably get the non-answer "because we said so." Saying that a constant speed of light is axiomatic is a scientific way of saying "because we said so." II. A bit more philosophical. Start of with a couple of simple observations. (1) There is this thing that we call "space" that appears to be described by Euclidean three dimensional geometry, and (2) there is this thing called "time" that appears to be the independent variable with which we can describe how things interact. Now add one simple hypothesis: There exists one speed such if any one observer sees a particle moving at this speed, all observers see that particle moving at that same speed. In other words, there exists some invariant speed. There are two basic choices for this universally-agreed upon speed. One choice is that this universally-agreed upon speed is infinite. This choice leads to the Newtonian universe. Time is absolute, and space is Euclidean everywhere. Should one observer see something moving at a finite velocity, there will always be some other observer who will see that thing moving at a different finite velocity. The only universally-agreed upon speed is that of a particle that is moving infinitely fast. The other choice is that this universally-agreed upon speed is finite. This requires recasting those simple observations of time and space in a new light. Just because those observations appear to be true locally does not mean that they are universal. All of the experiments that led to the conclusions that time is the independent variable that describes motion through Euclidean three dimensional space are ultimately local experiments. A physical theory must agree with these local experiments. This choice of a finite universal speed coupled with this local need for space to appear to be Euclidean three space and time to appear to be the independent variable leads to a radically different geometry of space and time. Couple this finite speed with some basic laws of physics results in massive particles always moving at a speed less than this universal speed while massless particles can only move at this universal speed. So now we have two competing hypotheses of the universe: The Newtonian hypothesis that the universe is globally Euclidean and time is absolute versus this non-Euclidean universe. There's a simple way to test one against the other: Is there a universally-agreed upon speed that is finite? Experiment after experiment says there is such a finite universally-agreed upon speed, and it is the speed of light. So why is the speed of light constant? Once again, the answer right now is that it is axiomatic ("because we said so"). Of course, those axioms have to agree with reality -- and they do.

No, there isn't. The Newtonian concept of an inertial frames is a fiction. A very useful fiction, but a fiction nonetheless. You can not feel inertial forces. Inertial forces, like the Newtonian concept of an inertial frame, are fictions. Very useful fictions, but fictions nonetheless. Another word for inertial force is fictitious force. Imagine you are standing still on the ground, watching someone on a merry-go-round. From that person's perspective, you are moving, and also accelerating. In addition to the vertical normal force and vertical gravitational force, you apparently are also subject to a horizontal centrifugal force and a horizontal coriolis force. Do you feel those horizontal forces? Of course not. They aren't quite real. From your perspective, you are subject to just the normal force (which you can feel) and gravitation (which you can't). From the perspective of a falling apple, the only force acting on you is the upward normal force. That gravitational force you think is pulling you Earthward is a fictitious force. You can't feel it precisely because it is an inertial force.

This thread is starting to verge into wronger than wrong territory. In the words of Isaac Asimov, “When people thought the earth was flat, they were wrong. When people thought the earth was spherical, they were wrong. But if you think that thinking the earth is spherical is just as wrong as thinking the earth is flat, then your view is wronger than both of them put together. So as to avoid being "wronger than wrong" myself, I'll first say that what Dr Rocket said in post #6 is basically correct: A slight refinement of this statement is needed to explain the equatorial bulges of the planets: An oblate spheroid minimizes the potential energy of a rotating, self-gravitating body. This explains the slight flattening of the Earth (flattening = 1/298.257) and the more extreme flattening of Saturn (1/10.2). The deviations from this oblate spheroid shape are quite small, even for Mars. Of the eight planets, it is Mars that is most "out of round" (i.e., exhibits the greatest deviation from an oblate spheroid shape). Something, or some things, quite powerful happened to Mars in the distant past that created features such as the Borealis basin, the Tharsis bulge, and the Hellas Planitia. But even Mars is more or less spheroidal.

Yep. Some of those far away galaxies are moving away from us at speeds greater than the speed of light. Closer to home, the Milky Way and Andromeda are approaching one another at 100 to 140 km/second, about 3 to 5 times the Earth's orbital velocity about the Sun. I did not want to raise the issue of gravitational lensing until the OP "got it". Now that that has happened, Those four blobs outside the central blob are the same object, the quasar Q2237+030, aka Einstein's cross. That central blob is the galaxy ZW 2237+030, aka Huchra's lens. This image depicts how gravitational lensing works. If a massive object such as a galaxy or black hole is directly between us and some remote galaxy, there can be multiple straight line (null geodesic) paths light from that remote galaxy to us thanks to the way mass causes spacetime to curve.

Apollo 13 was not in a free return trajectory at the time of the incident. Apollo 8, 10, and 11 used a free return trajectory that involved flying around the Moon and returning to Earth with no maneuver required (theoretically, at least). This severely limited the choice of landing sites on the Moon. Subsequent missions, including Apollo 13, did not use such a trajectory. Those later missions instead started their voyage to the Moon on a free return trajectory that would have had the vehicle fall far short of reaching the Moon should the vehicle undergo some massive failure. The first midcourse correction took the vehicle off of this sublunar free return trajectory and onto a translunar trajectory. This translunar trajectory was not a free return trajectory. A massive failure (worse than Apollo 13) at this stage was a irrecoverable CRIT1 (loss of mission, loss of life) failure, one of the very few irrecoverable CRIT1 failure scenarios that were waived. There was no maneuver as Apollo 13 went around the Moon. It was in free fall. There was a trans-Earth injection burn, but that wasn't performed until two hours after perilune. The Apollo 13 astronauts watched the Moon underneath, weightless. A point mass in a free fall trajectory cannot feel gravity. The Apollo capsule is not a point mass, but the tidal forces (which can be felt) on the Apollo 13 astronauts would have been very small.

Of course a Newtonian inertial frame is a convenient fiction. If there was no validity to the Newtonian concept of an inertial frame, the various space agencies around the world wouldn't use what is essentially a Newtonian inertial frame, the International Celestial Reference Frame, to plan, control, and analyze their space missions. Oftentimes they do this from a purely Newtonian perspective; no general relativity at all. The organizations such as the International Earth Rotation and Reference Systems Service (http://www.iers.org), the International Astronomical Union (http://www.iau.org), the U.S. Naval Observatory (http://www.usno.navy.mil/USNO) that develop this best guess at an inertial frame do consider relativistic effects, but as a small perturbation of an otherwise Newtonian universe. In other words, ultimately incorrect. There's a huge difference between a scientific hypothesis that is experimentally shown to be completely false and one that is shown to have limited applicability. Observations starting in the latter decades of the 19th century showed that something was wrong with Newtonian mechanics. Special relativity, general relativity, and quantum mechanics collectively showed that Newtonian mechanics is not universally true. This lack of universality does not mean that Newtonian mechanics is utterly false. There's a huge difference between a scientific hypothesis that is experimentally shown to be completely false and one that is shown to have limited applicability. IMO, this is where Popper got things a bit wrong with regard to falsification. Per Popper's naive falsification, we should be using quantum mechanics or general relativity to do all physics, almost all engineering. That isn't the case. Even though 20th century science showed that Newtonian mechanics is not the universal truth that physicists for the 200 years following Newton thought it was, Newtonian physics still remains a very useful concept precisely because the limited domain in which it is accurate is very important to us.

It's the acceleration relative to an inertial frame (specifically, with respect to a co-moving local inertial frame) that makes the twin situation asymmetric. There's something quite special about this acceleration. It is measurable with an accelerometer. An accelerometer is a local device; there's no need to look outside the window to measure this acceleration.