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Physics, Astronomy & Cosmology / Re: If I Was Able To Stop Time, What Would The Temperature Be ?

« on: 31/07/2020 23:06:18 »

This looks like saying that in the absence of change, time is meaningless, and in the absence of time, change is meaningless. 
Need to check that I’m not misinterpreting.

The assumption of causal symmetry is one of the vanities of philosophy, which is anathema.

Time being what separates sequential events, it is the dependent quantity. Change is not dependent on what we measure, but what we measure depends on change.   

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

« on: 12/05/2020 20:13:44 »

But it is molecules particles 'vibrating', right?

No.
The vibrational frequencies of molecules are roughly in the MHz to GHz range.

Sound is a variation in the pressure of molecules; a measure of how closely packed they are.

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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: Does light have mass?

« on: 03/05/2020 12:04:35 »

Again, since the m here refers to proper mass, it doesn't apply to a photon.  Instead, the momentum for a photon is found by
p = hf/c
And the general equation ends up giving you E= hf for the photon.

I understand both equations (surprise!), but am not clear as to how p = hf/c becomes E= hf.

Energy is momentum times velocity. Here it is pc. Since hf/c times c cancels out the speed of light you are left with E = hf.

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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: 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. 

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Physics, Astronomy & Cosmology / Re: Was the Big Bang the beginning of the universe?

« on: 16/10/2019 08:18:51 »

"It has a topology, it has a shape, it's a physical object," philosopher Jim Holt said

Yet another instance of pretentious drivel from a philosopher. Why does anyone bother to invite such people to speak? If you define "nothing"  as an absence of physical objects, it cannot be one. And there is no other meaningful definition.

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Physics, Astronomy & Cosmology / Re: Was the Big Bang the beginning of the universe?

« on: 09/10/2019 18:10:20 »

The Universe means everything. If I'm correct, that's the usual meaning of Universe with a capital 'U'. The Big Bang is not the beginning, it is just a phase.

Gravitational waves were postulated to conserved energy. And there we have them...

If you start with an infinite Universe, you won't find any satisfying solution because infinity cannot be rationalized. Yet, we have a constant speed of light in the vacuum, we have other constants and discreet particles. Let's start with a finite Universe, then we may add things to it only when no other solution is reasonable. In a finite and quantized Universe, there is a maximum to entropy...

Let's test a Big Bounce hypothesis where the Big Bang is just a phase transition.

Just before the Big Bang, all matter is condensed in an object having the lowest possible entropy. This could be a Schwarzchild black hole but it includes all space and time (no external space). This implies a prior Big Crunch which has condensed all matter in the previous cycle. This means there was an excess of attraction vs repulsion.

When the Universe reaches the bottom (the lowest entropy), the attractive force (whatever produces this force) passes by a symmetrical point where it becomes null and then this produces an excess of the repulsive force for a brief moment, something like a Planck time. Gravity disappears when the energy budget is 50-50, repulsion-attraction. But in fact, it never gets to this budget because it is a symmetrical point where attraction just disappears. It implies that there are asymmetries left to account for the structure. These asymmetries may be fundamental or related to a multiverse.

A finite Universe implies intrinsic asymmetries. Only an infinite Universe may have a complete symmetry. If you want a cause to our existence, it is the fact that there are irreducible physical asymmetries. The annihilation of an electron-positron pair doesn't result in nothing but two photons. This means there is no complete symmetry between them, though there are symmetries to be filled with the rest of the Universe.

Returning to the Big Bang, this results in a delay between repulsion and attraction.  The phase of repulsion is in advance of the attractive phase. This is dark energy.  Now the Universe has a much greater asymmetry in the form of a delay in the phase of the waves. Repulsion results in an increase in the degrees of freedom and the entropy. Attraction results in a decrease in the degrees of freedom. All forces should be mediated by particles. The known candidate for this effect is the photon which produces a delay of gravity in its direction of motion. Gravity moves at the speed of light. This adds to the original delay, though it is small, it means Dark Energy increases. But, as the Universe is finite, it will reach a maximum entropy and go through another phase transition when a symmetry of the repulsion force will be filled in. Dark matter has an important role to account for the ratio of gravitational mass vs repulsive mass. It could potentially have only an attractive component.

GR does not include the phase transitions or the Big Bang...

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Physics, Astronomy & Cosmology / Re: Do we go round in circles?

« on: 10/08/2019 14:16:49 »

I’ve put together a few questions, and made some tentative moves towards possible answers, or where they might be found.  Still a long way to go, but comments would be appreciated.

I put in some comments, but have not read the whole thread.
 

1. If infinity is not a number, how can you subtract anything from it?

It isn't meaningful to subtract a number from a not-a-number.  'Infinity' is more of an adjective, meaning 'without limit'.  Much confusion arises when its syntactic usage as a noun leads one to treat it as a number.

2. If the Universe is infinite, it contains an infinite number of galaxies.  How does one define an infinite number?

Again, it simply means there is no limit to the number of galaxies.

3. If one is subtracted from an infinite number of objects, is the remainder still an infinite number?  If not, what is it?

Per point 1, it is not meaningful to do addition and subtraction with 'without limit'.

4. Would an infinite number of (identical) objects contain all the examples of that object that could exist?

If they're identical, how is it not one object?  This sort of gets into the law of identity.
Perhaps you mean something like 'just because there are infinite points along a line doesn't mean that there are not other points that do not fall on that line.  So there are examples of points not in that set, but I'd not call any of the points 'identical' since they're all at different places on the line.

5 Is “absolute infinity” (sensu, Cantor) amenable to mathematical manipulation?

6.

The Absolute Infinite (symbol: Ω) is an extension of the idea of infinity proposed by mathematician Georg Cantor.
It can be thought as a number which is bigger than any conceivable or inconceivable quantity, either finite or transfinite.

How could this concept be expressed without referring to infinity as “a number”?

Since it isn't a number, not sure what you're asking.  It's expressed as Ω, but that's not your question. One infinity isn't larger than another since they're not numbers.  Perhaps it means 'highest cardinality', but cardinality isn't an expression of the magnitude of something.  Only numbers have magnitude.

I'd not trust wiki on this.  They call it a number, but any number is finite.  I think a proper mathematician would not word the description this way.

An infinite universe is possible only if the mean density of matter in the universe vanishes.

Why?  Simple explanation, please.
...
I'd really appreciate some guidance with #23.  I'm trying to tie up loose ends.

Since nobody replied to this, I'll just say that the mean density of matter in the universe is typically presumed to be the same as it is in the parts we see (cosmological principle).  yor-on's statement would only apply to a model where there is infinite space, but there is finite matter, presumably all clumped nearby.  I know of no such model that is seriously considered.

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Physics, Astronomy & Cosmology / Re: Do we go round in circles?

« on: 07/08/2019 14:56:34 »

differentiating between a sequence, which converges towards a terminal value that it can never reach, one that converges towards no identifiable limit and one that converges towards an endless decimal fraction, does seem to be making little more than a conceptual distinction, however valuable the concepts might be in mathematics.

It's enormously important!

In the first case I can write down a rational number (in this case 1) that is just a bit larger or in this case smaller than  any term in the sequence. So if you reported your position every hour as 1.1, 1.01, 1.001...miles from my house, I would know exactly where to look for you.

In the second case, I would know you are horribly lost and not recoverable because you are moving away from me at about 1 mph.

In the third case, although I couldn't write down a rational number that tells me where you would end up, I know it would be less than 2.7183 miles away.

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Physics, Astronomy & Cosmology / Re: what would connect chaos theory with multidimensionallity

« on: 31/07/2019 12:00:58 »

when I think of a fractal 'magnifying it' finding it not to end it's not about 'dimensions' being nested inside it. It's a self replicating system.
=

Maybe this is a good explanation of a Hausdorff dimension. https://www.quora.com/In-laymans-terms-what-is-the-Hausdorff-dimension-a-measure-of

Then again, I'm not that particular to the idea of dimensions and this opens for another interpretation of what it is. I see it as a result, not as an origin. But it definitely confuse me trying to imagine how to translate this mathematic into a universe consisting of 1.5 'dimensions'.
=

Ok, Hausdorff did not question the 'dimensions' we find around us. He just invented a new mathematical approach to describing it in where the result of describing certain types of fractals gives you a fractional number using his definitions. That does not state that you have 1.5 'dimensions' because that is meaningless practically. .5 dimensions doesn't mean anything, the translation of it into a real universe fail to even describe a line. As long as we use what we see around us.

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Physics, Astronomy & Cosmology / Re: Does a photon self propagate due to the interactions of the electric and magnetic fields?

« on: 19/05/2019 23:02:20 »

Even then, you need to remember that "observation" is introduced as an auxiliary and explanatory concept in physics, rather like "cross section" in engineering. Nobody is going to cut a ship in half to make it work, but drawing what it would look like if cut in half, is very helpful in explaining how it works.

The natural world does what it does, and we petty beings try to understand and model it as though we were observers to every action.

To go back a couple of paragraphs, the properties of longitudinal compression waves are pretty much independent of frequency, so they are all "sound" whether we, bats or transducers can sense them, but the interaction of electromagnetic radiation with matter is strongly dependent on its frequency so we assign specific names to parts of the spectrum,  and the tiny bit that excites transient chemical responses in the eye is called light.

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Physics, Astronomy & Cosmology / Re: Does a photon self propagate due to the interactions of the electric and magnetic fields?

« on: 15/05/2019 17:25:26 »

Electromagnetic radiation propagates according to Maxwell's equations,which are based on the observation that a moving charge creates a magnetic field (see any electromagnet) and a varying magnetic field induces charge (see any dynamo).

The electromagnetic spectrum is continuous but some sources of em radiation emit discrete quanta with a specific frequency.

The "observer" is irrelevant since the photon, phonon or whatever has no concept of observation and the source, which may be a few billion light years away, really doesn't care what happens to the energy it radiates.

Sound is longitudinal pressure waves of any frequency in any medium. The effect of a sound wave has nothing to do with the auditory system. You can play your violin in front of an audience of bats, humans and neurologically deaf humans - the effect on their tympani and auditory ossicles is exactly the same, and the same on a microphone diaphragm that converts it to an electrical signal. The violin and the connecting air have no interest in the nature or presence of a receiver.

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Physics, Astronomy & Cosmology / Re: Is all motion relative?

« on: 20/02/2019 14:50:35 »

As an electron has mass, it cannot be infinitesimal since that would imply infinte density - infinities are evenless palatable than infintesimals!

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Physics, Astronomy & Cosmology / Re: How fundamental is time?

« on: 11/01/2019 16:50:14 »

Change and time are not equivalent. Time is a dimension, and change refers to differences within one dimension.

This is most well defined within calculus (sometimes called the math of change), where the symbol ∂ (sometimes d) is used to denote microscopic change, often relating the change of one thing with respect to the change of another.

In this way, we can define instantaneous (microscopic) change of position along a spatial dimension (x) with respect to instantaneous (microscopic) change in time (t) as ∂x/∂t which is equivalent to the velocity in the x direction.

Although time is a very common dimension (variable) to compare changes against, it is not the only one. For instance we can look at the change in altitude with respect to change in forward distance traveled ∂y/∂dx (slope) or the change in cost of making chocolate with respect to the change in the cost of labor ∂$_chocolate/∂$_labor etc. etc.

What I am trying to learn in this thread is whether our concept of velocity (or any other rate of change with respect to apparent time) is best described as a ∂x/∂dt, or if there is a more fundamental variable (I believe I called it α earlier in the thread) such that ∂t/∂α is well-defined, and ∂x/∂α holds up better to the boundary condition that the big bang appears to impose on ∂t.

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Physics, Astronomy & Cosmology / Re: Has there always been a constant number of photons?

« on: 29/12/2018 18:10:15 »

Depends on the nature of the interaction. At low photon energies the photon generally disappears into heat, a chemical change, or the movement of charge in an electrical circuit, but at energies above the visible spectrum you can get all sorts of secondary emission including photonuclear reactions.

What is the likelihood that emissions arising from these interactions would influence the original beam?
Would there be any effect other than "attenuated intensity"?

I'm dealing with just such a case right now: the phenomenon of "buildup" as a photon beam passes through a concrete barrier.  Multiple interactions within the barrier means that you can end up with more photons coming out than went in, but at a lower mean energy per photon.

This is a useful phenomenon in industrial radiography where we use a thin sheet of tin or lead o "intensify" the image - more, lower energy, photons are captured by the x-ray film than if you just use the raw 300 kV x-ray beam. It's a pain when designing radiotherapy bunkers because you need to use an iterative process to determine the required thickness of concrete - more doesn't always mean less! And it caused great hilarity when a pompous and ignorant Health and Safety Inspector (he always spoke in Capital Letters) insisted that staff handling high-energy gamma sources should wear lead aprons, which not only slowed them down, but actually increased their instantaneous skin dose.

We use all kinds of filters to remove photons of specific energies from a beam to produce a more monoenergetic  (monochromatic) beam of lower intensity.

In these cases; would a filter equate to a “target”, in the original quote?

[/quote] Indeed. At the low energy end of the business, we use aluminum filters to remove the part of the x-ray spectrum that would cause skin burns without producing an image. Other folk use filters to remove high energy bits from the visible spectrum so that your room lights don't interfere with your TV remote control.

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Physics, Astronomy & Cosmology / Re: How does mass change when considering general relativity?

« on: 04/12/2018 12:01:31 »

I recently got into a debate at work regarding how mass can change in general relativity. In my feeble understanding of general relativity, an object with more energy bends spacetime to a greater magnitude, and so in terms of gravitation it behaves as if it has more mass.

It does have more mass.  Mass and energy are the same thing, so adding energy is the same as adding mass, and mass bends spacetime, so energy does as well.

But if this is true, does it also work for potential energy - If you lift an object up you put work into it.

Yes!  This is unintuitive, but if you lift an object, you have expended energy from elsewhere to the object, and it now masses more.
The energy needs to come from somewhere other than the object.  If it goes uphill due to its own kinetic energy (like a roller coaster), it is losing its own kinetic energy as it gains potential energy, and the mass is unchanged.  The lifting needs to be due to energy imparted from outside the object.

But does it now weigh more (or perhaps more than one would expect compared with simple Newtonian gravity)?

The weight doesn't go up since you've moved it away from whatever gravity well defines 'down'.  So the mass goes up a tiny bit, but the weight (gravitational force) still drops.  If you have impossibly accurate weight and mass scales (spring and balance respectively), you'd measure more more mass but less weight at the top of a building.

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Physics, Astronomy & Cosmology / Re: Would particles orbit a falling body?

« on: 17/11/2018 19:47:12 »

No, at that close to the Earth, even a steel marble would be within the Roche limit.   Tidal forces generated by the Earth would exceed the gravitational attraction of the marble. For a steel marble you would have to be at least 708 km above the surface for this to not to be the case. 

For objects to orbit the marble, they would have to be within the marble's Hill sphere. A 0.5 cm radius steel marble would have to be ~1689 km above the surface of the Earth in order for its Hill sphere radius to be larger than it own radius, so this is the closest it could be to the Earth and hold anything in orbit around itself.

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Physics, Astronomy & Cosmology / Re: Is the universe expanding faster than the speed of light?

« on: 23/09/2018 23:40:49 »

Is there any reason why it shouldn't?

Here's a thought experiment. Assume that the universe U is an infinite array of roughly isotropically distributed bits of stuff. Assume that for whatever reason (and Schwarzchild gives us a perfectly good one) we can only observe a finite sphere X within that array. And assume that "gravity sucks". Now since U >> X, there is more stuff outside X than inside it, so X will expand due to the "suction" of (U - X) - i.e.the stuff outside - so the density of X will decrease, and the rate of expansion will increase.

Suppose we have reached the stage  at which points on the boundary of X are receding from us at 0.55c. Nothing in that statement to contradict relativity. But diametrically opposite points, say to the north and south of us, are moving at 1.1c relative to each other. All this means, relativisitically, is that an observer at N will not have any information about S, which is fine, but the diameter of the observable universe is increasing faster than c.

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Physics, Astronomy & Cosmology / Re: Are there different types of gravity?

« on: 08/09/2018 20:14:04 »

It's tricky Bill. Einstein didn't consider it a geometry, as far as I get it,. He used calculus and thought of gravity and inertia in those terms. the geometric interpretation was something that came without him, not meaning that he didn't see the implications, just that he found it questionable. " he thought that General Relativity was no more and no less geometrical than Maxwells theory of electromagnetism; and that the important achievement of GR was the advancement of the unification programme in direct continuation of special relativistic electrodynamics. Einstein thought that the special theory unified electricity and magnetism, the general theory inertia and gravity. Yet, we shall see that, unbeknown to most scholars, Einstein was emphatic in his belief that this should not be interpreted as a ‘geometrization’of gravity, especially if ‘geometrization’was seen as a reduction of gravity/inertia to spacetime geometry

.... As early as 1926 Einstein insisted, explicitly, that his work should not be understood as reducing physics to geometry, either his work on GR or his (and Weyl's and Eddington's) work on a unified field theory of gravitation and electromagnetism. Interestingly, what seems to be Einstein's first clear statement on the matter was prompted by a letter from Hans Reichenbach. Reichenbach was at the time engaging with Weyl's and Eddington's theories, and wrote Einstein that he thought that seeing electricity as geometrical in Weyl's theory is not more than an illustration (Veranschaulichung), one that, he argued, is equally possible (and equally trivial) in GR. Einstein agreed wholeheartedly, writing ' You are completely right. It is wrong to think that ‘geometrization’ is something essential. It is only a kind of crutch (Eselsbrücke) for the finding of numerical laws. Whether one links ‘geometrical’ intuitions with a theory is a … private matter. '   "

So when speaking about a geodesic it's not a geometric path in his terms, " Already in the first papers in which Einstein starts making use of the metric tensor to give an account of gravitation, he is at pains to establish the status of the geodesic equation as describing the motion of particles as “straight and uniform” (geradlinig und gleichförmig) even when subject to gravity. This would lead him to call the geodesic equation a “generalized law of inertia”; redefining inertial paths such that the category includes motion under the influence of gravity. ..... Already in a note added in proof to Einstein (1912), Einstein had stated that equation (1) gives the equation of motion of point particles “not subject to external forces”.

Thus, it was clear that already in 1912, before even embarking on a metric theory of gravitation, Einstein thought of (static) gravitational fields not as invariant force fields diverting particles from inertial motion. Already, in 1912, he thought of equation (1) as describing inertial motion on one hand, and as describing motion in the presence of (static) gravitational fields on the other. "

"  Einstein effectively states that the very distinction between ‘gravity’ and ‘inertia’ is useful only for relating the theory to its predecessor theories; it is not a distinction from within the theory itself. Put differently, if one just looks at the theory without relating it to predecessor theories, there is no need whatsoever to distinguish ‘inertial terms’ and ‘gravitational terms’ in the geodesic equation. "

By that I understand he meaning that talking about gravity as a force directing the inertia (willingness to keep on indefinitely) of a test particle made little sense to him.

"  In Einstein's mind, the unification was very similar indeed, as the December 1919/January 1920 text on the development of relativity shows. There, he recalls the magnet-conductor thought experiment described in the first paragraph of his 1905 paper on special relativity, from which he concludes. ' The existence of the electric field is a relative one, depending on the state of motion of the coordinate system used; only the electric and magnetic field together can be attributed a kind of objective reality, independent of the state of motion of the observer, i.e. of the coordinate system. '

Einstein then describes how he worked on a review article of special relativity in 1907, and links the above realisation regarding the electric and magnetic field to another thought experiment regarding inertia and gravity: ' Then I had the most fortunate thought of my life in the following form: The gravitational field only has a relative existence in a manner similar to the electric field generated by electro-magnetic induction. Because for an observer in free fall from the roof of a house, there is during the fall — at least in his immediate vicinity — no gravitational field. Namely, if the observer lets go of any bodies, they remain, relative to him, in a state of rest or uniform motion, independent of their special chemical or physical nature '

quotes taken from " Why Einstein did not believe that General Relativity geometrizes gravity " By Dennis Lehmkuhl.