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Physics, Astronomy & Cosmology / Re: What is a moon of a moon called?

« on: 05/12/2018 13:46:16 »

Or one might be up the proverbial s***y  moonlet without a paddle  on an unmoonletlit night.

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Physics, Astronomy & Cosmology / Re: Can we say a bit about fields in physics?

« on: 27/11/2018 00:56:14 »

Thanks ,it looks very interesting.

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Physics, Astronomy & Cosmology / Can we say a bit about fields in physics?

« on: 26/11/2018 22:56:26 »

I understand that a field can be defined as a series of measurements over an area,the type of measurements involved  determining the nature of the field .

So how many types of field are there ? Is he number of them simply limited by our imagination or rather are there just  a  few corresponding to fundamental physical properties such as gravity , electro-magnetism ,temperature etc?

Could we imagine a density field for example ? An age field?
Are there circumstances where 2 kinds of field  "overlap" or even interact?

And is it true to say that ,even though a field can be seen as a mathematical model that this does not  mean that it does not stand as a representation of a physical object ?(so that ,for example ,the gravity field  can be seen to represent a physical object)

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Physics, Astronomy & Cosmology / Re: Can sound waves escape from a Black Hole?

« on: 26/11/2018 00:21:42 »

Is the mistake to imagine the field as a "thing" rather than a series of measurements?

No. The reason is that it is a thing ie a series of values in an area of space. It can be considered as an object which can be discussed, manipulated mathematically etc.

However, I suspect you are asking whether it has a life of its own. Well, take a gravitational field, would it exist if there were no mass in the universe? It would certainly have zero magnitude. Despite that, we can still consider the concept of a gravitational field even if there is no mass.

Can I  apply a similar logic to any body (my own as an example)?

Is it/can it be defined as  also a "series of values" , without inferring that it is  insubstantial on that account?

So the various fields can be treated as bodies in the same way as any everyday object except that they are particular (and extremely fundamental)  kinds of bodies.....

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Physics, Astronomy & Cosmology / Re: Can sound waves escape from a Black Hole?

« on: 25/11/2018 18:36:52 »

These do have a source within the particles that generate them.

Therein lies my problem. 
If a particle is a disturbance in a field, how can a particle exist without a field?
If a field has to exist before it can be disturbed; how can the disturbance be the source of the field?

A field is a series of measurements made in space - most have a zero value in the quiescent state. Along comes an electron carrying a charge, value of electron field is disturbed and increases to that of the electron charge; electron passes by, disturbance goes, value of field drops to zero.

Is the mistake to imagine the field as a "thing" rather than a series of measurements?

Am I into "interpretation territory"?

Or is the "measurement" definition the sine qua non of any subsequent (deeper?) understanding?

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Physics, Astronomy & Cosmology / Re: Does light move at different speeds through different materials?

« on: 13/11/2018 01:44:56 »

…..A photon does not slow down unless it interacts with something ie a material …..

If a photon reacts with something, doesn’t it cease being a photon?

I thought a photon only became a photon when the light wave interacted with something.

Or is the wave a classical description and the photon a quantum description ? (totally confused)

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Physics, Astronomy & Cosmology / Re: Does light move at different speeds through different materials?

« on: 02/11/2018 17:34:53 »

The "in a vacuum" qualification is important to relativity.   All formulas and equations relate to c.  c is the speed at which light will travel through a vacuum.  The fact that light travels slower through water will not result in the effects of Relativity being any different, because even in water c remains unchanged even if the actual speed of the light through the water has changed.

Light isn't the issue at all. it is the speed c.  Light in a vacuum happens to travel at this speed, so it makes a convenient reference.

Is it the case that light does travel at c in water? Is it a case of stop-start with the stops occurring every time the light wave interacts with the medium and the starts being periods where it continues at the speed of c?

It is the periods of interaction that appear to slow the light  down whereas it actually travels some of the time at zero speed and the rest of the time at c?

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Physics, Astronomy & Cosmology / Re: Are gravitational waves curves in spacetime that wobble?

« on: 12/10/2018 11:02:20 »

Did he ever come around to thinking of it as a geometry?

No. In fact he wrote to Lincoln Barnett on June 19, 1948, saying

I do not agree with the idea that general relativity is geomterizing physics of a gravitational field. The concepts of physics have always been geometrical concepts and I cannot see why the g_ik field should be called more geometrical han f.i. the electromagnetic field or the distance between bodies in Newtonian mechanics. The notion probably comes from the fact that the mathematical origin of the gik[/dub] field is the Gauss-Riemann theory of the metrical continuum which we are wont to look at as part of geometry. I am convinced, however, that the distinction between geometrical and other kinds of fields is not logically founded.

Were there any practical consequences to this disagreement?Is it simply a matter of interpretation ?

When theoretical physicists explore possible versions of a quantum gravity  do  these interpretations assume a greater importance?

Am I right to assume ** that the curvature of spacetime does not feature in any of the current (successful) quantum theories ?

**clearly I can only assume when I have very little personal understanding of those theories....

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Physics, Astronomy & Cosmology / Re: How many black holes are there in the Universe?

« on: 12/10/2018 10:34:26 »

Are there any educated guesses at all as to the ratio between the observed and the unobserved universe? Could it be an infinite ratio?

That question comes up later in the same interview with Prya Nataranjan.
As I recall, "an infinite universe is still on the menu".

thanks. I will give it a listen if I get time.

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Physics, Astronomy & Cosmology / Re: What's a black hole made of?

« on: 11/10/2018 14:33:34 »

Are you taking the piss, in what universe does matter get stuck outside the event horizon of a black hole. Do you have a citation?

I heard that phrase "Are you taking a piss?" in the movie Kingsman: The Secret Service. What does it mean? I live in New England. Is that phrase a British thing?

Citation? Sure On the Influence of Gravitation on the Propagation of Light by Albert Einstein, Annalen der Physik[/b], 35, 1911.

Taking the piss is different from taking a piss (although you could do both at the same time)
It means "are you joking?" broadly.(with a side of "are you winding me up" or  "getting a rise"? )

Also you could use it for someone only pretending to make an effort.

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Physics, Astronomy & Cosmology / Re: How many black holes are there in the Universe?

« on: 10/10/2018 15:32:34 »

Interesting stats, Evan. Multiply this by the number (and size) of galaxies and you have the number for the Universe.

Wait! We can't see all of the Universe; we don't know what is out there.  The answer to the OP must be something akin to the answer to "how many angels can dance on the point of a needle".

Are there any educated guesses at all as to the ratio between the observed and the unobserved universe? Could it be an infinite ratio?

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Physics, Astronomy & Cosmology / Re: Are gravitational waves curves in spacetime that wobble?

« on: 09/10/2018 14:07:03 »

late to this one but no. Einstein did not think of it as a geometry. He thought of it as a relativistic gravitational 'field'. In a similar way to how you can change your experience of a Electromagnetic field by your 'motion' relative it, a gravitational field also is observer dependent. It exist as long as you don't fall, as soon as you're in that free fall you negated it acting on you, locally defined. So he wanted to find a way to unify both into one field, as I understand it.

Did he ever come around to thinking of it as a geometry?

I know he was unhappy with Minkowski diagrams but later was happy with them....is that a very  different case?

I think you said earlier that he tried to unify gravitation theory with the other forces. Is this attempt still ongoing  or is the prevailing consensus(if it exists)  that gravity is entirely different from them?

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Physics, Astronomy & Cosmology / Re: How does Sean Carroll match up to Nostradamus as a prophet?

« on: 09/10/2018 12:15:57 »

Maybe not Geordie, maybe it's reaching a singularity. And that singularity by some reason seems to coincidence with our very possible demise, or survival. Depends on how much you trust the way we handled things so far? You're an optimist?

It does seem like it is approaching a singularity but I suspect it is a mirage.

In the 60s I used to go down to the water and imagine  that it would rise gradually  and destroy our civilization and put an end to our  lack of respect for nature (we had seawalls that  overflowed now and then)

That made me optimistic or pessimistic? If we can consider ourselves as stewards over this Earth then we have done enough to forfeit that position.

I also feel this coming climate change  disaster may be a way to force us to come of age**-which is why I find the obsession with colonizing Mars etc  to be a form of denial (or just plain desperation)

**implying no Gaia-like intention

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Physics, Astronomy & Cosmology / Re: What's a black hole made of?

« on: 08/10/2018 19:41:21 »

That is incorrect. Light is always moving when going into a black hole. It's merely slowing down. A particle like a photon can always be moving towards the event horizon and still never get there. Its sort of like Zeno's paradox. First its moving at c, then later at c/2 then c/4 then c/5 ....... At no time in that sequence is the photon at res

Does it follow from that that light slows down  (from an observer's perspective) whenever it goes into any gravitational well (eg the Sun) ?

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Physics, Astronomy & Cosmology / Re: How does Sean Carroll match up to Nostradamus as a prophet?

« on: 07/10/2018 23:52:32 »

"Very beginning" sounds like a hostage to fortune but I would not be surprised for there to be one or two astounding discoveries concerning that period of history.

But I wonder if the law of diminishing returns will set in and so each new discovery will raise more questions than it answers .

Is the scientific endeavour really ,against all appearances still in its infancy?

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Physics, Astronomy & Cosmology / Re: What's a black hole made of?

« on: 07/10/2018 14:23:54 »

Even light slows down in a gravitational field and so too for photons moving towards the black hole.

Does it slow down in a continuous way ?(ie if the gravitation increases it slows down more and more until the gravItational force is equal to that of a black hole and its speed is zero: do more powerful black holes make its velocity reverse?)

At what stage do photons (=em waves?) cease to exist in a BH?

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Physics, Astronomy & Cosmology / Re: In deep space, are spacecraft still in freefall?

« on: 04/10/2018 01:58:51 »

But if relativity is to be believed, how could space be curved and relative from the point of the observer simultaniously ? Surely the observer should see a straight line space due to acceleration being relative ? I think pedant is the word.

Pedant? Do you mean "dependent"?

Do you disagree that we should be discussing "spacetime" rather than "space"?

It is not my understanding that motion caused by gravity can be called "relative acceleration" but I am not really qualified to give an opinion and will defer to others  better able to do so.

Actually it is my understanding that acceleration is not relative but absolute -but again  I could be wrong.

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Physics, Astronomy & Cosmology / Re: In deep space, are spacecraft still in freefall?

« on: 04/10/2018 01:07:21 »

But space is curved and that is why the path is the shortest distance, that being the curved motion from the point of view of the observer. Due to the acceleration of gravity being greater one side as compared to the other. They are under there own relative motion. Only question is  which body is going under acceleration ?

I think neither are  since an accelerometer on either body will read zero.

I also think it is spacetime (rather than space) which is curved

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Physics, Astronomy & Cosmology / Re: In deep space, are spacecraft still in freefall?

« on: 03/10/2018 10:40:30 »

What do you mean by " A "straight line" in GR is the shortest path.... "  ?
Try the quotes I gave and see if they agree with it.
=

Maybe I did misread what you meant saying that " Isn't euclidean straightness an idealization ". The way I understood that sentence was in pointing out that it was a limited truth, much like Newtonian physics versus relativity? You need to define your thoughts again.

I haven't got around to reading  your  quotes (or PmbPhy's Wikipedia "model" page ) yet ,but I will soon)

Still ,just to clarify your question (with caveats on my strictly amateur  level of understanding) I was trying to claim  that the "straight lines" we use in mathematics  are idealizations in that whenever we try to draw one  in any physical situation they will be slightly off.

For the purpose of analysis  we ignore that  and so they are a mental construct really.

Not sure I can stand by my "they are a limit " comment  meaning they are what we get when a crooked line is straightened without limit.(it seems as if I am  trying to reinvent the concept of geodesics unnecessarily -too much time on my hands      )

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Physics, Astronomy & Cosmology / Re: In deep space, are spacecraft still in freefall?

« on: 02/10/2018 15:53:36 »

I don't think it was for Einstein geordief. The way I got it he didn't consider trajectories 'curved'. As I read him he thought of them as the shortest path, and how can the shortest path be curved? I know, you can argue the opposite, but that's what I got from reading him.
=

" 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.  ...   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. "

It's funny but even when not knowing this I wondered how SpaceTime would 'look' if 'folded out' into what we normally would call 'straight lines'. Which also lead me to wonder how many simultaneous 'geodesics' there could be at one point in the SpaceTime we see astronomically. The answer I got was innumerable.

Yes but those lines were not Euclidean ,were they?

A "straight line" in GR is the shortest path....

I find it interesting (insofar as I was able to understand it ) that some ( all?) of the proofs  demonstrating  the tensors determining spacetime curvature do seem to rely on plain Euclidean geometry.

The curvedness seems to be constructed from plain straight lines.

Euclidean geometry  seems to be like a mathematical lingua franca .