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Physics, Astronomy & Cosmology / Re: Will all moving objects with mass generate gravitational waves?

« on: 14/06/2019 15:34:33 »

If accelerated objects emit gravitational waves, and gravity is said to have an infinite reach, is it fair to say that all space would contain gravitational wave energy, coming and going in all directions, from a potentially infinite history of the relative motion of celestial objects?

All space does contain gravitational wave energy simply because all space is near accelerating objects.  But gravity waves travel at light speed, so a given event can only reach as far as the event horizon, just like light.
Gravity on the other hand is a field and does not propagate at some speed, so gravity has no finite bound to its effect.
Information can be transmitted via gravity waves, but not via gravity.

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Physics, Astronomy & Cosmology / Re: How much does the moon weigh?

« on: 08/06/2019 13:27:40 »

The moon is in free fall and is weightless.

That is the correct answer.  The Earth doesn't weigh anything either since it isn't sitting on anything.
OK, it is sitting on me, so the Earth must weigh something like 9 stone.

I was asked on the radio this week "how much does the Earth weigh?"

More accurately, we were of course discussing the mass of the Earth, and the answer, which I happened to know, is about 6 x 10²⁴kg.

That is a measure of its mass, which is commonly confused with weight, but they're very different things.

Later, driving to work, I began to think - as you do - what I would have said if the questioner had enquired instead about the mass of the Moon.

The questioner asked about weight, not mass.

Naively, I reasoned that gravity is about 20% of that felt on the Earth's surface, so the Moon must have a mass 20% that of Earth.

Intrigued, I later looked it up. The stated mass of the Moon is about 7 x 10²²kg. That's only about 1% of the mass of the Earth.

As alancalverd points out, this is correct, except moon gravity is about 16.5%, not 20%.

Is the acceleration due to gravity on the lunar surface as large as it is (almost 20% of the Earth's despite the lunar mass being only 1% of Earth mass) because the Moon is correspondingly smaller, so the inverse square law being what it is (GmM/r²) it works out that way?

Yes, exactly.

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Physics, Astronomy & Cosmology / Re: Impossible question about our Solar System, can anyone answer? Please help.

« on: 08/06/2019 00:51:28 »

If all of the planets in our solar system are gravitating towards the sun, and have been for billions upon billions of years. Then that would mean that planet Earth would have been in the same position (the same distance from the sun) that Mars is now

This doesn't follow.  Earth is accelerating towards the sun at all times, meaning Earth's trajectory is bent, from the straight line out of the solar system it would otherwise take, into the circular orbit that it actually takes.  All other factors not withstanding, that would make for each planet returning to the same spot after each rotation.

There are other factors, and the two making us drift imperceptibly away are stronger than the two making us drift even more imperceptibly closer.
Look at the moon's orbit for the best example.  Its average orbital distance is increasing by almost 4 cm per year, despite always 'gravitating towards Earth'.  That outward drift is due to gravitational tidal forces, a gravitational effect.

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Physics, Astronomy & Cosmology / Re: Can Special Relativity be derived from Newtonian Mechanics?

« on: 07/05/2019 02:53:02 »

Am I correct in thinking acceleration is to do with general relativity and not special relativity? Special relativity deals with none accelerating objects does it not.

Special relativity deals with acceleration just fine.  General relativity introduce the equivalence principle with allowed the properties of gravitational fields to be (locally) described in terms of accelerating reference frames.
Newton's equations did not cap speed of mass at c.  F=ma.  If you continue to apply force to an object, the acceleration will continue up to any beyond c.  With SR, acceleration (not proper acceleration) drops off and that same force just drives up m after a while.  They do this in particle accelerators where getting a particle to near light speed is easy, but then they just add mass to the thing until up to the limits of the magnetic field to hold it in its path.

Using simple algebra and tracing the path of a photon between two moving mirrors, will result in time dilation for an observer in a stationary reference frame.

Reference frames are not stationary.  Things are stationary relative to them.
That said, I see what you are getting at.  Newton probably wouldn't consider the light device to be a clock since without knowing its speed, there is no way to tell how far the light needs to travel.  Not like there is any real light clock anyway.  How would you get it to work?  You can't measure a photon going between mirrors, counting cycles.  A photon can be measured but once.

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Physics, Astronomy & Cosmology / Re: What would air and water do on a rotating space craft?

« on: 29/03/2019 11:53:48 »

The Avalon is an amazing space craft that uses rotation to simulate gravity. We know that air and water in a plane will move with the plane. What would air and water do on a rotating space craft? What would really to the water if somebody jumped into the Avalon's swimming pool?

Water would be 'down' just like water on Earth which is attracted to Earth by gravity.  So a spinning object would have 'down' feel like the outside of the craft and 'up' be towards the center of rotation.  The closer to the edge you get, the stronger the artificial gravity.  Thus there would be no 'gravity' at the center.

The ship would have to have a huge radius for air pressure to change much from one place to another.  I climb a 4 km mountain and the air is noticeably thinner, so if the spinning craft is several km in diameter and air is not restricted by decks from flowing anywhere inside, then the pressure at the center might be somewhat lower than at the edges.

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Physics, Astronomy & Cosmology / Re: Approaching An Event Horizon: Falling Observer vs Hovering Observer

« on: 20/03/2019 18:03:53 »

I re-read the OP, and if the event horizon is never crossed, then yes, the black hole eventually fades with Hawking radiation and the observer there (who cannot exist any longer since he has also faded away with the same radiation) will experience in no time a sort of finite discontinuity.
So let's say the black hole lives only for a billion years.  As I observe the universe as I fall in, outside time speeds up and abruptly jumps a nice finite billion years, and it turns out I never left the universe at all.  There isn't even a gravity well here anymore.  The black hole is gone, and my clock ticks at the same rate as the patient clock watching all this from a distance, except we're a billion years out of sync now.

So the answer is that the time of the rest of the universe is finite, not infinite.  The infinite answer is only a mathematical idealization which assumes black holes don't evaporate.

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Physics, Astronomy & Cosmology / Re: Approaching An Event Horizon: Falling Observer vs Hovering Observer

« on: 20/03/2019 17:50:04 »

However different perspectives, although can appear contradictory on a superficial level, can never lead to genuine paradox. In this case either answer to the first question seem to lead to a paradoxical result.

OK, I don't see a paradox.  OK, I see the title mentions a frame hovering very near the event horizon, which is a different frame in some ways than a falling 'observer', but both their clocks slow to essentially a stop from the perspective of the distant frame.

For instance, for an partner coming quickly towards me, a large amount of time must pass in my frame for a small amount of time to pass on his local clock, and yet in his frame, it is my clock that is running slow.  In both cases, if were were to read each other's clocks as they approach, they would appear to be running faster, not slower.  That's the difference between wording it as "in frame X or Y" vs as it "appears to observers in X and Y".

No, the distance between the observers has no effect on the time dilation of clock in the frame of the other observer.

I didn't say otherwise.  I left my quote there so you can see that.  Nothing is distance dependent, but the appearance of the non-local clock is dependent on the recession speed as well as the relative speed.  So the arriving clock appears to run fast if watch it.  This is essentially why light from approaching stars is blue shifted.  The frequency of the light (which is a clock) appears to go up despite the time dilation due to the motion of the approaching light source.

Each clock would be running slow from the perspective of the observer observer

From the frame of those observers, yes, but not from appearance.  I avoided the ambiguous term 'perspective' since it doesn't distinguish appearance from actuality.  You're obviously talking about actuality, relative to a frame, but I'm talking about appearance, what I see when I observe the approaching clock.

If you want to work out what they would actually see then you need to factor in Doppler shift

OK, you understand what I'm talking about.  Yes, appearance needs to account for that shift.

but that also remain constant at a constant relative velocity but is affected by direction of motion whereas time dilation only depends on velocity.

I disagree with both.  Relativistic dilation depends on speed, not velocity.  The velocity of a GPS satellite relative to Earth changes all the time, but its speed changes very little and its dilation is fairly constant due to this.  As for the Doppler effect, it changes slowly or abruptly as the watched thing passes by, all without ever changing relative velocity since I presume that both clock and observer are inertial.  The abruptness of the change depends on the proximity of the nearest approach.

We're not disagreeing with each other I think, just getting our terms straight.

What needs resolution?  From the outside frame, the event horizon is never reached.  The clock dropped in actually stops.  It also in theory appears to stop, since it can hardly move on if the next second is never reached.  In reality, there are only so many photons that escape the clock in the last moment, and so the dropped clock vanishes from view in a poof of red shift.

No it never stops. It continually ticks at a slower and slower rate but never actually stops ticking.

Sure.  By stopped, I mean there is a time on that clock that will never be reached in the history of the universe outside.  If it passes the event horizon at that time, then it enters a different universe with different laws of physics perhaps.  Since the clock is moving forward in time at the same pace it was before, it seems things are not all that different on the other side.  It cannot send light back to the outside any more than I can illuminate Einstein with my torch.  I have no way of pointing my torch in that direction.

This is directly equivalent to a constantly accelerating object approaching the speed of light but never reaching it from the perspective of an inertial observer.

I don't think so.  There is not a moment beyond which the accelerating clock will not reach, and once reached, light from that moment will makes its way back to the inertial observer after some finite time.

It could be seen as equivalent to an observer who is accelerating constantly (as opposed to the inertial observation of something else accelerating).  Surely our inertial observer dropping clocks into a black hole will experience constant g force of some magnitude, else he'd fall in himself.
If I drop a clock out of the window of a continuously accelerating craft, the clock will appear to stop as seen by that observer, and will actually stop in his frame.  There will be a moment in time which that clock never reaches in the accelerating frame of the observer.  So the situation is very equivalent to continuous acceleration, at least from the perspecitive of the accelerated observer.
From the perspective of the inertial clock thrown out the window, it notices nothing as it passes through the ship's event horizon and happily keeps on ticking. That is sort of an argument that things do indeed pass the event horizon, even if not in any time that makes sense to the hovering observer.

The two situations are not entirely equivalent since the black hole sets up a curved gravitational field and the accelerating object sets up a flat one.  The black hole clock gets torn apart by tidal forces and the one dropped out of the ship window experiences no tidal forces.  That difference I think invalidates the claim that the equivalence demonstrates that objects enter black holes.  But as I said, I await opinions from those taking the opposite stance.  I'm no expert.

The resolution needed here is that if the answer to the first question is no then in the falling observers frame the time on their watch when they reach the event horizon would show a different time on the same watch when they reach the event horizon in the distant observer's frame. That's something that shouldn't be frame dependent.

I don't see this issue.  I see the same final time as seen by both observers (the falling one and the distant hovering/orbiting one).

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Technology / Re: Why are Some Fridges Hard To Open Again Once Just Closed?

« on: 04/03/2019 15:08:15 »

Let's say that the fridge interior is kept at 8 °C (281 K), and the air in your kitchen is 24 °C (297 K), so there is a fairly significant difference between the two (otherwise you wouldn't need a fridge!). When you open the door, the air mixes--for the sake of argument let's say that the air inside reaches 22 °C (295 K) by the time you are ready to close it.

Experiment time.  It seems pretty unlikely that air would warm up to anywhere near room temperature in a box full of cold items.  Worst case is an empty fridge: all air and no objects to cool the incoming warm air.
How full/empty is the fridge?  It it hard to open right after just peeking into it for a moment, or only after having the door open a long time?  Are you forced to wait 30 seconds, or does sufficient effort get the door open despite the resistance?  What do you hear when you force the door open early?  Air sucking in (vacuum theory)?  The seal un-sticking (flagpole theory)?

It is a known energy saver idea to put a collection of boxes or bottles (empty or full) to occupy unused volume in a refrigerator.  This prevents there being a lot of space for the warm air to move in each time it is opened.

Feature theory:  Exert nominal force to open the door immediately after closing.  It doesn't open, but keep on pulling at this force.  30 seconds later, the door opens willingly.  Do you hear anything (like a relay click) just before the door opens?  If so, there is a feature that holds the door shut for some reason.

My car's power steering does that.  All my other cars had belt-driven power steering, meaning if the engine is turning, I have brakes and steering, even if I have the ignition off.  Not this new car, which has belt brakes but electronic power steering.  If I turn the ignition off, I have steering for 3 more seconds, and I hear this little click and the steering wheel abruptly gets a lot heavier.

I do these sorts of experiments as part of test drives.  I learn far more tooling slowly around a parking lot than most people find out by driving normally on the streets.  Knowing what works and what doesn't when the ignition fails may heavily influence my desire to purchase a car.  I drove my parent's Ford Torino off the road once due to a stall turning at an intersection.  Impossible to steer or stop.

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Just Chat! / Re: Puzzles From A Trampolining Upside Down Sheeps Bottom (PFATSB) :-)

« on: 04/03/2019 00:14:18 »

R

Always have driven a stick.

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Technology / Re: Why are Some Fridges Hard To Open Again Once Just Closed?

« on: 03/03/2019 23:19:36 »

A long shot:  Condensation forms a thin film on the seal which sticks the door shut like a tongue on a flagpole until 30 seconds go by to let the low-humidity from the inside absorb that moisture.

Or maybe the thing has a timer lock to prevent immediate opening again, so of a dietary feature for those who keep opening it in hopes something better will materialize there this time.

My fridge has the opposite problem.  Opening the door allows the contents to shift, making it hard to close again once just opened.

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Physics, Astronomy & Cosmology / Re: determining length of day on Jupiter

« on: 01/03/2019 12:06:57 »

It is indeed not a trivial task.  Sure, one can time how long the big spot goes around, but since it moves with the winds, that can't be accurate.
They did it with detection of radio waves from Jupiter's magnetic field, which have very regular variations with the rotation of the core of the planet.

A crude way to measure it would be to take a measure of its oblateness and calculate the spin needed to get that value.  This might get you a couple digits of accuracy at best, but the radio wave thing gets a much more precise measurement.

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New Theories / Re: Expanded: The dust and gas that completely envelops black hole is SOLVED

« on: 01/02/2019 00:29:52 »

Dont say centigrade, the correct term is Celsius.

Well, when talking about temperatures in the millions, the correct term is Kelvin.  Celsius is only for temperatures meaningfully compared to the human habitat.

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Physics, Astronomy & Cosmology / Re: Does gravity attract masses in space, or does it curve space between them?

« on: 28/01/2019 00:09:57 »

Two black holes spinning around each other, faster & faster...converge...beep!
It sends ripples of gravitational waves out into space, Right?

A pebble/stone thrown in a pond, generates ripples.
Hence even though Water being transparent & tasteless it's still a Substance/Medium.

If those gravitational waves are travelling through space, is not Space a medium or substance of sorts???

Of sorts, perhaps.  The difference is that I can reach out of a boat and feel the water and detect if I am still or moving in that water.  You can't do that with space, per the principle of relativity.  The best they've been able to do is note the mean velocity of everything in sight and assign 'stopped' to that otherwise arbitrary velocity.

Are G waves travelling thru a gravitational force field in space & not just simply space itself?

They travel through space.  A field is not a thing, it is a mathematical abstraction, a value assigned to the relative depth of the gravity at every point in spacetime.  That slope of that depth determines the force that accelerates something at that point.  A flat depth would entail no acceleration, but it would still be a nonzero depth.

Is there a grav field all over n evrywher in space, even in super voids?

Yes, there is.  The field may be relatively flat there, but it is anything but zero.

P.S. - n Y did or rather wen did Einstein say if u remuv d earth below our feet, v wuld xperience free fallin???
Dat dosnt sound rite, Right?
Y wud v or an object free fall, fall is downwards rite, but no up & down in space...y won't v or d object just float freely in space?

That's what free fall is: Not having a non-gravitational force acting on you.  Here we have the Earth below us putting a non-gravitational force upward on our feet and preventing our free fall.

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General Science / Re: How does an aircraft altimeter work?

« on: 25/01/2019 12:49:53 »

Cherished light planes still take people to work after 50 years, and one occassionally encounters "warbirds" comprised mostly of bits of WWI hardware in the sky. GPS? Some of them don't even have electricity!

They have a 1909 Bleriot near me, claimed the oldest flying aircraft in the western hemisphere. I've seen it in the air.  No, it isn't a reproduction, and indeed, it certainly doesn't have GPS or probably an altimeter of any kind for that matter.
Makes me wonder what the eastern hemisphere has got that is even older.

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Physics, Astronomy & Cosmology / Re: What shape will foam take in space?

« on: 21/01/2019 13:08:28 »

We know that liquids in space will be spherical and solids don't lose shape.

Liquid (water at least) turns into frozen mist in space that quickly sublimes to gas.  You're apparently thinking about zero-G such as the environment inside the ISS, which is not in space any more than I am.

What about foams? What shape will they take if they were in a zero gravity environment?

Most foams (shaving cream, the foam insert in my sofa cushion) retain the same shape in zero G.  A pile of shaving cream (and the cushion for that matter) will squash out under enough gravity/acceleration, while in zero-G it could probably be sculpted into whatever fragile shape you like.

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Physics, Astronomy & Cosmology / Re: In simple terms, what is the Multiverse theory?

« on: 11/01/2019 18:40:33 »

In simple words ' what is the multiverse theory'??

There are a lot of theories which claim the term.  In short, a definition of 'universe' is given, and then for that universe to be consistent with some theory or interpretation, it turns out that other such universes need to exist.

So for instance, the universe can be defined as all that we can see and that can ever effect us or be affected by us.  That defines a spherical region that is a bit larger than the Hubble sphere.  There are principles that say the universe is pretty much the same everywhere, so if the edge of our universe is just like here, there must be stuff beyond, that they can see but we can't.  That implies that there are other spheres of space just like ours, totally causally disconnected from us.  They don't exist to us, but they exist nonetheless.
That's multiverse theory.

The next guy can believe exactly the same things, but defines the universe as all of space, not just the portions that can affect us.  To that guy, the universe is much larger, and all those other spheres are part of it.  Same view, but different definition of 'universe', so this would not be multiverse theory.

I can think of at least 5 different ways that you can consider other existing things to not exist to us (the one above being 'too far away'), and thus are part of the larger universe, or part of the multiverse.  The only distinction is how 'universe' is defined.  If universe is defined as 'all there is', then I suppose there is no multiverse theory.

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Physics, Astronomy & Cosmology / Re: Does time stand still in the quantum world?

« on: 05/01/2019 16:51:30 »

  If you treat objects as mathematical points, then there is no combustion and no tidal drift.  The reversed solar system would be mathematically perfectly a rewind of prior state. 

True, but it would not be the solar system we experience; would it?                                                                                                         

Well, we don't experience living on a mathematical point, so no, it wouldn't be the solar system we experience.

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Physics, Astronomy & Cosmology / Re: Does time stand still in the quantum world?

« on: 02/01/2019 11:41:04 »

I have had ,in the past  the belief that time required motion  to exist.However I have been assured that   clocks that rely on radioactive decay show that this is not the case.(no motion)

I had responded to the posted question, but not so much to the thread title.  This is an interesting point.

Given a universe with but one unstable particle, it will decay at some point, and it would be indistinguishable if it took a year or a microsecond to do that.  Time would be meaningless.  But given two different particles, suddenly one has a measurably greater probability of decaying than the other.  Time is still sort of meaningless, but that very real probability of decay makes it not totally meaningless.  That's just two particles, and already we can see 'classic' time emerging.

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Physics, Astronomy & Cosmology / Re: Does time stand still in the quantum world?

« on: 01/01/2019 23:46:53 »

On a large scale, the world we exist in and the universe our world exists in has been, and as far as I am aware is forever changing. Using a second as a measurement for instance, our universe has been changing every second since the big bang, it is changing every second as I sit here typing this, and will continue to change every second into the future. Would this be a legitimate way of describing what is meant by a past, present and future? Past, present and future = change? If so, would the same be said of the quantum world?

Time is change.  Without change, there would be no meaning to time.
Past, present, and future are local relations to a specific moment: If 3 events are ordered A, B, and C, then B is in the past of C and in the future of A, and each event occurs in the present of itself.
This is no different than floors of a building, where floor 12 is below floor 18, but above floor 7, and all three floors are 'here' in reference to themselves.

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Just Chat! / Re: The DOGMA of science........

« on: 21/12/2018 01:53:22 »

So, a clock in the lab here ticks 0.9999999993 times for every tick of a theoretical clock completely outside of a gravity well (and stationary).

Completely outside Earth's well.  Earth is a very small fish in a huge pond.  That value is not 'completely outside of a gravity well'.  You lost track of the condition upon which the value was computed, which was "Considering only Earth's gravity".
Bogie got at least one more fish included:

However, if that clock is in the solar system, its distance from the Sun also matters! On the surface of the Earth, the contribution from the Sun's gravity means that clocks are ticking about 0.00000000494 times slower than in "outer space", far from the Earth and the Sun. So the Sun actually slows clocks down more than ten times as much as the Earth, relative to clocks in deep space.

Removing just one more object is still not 'deep space'.  Just like the sun as 10x the dilation effect here as does nearby Earth, the sun's effect is dwarfed by the additional mass of what you're labeling 'deep space'.  There is no such thing because it really is impossible to get anywhere close to being out of that well, even between galactic superclusters.
I point this out since you seem to be attempting to compute what the absolute dilation factor of local clocks is, and you're considering only some of the least significant components to that answer.