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Physics, Astronomy & Cosmology / Re: How does time at the Event Horizon differ for approaching vs Distant Observers?

« 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).

522

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.

523

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.

524

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.

525

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.

526

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.

527

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.

528

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.

529

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.

530

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.

531

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.

532

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.

533

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.

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Physics, Astronomy & Cosmology / Re: Is there a detectable direction in which the universe is expanding?

« on: 20/12/2018 22:13:13 »

If the universe was 'flat' then I assumed that there must be an observational spatial direction in order to determine it was flat.

Flat means that parallel lines stay the same distance apart, in any direction.  Yes, direction matters, but our space seems symmetrical this way.  With positive curvature (like the balloon), parallel lines meet.  With negative curvature (like kale), parallel lines diverge.

535

Physics, Astronomy & Cosmology / Re: Is there a detectable direction in which the universe is expanding?

« on: 19/12/2018 22:56:48 »

If the universe is flat then is it possible to detect a direction in which the expansion is travelling? Perhaps taking the earth/solar system as a point of reference.

From any spatial point of reference, like here, the universe spatially expands equally in every direction.

From an objective viewpoint, I suppose the balloon analogy is appropriate.  Earth is a dot on the balloon, and all the other stuff is other dots drawn elsewhere.  As the balloon expands equally, every dot moves away from a given reference dot in proportion to its distance from that reference dot.  This is exactly as we observe things.

So is there a direction to the balloon expansion?  Yes, but it is outward, not in any spatial direction that you can point with an arrow on the balloon surface.  In the same way, there is no arrow you can make showing the expansion direction.  The direction is outward in that 4th direction that you can't point with a spatial arrow.

536

Physics, Astronomy & Cosmology / Re: What does the term 'time' relate to in time dilation?

« on: 09/12/2018 17:43:30 »

A “timely” thought experiment

“In a galaxy far away”, there exists a star identical to our sun, with an identical “solar system”.
Earth is planet A with physicist A (PA) and clock A (CA).
Earth’s twin is planet B with physicist B (PB) and clock B (CB).
Each physicist can see his own clock and the other’s with no time delay for information transfer.

This last one is unrealistic unless both clocks are stationary relative to each other, in which case both clocks will run at the same pace in any frame.

If they're moving apart as distant things tend to do, then each clock will run slower in the frame of the other, but they will appear to run even slower than that due to Doppler effect.  That Doppler effect is why is it unrealistic to assume no delay in looking at each other's clocks.

2.  Each physicist sees the rate of time on the other’s clock as unaffected by gravity.

No.  If there was another clock in the same frame as the distant clock but not as deep in a gravity-well such as these planet-bound physicists, the clocks on any planet would appear to run slower.  Hence they all appear to be affected by gravity, but equally since both CA and CB are in similar gravity wells.

3.  Any difference between the perception of the rate of time between CA and CB, observed by either physicist, will be due to the degree of relative motion between the two galaxies; or relative motion resulting from differential motion within the individual galaxies.

Yes.

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Physics, Astronomy & Cosmology / Re: Do photons have mass?

« on: 04/12/2018 22:45:25 »

If a photon has mass does it too increase when it travels at light speed?

It doesn't increase with speed since it always moves at c.  It increases with frequency, which makes the photon mass still frame dependent.

I’m just trying to get my head round the idea of proper mass, inertia etc, in another thread; so I’m going to have a go at this, to see how I’m doing.

I may be confusing “rest mass” with “proper mass” (possibly they are the same), but let’s start with rest mass.

They're the same, yes.

This is the mass of an object as measured by an observer who sees that object as stationary. We can never observe a photon as being stationary, so we cannot assign it a rest mass.

However, the photon has inertia, so it adds to the inertia of any system of which it is a part.  Because of the relationship between inertia and mass, the photon must, therefore, contribute mass to that system.

All sounds pretty good to me.

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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.

539

The Environment / Re: Could the Earth ever be completely underwater?

« on: 29/11/2018 13:20:49 »

Another possible scenario is when the Moon was much closer than it is now.

Tidal forces follow an "inverse cube" law, so when the Moon was 10x closer, tidal forces would have been 1000x stronger - imagine a +/-1km tide! That would create an incredible amount of erosion - perhaps even enough to erode mountains faster than tectonic forces could push them up?

Interesting choice of distance.  At 10x closer, it puts the moon about exactly at geosync, so there would be no tides at all.  If it was just a little off, yes, there would be time to form tides at full height, but also the crust of earth would bend back and forth, heating it dramatically.  Maybe the heat of that would boil away the water.

Can you explain how that works please

From wiki on tidal force:
"The tidal force acting on an astronomical body, such as the Earth, is directly proportional to the diameter of that astronomical body and inversely proportional to the cube of the distance from another body producing a gravitational attraction, such as the Moon or the Sun.
...
Figure 3: [Graph showing a simple plot of Y=1/X²]
...
In this graph, the attractive force decreases in proportion to the square of the distance, while the gradient (slope) decreases in direct proportion to the distance. This is why the gradient at any point is inversely proportional to the cube of the distance.

The tidal force corresponds to the difference in Y between two points on the graph, with one point on the near side of the body, and the other point on the far side. The tidal force becomes larger, when the two points are either farther apart, or when they are more to the left on the graph, meaning closer to the attracting body.

For example, the Moon produces a greater tidal force on the Earth than the Sun, even though the Sun exerts a greater gravitational attraction on the Earth than the Moon, because the gradient is less. The Moon produces a greater tidal force on the Earth, than the tidal force of the Earth on the Moon. The distance is the same, but the diameter of the Earth is greater than the diameter of the Moon, resulting in a greater tidal force."

540

Plant Sciences, Zoology & Evolution / Re: Why do most humans have traces of Neanderthal DNA?

« on: 27/11/2018 17:31:01 »

It seems that human DNA (except native African) contains a small percentage of Neanderthal DNA

It is actually only a very small group in an isolated tribe in Africa that has remained free of Neanderthal DNA.
If the Germans wanted a pure race, that tribe has a far better claim to it than do the Aryans.  Fair skin and hair (especially red hair), blue eyes, and all that are traits from the Neanderthals, who evolved earlier on to colder climates.