1

Physics, Astronomy & Cosmology / Re: How did scientists measure the mass of the Earth and other planets?

« on: 14/10/2018 09:37:33 »

from: alancalverd on 12/10/2018 09:49:23Using F = GmM/r2 you can calculate the force on a falling object of mass m in terms of M, the mass of the earth, and G, which we assume to be a universal constant.

That doesn’t work.  We’re trying to compute at least a rough G and M here.  We don’t know either of them yet.  We do know force F is 9.8 newtons for a 1KG mass.  We can assume we know r.  We therefore know the product of G and M, but not either separately.

We don't "know" F = 9.8 N/kg. We define the newton as1 kg.m.s^-2, and have a measured value of g = F/m

We don't "assume" r: it is a measured value - see below.

As I said, Cavendish (1797) measured G directly. 

Maskeleyne estimated the density of the planet from an experiment in 1774 at Schiehallion, measuring the deflection of a plumb bob on two sides of the mountain. In fact, that was a measure of G rather than ρ_earth since the assumption that the mountain was homogeneous was more defensible. Newton had considered the possibility but rejected the experiment as too difficult.

As F = ma, we can measure the acceleration of a falling object or the period of a pendulum to get a value for F/m

F=ma works (F 9.8 = 1 (mass) * 9.8 m/sec acceleration), but that doesn’t yield either mass of Earth M nor G, which are the two things we’re trying to determine here.
The pendulum thing is a function of acceleration (9., not of the mass of Earth.  Put a pendulum in a rocket accelerating at that rate and it will have the same period as here on Earth.  It tells you nothing about the mass of the Earth under you.

You have missed the point. If we know F/m we know GM/r², and we have known r since 200 BC (Eratosthenes), so all that was required for an accurate estimate of M was Cavendish's determination of G. The reason for using a pendulum  is that we know the period T = 2π√(Lm/F) for small oscillations, which is a simpler experiment than measuring the acceleration of an object in free fall.

The most amazing fact in all this, hundreds of years and thousands of experiments later,  is that the gravitational masses of all bodies are identical to their inertial masses. Why?

Interestingly, Hipparchus estimated the radius of the moon's orbit around 130 BC, though his assumptions regarding the earth's orbit of the sun were somewhat wild.

2

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

« on: 13/10/2018 01:17:54 »

The illuminous region around the BH's gravitational field represents it's external boundary

There is a region called the "photon sphere", which is outside the event horizon (at 50% greater radius). If you emitted a flash of light here, you would see a series of light echoes as photons from the flash repeatedly orbit the black hole.

Light emitted within the photon sphere eventually finds its way into the black hole.
By itself, the photon sphere does not emit any light, so you could not call this region "luminous".
However, matter in an accretion disk around a rotating black hole is subject to severe shear forces, which does emit a lot of light (including infra-red at the outer parts, and X-Rays at the inner edge).
See: https://en.wikipedia.org/wiki/Photon_sphere

The illuminous region around the BH's gravitational field represents ... is a reflection of the incoming light striking a mass.

If there is no infalling matter (eg from a closely-orbiting companion star) then there is no mass between the photosphere and the event horizon (...if you ignore the hypothetical, microscopically thin, microwave-shifted skin of star matter that fell into the event horizon when the black hole first formed).
For a black hole that is not actively consuming matter, there is no luminous region around it.

This reflection is similar to what occurs on a star, The cooler substrata below the star's surface creates a barrier that the hotter corona reflect off of. During a coronal () we see the cooler substrata mass.

I am not sure what "coronal" event you are talking about - maybe a Coronal Mass Ejection, or the Corona that is visible during an eclipse by the Moon?
But in any case, the density of the corona is so low, and the corona's total light output is so low that it is only visible from Earth's surface during a total solar eclipse. (Space telescopes can see it at other times using a coronagraph - but they don't need to battle Earth's atmosphere...)

Apart from solar eclipses, all that we see of the Sun is the lower layers of the Sun's atmosphere - the photosphere (not to be confused with the photon sphere around black holes!). We don't need to wait for some special coronal event to see it. The photosphere is not solid, and it tends to absorb light rather than reflect it.

The photosphere has almost a black body spectrum of around 5500C, overlain with fine emission and absorption lines from elements in the Sun. It does not reflect the spectrum of the corona (which has a temperature in the millions C).
See: https://en.wikipedia.org/wiki/Photosphere

3

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

« on: 07/10/2018 22:29:18 »

(A Black Hole) cannot be made of normal matter.  That part is wrong.  Normal matter of that mass cannot support its own weight, and breaks down even before the black hole forms.

I think it is just a matter of grammatical tense here.
- We see mechanisms that form black holes from normal matter when a star implodes and/or explodes.
- However, once this matter approaches the singularity, normal matter should be torn apart - it's not clear what the components would be (eg would they be outside the Standard Model?)
- And once it reaches the singularity, normal matter should be mashed together - it's not clear what the result would be
- But really, we can't say much about what happens inside the event horizon of a black hole

Is a BH empty? If it rotates, the answer needs to be no.

We expect that most black holes will form with the angular momentum of their parent star, accretion disk or galaxy.
It is theoretically possible for a black hole to have near-zero angular momentum:
- If there was a merger of two black holes with opposite and equal angular momentum.
- Or transiently, if the black hole was tearing off the atmosphere of a nearby star which was orbiting in the opposite direction to the direction of black hole rotation.
- But I expect that most black holes would have considerable angular momentum

The black hole, is a singularity, it is comprised of unattached subatomic particles

A black hole singularity in general relativity is where all of the particles end up at a single point; all paths entering the event horizon end up at the singularity.

Some theories of Quantum Gravity may allow some quantum fuzziness about the "single point".

But Quantum Gravity is not yet sufficiently mature to assert that "it must be composed of subatomic particles that are not congealed in a structure".

a chiralty gravitational effect

Gravitational waves are polarized.

Once 3 gravitational wave observatories were online, it was possible to detect the polarization of gravitational waves.

The first one was: https://en.wikipedia.org/wiki/GW170814

But I don't understand  "a chiralty gravitational effect".

4

Physics, Astronomy & Cosmology / Re: Can the vacuum be affected by gravity?

« on: 23/09/2018 05:04:15 »

Yes. Recall Hawking Radiation.

5

Physics, Astronomy & Cosmology / Re: Does gravity have an infinite range?

« on: 21/09/2018 12:12:38 »

Some random thoughts: The Big Bang theory suggests a period of Cosmic Inflation, where for a moment, space expanded faster than the speed of light.

Gravity travels at the speed of light (in a vacuum). So for the moment before Cosmic Inflation, all matter was in the gravitational field of all other matter. This situation did not hold after Cosmic Inflation, nor does it hold for parts of the universe that are now beyond our Visible Universe.

So I suggest that:
- All of the universe once felt the gravitational pull of the rest of the universe, back at the Big Bang
- This was possible because the universe was not infinite, right after the Big Bang
- The effects of gravitational events that occur today (like colliding black holes) will propagate outwards at the speed of light, but will never reach the edge of the universe, as that is a receding boundary. So in this sense, the effects of gravity will have an infinite range in the infinite future (but also an infinitely small effect)...

Caveats:
- I really don't understand unification of the 4 forces in the early stages of the Big Bang.
- I really don't understand how quantization of gravity will affect this

6

Geology, Palaeontology & Archaeology / Re: To what extent can Biblical accounts of Noah's flood be believed?

« on: 20/09/2018 12:48:55 »

None of the stories in the old testament are to be taken as fact. My understanding is that when they were first written (over the course of multiple centuries, by different groups of people http://www.ancient-hebrew.org/articles_authors.html ), no one expected the stories to be taken literally (in fact the bible contradicts itself multiple times--allowing for much more complex analysis by the scholars--and contradicts physical possibility many times as well). Instead the stories are meant to be fables, and only in the centuries after the new testament did it become mainstream to believe that it actually happened exactly as it was written. (possibly due to the philosophical influences of christian and muslim schools of thought.)

7

Physics, Astronomy & Cosmology / Re: What do strange quarks do?

« on: 13/09/2018 11:54:33 »

the neutron lifetime puzzle

I wondered why I kept seeing different figures for neutron lifetime.

Not the 9 second difference between beam and bottle experiments.

But the larger difference between half-life and mean lifetime...

8

Physics, Astronomy & Cosmology / Re: What would a cold neutron star look like?

« on: 14/08/2018 23:20:42 »

create geometries very unlike such oxides at standard conditions

I agree that there will be crystal structures and molecules that would never be stable on Earth.

One unique thing that we don't have to worry about on Earth is that the gravitational potential energy on the surface of a neutron star is greater than the strength of mere chemical bonds. So elements would be sorted vertically by density. Chemical bonds would only be possible between atoms of very similar density.

The lightest element is hydrogen, and so it would be in the outer layer. Under these pressures hydrogen would probably form a metallic state, possibly superconducting?

A more speculative distorting factor is that the tidal forces and time dilation effects on the surface of a neutron star may be enough to distort the form of the electron's wave equation so that it takes on different forms than we have on Earth. What this would do to the interaction between light and matter is beyond me!

9

General Science / Re: Has special relativity been refuted?

« on: 10/07/2018 17:57:00 »

But the apparent speed of the CMBR is the same no matter which way we look at it. Velocity differs from speed in that it includes a direction. It looks to me that your reasoning would indicate that we are moving away from the source of the CMBR at roughly 368 km/s, but because the source is all around us, that presents a paradox: in which direction are we moving 368 km/s?

Not quite
https://en.wikipedia.org/wiki/Cosmic_microwave_background#CMBR_dipole_anisotropy

10

Physics, Astronomy & Cosmology / Re: Is this formula valid?

« on: 07/07/2018 21:35:20 »

So what if we have the following formula?

Δλ = 2πγ/k

Can we introduce gamma to modify the wavelength due to relative velocity? Can this be a valid way of viewing orbitals?

No
Orbitals have three dimensional shapes and also odd properties that have to fit with the uncertainty principle.

11

Physics, Astronomy & Cosmology / Re: The Graviton?

« on: 05/07/2018 21:46:55 »

However, real gravitons propagate away to infinity, as oscillations on the gravitational field. This is what LIGO detected. (And real photons propagate away to infinity, as oscillations on the electromagnetic field. This is what telescopes detect.)

LIGO detected gravitational waves as predicted by EFE not gravitons. Gravitons are theorized to exist and have such low energy levels that we can not detect them with todays technology.

If gravitons exist (Gravity can be explained with a quantum theory), then gravitational waves would consist of gravitons.

Bosons can not escape a BH, a graviton is a 2 spin boson how can it get out of a BH. A gamma ray cannot escape the event horizon of a BH and it has significantly more energy.

Gravitons can't,  but since gravity would be mediated by virtual gravitons, this is no different from the fact that photons can't escape a BH, yet a BH can have a charge and an electromagnetic field mediated by virtual photons.

Gravitational waves definitely exist gravitons might not exist, and might not have enough energy to escape a BH.

Gravitational waves can't escape a BH either (By this I mean that gravitational waves cannot get out from inside the event horizon anymore than electromagnetic waves can.)

12

Just Chat! / Praise for Evan_au

« on: 28/06/2018 11:16:09 »

@evan_au  just received on the Naked Scientists Twitter:

Project STREET WISE
@wise_project 50m50 minutes ago

I really enjoy reading evan_au comments on the @nakedscientists forum! 🤖 #isotoperesearch

13

Chemistry / Re: Is it possible to create a gelatinous metal?

« on: 26/06/2018 14:37:53 »

This may be of interest.
https://www.sciencedirect.com/science/article/pii/S0010854513000131

Interesting find, but I don't think it is directly relevant--the gelators discussed in the article contain metal atoms, but they are not metallic. Rather, they are organometallic or metal organic complexes (these are the sorts of compounds I study, though not for making gels).

14

Physics, Astronomy & Cosmology / Re: Why is energy observer dependent?

« on: 24/06/2018 13:15:33 »

This states that L = V + U.

Correction. L = T - V, where T = Kinetic energy and V = Potential energy. If the potential is a velocity dependent one then the letter U is used. This is particularly important in electrodynamics.

15

Physics, Astronomy & Cosmology / Re: Is it possible for something to absorb light without reflecting it?

« on: 21/06/2018 12:12:46 »

A "black body" comes close to a perfect absorber. If you drill a small hole in a large hollow sphere, almost everything that enters the sphere stays inside it.

16

Physics, Astronomy & Cosmology / Re: Can we measure the one way speed of light?

« on: 20/06/2018 17:41:33 »

the fabric of space

Please set down the physical characteristics of this wondrous fabric. Density? Elastic modulus? Viscosity? ε_FS and μ_FS?

If I can move relative to it, what is it fixed to?

17

Physics, Astronomy & Cosmology / Re: Can we measure the one way speed of light?

« on: 20/06/2018 07:30:06 »

Wholly different experiment, wholly different result.

In my experiment the source does not move during the measurement.The only moving part is the locally oscillating part of the receiver, and the measurement is of the Doppler shift between the fixed and moving parts of the receiver when the direction of the incoming beam is fixed.

Your "ring experiments" are also irrelevant. c is the speed of light in vacuo, not in a medium. c_m is subject to all sorts of variations.

18

Physics, Astronomy & Cosmology / Re: Can time emerge?

« on: 16/06/2018 13:32:11 »

A related concept (looking through the other end of the telescope): Can time disappear?

This is subject to the same proviso as is the original question; namely, how do you define time?

If time is just a measure of change, the question should be: can change "disappear"?

19

Physics, Astronomy & Cosmology / Re: Can we measure the one way speed of light?

« on: 15/06/2018 16:58:39 »

David c.#76

The house example is only intended to show all observers aren't required to have the same perception or point of view, even without motion.
---------

and with such a model you get event-meshing failures unless you go to a block universe model,

 
-----
There are no such things as 'event-meshing failures' (your invention).
-------

Stop pretending that SR makes no such claims. SR claims that there is no aether, and in doing so it ceases to be a mere theory of perceptions.

---------
I don't have to pretend. All experiments to detect any effect of an ether have failed. Einstein called it superfluous and developed SR without it, yielding the same coordinate transformations as developed by Lorentz.
If you understand perception, then you know the brain is a complex process which behaves according to rules of physics, specifically chemistry. That is also 'reality'. Medical science can monitor this activity and 'map' certain areas to a corresponding activity. It's not just inanimate clocks that run slow when moving fast, but also biological clocks.
---------

one-way speed of light is always c relative to any object it passes

---------
SR does not state that. It does state "the measured speed of light relative to an inertial frame is c. In #72  the relative speed of light was overlaid in the A frame to show the round trip time (2.31) would be the same.
If U and A had the same perception about everything, why have transformations?
------------

No - it's plain contradiction. If its true that light passes A at c relative to A and that it also passes B at c relative to B while A and B are moving relative to each other along the same line, you must have light overtaking itself as it travels between A and B because it's moving faster than itself.

---------
Your interpretation is flawed. You are not compensating for time dilation and length contraction for both A and B. Those motion induced effects alter perception.

20

The Environment / Re: Is the greenhouse effect a stupid mistake?

« on: 15/06/2018 03:40:59 »

So if a CO2 molecule absorbs outgoing radiation at x nanometers, it also emits at x nanometers,

This bit is essentially correct. (there are several complications having to do with non-radiative energy transfer, rotational-vibrational coupling, and doppler shifts, but these wrinkles don't change the overall effect)

so has no net effect on amospheric temperature. Doesn't smell right to me.

This bit isn't correct, and shouldn't smell right

The key here is that the direction of the emitted light is essentially random. This means that any EM of the appropriate frequency can't have a direct path from the ground our to space. Instead, it must take a random walk in which the mean free path is determined by the molar extinction coefficient of the absorber and the concentration of the absorber--and since the molar extinction coefficient is fixed, increasing concentrations of absorbers lead to shorter mean free paths, and increasing delays before the EM leaves the atmosphere.

To maintain thermal equilibrium, the rate at which energy leaves the Earth must be the same as the rate at which energy comes in (which is largely constant). So if it takes longer for a packet of IR energy to get from the ground to space, to maintain this same rate of energy going out, there must be more packets of IR energy in transit simultaneously. This is effectively the same as saying that the atmosphere must be hotter (at least where the increase photon density is). (ie it does have an effect on atmospheric temperature)