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Physics, Astronomy & Cosmology / Re: Are all the planets in the solar system the same age?

« on: 13/02/2019 08:33:03 »

The Solar System formed by a cloud of gas and dust contracting under its own gravitational field.
- It is a positive feedback system, in that the more the gas contracts, the more intense its gravitational field, so the more it contracts...
- This contraction is opposed by conservation of momentum, which tends to keep the gas in an orbit around the center of mass, rather than converging on the center of mass
- This orbiting gas is opposed by the many random directions of patches of the orbiting gas, which collide with each other, heating up. The increased temperature also opposes contraction.
- This excess temperature is radiated more easily if there are heavier elements mixed in (from carbon to iron), as a result of previous supernovas seeding the dust cloud with heavier elements
- The collisions cancel out different angular momentum, dumping gas closer to the center (which eventually forms the star)
- While some gas remains on the outskirts of a flattish disk, forming the planets
- This protoplanetary disk has been observed by radiotelescopes

So the whole gravitational collapse thing happens together - you don't need the Sun to form first.
- In fact, the orbit of a Jupiter-sized object is determined by the mass within its orbit
- It doesn't matter whether that mass is compact or fuzzy
- or whether it is fusing hydrogen or not

See: https://www.almaobservatory.org/en/press-release/almas-best-image-of-a-protoplanetary-disk/
https://en.wikipedia.org/wiki/Protoplanetary_disk

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New Theories / Re: There is no scientific method

« on: 03/02/2019 09:45:07 »

I have no idea on how a clock could have a memory of its history of acceleration

A clock does have a memory - the time now is 1 second more than it was 1 second ago (in the frame of reference of that clock).

If you subject a clock to varying acceleration, various gravitational fields and various velocities, then these changes will have an impact on the speed at which this clock runs (from the viewpoint of an external observer).

So the clock integrates the cumulative effect of all these effects over the operational lifetime of the clock.

So a clock does have a history of it's acceleration (and velocity, and depth in gravitational fields).

3

Geek Speak / Re: What software does the BBC use to transmit video over the Internet?

« on: 02/02/2019 22:09:46 »

Video quality is affected by the loss of data packets from the video stream, causing pixellation (blocky picture), pauses, misalignment of moving images and jumpy motion.
- This "Frame Loss Ratio" (FLR) is higher at times of peak data usage - typically around 7pm in residential areas (increasing from 4pm, declining after 10pm).
- FLR also can occur for reasons related to corrosion in your own home wiring, your microwave interfering with your WiFi signals, or activity of your neighbour's WiFi connection. Optical fiber connections are the best because they are immune to this type of interference.
- FLR can also occur because some device like a smartphone or a computer suddenly decides for itself that it is going to download a software update that may be 1 Gigabyte in size

Early types of video broadcasting on the internet just sent the packets at an encoding rate that was set for a particular show
- The rate varies with the content - rapid motion requires a much higher data rate than a static image. These peaks could exceed the capacity of your internet service, causing FLR during rapid motion of the image.
- There was no checking that your line is able to cope with these peak rates
- In contrast, Netflix and other more recent video services have been smarter
       - If frames are lost, they retransmit them (effectively no FLR)
       - they check the capacity of your line on an ongoing basis, and at times of congestion will reduce the data rate on your line. This makes the picture a bit more "blocky" for a few seconds, but this is less noticeable than the show stopping entirely

Most video services don't deliver the video themselves, but contract to a "Content Delivery Network" (CDN) to deliver the video.
- Akamai is one of the largest CDNs, but is almost unknown to the general public, despite the fact that the public use it every day.
- Amazon is also large, but only known because of their online store, not for CDN services

https://en.wikipedia.org/wiki/Content_delivery_network

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New Theories / Re: Biology is not a science

« on: 01/02/2019 21:48:36 »

I would say that "Biology is becoming a science".

40 years ago, before gene sequencing was really practical and efficient, biology was in a similar state to:
- Astronomy, before Kepler's Laws
- Chemistry, before development of the periodic table
- ie lacking a coherent basis for understanding what is going on

Now that gene sequencing is much more affordable, we can see the places where genes reside (and some non-coding regions), but we don't know what most of them do. This is like filling in the holes when:
- Astronomy: You have some planets you know about, but there are hints that there may be more out there...
- Chemistry: You have a periodic table, but it has gaps you are trying to fill

But, like other sciences, biology plays out at many levels - you can have a qualitative understanding of one level, which becomes a deeper understanding when you understand the adjacent levels:
- Biology: You can see and understand the behavior of whole organisms, but it becomes more complete when you understand cells - and even better when you can read the DNA
- Astronomy: You can see the stars, but you understand them better when you understand how nuclear fusion works
- Chemistry: You can understand mixing chemicals in the lab, but you understand it better when you know about atoms, and even better when you understand the quantum behavior of electrons and nuclei.

However, we are still making slow progress on the most complicated level of biology - whole ecosystems. We can see the individual organisms, but how they will interact in the future is beyond us...

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Question of the Week / Re: QotW - 19.01.28 - Why do candles smell once they're blown out?

« on: 31/01/2019 20:17:22 »

As a sidenote:
In a candle flame on Earth, Gravity+Convection plays an important role in taking away the hot products of combustion above, and feeding in cool oxygen from below, vaporising the wax and sustaining the flame.

However, in microgravity on the ISS, heavy fuel molecules must passively diffuse away from a wick, while light oxygen must passively diffuse from outside, producing a sphere of flame far from the wick. Deprived of heat from the flame, the wax stops vaporising, and the products of combustion smother the flame...

A recent experiment on ISS showed a new form of invisible "cool" combustion.... cool!

The action begins at 1:50

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Technology / Re: Can we make a robot that could make a smaller robot that made a smaller robot?

« on: 30/01/2019 08:46:32 »

You can't just scale up a robot:
- Let's say that you make it twice as big in every dimension
- It's mass increases by a factor of 8
- But the cross-section of its supports grows by only a factor of 4
- So eventually, it will collapse under its own weight (after a couple of doublings)
- Similarly, a larger robot will have increased trouble dissipating heat, as the surface area only grows by a factor of 4

You can't just scale down a robot:
- At small scales, the viscosity of air becomes more significant
- At the size of a bee, air acts much thicker - more like swimming than flying (this is related to the Reynolds number)
- At small sizes, overcoming viscosity is a major issue

So, in practise, you need to change the scaling of the robot to adapt to the changed conditions.
- And/or you may need to scale the environment too...
- If you reduce the size of the robot, you may need to reduce the air pressure
- If you increase the size of the robot, you may need to reduce the gravity(!)

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Physics, Astronomy & Cosmology / Re: How do we measure the energy of a photon?

« on: 28/01/2019 09:19:29 »

the uncertainty principle states either location or velocity can be determined, approximately.

There are several versions of the uncertainty principle.

But if you are only interested in the energy of the photon, and don't care too much about the position at which it arrived, or the time that it arrived, then you can measure the energy moderately accurately, even for (80% of) single photons.

Astronomers use CCD detectors that:
- Have a large fraction of the surface area available for detecting photons; the remainder is covered by wires, and some photons will be reflected rather than absorbed. Something like 80% of incoming photons can be detected.
- Are cooled to low temperatures, so there is a very low "dark current", and few spontaneous detections where there was no incoming photon (ie a "false alarm")
- Can be coupled to a large-area diffraction grating, which splits light into different wavelength/energy bands. There are several locations that a photon of a given energy could arrive, but if you put a CCD at all of them, you can estimate the energy regardless of where it arrives.

For this to work well, the energy of the incoming photons needs to be constrained within a reasonable range, otherwise a photon could be confused with one of a much higher or lower energy.
See: https://en.wikipedia.org/wiki/Charge-coupled_device#Use_in_astronomy
https://en.wikipedia.org/wiki/Diffraction_grating
 

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Physics, Astronomy & Cosmology / Re: Did inflation break the light barrier?

« on: 25/01/2019 21:25:05 »

The "light barrier" refers to the finding from Einstein's Relativity that it would take an infinite amount of energy to accelerate a massive object (like a proton or a planet) up to the speed of light - let alone beyond the speed of light.

However, space itself is not a massive object. So this "loophole" is used to explain how space itself could expand faster than the speed of light.

The motion of matter is measured relative to its position in space. So if space expands faster than the speed of light, the matter embedded in that space does not feel the acceleration, and matter does not violate the light barrier.

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

« on: 25/01/2019 10:01:37 »

Traditional altimeters measured air pressure.

Today's, most altimeters use GPS, which don't need recalibration every time the weather changes.

See: https://en.wikipedia.org/wiki/Altimeter

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Geology, Palaeontology & Archaeology / Re: How can you tell silicon from germanium non-destructively?

« on: 23/01/2019 09:47:42 »

The two elements have a different band-gap (Si 1.1V, Ge 0.7V), which means that they will absorb light of different wavelengths.
- The lustre is in the visible band
- But they become transparent at different wavelengths in the infra-red band

They also have a different refractive index.

If it is in the form of a semiconductor device, the forward voltage drop of germanium is lower than that for silicon (all other things being equal), as a result of the difference in bandgap.

Silicon is used more often than germanium in today's mass-production semiconductors as it is able to operate at higher temperatures with lower leakage currents.

11

Physics, Astronomy & Cosmology / Re: Optics/visibility question

« on: 16/01/2019 00:34:23 »

Our eyes detect the edges of objects by changes in color and brightness.
- Snow is a uniform color
- When there is an overcast sky, lighting is uniform, there are no shadows, and there is little variation in brightness

If we are close enough to objects (15 feet or 5 meters), our eyes can pick out additional cues due to binocular vision - but you need some variation in color, brightness or texture to allow your eyes to "converge" properly.

However, if you are near trees, that provides:
- A strong difference in color
- A shadowing effect, so that objects are no longer uniformly illuminated
- Some clear edges to assist convergence

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Physics, Astronomy & Cosmology / Re: Do we know the mass density of the observable universe?

« on: 15/01/2019 18:06:26 »

Do we know the mass density of the observable universe?

Yes, this is something that astronomers have attempted to measure and calculate many times over the past century.
- This is an important parameter in Einstein's General Theory of Relativity, since it dictates the future of the universe:
- Collapse back into a "Big Crunch"
- Or expand forever in a darkening sky

With increasingly powerful telescopes, it is possible to survey large parts of the universe for glowing gas - powered by stars
- Examination of the rotation rates of nearby galaxies showed that there was far more mass than could be accounted for in stars
- Radiotelescopes can detect clouds of neutral hydrogen at various distances in intergalactic space between us and distant quasars
- There have been surveys using gravitational lensing looking for black holes or free-floating planets. Some have been found, but not nearly enough to account for the missing mass
- We now call this missing mass "Dark Matter"
- Studies of nuclear processes in the Big Bang, and today's Cosmic Microwave Background Radiation allows cosmologists to place fairly tight limits on the density of matter in the universe, both normal matter and Dark matter

In all the studies, the density seems to be just above the magic value that would prevent a Big Crunch.

However, the more recent discovery of the accelerating expansion of the universe (attributed to Dark Energy) means that the density of matter in the universe is today not the dominant factor determining the future of the universe.

But astronomers are still looking for ways to determine the density of the universe, and the nature of Dark Matter and Dark Energy.

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Physics, Astronomy & Cosmology / Re: When Vacuum Is Not A Vacuum?

« on: 14/01/2019 09:08:35 »

As defined Vacuum is a space entirely devoid of matter

I guess that is a reasonable theoretical definition.

In practice, it is more useful to think of a vacuum in terms of the pressure of residual gas inside the chamber, it's temperature, and it's chemical/ionic composition.
- Whether you consider this a true vacuum depends on the application
- To the human body, the effects of exposure to the atmosphere of Mars (about 1% of the air pressure at Earth's surface) would be almost indistinguishable from the effects of exposure to interstellar space.
- If you are running the Large Hadron Collider, the air pressure of Mars would immediately block the proton beam.
- There are a variety of applications that require a vacuum between the atmosphere of Mars and the vacuum in the LHC
- There have been experiments put into space that delivered a better vacuum than that in the LHC. The simplest of these was a simple metal plate, orbiting the Earth at around 28,000km/h, blocking any gas molecules that might reach the experimental bay. (Of course, you need to make the metal plate out of substances that won't boil off atoms or molecules into the space behind the speeding plate...)

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Physics, Astronomy & Cosmology / Re: What would happen if the axis of the earth was tilted to up to 45ish degrees?

« on: 08/01/2019 19:36:08 »

The Tropics are reqions of the Earth which have a fairly uniform length of day throughout the year, and fairly uniform temperatures. There is no "Summer" or "Winter" as such - but there is often a "Wet" and a "Dry" season. This is all due to the fact that the Sun is never far from overhead throughout the year.

For convenience, the tropics are defined as places where the Sun is directly overhead at some time of the year.

If the Earth's axial tilt increased to 45 degrees, the length of day would change noticeably, and temperatures would vary significantly throughout the year, even for someone living directly on the equator (in fact, such a person would have 2 summers and 2 winters every year!).

Our current concept of "The Tropics" would be obsolete.

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Physics, Astronomy & Cosmology / Re: How remiss is our attitude towards entropy?

« on: 08/01/2019 19:11:03 »

If entropy is increasing universally, it is proceeding to equilibrium.

The universe at present is proceeding towards equilibrium with the CMBR at 2.7K.

However, in another 14 billion years or so, the CMBR will be down to perhaps 1.3K (depending on what Dark Energy gets up to in the meantime). The universe will then be proceeding towards equilibrium with this new, lower temperature.

This is described as the "Heat Death of the Universe", which, ironically, is currently expected to be cold and dark.
See: https://en.wikipedia.org/wiki/Heat_death_of_the_universe

the cosmic radiation background of the Universe's temperature is remarkably consistent. This to me, maybe not to you, presents an ordered system.

The great consistency of the CMBR temperature across the sky suggests that the early universe was in thermal equilibrium, at the time of last interaction between light and matter, which is thought to be around 300,000 years after the Big Bang.

The theory of cosmic inflation suggests one way that this thermal equilibrium could have occurred.
The very tiny variations in CMBR across the sky have been explained in terms of quantum fluctuations in this thermal equilibrium.
See: See: https://en.wikipedia.org/wiki/Inflation_(cosmology)

I recently read "Just 6 Numbers" by Martin Rees (year 2000), in which he suggested that some of the CMBR variation across the sky could be due to gravitational lensing by distant masses in the universe, effectively magnifying parts of the CMBR. I can see that this would increase the intensity of the CMBR, but should not change it's temperature? Is this still a current theory?

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

« on: 06/01/2019 08:35:41 »

In this perfect reverse universe with all the photons magically heading back to the sun

The trick of reversing time, and seeing if it makes sense says:
- Q1: If the Solar System were reversed in time, would the planetary orbits look normal?
- A1: Yes, The planetary orbits would look pretty normal.
          But if you looked closely at the orbit of the Moon, spiralling inwards is not normal, since tides dissipate energy as heat; the Moon should spiral outwards.

- Q2: If the Sun were reversed in time, would it look normal?
- A2: No, this would look very abnormal. Photons from across our galaxy (and beyond) do not spontaneously fly back towards the Sun. Heat does not spontaneously move from cooler places (the visible surface of the Sun) to hotter places (the center of the Sun). Helium does not spontaneously break down into Hydrogen. 

- Q3: If the Earth were reversed in time, would it look normal?
- A3: No, this would look rather abnormal. Heat does not spontaneously move from cooler places (the surface of the Earth) to hotter places (the center of the Earth). Cows do not regurgitate grass. Grass does not emit photons, turning complex chemicals like chlorophyl into carbon dioxide, water and nitrates. Random vibrations of the entire Earth's crust do not synchronise and converge at a point and move huge blocks of the Earth's crust.   

So parts of this macroscopic universe like the orbits of the planets are (close to) ideal, lossless, reversible systems.
But other parts like the Sun and the Earth have a clear direction to time's arrow, as dictated by entropy, and are clearly not reversible.

A Feynman diagram is time reversible

I agree that some simple Feynman diagrams with just 2 interacting real particles can be ambiguous in the direction of time.

But I suggest that by the time you get to the interaction of 3 or more real particles, a Feynman diagram provides a pretty clear idea of the direction of time (but not an absolute guarantee).

Take the decay of a neutron in a vacuum:
n⁰ → p⁺ + e⁻ + ν_e + 0.78MeV

1) The right hand side has 3 real particles which exist at the same time. The odds against these particles just happening to come together in the same place at the same time in a vacuum to form a neutron is so unlikely that it gives a clear direction to time's arrow.

2) The reaction releases 0.78MeV of energy, which is randomly split up amongst the products, and heads off in 3 different directions into the universe. Energy does not spontaneously gather from different parts of the universe and combine at a single point. This gives a clear direction to time's arrow.

See: https://en.wikipedia.org/wiki/Free_neutron_decay
This link shows a Feynman diagram for this reaction (with an arrow of time...)

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

« on: 05/01/2019 07:53:07 »

In spite of the presence of the word “tends”, the assertion “it never decreases” can still be found attached to many explanations.

If you have a small system, like the 4 coins mentioned by ChiralSPO, then entropy will (sometimes) decrease back to the original 4H. But it will be somewhat uncommon - the most common occurrence would be to have 2H + 2T, with other combinations being less common.

If you have a larger system, like 1 million coins, the most common combination (after allowing a few million flips for it to stabilise) will be around 500,000H+500,000T; combinations like 1,000,000H will almost never occur.

Now turn that into something practical that you can pick up with your hand, like 6x10²³ atoms, and you can say that returning to the original state will practically never occur. Hence the use of the emphatic "never".

It is possible to manually flip all the coins back to the original all-H state, but this takes information and hence energy. So you can reduce entropy by expending energy, but left to itself, entropy tends to increase in macroscopic systems (at any temperature above absolute zero).

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Physiology & Medicine / Re: Are there any naturally occurring proteins not encoded in DNA?

« on: 03/01/2019 08:38:45 »

A lot of proteins and enzymes found in the human body are not coded in the human DNA.
...because our microbiome has DNA code for more proteins than does human DNA

To a lesser extent, some proteins are coded in viral RNA (ie not DNA)

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

« on: 02/01/2019 09:19:05 »

Past, present and future = change?

"Change" tells you that time exists.

But "change", by itself, doesn't explain the other major characteristic of time: That it seems to travel in only one direction.

For this you have to look at entropy: Time flows in the direction where entropy increases.
- You can't tell the direction of time by looking at a single particle
- You often can't tell the direction of time by looking at the interaction of 2 particles
- But by the time you look at the interaction of 3 or more (real) particles, the direction of time is usually apparent

See: https://en.wikipedia.org/wiki/Entropy_(arrow_of_time)

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Physics, Astronomy & Cosmology / Re: Do our galaxies expand?

« on: 01/01/2019 20:08:48 »

what is the boundary of our galaxy?

That is a question that I am sure astronomers will debate for a long time, before drawing an arbitrary line
- Like the arbitrary Kármán line that says "Space starts at an altitude of 100km", even though Earth's atmosphere extends far beyond that, getting thinner and thinner the higher you go.

Stars get thinner and thinner, the farther you travel from the center of our galaxy
- There are some "dwarf galaxies" that are orbiting our Milky Way galaxy
- This list shows 38 of them closer than Andromeda galaxy
https://en.wikipedia.org/wiki/List_of_nearest_galaxies
- These dwarf galaxies passing through our galaxy would have left a faint trail of stars behind them, and would have tugged a faint halo of stars out of our galaxy
- Traditional telescopes can't see these individual stars, and can't even detect these low-luminosity patches of light, since they are fainter than the night sky, even on a good night in a good location
- But some new types of survey telescopes are better at detecting low-luminosity patches of stars, when looking away from the galactic disk, and may succeed in extending the fuzzy boundary of our galaxy
- but they won't extend the boundary very much in the galactic plane, since there are too many stars and patches of dust in the galactic plane, which will hide a faint outer band of stars
See: https://en.wikipedia.org/wiki/Low_Surface_Brightness_galaxy

When Andromeda galaxy collides with our galaxy (in about 5 billion years), this messy collision will spray stars in all directions, many of which will end up orbiting as a faint halo around the new, combined galaxy.
- We probably have a faint halo of stars from previous collisions