1

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: 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...)

3

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.

4

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...)

5

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).

6

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)

8

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

9

Cells, Microbes & Viruses / Re: Would Penicillin inhibit the growth of fungi?

« on: 29/12/2018 21:39:26 »

Yes, penicillin is produced by a fungus in its ongoing war against bacteria (which are always trying to digest the same piece of detritus as the fungus).
- Penicillin is optimised to attack the weaknesses of (some kinds of) bacteria
- Unfortunately, due to this long-running war, strains of penicillin-resistant bacteria exist in nature
- Historically, it has only been a year or two after introduction of a new antibiotic before cases of resistant bacteria are detected in humans.

I am sure that bacteria also produce anti-fungal compounds - and even fungi produce compounds that are toxic to other fungi
- However, in some respects, fungi are more closely related to humans than are bacteria
- So many anti-fungal compounds are toxic to humans as well as the fungi

The main problem with finding new antibiotics is that it is almost impossible to grow most strains of microorganism in a laboratory monoculture - they naturally live as part of an ecosystem, depending on each other for nutrients
- Some success has been experienced by taking a soil sample, and inoculating separate very thin membrane "pages"
- This "book" is then buried back in its original location, in contact with the soil
- This allows nutrients and signaling molecules to pass between pages and exchanged with the surrounding soil
- But limits each colony of microorganism to grow on only a single page
- Any compounds produced can then be tested for effectiveness, and the microorganism's genome can be sequenced.

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

10

The Environment / Re: Should we extinct malaria-spreading mosquitoes and other pests?

« on: 21/12/2018 23:42:04 »

Gene drive developer Kevin Esvelt has asked for a wide-ranging debate on the topic of gene drives before it is deployed in the wild. He has some ideas on how you could produce a new gene drive that would reverse a rogue gene drive, but it would be better not to let it go rogue in the first place!

Naturally, the military is interested/concerned about this topic. But like all biological weapons, it is almost impossible to control once it is released, so I think that all rational military organisations would agree to ban it as a weapon. Unfortunately, there are some non-rational para-military organisations out there that might ignore any international conventions.

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

11

The Environment / Re: Should we extinct malaria-spreading mosquitoes and other pests?

« on: 21/12/2018 21:55:16 »

Genetic control mechanisms can be made species specific, and there are plenty of insects and even mosquitoes that don't harm humans.

The mosquito Aedes Aegypti carries some nasty diseases, is attracted to humans, and has spread around the world from its Egyptian/African origins. It is an invasive species (following another invasive species, humans...)

I see few problems with wiping it out around the world - and even in its native habitat. Other mosquito species would expand to fill the ecological niche. But it needs a lot of debate before it is done!

A less severe form of control would be to genetically manipulate the mosquito so that it doesn't carry the pathogens. This would not exert an evolutionary pressure on the mosquito.

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

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

« on: 20/12/2018 10:42:52 »

The Solar System (and our local galaxy cluster) seems to have a measurable motion with respect to the Cosmic Background Radiation. The CMBR seems to be slightly red-shifted in one direction, and slightly blue-shifted in the opposite direction.

Whether this represents random motion of our galaxy, a local gravitational concentration or a random temperature variation in the CMBR is not known for certain.

See: https://en.wikipedia.org/wiki/Cosmic_microwave_background#CMBR_dipole_anisotropy

13

Cells, Microbes & Viruses / Re: How can we find a cell inside a water solution?

« on: 11/12/2018 19:11:25 »

Craig Venter's Sorcerer II expedition travelled around the world, sampling seawater every 200 miles.
They filtered the water through different sized filters, then just sequenced any DNA they found. This gave them a good idea of what types of cells and viruses were present.

if you ever make it to the sea, and you swallow a mouthful of seawater, keep in mind that each milliliter has about a million bacteria and on the order of 10 million viruses.

See: https://en.tiny.ted.com/talks/craig_venter_on_dna_and_the_sea

14

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

« on: 09/12/2018 10:32:59 »

In developing his theories, Einstein ran many thought experiments on relativity and time dilation.

After developing his theories, he used thought experiments to illustrate them.

Illustrations about time often referred to a clock which is next to an observer (ie stationary, in the observer's frame of reference, and at the same level in a gravitational field). This observer is then able to observe clocks that are moving relative to himself, or clocks that are at different levels in a gravitational field (or both).

Today, with accurate atomic clocks, we can do these experiments and measure the results.
- Every time you use GPS to find your destination or track your position on a map, this uses Einstein's equations to calculate the right rate of the atomic clocks in orbit (which is different from those same atomic clocks, sitting on the ground before they were launched).
- Some of the most accurate atomic clocks can detect a difference in elevation of 1 foot (30cm), due to the different time dilation.
- If we could put an atomic clock outside the Solar System (away from the Sun's gravitational well), the time difference would be more extreme
- If we could put an atomic clock outside our galaxy, the time difference would be even more extreme
- We have no way of traveling outside the galaxy (or communicating with a space probe outside the galaxy) with today's technology. But physicists expect that Einstein's predictions would hold to a high degree of accuracy.
- My observation is that it is a brave physicist who bets against Einstein (except when it comes close to a black hole - Einstein knew that he didn't have a solution there)

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

15

Physics, Astronomy & Cosmology / Re: Is the force of Gravity quantitised?

« on: 08/12/2018 09:38:16 »

Light is quantised, but there is no minimum energy of a photon. There is no maximum range for a photon (within its light cone) - the expansion of the universe merely reduces the energy of the photon towards zero.

If (as many modern physicists believe), gravity also turns out to be quantised,there would be no minimum energy of a graviton. Similarly, there would no maximum range for a graviton (within its light cone) - the expansion of the universe merely reduces the energy of the graviton towards zero.

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Physics, Astronomy & Cosmology / Re: Can UVA and UVB rays pass through a wood ply roof with a bitumen felt covering?

« on: 07/12/2018 11:04:22 »

The roof itself would be regarded as "SPF50+".

However, light reflected off a lake, or coming in through open windows and doors would carry a small amount of UVA & UVB around the roof.

17

That CAN'T be true! / Re: How do you melt gold at room temperature?

« on: 30/11/2018 23:45:20 »

An electron microscope at high magnification bombards the target with high-energy electrons.
- These electrons have an equivalent temperature, based on the Boltzmann constant of 8x10^-5 eV/K.
- Atoms at room temperature (about 300K) have a kinetic energy of 0.025eV.
- An electron in a 100kV electron microscope has a thermal energy equivalent to a temperature of around 4 million degrees (Kelvin or Celcius).
- So they would need to show that it was not thermal heating that caused the gold to melt.

Whatever the cause, this certainly puts a limit on the resolution of fine detail under an electron microscope - it is traditional to coat non-conductive specimens with gold before putting them in an electron microscope.
 
See: https://en.wikipedia.org/wiki/Boltzmann_constant

18

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

« on: 27/11/2018 08:56:28 »

No.
Two major reasons:
1) A child of a Neanderthal and Human would have a complete Neanderthal chromosome paired with a complete Human chromosome. As generations pass by, the large pieces of Human and Neanderthal DNA are broken up into smaller and smaller pieces due to crossover during production of egg and sperm.
2) Humans and Neanderthals share almost all their genes - but both the Human and Neanderthal genomes have unique changes that differ from each other (and from their most recent common ancestor - but that must be inferred, since we haven't sequenced their common ancestor). So by looking a the DNA of a modern Human, you can identify certain snippetts as having uniquely Neanderthal sequences, while other people have a uniquely Human variant of that same stretch of DNA. (In fact, one person could have both a Human and Neanderthal version of the same DNA segment).

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

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Cells, Microbes & Viruses / Re: Does telomerase benefit both healthy and cancerous cells?

« on: 26/11/2018 21:12:51 »

Human cells stop dividing after about 50-70 divisions - and many of these divisions occur before birth.
- Part of this is due to telomere shortening
- So a cell which turns cancerous in an adult (due, say, to smoking, or a severe case of sunburn) will be very limited in the maximum size to which it can grow - provided it is still subject to telomere shortening.
- and telomerase is turned off in most human cells

For a cancer to become dangerous, it must overcome telomere shortening, by turning on telomerase (a mutation)
- It must also turn off apoptosis: the process by which cells with uncorrectable mutations commit suicide (another mutation)
- It must also generate a blood supply (other mutations)
- And hide from the immune system (more mutations)

It takes quite a number of mutations before a cancer can become dangerous - unfortunately, some people are born with some of these mutations already present, which increases their lifetime risk of cancer.

Denying these cancerous cells the telomerase enzyme would stop their multiplying.

The gene for telomerase is present in every cell - but it is mostly turned off.
A treatment which suppressed telomerase may be one way to limit cancer growth.

Attempts to extend human lifespan by activating telomerase throughout the body would increase the chance of cancers developing.

Some large, long-lived animals like elephants actually have multiple copies of the gene that detects mutations, so that a single mutation in this gene is less likely to turn cancerous.
See: https://en.wikipedia.org/wiki/Peto%27s_paradox

20

Physics, Astronomy & Cosmology / Re: What was Georges LeMaitre's primeval atom?

« on: 20/11/2018 10:48:54 »

Georges Lemaître published his concept of the big bang in 1927, although he had previously corresponded with Einstein about it.
See: https://en.wikipedia.org/wiki/Georges_Lema%C3%AEtre#Career

The concept of anti-matter didn't really become concrete until 1932, so this probably didn't influence Lemaître.
See: https://en.wikipedia.org/wiki/Antiparticle#Experiment

The idea of radioactivity was discovered by Henri Bequerel in 1896, and the idea that many heavy elements were radioactive was established by Marie & Pierre Curie in the early 1900s. So the idea that a massively heavy atom would be unstable was probably the inspiration for Lemaître.
See: https://en.wikipedia.org/wiki/Radioactive_decay#History_of_discovery

Note that Lemaître was a Catholic priest, and so the idea of a beginning to the universe falls naturally out of Genesis 1:1: "In the beginning God created the heavens and the earth...". I don't think Lemaître would have been concerned about absence of a physical cause for the Big Bang.