21

Physics, Astronomy & Cosmology / Re: Are distant galaxies receding owing only to space expansion?

« on: 23/03/2018 21:44:41 »

(Oops! overlap with chris...)
Within our own solar system, motion of the planets is dominated by gravity.
Within our galaxy, motion of stars is dominated by gravity, but each star has its own independent speed and direction within the general flow.
Within our local galaxy group,  motion of galaxies is dominated by gravity, but each galaxy has its own independent speed and direction within the general circulation. Andromeda galaxy is heading towards the Milky Way galaxy at about 100 km/second.
See: https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision

The Hubble "constant" is currently estimated at around 73 km/s per Megaparsec.
So to see an expansion that overwhelms the random motion we see between Andromeda and our galaxy, you would need to look at least 3 Megaparsecs away (Andromeda is only 0.8 Megaparsecs away).

It is only when you look at other groups of galaxies that you start to see signs of the general expansion, so the M81 galaxy group is 3.5 Megaparsecs away, and has a recession velocity of about 300km/s.
See: https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters#Closest_groups

The Virgo Galaxy Cluster is 18 Megaparsecs away, and has a recession velocity of around 1100km/s.
See: https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters#Closest_clusters

22

Cells, Microbes & Viruses / Re: Could a virus or bacterium survive inside a rock?

« on: 23/03/2018 09:43:23 »

spent thousands of years protected inside this rock in a dormant form, finally be released into the environment when the rock breaks apart (erodes, falls off the side of a cliff, earthquake etc.) and go on to infect a new host?

This is certainly a concern as the tundra melts - it may release new strains of disease organism that have been frozen for thousands of years.

Would it be possible for a particularly hardy pathogen to land on some sediment, become incorporated into the rock

It is unlikely that a surface-dwelling bacterium could be incorporated in rock, and continue living - the change in environment is just too great. Living things require an ecosystem in which to live, and an ecosystem would not survive such a major change.

In tough times, some bacteria (and larger creatures like tardigrades) can go into a dormant state, and then resume growth when the necessary ingredients return - water, food, temperature, oxygen (or lack of oxygen), etc.

But the Desulforudis bacterium was found in a deep gold mine
- It is found as a monoculture, ie it is its own ecosystem.
- It lives and grows in rocks, using energy from the radioactive decay of elements in the rock.
- The fact that it is killed by oxygen suggests that it is not a surface bacterium that got buried, but perhaps one that has been living out its whole existence in deep rocks. (For comparison, some anaerobic bacteria in our gut can produce spores that survive exposure to oxygen.)
See: https://en.wikipedia.org/wiki/Desulforudis

23

Technology / Re: How dangerous are self driving cars?

« on: 21/03/2018 09:38:01 »

in cities where the GPS signal is distorted

Apparently, the cost of GPS chips have come down to the point where sometime this year, some cellphones should be able to start accessing a more accurate timing signal.

This should give much improved location resolution, and much better immunity to multi-path reflections (eg when driving through city canyons).

Unfortunately, due to launch delays, these signals will not provide full coverage until around 2022.
https://spectrum.ieee.org/tech-talk/semiconductors/design/superaccurate-gps-chips-coming-to-smartphones-in-2018
https://en.wikipedia.org/wiki/GPS_Block_IIIA#New_navigation_signals

24

Physics, Astronomy & Cosmology / Re: Which is the Closest Star to Planet Earth ?

« on: 20/03/2018 07:44:47 »

In terms of a runner-up, it depends on when you look...
https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs#Future_and_past

Scholz's star passed through the Sun's Oort cloud about 70,000 years ago, passing within 1 light year from the Sun. But it is a very faint star, and would not have been visible to the human eye, even at its closest.
https://en.wikipedia.org/wiki/Scholz%27s_star#Solar_System_flyby

25

Chemistry / Re: How does chlorine become chloride when it bonds?

« on: 19/03/2018 21:20:14 »

everybody knows chlorine as poisonous.  Why would we have chlorine in one of the most over-used foods...

What makes Chlorine so dangerous is that it is eager to grab 1 extra electron to fill up its outer shell of electrons.
- It will do this to the surface of your lungs, skin and eyes, which is what makes it so lethal
- But after grabbing this electron, it becomes much more benign - somewhat like Argon just to its right, which also has this many electrons
- The same is true for the other elements in that right-hand column of the periodic table

What makes Sodium so dangerous is that it  it is eager to give away 1 electron to empty its outer shell of electrons.
- It will do this to the surface of your skin and eyes, which is what makes it so dangerous (fortunately, it has more difficulty getting into our lungs than chlorine)
- But after donating this electron, it becomes much more benign - somewhat like it's neighbor Neon, which also has this many electrons (you read the periodic table across and down, just like English, so Neon is just before Sodium)
- The same is true for the other elements in that left-most column of the periodic table

Sodium+Chlorine→Sodium Chloride is bliss on the kitchen table:
- Chlorine desperately wants to grab an extra electron
- Sodium desperately wants to give away an electron
- They do an electron swap; this is called an ionic bond, and tends to happen between elements on opposite sides of the periodic table
- it makes a fairly safe salt which we sprinkle on our boiled eggs in the morning
- Warning: Do not react Chlorine+Sodium at home! They are both very dangerous, and their romance is explosive...

The fact that elements above and below in the periodic table have similar (but not identical) properties is one thing that makes the periodic table so very useful.

There are other ways of joining atoms together - the organic molecules in our bodies frequently make use of covalent bonds, which occurs between elements which are "closer together" on the periodic table*
See: https://en.wikipedia.org/wiki/Chemical_bond

*those actinides & lanthanides look like they push elements far apart, when they shouldn't....

26

Physiology & Medicine / Re: How near is a cure for all cancers?

« on: 19/03/2018 10:06:21 »

I know there are quite a few variants of cancer.

Even one person's tumour is not a single variant.
Cancerous cells have lost the ability to detect mutations and correct them - or at least block them via apoptosis.
This means that a single tumour is a mix of many cancer types and subtypes - a veritable family tree of cancers.

What is the likelihood that these cured cancers are simply in temporary remission?

It has been observed that once a cancer has been in remission, it often comes back more aggressively - and the previously effective medication is now ineffective.

This is because the original treatment killed off the branches of the cancer tree that were susceptible to the drug. The few remaining twigs that were immune to the treatment then continued to grow until they reached a size where they were detectable as a new tumour.

This is why most cancer treatments include a generic component like radiotherapy or chemotherapy that kills any rapidly dividing cells. Chemotherapy has the advantage that it can reach cancerous cells wherever they have spread (but this whole-body reach also causes whole-body side-effects).

Reaching 5 years in remission is a milestone that is often quoted in cancer statistics.
https://en.wikipedia.org/wiki/Cancer_survivor

27

Physics, Astronomy & Cosmology / Re: How would we calculate the time left to contact with an EH?

« on: 15/03/2018 20:35:46 »

How would we calculate the time left to contact with an EH?

To a distant observer, the location of the event horizon r_s  is where the escape velocity v_e = c.

However, to a closer observer - say, one about 53.6km from the center of the Cygnus-X1 black hole (10km outside the Schwarzchild radius r_s), the escape velocity from r_s to her current position is not c, but something lower.

Like most people, I have trouble visualising the distortions of space and time in relativistic environments (and even more trouble calculating them!).

So let's say that a space probe is now 53.6km from the center of the black hole, and 10km outside the Schwarzchild radius.
- To the person inside the space probe, the speed of light inside the space probe seems "normal"=c.
- I estimate the escape velocity from r_s out to 53.6km to be around  29,000km/s. This is a lot less than c
- So the observer in the space probe should be able to see light from inside r_s?
- For example, if another space probe was dropped just before her, and this probe had a bright beacon
- I estimate that the person in this space probe should be able to see light emitted from a radius > 12km from the center of the black hole.
- It is as if this observer sees a smaller event horizon than the more distant observer?
- If the observer in the space probe sees the black hole approaching at (say) c/3, then they would extrapolate impact with this smaller event horizon (41km away) in around 400μs (linear extrapolation)

So, my estimates are crude, but to the observer in the space probe, does the size of the event horizon shrink as they get closer?

Of course, if they blinked they would miss it! (A typical human blink takes around 200,000μs...)

28

Physics, Astronomy & Cosmology / Re: Will humans ever understand quantum physics?

« on: 15/03/2018 10:40:55 »

I think I can safely say that nobody understands quantum mechanics. So do not take the lecture too seriously, feeling that you really have to understand in terms of some model what I am going to describe, but just relax and enjoy it. I am going to tell you what nature behaves like.

I have observed in teaching quantum mechanics... The student begins by learning the tricks of the trade. He learns how to make calculations in quantum mechanics and get the right answers, how to calculate the scattering of neutrons by protons and so forth. To learn the mathematics of the subject and to learn how to use it takes about six months. This is the first stage in learning quantum mechanics, and it is comparatively painless.
The second stage comes when the student begins to worry because he does not understand what he has been doing. He worries because he has no clear physical picture in his head. He gets confused in trying to arrive at a physical explanation for each of the mathematical tricks he has been taught. He works very hard and gets discouraged because he does not seem to be able to think clearly. This second stage often lasts six months or longer. It is strenuous and unpleasant.
Then, unexpectedly, the third stage begins. The student suddenly says to himself, “I understand quantum mechanics,” or rather he says, “I understand now that there isn’t anything to be understood.” The difficulties which seemed so formidable have mysteriously vanished. What has happened is that he has learned to think directly and unconsciously in quantum-mechanical language. He is no longer trying to explain everything in terms of prequantum conceptions.

29

Physics, Astronomy & Cosmology / Re: How would we calculate the time left to contact with an EH?

« on: 14/03/2018 10:11:30 »

Are you saying that assertions such as: "To an outside observer any object approaching the Schwarzschild radius appears to take an infinite time to penetrate the event horizon", are incorrect, because the object would vanish before, in the observer's RF, it reached the EH?

Yes.

I hope I don't murder the maths too much, but:
- Let's take Kryptid's calculation of the escape velocity of a black hole at a certain radius: v_e = √((2GM)/r)
- If we are looking at a space probe falling "from infinity", it's velocity at radius r from the center of the black hole will equal the escape velocity given by this equation.
- Plug in the measured mass of the Cygnus X-1 black hole of 14.8 solar masses
- The event horizon is at a radius of r_s=43.6km

For an observer seeing the space probe fall away from him towards the black hole, there would be considerable relativistic doppler redshift, even without gravitational time dilation.
https://en.wikipedia.org/wiki/Relativistic_Doppler_effect#Motion_along_the_line_of_sight

But let's ignore Doppler shift, and put the observer at right angles to the infalling probe, so there is very little Doppler shift.
So now the main source of frequency change is due to gravitational time dilation. This frequency ratio is sqrt(1-r_s/r)

If the distant observer looks at the speed of the infalling space probe, and does a linear extrapolation of the time to impact with the event horizon:
D: Distance to event horizon, in meters
v/c: Velocity, as a fraction of the speed of light
T: Extrapolated time to impact event horizon (in seconds)
TD: Gravitational Time Dilation ratio = ratio of emitted frequency of light to frequency as seen by the distant observer


D(m)     v/c    T(s)   TD
10000   0.902   4E-05   0.432
 1000   0.989   3E-06   0.150
  100   0.999   3E-07   0.048
   10   1.000-   3E-08   0.015
    1   1.000-   3E-09   0.005


If the infalling space probe had a bright beacon on it,
- at 10km its frequency would be halved, and you would predict impact in about 40μs.
- at 100m, it's frequency would be reduced by a factor of 20, and you would expect impact in 0.3μs.
- at 1m, it's frequency would be reduced by a factor of 200, the received power would be reduced by a factor of 200, and you would expect impact in 3ns

You could extend the visibility for a nanosecond or two by using an X-ray beacon, or even setting off a gamma-ray burst with an atomic bomb.

I am sure there is some factor I have missed, but I think the general conclusion is valid - a remote observer would see the space probe approaching the event horizon at a relativistic speed, and around the expected time of impact, the frequency of the beacon would be reduced to undetectability within a very short time (effectively zero frequency and zero emitted power), so it would look as if the space probe had crashed into the black hole and disappeared
See: https://en.wikipedia.org/wiki/Gravitational_time_dilation#Outside_a_non-rotating_sphere

30

Physiology & Medicine / Re: Are 630% more flu virus particles emitted by people who receive flu vaccines?

« on: 14/03/2018 08:47:46 »

low self-reported influenza vaccination rate (22%)

We don't know what the level of influenza vaccination was in this general population.
- They don't describe the difference in age between the vaccinated vs non-vaccinated groups (immune system effectiveness tends to decline with age)

The influenza vaccine is not 100% effective - I have seen claims of around 75% effectiveness.
- But in those for whom the vaccine is effective, the immune response detects and crushes the viral invasion before it reaches a critical mass that will produce severe flu symptoms. I imagine the virus shed by this group would be very low.

...but probably not low enough to compensate for the observed increased viral shedding from the "ineffective vaccination" group.

31

Guest Book / Re: Please Ban me now and I promise never to return as a sock.

« on: 13/03/2018 21:20:48 »

There was some brilliant repartee in there, like:
Q: What do you think "subjective" means?
A: What do you want it to mean?

It is not our policy to delete all of someone's posts, if they are interleaved with useful answers from forum members. These good answers may help others with similar questions.
 
There was a very good description of escape velocity by Kryptid, and an implied commitment by TheBox to understand it.
This commitment seems to have been lost in the subsequent flurry of quotes of increasing length.

Mr Box: when you have processed this description, feel free to open another topic in New Theories to explore any new questions you may have.

32

Physics, Astronomy & Cosmology / Re: How would we calculate the time left to contact with an EH?

« on: 12/03/2018 08:51:05 »

Perhaps another way to measure your distance from a black hole, if you are in a circular orbit around it:
- Monitor the pulses from a number of millisecond pulsars, in different directions on the sky.
- By monitoring over many orbits, you should be able to determine the diameter of your orbit, and the mass of the black hole
- The center of the black hole is at half of the diameter
- You can work out the size of the event horizon from theory

If you were in an elliptical orbit, the maths gets more complicated, but still feasible.

Of course, if you are in a stable orbit, you will never reach the event horizon (until collisions with an accretion disk or gravitational waves steal your angular momentum...).

And the Delta-V to reach a black hole from a circular orbit would be... astronomical!

33

Physics, Astronomy & Cosmology / Re: How would we calculate the time left to contact with an EH?

« on: 11/03/2018 19:57:54 »

We then need a reliable way of determining our initial distance from the horizon.

What about looking at the distortion of the distant starfield caused by the gravitational field of the black hole?

If there were an accretion disk, you could use interferometry to measure the distance to the accretion disk.

If there was no accretion disk, you could fire a laser tangentially at the black hole event horizon, and measure the delay before the light appears on the opposite side of the black hole, after being bent in a U-Turn around the black hole. Then do a lot of complex maths that is beyond me....

34

Physics, Astronomy & Cosmology / Re: Do I weigh as much as the Earth?

« on: 10/03/2018 05:14:46 »

do I weigh the same as the Earth? Or am I missing something?

The concept of "weight" implies some standardised measurement conditions, usually "At sea level, on Earth.".

So if you change the measurement conditions, the measured weight changes.
- You weigh about 1/6 on the Moon as you do on the Earth
- If you are floating in space, and the Earth is floating in space, you are both weightless, and you weigh the same as the Earth
- If you measure the weight of you on the Earth, and the weight of the Earth on you, both come out as the same number. But you you are measuring both things under radically different conditions, so they are not really comparable.
- But if you put both the Earth and yourself level with the cloud-tops on Jupiter, the Earth would weigh far more than you. This is because you are measuring the weight of the Earth and the weight of yourself under the same conditions - ie at the same distance from Jupiter's center of mass. And the mass of the Earth is far greater than the mass of SymeAaro.

PS: The metric system sidesteps this dilemma by talking about the Mass of an object (measured in kilograms), which is a property of the object, regardless of whether it is on Earth, free-floating in space, on the Moon or on Jupiter.
There is a separate unit of Force (measured in Newtons) which measures the pressure you apply on the Earth, and which the Earth applies on you.

35

Physics, Astronomy & Cosmology / Re: Why do clouds go red at sunset? Why not green, or some other colour?

« on: 05/03/2018 10:26:37 »

why does a rainbow have seven colours right? That's the next question, right?

The podcast below had some interesting comments on colors in different languages.

Some linguists (Berlin & Kay) did a study of many languages, looking at common color words that would be used by children, for example.
- They excluded technical or specialist words that would be used in specific contexts

They found that every language had 2 words that were equivalent to light or white, and dark or black.
- But they found that other languages had 3, 4, 5, 6, 7 or more color words,
- And they concluded that languages generally added color words in a particular order:
- black+white, then red, green or yellow, then green + yellow, blue, etc (if I remember correctly)

The speaker suggested that Old English in the 1200s had words for white, black, red, yellow, and green.
- Purple was present, from the color of European royalty (but from the Romans)
- Violet was later added from the color and name of the flower
- Orange was added later, from the color and name of the fruit

Since white and black are not colors of the rainbow, one assumes that an Old English rainbow would have had perhaps 4 major bands of color.

I'm not sure I believe some of what he says, but listen to the last 10 minutes of: http://historyofenglishpodcast.com/2017/12/31/episode-106-an-illuminating-discovery/

If you listen to the whole episode, you will hear a bizarre explanation for why Romance languages like French use words like "blanc" for white, while English, a Germanic language uses "black" for black - and yet they are derived from the same Indo-European root word!

36

Physics, Astronomy & Cosmology / Re: What is the mean time dilation at the surface of the earth

« on: 03/03/2018 21:26:20 »

Can we determine, with respect to the centre of our galaxy, what the time dilation is on the surface of the earth.

Not really.
The center of our galaxy is believed to harbour a supermassive black hole with a mass around 3 million times the mass of the Sun.
- At the event horizon of this black hole, the time dilation of the Earth would be zero.
- Inside the event horizon of this black hole, the question almost loses its meaning, since spacetime is so twisted in there, and you cannot communicate the result back to the Earth anyway.

Normally the reference location for time dilation would be away from any local masses, such as outside our local galaxy cluster. Kryptid implied this when he talked about "escape velocity of the Milky Way".

37

Physiology & Medicine / Re: What happens to sperm inside the female body?

« on: 01/03/2018 21:07:20 »

Macrophages are part of the immune system; they engulf and destroy foreign invaders, usually pretty successfully (apart from a few bacteria like Listeria and viruses like HIV that actually thrive and spread inside the immune system).

If a woman's immune system is overactive, or previously sensitised to sperm, it may engulf and destroy all sperm, rendering the woman infertile.
Assisted fertilisation techniques ("test tube babies") may assist in such cases.

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

38

Plant Sciences, Zoology & Evolution / Re: What do rats find rewarding in play fighting?

« on: 01/03/2018 20:39:43 »

What do rats find rewarding in play fighting?

Probably similar to human children:
- Exercise
- Social Interactions
- Playing out scenarios that will become deadly serious as adults
- Finding their niche in the social hierarchy before they become serious adults

How precisely do ultrasonic vocalizations (USVs) in the 50kHz range may elicit play-fighting and aggressive behavior in the juvenile rat? 

Probably similar to the way human children elicit play-fighting and aggressive behavior with sonic vocalisations in the 300Hz-10KHz range.

39

Physics, Astronomy & Cosmology / Re: Why do clouds go red at sunset? Why not green, or some other colour?

« on: 27/02/2018 21:03:16 »

As @chiralSPO said, reddish hues are the dominant light left after a long passage through the atmosphere at sunset & sunrise.

But sometimes, other colors do appear at Sunrise or Sunset - like the fleeting "green flash".
Warning: Do not stare at the Sun - you can easily damage your eyes. If it's uncomfortable, it can do damage.
A fleeting glance after the disk of the Sun has gone should be ok.

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

why does a rainbow have seven colours right? That's the next question, right?

As all our US readers will know, it doesn't. It has 6.
- Only British rainbows have 7 colours (plus a few of their non-US colonies, like New Zealand).
- Colour is partly a linguistic construct - I understand Russian has separate words for Light Blue and Dark Blue, while Vietnamese does not distinguish Green and Blue.
- And these linguistic constructs mean that some cultures can consistently see different colours where other cultures see only one. It's bizarre when you see colour swatches selected to have these characteristics!

But I can explain why humans would see the rainbow has around 6-8 broad bands of colour:
- The human eye has 3 sensors in cone cells, with a broad range of sensitivity peaking in the Red, Green and Blue parts of the spectrum.
- The Red and Green ranges overlap strongly (fine gradation of colours), but the blue doesn't overlap much at all (coarse gradation of colours).
- A rainbow breaks sunlight into a continuous range of pure colours (somewhat muddied by the Sun's apparent 0.5° width in the sky)
- At the Red end of the spectrum, it is mostly the Red cones that are triggered
- In the Orange zone, both Red and Green cones are triggered strongly, but almost no Blue
- In the Yellow zone, Green cones are triggered strongly, and Red cones weakly
- In the Green zone, Green cones are triggered strongly, and Red & Blue cones weakly
- In the Blue Zone, the Blue cones are triggered strongly, and Green weakly
- In the Purple/Violet/Indigo zone, the Blue cones are triggered strongly, but Red and Green hardly at all
See: https://en.wikipedia.org/wiki/Color_vision#Physiology_of_color_perception

If you want to see all the colours of Sunlight clearly, you need to pass it through a narrow slit before passing it through a prism. The narrow slot avoids the blurring effect of the Sun's apparent 0.5° width.
See: https://en.wikipedia.org/wiki/Fraunhofer_lines

Some women have two Red sensors with different spectral sensitivity, and can see more colours at the red end of the spectrum than us poor males (but see linguistic construct, above).
Other species have up to 16 color sensors (some of them farther into the infra-red and ultraviolet than ours), so these creatures would see far more bands of colour than humans.
See: https://en.wikipedia.org/wiki/Mantis_shrimp#Eyes

40

Physics, Astronomy & Cosmology / Re: How can Dark Energy make the Universe expand at faster than light speed?

« on: 26/02/2018 20:00:20 »

I feel like the language used to describe this seemingly contradictory nature of things is a bit confusing for a layman like myself.

An analogy often used to describe the expansion of the universe is to imagine that the galaxies are like little dots on the surface of a balloon. As you inflate the balloon, the dots move away from each other - but the dots are still where you originally put them.

And the dots that were originally farther apart are moving away from each other faster than dots that are nearby.

Like all analogies, it has severe limitations (like the surface of a balloon is 2-dimensional, while our familiar space is 3-dimensional), But if you imagine that light travels at a finite speed across the balloon surface, there is a certain distance beyond which light from these galaxy dots will never reach us, because they are expanding away from us faster than light can travel.

For a more in-depth introduction, see: https://en.wikipedia.org/wiki/Metric_expansion_of_space