1

Physics, Astronomy & Cosmology / Re: If nothing is faster than light, how can we look back in time with a telescope?

« on: 24/05/2018 12:39:03 »

If nothing is faster than light, how can we look back in time with a telescope?

An example: Chinese observers saw a "guest star" in the sky in the year 1054.
- It was bright enough to see in the daytime.
- We now know that this was caused by a supernova about 6,500 light years away.
- The expanding fragments are now known as the Crab Nebula.
- The supernova actually took place around 5,500BC (roughly 1054 minus 6500 years).
- So when the astronomers saw it in 1054 AD, they were looking back in time about 6500 years.

But supernovae also emit some other particles that travel at well below the speed of light. This probably includes radioactive iron nuclei, ⁶⁰Fe.
- If these are traveling at (say) 2/3 the speed of light, they will take about 10,000 years to arrive at Earth (compared to 6,500 years for light)
- We can expect these to start arriving around the year 4,500 AD.
- So assuming that scientists then have radiation detectors in space, they would see this increase in subatomic particles.
- They would be effectively looking backwards in time at the same event - only now they will be looking back almost 10,000 years
- We can see a sprinkling of ⁶⁰Fe on the sea floor, hinting at nearby supernovae in our galaxy. This isotope has a half-life of about 2 million years, so we know that this did not occur naturally on Earth.

So far as we know, there is nothing that travels faster than light, so no high-technology astronomers on Earth could have seen the Crab Nebula explosion before 1054.
- There is one subtle exception: Neutrinos. It is estimated that a supernova in the core of a giant star may take from a few hours to a day to break through the surface of a star, emitting an intense flash of light, X-Rays and gamma rays. However, neutrinos (which travel imperceptibly less than the speed of light) can make the same trip from core to surface in a few seconds. So neutrinos might arrive at the Earth from the core of the supernova a few hours before the flash of light from the surface of a supernova. With today's neutrino detectors and X-Ray/Gamma ray detectors, I expect we would be able to measure this time difference if another supernova exploded to us as close as 6500 LY.

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

edit: Correct formatting of the iron isotope, as highlighted by PmbPhy.

2

Physiology & Medicine / Re: How to increase or improve cognitive neuroimmunity?

« on: 19/05/2018 09:02:45 »

neuroimmunity and feedback on novel ways to defend/protect cognitive functions of the central nervous system (CNS)

A few years ago, I heard of a bizarre medical case where a woman suddenly lost her mind, over a period of weeks.

It turned out that she had an ovarian cyst called a teratoma. Teratomas can contain many kinds of bodily tissues, but all scrambled together.

Apparently, this teratoma contained some neurological tissue (normally found only in the Central Nervous System, or CNS); her immune system was triggered by the teratoma to attack her CNS.

Eventually, after a long hunt for the cause, finding and removing the Teratoma allowed her a gradual recovery; today neurologists are aware of this trigger, and know to remove the teratoma promptly, producing rapid recovery.
See, for example: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4688824/

But tk, I think you are largely protected from this form of neuroimmune attack by your male gender.

3

Geek Speak / Re: Does an HDMI cable work with a Display Port connector?

« on: 19/05/2018 08:48:12 »

VGA is quite capable of supporting HDTV (1080p). So unless you are planning to go to a new 4k TV, you could stay with VGA.
See: https://en.wikipedia.org/wiki/Video_Graphics_Array

HDMI and DisplayPort are mechanically incompatible, but the protocol between computer and display is compatible, so you can get adapters.

My old work laptop (about 5 years old) had DisplayPort - but things have moved on from then.
- 2 years ago, when I needed to buy a new home laptop, it had HDMI only
- Last year, when I wanted to buy a new, larger, screen, economical models had HDMI & VGA, but not DisplayPort.
- My new work laptop (this year) has a HDMI connector, but not DisplayPort.
- So I think HDMI has won, and DisplayPort has lost
- You can also get USB to graphics adapters, and this bypasses some limitations in Windows, and lets you run 3 screens at once - so you get an even tan on all sides of your face 

For the year when my work computer had DisplayPort and the display had HDMI, I saw that you can get quite economical video converters on eBay (but I bought a somewhat more expensive one the same day from my local office supplies warehouse).
See: https://en.wikipedia.org/wiki/DisplayPort#Comparison_with_HDMI

4

Marine Science / Re: How do denticles make a shark swim faster?

« on: 15/05/2018 11:24:06 »

There is a major difference between denticles and dimples in a golf ball - the golf ball is not alive.

I read many years ago that each shark denticle is coupled to an underlying muscle, making it seem possible that the sharks are able to control the buildup of turbulence by active control of their surface. But modern research is not so clear on this finding.

Dolphins have a slightly grooved skin, which develops large, moving dimples when they swim quickly. This definitely reduces turbulence, and it is definitely controlled by muscles.

The following article in Nature is a bit technical, but it illustrates that both shark skin and dolphin skin significantly reduce friction, by controlling turbulence.
See: https://www.nature.com/articles/srep06650

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Physics, Astronomy & Cosmology / Re: Does your height above sea level affect your weight?

« on: 15/05/2018 10:52:20 »

In an aircraft at 20,000 feet (3.8 miles), you weigh less than you would at sea level.
The radius of the earth is 4,000 miles, so your weight 3.8 miles up in the air (Wa) can be calculated from your weight at sea level (Ws) as:
Wa = Ws * (4000/4003.8 )^2 = Fs * 0.998

So your weight at 20,000 feet would be 99.8% of your weight at sea level (this includes the inverse square law of gravity, but ignores centrifugal force from Earth's rotation).

If, however, you had reached 20,000 feet by climbing to the top of a high mountain, your weight would still be lower than at sea level, but by a slightly smaller margin, since there is now a mountain of mass immediately under your feet, attracting you.

For those in countries using the metric system, this is not a "get thin quick" scheme, since your mass in the plane is identical to your mass at sea level.

6

Technology / Re: Are we heading for a driverless car disaster?

« on: 14/05/2018 10:53:52 »

So why is this? What's the problem with determining that an object is not moving relative to you, so you should stop...?

The problem is that the wavelength of practical car radars is too long to form an actual image. You just get a lot of reflections from the things around you - the best you can do is to have a beam that projects most of the energy straight ahead, and gets the best returns from straight ahead.
- This is great for cruise control that can slow down to match the speed of the car ahead of you
- But it is always getting reflections from the road, signs, trees, poles - stationary stuff
- So to reduce this "ground clutter", the designers suppress all the signals coming from "stationary" things (apparently including a stationary fire truck, in this unfortunate incident)
See: https://en.wikipedia.org/wiki/Clutter_(radar)

In contrast, cameras use very short wavelengths (visible light) and are able to form an image; in principle, by detecting objects that are growing larger in the visual field, a camera should be able to detect imminent collisions, and put on the brakes, regardless of whether the object is stationary, moving in the same direction or the opposite direction. Just don't trust it in rain or fog.

But we aren't quite there with AI, yet...

7

New Theories / Re: Rotating earth theory of two high tides a day

« on: 13/05/2018 22:37:43 »

Four tidal hills per day

The total tidal effect of Moon (larger) and Sun (smaller) produces a tidal force whose magnitude depends on the vector sum of the tidal force from Sun & Moon.

This vector has a maximum, not 2 maxima.

This single maximum generates 2 hills/humps, not 4.

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New Theories / Re: Rotating earth theory of two high tides a day

« on: 13/05/2018 08:07:59 »

List of Questions

I can only address a few of your questions from general knowledge, so here goes...

2. How to explain the fact that in southwest Australia in the port of Fremantle the tides are part of the year significant, and then disappear.

In fact, this happens everywhere, just with different amplitude and timings in different places.
There is a peak tide, which occurs about 3-7 days after the Sun and Moon line up. Then there is also a low-tide period abut 2 weeks later.
This can be complicated by other tidal drivers that are more significant in some places than in others, due to local resonance effects.

3. Why abnormally high tides are formed only on the northern and southern shores of the seas and oceans and not on the eastern.

Mont St Michel (included in your list below) is on the eastern side of the Atlantic.
Alaska (included in your list below) is on the eastern side of the North Pacific.
Colombia is on the eastern side of the equatorial Pacific.

4. Why are tidal waves moving strictly along the perimeter of lakes, seas and oceans, and not from east to west, following the Moon?

This was only true in the days when tides were measured on the shore.
Now, with radar satellites, we can measure tides moving in the middle of the ocean.

5. As the current of the western winds manages to move from west to east, at a speed of 5 km / h, wiping the ocean floor

I am not sure what you mean by "current of the western winds"; are you talking about winds, or ocean currents?
The Gulf Stream is an ocean current of warm water that flows from the Gulf of Mexico towards the UK.
This was used to assist sailing ships traveling from the Americas back to Europe.

, while the tidal wave flows from east to west at a speed of 800 km / h, wiping the ocean floor.

Note that in English, "tidal wave" has two distinct common meanings, both of them somewhat misleading: One is a sudden wave created by an undersea earthquake, the other (wrongly) imagines that the tide is a wave which propagates around the world. It is probably better to use:
- "Tsunami" (a Japanese word) for earthquake waves
- "Tide" for the second one (I have used tidal "hump", in an attempt to be descriptive).

It is true that tidal waves (tsunamis) do travel with a speed of around 800km/h in the deep ocean, although that drops to perhaps 80hm/h near the coast.
See: https://en.wikipedia.org/wiki/Tsunami#Characteristics
What this reference tells you is that once the wavelength of a wave exceeds the depth of water, it's velocity is roughly the same - around 800km/h in deep water.
- This applies to tsunamis (period of a few minutes to a few hours)
- And also the tide (period of a bit over 12 hours in most places)
The circumference of the Earth is about 40,000km, so for the tide to propagate as a wave would take about 50 hours, or about 4 times what is observed. This tells us that the tide does not propagate as a wave, but is driven by the gravity of Moon+Sun.

6. Why is not tidal current used to move ships from east to west.

Because the tide is a passing wave, which produces no net movement of water, and so cannot produce a net movement of shipping.

7. It is believed that the cause of the second hump is the barycenter.
And what is the cause of the solar second hump?

Two mixed-up questions here:
- It presumes that the Sun's gravitation will produce a tidal "hump" which is different from the Moon's tidal hump. In fact, it is the sum of the tidal effects of Sun and Moon that produces a pair of humps, roughly on opposite sides of the Earth (a physicist would describe it as a "vector sum" of the tidal forces). There are not 4 distinct humps, 2 for the Sun and 2 for the Moon.
- It presumes that there is a single barycenter. In fact, there is a barycenter of the Earth-Moon system (which is inside the Earth, and can be ignored when calculating the height of tides driven by the Sun), and a separate Sun-Earth/Moon barycenter (which is inside the Sun, and can be ignored when calculating the height of tides driven by the Moon). The total motion of the Earth and Moon includes the motion around both barycenters.

8. As long as the tidal wave makes one revolution around the Earth, the Moon makes two turns, how is the gravitational connection between the Moon and the tidal hump?

You have this backwards. In most places on Earth, there are 2 tides per day.
The two tidal "humps" pass a given point while the Moon makes one rotation of the Earth.

9. According to the lunar theory of tides, the earth's crust at the latitude of Moscow, with a frequency of two times a day, rises and falls with an amplitude of about 20 cm, at the equator the swing of oscillations exceeds half a meter.
Then why, the highest tides are formed in the temperate zones and not at the equator?

Two mixed-up questions here:
- When we measure tides, we are measuring a difference in height: "sea tide" - "land tide".
- Colombia is on the eastern side of the equatorial Pacific, so extreme tides can occur near the equator, if local resonances are favorable. And Madagascar is not that far from the equator.

The highest tides on Earth are formed in the Fandi Bay in North America - 18 m, at the mouth of the Severn River in England - 16 m, in the Bay of Mont-Saint-Michel in France - 15 m, in the mouths of the Sea of ​​Okhotsk, Penzhinskaya and Gizhiginskaya - 13 m , at the cape Nerpinsky in the Mezensky Bay - 11 m.

10. Tidal forces have been used successfully to date, as a plug from many of the mysteries of nature.
In fact, they exist only theoretically, but practically, they have not been fixed by any gravimetric device.
Acceleration of free fall at the equator is less than at the poles: at the equator is 978 gal, and at the poles - 983 gal.
There is no data on the fluctuation of tidal forces, if there were fluctuations, then there would be data.

There is such data, see this story from 2016, where researchers were able to measure the daily gravitational tugs of both Sun and Moon, in the basement of their laboratory.
https://spectrum.ieee.org/energywise/energy/fossil-fuels/stampsized-gravity-meter-could-have-big-impact-on-oil-exploration

11. The way to refute the whirlpool theory is to show coves and bays where there are fast rotating whirlpools, but there are no abnormally high tides.

An alternative is to show that high tides occur where there are no large whirlpools (tiny whirlpools are inevitable wherever there is turbulence, such as along a coastline).

12. Frequently Asked Questions Concerning Tides:
http:/.../mypages.iit.edu/~johnsonpo/smart00/lesson4.htm#tidefaqs

This web page and it's links seem to present well-described science. You should try reading it, Fermer.

9

Physics, Astronomy & Cosmology / Re: How do we know what the time is?

« on: 12/05/2018 23:18:21 »

Listen to physicist Carlo Rovelli talk about why time should be removed from physics (23 minute podcast). Carlo works on quantum gravity.
- He suggests that only entropy gives us a direction to time.
- In response to a listener question about what does this do to 4-dimensional spacetime (a la Einstein), he responded that quantum gravity also does away with the concept of continuous space...

https://www.sciencefriday.com/segments/a-case-for-why-time-may-just-not-exist/

This is described more in his new book, The Order of Time.

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New Theories / Re: Rotating earth theory of two high tides a day

« on: 12/05/2018 03:42:39 »

How to explain the fact that in southwest Australia in the port of Fremantle the tides are a part of the year significant, and then disappear.
What is lost, gravity or whirlpools?

As it happens, soon after your post about Fremantle, @chris flew from the UK to Fremantle to check it out*.

He reported no signs of giant whirlpools in the southern Indian Ocean, and so he can neither confirm nor deny your hypothesis that tides in Fremantle are caused by such whirlpools.

* Actually, he was there for an entirely independent reason...

11

Physiology & Medicine / Re: What is the best way to stay healthy in a sedentary office job?

« on: 08/05/2018 11:22:37 »

You could go extreme, and get a hamster-wheel desk...

But more conveniently, you could go for a walk at lunch time, or walk to the train station.

12

New Theories / Re: Rotating earth theory of two high tides a day

« on: 07/05/2018 22:32:56 »

sailors manage to measure (the height of tides)... without having an idea of the real causes of their occurrence.

Measuring the height of tides just needs a fixed scale against which to measure the height of the water, and a low-frequency filter that can remove the higher-frequency "noise" of the waves.

Unfortunately, there is no "fixed scale" in the middle of the oceans, since your boat tends to move up and down with the tide. So sailors could not measure tides when they were out to sea.

Seeing where tides originate had to wait for the introduction of radar satellites that could accurately measure the height of the tides at all points on the ocean's surface. Now we can model global tide heights on computers, we feel we have a good picture of the influences on the tides - and whirlpools aren't the primary driver.

13

New Theories / Re: Rotating earth theory of two high tides a day

« on: 28/04/2018 01:35:43 »

the height of the tidal wave depends on ...the orbital velocity of the Earth

Is this "orbital velocity":
- The velocity of the Earth in its orbit around the Sun? =100,000 km/h
- The velocity of the Earth's surface in its rotation around the Earth's axis? = 1700 km/h at the equator (and lower towards the poles)
- The velocity of the Earth's center around the Earth/Moon barycenter? = 35 km/h

Since tidal circulation is affected by Coriolis forces, then I suspect that the most important factor is the Earth rotating around its axis, as the Earth rotates through 360° in just 24 hours.
- The Moon's orbit takes 28 days to rotate 360°
- The Earth's orbit around the Sun takes 365 days to rotate 360°

14

New Theories / Re: Rotating earth theory of two high tides a day

« on: 26/04/2018 11:03:19 »

2...the wedge-shaped bays and bays in the east of the Gulf of Maine are quite numerous?

To get extreme tides, the shape, cross-section and depth of the bay must combine to provide the right resonant frequency.

Most bays won't meet this criterion.

3. How the Coriolis force works in the Gulf of Maine.

In general, the tidal bulge will rotate anti-clockwise in the northern hemisphere, driven by the Coriolis effect.
However, when you have several interacting tidal basins (eg in the Mediterranean), the adjacent tidal bulges rotate in opposite directions.

See: https://en.wikipedia.org/wiki/Amphidromic_point
Expand the world map to see the Amphidromic points close to Maine and Quebec.

Also see: http://www4.ncsu.edu/~ceknowle/Envisions/chapter11copy/part3.html

15

New Theories / Re: Rotating earth theory of two high tides a day

« on: 25/04/2018 22:48:00 »

Then why (are) the highest tides are formed in the temperate zones and not at the equator? The highest tides on Earth are formed in the Fundy Bay in North America...

I would hardly call the Bay of Fundy "temperate zone"... I wouldn't want to go swimming there!

The reason that the tides are so high in these regions is because the natural resonance frequency of the water sloshing in and out of these bays is similar to the driving period of the Moon's orbit, which is a little over 12 hours (or the Sun, which is 12 hours).

Just like a child's swing in the playground: if you push it at a rate close to the resonant frequency, you can get a large amplitude of motion. However, if you push it at a very different rate, the swing will still move, but it will be much lower in amplitude.

These locations with extreme tides cannot be due to centripetal force from whirlpools - a long, narrow channel like the Bay of Fundy will not support circular motion of the water with a large radius. The same goes for Mont St Michel, which is on the English Channel/La Manche.

PS: I have seen that in Brittany (North-Western France), small boats are built with little "feet". This is so that when the tide goes out an enormous distance, the boat remains upright on the sand, and refloats on the rising tide. Without this, the boat would sit at an angle on the keel, and be swamped by the rising tide...

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Chemistry / Re: Why Mass <> moles<> no of molecules are interconverted?

« on: 24/04/2018 11:31:08 »

Equation #2 describes the following reaction (molecule by molecule):
CH₄ (g) + 2O₂ (g) = CO₂ (g) + 2H₂O (g)

Equations #1 (mole by mole), #3 (liter by liter) & #4 (gram by gram) are just ways of expressing the following reaction (where N_A is Avogadro's constant):
N_ACH₄ (g) + 2N_AO₂ (g) = N_ACO₂ (g) + 2N_AH₂O (g)
#1: Avogadro's number of molecules = 1 mole (regardless of whether is is solid, liquid or gas)
#3: Avogadro's number of molecules of an ideal gas takes up 22.4 liters at standard temperature & pressure
#4: Avogadro's number of molecules has a mass in grams which is equal to the atomic mass in grams (regardless of whether is is solid, liquid or gas)

These are some interlinked definitions.
#4 is the definition, which determines the value of N_A
#3 is a measurement, which can be extrapolated to other temperatures and pressures (provided the pressure doesn't get too high, or the temperature get low enough for the gas to start condensing into a liquid).

These are all ways of expressing Proust's Law, which dates from 1779:
https://en.wikipedia.org/wiki/Law_of_definite_proportions

17

General Science / Re: How do seizures occur?

« on: 24/04/2018 11:17:07 »

What fail-safes did evolution manifest?

The harshest fail-safe is that an animal which has regular seizures is more likely to become lunch.

At the electrical level, neurons are affected by signals that encourage firing, and those that inhibit firing. This is a complex, non-linear system which can sometimes exhibit cyclic and/or chaotic behavior - especially if an injury or stroke damages the normal circuitry.

This abnormal electrical behavior is not just restricted to the brain - similar faulty circuits in the heart can result in arrhythmia or fibrillation - which can be fatal.

The brain can't tell the impulses are coming from an injured cell vs a healthy one.

An injured cell is more likely to die, and produce no impulses.

It is estimated that about 1 adult in 8 suffers from tinnitus, which is caused by damage to the inner ear, causing cells in the cochlear to misfire, producing spurious signals in the brain which are interpreted as sound.

Most people manage to ignore it to some extent - but for others it can be quite debilitating.
See: https://en.wikipedia.org/wiki/Tinnitus

Tinnitus is a case where the spurious signals are mostly restricted to the auditory section of the brain, disabling one sense, but leaving muscle control unaffected.
However, in epilepsy, the spurious signals spread out to involve large sections of the brain, causing loss of consciousness and convulsions or rigidity. In extreme cases, surgeons have separated the left and right side of the brain by severing the corpus callosum, in an attempt to prevent these electrical storms spreading from one side of the brain to the other.

See: https://en.wikipedia.org/wiki/Epilepsy#Mechanism

18

Physics, Astronomy & Cosmology / Re: What has been learned about Venus from its recent transit?

« on: 23/04/2018 12:04:55 »

Recently a twice in a lifetime opportunity occurred

If we were orbiting a star 100LY away, which was on the plane of Venus' orbit, it would not be a "twice in a lifetime" opportunity, it would be an opportunity which repeats every 7 months.

What makes it so rare on Earth is that there is a narrow band where the plane of Earth's orbit crosses the plane of Venus' orbit, and it is rare that both Venus and Earth are in that narrow band on the same day.

if the transits had been across a star say 100 LY away how much would we have learnt ?

If we had a telescope, it had better be above any CO₂ or N₂ in the atmosphere, otherwise we would miss 96% and 4% of Venus' atmospheric composition, respectively. A satellite observatory is strongly recommended!

The amount of light blocked by Venus is (0.95/109)², or 0.0076% of the Sun's area. With a decent model of the age, structure and diameter of the Sun, this would give you an estimate of the diameter of Venus.
 
Of the light blocked by Venus, a tiny sliver of a percentage would have been modified by passing through the atmosphere of Venus. It would be very hard to detect this change in spectrum, even if your space telescope had a very large diameter.

It may be better to take measurements about 4, 5 and 6 months after the transit, where a considerable amount of the light falling on Venus would be reflected in our direction as the illumination of Venus changed from crescent to almost full. Venus reflects about 70% of the light falling on it, which gives a much better chance of working out its atmospheric composition from the subtle changes in spectrum (compared to the transit).

Unfortunately, these spectrum readings would be muddied by the reflection from Jupiter: Jupiter has 136 times the visible area of Venus, but only 1.8% of the light intensity, and half the albedo, producing a similar brightness to Venus.

But the most successful approach may be to use a coronagraph, which blocks out the light from the star, allowing imaging of multiple orbiting planets at once. Unfortunately, an effective space-based coronagraph would require two satellites in orbit, maintaining a precisely fixed orbital relationship relative to the distant star for a long exposure - a very difficult problem in orbital mechanics! A coronagraph has the advantage that it does not rely on the observer having a particular orbital alignment with the planetary system.

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

19

Chemistry / Re: How does a non-polar solvent dissolve a non-polar solute?

« on: 22/04/2018 05:39:27 »

Solid paraffin consists of long hydrocarbon chains that get easily tangled around each other, so the weak intermolecular bonds are enough to make it go solid at quite a high temperature.

In contrast, a small-molecule organic liquid consists of many small molecules that are too small to get easily tangled with each other, so they are liquid at room temperature. In fact, ethane and methane are gases at room temperature, so adding some hydrogen bonding (eg ethanol) or a big chlorine molecule (eg dichloromethane) keeps them liquid at room temperature, and easier to handle. More importantly, there is a much greater density of molecules in a liquid., and this makes it easier to overcome the molecular attraction between the paraffin molecules.

The small organic molecules will have a similar affinity to the long-chain molecules as the paraffin molecules have to each other, so they will become more mobile. When a long-chain hydrocarbon is completely surrounded by short-chain hydrocarbons, it can no longer get tangled with others, and it will be a liquid at temperatures where the pure paraffin would be solid.

20

Chemistry / Re: Does 1 volume of hydrogen + 1 volume of oxygen make 2 volumes of water vapour?

« on: 20/04/2018 11:41:04 »

The question talks about "volumes" of a gas, eg liters, cubic meters or footballs.
- They don't need to specify how much volume is present because gases combine in certain ratios of volume, and as long as the temperature and pressure is the same when you take the measurements, you will get the same ratios.
- This rule of ratios was discovered before scientists discovered the nature of atoms and molecules
- But chemists like to talk about a particular volume called a molar volume of a gas, because you can tell how much mass is present by just looking at a periodic table.
- A molar volume occupies 22.4 liters at 25C and normal atmospheric pressure, independent of what type of gas is present, and has the same number of molecules = 6.022 × 10²³ (Avogadros's constant, abbreviated N_A).
See: https://en.wikipedia.org/wiki/Molar_volume
https://en.wikipedia.org/wiki/Avogadro_constant

So a "mole" of oxygen will occupy 22.4 liters (at 25C) and contains 6.022 × 10²³ molecules of O₂. If you look at a periodic table, you will see that the atomic mass is 16 (or 15.9994 if you want to be really precise). This mole of oxygen will have a mass of 2x 15.9994 = 31.988 grams.
- And a mole of hydrogen will occupy 22.4 liters (at 25C) and contains 6.022 × 10²³ molecules of H₂. If you look at a periodic table, you will see that the atomic mass is 1 (or 1.00794 if you want to be really pedantic). This mole of hydrogen will have a mass of 2x 1.00794= 2.01588 grams.

Now, to address your questions:

1. How many hydrogen molecules are in 2 volume of hydrogen?
2. How many oxygen molecules are in 1 volume of oxygen?
3. How many water vapor molecules are in 2 volumes of water vapor?
please explain. I am confused.

Since we don't know (and don't really care) how big this "volume" is, we can't say how many hydrogen and oxygen molecules are present. But we can say:
- There is the same number of molecules in 1 volume of hydrogen as there is in 1 volume of oxygen
- There are twice as many molecules in 2 volumes as there are in 1 volume
- Assuming that they are all gases, and all measured at the same temperature and pressure

''1 volume of hydrogen + 1 volume of oxygen = 2 volumes of water vapor''

I'm afraid that this doesn't quite stack up.

Now that we do know about atoms and molecules, we can write an equation like:
2H₂ + O₂ → 2H₂O
Which can be read as ''2 volumes of hydrogen + 1 volume of oxygen = 2 volumes of water vapor''.

This is a little tricky, because:
- If you conduct this reaction, it releases a lot of heat, and the temperature is far higher than 25C. So rather than causing a net reduction in volume, it causes a massive increase in volume, quite possibly causing glass to shatter and injuring the person conducting the experiment.
- You will know that at a temperature of 25C, water is not a gas, but is a liquid. So if you collect the hot gases and cool them down, it turns into a liquid which takes up far less space than 2 volumes of gas.
- This is why they have to specify water vapor. But to measure the volume of water vapor at 25C you have to do tricks like measure the volume at increased temperature and/or reduced pressure, and extrapolate to 25C and atmospheric pressure.

Warning: Do not try this experiment at home until you have done it safely, when supervised in a science class. And do wear safety goggles!