1221

Physics, Astronomy & Cosmology / Re: gravity and time dilation

« on: 30/11/2015 20:24:36 »

I assume that this question is about time dilation due to gravitational fields (ie general relativity), but also time dilation due to velocity (ie special relativity)?

Both effects affect the atomic clocks on the GPS satellites, and they work on opposite directions, the two effects partially canceling each other. The clocks on GPS satellites run faster in orbit because they are further out of Earth's gravitational well; but they also run slower because they are traveling at orbital speed.

a (huge) artificial satellite... a beam of light reaches the satellite surface (and) bounces back from a mirror at its center

From this description I am imagining something like a huge partially reflective balloon in a circular orbit around Earth, inflated by a low-pressure gas, with something like a retroreflector at its center?

A laser pulse from Earth's surface is partially reflected from the metalized surface of the balloon and the remainder is reflected back to Earth from the retroreflector at the center. Someone on Earth can measure the delay between the two light pulses.

A similar set of tools on the ground is used to measure the distance to the Moon.

Since this satellite is a big balloon, I assume that it has negligible mass, so it will effectively have no gravitational time dilation of its own (just the time dilation due to the Sun and the Galaxy, which is shared with the Earth).

Since the retroreflector is moving in a circular orbit, it is not traveling radially towards or away from the laser, so there will be no doppler shift, and no relativistic frequency shift (ie there is no laser source on the balloon).

For someone in the earth, the time would be bigger than 2r/c due to the satellite orbital speed.

I don't expect so. There is no clock on board the satellite, so we are really just talking about the speed of light in an (almost) vacuum.

I don't think the satellite orbital speed will have any impact on the result.

The beam should be travelling a curved path from the surface to the center and back to the surface from earth perspective.

I don't expect so. The photons in the laser beam travel in a straight line. Some will be reflected from the metallic surface of the balloon; others will bounce off the mirror. Because they are traveling radially out from the Earth and returning radially to the Earth, any curve will be negligible. (When Eddington was trying to test the General theory of Relativity, he used light traveling tangentially to the Sun's surface, close to the Sun's much larger mass, and was able to demonstrate a small bend in direction.)

I expect that the light will travel in straight lines, for all practical purposes.
 

For someone in the earth, the time would be bigger than 2r/c

Since the Earth is in a gravitational well, time will pass more slowly on Earth. So I expect that the measured time will be smaller than 2r/c.

But in reality, light is very fast, and gravitational time dilation on Earth is very small. The ability to measure r accurately is very limited, since a balloon is very stretchy, it's shape will be slightly distorted by the heat of the Sun, the Solar Wind and/or any drag from Earth's outer atmosphere, and it is very hard to mount a retroreflector at the precise geometric center. The proposed experiment is trying to measure something very precisely (gravitational time dilation), using something which can be known only very approximately (r).

So I suggest that the best way to measure r is to measure the time difference between a pulse of light reflected from the surface, and one measured from the mirror at the center. Forget measuring gravitational time dilation this way. Much better measurement of relativistic effects are available from GPS satellites and Gravity Probe B.

1222

Physics, Astronomy & Cosmology / Re: Can we put an end to the nonsense of "duality"?

« on: 22/11/2015 04:53:29 »

...and just when I think I've understood light as a wavicle, it behaves like a particave.

1223

Physics, Astronomy & Cosmology / Re: Would Dark Matter be needed if everything was bigger and closer than it is?

« on: 18/11/2015 10:44:42 »

23% closer in distance

The distance to the nearer stars can be measured from the ground, by using parallax. Some recent space probes have measured the distance to many stars using this method.

So I think a 23% error is unlikely, at least for our neighbourhood in the galaxy.

Greater astronomical distances are measured by a variety of methods, including Cepheid variable stars and supernova explosions.

all bodies of mass in the universe..were 23% bigger than we estimate?

By "23% bigger", I assume that you mean "23% more massive"?

It is hard to measure the mass of a star directly, unless you can measure the period & radius of a planet's orbit. The Kepler space telescope has given us the period of many planets, but I am not sure if the radius of the orbit can be estimated from this. Some proposed future space probes should be able to image planets directly.

Mathematical models of stars show that the light output is strongly dependent on mass. So by knowing the distance and brightness, it is possible to constrain the mass of visible stars fairly closely. I expect an error of 23% is unlikely.

what percentage of dark matter would then be necessary?

The anomalies we attribute to Dark Matter were first detected in the rotation curves of other galaxies.

Because the stars in another galaxy are all at roughly the same distance from us, the "23% closer" does not make a difference.

Even if the mass of the galaxy were "23% greater", that would not account for the velocity of rotation of the galaxy remaining fairly constant at different radii. Based on the distribution of visible matter (stars), the more remote portions of the galaxy should rotate more slowly than what is observed.

So these two "23%" assumptions do not eliminate the need for some mass that we just don't see, ie "Dark Matter". We just need to find out what the Dark Matter is; there are a number of theories, and the answer is probably some mixture (and probably some surprises too).

There are even more theories that have been disproved; unfortunately, I think the 23% hypothesis must go into this category.

1224

Cells, Microbes & Viruses / Re: Can humans survive a 0.13% atmosphere?

« on: 17/11/2015 09:27:42 »

I assume this means 0.13% of normal atmospheric pressure?

Short answer: Unconscious in about 30 seconds, dead in 5 minutes.

Longer answer: Earth's surface has an atmosphere which is about 20% oxygen, 80% nitrogen and 1% miscellaneous (but important). The pressure is about 100 kiloPascals at sea level.

The nitrogen doesn't do very much directly for us, so spacesuits often don't carry it.
You can survive on the 20 kiloPascals of Oxygen, which is about 20% of atmospheric pressure, provided you also have some of the miscellaneous: CO₂ to trigger the breathing reflex, and some water vapour to keep your airways moist.

The oxygen pressure at the top of Mount Everest is about 7 kiloPascals. Some mountain climbers have made the summit without oxygen, but they would die if they stayed up there too long.

So you would slowly die if the air was 100% oxygen, but at 30% of normal pressure.
Lower pressures mean you die faster.

At very low atmospheric pressure, other effects occur like water would boil out of your lungs and skin. Some innovative spacesuit designs are planning to apply mechanical pressure to the skin, without actually applying air pressure. (Your lungs need genuine oxygen pressure!)

1225

Physics, Astronomy & Cosmology / Re: Human footprint in space: What are we emitting from earth?

« on: 17/11/2015 09:12:11 »

It depends what you are trying to hide.

For an alien at the distance of Pluto, it would be fairly easy to tell that Earth has an oxygen-rich (unstable) atmosphere, hinting at the presence of life. This alone may trigger a closer look by curious aliens. Hiding the signal would almost take a Dyson sphere.

If you want to hide advanced technology without going entirely back to the bronze age, you could communicate via medium wave and shortwave radio; the ionosphere reflects it back to Earth.

Higher frequencies (such as used by TV, FM radio and radar) go straight through the ionosphere into space, giving away the presence of a technological society.

If you want an internet, optical fibers work at high speeds, and don't emit telltale signals.

But really, why would your average alien be interested in anything happening on Earth?
- We see a lot of "Hot Jupiter" exoplanets; for aliens from these worlds, Earth is too cold and too small, and doesn't have enough atmosphere.
- On the other hand, aliens from the Oort cloud comets would find Earth much too hot and heavy.

1226

Physics, Astronomy & Cosmology / Re: Has there been only ONE Big Bang???

« on: 13/11/2015 15:49:32 »

it may be safe to say that this thread belongs in the New Theories section

This is not a particularly new theory - Einstein considered a cyclic universe to be a possibility (presumably, after he had dispensed with his "Cosmological Constant").

More recent researchers have also taken a cyclic model seriously, including models related to string theory.

The recent discoveries of Dark Energy (the accelerating expansion of the universe, restoring something like Einstein's Cosmological Constant) make it unlikely that the matter in our universe will clump together again.

Other theories relating to the Multiverse suggest that there may have been other Big Bangs, elsewhere in the Multiverse.

1227

Physics, Astronomy & Cosmology / Re: Can a photon be length contracted?

« on: 05/11/2015 01:44:05 »

Can the concept of length contraction be applied to photons?

Yes. This works with photons (light) as well as phonons (sound).

But it is a Doppler contraction of the wavelength (or it's relativistic equivalent at very high relative velocities between source and receiver).

Unlike relativity, you can also get wavelength expansion, if the source is moving away from you.

1228

General Science / Re: How does crowdsourcing work in scientific projects?

« on: 02/11/2015 08:48:23 »

I guess crowdsourcing has always been used by scientific journals to peer-review papers submitted for publication.

There was recently another type of crowdsourcing scientific research in Australia.

The MOPRA telescope is mapping molecular dust clouds in the plane of the Milky Way. But the government decided not to fund them to finish the task they had set themselves.

So they appealed to the general public, and managed to raise enough money to finish their initially planned survey (and a little bit more). So they survive another year, and will start to map molecular dust clouds further outside the plane of the galaxy (hopefully detecting any that might be heading our way...). http://mopra.org/

Crowdfunding is a little different than traditional science grant applications (eg "What design will be print on the T-Shirts?" and "How much should we charge to 'name' a molecular gas cloud?").

Crowdfunding has been used by the Planetary Society to fund a pilot solar sail, and another project proposing a satellite looking for Near-Earth Asteroids is also looking for public funding.

I guess anything that inspires the public to dip into their pockets to help science is a good thing (and more productive than the horse race that will have most Australians dipping into their pockets tomorrow).

1229

New Theories / Re: Can the speed of light be faster than time itself?

« on: 02/11/2015 08:26:31 »

Thebox, are you saying that anything that travels faster than (1035) mph is travelling faster than time?

What does  travelling faster than time mean?

So it seems that this question is not about Einstein's relativity, but it's all about the speed of the Terminator: the day/night dividing line (not the time-traveling cyborg).

So with the deduction that the speed of the terminator at the equator is 1035 mph, this means you could get in a rocket, and travel faster than the terminator, and make the Sun "rise" in the west!

This has already been done. But they cheated, because at high latitudes, the terminator moves slower - and at the equinox at the North and South Poles, it does not move at all!

On one occasion, the Concorde supersonic passenger jet picked up passengers after their midnight New Years Eve celebrations in London, and delivered them to New York just in time to do it all again! They did not turn inside out, become frozen, become gods, or even zombies (although they may have felt like the latter the next morning).

In the extreme, a polar explorer could easily do a quick jog around the South Pole in 5 minutes, but its not like they will get any younger, or can change the future or anything.

1230

Chemistry / Re: Does burning candles improve air quality?

« on: 30/10/2015 23:50:53 »

Many smelly organic chemicals are flammable, so if they pass by a flame, they are likely to be broken down into non-smelly chemicals like H₂O and CO₂. However, they are also likely to  produce more noxious substances like NO_x and SO₂.

Incomplete burning of the wax can also create nanoparticles of soot, which may find their way deep into your lungs and cause irritation.

But if you are so familiar candles that you "like" the smell, or you find them "romantic", then you probably will overlook some of the negatives.

1231

Physics, Astronomy & Cosmology / Re: What is the largest possible rest mass of the photon?

« on: 30/10/2015 07:44:28 »

It is very hard to detect radio waves using an antenna which is much smaller than the wavelength. So in practice, it is hard to measure propagating radio waves with a frequency of < 1Hz, where the wavelength is larger than the Earth.

However, the Sun is effectively a low-frequency transmitter, producing 1 cycle of radiation from its solar magnetic field about every 22 years (approximately). The amplitude of this 22-year cycle is reflected in the 11-year sunspot cycle. However, due to the size of the Sun's magnetosphere, only a small fraction of the energy in this oscillating magnetic field would be released as freely-propagating EM waves.

1232

Physics, Astronomy & Cosmology / Re: Could you become a God by traveling at the speed of light?

« on: 26/10/2015 11:16:26 »

Imagine with me if we can move as fast as time what will happen, time will get slower and slower and when you reach to its speed times stops for you [/size][/font]
This seems to be talking about some type of time dilation.
I can see a couple of problems with this speculation:
- If you wish to make some changes in the frame of reference of Earth, moving very rapidly relative to Earth will make it appear to someone on Earth that time is moving slowly for you, and you are aging slowly.
- But from your viewpoint, everything is happening at the usual rate, so you aren't long-lived at all!
- From either frame of reference, if you are moving very rapidly, that gives you little visibility of what is happening on Earth, and little opportunity to change it in a controlled way.]

at the very least we can change the course of time

[/size]Time travel has always been a favorite topic of science fiction (some of them more serious than Terminator or Doctor Who), because then you can dream up all sorts of mayhem in which to engage.
[/size]
[/size]But so far, anything more than unidirectional time travel has remained out of our grasp; some claim that it is impossible, as it would violate the principle of Causality (which seems to be a good principle, in the absence of a time machine or faster-than-light warpdrive).

1233

Geology, Palaeontology & Archaeology / Re: When during human evolution did the first scientist appear?

« on: 25/10/2015 20:25:13 »

when chemical entropy changes near neurons and synapses new states of matter will appear that reflect this value.

In humans, neurons in the brain operate pretty much at 37C (if it diverges by more than a few degrees, you die).

When a nerve impulse propagates through a neuron, the temperature remains at 37C (because the brain has an effective liquid-cooling system).

No new states of matter appear because the temperature does not approach 0C (where ice could appear) or 100C (where steam could appear) or the air pressure on Mars (where steam could appear). The neuron operates in a liquid water environment.

The only change of state is the transient depolarization/repolarization change which propagates through the cell, plus possibly some more permanent adjustment in the synaptic linkage to adjacent nerves.

1234

Physics, Astronomy & Cosmology / Re: Do photons and massive particles behave the same in double slit experiments?

« on: 25/10/2015 03:22:50 »

Quantum effects have been demonstrated for particles as large as buckeyballs (C₆₀).

There is a claim that the dual-slit effect has been demonstrated on molecules having over 800 atoms.

1235

Physics, Astronomy & Cosmology / Re: is it possible to change the rotational speed of earth?

« on: 24/10/2015 02:10:52 »

Yes. The earthquake that destroyed Fukushima changed the rotation of the Earth by a measurable amount.

Less dramatically, Earth's tides are affecting the rotation of the Earth.

Humans are a long way from wielding the energies involved in all of Earth's tides or a large earthquake.

1236

Physiology & Medicine / Re: Why can't we remember everything?

« on: 24/10/2015 01:59:50 »

The human brain and sensory system has a very impressive form of data compression.

For example, to provide a video display that fills the human visual field with the maximum resolution of human vision would require somewhere around 2Gbps.

However, the processing in the back of the eyeball reduces this to an estimated 100kbps in the optic nerve. It is this compressed data stream that is processed by the brain to recognize people we know, and the child about to step in front of our car.

Similar processing occurs in our ears.

It takes previous experience to recognize these patterns - recognizing a mother's language and food appears to start before birth, and recognizing a mother's face and scent occurs in the first few months after birth.

The human brain does not store a video recording of these data streams, but integrates this with previously stored events. It is thought that this occurs by strengthening certain synaptic links, and weakening others. When we recall an event, our brain fills in the gaps with what "probably" happened, based on the sum total of our previous experiences.

These linkages are not permanent, but are revised every time we recall an event, and the memory can be changed by the context at the time we recall it. This undoubtedly saves a lot of neurones!

1237

Physics, Astronomy & Cosmology / Re: If an object is moving through space and no force is applied, where does it go?

« on: 21/10/2015 11:34:08 »

is it true that an object set into motion in outer space will travel in a straight line until a force alters it's direction?

One aspect that has not really been addressed: When you are near a massive object like the Earth or the Sun, gravity provides a force that attracts the two, changing both their paths. (Although the change in motion of the Sun would be very small and hard to measure for any object smaller than a planet.)

take that cannon ball up to the space station, and just 'shove' it off into space... it would just keep traveling in a straight line.

The space station is in orbit around the Earth, taking about 90 minutes to go around once. The Earth's gravitational field bends the path of the space station into a circle.

If you shove a cannonball off the space station, the cannonball will also be in orbit around the Earth, also taking about 90 minutes for one complete circuit.

If you gave it a really big shove (with a powerful rocket), you could take it out of Earth orbit, and into orbit around the Sun. It would take about 1 year for an orbit around the Sun.

from what I now understand the atmosphere in space wouldn't offer any resistance to the ball

The very thin atmosphere at the height of the ISS does provide some drag, causing it drop in altitude every month. To avoid the ISS crashing back to Earth while it is still in use, they use a rocket to boost its height every month or so.

A solid cannonball is denser than the hollow ISS, so it would take longer to crash, but it would eventually hit the thicker part of the atmosphere, forming a little fireball as it plummeted towards Earth.

1238

Physiology & Medicine / Re: Could you urinate on the moon?

« on: 18/10/2015 21:50:05 »

Given that water is a pretty scarce resource over most of the Moon, peeing into the Solar wind would be a bad move for anyone planning a long-term stay.

1239

Technology / Re: Can a powder make a powerful magnet?

« on: 14/10/2015 21:41:19 »

Neodymium magnets are quite strong, and they are formed from a powder which is rapidly cooled - the temperature is dropped by millions of degrees per second to "freeze in" a metastable structure. Obviously, this cooling does not continue for anywhere near a second!

Then a strong external magnetic field is applied while slowly cooling the material down through the Curie temperature, "freezing in" the external magnetic field in the orientation of the atomic-level magnetic fields.

As for attracting objects from several meters away, the mass and friction of an iron or other magnetic object would prevent it moving closer to the magnet, unless it were mounted on an air-hockey puck. It would probably work OK in the International Space Station, in a microgravity/microfriction environment.

You really need a superconducting magnet to attract objects from meters away in Earth's gravity. Some types of superconducting wire are manufactured from a powder, but the magnetic field is generated by the supercurrent, not by the wire itself.

See:

...but do not try this when you go for an MRI!

1240

General Science / Re: How would you operationalize Authority?

« on: 12/10/2015 21:24:03 »

If you need to do an experiment, you could google previous work in Psychology on the subject of Authority (referring to a few relevant papers in your report shows that you did some homework).

Then extend their work into a new area (perhaps one relevant to you, or in your locality).

A quick search turned up a questionnaire on Parental Authority - but there are many kinds of authority.
http://www.tandfonline.com/doi/abs/10.1207/s15327752jpa5701_13

I was recently talking to a psychology student who was able to engage many people to take part in his thesis study by connecting with them on social media, and pointing them to his questionnaire on-line. Tools like Survey Monkey are free, and will collect the results for you.