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Let's start by talking about Newton's First Law of Motion:

An object at rest will stay at rest unless acted upon by an unbalanced force. An object in motion will stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.


So let's look at the Moon as it orbits around the Earth.


So there's the Moon moving around the Earth (the black dot with the two arrows). Newton's First Law, if there was no other forces acting on the Moon, would follow the Forward Motion arrow - it would fly off happily, in a straight line at a constant speed, forever.

But it doesn't. Why is that?

Because there is another force acting on the Moon - which is the Pull of Gravity arrow. Earth's gravity, if there were no other forces acting on the Moon, would have it plunging down onto (and into) the Earth - resulting in the biggest collision the world has ever experienced.

Thankfully, the balance of the forces, the Moon's inertia and Earth's gravity, act on the Moon to keep it in orbit (one of my professors described it as "the Moon is continually falling towards the Earth and missing").

And so the interaction of the two forces creates accelerated movement - Earth's gravity constantly pulls on the Moon and that is the source of the acceleration (that constant change in direction).

So now to the Kepler portion of the question - it is true that orbits are slightly elliptical and not purely circular. In the case of the Moon, this ellipse is quite elongated compared to the orbit of the Earth revolving around the Sun. However, it is not the elliptical quality of the orbit that makes the orbiting motion accelerated motion.



At least #4.404# billion years, probably #4.54# billion years.


Date estimates are obtained by radiometrically dating very old rocks.

OK so far, but:

  • How can you find a very old rock when rocks are typically recycled by tectonic processes?

  • How do you know that a rock is actually part of the Earth and not part of a meteorite?

  • What is this radiometric dating anyway and how do we know that it is accurate?

Firstly, though rocks are subsumed in the mantle and new rock formed from volcanic eruptions, there are some samples of very old zircon found in Austrailia that managed not to be recycled.

Meteorites have specific characteristics that allow us to differentiate them from other rocks. They may have fusion crust - though that does tend to break off after time. They may differ in density from terrestrial rocks, or exhibit particular kinds of fractures, etc.

Radiometric dating measures the relative proportion of different isotopes. For example, over time uranium turns into lead. So by measuring the relative proportions of lead and uranium in a sample you can date it. That's a very crude description of what is a precise and carefully executed technique.

By means of radiometric dating we have measured the age of the old zircon in Australia at #4.404# billion years and meteorite material at #4.54+-0.05# billion years.

So what does the age of meteorites tell us about the age of the Earth? According to our understanding, they are kind of left over material from the formation of the solar system. Unlike rocks on Earth they have not been subject to volcanic activity, but were formed at about the same time as the Earth. So it is generally reckoned that their age is the same.

How sure are we?

Radiometric dating techniques are well corroborated. Uranium based dating has the advantage of the ability to cross check between #""^235U -> ""^207Pb# and #""^238U -> ""^206Pb# decay processes for extra confidence.

So I would say we are pretty sure.


Beware of some internet sites which claim the radiometric dating is faulty. They often have unscientific agendas.


Astronomers use the Doppler Shift to determine the speed at whih a source of light is moving. The faster the source moves, the greater the shift in the colour of the light we observe (compared to the colour of a stationary source).


Anyone who has listened to a train approaching and passing on a track has been aware of the change in sound pitch that occurs. This effect is known as the Doppler Shift, and is well understood for all types of waves, including sound and light.


The image is meant to show that if the wave source is moving toward us, the waves will be compressed ahead of the source, and a high frequency is heard. Behind the source, the observed pitch is lower than that of a stationary source. The faster the source moves, the greater the change in pitch that is observed.

With light, the effect is not in pitch, but colour. If a source of light moves toward an observer, the waves will be shorter in wavelength and higher in frequency. The light will appear to be shifted toward the blue end of the spectrum. If the source is moving awat from the observer, the shift in frequency causes the colour to appear more red. As in the case of sound, the faster the source moves, the greater the change in colour that is observed. This is the red shift that is seen in all galaxies.

So, to finally answer your question, when we measure the light from more distant galaxies, we routinely note larger shifts in the colour of the light they emit. This tells us they are moving at greater speed than the galaxies that are "close by".


The Doppler effect, which changes the spectra of of stars and galaxies toward the red or longer wave lengths, tells scientists that the universe is expanding at an ever increasing rate.


The red shift first observed by Edwin Hubble changed the way scientists viewed the universe. The red shift is part of the Doppler affect waves coming toward the observer move toward the blue side of the spectrum as the waves get closer together, and waves move toward the red side of the spectrum as the waves get further apart. Before Hubble's observations of the change in the spectra of stars scientists believed that the universe was static. The belief was that the universe had always existed in its "present" observational state. This view was consistent with the philosophy of material realism that matter and energy are all that has ever existed or ever will exist.

After the observations of the red shift doppler effect were widely accepted the static steady state view of the universe was abandoned. The Big Bang theory began to be developed in response to the empirical evidence of an expanding universe.

Recently ( in 1998) observations of the rate of expansion of the universe is increasing. The observations were based on the spectra of super novas, made by the Lawrence Berkley National Laboratory.
The hypothesis was that the rate of the expansion should be slowing down. The

The hypothesis was based on the theory that the universe was eternal and would recycle alternating between Big Bangs and Big Crushes. The Hypothesis was proven wrong by the observations of the changes in spectra of the galaxies.

The changes of the spectra of the galaxies tell scientists that our present universe had a beginning, will have an ending and that matter and energy are not eternal or self existent.


This is a brilliant question, but the answer isn't simple (I understand some of it!)


Essentially astronomers think that on the largest scale the structure of the universe resembles a foam (weird, eh?) It seems there are filaments and sheets of galaxies in 3D that surround huge voids.

The evidence for this comes from experiments and theoretical computations that seem to match exceptionally well. Have a look at these two, the first is a simulation, the second is a map:

enter image source here

Taken from : http://www.astronomynotes.com/galaxy/s9.htm [the chap states his material is copyrighted.... hope this does not constitute any infringement]

And the map,

enter image source here

Taken from: http://www.abc.net.au/news/2011-09-29/milky-way-hangs-by-a-cosmic-thread/3050586

There is much debate about why this is so, but the leading proponents seem to be persuaded that a model of the Universe termed LCDM (for 'lambda cold dark matter' I think) is essentially correct.

This states that the current structures we observe are due to the quantum fluctuations present in the first shavings of a nanosecond after the Big Bang and were "inflated" to relatively huge sizes in the very brief period that followed. This implies that that the same sort of density fluctuations (or imprints of those fluctuations) are or should be visible in the cosmic microwave background radiation (CMBR). The latest data from the Planck satellite launched in 2013 seems to bear this out (very tempted to include a comparison of COBE, WMAP and Planck data here, but shall restrain self.)

So you've seen it, here's an image of the data taken from the University of Cambridge Centre for Theoretical Cosmology (http://www.ctc.cam.ac.uk/news/130322_newsitem.php)

enter image source here

The idea is that slightly cooler parts of the CMBR (we're talking one part in 10,000 I believe) contained particles that were moving slightly slower, so gravity had more chance of binding them into structures that would later become stars, and galaxies.

Slightly warmer parts, coloured orange & red in the image above, became the voids we now see because the thermal wriggling of particles meant they were less likely to be bound by gravitational attraction.

Sorry if the answer is very long, someone somewhere will get to this point and hopefully have another 28 questions fizzing through their head as a result. Like I said, simple it isn't, but it is amazing.

Question #57f3c

Mark C.
Mark C.
Featured 4 months ago


OK, we need to understand a bit of quantum theory and the standard model.


The universe is best understood by reducing everything to the smallest possible number of entities (the goal of physics.) The “standard model” is our current best attempt and is based on quantum theory (more specifically, quantum electrodynamics or Q.E.D or it’s rather more bohemian cousin quantum chromodynamics, Q.C.D.)

The standard model is much like a family tree the way I teach it ... and the first split down from “everything” is into two groups called ‘fermions’ and ‘bosons’. Fermions include all matter. That means solids, liquids, gases, ceramics, polymers, you, me etc. Every single particle we know of including electrons (and there are hundreds) is a fermion. They all have one fundamental thing in common - they obey something called the “Pauli exclusion principle.”

The other group, the bosons, concerns us less in the context of your question, but includes all the known forces (called strong, weak, electromagnetic and gravity, though this last one is troublesome.)

All the known particles can be described as having a set of “quantum numbers” that determine all the properties that exist in quantum theory. Effectively this states that energy, momentum, spin and possibly even space and time exist as quantised states, meaning they cannot take any value, but only discrete ones (i.e. they are like an uneven ladder where you have to be on one rung or another, you cannot exist in between, so we would say your height is then “quantised”). The opposite would be something like an escalator, where your height could vary continuously.

The Pauli exclusion principle simply states that any fermion (particle to us, including the electrons in your question) cannot exist in the same state (complete set of quantum numbers) as another fermion. In other words (rather loosely stated, but it helps) they can be in the same place, but not at the same time, or they can be in the same time, but not at the same energy etc. It is forbidden for two or more to occupy the same complete set.

This explains why two electrons can be in the same orbit (same energy, possibly even the same position at the same time) but then cannot have the same spin. One must be spin “up” and the other spin “down” to prevent violation of this fundamental principle.

Finally, no you don’t get anything like a black hole, it is not even dangerous, just forbidden by the laws of physics.

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