1

Physics, Astronomy & Cosmology / Re: How are particles monitored for position and velocity in the LHC?

« on: 27/07/2020 09:41:37 »

It's not that the particle doesn't have a definite position and velocity, it's just that we can't measure or determine both simultaneously?

No it's that they don't  have a definite momentum or position.

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Physics, Astronomy & Cosmology / Re: Why are we not covered in starlight?

« on: 24/07/2020 23:51:56 »

Or, equivalently. much more of the light from the other stars misses your eye.

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Physics, Astronomy & Cosmology / Re: Why are we not covered in starlight?

« on: 24/07/2020 23:41:42 »

In short, the other stars are a lot further away.

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Physics, Astronomy & Cosmology / Re: Why are we not covered in starlight?

« on: 24/07/2020 21:59:03 »

If I look at a star the size of our sun in the night sky, what happens to the light it emits that results in it being just a pinprick by the time it reaches my eye?

Let's for a moment ignore the important questions raised by Olbers' Paradox, and just consider our nearest 5 stars....

The visual height of an object on my retina halves every time I double the distance.
The visual width of an object on my retina also halves every time I double the distance.
The visual area of an object on my retina reduces to 1/4 every time I double the distance (area ≈ height x width).

Our nearest Sun-like star is Alpha Centauri A.
- It's diameter is 1.1 times the diameter of the Sun
- It's distance is 4.4 light years, compared to 8.3 light minutes for the Sun.
- So the diameter of Alpha Centauri A as seen from Earth would be 300,000 times smaller than the diameter of the Sun.
- And the area of Alpha Centauri A as seen from Earth would be 80 billion times smaller than the area of the Sun.
- If you had a big enough telescope to see a visible disk at all!
- That's a pretty small pinprick...
See: https://en.wikipedia.org/wiki/Alpha_Centauri

For a glowing object like a star, the brightness reduces to 1/4  every time I double the distance; this is called the "inverse square law".
See: https://en.wikipedia.org/wiki/Inverse-square_law

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Physics, Astronomy & Cosmology / Re: Why are we not covered in starlight?

« on: 24/07/2020 21:09:35 »

This is actually a more interesting question that it first seems.
The fact that it gets dark at night offers a profound understanding of the Universe.
The fact was considered  a long time ago and there's  a pretty good summary here

https://en.wikipedia.org/wiki/Olbers'_paradox

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Physics, Astronomy & Cosmology / Re: Expansion of the universe - what is being measured?

« on: 17/07/2020 13:01:51 »

If I was watching a video of a river, I couldn't actually see how fast the water was moving.
But, if I watched the fish + bits of weed +twigs and then averaged the speeds that they were moving, I'd get a fair idea of the water speed.

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Physics, Astronomy & Cosmology / Re: Expansion of the universe - what is being measured?

« on: 17/07/2020 12:24:52 »

This telescope is trying to measure the CMB with very precise angular resolution across the sky.
- Some "Hotspots" in the CMB are due to galaxy clusters, whose high-energy electrons interact with the CMB to increase its apparent temperature (by millionths of a degree Kelvin)
- By identifying these distant galaxy clusters, they can take a measure of the expansion of the universe
- One unique thing about using the Sunyaev-Zeldovich effect is that it does not suffer from the inverse square law, like a normal telescope
- It is able to detect galaxy clusters with a redshift of 3 almost as easy as galaxy clusters with redshift of 0.2
- My simplistic understanding is that more distant objects made up a larger part of the universe (at the time the light was emitted), and so leave an imprint on the CMB that is as large as the imprint of nearer objects(?)
- A better explanation would be appreciated!

See: https://en.wikipedia.org/wiki/Atacama_Cosmology_Telescope
https://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zeldovich_effect
 

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Physics, Astronomy & Cosmology / Re: Expansion of the universe - what is being measured?

« on: 17/07/2020 11:37:47 »

I was just reading about the Atacama Cosmology Telescope in Chile. Research is as active as ever in trying to determine the age of the universe and the rate at which the universe is expanding. Does this research measure the rate at which matter is travelling through the universe or measure the actual expansion of the fabric of the universe?

The research is intended to determine the actual expansion rates of the 'fabric' of the universe, but since that cannot be measured directly, they instead measure the rate of increase of proper distance between us and distant galaxies over time.  These distant galaxies are generally not travelling very fast through the universe (usually less than 0.02c) relative to said fabric, and thus make a good way to indirectly measure the expansion rate.

The rate has not been constant over time, so for instance some distant galaxy might be receding today at twice light speed, but 6 billion years ago it might have been receding at 1.9c and 13 billion years ago it might have been receding at 3c.  Expansion had been slowing considerably since the early times, and measuring those rates from all the points along that curve is critical to estimating the age of the universe.  Problem is, for any one galaxy, one can only see it at one moment in time, depending on how far away it was then.

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Physics, Astronomy & Cosmology / Re: An expanding flat universe?

« on: 14/06/2020 20:55:50 »

The trumpet shape has been around for a while.  Eg.  https://www.independent.co.uk/news/science/the-universe-is-not-round-say-scientists-it-is-shaped-like-a-trumpet-560046.html. 

I have always assumed it was a “mathematical” model, intended as an aid to understand; rather than something that was meant to be taken too literally. 
No one will be surprised that I have grave reservations about the physical demonstration of:-
 
“The thin end would be infinitely long - but so narrow that it would have a finite volume.”

OK, Mr Arthur gets a good prize for bad reporting of science.  No scientist claims the universe is shaped like a trumpet, an object with edges.  The bit about finite volume comes probably from Gabriel's horn ( https://en.wikipedia.org/wiki/Gabriel%27s_Horn#:~:text=Gabriel's%20horn%20(also%20called%20Torricelli's,surface%20area%20but%20finite%20volume.&text=The%20properties%20of%20this%20figure,Torricelli%20in%20the%2017th%20century. ) a 3D shape which has zero to do with the shape of the universe (a 4D thing).

What the scientist are doing is taking a circle of space and plotting just that circle as it expands over time.  The circle can be any size, and nobody suggests that the universe is bounded at that circle, so the actual shape is something 3D (not a 2D circle) without bounds, expanding in a non-linear curve over the 4th dimension, time.  The 'trumpet' shape is just what you get if you graph the expansion rate (the scalefactor to name it more correctly) and twirl that graph on its time axis.  It is the shape of expansion at best, not the shape of the universe at all.

Oh, and this news is decades old.  I knew this expansion curve 30 years ago, even though it definitely has been tweaked since then. I think it was merely published as a way to get hits to their hostile site, which I found impossible to read without ad-blocker on.

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Physics, Astronomy & Cosmology / Re: An expanding flat universe?

« on: 14/06/2020 19:45:11 »

The trumpet shape has been around for a while.  Eg.  https://www.independent.co.uk/news/science/the-universe-is-not-round-say-scientists-it-is-shaped-like-a-trumpet-560046.html. 

I have always assumed it was a “mathematical” model, intended as an aid to understand; rather than something that was meant to be taken too literally. 
No one will be surprised that I have grave reservations about the physical demonstration of:-
 
“The thin end would be infinitely long - but so narrow that it would have a finite volume.”

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Physics, Astronomy & Cosmology / Re: An expanding flat universe?

« on: 12/06/2020 14:27:00 »

I get the Euclidean non Euclidean thing after reading up on it. The universe is flat and I'm seeing the parallel lines. The universe is expanding in all directions and I'm seeing those parallel lines expanding into a trumpet shape. So where am I going wrong?

The trumpet shape is probably due to something graphing space vs time.  Flat space should depict parallel lines in a graph of x vs y (both space), not x vs t.  If you expand that graph, the lines remain parallel.

The surface of Earth is a nice example of non-Euclidean 2D space.  It is difficult to draw it on a flat paper map, and any attempt to do so ends up distorting continents and such.  Parallel lines drawn on the ground always meet.  Triangles have their angles add up to more than 180°.  That's a great way to show that the surface of Earth is not flat.
They've actually attempted to do the triangle thing to measure curvature of space.  It does actually curve locally due to gravity, but on a large scale it seems to be flat.

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Physics, Astronomy & Cosmology / Re: An expanding flat universe?

« on: 12/06/2020 11:12:24 »

I'm seeing those parallel lines expanding into a trumpet shape

You could imagine the universe as a sheet of rubber. Draw two parallel lines on it while it is relaxed.
 - If you grab the rubber sheet by 2 edges and pull, it will stretch on one direction, and shrink in the other direction. This will make the lines bend; closer to each other in some parts, farther away in other parts. They are no longer straight lines.
- However, if you hold the rubber sheet all along all 4 edges and pull equally outwards, the two lines will remain straight, and will be the same distance apart all the way along (just further apart than they were originally).
 

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Physics, Astronomy & Cosmology / Re: An expanding flat universe?

« on: 12/06/2020 08:20:02 »

What does 'flat' actually refer to when discussing the expanding universe?

"Flat" means that it follows Euclid's geometry that you learned at school (on large scales, like bigger than galaxy clusters):
- If you have a line, and a point not on that line, there is only 1 parallel line you can draw through that point
- The sum of the angles of a triangle add up to 180°
- The circumference of a circle is π times the diameter

There are also varieties of non-Euclidean geometry where:
- There is more than 1 parallel line through the point (or none at all)
- The sum of the angles of a triangle add up to more than 180° (or less than 180°)
- The circumference of a circle is more than π times the diameter (or less than π times the diameter)

These ideas seem strange, until you realize that the surface of the Earth is a non-Euclidean surface.
- There are no parallel lines of longitude (they all intersect at the poles)
- You can draw a right triangle with internal angles adding up to 270° (one side is on the equator, and one vertex is at the North pole)
- You can draw a circle around the equator whose circumference is just twice the diameter

See: https://en.wikipedia.org/wiki/Non-Euclidean_geometry

In the vicinity of a black hole there are also small-scale distortions, in addition to any large-scale distortions.

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Physics, Astronomy & Cosmology / Re: An expanding flat universe?

« on: 11/06/2020 23:22:16 »

Flat, here, does not mean like a pancake or CD.
It's about whether it's curved.
https://en.wikipedia.org/wiki/Shape_of_the_universe#Curvature

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Physics, Astronomy & Cosmology / Re: An expanding flat universe?

« on: 11/06/2020 20:28:55 »

Can anyone help me visualise a flat universe that is expanding in every direction? It would seem to me that anything that is flat could not retain its flat shape if it were expanding in every direction.

Don't understand your inability to visualize it.
Consider any shape, like a heart of size 1.  Now double every dimension in it, so it is twice as wide, tall, and thick, but size 2 now.  It is the exact same shape, just expanded.  Ditto for the flat universe.  If it was flat before, doubling its space in all dimensions does not affect the shape at all, so it's still flat.  Expansion does not inherently change the shape of something.

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Physics, Astronomy & Cosmology / Re: Wave-particle duality theory and the observable universe.

« on: 04/04/2020 22:59:49 »

Can the outcomes of wave-particle duality experiments be reflected in the world I observe and experience around me?

Yes - the wave and particle nature of light has been observed over many centuries, and has had a vigorous debate over those centuries.
- The "winner" has swayed backwards and forwards over that time:
- Descartes explained refraction in terms of waves
- Newton argued that light was composed of particles
- Hooke & Huygens put the wave theory on a mathematical basis
- The Michelson-Morley experiment seemed to show that light could not be a wave (particles can travel through a vacuum, the familiar types of waves could not)
- As we now know, both ideas are correct; this synthesis happened in the early 1900s, through the work of Einstein, Bohr, Schroedinger & others
See: https://en.wikipedia.org/wiki/Light#Historical_theories_about_light,_in_chronological_order

One simple synthesis is "Light travels like a wave, but hits objects like a particle".
- So when you look at anything (eg a star), the light has spread out like a wave, following the inverse square law.
- When the light wave hits the lens of your eye, the wave is diffracted by the entire surface of the lens into a small area on your retina
- Within this small area, the light particle will hit a particular light-sensitive molecule in a particular rod cell in your retina, which will trigger an electrical signal to your brain
- So wave/particle duality applies to everything you see.

According to wave-particle duality the particle is a wave of potential until it is observed. ... there are potentially multiple me's, multiple universes??

The duality of light as a wave and a particle has been apparent for centuries.
- The possibility of "other me"s comes up in the possibility of an infinite universe - "if the universe is infinite in extent, won't there be other mes out there?". In principle, you could fly into space, and meet these other mes (if they weren't too far away).
          - Whether the universe is infinite in size is still open; having infinite amount of matter seems less likely to me.
- But you are talking about the "Many Worlds hypothesis", proposed by Hugh Everitt in 1957.
        - This does imply that there are many mes
        - but separated from each other in different universes, so they don't meet each other.
See: https://en.wikipedia.org/wiki/Many-worlds_interpretation

If you want to know more about this hypothesis, I suggest you listen to Sean Carroll's Mindscape podcast.
- Sean Carroll does research into the Many Worlds Hypothesis (but the podcast ranges much wider than that)
You could start here: https://www.preposterousuniverse.com/podcast/2019/12/02/75-max-tegmark-on-reality-simulation-and-the-multiverse/

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Physics, Astronomy & Cosmology / Re: Wave-particle duality theory and the observable universe.

« on: 04/04/2020 19:12:24 »

Duality is not real and has no implications for the behaviour of the universe. It simply means that we have two different mathematical models for describing and predicting the results of certain experiments. The art of physics and chemistry is knowing when to use which model. Generally the wave model tells you where things will happen, and the particle tells you what will happen and when.

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Physics, Astronomy & Cosmology / Re: Wave-particle duality theory and the observable universe.

« on: 04/04/2020 17:52:00 »

Can the outcomes of wave-particle duality experiments be reflected in the world I observe and experience around me?

Of course. If you can perform an experiment and measure the result then it is part of the reality around you.

If so, does the universe I know and experience only exist because at every moment in space and time I am able to observe it?

No, it has nothing to do with you, or anyone else, observing anything

Or does wave-particle duality theory only refer to the world of quantum?

to be honest, the idea of wave particle duality is just used to describe that in certain circumstances a wave or a particle can each appear to have the properties of either a wave or a particle. Although first described of light and then electrons, some very large molecules have been shown to behave with wave properties.
In theory any object will exhibit wave properties, but for really large objects eg a car the effect is so small as to be unobservable.

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General Science / Re: Manoeuvrability in space travel?

« on: 02/04/2020 18:12:11 »

It propels against the mass of it's own fuel! Brilliant!

It is simply the law of action and reaction.  In order to eject the fuel at high speed out of the nozzle, the rocket has to exert a force on it in that direction.  This, in turn, also results in an equal force pushing the rocket in the other direction.
There are a couple of misconceptions going around about rockets.  One is that they need something Air or something outside of the rocket to "Push against" in order work, which is not true.  If you were floating in the vacuum of space, holding a brick, and threw the brick away from yourself, you would move start moving in the opposite direction to the brick.  It's a matter of conservation of momentum.  Both you and the brick together start with zero momentum total.  You toss the brick to the right, giving it a non-zero momentum to the right.  In order for the combination of you and brick to maintain a total momentum of 0, you need to move at some speed to the left so your momentum to the left cancels out the brick's momentum to the right.
In reality, it is a disadvantage to have an atmosphere outside of the rocket. The outside air pushing in on the rocket nozzle decreases the speed at which the gasses can escape, and reduce the effective thrust of the rocket.

The other misconception is that a rocket's top speed is limited to the to its exhaust speed.   As long as a rocket is firing and expelling exhaust, it will continue to accelerate.  The only thing that limits its final velocity ( other than the Relativistic limit of the speed of light), is how much fuel it carries.

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General Science / Re: Manoeuvrability in space travel?

« on: 02/04/2020 13:35:14 »

Just a question I was asked by a friend that I didn't have an answer for! He asked, if space beyond our atmosphere is empty, just how does a vehicle alter it's speed or direction? Assuming a vehicle has some kind of propulsion and method of changing direction, what does it propel against?

It propels against the mass of its own fuel, simple as that.
The faster it can eject this reaction mass, the more thrust it gets from a given Kg of fuel.