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Physics, Astronomy & Cosmology / Re: Which twin is older when they meet again?

« on: 06/04/2020 20:40:34 »

At what velocity do you have to be moving towards an observer to see their watch ticking at the same rate as yours after taking both time dilation and Doppler shift into account?

Both observers would have to be stationary relative to each other.
Mind you, that's not the only solution.  Two observers could circle each other at some rate, and thus have a relative velocity.  You can get really creative and plot a funny spiral path in so the Earth guy sees a 1-1 rate between them, but then the incoming guy would still see a different rate.

I'm not quite understanding this.

First of all, my reply concerned the point of view of the observer, who we're trying to get to see an incoming clock tick at normal rate.  You kind of worded it the other way around, but it can work that way as well.

If two observers are circling each other without the distance between them changing then of course they would each see the other's watch ticking at the same rate as their own watch but how could following a path that spirals inwards cause a different result than a direct path?

Easiest case: From PoV of Earth, the remote clock is getting closer (blueshift) but is moving (redshift).  If you adjust the angle just right, the two cancel.  The relativistic effect is a function of speed, but the Doppler effect is a function of the rate of reduction of separation.  A circular path would be red shifted (all dilation), but a direct path is dominated by Doppler.  Somewhere between is a balance.  The math isn't too hard to work out.

Imagine they are the only two objects in the universe. Any spiral path is now meaningless because there's no point of reference to create a spiral

Earth is the reference.  Remember, rotation is still absolute, not relative.
So no need for a 3rd object for it to be a spiral.

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Physics, Astronomy & Cosmology / Re: What was most shocking about the dark side of the moon?

« on: 03/04/2020 19:30:25 »

Interestingly I've heard it argued that they are actually nine planets in the Solar system and the moon is one of them because it's so big compared to us that it doesn't just cause a wobble, it causes us to both orbit the centre of gravity between us that's well away from the Earth, making us a dual planetary system. We have nine planets after all.

The center of gravity of the Earth-moon system is about 25% of the way down to the center of Earth, not well away from Earth.

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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.

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Physics, Astronomy & Cosmology / Re: Which twin is older when they meet again?

« on: 27/03/2020 12:38:06 »

With D moving at 0.5c towards the Earth and the Earth receding at 0.2c, the closing speed between D and the Earth, according to B is 0.3c,  at which rate, it will take 20 years for D to reach the Earth.

That doesn't seem like relativistic velocity addition.

It's not.  Relativistic velocity addition is used in situations where you add Y's velocity relative to X, and Z's velocity relative to Y.  In other words the velocities are measured in different frames. Here, all velocities are relative to B's frame, so regular arithmetic is used to add/subtract them. Here, Janus was expressing the difference in velocities as measured in B's frame. Notice that he said 'according to B'.  To compute that same relative velocity relative to either Earth or D would require the relativistic addition.

To make the scenario perfectly symmetrical using those numbers, in B frame D should aim to planet Y which is 10 lightyears away. At speed of 0.5c, it will take 20 years to reach planet Y.

Right, and if you look at how I did the math at the bottom of post 43, you see I computed it just like that: how long Dewey takes to get to Y.

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Physics, Astronomy & Cosmology / Re: Which twin is older when they meet again?

« on: 25/03/2020 11:55:31 »

My question is, why the separation period of the twins is measured lower in B's frame compared to earth's, while both of them are equally inertial. The only difference I can find is that planet X is stationary in earth frame while moving in B frame.

The end of the separation period is defined by event R (reunion) which is on B's worldline (and A's as well), but not Earth's. In any frame, B is 48.99 years old at that event. In Earth frame, Earth is age 50 at an event simultaneous with R.  In B's frame, 48 years have gone by on Earth when the reunion takes place.

Is that what you're asking?  The separation period is lowest from A's POV actually.  Earth's frame doesn't really count since Earth is not present at event R.

None of this has to do with the frame of Planet X, since X's frame does not effect the reunion.  If both A and B assume frame X when they arrive together, then we can do it in X's frame, but I'm talking about the frame of B enroute when referencing B's frame above.

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Physics, Astronomy & Cosmology / Re: Neutrinos travelling faster than light?

« on: 24/03/2020 18:35:46 »

So in simplistic terms is it fair to say that the expanding universe isn't expanding at a particular speed but rather, everything is receding away from everything else at an increasing rate ?

Everything is receding from any given thing at a rate proportional to the distance between the two things.

This rate depends on certain universal factors.

Mostly only on the age of the universe.  I said the rate was the reciprocal of time: In particular, it is the reciprocal of the age of the universe.  Not exactly because the expansion rate is not constant, but accelerating, which mucks up the simplicity of it.

Ha ha you threw it right back at me, what would happen if a neutrino reached the edge of the universe? I'm guessing there might be an edge to the universe but not an edge to space.

That sort of conjures an image of the big bang happening at a particular location in space, which isn't the model at all.  No, I know of no model where there is an edge to the stuff, and your view might have stars only on one side.

Everything that reaches the edge of the universe just expands the universe into space. I'd love to know if that makes sense to you or not.

No, the model is that the big bang happened literally everywhere, but there was no separation between any two places at the event, so time and space really have no meaning at such a singularity. Space, measured along lines of equal proper age, has no finite boundary.

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Physics, Astronomy & Cosmology / Re: Neutrinos travelling faster than light?

« on: 21/03/2020 00:41:01 »

What developed first after the big bang , neutrino's or light?

I cannot see how there would not be both from pretty much the earliest times, but the universe was completely opaque, so none of the early light got anywhere before being absorbed.  Only 380,000 years later did hydrogen form, turning the universe transparent and allowing the light from that time to reach us today as the CMB.  The cosmic neutrino background we see today is, well, 380,000 years older than that.

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Physics, Astronomy & Cosmology / Re: Neutrinos travelling faster than light?

« on: 10/03/2020 11:09:46 »

I read that  supernova neutrinos 'reach Earth hours before the light produced in the explosion'. Of course my question is going to be how do these neutrinos travel faster than the speed of light and reach Earth 'hours' ahead of the light?

The neutrinos begin their journey to Earth immediately upon the core explosion, since neutrinos have almost no interaction with matter.  Light cannot do this, and the light you see is from the explosion shock wave reaching the surface of the star hours later, which generates the light you eventually see.  The light from the initial explosion is long since absorbed by the material of the star.

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Physics, Astronomy & Cosmology / Re: What would I find in a square centimetre of space?

« on: 07/03/2020 22:12:57 »

And ... how much of that square centimetre is 'empty'? Is there such a thing as empty space?

In a way, all space is empty.  Take for instance an apple.  How much of some cubic centimeter of that apple is empty?  Well, no known fundamental particle occupies a finite volume.  It cannot, else it would have the left side and the right side and it would not be a fundamental particle.  Hence the entire apple is empty space.  Sure, the cc has plenty of electrons and other things in it, but none of them occupy any of the volume in it.

OK, that's one answer.  The QM answer gives a different way of looking at it.

Quantum mechanics has interpretations that kind of fall into two camps: Local and counterfactual ones.  Some are neither.  A counterfactual interpretation (state exists even if not measured) would say that there is actually a measurement independent state to any system, which in our case is some cubic centimeter in a random spot in space, likely well between galaxies.  For that, the lower bound of the numbers given by Janus would be a fair description.  Such interpretations need to posit information transfer faster than light, and so are not considered local interpretations.

On the other hand, a local interpretation (denying faster than light cause and effect) would not say there is in fact an electron at location X inside that cc. An electron's position only exists when measured, and nothing is likely getting measured there.  So such interpretations would simply say that were a measuring device to be placed there, it would likely detect particles and radiation at the levels given by Janus.  But by no means can it be said that a given particle is in fact within that cc if not measured.
So the wave function of that little volume of space might be wildly complex, describing all manner of probabilities of things that may or may not be measured if something were to put a detector there, but if there was a detector there, it wouldn't be empty, would it?

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New Theories / Re: Weakest point of special relativity

« on: 07/03/2020 00:27:50 »

Voyager sometimes approaches forward the Earth.

Did you mean towards the Earth? What evidence do you have for this?

His syntactic travesty aside, the distance between us and voyager does regularly decrease.
It is moving out of the solar system at something on the order of 16 km/sec (and slowing), while Earth orbits at 30 km/sec, so part of every year our orbit outruns the receding probe, thus bringing it closer to us.

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The Environment / Re: Do wind turbines affect the speed of Earth's rotation?

« on: 28/02/2020 12:16:08 »

With all the dams in the world (I believe its a third of the rivers in the world), we have slowed down the speed the world spins around, only a tiny tiny tiny amount!

Will something similar happen with wind turbines?

The supposed slowing of Earth's spin is theoretically due to the change in the rotational inertia of the planet by redistributing some of the mass of the water into the basins of these dams. That means that some dams must speed the rotation if this is true, which is questionable.

Nevertheless, windmills do not have any measurable effect on angular inertia of the planet since they don't work by hoarding all the air into a pressurized ball somewhere, so the argument put forth for the dams does not apply at all to the windmills.

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General Science / Re: Enlarged prostate treatment in North Delhi

« on: 24/01/2020 12:30:41 »

Our problem here is that we've an enlarged list of banned users.  You seem to be contributing to the problem instead of helping it.

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Physics, Astronomy & Cosmology / Re: If you throw a mouse or elephant at someone in space, which will move them more?

« on: 07/01/2020 21:59:58 »

What is the cutoff mass for [projectile returning to first person]?  This is not the same mass as the answer to my post 12 question.

I don't know how to calculate that without using trial and error. You likely use calculus

It's actually just complicated algebra, but I did my reply-12 thing by guessing, and hit it in my 2nd guess, the first guess being 50kg. I had a hunch and it turned out to be correct. This is the case of the 2nd guy returning the projectile with the same energy as the first throw, not at the energy of the incoming projectile.

One more thing on a different but related issue. If I wanted to write a snooker simulation but with variable-mass balls and with the angles of impacts leading to the balls going off at different angles, can the same maths as above still be used just by breaking things down into vectors?

Snooker is complicated.  It isn't frictionless balls in a vacuum, which is actually pretty easy arithmetic.  Snooker has to deal with table friction, especially immediately after the impacts where the balls are moving at a different rate than their spin.  That spin deflects both balls from their initial trajectories, and I'm not up to the challenge of computing the angular inertias and funny spins with non-horizontal axes.

Yes, it's quite easy to do with frictionless balls in space.  Target ball moves directly away from its impact point and incoming ball is deflected in a way that preserves momentum and energy.

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Physics, Astronomy & Cosmology / Re: If you throw a mouse or elephant at someone in space, which will move them more?

« on: 05/01/2020 01:55:25 »

I'm still trying to find out how you work out how fast the two objects will move apart for a fixed amount of input energy.

I started with the elephant which is posited to mass 10x myself.  So I chose a speed for myself that was 10x the elephant speed, or 1 and 0.1 m/sec respectively.  This is an arbitrary speed but not an arbitrary ratio.  Then I computed the energy (1/2mv²) for each.  50 + 5 totals 55 joules, so that's the energy of the spring that would impart those speeds on those two objects.
Then I had to do the same with the mouse, but starting with a known energy this time, and I got 0.104 and 10.4 m/sec for myself and the mouse respectively, a ratio of 100 this time.  It still adds up to 55 joules.

If spring efficiency wasn't an issue, you could simply divide the total energy equally between both objects.

You can't do that.  The elephant gets a 10th the energy that you do when you throw it.  The mouse gets 100x the energy when you throw it.  Energy can only be divided equally when an equal mass (the wife) gets thrown.
Momentum gets divided equally.  Both total momentum and energy is preserved on the throwing event.  The former is preserved upon catching event and the energy is lost to friction (or possibly captured if the friend has a smaller spring).

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Physics, Astronomy & Cosmology / Re: Can Atoms Touch Each Other?

« on: 18/12/2019 18:21:13 »

Atoms (in molecules say) keep each other at a distance to the the repulsion of their electron clouds.
For electrons to literally 'touch', they'd have to be at the same location, and the EM repulsion force (not to mention the even stronger nuclear forces) would repel at pretty much infinite force. Things can get arbitrarily close, but to touch they'd have to be at the same location, which the forces don't allow.

Hence the definition of 'touch' typically being more reasonable than that, as chiralSPO points out.  Atoms are considered to be touching if their repulsion due to their electrons makes a significant contribution to the distance between the atoms.

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Physiology & Medicine / Re: Is your perception of colours the same as mine?

« on: 16/12/2019 19:53:45 »

Dearest Peeps Of Klevur Knowing Stuff,

Peeps with peepers is more like it.

How do I know that my eyes perceive colours exactly the same as yours ?  For all i know your orange might be my green ?

You don't.  No way to compare subjective experience, but there are some people that see more colour than typical, sort of the opposite of being colour blind.  They see patterns in flowers that might appear white to me.  They get offended that the crayon box never seems to be anywhere near complete.

Squirrels see yellow.  Humans do not, and the banana appears the same colour as an object that emits only red and green light.  Squirrels could tell the two apart effortlessly, not because they see better, but because they see actual yellow.

Do both eyes see colours exactly the same  ?

I think one would notice if otherwise, so barring health issues, yes.

Does the colour of eyes have any effect on colour perception ?

No

Will I still perceive colours the same when I'm 80 like i do now ?

Again, barring health issues, yes.  I've had cataracts, so my world sort of faded to foggy white.  My colours were not off, but I needed 3cm high font to read anything.  All fixed now.  My mother lost most of here colour vision and was amazed when it was restored.  All the vivid hues and such that she'd forgotten about.

*What's your favourite colour ? (US translation service: "colour" = "color" sheesh !!)

Boring if there's only one.  The prior owners of my house liked gold.  All the drapes, carpets, hardware, and even the cars.  Sheesh indeed...

Why do we prefer certain colours over another ?

I don't, so don't know.

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General Science / Re: Can I turn Pixie into a diamond?

« on: 16/12/2019 14:38:31 »

Your Pixie seems to have walked through (and face-planted in) a shallow dish of bleach.
Nevertheless, she has proven her abilities by prevailing over her leopard-trainee foe in the corner there.

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General Science / Re: Can I turn Pixie into a diamond?

« on: 16/12/2019 13:55:24 »

My childhood cat had the same name.  Thing was all hair, making it sort of a challenge to find the cat within.

That said, she was regularly threatened with being made into either a toilet rug or a tennis racquet, but never a diamond.  I suppose that being a carbon based life form, one could isolate the carbon and then squeeze it into a diamond, but it takes warm hands.

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General Science / Re: Can fire cast a shadow?

« on: 16/12/2019 13:49:45 »

Fire very much does block some light as it consists of glowing particles in part.  Other light gets deflected by the heat, but that effect will not produce a crisp shadow.

So put a spotlight to the side of a fire and it will indeed cast a shadow on a wall on the opposite side.

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Physics, Astronomy & Cosmology / Re: What Do You Call A Moon Of A Moon ?

« on: 16/12/2019 03:05:30 »

Does this then mean that the Sun is a moon of Sagittarius A ?

Few stars actually orbit Sgr-A.  The sun is not one of them as its orbital speed is far too large at this distance from such a small object.  No, we go around the galaxy in a non-elliptical path and if we orbit anything, it is the general cloud of assorted objects and (mostly) dark matter that exists mostly (but not entirely) within the radius of our circuit.  Sgr-A is only a tiny percentage of this mass, and is thus just one of those assorted objects.