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Physics, Astronomy & Cosmology / Re: What is 'Zero Time'?

« on: 05/03/2018 21:00:57 »

Being able to discuss theoretical or philosophical mathematical meaning of zero time is a different matter from making a physical measurement of it.
An physical example of zero time is the time that elapses in a photon's frame of reference as it propagates through a true vacuum. A possible problem with that example is that there is no such thing as a true vacuum due to a ubiquitous quantum foam of virtual particles.

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Physics, Astronomy & Cosmology / How accurate are dating methods?

« on: 18/06/2011 22:16:34 »

It seems almost entirely random whether she and I get on or not ;-)

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Physics, Astronomy & Cosmology / ?Stellar bidet?

« on: 18/06/2011 21:47:53 »

What is the dilemma there, the fusion process is not creating the oxygen, it is simply providing energy that is causing its ejection.

Quite a phenomena, though, I'll second that.

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That CAN'T be true! / ...that the sun generates about as much energy as a compost heap.

« on: 14/04/2011 22:46:34 »

Merits of wikipedia are worthy of more careful debate. Plenty of problems with peer review.

I thought this forum was for established facts that are hard to believe - just for fun. I wasn't looking for an explanation.

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New Theories / Aether Displacement

« on: 14/04/2011 21:03:32 »

1) so is matter "able to change into" aether or is it "continuously changing into aether everywhere throughout the universe" the word "evaporate" leads me to think you are talking about all matter having some kind of half-life.

2) Alright - the theory is clear.

3)OK, it's frictionless, so it just gets pushed out of the way without any work being done. But there must be a force doing the pushing, even so.

4) The gravitational attraction between two galaxies in orbit is stronger because of dark matter. How can aether displaced by one galaxy result in increased gravitational attraction towards a different galaxy (like its binary counterpart)?

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Cells, Microbes & Viruses / How does epigenetics impact our DNA inheritance?

« on: 14/04/2011 19:59:42 »

TBH, that is what I expected even 20 years ago.

I guess I'm secretly looking to undermine the public perception of DNA as the sole source of inherited information.

Epigenetics is complex; but I don't need to fully understand it. I just want some-one who knows to tell me if we have yet discovered any mechanism by which epigenetic information can persist from one generation to the next without any of the information being "encoded in" the DNA itself.

I don't feel I've explained that as well as I would have liked.

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New Theories / Aether Displacement

« on: 14/04/2011 01:04:18 »

OK, fair enough, that I rescind.

Which leaves me uncomfortable with
1)"Matter evaporates into aether."
Einstein was talking about energy loss through radiation in the paper you quoted. So why do you need this bit of the theory? Is the displaced dark matter in halos not heavy enough that you need to generate more

2) The expansion of the universe must be resulting in a massive creation of energy as more massive aether? 

3) On the scale of galaxies I don't think quantum effects would not apply due to de-coherence (the super-fluid opener was just an analogy, right). So what is causing the displacement between matter and dark matter which don't interact in any known way except (attractive) gravity?

4) "Aether is not at rest when displaced. Displaced aether exerts force towards matter. " - seems like there needs to be a theoretical mechanism by which displaced aether suddenly exerts gravity, but undisturbed aether distributes itself evenly throughout the universe?

5) http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#The_experiment - its over the limit of my current understanding on QM

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New Theories / How brain-cells store memoryes?

« on: 13/04/2011 23:34:05 »

You probably want to read up on;

http://en.wikipedia.org/wiki/Neural_network

Another interesting aspect is the maths of inductive reasoning;

http://en.wikipedia.org/wiki/Inductive_logic#Inductive_inference

In terms of physiology, the indications are that adjustment in synapse transmission of neural impulses and the growth of new neural synapses constructs a neural network that adjusts to make rewarding inductive inferences. Different parts of the brain inherit different physiology and structure genetically to some extent, which gives them different roles. The learning process tunes the synapses to make them good at the roles.

(I hope you didn't want a short answer... ...they say it is the most complex "machine" known to man.)

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Physics, Astronomy & Cosmology / What is the simplest explanation of gyroscopic precession?

« on: 13/04/2011 23:04:53 »

If it was subject to a cross axial-torque it would.
If it was not subject to a cross axial-torque it would not.


Why is anyone talking about friction, that's what confuses me.

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Physics, Astronomy & Cosmology / Do sound waves get transferred in a Newton's Cradle?

« on: 13/04/2011 22:52:40 »

I suspect Luke knows that sound travels down threads because of the paper cup telephone.
But it is also a kind of sound wave travelling through the marbles that transfers energy from one marble to another. It's a shock wave and cannot travel faster the speed that sound travels at in the marbles.

The marble collisions create lots of noise, which could be picked up by a detector.

But what Luke really seems to want is an amplifier, something like a megaphone that will take a small sound and make it larger. Amplification requires an energy source like some batteries.

(Focusing the sound into one direction also makes it louder - in that direction.)

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Physics, Astronomy & Cosmology / What is the simplest explanation of gyroscopic precession?

« on: 13/04/2011 22:34:41 »

In a toy gyroscope or toy spinning top the torque that causes the precession is due to gravity in one direction and the force of support from the floor (through the axle and, yes, any bearings) in the other. (Gravity pulls the whole body in one the same direction).

The non-vertical initial alignment of the toy, and asymmetries in its mass mean that it can never run perfectly vertical.

Friction slows the angular velocity of the spinning part (as does air resistance). These are torque vectors parallel to the angular momentum. There is no mechanism by which they can drive/cause precession.

Many examples are given above of precession in the absence of friction (but not in the absence of cross-axial torque of some form).

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Physics, Astronomy & Cosmology / How do they measure very low levels of things?

« on: 13/04/2011 22:19:03 »

They put sample (the grain of salt or seawater) and detector in a big lead box to get the readings - at least that is one method.

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Physics, Astronomy & Cosmology / What is the simplest explanation of gyroscopic precession?

« on: 13/04/2011 22:10:42 »

We'll hopefully get to the intuitive explanation now.  Keep in the back of your mind the idea that angular acceleration needs to be perpendicular to the plane of rotation.  We know intuitively that a force applied along one of the spokes of the wheel won't make it rotate any faster or slower.  A force applied along it's axle won't make it rotate any faster or slower.  Only a force applied tangentially along the wheel's surface makes it rotate faster or slower. 

Sure thing

So let's say we have a force applied tangentially to the wheel so that it speeds up the wheel.  When the wheel speeds up, this means it has a positive angular acceleration.  This means that somehow this force which was applied in the plane of the wheel has created an angular acceleration which is directed perpendicular to the plane of the wheel.  We want some other quantity which takes into account the force and the point on the wheel at which it's applied and tells us the resulting angular acceleration due to that force.  This quantity has to somehow "know" that only components of the force directed tangentially along the wheel contribute to it's rotation and it's resulting direction should be along the axle, just as the angular acceleration is: this way you can equate this quantity with angular acceleration.

err and it also has to somehow "know" how far the force is applied from the axis of rotation. torque=force*distance
Oh hang on, you I'll assume you meant Torque not force. Makes sense.

The quantity that does this is the cross product, which is why torque is equal to the cross product between a vector pointing from the axis to the point of contact of the force and the force vector itself.

So that would be the cross product of the Torque and the normalised/unitary angular momentum? Which is the change in angular momentum. OK I'm still aboard

Now you might argue that this is unnecessarily complicated or unintuitive.

Perhaps, but I do get it. Hopefully you are going to go on to precession...

That's certainly true for the case of a single flywheel rotating on it's axis.  You can do the entire analysis using linear acceleration of points on the wheel without needing right hand rules and torques.  But when you end up with much more complicated systems with multiple degrees of freedom for both rotational and translational motion, torques and angular velocities/accelerations are incredibly useful. 

This is my experience also. Try working out how two "nested" gyroscopes behave using finite elements - but with angular momentum you just add the vectors and treat it as a single one. Whereas in the past I might have felt that that operation was a bit of a trick, you've explained it well enough for me.

And they're like many things physics--the concept is not intuitive at first, but once you understand it (usually through practice using it), it becomes intuitive.  Once it's intuitive to you, it's a simpler way of dealing with even simple problems like the flywheel or gyroscope.  It's a lot easier to me at least to justify precession in terms of torques and angular momentum than it is to justify it by breaking it into tiny chunks of mass and doing F=ma on each piece.  And if you want someone to actually compute precession rates, it's going to be far, far more difficult without going to torques and angular momentum.

OK seems to work that way for me also.

So if you've followed all that, the final little bit is angular momentum.  After all this work and defining cross products, you end up with the quite elegant equation τ=Iα, where τ is the torque vector, I is the moment of inertia about a particular axis of rotation and α is the angular acceleration vector.  If there are no torques applied, then angular acceleration is zero, which means that the angular velocity is constant.  If you construct the quantity L=Iω, where ω is the angular velocity, then this quantity is only changed when a torque is applied. 

...by integration of τ=Iα, yes. I'm no stranger to calculus.

So you can state this as a law: that L is conserved unless the system is acted upon by an outside torque.  This L is called angular momentum.  (It's called that because linear momentum is arrived at in the exact same way using linear velocity, acceleration, and Newton's second law.)  Again, since angular velocity, acceleration and torques are vectors which have to point (for simplicity) along the axis of rotation, angular momentum has to as well, since it points along the direction of angular velocity.

Sorry this is long-winded, but rotational kinematics usually takes weeks in an introductory physics course. 

Well, good recap on foundations I feel less rusty, but you haven't got to the bit that was my question yet. Which is about precession.

If you have followed all of this, then the real payoff is the elegance of the expressions you get out: if θ is rotation angle, ω is angular velocity and α is angular acceleration, τ is torque, I is moment of inertia, L is angular momentum and t is time then:

θ(t)=ω t+1/2 α t² tells you how far the wheel's rotated,

τ=Iα tells you how the wheel's acceleration and velocity change with applied torques,

dL/dt=τ tells you that angular momentum only changes over time along the direction of an applied torque.

This is all analogous to the linear equations which are usually considered much more intuitive.  Here x is position, v is velocity, a is acceleration, F is force, m is mass, t is time and p is momentum.

x(t)=vt+1/2 at²
F=ma
dp/dt=F.

Which is a great recap of the area of the subject. I like the bit at the start about tangential forces (or Torques) as it is the sort of terms I am looking for in the promised explanation of

τ=ω_pXL

using all these vector quantities we have discussed. I hope the next post will be able to do that - but it is significantly more difficult.

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Physics, Astronomy & Cosmology / What is the simplest explanation of gyroscopic precession?

« on: 13/04/2011 21:33:57 »

This is interesting:

"...Gyroscopes would do nothing in outer space.  With no gravity to exert the
torque, there would be no reason for angular momentum to change direction.
The spinning gyroscope would not turn..." ~ask a Scientist.

Gyroscopes are used to orient satellites in space. You don't need gravity to apply a torque - it can be applied another way (including through friction, I suppose).
Torque -> precession.

The axis of the earth precesses, but I'm not sure what is applying the torque. I assume it is due to moving round the sun with an inclined axis.

15

Physics, Astronomy & Cosmology / How does coal compare to fission for radioactive waste?

« on: 13/04/2011 20:58:43 »

Did you read the editor's note?

"As a general clarification, ounce for ounce, coal ash released from a power plant delivers more radiation than nuclear waste shielded via water or dry cask storage."

So, it's a saying coal ash is more radioactive than shielded nuclear waste.

...from this we conclude that the shielding works. Not that coal ash is dangerous. Although it may be. Or not.

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That CAN'T be true! / Where do sound waves finish?

« on: 12/04/2011 00:07:50 »

Sound waves attenuate rapidly beyond detection by the human ear. The energy in the sound wave ends up as thermal energy in the medium through which it passes (the air is warmed up).

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That CAN'T be true! / Rain falls with uniform velocity in my frame and accelerates in yours?

« on: 11/04/2011 23:59:50 »

The point is that the raindrop does not change at all.

Observer A (stationary) sees the rain fall straight down, including, say, a particular raindrop that I shall, for want of a better name, call Bored chemist.

Observer B (on his bike moving towards A) sees the same rain completely unchanged but because he is moving forwards the rain appears to move towards him.

Observer A and B can both look at exactly the same raindrop, Bored chemist, at exactly the same time and both observe a different velocity relative to them. The raindrop does not change. Is it accelerating? No it is not.

Also note that in A's frame of reference B is travelling towards him, but B has not hoped on a bike.

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Physiology & Medicine / Are astrobiologists a little short sighted?

« on: 11/04/2011 20:52:50 »

Not unreasonable, but you can't stop them spending time on it unless you have a better plan.

have a look
http://en.wikipedia.org/wiki/Fermi_paradox

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Marine Science / There are increased radiation levels in the sea from fukushima, Implications?

« on: 11/04/2011 20:36:30 »

As long as they mix it up real good.

Certain chemicals can get concentrated into the food chain, but uranium isn't one of them as far as I know?

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Marine Science / Ways to remove Radioactive material from water?

« on: 11/04/2011 20:31:31 »

There's often a heat exchanger to a secondary loop to drive the turbine; which further contains the radiation.

Are we talking about how to clean up the sea near Fukushima?
Would dispersion do it, the ocean is pretty big.