[Bored chemist]

10¹⁹ annihilations

That's quite a lot.
About 10 micromoles  so something of the order of 30 nanograms of positronium.
If you could get that together in one place you could weigh it of a good balance (apart from the tiny little radiation hazard)

[evan_au]

each unit contains the power = hf Cos ϴ.of energy per second.

This sounds like a solar panel.

I think you know how neutrino was discovered. How do you make sure that no neutrino is produced in the process?

I didn't know how neutrinos were first discovered in practice, so I had to look it up. There was a gap in my knowledge between Pauli and the Homestake experiment:
https://en.wikipedia.org/wiki/Neutrino#Direct_detection

Apparently, the original discovery of neutrinos was adjacent to a nuclear reactor; Uranium fission produces many kinds of radiation, including a broad spectrum of neutron, neutrino and gamma ray energy.

The actual detection of a neutrino absorption (extremely rare) involved not just the emission of a positron (which was detected by its characteristic gamma ray energy), but by emission of a neutron and its subsequent absorption.
https://en.wikipedia.org/wiki/Cowan%E2%80%93Reines_neutrino_experiment#Setup

Typical PET reactions emit a positron only, which then produces a pair of gamma rays with a specific energy, and opposite directions to conserve energy and momentum.
https://en.wikipedia.org/wiki/Isotopes_of_oxygen#Oxygen-15

So the reaction that first detected a neutrino was a more complex reaction than the reactions used in PET scanners.
PET scanners don't try to detect neutrinos  - and the energy spectrum and direction of the gamma rays shows that no neutrino is involved.

[alancalverd]

If you could get that together in one place you could weigh it of a good balance (apart from the tiny little radiation hazard)

...and the fact that it has a halflife of ~100 picoseconds, so there's buggerall left of the stuff we made in 1980, or even yesterday.

[hamdani yusuf (OP)]

Caltech made a documentary video about this

Episode 50. Particles and Waves: Evidence that light can sometimes act like a particle leads to quantum mechanics, the new physics.

“The Mechanical Universe,” is a critically-acclaimed series of 52 thirty-minute videos covering the basic topics of an introductory university physics course.

Each program in the series opens and closes with Caltech Professor David Goodstein providing philosophical, historical and often humorous insight into the subject at hand while lecturing to his freshman physics class. The series contains hundreds of computer animation segments, created by Dr. James F. Blinn, as the primary tool of instruction. Dynamic location footage and historical re-creations are also used to stress the fact that science is a human endeavor.

[hamdani yusuf (OP)]

No, Changing Electric Fields DON'T Cause Magnetic Fields; The Real Origin of Electromagnetic Waves

Is the explanation here correct, according to current understanding of electromagnetic wave?

Doesn't it contradict following statement?

In addition, E and B are perpendicular to each other and to the direction of wave propagation, and are in phase with each other. A sinusoidal plane wave is one special solution of these equations. Maxwell's equations explain how these waves can physically propagate through space. The changing magnetic field creates a changing electric field through Faraday's law. In turn, that electric field creates a changing magnetic field through Maxwell's addition to Ampère's law. This perpetual cycle allows these waves, now known as electromagnetic radiation, to move through space at velocity c.

https://en.wikipedia.org/wiki/Maxwell%27s_equations#Vacuum_equations,_electromagnetic_waves_and_speed_of_light

[hamdani yusuf (OP)]

https://science.nasa.gov/ems/02_anatomy

In the 1860's and 1870's, a Scottish scientist named James Clerk Maxwell developed a scientific theory to explain electromagnetic waves. He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. He summarized this relationship between electricity and magnetism into what are now referred to as "Maxwell's Equations."

The diagram from NASA website seems to suggest that there is phase shift between electric and magnetic field in a propagating electromagnetic wave.

[hamdani yusuf (OP)]

Here is an excerpt from University Physics Volume 2

Summary

• Maxwell’s prediction of electromagnetic waves resulted from his formulation of a complete and symmetric theory of electricity and magnetism, known as Maxwell’s equations.
• The four Maxwell’s equations together with the Lorentz force law encompass the major laws of electricity and magnetism. The first of these is Gauss’s law for electricity; the second is Gauss’s law for magnetism; the third is Faraday’s law of induction (including Lenz’s law); and the fourth is Ampère’s law in a symmetric formulation that adds another source of magnetism, namely changing electric fields.
• The symmetry introduced between electric and magnetic fields through Maxwell’s displacement current explains the mechanism of electromagnetic wave propagation, in which changing magnetic fields produce changing electric fields and vice versa.
• Although light was already known to be a wave, the nature of the wave was not understood before Maxwell. Maxwell’s equations also predicted electromagnetic waves with wavelengths and frequencies outside the range of light. These theoretical predictions were first confirmed experimentally by Heinrich Hertz.

https://opentextbc.ca/universityphysicsv2openstax/chapter/maxwells-equations-and-electromagnetic-waves/

And another physics course says:

Maxwell’s equations encompass the major laws of electricity and magnetism. What is not so apparent is the symmetry that Maxwell introduced in his mathematical framework. Especially important is his addition of the hypothesis that changing electric fields create magnetic fields. This is exactly analogous (and symmetric) to Faraday’s law of induction and had been suspected for some time, but fits beautifully into Maxwell’s equations.

Since changing electric fields create relatively weak magnetic fields, they could not be easily detected at the time of Maxwell’s hypothesis. Maxwell realized, however, that oscillating charges, like those in AC circuits, produce changing electric fields. He predicted that these changing fields would propagate from the source like waves generated on a lake by a jumping fish.

The waves predicted by Maxwell would consist of oscillating electric and magnetic fields—defined to be an electromagnetic wave (EM wave). Electromagnetic waves would be capable of exerting forces on charges great distances from their source, and they might thus be detectable.

https://courses.lumenlearning.com/physics/chapter/24-1-maxwells-equations-electromagnetic-waves-predicted-and-observed/

[Bored chemist]

Well, that was interesting.
I hadn't heard about the LW equation.
Wiki shows me how it can be derived from Maxwell's equations so it must be equivalent to those equations.
Re the video: "No, Changing Electric Fields DON'T Cause Magnetic Fields; The Real Origin of Electromagnetic Waves"
At about 06:20 it shows an animation of the magnetic field round a single moving charge.
That's quite interesting in a video that starts by saying that magnetic fields are not created by moving  charges.

At 13:44 he says the acceleration dependent part of the  equation is usually called the radiation field.
Again, that's a brave statement for someone who kicked off by essentially saying that radiation is not caused by accelerated charges.
This is reiterated at 16:00

At 10 minutes, he says that, if the velocity an acceleration aren't zero then the electric field won't point at where the charge "was".
He then says, when that happens there will be a magnetic field.

Again, that's an interesting claim if you start by saying that a moving charge does not create a magnetic field.

He's right in saying that fields are just bookkeeping and that any "field meter" actually just measures the effects of distant charges on a "test charge".
But that';s pretty much what the definition of a field is.
The gravitational field points "down" because that's the direction in which a test mass will accelerate as a result o the field.

He's restating classical electromagnetics- derived explicitly from Maxwell's equations.
In doing so, he's tying to be controversial, but he doesn't actually seem to say anything new.

And, of course, it's a classical model.
It does not handle QM.
So, while it's fine for a radio transmitter and you can use it to model reflection, refraction and diffraction of light, it's no use at all for interactions with atoms.
And, since it's not relativistic (in the E=MC² sense) it doesn't deal with the mass or (I think) the momentum of light.

It certainly doesn't cover the creation or annihilation of a positron; and that limits it.
We can observe pair production directly in a cloud chamber.
https://physics.stackexchange.com/questions/127855/is-it-possible-to-produce-images-of-pair-production-in-home-made-cloud-chamber

A model which does not include it is less use than our current model.

[alancalverd]

We know from laboratory experiments at any convenient frequency that a changing magnetic field B induces an electric field E (that's how we generate electricity) and a moving charge (i.e. a flowing current) generates a magnetic field. Maxwell pointed out that a moving charge was equivalent to a changing electric field.

So in one dimension we have E = -dB/dt and B = dE/dt (with a few constants that are not relevant at this stage)

Now wiggle one parameter (say E) sinusoidally. E =  a sin ωt, so B = dE/dt = a cos ωt, which is the same shape but phase-displaced by π/2.

[alancalverd]

I'm a bit baffled by the suggestion that

Since changing electric fields create relatively weak magnetic fields, they could not be easily detected at the time of Maxwell’s hypothesis.

as the deflection of a compass needle by a current-carrying wire was a standard demonstration and the empirical Biot-Savart law that describes it, predates Maxwell.   

And whilst I'm riding my pedantic hobby-horse,

Evidence that light can sometimes act like a particle

should be struck from any syllabus! Light always behaves as light, but we need a particle model  to predict that behavior in certain circumstances.

[Bored chemist]

Oddly, the textbooks get the phase right when they talk about circularly polarised light
https://en.wikipedia.org/wiki/Circular_polarization#/media/File:Circular.Polarization.Circularly.Polarized.Light_With.Components_Right.Handed.svg

And we know that photons always carry angular momentum.
The combination of a left and right handed circular polarisation gives a linear one.
For some reason, people seem to think that "ordinary" light is composed of plane polarised light.
In some ways, it works better if you assume it's a 50:50 mix of two sorts of circular polarisation.

[Bored chemist]

Light always behaves as light,

I can see why you were keen on the idea that "Brexit means Brexit".
Both statements are true but meaningless.

Saying that light acts like light is pretty pointless.
Saying that it sometimes seems to act like a particle (and by implication sometimes it doesn't) actually imparts information.

[alancalverd]

Please don't ascribe the vapid drivel of a politician to me.

It is absurd to think that light somehow decides how it is going to behave (wave approaching a grating) and then changes its mind (particle detected at the exit).  The behavior of light is absolutely consistent (unlike that of a politician) but is not completely modelled by either a particle or a wave formalism. You can't blame the photon! "Seems to " behave is excusable in a public lecture but I wouldn't teach it to an aspiring scientist.

[Bored chemist]

It is absurd to think that light somehow decides how it is going to behave

And nobody said it does "decide".

However, in point of fact, it sometimes behaves like a particle would be expected to and it sometimes behaves like a wave would be expected to.

The fact that it always does what light does is true, but tautology, and thus unhelpful.


The light does not "decide" how to behave.
But we might have to decide which model to use.

[alancalverd]

Agreed.

[hamdani yusuf (OP)]

Meanwhile, Fourier transform tells us that infinitesimally precise frequency of a wave requires infinitely long sine wave. The most compact waveform in both frequency and time domains is Gaussian. It contains a range of frequency, and has non-zero time duration.
I've seen some sources said that uncertainty principle is not a problem of measurement. Instead, it's a fundamental principle inherently embedded to the math of wave models through Fourier transform.

Here is an example.

[hamdani yusuf (OP)]

At about 06:20 it shows an animation of the magnetic field round a single moving charge.
That's quite interesting in a video that starts by saying that magnetic fields are not created by moving  charges.

What he actually said is

Changing Electric Fields DON'T Cause Magnetic Fields

In other words, magnetic fields are not created by Changing Electric Fields.
Around 6:00 he does say that charge movement causes magnetic "situation".
Thus, the magnetic field in a spot of space is not generated by changing of electric field in that spot, but instead, by moving charges at some distance away from that spot. Both changing of magnetic field as well as electric field are caused by moving charges. Both field changes are the effect of the same cause.

[Bored chemist]

First of all we need to accept that an electric field may or may not be a "thing"- that's a matter of definition. You certainly can't got to the supermarket and buy some electric field.

However, we can have a charged body. And we can say that, if you put a small test charge near that body there will be a force on it.
And you can plot out the direction of that force as a function o location.

And you can cal that map of "what direction would a test change accelerate if you put it here" an electric field.

You can  play the same game  (in principle) with a magnet and a hypothetical magnetic monopole.
And that gives you a magnetic field.


If you do something that moves a  charged object, then you will change the electric field and if you move a magnet you will change the magnetic field.

So there is no practical difference between moving a charged object and changing the field round the "test charge".

However, if you choose to make this distinction:

Thus, the magnetic field in a spot of space is not generated by changing of electric field in that spot, but instead, by moving charges at some distance away from that spot.

you need to explain how the effect on gets from the point where the charge was to the point where you are measuring it.
There must be "something" in between.
Unless you propose to reinvent the luminiferous ether, that "something" is an EM field.

He might say that the field is just "bookkeeping", but there must be something that tells the point A that the charge has moved.
He says the charged thing is sending out it's location velocity and acceleration.
But he skips over how that's sent out.
Is it sent by pigeon?
Given that it travels at C, it's hard to see what it could be apart from EM radiation.

So his model of light conveniently overlooks the fact that it depends on light to make it work.
That's a circular argument.

[alancalverd]

I've seen some sources said that uncertainty principle is not a problem of measurement. Instead, it's a fundamental principle inherently embedded to the math of wave models through Fourier transform.

Neither is true, and the term "uncertainty" is misleading.

It is a principle of indeterminacy: a particle cannot simultaneously have a position and a momentum since  position implies stasis and momentum implies continuous motion.

If we arbitrarily assign a limit to the determinacy of both and say Δp.Δx = h, we can determine h by experiment since the indeterminacy defines for example the effective radius of a hydrogen atom.

[hamdani yusuf (OP)]

Neither is true, and the term "uncertainty" is misleading.

Did you say that it is a problem of measurement?