Genady

Just now, swansont said:

Owing to dispersion/diffusion/dissipation, wind has a better defined source than sink and water flow in rivers has a well-defined sink, but not source. Makes sense the direction would be in terms of the component that’s more clearly known.

Yes, this is the "hypothesis 2" above. (I guess we cross posted.)

1 hour ago, exchemist said:

Wind direction is specified according to the direction it comes from, as that is what you feel on your face and how you relate to the wind to set your sails and steer your vessel. I couldn't quickly find a reference to a "northern current", but I imagine it might make sense to refer to that according to the direction in which it causes your vessel to drift. 

31 minutes ago, TheVat said:

That sounds about right.  Water, for humans, is all about where it is flowing to.  A river or stream flows south, it isn't a "northerly stream."  Maybe earliest human experiences were with freshwater, simple rafts on water flowing between banks, where the concept is very clear.  With wind, otoh, it matters where it comes from - north wind brings cold, south wind brings milder temps, west wind (in a particular area) might bring more rain, or dust, or distant sea air.

Let it be "hypothesis 1". Here is "hypothesis 2":

From an everyday physical experience, air needs to be blown in order to make it move, i.e., its direction is determined by its source. Water, OTOH, flows by itself, e.g., downhill or towards the ocean, i.e., its direction is determined by its destination.

45 minutes ago, StringJunky said:

Here we say "Northerly, Westerly, Easterly, Southerly". The convention seems to name them where they come from i.e. the source.

Winds but not currents, AFAIK. For example, (Sea current direction @ Windy Community)

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In the nautical world the sea current direction is named as:

The direction of ocean currents is the direction they're headed for or where the current is flowing towards, and the direction of wind is named as the direction from where it's blowing. So an easterly current and westerly wind is going the same way or has the same vector - from west to east, but is named differently.

Why are the directions of wind and current designated in an opposite way, as in "Northern wind" vs. "Northern current"?

6 hours ago, Elmore said:

Zero is not fundamental and nonzero numbers are fundamental (Newton/Einstein calculus).

There is no such distinction in calculus.

BTW, there is no thing called "Einstein calculus".

6 hours ago, Elmore said:

In Geometry any new dimension has to contain within it all previous dimensions.

It does not seem to be correct. For example, the dimension "height" does not contain within it the dimensions "length" and "width".

7 hours ago, MasterOgon said:

Let's imagine a homogeneous medium that consists of individual particles.

Unimaginable. If it consists of individual particles, it is not homogeneous.

1 hour ago, Oryza sativa said:

More clearly stated, information is an increase in uncertainty or entropy.

If we take entropy as being information

The two statements above are contradictory. The first says that information is an increase in entropy: \(I=\Delta S\). The second says that information is entropy: \(I=S\). Before continuing with the argument, this contradiction needs to be cleared out. Which one is true, \(I=\Delta S\) or \(I=S\)?

13 minutes ago, joigus said:

You could, of course argue that there are <T²>-<T>² quantum fluctuations of these quantities, and thereby similar quantum fluctuations in the Einstein tensor.

I am now in the first chapters of Penrose's The Road to Reality. I know that he has his ideas on this topic, and I am curious but not there yet. 

4 minutes ago, ovidiu t said:

a neuropsychological standpoint

How does it relate to Astronomy and Cosmology?

10 minutes ago, Time Traveler said:

Images of stars from Milky Way are from a mix of past times from 4.2 years Proxima Centauri until 10 000 ,30 000 and 100 000 years from stars at  the borders from Milky Way

When we look at the image of, for example, a spiral galaxy, we do not look at it in the disk plane, but close to perpendicular to the plane. The thickness of Milky Way's disk is about 1000 ly. We don't see through the disk, so we see about 500 ly thick image. The maximal difference in "past times" is thus about 500 years.

25 minutes ago, Time Traveler said:

We can't make a correct image from a mixture of different past times 

We can take in account the motion of stars in a galaxy and estimate their positions at one instant of time. As the past times you mention are only different by several hundred years, we find that on a galaxy scale this does not have any visible effect on the image we get.

We receive at present the signals from different times in the past and we separate them if we know relative distances to the sources. For example, we can see simultaneously the Sun and the Moon, but we know that we see the Moon as it was one second ago and we see the Sun as it was 8 minutes ago. The same with galaxies and all.

5 hours ago, joigus said:

Every direction I know in which people are thinking has to do with generalising QFT to the appropriate degrees of freedom accounting for gravitation or proposing a unifying principle (EPR = ER) that achieves the concept-bridging between GR and QFT that everybody dreams of.

And then, there is Freeman Dyson (F. Dyson, “The World on a String,” New York Review of Books, May 13, 2004)

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[Gravitons] are like the ether, the elastic solid medium which nineteenth-century physicists imagined filling space. Einstein built his theory of relativity without the ether and showed that the ether would be unobservable if it existed. He was happy to get rid of the ether, and I feel the same way about gravitons. According to my hypothesis, the gravitational field described by Einstein’s theory of general relativity is a purely classical field without any quantum behavior.

1 hour ago, MigL said:

At this point its position is exact; it is a point.

From Susskind & Cabannes, General Relativity: The Theoretical Minimum (p. 227):

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the singularity at r = 0 is not a place, it is a time! More accurately, it is not a single place but many places all on a time-like curve.

10 minutes ago, sethoflagos said:

I don't think you can use the word 'generally' when your justification is based on the specific case of a boolean relationship. Applying this reasoning to a continuous variable like photon frequency seems too big a jump for me.

The direction of spin is continuous. The spin of the electrons can have any one of continuous 3D directions.

Generally, an observable value can be uncertain AND conserved - there is no contradiction. For a simple example, consider an entangled Bell pair of two electrons with opposite spins. The spins of the electrons are maximally uncertain but they certainly sum to zero, i.e., are certainly opposite to each other.

2 minutes ago, swansont said:

Vacuum fluctuations are not an example of variations about zero energy, though. The energy of the vacuum - zero-point energy - is not zero. 

Exactly. This was my message:

7 hours ago, Genady said:

its average cannot be zero.

4 minutes ago, swansont said:

What’s an example of a fluctuation about zero?

We were discussing this example:

6 hours ago, exchemist said:

for instance, vacuum fluctuations imply a temporary violation of conservation of energy, I think, which averages out to zero.

20 minutes ago, swansont said:

The linewidth of a de-excitation transition is related to the lifetime, owing to the uncertainty relation. The nominal value of the transition might be e.g. 1 eV, but the value of any particular photon might be slightly higher or lower than 1 eV. Not negative, since the linewidth is on the order of MHz to GHz. 

Right. The fluctuations are above and below a positive average, but not below zero average.

6 minutes ago, swansont said:

The contribution can average to zero; the value being slightly larger or smaller than the average.

Please explain: if the average is zero, then a slightly smaller value would be negative, wouldn't it?

5 minutes ago, exchemist said:

Hmm, fair point about energy. I'm afraid I don't know QFT, so I am not sure of the connection between the path integral formalism and the uncertainty principle. AS I understand the concept of expectation values of a property it is the average result one would get from a series of measurements on a series of identical systems. Some individual members of the series would be below and some above. So if the expectation value corresponds to the value predicted by conservation, some of the results might not. Is this a wrong picture?

Energy is conserved in every possible outcome, not only on average. In no quantum or particle experiment conservation of energy was ever violated.

Momentum is conserved on every line and at every vortex of every Feynman diagram. 

The path integral calculation sums up all available paths, not only straight ones.

1 hour ago, exchemist said:

vacuum fluctuations imply a temporary violation of conservation of energy, I think, which averages out to zero

There is an inconsistency in this claim: energy is non-negative, so if it becomes positive even temporarily, its average cannot be zero.

In QM, as well as in QFT, momentum is conserved. Lagrangians are translationally invariant. Momentum conservation follows by the Noether's theorem.

19 minutes ago, TheVat said:

In deep diving, nitrogen narcosis has been described as like being drunk and rather pleasant, hence the slang term "rapture of the deep."  (  @Genady would probably know more about that.)

I've had an experience with the nitrogen narcosis, some myself and more in other divers (students and those I was guiding.) I would not describe it as being drunk, but rather as being euphoric and very unfocused. As the divers ascend and the effect disappears, they often don't have any recollection of what they were doing and what happened while they were affected, similarly to what happens immediately after waking from being sedated by a drug.

11 minutes ago, Airbrush said:

In a multiverse model there may be multiple, or an infinite number of, big bangs.  In that case each "universe" is finite in size, has a center, and edges expanding outward. The outer edge may have ANY shape and be moving at ANY speed since the edge is not constrained by space.

Not necessarily. There are ways to have even infinite number of infinite universes in a multiverse. And if some of them are finite, they do not necessarily have a center, edges, etc.