1

**Physics, Astronomy & Cosmology / Re: What is a tensor?**

« **on:**18/04/2018 18:09:24 »

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.

2

@Thebox @atbsphotography

These long quotes in the last few posts use up a lot of bandwidth and make the threads hard to follow. Please edit your quotes to the specific point you are answering.

Thank you

These long quotes in the last few posts use up a lot of bandwidth and make the threads hard to follow. Please edit your quotes to the specific point you are answering.

Thank you

3

@PmbPhy and @jeffreyH understand tensors and will give you very sound answers with no waffle, but as @Bill S says, perhaps you need to step back a bit.

Vector is a simple tensor rank 1. Examples can be a force+direction or speed+direction.

Tensors aren’t limited to these values because a tensor is just an array of numerical values that can be used to describe the state or properties of a material or point in space.

Let’s take a simple example of stress. Stress, like pressure is defined as force per unit area and a material can support different forces applied in different directions. Imagine a cube of material with forces acting on it in three dimensions, the stresses on the cube can be obtained and stress state can be represented by 9 components - 3 for each face, force component and 2 components for the area of the face. Hence a second rank tensor.

Stress, strain, thermal conductivity, magnetic susceptibility and electrical permittivity are all second rank tensors, piezoelectricity is described by a third rank tensor, elasticity of single crystals is described by fourth rank tensor.

So very useful, particularly when you add the transformations and operations that @PmbPhy and @jeffreyH mention.

So ,if in any medium (or apparently also even in a vacuum) any point is associated with more than one force , these forces can be represented by vectors and they are called tensors when they all originate and combine at this one point....Let’s start by forgetting medium, vacuum, forces, and combining at one point.

Vector is a simple tensor rank 1. Examples can be a force+direction or speed+direction.

Tensors aren’t limited to these values because a tensor is just an array of numerical values that can be used to describe the state or properties of a material or point in space.

Let’s take a simple example of stress. Stress, like pressure is defined as force per unit area and a material can support different forces applied in different directions. Imagine a cube of material with forces acting on it in three dimensions, the stresses on the cube can be obtained and stress state can be represented by 9 components - 3 for each face, force component and 2 components for the area of the face. Hence a second rank tensor.

Stress, strain, thermal conductivity, magnetic susceptibility and electrical permittivity are all second rank tensors, piezoelectricity is described by a third rank tensor, elasticity of single crystals is described by fourth rank tensor.

So very useful, particularly when you add the transformations and operations that @PmbPhy and @jeffreyH mention.

The following users thanked this post: Bill S

4

I understand Relativity assumes space to be unchanging volume.Most of the problems GR is used to solve take place in none-expanding space. That’s not the same as constant volume. Look at the Schwarzschild metric, if distance changes so must volume!

It includes the cosmological constant (Dark Energy) to explain away the expansion of space.More to stop it expanding or contracting.

But you can take that out or use a different value. See https://map.gsfc.nasa.gov/universe/uni_accel.html

It does not explain the mechanism behind the apparent expansion(dark energy) and contraction(gravity) of space.That depends on what is causing these phenomena, at the moment we don’t know.

Can space

1) be considered a super fluid, by most theories,

2) exist without quantum fluctuations.

3) be multidimensional if non local effects can not be explained in any other way.

4) allow both real and virtual particles to exist in it.

5) expand due to dark energy( possibly caused by virtual particles/quantum fluctuations)

6) contract due mass and gravity( possibly due to the absorption of quantum fluctuations/gravitons)

1) be considered a super fluid, by most theories, - most theories just ignore this. As I said in @jeffreyH thread on inertia, space doesn’t offer any resistance to mass. Problem is how to detect superfluid, how would we know if there were permanent eddies? Best you can say is behaves like it when considersd in specific ways.

2) exist without quantum fluctuations. - that depends whether you consider space and the vacuum to be one and the same, or separate.

3) be multidimensional if non local effects can not be explained in any other way. - depends what you mean by “can”. Do you mean “is” if no other

4) allow both real and virtual particles to exist in it. - generally consider it is the field that supports particles, yes both exist but be careful what you interpret virtual particles to be. Fields exist in space.

5) expand due to dark energy - apparently. No comment on cause

6) contract due mass and gravity - it can, no comment on your cause.

Edit I found this link indicating non locality is a fact, but it may be able to be explained away via a wave function, I think that is what its conclusion was https://arxiv.org/ftp/arxiv/papers/1402/1402.4764.pdfYes, that is how the author tries to describe it.

The problem is the one I raised in the entanglement thread. The wavefunction is the description of probability distribution of the particles and those probabilities remain the same whether the 2 particles are in the same room or light years apart. So, you could argue both ways.

Depends whether you believe QM etc describes an underlying reality Eg I send you a playing card, in the post, drawn at random. I can either say that it takes all 52 values until you look at it, or I can say it has a value but we won’t know until you look. The probability of a specific card is the same no matter which description we choose.

There are arguments both ways at the moment. Have a look at this https://arxiv.org/pdf/1205.4636.pdf

Interesting discussion. Won’t be around all the time over next 2 weeks, but will look in when I can.

The following users thanked this post: disinterested

5

Why relative motion is not necessary?I think you are forgetting gravitational effects. I know you know, because we’ve discussed before.

Here is a different point of view, with more details:As it says in the link “None of this is really a contradiction between general relativity and quantum mechanics. ”

https://physics.stackexchange.com/questions/387/a-list-of-inconveniences-between-quantum-mechanics-and-general-relativity

The following users thanked this post: nilak

6

No one talks about that possibility. Why? Is it illogical?Lots of people talk about it, but not on this thread which is about something else.

Does 1 asteroid traveling at X speed have the same impact force as 3, 1/3 sized objects travelling at 3X speed?The energy is the same but its distribution will be different.

Basic Force equation says yes, but does material makeup and the fact that less material is lost on contact with the atmosphere change that?

Smaller lumps are more likely to burn up on entry giving lots of downward heat, as in a nuclear air burst. If the entry points are widely spread then the heat will be dissipated over a wider area giving less overall effect. There will also be significant shockwaves flattening trees etc.

A single lump which loses mass on entry might have sufficient remaining mass to kick up a significant dust cloud and and create a nuclear winter.

Useful to note that the nucleus of a comet is mainly ice and frozen gas with a small amount of solid material.

The following users thanked this post: Xen

7

However, we know that a particle has higher energy when the associated wavelength is smaller. ......, therefore higher wavelength apear only if there is relative motion between the observer and the particle.Relative motion is not necessary.

We are dealing now with effects on quantum fields, which GR cannot handle,I don’t understand what you are saying here. GR can handle quantum fields, but the 2 work at different scales. You wouldn’t use a micrometer to measure a football field.

The following users thanked this post: nilak

8

Or is it more accurate to state that the state of neither photon is known until one is observed, and once one is observed then you instantly know the state of the other photon, regardless of separation distance.I think you are getting close to the crux of this question, because the above is certainly true.

Looking at the article you quote:

Edit: https://www.livescience.com/28550-how-quantum-entanglement-works-infographic.html (The transfer of state from photon A to Photon B takes place at a speed at least 10000 times faster than the speed of light)In there they say:

Qouting from the above link

In quantum physics, entangled particles remain connected so that actions performed on one affect the other, even when separated by great distances. The phenomenon so riled Albert Einstein he called it "spooky action at a distance."

“The rules of quantum physics state that an unobserved photon exists in all possible states simultaneously but, when observed or measured, exhibits only one state.”

If you believe the above, then you must also believe that both photons exist in all possible states until measured, and when one is measured then somehow the other one must be informed of that measurement.

If, on the other hand you believe that entanglement puts two photons into related states such that measuring one reveals the state of the other, then no communication is necessary.

If you stick to strict QM rules then you must believe the former, and use that terminology.

However, QM does not pretend to deal with an underlying reality, it has a set of rules and terminology for handling wavefunctions and probabilities in such a way as to predict the behaviour of quantum systems, that is all. The terminology can lead to a number of paradoxes.

Feynman once said to his students, shut up and calculate

9

If entanglement is an intrinsic feature of quantum particles, surely, there needs to be no transfer of information at the quantum level.I think this depends on what you are calling information.

When 2 particles collide they transfer information about their momentum.

Again we have to be clear what we mean by information.QuoteLater the three get together to compare notes

As the article states; information is shared later.

The article states “Charlie generates a pair of entangled photons, and records the time.

When Alice detects a photon, she records the polarization and the time at which the measurement was made. When Bob measures his photon, he also records the polarization and the time of arrival.”

So we can see that all 3 collect a lot of data and they get together to analyse it.

Yes, data is information, but don’t confuse it with the idea of spooky transfer of information.

The following users thanked this post: Bogie_smiles

10

In a weak field and small mass the equations approximate to the Newtonian U.

Near a black hole you would need to use the full GR calculations.

Near a black hole you would need to use the full GR calculations.

The following users thanked this post: jeffreyH

11

Is Preaching Christianity allowed, promoted or overlooked on The Naked Scientists Forum?No

(If any Mod could provide a simple yes/no answer, I'd be much Obliged.)

Thanks.

😁

The following users thanked this post: Zer0

12

13

LOL no particles were harmed in the making of this experiment - NOT!

The following users thanked this post: PmbPhy

14

"Where do the laws of physics come from?" My supposition is that they have always existed, and are a set of invariant natural laws ...I would agree with that. The scientific process is one of discovery rather than invention.

Words or maths? Well, you can describe Pythagoras in words, but it is the maths that makes it useful. Both are language, choose the right tool for the job.

The following users thanked this post: Bogie_smiles

15

It isn’t really related to distance, but to the sensitivity of the 2 types of receptor in the retina. The cones are the colour receptors sensitive across the range of colours but need quite a bit of light to work, the rods are better at night - low light - but are less sensitive to the red end. So as it gets darker you will start to see the reds getting duller and at night blues are easier to see - assuming the same level of illumination.

As regards lights in the distance, it will depend on the relative level when the light reaches your eye, so a really bright red light will be easier to see than a dull blue one and vise versa, but if they are the same brightness the bluer one will be more easily seen.

If you want to know more tech detail look up Purkinje effect.

As regards lights in the distance, it will depend on the relative level when the light reaches your eye, so a really bright red light will be easier to see than a dull blue one and vise versa, but if they are the same brightness the bluer one will be more easily seen.

If you want to know more tech detail look up Purkinje effect.

The following users thanked this post: doninover

16

Which bit is incorrect? All of it?The bit about how he thought about it is. As Pete says, and I’ll quote Einstien “I should observe such a beam of light as an electromagnetic field at rest.” In other words, the wave would seem stationary, but this was not possible according to Maxwell’s equations, which describe the motion and oscillation of electromagnetic fields.

So he either believed Maxwell was correct or the wave was stationary. He chose Maxwell for the same reason we do, the maths is correct and consistent with experiments of Faraday, Gauss etc.

The following users thanked this post: Bill S

17

True or false:False

New study PROVES vaccines SPREAD the FLU

I suspect the ‘reporters’ haven’t read the original report.

Everyone tested had symptoms “We screened 355 symptomatic volunteers with acute respiratory illness and report 142 cases with confirmed influenza infection” so this isn’t a general population. To test the hypothesis you would have to take a random sample of the population and see whether they were emitting flue virus.

It is interesting that people who had been vaccinated had flue, so for them the vaccine had not worked. The suggestion made in the paper is:

“The association of current and prior year vaccination with increased shedding of influenza A might lead one to speculate that certain types of prior immunity promote lung inflammation, airway closure, and aerosol generation. This first observation of the phenomenon needs confirmation. If confirmed, this observation, together with recent literature suggesting reduced protection with annual vaccination, would have implications for influenza vaccination recommendations and policies.”

The following users thanked this post: FuzzyUK

18

You can post whatever you like here as long as it is legal and friendly.

However, there is no benefit in bringing entanglement into a discussion that clearly isn’t giving you the answers you are expecting, it will just create more confusion.

However, there is no benefit in bringing entanglement into a discussion that clearly isn’t giving you the answers you are expecting, it will just create more confusion.

The following users thanked this post: opportunity

19

Valid as this might be mathematically, it involves imaginary time. Is there a fundamental concept of imaginary time in physics?As you know imaginary numbers were very much misnamed when discovered, but they do have a physical significance in many situations. In the case of imaginary time there is no physical significance needed, as used by Hawkins, it is a mathematical work around by changing the metric to a Euclidian space which is far easier to handle. If you want to imagine what has been done, it is like rotating the time axis 90 so it is still orthogonal to the 3 space dimensions but now has the same sign (using East Coast convention). It’s known as a Wick rotation if you want to look up details.

I fail to see how drawing an analogy between the two-dimensional surface of the Earth and three-dimensional spacetime solves the problem of something emerging from nothing.Hawkins was interested in a slightly different issue which is whether the start of time for our universe is from a point eg like a singularity or from a smoother surface - a boundary. A Euclidian space can be modelled as if it is surface of the earth (but you have to imagine that surface is 3 space dimensions) and if you imagine the start of time as the S Pole (in some lectures he uses N Pole) with time axis pointing to N Pole and 2 of the 3 space dimensions at rt angles , then the expansion from the s pole moves up the surface of the earth as a series of time-slice circles (but in 3D). The point about starting from the s Pole is that it is a smooth curve (boundary) rather than a sharp discontinuity like the point of a cone.

Hope that makes some sense.

The following users thanked this post: Bill S

20

Most of the universe is too far away to influence the rate of passage of time.@jeffreyH what @Bored chemist says is equivalent of taking the std gp calculation removing test particle to ∞. You could start with that as a working assumption which would give you a magnitude to work with.

The following users thanked this post: jeffreyH