[Eternal Student]

Hi.

An entity is a distinct object - electron, motor car, whatever

   Do physicists really use the term "entity" that much?   Sounds a bit metaphysical.
   According to Wikipedia we have this definition:
An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not. It does not need to be of material existence.

   I can't find any specialist scientific definition of the term "entity" on a quick search of the internet -  except for computer science where it is considered to be one object about which you can store data in a database.  It has a meaning in the English Language and you could obviously chose to use it in a piece of scientific text just as that.

OK. I got into this a bit with other people who seemed keen to point put that once you define an entity, it has attributes or properties. But are the properties the things that are identified and in what way is an entity separate from its attributes?

     It depends how much science you want to discuss.   To keep it simple, then yes - real objects have properties and it makes some sense to ask further questions along the lines you have presented.   However, if you wanted to consider Quantum Mechanics (QM) then it is no longer so obvious that objects must have properties.

     In QM the notion of "local realism" cannot be sustained.  As such one answer to your question "in what way is an entity separate from its attributes?"  is that an object and its attributes could be separated by many thousands of miles.  There are many ways in which an object can be separated from its properties or attributes.   Moreover, an object does not need to have any definite value of its attributes until those are measured.   Furthermore, it cannot be assumed to have properties much like hidden variables that existed but were just unknown to you.  In some situations it is just not possible to explain the behaviour of some objects if their attributes were fixed and established by nature prior to the measurement being made.
    If you're interested look up  the EPR paradox   and   Bells inequality.   (Glance at these Wikipedia entries for example:   https://en.wikipedia.org/wiki/Einstein%E2%80%93Podolsky%E2%80%93Rosen_paradox         and also     https://en.wikipedia.org/wiki/Bell%27s_theorem ).     However they aren't easy going or an ideal introduction.    This video:  "Bell's Theorem: The Quantum Venn Diagram Paradox" by minute physics is available on YouTube and it's a much lighter introduction to the topic  (it will still take about 15 minutes, even "minute physics" didn't seem able to keep it short).

Best Wishes.

[alancalverd]

Do physicists really use the term "entity" that much?

The high priests of pedantry, the International Standards Organisation, use the term exactly as I did. It's been a while since I was involved in such matters but I don't think energy is an entity in ISO - it is a quantity. Feynman was a good musician but I think he got this one wrong.

[varsigma (OP)]

I've walked across a nylon carpet and am sitting in my rotating office chair. I have mass, charge and spin. I am not an electron. Nor is an electron a proton.

OK, let's suppose the quantities are identical. I have £50 and The Boss has £50. I am not a woman, and she is not Alan.

More identities: John and Tom are identical twins, down to the last atom of their DNA. They are still distinct entities.

But if you want to be mystical, conduction electrons really are indistinguishable!

Ok. Sort of.
You can't really compare classical objects with quantum particles, can you?

Conduction electrons are indistinguishable because all electrons are identical. Identical in a very exact way, whereas classical copies are only identical in an approximate way.
Print two copies of a document; close examination of the characters will show some differences, never mind how different the sheets of paper might be.

Copying information classically never gives you identical copies--even in a digital computer with very tight manufacturing tolerances. Classically we have to deal with approximate copies.

It depends how much science you want to discuss.   To keep it simple, then yes - real objects have properties and it makes some sense to ask further questions along the lines you have presented.   However, if you wanted to consider Quantum Mechanics (QM) then it is no longer so obvious that objects must have properties.

The mass, charge, and spin of the electron are things we can safely assume all occupy the same place, in classical experiments. But those are what we might term the local properties or attributes; the position and momentum are also "properties" because--spacetime . . .

It's difficult to put nonlocality into the frame when it comes to the location of an electron, then the location of its charge, its mass, and its spin. Most electron experiments I've looked at appear to assume these are all localised. For instance they propagate together in a beam of electrons.

The only real evidence I've seen of de-localised properties is the appearance of quasiparticles on the surface of certain materials at low temperatures. Here you see electrons interacting with the material and each other, and appearing to separate into localised quanta of charge (chargons), spin (spinons), and mass (magnons). This result confirms my suspicions that electrons are a superposition of field quanta. Since the result also yields some information about geometry and dimensions, that has to be a measurement.

[alancalverd]

An electron is a classical particle, and like all particles it is subject to quantum mechanics.

Now Pauli says you can't have two fermions with identical quantum numbers, so electrons are distinguishable.

Except when they aren't.

[Bored chemist]

nobody can say what energy is.

A dictionary editor can.

[Bored chemist]

To measure a distance you MUST actually measure THE TIME it would take for light to travel travel there,  you cannot do it just by putting some sticks between the two points.

No
Nobody does that.
Apart from anything else, how do you measure time?

Also there's a reason for the weird number in

how far light would travel in a vaccum after a small fraction ( 1 / 299 792 458 ) of a second. 

And it's arbitrary.

[geordief]

Apart from anything else, how do you measure time?

Is  a time unit arrived at  by counting the spontaneous and     random emissions in the radioactive decay of  an atom?

[Eternal Student]

Hi.

The mass, charge, and spin of the electron are things we can safely assume all occupy the same place, in classical experiments.

   The spin of an electron is exactly the sort of property that could become entangled with the spin of another particle and illustrate behaviour similar to photons with entangled polarisation that were described in Bells inequality.
    You can't prevent it just by declaring that an experiment was "classical", all you can do is choose to do an experiment where it wouldn't be important.  However, you can certainly choose not to include it in your discussion, all physics is just a model and a decision must be made about what is sufficient.

ES said:    To measure a distance you MUST actually measure THE TIME it would take for light to travel travel there,.....
BC replied:    ...Nobody does that.

   1.   Nobody eats 5 portions of vegetables every day but they should.  Distance is defined a certain way, that's not my fault, that's just how it is in the SI system.
    2.   Some people do measure distances just by measuring the time for light to travel.  Estate agents have radar guns to measure the dimensions of a room quickly.  As you know, estate agents are paragons of truth.

Also there's a reason for the weird number in   ( 1 / 299 792 458 ) of a second.
And it's arbitrary.

    Yes.   Setting it to 1 would have been easier but, I suppose, there were limits on what number they could chose without putting the modern definition too far out of tolerance with the older standards already in use.  It's an integer, so... glass half full.

Apart from anything else, how do you measure time?

   Well, as I expect you already know there is a standard established for the second.  It looks like @geordief has already asked about this.   I was tempted not to even make a start discussing time, it's too late in the day.   For practical purposes I use the elephant method ( just count 1 elephant, 2 elephants, 3 elephants...etc. ).

Best Wishes.

[Bored chemist]

Apart from anything else, how do you measure time?

Is  a time unit arrived at  by counting the spontaneous and     random emissions in the radioactive decay of  an atom?

No, it's defined in terms of microwaves emitted by caesium atoms.
But the tricky bit is that, in principle, anything perturbs that emission- even if it's only by gravitational shifts.
So, if you have anything nearby that you want to measure the length of, your clock is wrong and so is your ruler.

[Bored chemist]

ome people do measure distances just by measuring the time for light to travel.  Estate agents have radar guns to measure the dimensions of a room quickly.  As you know, estate agents are paragons of truth.

Those are interesting gadgets, but they don't work by time of flight of photons.
On the other hand, if you use them to measure the size of your swimming pool, you get the answer wrong by a factor of the refractive index of water.

[alancalverd]

nobody can say what energy is.

A dictionary editor can.

And I have, at least twice in this thread.

[alancalverd]

The mass, charge, and spin of the electron are things we can safely assume all occupy the same place, in classical experiments.

The sentence is meaningless, so you can't "safely assume" its validity.

[geordief]

So, if you have anything nearby that you want to measure the length of, your clock is wrong and so is your ruler

Why is that important.?We can never attain absolute accuracy in measurements ,can we?

If the clock was right but the spatial distance was wrong that really would be a problem,surely.

[alancalverd]

But the tricky bit is that, in principle, anything perturbs that emission- even if it's only by gravitational shifts.
So, if you have anything nearby that you want to measure the length of, your clock is wrong and so is your ruler.

Not true, my friend! Both space and time are distorted by the same phenomenon so your measurement is correct!

[alancalverd]

2.   Some people do measure distances just by measuring the time for light to travel.  Estate agents have radar guns to measure the dimensions of a room quickly.  As you know, estate agents are paragons of truth.

I rarely deal with estate agents, but I frequently rely on the opinion of radar operators who use exactly this method to tell me about conflicting traffic.

[Eternal Student]

Hi.

Those are interesting gadgets, but they don't work by time of flight of photons.

    I thought most radar systems measure distance this way.   I'm almost tempted to ask how estate agents devices do work - but I have only a passing interest so a short answer would be fine.
 

But the tricky bit is that, in principle, anything perturbs that emission- even if it's only by gravitational shifts.
So, if you have anything nearby that you want to measure the length of, your clock is wrong and so is your ruler.

    I've got to agree with @alancalverd  and @geordief  here.    It's not "wrong" as such,  it just is the local flow of time.   It's not the same rate as you might obtain elsewhere but since Einstein's results we were expecting or counting on that.   It is exactly how we would want time to be measured or defined.   
    There are a few caveats:   1.  We assume you can keep an atomic clock small enough so that all of it is effectively in one location and experiencing the same local conditions.   For example, in the vicinity of the event horizon of a small black hole the curvature of space could change significantly even over the width of just a few atoms and not all of your Caesium atoms will be under the same conditions.   This is only a problem for practical use of the equipment, theory can have time defined this way without problems.
    2.  It does assume that no significant forces or fields exist which would influence the electron orbit energy levels but a good clock should be designed to control the E and B fields at the Caesium atoms (along with everything else we think is important and know about).   
    3.  There could be unknown unknowns.  If there is some currently unknown physics that could influence these electron orbit energy levels  OR  shift the frequency of radiation before it's detected by the detectors - then it isn't measuring exactly what we wanted.

Best Wishes.

[Eternal Student]

Hi again,

Is  a time unit arrived at  by counting the spontaneous and     random emissions in the radioactive decay of  an atom?

    Are you happy with the answers / discussion so far,  or did you want a reference to the way that time is measured?
There are some references and discussion in Wikipedia or on the internet.   Just use sensible keywords like  "second", "time",  "caesium", "atomic clock"   etc.
     It isn't really about the radioactive decay of an atom.   It is just about a Caesium atom being allowed to have an electron make a transition from one energy level to another.  As you probably know, for any given transition between two energy levels, a photon of precisely one frequency is emitted (or absorbed).   We get the equipment to make a specific transition happen and assert that 1 second is however long it took for 9192631770 cycles of that electromagnetic wave to pass a point in space (e.g. a detector).    So, as opposed to a radioactive decay process, where your clock substance would be used up and depleted,  the Caesium atoms are re-useable,  you just excite the electrons back to the high energy state again.

    There are other systems and atomic clocks but the general concept allowing us to define a second is as described above.   Some of the more reliable and practical methods use some simpler device (like a quartz crystal) to keep time and the Caesium atoms (or whatever substance they have chosen) just become a test.  The entire system can be imagined as something which guesses the time and then applies corrections.  For example, you can bombard atoms with radiation which is just guessed to be the right frequency (based on the quartz crystal system) and then adjust the frequency upwards or downwards slightly until enough of the Caesium atoms are detected as being moved to the higher energy state.   Computer algorithms automatically apply adjustments to the time keeping that has been done in between the bombardment tests.

Best Wishes.

[Zer0]

Hopefully you aren't thinking in Metaphysical terms, are you?

No. I think I might be thinking in terms of: If Feynman is correct, nobody can say what energy is.
If someone else says energy isn't physical they need to explain how they aren't saying they know what energy is.

It's just logic, really. I don't think metaphysics comes into it.
Or perhaps there's an idea that if you can't say what it is, because nobody knows, you can still say what  it isn't.
Like, you can say energy isn't time, or distance. Can you say energy isn't physical? What does that mean?

If it isn't meant to be Metaphysical, but purely Logical...

Then Energy seems to be a Calculable entity.

It's a Real thing.

It can be Measured.

We can Control & Utilize it.


(I do Not feel the need to bring Prof Feynman into this.
& I do Not believe He ever made such an Absolutist statement.
Maybe for QM, but not ' Energy ' )

Apologies i Misunderstood the Topic to be based on Abstractions, hence felt it was a Metaphysical dialogue.

[varsigma (OP)]

    The mass, charge, and spin of the electron are things we can safely assume all occupy the same place, in classical experiments.

The sentence is meaningless, so you can't "safely assume" its validity.

If the sentence is meaningless, how did Milliken do his experiment? What did he assume?

[geordief]

decay of  an atom?
    Are you happy with the answers / discussion so fa

I am not sure I agreed with you regarding yard sticks.

If you create a yard stick for use as a universal unit of measurement  and then wait until (let's hypothesise) the speed of light slows appreciably.

As per what I understand you to be saying  those yard sticks remain unchanged in length but I am wondering  whether their length is dependent on the speed of light  insofar as the bonds holding the Atoms together  are  those of electric attraction.

Since light is em radiation  I was wondering if the force of electric attraction was proportional to the speed of em radiation -ie the speed of light.

If so ,the yard sticks  would change in length.

Am I right?