What is "real" in physics?

[swansont]

Swansont makes the point that being real and being fundamental are not the same thing. I think this is wrong, but then I use the words differently.

This came up in another thread, and I thought this would be an interesting discussion, so I wanted to set this up before I forgot, and will weigh in when I have a chance.

[studiot]

The statement was pithy, too the point, accurate.

Agreed 110%.

 

What more is there to say?

Edited May 7, 2014 by studiot

[Greg H.]

I'm reserving comment until Swansont expands on his initial thought. I am pretty sure I know what he's getting at, and I'm relatively certain I agree with the thought process behind it, though.

[swansont]

One aspect of this is the question if electrons are real. What about photons? For me this came up in a discussion some time ago, with the question of whether phonons were real — my answer was no, that's just a model for how we describe the vibrational modes of a lattice. The model is powerful and reasonably simple, so that's why it is used. It's similar to modeling the lack of an electron as a positively charged "hole" when looking at semiconductors. A hole isn't physical object. Which raised the question: maybe a photon isn't, either — it's just a vibrational mode of the universe's electromagnetic field. Electrons are a vibrational mode of another field. Is there a way to falsify these ideas?

 

There are plenty of constructs in physics that aren't fundamental, and aren't physical objects. We speak of electric and magnetic fields, for example, but these do not physically exist.

 

If you can find a copy of David Mermin's "What's bad about this habit" (paywalled at Physics Today but probably copies can be found elsewhere on the internets), I'd recommend it. The bad habit he refers to is treating our abstractions as real things, and discusses a bunch of things that are probably not real, because there's some underlying idea from which the abstractions emerge. The bottom line being that very little of what we discuss in physics might end up being real — physics real makes no claim to that. What we describe is behavior, so all we can say is that nature behaves as if these things were real.

 

So being fundamental makes no real claim on being real. It doesn't matter if the concept is emergent or not. Most of these are tools to understand how things behave. Personally, I don't get hung up on it — other people have much more passion for dividing concepts into real or not real, as if that made a difference. I can't recall ever being convinced that it mattered for the bulk of scientific inquiry. To me it seems almost all of physics can proceed without answering the question.

[studiot]

Let us look at English, which offers the classification

 

Concrete Nouns such as , well concrete

 

Abstract Nouns such as symmetry.

 

The first is what one might identify with 'Real' in the OP but is far from fundamental in Physics, although it has a place.

 

The second is non real, but is certainly fundamental in Physics.

 

But I agree with swansont, a bear can overclassify.

[Orodruin]

So being fundamental makes no real claim on being real. It doesn't matter if the concept is emergent or not. Most of these are tools to understand how things behave. Personally, I don't get hung up on it — other people have much more passion for dividing concepts into real or not real, as if that made a difference. I can't recall ever being convinced that it mattered for the bulk of scientific inquiry. To me it seems almost all of physics can proceed without answering the question.

 

I would tend to agree with you if you are saying what I think you are saying. As a scientist, I am interested in how well a theory describes the data. If it does a good job at this I am going to be happy with it. Whether or not the underlying constructs of that theory are "real" (whatever meaning is given to that word) are of less importance.

[Greg H.]

And as I suspected, I agree with Swansont for the most part. It gets a bit muddier for me, though, and I wanted to bring this up for clarification.

 

Let us take, for example, gravity. Is gravity real? Certainly the effects of gravity are very real - we feel those every moment of our lives, and they are, in fact, necessary for proper development of the human body. But does that make gravity itself a real thing, or an abstract concept that is used to provide a simpler description for those particular effects?

 

I tend to believe it's the latter. Gravity itself is a concept - it's a way we describe a particular set of physical interactions between objects. The effects are real - they can be felt, measured, calculated, and compared. We talk about measuring gravity or gravitational attraction as if it were real, but what we're really measuring is the effect of that attraction - it's just a shorthand way of speaking, in my opinion.

[swansont]

And as I suspected, I agree with Swansont for the most part. It gets a bit muddier for me, though, and I wanted to bring this up for clarification.

 

Let us take, for example, gravity. Is gravity real? Certainly the effects of gravity are very real - we feel those every moment of our lives, and they are, in fact, necessary for proper development of the human body. But does that make gravity itself a real thing, or an abstract concept that is used to provide a simpler description for those particular effects?

 

I tend to believe it's the latter. Gravity itself is a concept - it's a way we describe a particular set of physical interactions between objects. The effects are real - they can be felt, measured, calculated, and compared. We talk about measuring gravity or gravitational attraction as if it were real, but what we're really measuring is the effect of that attraction - it's just a shorthand way of speaking, in my opinion.

 

 

I agree that gravity is an abstraction. It's certainly not a physical thing I can hand someone, (similarly with linear/angular momentum or energy), and part of this conversation always seems to bog down when some of the participants use real in the sense of physical object or not, vs the use in the sense of illusion or not.

[Sensei]

One aspect of this is the question if electrons are real.

IMHO they are real.

 

And energy is real.

It flows from one particle, or body, to other during interaction between them.

Hot-fast particle shares energy with colder ones until they're all in equilibrium.

That's why it's always conserved. At least in classic & high energy physics.

 

What about photons?

The problem with photons, as with the most neutral particles, is that until they're absorbed, or decay (pion0, neutron etc), we have no idea about their existence.

Once we know about it, photon is gone from the system, making illusion it was not real.

Instead we have increased energy in our eye or detector.

 

Which raised the question: maybe a photon isn't, either — it's just a vibrational mode of the universe's electromagnetic field. Electrons are a vibrational mode of another field.

 

Such splitting fields is kinda artificial.

After all electron and positron can be made of photon(s) in pair production. And reverse in annihilation.

Muon will have it's own field, and tau yet another?

 

Is there a way to falsify these ideas?

 

IMHO the key is f.e. answering why exactly photons in pair production must have frequency equal Compton frequency 1.23559*10^20 Hz

Why it's not any other value?

What is so special in this frequency.. ?

 

(Yes, I know that m_e=9.11*10^-31 kg, or m_e=510999 eV/c^2 and E_e=510999 eV, 6.62607*10^-34*1.23559*10^20/299792458^2=~9.11*10^-31 etc)

 

String theory author Leonard Susskind, tries to answer why physical constant have value we know by introducing infinite number of universes, each one with different properties.

But again IMHO it's road to nowhere. Answer without answering question.

 

There are plenty of constructs in physics that aren't fundamental, and aren't physical objects. We speak of electric and magnetic fields, for example, but these do not physically exist.

 

How to visualize magnetic field? Use array of compasses. And each of them will be pointing in direction defined by magnetic field. But what was first, egg or chicken? To have some effect we need magnet, electromagnet or other magnetized source. Which are all made of particles. Particles in compasses will show us where are other particles and their attributes.

 

XIX century authors introducing electric and magnetic fields had no idea about particles.

 

No particles in the first place, no electric or magnetic field around them. No attraction or repelling between them.

[swansont]

And energy is real.

 

Can I borrow a cup of energy?

 

That's why it's always conserved.

 

Nothing to do with time translation symmetry, then?

 

How to visualize magnetic field? Use array of compasses. And each of them will be pointing in direction defined by magnetic field. But what was first, egg or chicken? To have some effect we need magnet, electromagnet or other magnetized source. Which are all made of particles. Particles in compasses will show us where are other particles and their attributes.

 

XIX century authors introducing electric and magnetic fields had no idea about particles.

 

No particles in the first place, no electric or magnetic field around them. No attraction or repelling between them.

 

Are you arguing for or against the fields being real? I can't tell from this.

[Greg H.]

 

Can I borrow a cup of energy?

Here's 8 liquid ounces of gasoline. Admittedly, the energy is in a somewhat compressed form, but the application of an appropriate catalyst can take care of that.

Sorry, I couldn't resist.

[Sensei]

Here's 8 liquid ounces of gasoline. Admittedly, the energy is in a somewhat compressed form, but the application of an appropriate catalyst can take care of that.

Sorry, I couldn't resist.

 

You can't beat protons & antiprotons in magnetic traps..

 

Today I read economic article that in the next 25-50 years there will appear the first trillionaire (10^12).

And the first thought was that it could be somebody who will find a cheap & fast way to produce antiprotons.

Edited May 8, 2014 by Sensei

[Orodruin]

As I said earlier, I do not want to put the label "real" on any concept in physics. However I do not see how one could consider the electron real without considering the photon real. Both are just excitations of different quantum fields, what would make a fermion field more real than a boson one? Or a fermion field in a certain representation more real than a gauge field? If you cannot call a photon real because it is always absorbed in its interactions I would not call electrons real because of the same reason. That another lepton is created in its stead in order to conserve lepton number I would consider inconsequential.

 

 

Muon will have it's own field, and tau yet another?

 

Yes, (simplistically) each particle is an excitation of the corresponding quantum field.

[swansont]

Here's 8 liquid ounces of gasoline. Admittedly, the energy is in a somewhat compressed form, but the application of an appropriate catalyst can take care of that.

Sorry, I couldn't resist.

 

Without the molecules — I just want the energy, and only energy. I'm a purist.

[moth]

Is a photon made of photons? Virtual photons are used to describe electric and magnetic fields, and the fields define the photon.
If they are circular how can they be real. Photons are just a map, not the territory.

[imatfaal]

 

Without the molecules — I just want the energy, and only energy. I'm a purist.

 

Ok - So I will accelerate the cup up to relativistic speeds, fire it off in the direction of your secret hideout, when you have utilised the energy you can give me the cup back (please handle with care as it was a present). You get the energy I get the cup.

[swansont]

 

Ok - So I will accelerate the cup up to relativistic speeds, fire it off in the direction of your secret hideout, when you have utilised the energy you can give me the cup back (please handle with care as it was a present). You get the energy I get the cup.

 

That's a property of the cup, then, not a substance held by the cup. It also disappears when when we're in the same frame. That's not a substance. How can a real, physical thing be frame dependent?

[Sensei]

We have protons at rest. Accelerating part of them to v > 0.99c and hitting them to other protons at rest.

 

Result is production of protons and anti-protons:

p+ + p+ -> p+ + p+ + p+ + p-

 

Like this has been showed in this article at the bottom:

http://galileo.phys.virginia.edu/classes/252/particle_creation.html

 

And from frame-dependent kinetic energy of particle, two new real particles appeared..

Edited May 9, 2014 by Sensei

[swansont]

How does that draw a distinction between energy as some substance and energy as an abstraction and a property of things?

[SamBridge]

If something isn't real, how is it being measured?

[moth]

If something is measured, how can we tell if we are measuring an emergent phenomena or a fundamental phenomena?
We can measure temperature, but that does not make degrees real.
We can detect light, but that does not mean Photons are real.

[studiot]

You guys are arguing with your instincts, not your intellects.

 

In ordinary English we can identify non material nouns ie separate out some quality from an object eg roundness or hardness.

 

In science or maths we can be more specific about these and even develop ways of quantifying these non material nouns.

 

Reality includes both material and non material nouns. Neither type is more real than the other.

 

In fact as soon as you have a single material noun, that you can declare to be 'real', I can find a non material property it possesses which must therefore be as real as the object itself.

Edited May 9, 2014 by studiot

[`hýsøŕ]

ignore me if this has already been said (have yet to read through the other posts in the topic) depends on your definition of real i think

rainbow's are real in the sense of them being something you can see, but not real in the sense of them being a physical object made of regular matter hanging around in the sky, real in the sense that they are due to some physical phenomenon, namely total internal reflection of light....

also you could say it's not real because it's appearance changes depending on where you are relative to the 'curtain' of moisture causing the rainbow, or you could view that as being some kind of varying physical system, like how the colour of an object changes depending on it's state of motion relative to you (doppler effect).

[SamBridge]

You guys are arguing with your instincts, not your intellects.

An unanswered question is a logical argument?

 

You guys are arguing with your instincts, not your intellects.

 

In ordinary English we can identify non material nouns ie separate out some quality from an object eg roundness or hardness.

 

In science or maths we can be more specific about these and even develop ways of quantifying these non material nouns.

 

Reality includes both material and non material nouns. Neither type is more real than the other.

 

In fact as soon as you have a single material noun, that you can declare to be 'real', I can find a non material property it possesses which must therefore be as real as the object itself.

But, how do you measure something that's "immaterial"? And since can't measure it, then how do you know it exists?

 

I can find a non material property it possesses which must therefore be as real as the object itself.

Physical and immaterial are antonyms, something cannot be physically immaterial just as a red object can't be blue and going left isn't the same as going right. The fact that you can say something is immaterial proves there is a difference between something physical and something that is immaterial.

Edited May 10, 2014 by SamBridge

[studiot]

Sam, I can't make out if you are agree or disagreeing with me?

 

 

But, how do you measure something that's "immaterial"? And since can't measure it, then how do you know it exists?

 

I can tell by looking at a stone or star shape if it is round or pointed. I can say an apple is round, but a banana is stick shaped.

I said that science devises methods of measuring the roundness or angularity or other shapes, where the assessment is more vague in common English.

 

Furthermore I said that If the object exists, its shape exists.

 

I can prove that by tracing round the outside and removing the object from the universe (ie destroying it)

But I still have the shape.

 

Again maths provides us with ways of measuring. It is not usual to describe a circle by the emthod of envelopes, but it can be done.

Such dual methods provides useful alternatives in some situations.