3 hours ago, exchemist said:

But for relativity you need the speed of light, which has a particular physical value, which in turn implies particular physical values for permittivity and permeability of space. Though those do not matter for relativity calculations of course, only c itself. But the very fact that c is a particular, finite number means space has physical properties.

This being a phil thread, one could ask if space is anything more than a bundle of properties around matter. Without matter/energy, and translation or rotation, where would this bundle go? -- volume, permeability, permittivity... would be meaningless.

16 minutes ago, TheVat said:

This being a phil thread, one could ask if space is anything more than a bundle of properties around matter. Without matter/energy, and translation or rotation, where would this bundle go? -- volume, permeability, permittivity... would be meaningless.

But is not true of any material entity as well? Are not all their properties aspects of them that are manifest only in an interaction with something else?

(Just realised I may be starting to channel Rovelli here😄)

Edited August 29Aug 29 by exchemist

2 minutes ago, exchemist said:

But is not true of any material entity as well? Are not all their properties aspects of them that are manifest only in an interaction with something else?

Fair point. I figured that might be mentioned. For some reason, space seems more that way, perhaps because I am unclear as to what properties of space are intrinsic to it. An electron has a particular charge even in an otherwise empty universe, or does it? (so ridiculously hypothetical I know, but hey it's the phil sandbox haha) So would one say that space has a particular property in an empty universe? Would vacuum energy still mean anything? I admit I'm floundering a bit, so any flotation device welcomed.

13 hours ago, TheVat said:

I am unclear as to what properties of space are intrinsic to it

Of the top of my head, so probably not an exhaustive list, and assuming you mean actual physical space:

1. Topology, ie properties such as dimensionality, connectedness, orientability etc

2. Geometry. Pick any set of points in your space, and there will be well-defined notions of distances, angles, volumes etc. IOW, a metric. Note that the absence of gravitational sources does not necessarily imply that this geometry is trivial.

3. Permittivity and permeability, ie an ability to support EM fields, as well as a particular value of c.

13 hours ago, TheVat said:

Would vacuum energy still mean anything?

Yes. In an empty universe all quantum fields are in their ground (vacuum) state, and that state may be associated with non-zero vacuum energy.

Could one conceive of a universe that does not contain any quantum fields? I don’t know. Possibly.

16 hours ago, TheVat said:

Fair point. I figured that might be mentioned. For some reason, space seems more that way, perhaps because I am unclear as to what properties of space are intrinsic to it. An electron has a particular charge even in an otherwise empty universe, or does it? (so ridiculously hypothetical I know, but hey it's the phil sandbox haha) So would one say that space has a particular property in an empty universe? Would vacuum energy still mean anything? I admit I'm floundering a bit, so any flotation device welcomed.

I was, I now realise, playing with Rovelli's idea that QM entities only become "real" in the course of an interaction. In between, who knows? We say an electron has a charge, mass, spin etc. because when it interacts, that's how it behaves. But how can we know if it has a continuous existence in between? There's a wave function that describes how it will interact, but that's just maths.

4 hours ago, Markus Hanke said:

associated with non-zero vacuum energy.

This is the predicted number that is out by many orders of magnitude?

I have read this in popsci, not the best source granted.

Where is it written that the ground state must necessarily be the vacuum state ?

1 hour ago, studiot said:

Where is it written that the ground state must necessarily be the vacuum state ?

All systems tend to their ground state, and all 'excited' systems tend to decay.
Least action principle.

2 hours ago, pinball1970 said:

This is the predicted number that is out by many orders of magnitude?
I have read this in popsci, not the best source granted.

Try this video by an actual Physicist

9 minutes ago, MigL said:

All systems tend to their ground state, and all 'excited' systems tend to decay.
Least action principle.

Is that why they got the first quantum theory wrong ?

Nor is that comment (though true) any reson why the ground state has to be in a vacuum.

11 minutes ago, studiot said:

Is that why they got the first quantum theory wrong ?

Nor is that comment (though true) any reson why the ground state has to be in a vacuum.

Is that not just a definition for a "vacuum"? (a vacuum as otherwise defined** may not exist by its own definition)

**"absence of anything"

28 minutes ago, studiot said:

Is that why they got the first quantum theory wrong ?

Are you suggesting the vacuum has quantized states ?
Please elaborate.

29 minutes ago, studiot said:

Nor is that comment (though true) any reson why the ground state has to be in a vacuum.

No reason the ground state has to be in a vacuum, but an appropriate reason why the vacuum will tend to a ground state.

13 minutes ago, MigL said:

Are you suggesting the vacuum has quantized states ?
Please elaborate.

I honestly don't know the answer to that.

I was alluding to what is known as the old quantum theory, before the 1/2 was added.

This was developed from the states of a classic harmonic oscillator (Morse curves I seem to remember)

A classic oscillator not only has harmonics it has sub harmonics and a fundamental, which corresponds to the ground state.

As far as I know there is no suggestion in QM of equivalent sub levels.

17 minutes ago, MigL said:

No reason the ground state has to be in a vacuum, but an appropriate reason why the vacuum will tend to a ground state.

Whilst it is possible to have a conduction current by electrons in a vacuum,

again as far as I know, holes can't even exist in a vacuum, let alone conduct.

36 minutes ago, geordief said:

Is that not just a definition for a "vacuum"? (a vacuum as otherwise defined** may not exist by its own definition)

**"absence of anything"

The trouble is that the earlier idea of a vacuum as the absence of anything now seems to be rather oversimplistic.

We should perhaps be more specific than anything.

13 minutes ago, studiot said:

The trouble is that the earlier idea of a vacuum as the absence of anything now seems to be rather oversimplistic.

We should perhaps be more specific than anything.

I wonder do any physicists use the term "vacuum" in that "old" sense nowadays?

It is used ,but as you say I would like to know what it is meant to mean specifically. (Or whether it is perhaps just a portemanteau for something we don't /can't understand whereas we do seemingly have models for things pertaining to the old idea)

When you suck as much the energy out of a system as possible do you approach one universal configuration or do more than one such configurations open up?

Edited August 30Aug 30 by geordief

1 hour ago, geordief said:

I wonder do any physicists use the term "vacuum" in that "old" sense nowadays?

It is used ,but as you say I would like to know what it is meant to mean specifically. (Or whether it is perhaps just a portemanteau for something we don't /can't understand whereas we do seemingly have models for things pertaining to the old idea)

When you suck as much the energy out of a system as possible do you approach one universal configuration or do more than one such configurations open up?

Good question. +1

Here is a rough explanation of the difference between a classical oscillator and a quantum oscillator.

Consider first a classical pendulum hung up on the wall, perhaps in a clock.

As it swings to and fro it has oscillatory energy.

If it is not swinging but hanging stationary , it has exactly zero oscillatory energy.

Of course it also has potential energy by virtue of its position in a gravitational field, but we ignore that.

There are lots of other things about it we could analyse, but that is the nub of it.

Just by being there it has no oscillatory energy and could hang there indefinitely where it naturally comes to rest at the lowest point of its swing or its ground state.

Now consider an electron in an atom.

Whatever model you choose the electron has to have oscillatory energy just to remain in the atom.

This is true for particle in a box wave functions or orbiting electron particles.

Beyond that both models allow for a series of increasing oscillatory energy levels.

But it must have some eneregy to be in the lowest one, unlike the pendulum.

This is called zero point energy.

Outside the nucleus, the 'atom' is basically empty ( of material objects) space but with an electric field in it.

The pendulum could equally well be in a vacuum or air.

Does this help ?

1 hour ago, geordief said:

I wonder do any physicists use the term "vacuum" in that "old" sense nowadays?

Current understanding, as per QFT, is that some fields permeate all space, even in the absence of the particles, charges, or energy-momentum, that were once thought to give rise to these fields.
Even in totally empty space, these fields endow the vacuum with certain properties and an energy density.
The energy of these fields gives rise to two conditions termed off-shell ( virtual particles with less than a quantum of action, that do not satisfy the energy-momentum relation ) and on-shell ( real particles with more than a quantum of action , that adhere to the energy-momentum relation ). The vacuum on either side of the Event Horizon of a Black Hole has some peculiar properties as well; it can turn off-shell particles into real ( Hawking Radiation ) by 'borrowing' mass-energy from the BH just outside the Event Horizon, and, turn off-shell particles into 'exotic matter', with negative energy, just inside the Event Horizon.
The Higgs field has also added another scalar component to previous vector and tensor fields, again modifying our understanding of the properties of the vacuum. And these are just some of the properties I can think of, off the top of my head; there may be more.

So, you can see the current understanding of the vacuum, what used to be simple empty space, has gotten fairly complex, and some, or all, of these properties need to be considered, depending on what you are attempting to model, or predict

1 hour ago, MigL said:

Even in totally empty space, these fields endow the vacuum with certain properties and an energy density.

Three of you have asserted this here (and thanks all for replies), but my earlier question is still broadly: how would we empirically verify the nature of truly empty space. Even the voids between galaxy clusters are still within a particle horizon of something material and so can be affected. Can we assert that a void such that all matter is beyond a particle horizon, where nothing is causally connected to that void, must still have topology and fields and vacuum energy and permittivity and so on? It sounds like I'm going off on a Rovelli limb and a bit beyond, asking if even topology or geometry can be real where there is nothing to interact or have a metric applied.

17 hours ago, studiot said:

Where is it written that the ground state must necessarily be the vacuum state ?

You’re right, the two are not necessarily the same - you can eg have “false vacuums”.

9 hours ago, TheVat said:

Can we assert that a void such that all matter is beyond a particle horizon, where nothing is causally connected to that void, must still have topology and fields and vacuum energy and permittivity and so on?

So long as you assume that the laws of nature also apply to a “perfect void” of the type you describe, then yes, the void must have these properties.

The thing about fields is that they need a source and / or a sink or the go on to infinity.

So if the soruce / sink is within you 'empty space' or vacuum is that space still empty ?

But if it is not, then it must be external or on the boundary yet infinity has no boundary.

5 hours ago, studiot said:

The thing about fields is that they need a source and / or a sink or the go on to infinity.

So if the soruce / sink is within you 'empty space' or vacuum is that space still empty ?

But if it is not, then it must be external or on the boundary yet infinity has no boundary.

Still working through your (and @MigL 's ) earlier replies but I just wonder if the "universe creating space as it expands" concept covers this?

5 minutes ago, geordief said:

Still working through your (and @MigL 's ) earlier replies but I just wonder if the "universe creating space as it expands" concept covers this?

How does that provide a source / sink ?

If you think about field lines the condition I described is equivalent to saying the are no 'loose ends' to any field lines.

This is different from streamlines in a fluid which can have loose ends, where the flowing fluid comes to a complete halt and a bifurcation occurs.

18 minutes ago, studiot said:

How does that provide a source / sink ?

If you think about field lines the condition I described is equivalent to saying the are no 'loose ends' to any field lines.

This is different from streamlines in a fluid which can have loose ends, where the flowing fluid comes to a complete halt and a bifurcation occurs.

I don't know.

On 8/30/2025 at 6:59 AM, studiot said:

Where is it written that the ground state must necessarily be the vacuum state ?

Oh, now I understand what you were getting at ( Markus worded it more clearly 🙂 ).
False vacuum states are necessarily unstable, and tend to decay, eventually reaching the ground state.

7 hours ago, studiot said:

The thing about fields is that they need a source and / or a sink or the go on to infinity.

Would it not be these fields that go on to infinity ( Space-Time Geometry or Gravitational field, Higgs Field, and possibly EM Field or Photon field ) ) that provide the global energy density ?
The source/sink fields only provide a local energy density.

On 8/30/2025 at 1:39 PM, MigL said:

Try this video by an actual Physicist

I think I need to watch that again with a note book this time.

21 hours ago, MigL said:

Would it not be these fields that go on to infinity ( Space-Time Geometry or Gravitational field, Higgs Field, and possibly EM Field or Photon field ) ) that provide the global energy density ?
The source/sink fields only provide a local energy density.

No one knows how the Higgs field works or has actually detected it, only that if there is a field with certain properties, there is a recognisable mechanism to acount for mass ie the massive higgs particle.

I think we also have to be more careful with energy as opposed to force.

Most fields in question are fields of force and no energy needs be extended to maintain or exert said force(s).

Lines of fluid flow that end at stagnation points have a residual stagnation pressure, unlike lines of electric force that end on a charge.

2 hours ago, pinball1970 said:

I think I need to watch that again with a note book this time.

This guy's channel ( Physics Explained - YouTube ) has some very interesting videos.
He got his PhD at Queen Mary in London, and lectured at Liverpool, before recently quitting to pursue YT interests.

Interestingly ( @studiot ), one of his videos relates to the empty space, or 'nothingness', within the atom, and what the fields of QFT tell us about it.