I understand we have the concept of a universal heat death where (I think) all that remains is random interactions between objects (ie particles?) that never lead to anything of more consequence.

Could those random interactions lessen in frequency over time so that eventually there are none and we can say nothing is moving ?(if there are no interactions how can an object "move" just with respect to itself?)

Does an end of absolutely everything become possible in those circumstances?

If it does ,would/could that rule out any restart?

BTW the forum seems to be getting very ,very slow also- ironically.

“There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.

There is another theory which states that this has already happened.”

― Douglas Adams, The Restaurant at the End of the Universe

Assuming absolute zero means no movement, would that mean electrons stop orbiting the protons?

1 hour ago, geordief said:

Could those random interactions lessen in frequency over time so that eventually there are none and we can say nothing is moving ?(if there are no interactions how can an object "move" just with respect to itself?)

Motion is always with respect to something else; it doesn’t require an interaction. Having no motion isn’t possible, since absolute zero isn’t attainable

57 minutes ago, dimreepr said:

Assuming absolute zero means no movement, would that mean electrons stop orbiting the protons?

Electrons “in orbit” aren’t moving; they have no trajectory. This is the domain of quantum mechanics. Electrons are not in classical, planetary orbits

1 hour ago, dimreepr said:

“There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.

There is another theory which states that this has already happened.”

― Douglas Adams, The Restaurant at the End of the Universe

Assuming absolute zero means no movement, would that mean electrons stop orbiting the protons?

At absolute zero there is still energy in the ground states of many quantum mechanical systems. Some of this energy is kinetic energy, so one could say it is associated with “motion”. However, as @swansont points out, you can’t define a path along which these QM entities move, so it is not motion in the classical sense.

This residual energy of the ground state does not contribute to temperature, which is why it is called “zero point energy”.

12 minutes ago, swansont said:

Electrons “in orbit” aren’t moving; they have no trajectory. This is the domain of quantum mechanics. Electrons are not in classical, planetary orbits

1 minute ago, exchemist said:

At absolute zero there is still energy in the ground states of many quantum mechanical systems. Some of this energy is kinetic energy, so one could say it is associated with “motion”. However, as @swansont points out, you can’t define a path along which these QM entities move, so it is not motion in the classical sense.

This residual energy of the ground state does not contribute to temperature, which is why it is called “zero point energy”.

That's what I thought, just not why I thought it...

I was thinking of the beginning, and where the energy came from, for a quantum fluctuation to be significant...

24 minutes ago, swansont said:

Motion is always with respect to something else; it doesn’t require an interaction. Having no motion isn’t possible, since absolute zero isn’t attainable

If absolutely nothing in the entire universe interacts with anything else ,is it not possible to view that as all particles effectively ceasing to exist (having no motion being incidental at that point)?

Is the unattainability of absolute zero contingent upon there being an overall system within which sub systems exist?

None of the subsystems could individually reach absolute zero but if the overall system itself reaches absolute zero (I don't know how) then all the subsystems would likewise reach that state.

Is the temperature of a system something we model anyway and ,if the universe did come to a complete end would the model break down at or before that point?

Is the OP "Can the universe die?" a legitimate question or is such an end nonsensical (the same applying to any beginning?

Personally I am more comfortable with no beginning ,no end but continuous change.

34 minutes ago, exchemist said:

At absolute zero there is still energy in the ground states of many quantum mechanical systems. Some of this energy is kinetic energy, so one could say it is associated with “motion”. However, as @swansont points out, you can’t define a path along which these QM entities move, so it is not motion in the classical sense.

This residual energy of the ground state does not contribute to temperature, which is why it is called “zero point energy”.

Does that apply to an ultimate heat death scenario? Is that scenario so theoretical that we can say little about it? (I think Penrose has a theory about it though...)

20 minutes ago, geordief said:

If absolutely nothing in the entire universe interacts with anything else ,is it not possible to view that as all particles effectively ceasing to exist (having no motion being incidental at that point)?

Is the unattainability of absolute zero contingent upon there being an overall system within which sub systems exist?

None of the subsystems could individually reach absolute zero but if the overall system itself reaches absolute zero (I don't know how) then all the subsystems would likewise reach that state.

Is the temperature of a system something we model anyway and ,if the universe did come to a complete end would the model break down at or before that point?

Is the OP "Can the universe die?" a legitimate question or is such an end nonsensical (the same applying to any beginning?

Personally I am more comfortable with no beginning ,no end but continuous change.

Does that apply to an ultimate heat death scenario? Is that scenario so theoretical that we can say little about it? (I think Penrose has a theory about it though...)

I can't immediately see why quantum mechanics would cease to apply in a heat death scenario.

21 minutes ago, geordief said:

Does that apply to an ultimate heat death scenario? Is that scenario so theoretical that we can say little about it? (I think Penrose has a theory about it though...)

Doesn't that mean a theory is less than?

This is more clearly understood from an entropic consideration.
When the whole universe achieves maximum entropy, there is no more useable energy to do any work, however there is still ground state, or xero point, energy, as that is a property pf quantum systems, and this implies that the universe keeps expanding, making interactions ( of any kind ) more and more rare.
One of the last interactions that may supply useable energy is proton decay, but that is hypothetical in some theories, and protons are expected to have lifetimes about 25 orders of magnitude greater than the current age of the universe.

1 hour ago, swansont said:

Electrons “in orbit” aren’t moving; they have no trajectory.

Does that mean you don't subscribe to the commonly stated view that atoms are mostly empty space?

15 minutes ago, geordief said:

Is the temperature of a system something we model anyway and ,if the universe did come to a complete end would the model break down at or before that point?

I suspect you are confusing local and non-local effects. In any freely expanding system, there's a tendency for any group of like particles in a local comoving space to approach parallel-ish trajectories with zero relative velocity (all others will have migrated into adjacent spaces with or without the assistance of a declining frequency of collisions). From an arbitrary frame of reference, they may still be moving fast as billy-o, only not with respect to their immediate neighbours. So the local thermodynamic temperature asymptotically approaches absolute zero.

What happens then depends I guess on exactly how flat the universe is at the largest scale. If the universe is closed and finite, they may ultimately run head first into something coming fast in the opposite direction. That would warm them back up a bit.

16 minutes ago, exchemist said:

I can't immediately see why quantum mechanics would cease to apply in a heat death scenario.

Aren't all models liable to break down in extreme situations?

If a heat death scenario lasts for ,say a million or more times the length of the universe to date(just picking a figure out of the air) might that approach a circumstance we can't describe with a model that works in current conditions?

I have no idea what in particular might cause the model to break down but it might not be a surprise if it did,would it?

49 minutes ago, KJW said:

Does that mean you don't subscribe to the commonly stated view that atoms are mostly empty space?

No. That’s a reasonably accurate pop-sci description; the electron is nominally a point particle and if you detect it, it is localized. But as you know, the actual science is more nuanced than popular, qualitative descriptions.

28 minutes ago, geordief said:

Aren't all models liable to break down in extreme situations?

If a heat death scenario lasts for ,say a million or more times the length of the universe to date(just picking a figure out of the air) might that approach a circumstance we can't describe with a model that works in current conditions?

I have no idea what in particular might cause the model to break down but it might not be a surprise if it did,would it?

But there is nothing extreme about heat death. It's just about temperature evening out. So mild conditions, well within the scope of current theory. In fact, it is their very mildness that removes all capacity for dynamic change.

Edited January 10Jan 10 by exchemist

2 hours ago, exchemist said:

At absolute zero there is still energy in the ground states of many quantum mechanical systems. Some of this energy is kinetic energy, so one could say it is associated with “motion”. However, as @swansont points out, you can’t define a path along which these QM entities move, so it is not motion in the classical sense.

And temperature in kinetic theory is the center-of-mass kinetic energy. When QM is incorporated it shows up as a fraction of atoms in excited states, so at absolute zero everything is in the ground state

2 hours ago, dimreepr said:

I was thinking of the beginning, and where the energy came from, for a quantum fluctuation to be significant...

AFAIK, it’s not clear that the energy isn’t zero. Gravitational potential energy is negative.

1 hour ago, geordief said:

If absolutely nothing in the entire universe interacts with anything else ,is it not possible to view that as all particles effectively ceasing to exist (having no motion being incidental at that point)?

Some interactions have infinite range, so I don’t see how you get there. The interactions may be quite small, but electromagnetic and gravitational effects don’t actually cease.

1 hour ago, geordief said:

Is the unattainability of absolute zero contingent upon there being an overall system within which sub systems exist?

None of the subsystems could individually reach absolute zero but if the overall system itself reaches absolute zero (I don't know how) then all the subsystems would likewise reach that state.

How would an “overall” system get to zero but a subsystem doesn’t? Temperature in any equilibrium state is positive.

1 hour ago, geordief said:

Is the temperature of a system something we model anyway and ,if the universe did come to a complete end would the model break down at or before that point?

I don’t think we have any evidence that models would fail

1 hour ago, geordief said:

Is the OP "Can the universe die?" a legitimate question or is such an end nonsensical (the same applying to any beginning?

It’s not nonsensical to ask - that leads to discussion of what’s happening. (it would be like asking if lacking knowledge/not understanding something is nonsensical; of course not. Everybody has things they don’t know or understand)

It’s asserting things that can be.

55 minutes ago, swansont said:

Some interactions have infinite range, so I don’t see how you get there. The interactions may be quite small, but electromagnetic and gravitational effects don’t actually cease.

I hadn't considered that

43 minutes ago, swansont said:

Is the temperature of a system something we model anyway and ,if the universe did come to a complete end would the model break down at or before that point?

I don’t think we have any evidence that models would fail

That (lack of evidence)wouldn't prevent them from failing,would it?

It is an argument from incredulity but I would be extremely impressed if the models (note I don"t claim to be at all versed in those models -as I am sure you already know) continued working indefinitely

@exchemist points out that we are not talking about extreme conditions but I am not so sure.They seem extremely different to what would have preceeded but I am in no position to argue that (semantic?) point.

59 minutes ago, swansont said:

How would an “overall” system get to zero but a subsystem doesn’t?

I think I might have be saying or implying more or less the same.

Edited January 10Jan 10 by geordief

Keep in mind heat death is only one possibility. One that relies on the cosmological constant remaining constant.

Its still viable at some point that this may no longer hold true and the universe could start to collapse.

The key equation being the critical density relation which was originally used to determine the inflection point from and expanding universe to a collapsing universe.

Aka cyclic bounce models

Using Planck 2018+BAO dataset values roughly 45 B years into the future the Hubble constant will hit 55.7 km/Mpc/sec. It will remain roughly this value up to universe age 93 B years old. Thats as far as the cosmological calc in my signature goes. At that time the CMB balckbody temp will be roughly 0.0273 Kelvin.

It will never hit absolute zero but that temp is still too warm for Bose-Einstein and Fermi-Dirac condensates so you will still have particles not in thermal equilibrium as per the standard model today.

That of course is under the assumption the cosmological constant remains constant.

Edited January 12Jan 12 by Mordred

29 minutes ago, Mordred said:

Aka cyclic bounce models

And how does entropy reset itself when everything collapses to the same temperature ?

Will it need to have a quantum fluctuation restart the cycle ?

28 minutes ago, Mordred said:

Keep in mind heat death is only one possibility. One that relies on the cosmological constant remaining constant.

Its still viable at some point that this may no longer hold true and the universe could start to collapse.

The key equation being the critical density relation which was originally used to determine the inflection point from and expanding universe to a collapsing universe.

Aka cyclic bounce models

Using Planck 2018+BAO dataset values roughly 45 B years into the future the Hubble constant will hit 55.7 km/Mpc/sec. It will remain roughly this value up to universe age 93 B years old. Thats as far as the cosmological calc in my signature goes. At that time the CMB balckbody temp will be roughly 0.0273 Kelvin.

It will never hit absolute zero but that temp is still too warm for Bose-Einstein and Fermi-Dirac condensates so you will still have particles not in thermal equilibrium as per the standard model today.

That of course is under the assumption the cosmological constant remains constant.

Yes ,I understand that a heat death is just one possibility (very recent findings apparently even suggesting an eventual gravitational collapse may be on the cards again)

What is the reason that an absolute zero temperature cannot be reached ?(my OP was assuming that it would)

Is to do with the "ground state" energy that has been mentioned by other posters -or does the system not even reach that state where absolute zero still contains energy of some kind.

Or is there another reason that the system cannot attain absolute zero even with an infinite amount of time at its disposal?(could it be that ,time being relative that that is the wrong way to look at it?)

Assuming all particles reach thermal equilibrium the entropy can be safely described strictly via temp. As the mediator for temp is the photon entropy will end up being S=2 same as the entropy at 10^-43 seconds. However the problem at the low temperature end is that only massless particles travel at c and all massive particles will likely remain massive so wouldn't be in thermal equilibrium such as our universe beginning.

Too many variables with regards to how particles would remain coupled for the mass terms to give any good guess. Will the coupling constants operate the same is anyone's guess.

According to QM zero point energy you will always have quantum fluctuations hence absolute zero is impossible via current understanding of QM.

Then there is still BH evaporation times to consider lol which is far greater than the time frame I mentioned above for a one solar mass BH.

One could consider we understand electroweak symmetry breaking processes at the hot end better than we understand thermal equilibrium states on the cold end.

Edited January 12Jan 12 by Mordred

6 minutes ago, geordief said:

What is the reason that an absolute zero temperature cannot be reached ?

From a QM standpoint, if all motion ceases at absolute zero, quantum particles have no momentum and their position is fixed.
An impossibility according to Heisenberg.

In Quantum Field Theory, this video does a better job than I could of both, QFT fundamentals, and minimum allowed energy.
( and I like his videos )

Edited January 12Jan 12 by MigL

I Agree excellent video one of the better ones Ive seen. I love how he went from classical wave theory, included SR to QM and then QFT in a very well laid out format.

Lol literally covered several chapters of most textbooks in a short video.

One added detail however the Schrodinger equation isn't lorentz invariant it doesn't work well with SR however the Klein Gordon equation used by QFT is. It does so by factoring in the mentioned energy momentum relation into its equations (in essence employs the 4 momentum.)

Its an important distinction between QM and QFT. Some of you may have heard me mention the term canonical ( this is a quantized field theory) a conformal theory however isn't quantized. ( string theory as one example).

Just some side tid bits

Edited January 12Jan 12 by Mordred

11 hours ago, geordief said:

Yes ,I understand that a heat death is just one possibility (very recent findings apparently even suggesting an eventual gravitational collapse may be on the cards again)

What is the reason that an absolute zero temperature cannot be reached ?(my OP was assuming that it would)

Is to do with the "ground state" energy that has been mentioned by other posters -or does the system not even reach that state where absolute zero still contains energy of some kind.

Or is there another reason that the system cannot attain absolute zero even with an infinite amount of time at its disposal?(could it be that ,time being relative that that is the wrong way to look at it?)

The point is that the energy of the ground state (the zero point energy) cannot contribute to temperature, as there is no lower state and therefore this energy can never be extracted. It is the energy that still remains AT absolute zero. Absolute zero, remember, is just the lowest temperature you can get, i.e. there is no lower temperature.

Lowering the temperature of a system in which every degree of freedom was in the ground state would require extracting heat from it - which would mean getting it to a state lower in energy than the ground state - a contradiction in terms.

So the residual energy of the ground state has nothing to do with why you can't get to absolute zero. That's simply a matter of not being able, in practice, to extract 100% of the extractable energy (which is the energy of states above the ground state).

Edited January 12Jan 12 by exchemist

1 hour ago, exchemist said:

The point is that the energy of the ground state (the zero point energy) cannot contribute to temperature, as there is no lower state and therefore this energy can never be extracted. It is the energy that still remains AT absolute zero. Absolute zero, remember, is just the lowest temperature you can get, i.e. there is no lower temperature.

Lowering the temperature of a system in which every degree of freedom was in the ground state would require extracting heat from it - which would mean getting it to a state lower in energy than the ground state - a contradiction in terms.

So the residual energy of the ground state has nothing to do with why you can't get to absolute zero. That's simply a matter of not being able, in practice, to extract 100% of the extractable energy (which is the energy of states above the ground state).

Is that because ZPE is sub-quantum in value? Nothing in the sub-quantum domain can normally affect the quantum domain.

Edited January 12Jan 12 by StringJunky

10 hours ago, Mordred said:

however the Klein Gordon equation used by QFT

I thought Klein-Gordon was virtually un-useable, and the Dirac Equation replaced it.
See here ...
( since you like his videos also )