[Azhar (OP)]

Hello,

I heard recently that spacetime is "granular".  Leaving time aside for a minute, what exactly is "space" made of? What does it mean when we say "space" anyway? Is space formed by all the matter/non-matter particles and "sum or collection" of those is space?  How is it granular?

Thanks in advance!

[evan_au]

spacetime is "granular"

By "granular", I assume you mean "quantised", and can only take on certain specific values?

Many quantities in subatomic physics are quantised, such as electric charge, the energy of electron shells in an atom, photons (particles of light), and the energy of photons emitted by an atom. The discovery that these quantities can only take on certain values allowed scientists to avoid some nonsensical infinities in their theories.

In 1899, Max Planck proposed some units that made many quantities in physics come out as "nice" numbers. In some cases, these represented quantities where it is thought that the laws of physics might break down (eg the Planck unit of time). In other cases (the Planck mass), scientists often deal with smaller quantities.
See: https://en.wikipedia.org/wiki/Planck_units

More recently, scientists have produced mathematical theories that suggest that spacetime might be quantised. This may allow scientists to avoid some of the stubborn infinities associated with black holes. But at this time, there is no solid evidence to support these theories.
See: https://en.wikipedia.org/wiki/Quantum_spacetime

[Bill S]

Hi Azhar,

Hopefully, the experts will give you a better answer than I can, but, as I understand it, there is often overlap; even confusion; between the terms space, the vacuum and nothing.

My (simple) understanding is that space is the nearest we can come to a complete vacuum, having just a few hydrogen atoms pre m³ .

Vacuum is generally used in its QM interpretation.  That is the lowest possible energy state.  As such, it is seen as a roiling “mass” of virtual particles/energy, that can never be zero as the rules of uncertainty must be observed.

Nothing: that’s a can of worms that might best be left unopened here.

[jeffreyH]

While energy may be quantised, time and space are both continuous in nature. The nature of indeterminacy means this has to be the case. Since you cannot make an absolutely accurate measurement of a space or time interval. It is not quantised.

[Bill S]

 

time and space are both continuous in nature.

I thought this was still open to debate.   As far as I am aware, the general feeling is that they must both be either quantized or continuous, but there is no firm evidence either way.

Am I way out of date?

[opportunity]

Some of my replies may seem outside the square, and I'm hoping this is no different:

What's a point made of, or even a line?

(Hopefully someone gets this)

Successfully defining the lines and points as topographical concepts of time and space
 roughly gives a good answer.

[Colin2B]

As far as I am aware, the general feeling is that they must both be either quantized or continuous, but there is no firm evidence either way.

You are right, at the moment no one knows, but it doesnt have to be either or, remember the energy of a free electron is not quantised.
We know that the classical theory of general relativity breaks down at very high-energies and very tiny scales, where quantum effects become significant, but whether quantisation would solve the problem is unknown.

While energy may be quantised, time and space are both continuous in nature. The nature of indeterminacy means this has to be the case. Since you cannot make an absolutely accurate measurement of a space or time interval. It is not quantised.

Jeff makes a good point however.

[opportunity]

The fundamentals of science, how we judge events we perceive, require a tapestry. For millennia (perhaps maybe more correctly, centuries, thinking here of the Cartesian coordinate system, brilliance), the idea of 3-d space and 1-d time has sufficed. Upon that tapestry we develop with the aide of mathematics topographical models of what we are trying to uncover, explain, theorise, and thus ideally predict.

3-d space and 1-d time though in the past century has run into problems; it's as like the ideas we explain are more complex than the tapestry we use to write these ideas upon.

Simple question: "Why not make the tapestry of space and time more complex as an a-priori, and here "why not time"....is there a mathematical a-priori between space and time that we're missing, haven't considered, an algorithm the defines the existence of the play on its tapestry?"

Indeed we can strategize new dimensions of 3-d space, folded space, quasi-crystal space, and so on, yet no one touches on time.

Yet there's another element to this, what if there is a natural dynamic, a dynamic that makes the natural world evident, "purely based on" the code of space interacting with time?

To thus say what spacetime is made of, we can only answer in the affirmative for the current a-priori of time and space, "while also" thinking that there is an ultimate "level, which we have labelled the Planck scale, based on relativistic equations. "In that event though", we are still using 3-d space and 1-d time as per relativistic theory. To then say, "space and time", spacetime, is made of features outlined in the idea of gravity being a curvature of spacetime, "is that actually correct"? Is that what spacetime is "made of"? It's like saying those features are more prescient than space and time itself.

I hope this didn't read as a poor reprint of abstract art, "yet" getting to the bottom of what time and space "is", well it "would" represent "the key", "it would have to", of all events that occur on those manifolds of congress, not vice-versa.......right?

Maybe we should ask, "are the space and time manifolds currently in use "adequate""? Yeah, they work well; they've worked well for an overpopulated and naturally starved planet looking for new planets to colonise for a better chance of survival given all our achievements in science....yet, what if we're missing something more fundamental that can help use a best answer for, "what is spacetime made of"? Maybe the right answer can help us solve the greater problems we face as a species?

I have a feeling though that current contemporary theory is running towards the idea that there is a fundamental concept that "makes" space and time "happen", that demands spacetime exists. Can anyone shed light on that undercurrent? One simple idea is, "was the big bang merely a curvature of spacetime that unfolded, or more precisely, was the pre big bang a massive curvature as gravity that when exploded created the idea of space-time"?. The point I'm making here is that if you say spacetime is made of something, that something "makes" spacetime. Is that correct though? If something existed pre big bang, something that makes spacetime, can it sill exist if it was a pre big-bang concept?

[geordief]

Well these are interesting questions (and it is helpful not to suffer misconceptions vis a vis what has already been established as a working model) but asking these questions is  akin to asking "what will we have for supper" when there is no food in the house.

Observation can be expected to precede (useful ) theory  and these observations are (hopefully) a work in progress </statement of the bleeding obvious  >

[opportunity]

Well these are interesting questions (and it is helpful not to suffer misconceptions vis a vis what has already been established as a working model) but asking these questions is  akin to asking "what will we have for supper" when there is no food in the house.

Observation can be expected to precede (useful ) theory  and these observations are (hopefully) a work in progress </statement of the bleeding obvious  >

You're absolutely correct, and insightful. In a developed understanding beyond what we have though, as scientific norms, the future "has to be" brighter. Although that may sound like flim-flam, every new discovery was born with the idea of something better. To propose the implications of what I suggested above minus the criticism is sad, as it basically suggests we're stonewalled "as that theory" that no one is willing to challenge "fundamentally so".

As for supper, I know you're point. Think of it this way........you've been flying planes for years.......then you find rockets. Physicists will always apply their quest for the appreciation and utilisation of space and time despite advancements more than those who would rather do something else. Any next step is not easy, trust me. It "never has been"....it's "always" required developing minds from previous levels of aptitude. To say that there will be no jobs in physics anymore with a new a-priori standard for space and time is like saying men would no longer be interested "a new quest.....undiscovered, built on all that physics so far knows".

Bottom line, there's more wealth in physics with a new fundamental theorem and associated tech discovery than continuing to hit the current status quo.

If physics finds the answer, understands the answer first, you know, properly answering questions like "does a field-effect have a source"...?....it's up to physics to define that......why be worried that gen-x's or y's will just float the boat? They will, of course they will. But that's cut and paste for them if you think they're just cutting in. The granularity of understanding is historical.......and there lies purpose. No one can cut and paste that....working out the purpose.

You know, I'm happy to have a few things explained, but to put up with rubbish is something else. I think its rubbish, for instance, to submit to ideas held in folklore by giants, not people open to criticism. What giant in science has said for instance, recently, that the tapestry of 3-d space and 1-d time is their tapestry for their theory? What is spacetime made of? They're using a tapestry to put things there to say that presumption is over-ceded by new findings?

Physics is the greatest quest. It unlocks how everything works. Don't get me wrong. But "we have to" ask ourselves if there's more to the a-priori of space and time than reverse engineering what we already know as an "application to" space and time as an answer to what spacetime is made of, for instance.....we should get real and ready for that answer. This is not worrying about pay checks, this is standing on the shoulders of giants.

[oilinki]

I would think the spacetime, as far as we know it, is made of force fields. Similar to the old idea of Ether.

We have gravity fields, which span across the Universe. It affects the everywhere, while it's strongest close to the masses.

We likely have other force fields, which we simply have not discovered yet.

Some effects of these force fields can be measured, like the speed of light through the 'empty' space. Likewise how the mass of black hole can bend the trajectory of light.

Just thoughts.

[Bill S]

Since you cannot make an absolutely accurate measurement of a space or time interval. It is not quantised.

I can see that being unable to make an absolutely accurate measurement would prevent you from knowing if it is quantised, but would that necessarily mean that it was not?

[jeffreyH]

Well Planck's constant has a well defined value as a quantum mechanical action. If either space or time were quantised it would have to be in Planck units. We have the Planck length and Planck time that could fit the bill. Does it make sense to consider motion to be defined in multiples of these units? Well the speed of light is defined as 1 Planck length over 1 Planck time. So your quanta has to be less than that. Otherwise everything has to move at the speed of light. Therefore the quanta, if they exist, are impossible to measure.

[Bill S]

Therefore the quanta, if they exist, are impossible to measure.

It was the "if they exist" that I was wondering about.  I have no problem with "impossible to measure"; therefore' in the present state of our knowledge, they might as well not exist. 

Does that qualify as keeping an open mind to the point where one's brain falls out?

[jeffreyH]

Therefore the quanta, if they exist, are impossible to measure.

It was the "if they exist" that I was wondering about.  I have no problem with "impossible to measure"; therefore' in the present state of our knowledge, they might as well not exist. 

Does that qualify as keeping an open mind to the point where one's brain falls out?

You cannot verify a negative assertion. God does not exist, space and time are not quantised.

[Bill S]

You cannot verify a negative assertion. God does not exist...

Whilst I might agree with that, I know from discussions elsewhere that Alan doesn't, but let's not get into the agnostic debate, 

I'm happy to say: Don't know, but some more information would be good.

[Bill S]

If time were quantised, it might seem reasonable to consider an “instant” as a quantum of time, (assuming one could define an instant).

Lynds  (Time and Classical and Quantum Mechanics: Indeterminacy vs. Discontinuity. Obtainable from the CERN document server) makes a sound argument for there being no static instant of time, during which change could be considered as having paused.  He reasons that if there were such an instant, then all change, all motion, would come to a permanent standstill at that point.                       

[jeffreyH]

I would agree with Lynds.

[Bill S]

You mentioned Plank time. Considering the implications of that raises some interesting thoughts.

For example, if this is defined as the smallest interval of time that has any meaning, beyond which it is not possible to further sub-divide time, then, surely, we must regard this as the quantum of time.  However, even 10^-43 s has a duration, at least, in principle.  That being the case, it must be possible to attribute a degree of change, during that period, to any object in relative motion; otherwise that object would be stationary. 

Defining an object as being, at once, moving and stationary does seem somewhat contradictory. 

[jeffreyH]

You mentioned Plank time. Considering the implications of that raises some interesting thoughts.

For example, if this is defined as the smallest interval of time that has any meaning, beyond which it is not possible to further sub-divide time, then, surely, we must regard this as the quantum of time.  However, even 10^-43 s has a duration, at least, in principle.  That being the case, it must be possible to attribute a degree of change, during that period, to any object in relative motion; otherwise that object would be stationary. 

Defining an object as being, at once, moving and stationary does seem somewhat contradictory. 

I have pondered upon that one quite a bit myself. We can follow this with the idea of quantum jumps.