Measurements on the entire universe

  • #1
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I can not claim that I fully understand my own question, but I feel it is worth sharing.
Suppose there is a physical system composed of MxN degree of freedom where M is the number of the states and N the number of particles. When a measurement is undertaken, the physical system collapses to well defined state that represents the eigen value of the physical observable. The result is an inevitable increase in the total entropy of the (system+the measuring machine+the universe) in the form of heat. What if MxN is the physical universe itself, where does the heat go?
 

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  • #2
phinds
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Since measurements on "the entire universe" are not possible I don't see how your question makes any sense. Measurements are local.
 
  • #3
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Since measurements on "the entire universe" are not possible I don't see how your question makes any sense. Measurements are local.
Sorry if I still do not see why it is not possible. If the physical reality can not be measured, then it is not a physical reality. Similarly, if the entire universe can not be measured then it is not physical.
 
  • #4
phinds
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Sorry if I still do not see why it is not possible. If the physical reality can not be measured, then it is not a physical reality. Similarly, if the entire universe can not be measured then it is not physical.
Since the vast majority of the universe is not in causal contact with our part, just how would you propose to measure it?
 
  • #5
Drakkith
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What does it mean to "measure physical reality"? We can perform measurement on particles and systems of particles to determine what states they are in. Talk of "reality" is fraught with problems as nowhere in science do you find a non-ambiguous, rigorous definition of reality. That tends to fall under metaphysics and perhaps other areas of philosophy.
 
  • #6
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Since the vast majority of the universe is not in causal contact with our part, just how would you propose to measure it?
We wait until every single light photon reaches us. Or to have so many observers everywhere.
 
  • #7
Drakkith
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We wait until every single light photon reaches us.
They won't. Because of the expansion of space, photons emitted beyond a certain distance will never reach us. Ever.
 
  • #8
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They won't. Because of the expansion of space, photons emitted beyond a certain distance will never reach us. Ever.
So how did we come to know that it is expanding without observing the red shift from those photons?
 
  • #9
Drakkith
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So how did we come to know that it is expanding without observing the red shift from those photons?
We observe the redshift of the photons that weren't emitted past this "horizon". We can currently see about 45 billion light years in all directions, which represents a redshift from z=0 (for nearby objects) to z=1100 (the CMB).
 
  • #10
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We observe the redshift of the photons that weren't emitted past this "horizon". We can currently see about 45 billion light years in all directions, which represents a redshift from z=0 (for nearby objects) to z=1100 (the CMB).
This might drive me away from the question but I am just curious to know. If there is a horizon where no information about the universe could ever reach us, then how do we know that it exits beyond it at any point of time? And if my assumption about its definite states is correct, then how would that conceal with the fact that its full extent can never be observed beyond this horison?
 
  • #11
Drakkith
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If there is a horizon where no information about the universe could ever reach us, then how do we know that it exits beyond it at any point of time?
The problem is that there isn't a single horizon. You and I both have different horizons by virtue of being in different places. It may only be a few feet, a few miles, or a few thousand miles, but our horizons are different. This means that, in this context, it is impossible to have some sort of "hard" horizon beyond which there is nothing because every observer will have a different horizon. Any such hard horizon would require that cosmology be significantly different than how we currently model it.

And if my assumption about its definite states is correct, then how would that conceal with the fact that its full extent can never be observed beyond this horison?
I'm afraid I don't understand what you're asking. I'm not very knowledgeable about quantum physics and thermodynamics beyond a very basic understanding. I also don't know what "conceal with the fact" means.
 
  • #12
Vanadium 50
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I can not claim that I fully understand my own question
That's fair enough, but you seem to be getting unhappy with us that we don't understand it either. I think your best bet is to take a step back, and take as much time as you need to try and ask it more clearly.
 
  • #13
phinds
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Since the vast majority of the universe is not in causal contact with our part, just how would you propose to measure it?
We wait until every single light photon reaches us. Or to have so many observers everywhere.
Clearly you do not understand the meaning of "not in causal contact", or perhaps I should have said "is not AND NEVER WILL BE in causal contact..." although I think that's redundant.
 
  • #14
russ_watters
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The result is an inevitable increase in the total entropy of the (system+the measuring machine+the universe) in the form of heat. What if MxN is the physical universe itself, where does the heat go?
Entropy isn't new heat, so the better question is where did it come from?
 
  • #15
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Clearly you do not understand the meaning of "not in causal contact", or perhaps I should have said "is not AND NEVER WILL BE in causal contact..." although I think that's redundant.
I considered "not in causal contact" a vague expression which, unsurprisingly, leads to misunderstanding.
It is getting complex here, in part because I have not well elaborated what I asked and in other because of misunderstanding from some.
The essence of the topic was; how far can a measurement be made?
Consider Maxwell demon, he made his thought experiment to challenge the second law of the thermodynamic. The resolution was that any measurement made by the demon himself will increase the entropy which is not violating the law. Now, what if the demon measures the whole universe instead of just a small box with a hole between its 2 parts; what states would accommodate the resultant increase in the entropy?
Consider my own example; you are using a computer program designed to analyse particular set of data in certain way. Every time the program analyses a new entry, it makes measurements by deleting redundant values and the entropy increases in the form of heat from the processor. What if the data set, is the universe itself, where will the heat go?
 
  • #16
phinds
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. Now, what if the demon measures the whole universe ...
AGAIN, this is just not possible. "Not in causal contact" is not a vague or ambiguous phrase at all. It means we cannot EVER touch/see/smell/measure those things that are not it causal contact with our part of the universe.
 

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