Boltzmann Brains

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Main Question or Discussion Point

‘Everything can happen, will happen’ so the saying goes. So given infinite time a universe is meant to generate infinite Boltzmann brains. But I was thinking about this:

- Things that are possible can become impossible (example: heat death making Boltzmann brains formation impossible)

- Things that are impossible can’t become possible again

- So the probability space shrinks with time

- The probability space shrinking to exclude Boltzmann brain formation will probably happen before any brains are formed?

- After a certain period of time elapsed, Boltzmann brain formation becomes impossible?

- Argument assumes a universe with infinite time and space but finite mass.

Does this make any sense to anyone?
 

Answers and Replies

  • #2
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It may be a saying that if something is possible then it must happen, but it isn't necessarily so.
It's within the realm of possibility that the same person could win the lottery every week
 
  • #3
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The temperature in the universe decreases - the probability for thermal fluctuations above some threshold goes down. It is unclear if a large energy is possible at all in an essentially empty universe (0-1 massive particles plus possible radiation from the expansion of the universe).
 
  • #4
phinds
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Things that are impossible can’t become possible again
I doubt that.
 
  • #5
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I doubt that.
I mean if they are impossible then by definition they can’t become possible.

For example once all the stars have burnt out, and no more can form then certain things in the universe become impossible...

So
 
  • #6
phinds
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I mean if they are impossible then by definition they can’t become possible.

For example once all the stars have burnt out, and no more can form then certain things in the universe become impossible...

So
I agree that SOME things that become (or are) impossible will not again become possible. I do NOT agree that that it is always true. For example, it was impossible to Michael Jordon to do a dunk when he was 3 years old. But now ...
 
  • #7
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I agree that SOME things that become (or are) impossible will not again become possible. I do NOT agree that that it is always true. For example, it was impossible to Michael Jordon to do a dunk when he was 3 years old. But now ...
By possible / impossible I mean everything possible / impossible in the present and future, rather than just the present as in your example.

So it was always possible that a grown up Michael Jordon would be able to do a dunk.

Contrast that to a universe where there are one way irreversible processes like the 2nd law of thermodynamics and density reducing with time - these would reduce the future possibility space I think.
 
  • #8
PeterDonis
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By possible / impossible I mean everything possible / impossible in the present and future
these would reduce the future possibility space
Do you see the contradiction between these two statements?
 
  • #9
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Do you see the contradiction between these two statements?
No please enlighten me.

Things that are possible can become impossible
AND
Future possibility space becomes smaller
Are not contradictory?
 
  • #10
PeterDonis
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No please enlighten me.
You're saying that your "possibility space" already includes all of the future; but then you say the future possibility space becomes smaller. Those two statements are contradictory. If your possibility space already includes all of the future, then the possibility space can't change with time.

Things that are possible can become impossible
AND
Future possibility space becomes smaller
Are not contradictory?
Those weren't the statements I quoted and said were contradictory. See above.
 
  • #11
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If I rephrase it maybe it makes sense:

At time t+1 the set of all possible future events is smaller than the set of all possible future events at time t.
 
  • #12
PeterDonis
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If I rephrase it maybe it makes sense:

At time t+1 the set of all possible future events is smaller than the set of all possible future events at time t.
If you rephrase it this way, first, you are explicitly rejecting your previous statement that your "possibility space" includes all future possibilities. Which is fine, but it indicates to me that you're confused about what actual model you are using.

And second, rephrased this way, your claim is false, because, if you are allowing your "possibility space" to change with time, you have given no argument to show why the "size" of that space can't increase with time. The only argument you gave relied specifically on your definition of "possibility space" as already including all future possibilities. Once we drop that definition, your argument no longer works. Again, this indicates to me that you're confused about what actual model you are using.
 
  • #13
kimbyd
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- So the probability space shrinks with time
Provided the laws of physics are unitary (which is usually expected to be the case, as otherwise causality becomes poorly-defined), the probability space cannot ever change.
 
  • #14
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And second, rephrased this way, your claim is false, because, if you are allowing your "possibility space" to change with time, you have given no argument to show why the "size" of that space can't increase with time. The only argument you gave relied specifically on your definition of "possibility space" as already including all future possibilities.
- The size of the probability space can’t increase with time because ‘things that are impossible can’t become possible’
- The size of the probability space can decrease with time because ‘things that are possible can become impossible’
 
  • #15
PeterDonis
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The size of the probability space can’t increase with time because ‘things that are impossible can’t become possible’
Which is false if we allow the possibility space to change with time. Conditions can change that make something previously impossible possible. You have already been given examples of this in this thread; your only response to those was to claim that your "possibility space" already included all future possibilities--but if that's the case, then this...

The size of the probability space can decrease with time because ‘things that are possible can become impossible’
...is also false. You can't have it both ways.
 
  • #16
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Provided the laws of physics are unitary (which is usually expected to be the case, as otherwise causality becomes poorly-defined), the probability space cannot ever change.
Probability space can change: think about the 2nd law of thermodynamics - closed systems start energetic and full of possibilities and over time die a heat death and tend to stillness and lack of possibilities.

Think of sturing a cup of coffee. Lots of motion and possibilities to start with tending to stillness and lack of possibilities.
 
  • #17
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Conditions can change that make something previously impossible possible.
- If it was impossible to start with, nothing can make it possible (assuming laws of nature invariant over time).
 
  • #18
PeterDonis
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Probability space can change
Yes. Which means that things that were possible can become impossible, and things that were impossible can become possible. Again, you can't have it both ways; "change" can happen in either direction, you can't just arbitrarily exclude one.
 
  • #19
kimbyd
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- If it was impossible to start with, nothing can make it possible (assuming laws of nature invariant over time).
If that's true, then something that's possible can't be made impossible, by the same reasoning.
 
  • #20
PeterDonis
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If it was impossible to start with, nothing can make it possible (assuming laws of nature invariant over time).
You have given no argument or reference to support this, and you just keep repeating it without even addressing the responses you have been given. That is not the way to have a productive discussion.

Thread closed.
 

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