- #1
Coin
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So I'm reading "Three Roads to Quantum Gravity" by Lee Smolin, and at one point he brings up something called the Beckenstein Bound which is confusing the heck out of me.
The way Smolin basically describes this (this is a popular, not a technical book, so maybe he left out some details...) is that if you have a block of space, the maximum bound on the entropy within that space is given by the area of its boundary, not its volume.
The proof given for this is that it is known that black holes have a specific entropy which is based on the area of its boundary. So therefore it is argued no region of space could have an entropy higher than the equivalent black hole entropy, because there are processes by which that region of space could become a black hole (maybe you drip in energy until a black hole forms, or maybe a black hole is just passing by and it falls in) and it's not possible that the process of becoming a black hole could cause the region's entropy to decrease.
Alright, fine. Here's the part that confuses me: The choice of "boundary" seems to me to be arbitrary. Let's say that we have a region A of space within a certain sphere. Because we have this sphere boundary, we have a bound on the region's entropy. Now let's say that we have another region B of space of the same volume, but instead of its boundary being a sphere the boundary has folds, like a brain. Although the volume of region B is the same as region A, the entropy bound is much higher because it has more surface area.
Okay, now let's say we draw another boundary, just around region B, which is perfectly spherical. This boundary defines a region C which contains region B, but which would have a lower surface area and thus a lower entropy bound. It seems like we've now lowered the amount of possible entropy within that space just by considering a different boundary.
What am I missing here?
The way Smolin basically describes this (this is a popular, not a technical book, so maybe he left out some details...) is that if you have a block of space, the maximum bound on the entropy within that space is given by the area of its boundary, not its volume.
The proof given for this is that it is known that black holes have a specific entropy which is based on the area of its boundary. So therefore it is argued no region of space could have an entropy higher than the equivalent black hole entropy, because there are processes by which that region of space could become a black hole (maybe you drip in energy until a black hole forms, or maybe a black hole is just passing by and it falls in) and it's not possible that the process of becoming a black hole could cause the region's entropy to decrease.
Alright, fine. Here's the part that confuses me: The choice of "boundary" seems to me to be arbitrary. Let's say that we have a region A of space within a certain sphere. Because we have this sphere boundary, we have a bound on the region's entropy. Now let's say that we have another region B of space of the same volume, but instead of its boundary being a sphere the boundary has folds, like a brain. Although the volume of region B is the same as region A, the entropy bound is much higher because it has more surface area.
Okay, now let's say we draw another boundary, just around region B, which is perfectly spherical. This boundary defines a region C which contains region B, but which would have a lower surface area and thus a lower entropy bound. It seems like we've now lowered the amount of possible entropy within that space just by considering a different boundary.
What am I missing here?
