[extropy-chat] Re: Time Travel

The Avantguardian avantguardian2020 at yahoo.com
Wed Feb 25 08:31:47 UTC 2004

Rob Wilkes <robwilkes at satx.rr.com> wrote:
If there were a technology that could backtrack the "entropic" disassembly 
of human remains up to the time of death then the information could be 
captured. Is there some law of physics that precludes back-tracking 

* Yes it is the second law of thermodynamics. In one sense entropy is randomness and therefore *defined* as the loss or lack of information. For example if you have a gas in thermodynamic equilibrium with its surroundings in a flask lets say, it has a temperature characteristic of its heat content or enthalpy. But on a molecular level, the gas is actually composed of particles that have a wide range of velocities. Heat is defined as the sum total of the kinetic and vibrational energy of the molecules in the gas. In this sense on an individual level, some of the molecules are "hot" and some of the molecules are "cold" but  the gas as a whole however would be "lukewarm". In order to separate the "cold" molecules from the "hot" molecules would require that work from an outside source be done on the system. Thus the hodgepodge of fast and slow molecules would never (defined as a probability so low as to be considered zero) spontaneously separate on their own. This is why refrigerators
 and air conditioners for example need electricity to operate. But in a sense, the reason we need to do work on the system is because we are completely ignorant of the velocities (and therefore kinetic energies) of the individual molecules. Thus this ignorance is an alternate definition of entropy. If we knew the position and speed of every molecule in the flask, we could simply open a tiny hole in the flask whenever a fast moving molecule was heading toward it and block the hole when a slow moving molecule was heading towards it. Thus cooling the gas with a neglible expenditure of energy in violation of the 2nd law. Free airconditioning would be really cool, pun fully intended, but it isn't going to happen. And for this same reason, we can't backtrack entropy in the sense Rob intends. Entropy is in this regard an irreversible process. It may be possible to overcome the entropy and reassemble the person through brute force but that would require both a tremendous amount of
 information AND energy.  

Acy James Stapp said:
You cannot make a measurement without interfering with the
system. Not only does the measurement interfere with the
system, but your scenario seems to violate causality because
the information needed to perform the scan must be sent back
in time. 

* In order to actually travel back in time at the same point in space as the death of the person in order to gather the information would not be possible without faster than light travel. But someone at alpha centauri for example could use a ridiculously powerful multispectral telescope and in theory get all the information needed to reconstruct the person some four years after their death provided they had a lot of energy and computing power at their disposal. This would not violate causality because there is no time travel involved and therefore no causality paradoxes. Moreover any perturbations in the system due to this measurement would be neglible since we are talking about macroscopic system and not a quantum particle. (Not to mention that the person is already dead and you don't get anymore perturbed than that.) General relativity would also allow such feat if for example, through gravitational lensing around the event horizon of a black hole, the light and therefore the
 information from the person before death could be viewed by someone on earth with the aforementioned ubertelescope. This of course would have to wait until such a time that the light would have been able to travel all the way to black hole and back again. Ok... well that's my two cents. :)


The Avantguardian 

"He stands like some sort of pagan god or deposed tyrant. Staring out over the city he's sworn to . . .to stare out over and it's evident just by looking at him that he's got some pretty heavy things on his mind."

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