MadSci Network: Physics

Re: Is there only 'Past' & 'Future' ?

Date: Tue May 11 09:32:54 2004
Posted By: Guy Beadie, Staff, Optical sciences, Naval Research Lab
Area of science: Physics
ID: 1082055093.Ph

My, that’s quite a question.  I’ll split your question up into two parts: 
1) what is the ‘present’ for our subjective experience, and 2) what is 
the ‘present’ from an objective perspective.

Subjective ‘Present’:
I was reading Penrose’s “The Emperor’s New Mind” the other day.  Turns out 
it has a chapter that goes to the heart of some of your questions about 
the order in which the brain processes signals.  First of all, it confirms 
your suspicions about the processing time necessary to interpret stimuli.  
Your perspective of the present is actually several milliseconds delayed 
in ‘real’ time.  However, the brain also interleaves different signals in 
different ways.  You can react to certain stimuli (touching a hot stove) 
much more quickly, so there are some signals that you can process before 
you’re even aware of them.  There were also experiments described in which 
carefully chosen stimuli occurred in one order, but were perceived in 
reverse order, and then vice versa.

Therefore, I would conclude that our perception of the ‘present’ is 
garbled to the point where it really serves no meaning except to describe 
your current state of consciousness.  Furthermore, I would argue that my 
perception of the ‘present’ has only a tenuous relationship with anyone 
else’s perception of the ‘present.’

[Roger Penrose, “The Emperor’s New Mind: Concerning Computers, Minds, and 
the Laws of Physics” (Oxford Press, 1999, ISBN 0192861980).  I cannot 
remember details, but the experiments I’m talking about are described in 
his chapter on the structure and operation of the brain, and he includes 
full reference information there.]

Objective ‘Present’:
As for an objective version of the ‘present,’ this too is tricky.  As you 
hinted in your question, this goes beyond physics into philosophy or 
semantics.  Whether a person can point to the existence of a ‘present’ is 
going to depend entirely on how that person defines the concept.  Whether 
anyone else will believe the argument will depend first on whether the 
concept was communicated clearly enough, and then whether they believe the 
definition, too.

Extemporaneously, I’ll present an argument for an objective ‘present’ in a 
local sense, based on the ideas of physics.  Arguing from a physics 
perspective, however, does not necessarily mean that the arguments would 
be accepted universally.  It’s merely the most comfortable framework for 
me.  (Note also that this description is filled with just as much jargon 
as would be required of a philosopher – “inertial reference frame” 
and “relativity” are phrases that require an awful lot of background 
baggage before they can be appreciated, just as do technical terms from 

Provided we do not approach a gravitational singularity, any event in 
space can be labeled with a local position and local time.  Furthermore,
physics tell us the order of events which take place in the same location 
will always be the same: if one person sees A before B, then everyone else 
will see A before B (if A and B occur in the same spot in our local 
inertial frame).  Relativity says that time and space _between_ events are 
different for different observers, but that the order in which they occur 
is always observed to be the same.  Furthermore, we can trace the flow of 
events down to very, very small increments.  Near as we’ve been able to 
measure, time is continuous.  So, I argue for a ‘local present’ in the 
following way. 

As we record events with ever-smaller increments and ever-faster speeds, 
the ‘present’ is the limit of the sequence of past events.  I think you’re 
right in that we cannot ever experience the present, as both measuring 
devices as well as our brains take time to record events, but I think one 
can define a present as this limit.

Interesting question – it certainly made me think,

  - Guy 

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