Re: A question of discrete space-time, part 3



robert bristow-johnson wrote:

<snip>

> i just want to add that Ed Fredkin seemed to have no use for the
> gravitational constant, G, in his theory (not sure why). his unit of
> discrete-time was many orders of magnitude larger than the Planck Time.

Dear Robert,
I'd gladly yield to a more convincing argument than the one I happened
to stumble across. Do you know if Ed Fredkin is still active?

> > It bears repeating that I'm not trying to convince anyone of the merits
> > of a discrete space-time over continuum physics,
>
> i am an agnostic regarding discrete vs. continuous physics, but if reality
> turned out to be discrete, i would not be surprized if the discrete units of
> time and length turned out to be the Planck units, possibly scaled by
> sqrt(4 pi) ("rationalized Planck units").
>
> i am not an agnostic about the notion of Planck units (or better yet,
> rationalized Planck units). i truly believe that nature is inherently
> scaled by them and there is no magic universal quantity defined by c or G or
> h_bar or epsilon_0 (or Boltzmann, either, for that matter). on the Planck
> scale they just don't exist as concepts of quantity which is one reason i
> think that Michael Duff is absolutely right about the "operationally
> meaninglessness" of VSL or varying G theories (or a varying h_bar theory if
> anyone puts one out). there is no way we could know the difference if they
> did vary, we would still measure them and perceive them to be the same as
> before.
>
> .
>
> > This seems to provide hope of
> > finding a basis for unifying relativity and quantum mechanics, by
> > proposing a discrete space-time at the Planck scale.
>
> the Planck Time and Planck Length are so ridiculously small (from the POV of
> our human existence) that i can't see how any meaningful accuracy is lost if
> all of these differential equations (Schrodinger, Maxwell's, GR or GEM) were
> expressed in discrete form (a la Euler) as one would do if trying to
> simulate these equations on a computer. might not be the true
> representation of reality, but i can't see how it would be measurably
> different. well, i wouldn't want light or gravitational propagation to be
> in the shape of an expanding cube or tetrahedron (instead of spherical
> expansion), so somehow i think these discrete "cells" could not have some
> absolute fixed orientation. i dunno how to model it so that no direction in
> space appears to be preferred over another.

I'd like to wait a bit before talking about geometric issues, but I
agree that symmetry is the key, as nature seems to prefer the highest
possible levels of symmetry. If space-time were in fact some sort of
discrete lattice, the individual cells ought to be highly symmetrical,
and if it were possible to tile space with tiny spheres, nature would
probably have done so.

<snip>

> admittedly, JJ, this is more philosophy than physics. but it gets me
> wondering. sorry to pile on a little more speculation onto your thread, Ed.

Philosophy with experimental backing is most likely to be useful to the
physics community at large.

All points of view are welcome. Diversified views (including mine) are
useful only to the extent that they can do something to predict or
explain some physical phenomenon. For instance, I like the idea that a
discrete space-time lattice with no gaps, can automatically explain why
there is a maximum speed limitation, even though I realize that
"explanations" are passe in today's physics. (And I just lost 10
points on the Baez crackpot index.)

But in the end, space-time is either discrete or continuous, or
possibly somewhere in between. I happen to be hacking my way through
the underbrush, following the trail of a discrete space-time fixed
lattice with probabilistic motion (as opposed to deterministic motion).
There may be a brick wall somewhere in from of me, but I haven't run
into it yet.

Regards,
Ed Hanna

.

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