12/15 Anyone know how laser distance measurement work? Intensity of
reflected light? But doesn't that depend on the reflecting
surface? Curious. Thanks.
\_ I thought it was the time-to-travel of a beam of light. I assume
you mean far distances.
\_ I recently tried a Leica Disto laser measurement device, it
can measure from 1 ft up to 600 ft. Just wondering how it
works because it seems to be pretty accurate and works on most
surface I point it at, even tree leaves at night.
\_ It modulates the laser's strength to produce pulses, and then with
a high-resolution timer it can tell how long it took for the
roundtrip. An advantage of this is that the shininess and distance
of the target do not effect the measurement as long as they are not
too far and too dark. If you shone it on a smooth enough mirror
at the wrong angle, you could disrupt the measurement though.
\_ wow, how high resolution of a timer are we talking about?
This is speed of the light we are talking about. Something
comparable to timer used in GPS? But those are much further
away...
\_ For ~1-meter resolution, you need 3ns resolution. I know
higher-resolution timers are available, as for price, no idea.
\_ The clock period on a 2GHz Pentium is 0.5ns, and the chip
costs only a couple hundres dollars. So I guess a timer
with 0.5ns resolution would cost much less than that.
Come to think of it. Light is not really that fast.
\_ If you mapped all speeds onto the domain [0,1], light
would be 1.
\-"we have measured the charge of the electon ... and
it is 1" --psb
\_ I suppose it can be the fastest and still not be
"that fast". After all, the universe seems a lot
bigger than light seems fast. Or perhaps it's just
that our sense of time is too fast, since we live
so short.
\- gee what other free parameters seem too
big/too small?
\_ Size of universe = age * speed of light.
If you think the universe is bigger than light is
fast, then that's just saying the universe is old.
Your preception would remain the same no matter
how fast light it, because the universe would be
bigger.
\_ If size of univese = age * speed of light, why
\- hello, even without a discussion about
inflationary theories [i mean inflation
in the sence of alan guth et al] this
simple notion doesnt work because the
universe was not opaque for a long time,
meaning a photon would not have been
able to cross the diameter of the
universe [or even get far at all].
you can probably GOOGLE for "opacity
cosmology" or something like that.
so the speed of light in a vaccum was
not always the speed at which photons
moved through the universe. ok tnx.
\_ While the speed at which photons can
cross the universe is not always C,
with the exception of hyperinflation,
the outermost dimension of the
universe grows at C, modified by the
geometry of space.
\-saying "assuming expansion is space-
like, then it would fit inside the
light cone" is not 'interesting'.
positively asserting that inflation
is, always was, and always will be
spacelike, i suppose is interesting.
\_ I'm just trying to make the point
that opacity/optical depth has no
effect on the size of the universe.
Want 'interesting'? I like the
fact that assuming linear
expansion, the age of the universe
is the same as the inverse of the
Hubble Constant.
\- ok, now tell us about
zero-point energy.
\- one is a boundary condition
the other is an approach to
answering the empirical Q and
and attempt to do better. the
real point of course is we have
some observational data for size
so really what we are trying to
figure out is age.
is there debate among scientists on whether the
universe is growing at an accelerating rate,
constant rate, or decelerating rate?
\_ It *is* more complicated than that, but I
wanted to gloss over that fact because for
purposes of comparing non-comprable huge
values (light-speed vs. universe size), it's
about right. If you want to do actual
\_ You are assuming that the expansion
of space is limited by the speed
of light, correct?
cosmology, you need to think about tensors
of 4-dimensional non-euclidean geometry, but
that seemed beyond the scope of this debate.
\- um without looking for explanation that
involve really exotic theories and
fancy math like M-theory and supergravity
the two big Qs in cosmology today are
1. the missing mass question and the
2. the hubble constant/cosmological
constant question ... some recent
observation are seeing some curious
phenomenon in high red shift objects.
in both cases there has been a lot of
study to rule out dumb mistakes but now
a lot of physicists believe something
big is missing from our theories and
models. on a parochial note on topic #1
the dark matter studies are a major
funding priority for the govt and on #2
a lot of the seminal work is being done
at lbl (smoot, permutter, borrill etc).
there are a lot of decent and fairly
accessible books on these topics
as well as many good WEEB pages at
various levels. s. weinberg is a really
good writers if you are looking for a
specific recommendation. ok tnx.
\_ It turns out that time is what we're best at measuring.
\_ How do you tell pulse from one another? How do you identify
the return pause is the one you sent x time ago?
\_ Imagine you space your pulses out by, say 1ms. This lets you
measure up to 1000 pulses per second, each can have a maximum
roundtrip distance of 300km, which is way more than you can
measure in practice.
\_ I thought reflection is absorption and re-emission of photons.
Does that happen instantaneously? If not, does the delay depend
on the surface material of the target?
\_ It's not instantaneous, and it does depend on the elements
present in the surface, but except for a few special cases,
the delay is inconsequential in this type of measurement.
\_ Yes, if what you're pointing at is a black hole, you're scr00ed.
\_ Has scientists confirmed that black holes exist?
\-yes, essentially. --psb
\_ Black holes? Humbug! I've never seen one!
\_ Black holes, white holes, Asian holes. I've seen them all.
I've even gone inside a few Asian holes. |
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