- 4293 - LOST
IN SPACE! One of the hardest things for many people
to conceptualize when talking about how fast something is going is that they
must ask, "Compared to what?" All motion only makes sense from a
frame of reference, and many spacecraft traveling in the depths of the void
lack any regular reference from which to understand how fast they're going
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4293 - LOST IN
SPACE!
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- There have been
several different techniques to try to solve this problem, but one of the ones
that have been in development the longest is “StarNAV”, a way to navigate in
space using only the stars.
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- The technology is
based on a specific feature of stars known as “stellar aberration”. As defined
in the “Special Theory of Relativity”, stellar aberration occurs when the
velocity of an observer changes the apparent distance between it and a star.
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- This technique has
been used before; however, it has had wide error bands when calculating a
spacecraft's instantaneous velocity. Typically, existing solutions would use a
large telescope to measure a property known as an "inter-star angle"
between two stars in a relatively narrow field of view precisely. If it is
precise enough, complex math can produce a spacecraft's velocity from only one
inter-star angle.
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- Getting a
measurement that is precise enough is the tricky part. To accurately detect the
position of an individual star in an inter-star pair, many telescopes have to
have a narrow “field of view”. That
narrow field of view means that only one star can be tracked per telescope,
which requires a second telescope and a complicated metrology system to track
the relative alignment of these telescopes.
-
- The NIAC
researchers came up with a method of using slightly less precise inter-star
angle measurements but multiple measurements, and once again using fancy math
to calculate an accurate velocity measurement without the complicated tracking
systems.
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- The system consists
of three different telescopes offset from each other at known angles, each
observing a different pair of stars. With these three slightly less precise
measurements, an algorithm can still calculate an average stellar aberration
and a reasonable estimate of spacecraft velocity.
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- As more and more
spacecraft start venturing into deep space, improving how they calculate their
velocity will become an ever-increasing problem. StarNAV seems well placed to
do so, it just needs a bit more of a push into the prototyping stage to get
there.
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- The most distant light
we can see is the cosmic microwave background (CMB), which has taken more than
13 billion years to reach us. This marks the edge of the observable universe,
and while you might think that means the Universe is 26 billion light-years
across, thanks to cosmic expansion it is now closer to 46 billion light-years
across.
-
- Most cosmologists
think the Universe is much larger than our observable corner of it. That what
we can see is a small part of an unimaginably vast, if not infinite creation.
There are several reasons why cosmologists think the Universe
is large.
- One is the
distribution of galaxy clusters. If the Universe didn’t extend beyond what we
see, the most distant galaxies would feel a gravitational pull toward our
region of the cosmos, but not away from us, leading to asymmetrical clustering.
Since galaxies cluster at around the same scale throughout the visible
universe. In other words, the observable universe is homogenous and isotropic.
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- A second point is
that spacetime is flat. If spacetime weren’t flat, our view of distant galaxies
would be distorted, making them appear much larger or smaller than they
actually are. Distant galaxies do appear slightly larger due to cosmic
expansion, but not in a way that implies an overall curvature to spacetime.
Based on the limits of our observations, the flatness of the cosmos implies it
is at least 400 times larger than the observable universe.
-
- Then there is the
fact that the cosmic microwave background is almost a perfect blackbody. There
are small fluctuations in its temperature, but it is much more uniform than it
should be. To account for this, astronomers have proposed a period of
tremendous expansion just after the Big Bang, known as early cosmic inflation.
-
- We have not
observed any direct evidence of it, but the model solves so many cosmological
problems that it’s widely accepted. If the model is accurate, then the Universe
is on the order of 1,026 times larger than the observable universe.
-
- Although “string
theory” is often presented as a physical theory, it’s actually a collection of
mathematical methods. It can be used in the development of complex physical
models, but it can also just be mathematics for its own sake.
-
- One of the
problems with connecting the mathematics of string theory to physical models is
that the effects would only be seen in the most extreme situations, and we
don’t have enough observational data to rule out various models. However, some
string theory models appear much more promising than others.
-
- One way to get
around “early cosmic inflation” is to look at higher-dimensional structures.
Classic general relativity relies upon four physical dimensions, three of space
and one of time, or 3+1. Mathematically you could imagine a 3+2 universe or
4+1, where the global structure can be embedded into an effective 3+1
structure.
-
- This is a common
approach in string theory since it isn’t limited to the standard structure of
general relativity. Under just the right conditions, you could construct a
higher-dimensional structure within string theory that matches
observation. Based on their toy models,
the Universe may only be a hundred or a thousand times larger than the observed
universe. Still big, but downright tiny when compared to the early inflation
models.
-
- If early cosmic
inflation is true, we should be able to observe its effect through
gravitational waves in the somewhat near future. If that fails, it might be
worth looking more closely at string theory models that keep us out of the
theoretical swamp.
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December 29, 2023 LOST
IN SPACE! 4293
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Friday, December 29, 2023 ---------------------------------
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