WHAT IS SPACETIME ?
.. perhaps the main point is that these two entities always
exist no matter what.
What is
meant by spacetime ? Isn’t ‘space’ one thing - and
‘time’ another ? Why do some scientists say ‘spacetime’ or
‘curved spacetime’ ?
When a scientist says ‘spacetime’ he
is not actually referring to a parameter that can be measured
such as force or distance, which makes it bit confusing when
using the term ‘spacetime’. Time itself, however, is
a physical parameter that can be measured, but 'space' is
not considered a physical parameter unless you mean the distance
between two points or a volume in space, for instance. If
one assumes that when referring to the term ‘space’ that it
means any of the parameters of distance, volume, points, or
planes in space – then ‘space’ can be considered a parameter too
(a type of loosely stated variable).
The purpose of the above reasoning
is that time and space can be considered “base variables” for
any type of study in Space Science.
Now consider this: Take everything
out of the picture. No masses, no matter, no forces, no
radiation. Nothing. Take everything out – but you are
still left with space, and of course time is still passing.
It is as if we are always “stuck” with these two variables (or
entities): space and time. These can be considered the
“default variables" that are always present and always exist.
A scientist may say ‘space’ and ‘time’ are the “base” variables
to any scientific study. (Also “raw” variable or
“stranded” variable).
Being the
type of precocious intellect that he is, the scientist tends to
be “one-step ahead” of the layperson, and therefore decides that
these two default variables must also be intimately related, and
infers that a “space-time” exists. Since we have now
determined that space and time are the “default variables” in
the universe, and if we refrain for a minute from measuring a
distance in space or timing an event on a stopwatch (we
“interfered” with our measuring sticks and stopwatches)
1, these two entities
begin to take on an absolute characteristic, and it is said that
‘space’ and ‘time’ comprise a “space-time continuum” - and
become an absolute background for all further measurement and
study in space.
It is
still okay to think of space and time as separate (you don’t
have to be a precocious scientist); perhaps the main point is
that these two entities always exist no matter what. As
previously stated, we can remove all physical entities from a
certain situation: forces, masses, heat, light, etc. But
we can’t remove ‘space’ or ‘time’ – they will always exist in
any example we construct. This gives space and time an
absolute quality.
The issue of how to use these terms
can even be considered mnemomic, which is whatever way helps you
understand or remember things. The theoretical physicist
may say “spacetime” when speaking with his like-minded
colleagues, and the applied physicist will prefer to only
mention ‘time’ or ‘space’ (i.e., ‘distance’) for making
practical measurements.
The use of the term ‘curved
spacetime’ is simply a curved path that something takes through
space but since time is always present, it is referred to as
traveling through “curved-spacetime”. Observe a house fly
flying about a room. It flies anywhere but straight.
It is traveling in curved-spacetime.
As a helpful tip, to better
understanding the theoretical scientist’s usage of ‘spacetime’,
it can be helpful to think about the variable
time itself, which is thought to be one the more abstract
quantities in science.
It may
sound a bit like “mumbo jumbo”, but singling-out these absolute
variables can help one think about certain problems in Space
Science, most often dealing with relativity (Einstein).
1.
In making measurements, it
follows in the scientific method of taking empirical data, that
the measuring device itself can introduce an error into the
results. The measuring instrument itself “interferes” with the
data being measured. This is often accounted for and data
compensated with a correction factor (‘raw’ data versus
‘corrected’ data).