1.1 Relativity Terminology (Relativity and FTL Travel)
Description
This article is from the Relativity and FTL Travel FAQ, by Jason W. Hinson jason@physicsguy.com with numerous contributions by others.
1.1 Relativity Terminology (Relativity and FTL Travel)
As we begin our discussion, I want to first introduce the reader to
some terms which will be used. The first term to consider is the obvious
one, "relativity". Why is this field of study called relativity? Well, it
involves considering how an event or series of events would look to one
observer given that you know how it looks to another observer who may be
moving with respect to the first. This is called "transforming" the
observation from one frame to another, and relativity tells us how to do
that. Thus, we are concerned with the way something seems to one observer
RELATIVE TO how it seems to another. Certain measurements or calculations
will be the same regardless of your frame of reference. They are "frame
independent" or "absolute" or "invariant" in nature. Other aspects of our
universe depend greatly on your frame of reference, and they are thus "frame
dependent" or "relative" in their nature. Relativity is thus study of the
relative nature of things in our universe.
In that last paragraph, I use the term "frame of reference," and I
should take a moment to explain what it is I am talking about. By "frame of
reference" I sort of mean the "point of view" of a particular observer.
Essentially, your frame of reference is what decides your relative "view" of
things, so observers in different reference frames will have different
relative "views". In special relativity, moving with respect to another
observer is what makes your frame of reference different from his. Note too
that frames of reference are relative, so that what we are really concerned
with is what one frame of reference is like with respect to another frame of
reference. Thus, we would say that your frame of reference relative to
another frame depends on your velocity in that other frame of reference.
Now it is very easy for a newcomer to relativity to get mislead by this
concept of frame of reference. The sticky phrase in the above explanation is
"relative 'view' of things." You see, whenever I talk about when something
occurs in some frame of reference, I DO NOT necessarily mean what the
observer in that frame would actually see with their eyes. This is because
the observer only sees the event after the light from the event reaches him.
To figure out when the event actually occurred for that observer, one must
account for the "signal delay". For example, an observer may see an event
today, but if the event occurred on some star ten light-years away (the
distance light would travel in 10 years) in this observer's frame, then we
must realize that the event actually occurred ten years ago in this
observer's frame of reference (because then light from the event would just
be reaching the observer today). I mention this because it is sometimes
tempting for newcomers of relativity to conclude that its odd effects (like
time dilation--which we will discuss later in this chapter) are only
illusions created by the fact that light from an event may reach one
observer before it reaches another. However, here I am clearly stating that
when we talk about when an event occurs in a frame of reference, we are
talking about when it ACTUALLY OCCURRED in that frame after all light signal
delays are taken into account.
Similarly, if I say that event A and event B occur simultaneously in
some frame of reference, I do not mean that an observer in that frame would
necessarily see them occur at the same time, but rather that they actually
happened at the same time. For example, if two explosions really happened at
the same time in our frame of reference, and one occurred on the moon while
the other occurred on the sun, then we would see the one from on the moon
first (because it is closer). However, we must take into account the time it
takes the light to get to us. We must note that it would take longer for the
light from the explosion on the sun to get to us, and we can then understand
why we saw the explosion on the moon first. Then, with the proper
calculations, we could conclude that the explosions actually happened at the
same time in our frame. It will be important to remember that this is what
we mean as we talk about when and where events occur in different frames of
reference (especially in Chapter 2).
Now, with these terms and considerations in mind, we can go on to
reason as to why the theory of relativity exists as it does today.
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