|I know this question had often been asked but I
don't get to understand it. What do mean when we
say that space is expanding? I think that space is
not a physical entity that can expand. Space is
just (apparently) "nothingness". Please solve my
Good question! Yes, before the early twentieth
century, we thought of "space" not as an actual
object, but just a bunch of nothing in which
everything else existed. But with the development
of special relativity in the early 1900s, we
started to combine time and space into a single
thing: spacetime. And with the advent of general
relativity (Einstein's theory of gravity) in 1915,
spacetime became dynamical: that means that we
started thinking about spacetime as a real "thing"
that can bend, stretch, squeeze, wave, and do all
sorts of things. This bending of spacetime is
what gravity is.
So spacetime (and therefore also space) really
is a physical entity that can expand! What does
this mean? Well, "space" is the thing through
which we measure the distance between two objects,
so if space is expanding, that really means that
distances are expanding! This is hard to imagine:
say I'm in a car and driving away from you towards
my house. Then it's true that the distance
between you and me is increasing, but is that
because space is expanding? No - I'm moving away
from you, but you're not moving relative to my
house, and I'm moving closer to my house. In an
expanding space, objects tend to ALL move farther
apart - so I would be moving away from you, you'd
be moving away from my house, and I'd be moving
away from my house.
When we say our universe is expanding, this is
what we mean: when we measure the velocities of
galaxies relative to us, we see that everything is
moving away from us, no matter what direction we
I hope these help!
Imagine you are a tiny organism living on the
surface of a balloon. To you, space would be the
surface of the balloon and you would measure the
distance between two points on the surface of that
balloon as the distance it took you to walk from
one point to another. Now imagine that the balloon
gets inflated so that it is larger. Now you would
be required to walk a farther distance between the
same two points than you would have had to walk
before when the balloon was smaller. If you were a
tiny organism living on the surface of this
balloon, you would see that your universe has
expanded. This is similar to what physicist mean
when they say that our universe is expanding.
This is a tricky question to answer from far away.
Space may be 'empty,' but it isn't actually
nothing. It can be measured and has a size,
despite being empty. Nothing is ...nothing.
Nothing has no volume, and does not experience
time. Many scientists say space (or spacetime) is
like a fabric on which all things exist. I prefer
to think of space is the surface of a balloon. To
demonstrate this to yourself, blow up a balloon
partially, and draw two galaxies on it, a few
inches apart. Now, inflate the balloon more. The
galaxies move apart, but have they really moved?
They are situated on the same balloon surface
places as before, and the drawings haven't moved,
but there is more space in between them due to the
balloon stretching out. This is the same way space
expands. All we can see or interact with are the
galaxies or the drawings, but the space underneath
it all is expanding. Although the galaxies look
like they are stretching a bit too, in reality,
the universe is so big that it isn't noticeable to
humans without very strong telescopes. Some
estimates suggest that there are more stars than
there are grains of sand on all the beaches in the
world put together. The balloon example is also
good, because many scientists believe that space
is naturally curved so that if you go long enough
in one direction you end up back at where you
started, like on the balloon. To do this though,
you would have to travel for billions of years at
the speed of light. The universe is a big place.
If you're interested in questions like this,
you may want to read Stephen Hawking's book A
Brief History of Time. I read it in 11th grade,
and it is written at a simple and interesting
level. It is also actually pretty short (100
pages) and has sketches like the example I used
above to explain these cool astronomy concepts.
I'm sure he does a much better job than me :)
Simply put, space *is* a physical entity that
*can* expand. It may be empty (i.e. devoid of
matter), but it is not nothingness.
A way to visualize it is to think of a loaf of
bread in an onion that has raisins in it; as the
bread bakes, it expands, which causes the raisins
to become farther apart. The farther apart the
raisins are, the faster they move away from
each-other as the loaf expands. Now, imagine that
the bread is "empty" space, and that the raisins
are galaxies. This is basically what the universe
is doing. The only difference is that the bread
has a limit to its dimensions, whereas space, as
far as we know, extends to infinity in all
directions. This means that the distance between
two points in space is getting larger over time.
Space can contract, too. The presence of mass
causes space around the masses to contract. This
exerts a drag-like force on all other objects
caught in the contracting space. This force is
what we know of as gravity.
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