Time and the speed of light are inextricably linked. No matter how fast or slow you go or where you are or anything you will always measure the speed of light as 186,000 mph in a vacuum.
In a gravity well light slows down…the further away from the gravity well the faster it can go. However, since you must always measure the same speed something else has to give. In this case it is time. Speed is merely distance travelled over time. In order to get the same results for the speed of light over the some distance when light is moving slower you have to adjust your clock. As a result time moves slower in a gravity well.
This has been proven right here on earth by several experiments. In one they took two hyper-accurate clocks and placed one atop a water tower and one at the base. After awhile (I don’t remember how long…at least a few months I think) the two clocks started disagreeing as to what time it was (time moved faster on top of the water tower since it is further away from the earth’s center of gravity).
Of course, the effect is very small here on earth and not something a human could ever notice without using very sensitive equipment.
Well lets see here. I will try to put a **little **perspective on a large Idea.
Picture the milky-way Galaxy, we are a spec of sand somewhere in the mush of the galaxy. That Galaxy is one of
a couple trillion in a ‘strain’ of Galaxies in this ‘Node’ of the Universe. That said, this little ‘node’ is part of a cloud of matter, the density of the cloud comes from trillions of other nodes all together in a spiraling ‘strand’ in a larger scheme. I could go on but the current theories would enable me to go on for infinity. So when you speak of size and “ludicrously large” you are exactly right. But even that is an understatement. So a Black hole stretching for 10 billion light years across is reletively speaking entirely probable and plausible.
Try not to think about the size of the universe for more than a couple minutes at a time, there may be a danger of cranial implosion.
You might just as well ask why the Sun doesn’t suck all the planets in, and become larger, and suck more in. (It doesn’t matter what the body is, it still has the same effect; although the Sun is losing tons of matter daily to the solar wind…)
In our solar system, as in one where the star has become a black hole, most of the non-stellar matter has settled down into stable orbits around the star, so it will never fall in. It’s just the odd comet or asteroid with a highly eccentric orbit that tips in every now and then.
You do need to think about the size of the universe. Ten billion light years is really big! As I already mentioned a black hole with a 10 billion light year diameter would encompass most of the universe as it currently is (anywhere from all of it down to 2/3 of it).
The math is well beyond me but I would guess you’d need to take all of the baryonic matter in the universe to form a black hole with a 10 billion LY diameter. Even then you might not have enough.
I could very easily be wrong, but I always thought the universe to be infinite. Therefor thoroughly impossible to think of as having an ‘end’. So mileage, light years and giving something a distance, is a complete human construct, giving us the ability to formulate thought about something that can not be successfully explained. No?
I will grant that the following are just theories and as such do not constitute proof but they are the best our scientists have come up with so far so I’m going with it.
I believe the current view of the universe is that it is finite in size but without boundaries. That’s to say, although the universe is not infinite you could travel around it forever and never find an edge. The example given for this is to think of the earth as a smooth sphere that you are standing on (it doesn’t have to be smooth…that’s just for ease of thinking about the following). We know that the earth is finite in size yet you could walk around it forever and never come to an ‘end’.
The other way to think about it is that the Universe had a beginning in the Big Bang at some time in the past. Although the age of the Universe is still up for debate most numbers I have heard put it in the 10-15 billion years old range. So, starting from a point (singularity) the Universe starts expanding outwards at light speed. In 15 billion years the Universe would have a radius of 15 billion light years (some theories postulate a faster expansion rate early on that might allow for a bigger Universe today).
After thinking about it I realize that a 15 billion year old universe would actually have a 30 billion light year diameter (if 3-D geometry can be applied which I think is not quite up to the task as regards the Universe). Also, the event horizon of a newly formed black hole moves away from the black hole at light speed till it reaches its full size. It would take 10 billion years for the BH we are talking about to reach out that far in which time the Universe itself would have contiunued to grow for 10 billion years. Still, a 10 billion LY black hole would constitute an enormous portion of the Universe and is a little overstated…especially when you have only (relatively speaking) the mass of the Milky Way Galaxy to work with.
Never come to an end? Or Never come to an Edge? Or Both? Doesn’t that mean it is infinite.
Finite in it’s very essence means ‘…completely determinable in theory or in fact by counting, measurement, and thought…definable limits…’
So in a finite universe how could there be no boundries? Wouldn’t having no boundries constitute infinite? You’ll have to excuse me Whack, I find it amazingly difficult to contemplate a finite universe, my brain automatically tells me that finite means there is a boundry, an edge if you will. Had I been able to accept this I would constently want to know what would be on the other side of that ‘limit’. Such is the reason this is one of the most debatable subjects in this forum. However, I am also the one to shoot holes in the big bang theory as well. I am going to look up a quote from Carl Sagan. If I try to remember it now without the text I will butcher it. I’ll be back.
Standing on the Earth is not a good analogy since we now have the means to transcend our planet. Imagine a 2-dimensional creature, however, on a 3-dimensional sphere. It can not transcend or even see or comprehend beyond the sphere. Similarly, our Universe exists in a multi-dimensional Something that contains many universes. However, we cannot see them or even comprehend them as they exist in other dimensions that we can have no inkling of.
The above is not verifiable by any known means, as it is purely my idea.
No boundary does not mean infinite. Again, think of the example of Earth’s surface. There is no edge, but it’s finite.
You have to imagine the concept as a 2D surface in three dimensions, because your brain isn’t wired to imagine a 3D surface in four dimensions. The 2D surface is as close as our heads can get to it, then we have to rely on the fact that the math is identical to accept the reality.
The speed of light is constant if and only if you are in an inertial reference frame where Special Relativity holds. In a general reference frame, where you need General Relativity, the speed of light is not constant. Near a strong gravitational field, light “falling” travels faster than light rising. Light at the event horizon of a black hole trying to travel outward is stationary. In General Relativity, you still have the weaker condition “Nothing travels faster than light”. Whenever you have acceleration or gravity you need to invoke General Relativity.
No. Even in an infinite universe, it is still meaningful to have finite distances. The units we use are human constructs, but distance is not.
I believe that the current thinking is that the universe is infinite. But a finite yet boundless space does make sense. The 2D surface of a sphere is finite, yet it has no boundray. If 3D space is finite it could have the topology of the surface of a 4D sphere. It would be finite, yet have no edges or boundaries.
For the Universe to be infinite wouldn’t it have to come into existence all at once? That is, no growing from a Big Bang?
I believe the Universe is now considered to be open. That is, it will expand forever so in that sense it is infinite since it would have no end (it will expand forever). However, if you believe the Universe had a beginning then in order for it to be infinite it would have to pop into existence all at once, everywhere, simultaneously. Wouldn’t it?
[sub]NOTE: I know Big Bang theory isn’t universally accepted but I thought it is the most widely accepted theory (for now).[/sub]
If gr8rguy has his equation right (it looks right to me, but can’t find a ref) a black hole the mass of the Galaxy (4x10^41 kg) would have an event horizon a measly 0.06 light-year. So much for Cecil’s infallibility. A BH the size of the observable universe is in principle possible. Living inside its event horizon would not be too different from living in a universe that is collapsing. Eventually, everything smacks into the Big Crunch (= center of BH).
Waverly, this is not quite right. As seen from the outside, something falling into a black hole seems to fall more and more slowly as it approaches the event horizon. To the unfortunate faller, though, the trip takes a finite (rather short) time. How long depends on size of hole, starting point, etc. But, sooner or later, you run into the singularity.
Essentially, yes. The Big Bang happens at all points in the Universe simultaneously - it doesn’t “grow” from a point. This is true whether the Universe is finite or infinite. This can’t really be pictured, and for a good reason: the Big Bang is a “singularity”, i.e., the equations stop working at that point. We can only predict from some arbitrarily small time after the BB to the present, and future, of the Universe.
Current observations indicate that the Universe is probably flat (between open and closed), which means that it will expand forever and is infinite in size. Were it closed, however, it’d be finite in time and space, and the results would, in fact, be identical to those for a black hole of that size.
Cecil’s quote is definitely wrong on the size of a whole-galaxy hole. I don’t have time to check FriendRob’s calculations, but it looks to be in the right neighborhood.
If the universe is infinite why do we so often hear about the Universe expanding? Is it just me or is the notion of an infinite space expanding somehow silly?
Two things:
Why do clocks (very accurate ones) disagree when one clock is further from a center of gravity than the other clock. Light going out is slowed down but light coming in is sped up. Shouldn’t the two cancel each other? Why would the clocks have a preference for measuring the speed of light exiting the gravity well rather than the light coming in (and by measuring I am saying that the differences as their times diverge equates to how they would have to run to measue the speed of light in their location and get the same results)?
Light stopping in a black hole? I thought light always kept moving but space was curved in such a fashion that light couldn’t find a path out of the black hole. Instead it always manages to get turned back in on itself. Light stopping seems like actually hitting absolute zero. One of those things that just isn’t ever quite possible even if you can get arbitrarily close to it. If light actually did stop does that mean that time stops too? If light stopped (and thus time with it) wouldn’t everything get stuck right at the event horizon of a black hole since there is no ‘future’ where the object is closer to the center of the BH?
I make this same mistake, apparently, when thinking of the influence of black holes upon neighboring bodies and clusters of mass. For me, this stems from Dr. Sagan’s example of weighted balls on a trampoline-like surface of rubber(?). Scatter around a couple of baseballs and they will attract marbles set near them but will not affect each other. Drop a bowling ball in the center and everything on the surface will ‘fall’ into it. It would seem a black hole with an event horizon of 10 billion LYs would have a little more influence on gravity and the ‘warping’ of space than the sun.
And, Sea Sorbust, as I understand it, this is a place of learning. I think darkcool has missed that point. I, for one, welcome your questions.
Might that ‘stationary’ photon only appear to be so from your arbatrary vantage point relative to it? Wouldn’t matter be rushing past it at great speed? If you were on the matter in question, the light would be moving at great speed relative to that vantage point.
You’re fairly close to a good analogy but you’re forgetting one thing…movement.
If the earth were to just stop in its orbit it would get sucked into the sun just as your marbles and baseballs go towards the bowling ball.
Picture your bowling ball on a rubber sheet and now shoot a marble near it across the rubber. The marble will orbit the bowling ball. Due to friction (and air resistance and such) the marble loses energy and eventually spirals into the bowling ball. However, if we assume a frictionless environment such as the earth is in and give the marble the correct energy and angle it will spin around the bowling ball forever (too much energy and the marble will partly circle the bowling ball and shoot away…too little energy and it will spiral in to the bowling ball eventually).
In reality the earth does lose energy via gravitational waves (like a cork bobbing in water) and will spiral into the sun someday. Luckily for us this energy loss is very small and the sun itself will die long before the earth crashes in to it.
General Relativity does not state “Nothing travels faster than light.” It says nothing can be accelerated faster than light, and if there is something that is traveling faster than light at creation, it cannot be decelerated slower than the speed of light.
