Is the universe infinite or finite?

[ossumguywill]

If we look at the law of universal gravitation we know that regardless of the distance between masses there is some gravitational influence, albeit very very weak at galactic distances. This law describes the influence of gravity. When we look at relativity we find that gravity is represented by warped space-time. Knowing that every object affects every other object in the universe inversely proportional to the distance squared we an postulate that the shape of space-time is no where near flat and most likely not spherical. It is a convoluted, twisting 4-dimensional entity with no consistent shape or central frame of reference. Stepping outside of basic physics and mathematics we begin to enter the realm of manifolds to describe various components of space-time.

In the early 1900s there were several observations of eclipses to verify Einstein's theory of general relativity (the melding of universal gravitation and special relativity). Basically they were verifying gravitational lensing due to warped space-time. As it was clearly verified we are left with a non-euclidean universe. If, while traveling on the a light ray you encounter a sufficient gravitational field the ray would bend to follow the warp of space-time. From your reference point, however, you would appear to continue in a straight line, never feeling any effects of inertial change as there is no inertial change.

I think this is all correct-what I was suggesting was the properties of the universe without gravity. Now obviously the universe is pockmarked with holes, but the general structure of the universe stays the same to some extent. I still have to ask my physics teacher about the speed limit of light no matter the starting speed thing....

 

[photonaholic]

I clocked it at 186,000 miles per second Will.

5,865,696,000,000 miles = 1 light year. Wow that will leave some jet lag.

 

[ossumguywill]

^no, I mean how even if you are moving at a speed and if you point a laser at an object you are approaching, the laser will still only go the same speed as if you were standing still. But, if you were moving at a speed and you threw a rock at the object you were approaching, then the rock would move faster than if thrown while standing still.

 

[photonaholic]

OK I get it now...

so if I was moving 186,000 miles per second, and I pointed my laser in front of me, It would appear to be off, since I am theoretically moving at the same speed as the light coming from the laser.

However, here is where it gets different, a rock has mass, light has none.

 

[ossumguywill]

OK I get it now...

so if I was moving 186,000 miles per second, and I pointed my laser in front of me, It would appear to be off, since I am theoretically moving at the same speed as the light coming from the laser.

However, here is where it gets different, a rock has mass, light has none.

so the laser would simply pool up next to you. How weird is that... I might have it, though. Photons don't take any kinetic energy to move because they have no mass, right? So you can't impart any kinetic energy into a photon because it has no capacity for kinetic energy, and you can't take any away because there is none to begin with. Perhaps you could slow down a photon, but that's different.

 

[ninja_tux]

OK I get it now...

so if I was moving 186,000 miles per second, and I pointed my laser in front of me, It would appear to be off, since I am theoretically moving at the same speed as the light coming from the laser.

However, here is where it gets different, a rock has mass, light has none.

exactly-the laser would simply pool up next to you! How weird is that... I might have it, though.

No, this is not true. If you are going 186,00 miles per second and turn on your laser you will still see the beam go forward as if you weren't moving at all. This is because of the Second Postulate of Special Relativity which states that the speed of light is INVARIANT in any frame of reference. So basically, no matter how fast you are going the speed of light is still going to be the same relative to you.

The situation with light is NOT the same as if you were to, say, throw a rock forward on a moving train. If the train is going 10 m/s and you throw the rock forward at a speed of 5 m/s (relative to the train) then the speed of the rock relative to the "stationary" ground is now 15 m/s; this situation is fairly intuitive and most people can get this concept.

However, light is special, since the speed of light in vacuum is defined by certain laws of physics, it MUST be constant in all points in the universe, so in each "frame of reference". Considering the train scenario once more, if the train is going 10 m/s, and someone points their laser forward, the observed speed of the photons relative to the train AND the "stationary" ground would be 3*10^8 m/s (assuming it's in a vacuum). What happens is that the very fabric of space-time changes to allow for the speed of light to remain constant for both frames of reference. It's some pretty mind-boggling stuff, as was the case with most of Einstein's work

I'm not sure if that explanation makes any sense...I may not have done a great job haha but if you want a more complete explanation I would suggest reading the howstuffworks.com article I linked to earlier.

And of course you can slow down photons, it happens all the time. For example, when you shine your laser in a room, what is actually happening is: a photon exits the laser, interacts with an air molecule and is physically absorbed by the electrons, which then "jump" to a higher energy level; the electron then falls back down to the original energy emitting a new photon. There is a minute time between this emission and absorption which effectively slows down all the photons. Different materials have varying speeds at which light pass through them, some (most notably "Bose-Einstein Condensates) slow down the emission/absorption process so much that you could literally walk faster than the light passes through them.

 

[zaery]

exactly-the laser would simply pool up next to you!

No.

If you were an observer on the outside, it would look like the laser was just pooling up in front of the laser-user. If you actually were the laser user, then it would look like the laser is working just like any normal laser.

Light always moves at the speed of light relative to the observer.


Edit: Man, ninja_tux being a ninja_poster..... *grumble*

 

[ossumguywill]

^that sounds like a huge pain in the ass for space-time to do, and a pretty damn complex solution to a problem... Why would the light pool up in front of the user to the observer, but not to the user? That seems counter intuitive... Won't the light appear to be in the same place, and if not why is that?
and please explain this to me, ninja_tux, if the user is moving at the speed of light and to him the laser is moving at the speed of light, then to a person standing still is the light not moving at twice the speed of light?

 

[zaery]

^that sounds like a huge pain in the ass for space-time to do, and a pretty damn complex solution to a problem... Why would the light pool up in front of the user to the observer, but not to the user? That seems counter intuitive... Won't the light appear to be in the same place, and if not why is that?
and please explain this to me, ninja_tux, if the user is moving at the speed of light and to him the laser is moving at the speed of light, then to a person standing still is the light not moving at twice the speed of light?

It's not a solution to a problem, it's just the way things are.

It's called special relativity:
The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.
It's not my fault, this guy said so:

 

[photonaholic]

my hero!

 

[zaery]

my hero!

He's the best, he made a ton of paradoxes. Most of them had something to do with time dilation or special relativity.

By the way, time dilation makes faster-than-light travel impossible :cryyy:

 

[ninja_tux]

^that sounds like a huge pain in the ass for space-time to do, and a pretty damn complex solution to a problem... Why would the light pool up in front of the user to the observer, but not to the user? That seems counter intuitive... Won't the light appear to be in the same place, and if not why is that?

Well Zaery was half wrong, it DOESN'T appear to pool up to anyone, light from a laser beam will always look like a laser beam, and it will always appear to go at the speed of light no matter how fast any observer is going.

I agree in that it's rather difficult to grasp, but it's just how the universe works. (If you choose to believe that crackpot Einstein)

and please explain this to me, ninja_tux, if the user is moving at the speed of light and to him the laser is moving at the speed of light, then to a person standing still is the light not moving at twice the speed of light?

Yes, so if you are moving at the speed of light, fire your laser forward, relative to you it is moving at the speed of light. If the person standing still were to measure the speed of the photons in the beam they would also be observed to be the speed of light as well. This does seem counter-intuitive, but it physically cannot happen any other way, otherwise the photons would be violating the laws of physics. The speed of light is defined by these laws, it is a universal constant that never changes.

Now, you might think about it in this sense "Well the speed of light isn't actually changing...just the observed speed is!" This brings us back to Einstein. Einstein postulated (and just about everyone in the last hundred years has agreed) that the laws of physics hold true in all frames of reference, which essentially means that no matter where you are in the universe, the same laws of physics apply to you. Since the laws of physics must remain constant, then so to must the speed of light. In order to compensate, according to Einstein, space-time must change such that the speed of light is always observed to be the same value.

I'm sorry if that wasn't a good enough explanation again...haha it's an admittedly strange concept which many people have difficulty comprehending. But if you take nothing else from that, just remember that the speed of light is the same no matter who observes it or how fast they are going, light is just THAT special

 

[ninja_tux]

He's the best, he made a ton of paradoxes. Most of them had something to do with time dilation or special relativity.

By the way, time dilation makes faster-than-light travel impossible :cryyy:

Well to be fair most of those paradoxes have solutions

I vote that we don't derail this thread further by turning this into a "faster-than-light" debate, since in my experience it becomes more an issue of semantics than anything else, and really who wants that?

 

[zaery]

Well Zaery was half wrong, it DOESN'T appear to pool up to anyone, light from a laser beam will always look like a laser beam, and it will always appear to go at the speed of light no matter how fast any observer is going.

To the observer, who is presumably not moving, the laser light looks like it's going at the speed of light, but it's also followed by a person holding a laser pointer, creating more laser photons right behind it, at the speed of light.

And as my final statement, i say go read up on things like this, it's great fun to try to wrap your brain around things like this, if I have anything more to say about it, i'll just make a new thread

 

[ossumguywill]

^OK, I've read like the first 10 pages on the howstuffworks thing on special relativity, but I'm tired and it's not really making sense right now. maybe I'll try again later.

 

[photonaholic]

if I have anything more to say about it, i'll just make a new thread

Why, this topic fits well right here.....