# What happens to light when it reaches the edge of the universe?

Tags: edge, light, reaches, universe
 P: 40 What happens to light when it reaches the edge of the universe? it couldn't keep on travelling into nothingingness could it? becuase that nothingness would be space. it couldn't reflect off something unless there was matter at the edge to reflect off. the light must do something, it can't disappear can it? that would contradict the 'can't create or destory energy' law.
 Sci Advisor P: 2,507 It would keep going. "The edge of the universe" is the distance to which this light has gotten, (at least).
 P: 653 If the universe is the same everywhere - as stephen hawking says it must be to make calculations relevant -then light can't reach an "edge" because an edge would be different from the rest of the universe! On the event horizon of a black hole the kinetic energy of a mass = potential energy and this is the same for the furthest distance light is from the Earth - using the Newtonian idea that the universe is a sphere.The universe isn't a black hole, this just shows that the universe is like a black hole in that it won't allow a mass to escape from it.
P: 40

## What happens to light when it reaches the edge of the universe?

maybe space is so curved that light just gets bent when it reaches the edge and flows along the edge of the universe.
 Sci Advisor PF Gold P: 2,226 The universe (in all credibel cosmological models) doesn't have an edge, it's as simple as that.
P: 1,382
 Quote by fbsthreads What happens to light when it reaches the edge of the universe?
The universe's three dimensional space is curved back on itself in the same way that the 2-dimentional surface of the earth is curved back on itself. If you keep walking in one direction on Earth, eventually you will reach your starting point. Similarly, light travelling in one direction through the universe will eventually reach its starting point (if it travels long enough and outruns any expansion of the universe).

Because the universe is curved, if you could see far enough, and if there was a clear view in front of and behind you, you would see the back of your head. If you could see far enough and looked up, you would see the other side of the earth.

The implication here, though, is that whatever direction you look in you will see your starting place. Since this Earth-in-the-sky would logically have to be continuous, wherever you happen to be located at any given moment will be continuously projected in reverse against the "sky" of the universe.
 Emeritus Sci Advisor PF Gold P: 4,005 hitssquad's description applies if the universe is closed; if it's open you wouldn't be able to see yourself (and flat is in between). There's the small matter of the speed of expansion, which is not a question of the geometry of the universe. The WMAP site has a nice alien illustration of closed, flat and open.
 P: 196 Correct me if I am wrong, but there is no requirement for the universe itself to be moving(expanding) at or less than the speed of light, and with the recent discoveries concerning dark energy, it appears that its expansion is accelerating. Hence, the "edge" of the universe could be moving faster than the speed of light, and a "beam" of light couldn't reach it!
P: 40
 Quote by geometer Hence, the "edge" of the universe could be moving faster than the speed of light, and a "beam" of light couldn't reach it!
but if it is accelerating all the time then there must have been a time when light could easily reach the edge.

also, if big bang is to be believed then any light from the big bang 'explosion' would have been at the edge of the small universe in no time.

if light could never reach the edge, you could still see the big bang if you were at the edge.
 P: 196 The initial expansion of the universe was much faster than the speed of light (the inflationary period), and so the "edge" has probably always been receeding at faster than the speed of light.
PF Gold
P: 2,226
 Quote by geometer The initial expansion of the universe was much faster than the speed of light (the inflationary period), and so the "edge" has probably always been receeding at faster than the speed of light.
No:

It is possible for objects in the visible universe (with respect to some obsever) to have recession velocities faster than the speed of light, but, you have to relaize that the recession velocity of an object is depenednet on the distance of the observer from that object. A beam of light emitted by the observer would never recah your hypothetical edge, but it would be possible for a beam of light within the observable universe that is considerably closer to the edge to reach and go past that edge.

As I said before, whatever the geometry of the universe is (whether it is postive, negative, or flat curavture or whether it is infinite or finite), it doesn't have an edge, it really is as simple as that.
 Sci Advisor PF Gold P: 2,226 Basically expansion is global, not local.
 P: 358 When the vu the BB-scientists calculate the universe to expand in equals the speed of light, c, the ether will fall back to it's previous energylevel, and the gates god built will not stay in the condition it had earlier.
 Sci Advisor PF Gold P: 2,226 Sariah, though I siad exapnsion, is global not local, the measure rate of expansion is local not global, so the expansion rate is not a velocty, but a function of distance and velocity (it will give you the recession velocity v of two objects in a co-moving sphere of radius r) and therfore can't equal c as it has different units.
 P: 358 time went and radius grew. r/t is not c yet.
 P: 358 though the universe is not really expanding, i still think this is what happens when dr/dt = c
 Sci Advisor PF Gold P: 2,226 The rate of expansion of the universe is measure by Hubble's constant, which is equal to vr/d, so you should be able to see that even if Hubbles constant is very small, as long as the distance between the two objects (d)is large enough the recession velocity (vr) will exceed c. In an infinite expanding universe there must be a distance where vr exceeds c.
P: 26
 Quote by hitssquad The universe's three dimensional space is curved back on itself in the same way that the 2-dimentional surface of the earth is curved back on itself. If you keep walking in one direction on Earth, eventually you will reach your starting point. Similarly, light travelling in one direction through the universe will eventually reach its starting point (if it travels long enough and outruns any expansion of the universe).
if this was the case, would the darkness of space even exist? wouldnt there be everlasting light in every single direction at every single point of time?

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