Lasers in Space

[Bill Thompson]

Two lasers are fired into space at 90 degrees to each other. They will remain at 90 degrees as far as we can observe. Across the length of the galaxy they will seem to be exactly at 90 degrees.

But things on a large scale in the universe cannot really be represented in tangible models. They can really only be represented in equations.

After time, although the lasers are traveling in precise straight lines, they will be parallel to each other. In time, they will even cross. After traveling 156 billion light years, they will even return to where they started.

Also, if we have two lasers pointed in opposite directions to each other fired into space, they will eventually come crashing into each other head-on. then, after they have traveled 156 billion light years, they will return to where they started.

Also, if we have two lasers launched into space parallel to each other, they will eventually cross paths. Then they will cross paths again. Then they will end up exactly where they started.

Something else. Space has no middle or edge. And the big bang happened everywhere. The background radiation from the big bang is everywhere and is constant. Space and time were created durig the big bang. The blast did not "go out", in a 3D space way, into anything.

[DC]

After traveling 156 billion light years, they will even return to where they started.

what? why?

:confused:

[bluesjnr]

Exactly what is your point?

[Dave Rogers]

Two lasers are fired into space at 90 degrees to each other. They will remain at 90 degrees as far as we can observe. Across the length of the galaxy they will seem to be exactly at 90 degrees.

No, they won't, unless there is no mass present anywhere in the galaxy. The presence of gravitational fields will alter their directions of propagation according to General Relativity.

Dave

[Andrew Wiggin]

The 'asteroids video game' model of space... When the little triangular spaceship flies off one edge, it reappears on the other.

In other news, can we say 'unsupported assertions not backed up by fact or logic'? I've often heard this assertion, with equal amounts of nothing to back it up, but I've never heard any convincing argument that requires space to be this curved or to have this property.

I wrote a lot more, but then deleted it. I'll save it for later if and when it becomes apparent that this isn't equivalent to the iron sun assertions and such.


A

[ArcturusA]

Back when I was taught cosmology and GR, I remember being told that if space was curved such that it was closed, so that in theory it would be possible to set off in one direction and return to where you started, it wouldn't be possible in practice.

The idea is that if the universe is closed, it will eventually collapse and lead to a Big Crunch. The time from Big Bang to Big Crunch is necessarily the same time for light to traverse the entire universe and return to its original point. I'm sure I could dig up the proof if anyone's interested.

[edd]

ArcturusA - that assumes that there's no odd stuff going on. Unfortunately odd stuff is going on (http://en.wikipedia.org/wiki/Dark_energy) and a closed universe need not recollapse.

Bill Thompson - there's no good evidence the universe is closed. It looks flat. Your 156 billion light year number is an incorrect lower bound.

And
Also, if we have two lasers launched into space parallel to each other, they will eventually cross paths. Then they will cross paths again. Then they will end up exactly where they started.
is also potentially ill-founded even if the universe is closed. It might be true on some kind of 3-sphere but it isn't true on, for example, a 3-torus.

[ArcturusA]

ArcturusA - that assumes that there's no odd stuff going on. Unfortunately odd stuff is going on (http://en.wikipedia.org/wiki/Dark_energy) and a closed universe need not recollapse.

Good point. It was all very idealised and near the start of the course, so we must have omitted dark energy at that point.

[Complexity]

(snipped for silliness)

But things on a large scale in the universe cannot really be represented in tangible models. They can really only be represented in equations.

(Snipped for irrelevance and silliness)


Give us an example of a 'tangible' model.

What is the distance threshold beyond which 'tangible' models can no longer be used to represent 'things'.

'only' represented in equations? What nerve! What do you think is used for modeling/describing reality?

How much mathematics do you understand?

[HansMustermann]

Who cares about lasers in space. I want mine on frikken sharks. Is it so much to ask? ;)

[Mirrorglass]

Who cares about lasers in space. I want mine on frikken sharks. Is it so much to ask? ;)

Would it be fair to grab a laser-wielding shark by the tail? (http://forums.randi.org/showpost.php?p=5769463&postcount=13)

[theprestige]

But things on a large scale in the universe cannot really be represented in tangible models. They can really only be represented in equations.
So you'll be showing us a bunch of equations, then?

After time, although the lasers are traveling in precise straight lines, they will be parallel to each other. In time, they will even cross. After traveling 156 billion light years, they will even return to where they started.
Wait, where's the equations?

Also, if we have two lasers pointed in opposite directions to each other fired into space, they will eventually come crashing into each other head-on. then, after they have traveled 156 billion light years, they will return to where they started.
That's funny, I was certain you said this outcome could only be expressed by equations. Where are the equations?

Also, if we have two lasers launched into space parallel to each other, they will eventually cross paths. Then they will cross paths again. Then they will end up exactly where they started.
Equations! Do you have them?

Something else. Space has no middle or edge. And the big bang happened everywhere. The background radiation from the big bang is everywhere and is constant. Space and time were created durig the big bang. The blast did not "go out", in a 3D space way, into anything.
Protip: Non sequiturs are not equations.

[Skeptical Greg]

Has anyone mentioned that lasers don't go anywhere ?

But assuming that Bill means " light from a laser " , where does Bill plan to get a laser with the energy to accomplish what he describes ?

The Mote In God's Eye anyone ? (http://www.amazon.com/Mote-Gods-Eye-Larry-Niven/dp/0586217460/ref=sr_1_1?ie=UTF8&s=books&qid=1271432234&sr=8-1)

[Brian-M]

In time, they will even cross. After traveling 156 billion light years, they will even return to where they started.

http://en.wikipedia.org/wiki/Laser
Thus, a beam generated by a small laboratory laser such as a helium-neon laser spreads to about 1.6 kilometers (1 mile) diameter if shone from the Earth to the Moon.


If a small laser beam is a mile wide after only 1.3 seconds of travel, I wonder how wide they'll be after 156 billion years?