[stevemc2]

I wonder what would happen if one somehow lived on a planet a few meters from the edge of the universe, and then threw a baseball towards the edge of the universe, at a velocity faster than the expansion rate.

Would the baseball then:

a) bounce back

b) disappear

c) get stuck at the boundary

d) cross the boundary and cease to exist

e) none of the above

[Profpat]

Originally Posted by stevemc2 I wonder what would happen if one somehow lived on a planet a few meters from the edge of the universe, and then threw a baseball towards the edge of the universe, at a velocity faster than the expansion rate. Would the baseball then: a) bounce back b) disappear c) get stuck at the boundary d) cross the boundary and cease to exist e) none of the above

Hi Steve, welcome to the TOE Quest forum.

A good question at the start of baseball season.

I guess I would pick e) none of the above. I vision right outside our universe is the void, which becomes part of the space/time continuum as soon as something enters it. Purely a guess mind you, since I've never hit, or threw, a baseball that far.

In other words if the baseball is going faster than the expansion rate, that would become the new expansion rate for our universe, since it would be part of our universe.

Best to you,

Pat

[neutralino]

Unfortunately, there is no such thing as the "edge of the universe" so the question is not defined.

[Mohan.C]

Originally Posted by stevemc2 I wonder what would happen if one somehow lived on a planet a few meters from the edge of the universe, and then threw a baseball towards the edge of the universe, at a velocity faster than the expansion rate. Would the baseball then: a) bounce back b) disappear c) get stuck at the boundary d) cross the boundary and cease to exist e) none of the above

Steve,
What is happening to the planet as the universe expands. Is it staying still or is it moving away with the same velocity as that of the expansion.
Anyway I don't know why it should cease to exist if it is swallowed by our universe. It can exist in our universe.
And I think it will not be retrievable by the guy on the planet if the gravity of our universe acts only within the confines of our universe.
It may get stuck you know if the gravity on the planet is same as that of the gravity of the universe then both will be contending who wants the ball.
It would bounce back if the gravity exerted by the planet is more than the gravity exerted by the universe on the ball.

Who knows that planet may just be another universe in that case we will have to make a universal system like the planetary system.
Good one though, for a thought experiment!!

[neutralino]

Hmm.. no-one seems to be listening to me today . Anyway, like I said above, there is no edge/boundary to the universe. Furthermore, there cannot (by definition) by anything existing outside the universe-- by definition, the universe is all matter, energy and spacetime that exists. Thus, if you say there is some matter, or even spacetime existing outside the universe, then this is a contradiction.

[stevemc2]

Originally Posted by neutralino Unfortunately, there is no such thing as the "edge of the universe" so the question is not defined.

Originally Posted by neutralino Hmm.. no-one seems to be listening to me today . Anyway, like I said above, there is no edge/boundary to the universe. Furthermore, there cannot (by definition) by anything existing outside the universe-- by definition, the universe is all matter, energy and spacetime that exists. Thus, if you say there is some matter, or even spacetime existing outside the universe, then this is a contradiction.

I spread out my thoughts here for clarity...

I don't understand the concept that there is no "edge/boundary" of the universe.

If space-time has been expanding since the big bang, then by inherent definition it has an outer edge of it's expansion.

To say there is no "edge of the universe," then to me that means the universe is infinite.

But, it is not infinite, but has an age of 14 - 15 billion years old.

One can calculate that the size of the universe at 1 second was about the size of the earth. So, where I am sitting now as I type, this point in space-time 1 second after the big bang was non-existent, yet to be created by the expansion of space-time. (note I am interchanging space-time as a synonynm for "universe.")

And so let's call what's on the other side of the edge "The Void."

For the sake of argument, let's define The Void as "the region empty of everything including matter, energy, space and time, and importantly, no quantum mechanical zero-point "jittery" vacuum energy of particle/anti-particle oduction/annihilation.

And, I'll concede this "edge" may also represent in a way the "edge" or boundary between physics and metaphysics, thus the question could be indeed illogical in a strictly physical sense of the word, at this point the argument could be sent out as a GOTO instruction to a metaphysics forum.

But I'll keep harping on it as follows:

So, what's on the other side, The Void?

On a similar note, prior to the big bang, was there only "The Void"

(I won't confuse things here, although I'm tempted to put in one of Hawking's arguments that the big bang arose from quantum tunneling, but that would negate my proposed definition of The Void, as the Void is completely empty, including of something to start quantum tunneling. Hawking's theory may be fruit for another thread topic....)

Well, I hope to convince neutralino, or anyone else, that the universe has "an outer edge or boundary".

Thus, to restate my argument: The universe has an edge because it has been shown by measurements of the cosmic background radiation to have arisen from the big bang around 14-15 billion years ago, and has a finite size, which is expanding (for now.) The only way there is no edge to the universe is if can be proved the universe is infinite (i.e., remember the Steady State Theory?)

Thanks to everyone for the other replies.

About 15 years ago I asked Dr. Robert Kirschner - of Harvard-Smithsonian Center for Astrophysics - this question when I was at one of his Smithsonian Institution lectures, but forgot his answer, thus I am asking it again!! Maybe I'll shoot him an email and ask him also.

Stevemc2
(I'm tempted here to add a link to the Doors' song "Break On Through to the Other Side! )

[stevemc2]

Originally Posted by Mohan.C Steve, What is happening to the planet as the universe expands. Is it staying still or is it moving away with the same velocity as that of the expansion. Anyway I don't know why it should cease to exist if it is swallowed by our universe. It can exist in our universe. And I think it will not be retrievable by the guy on the planet if the gravity of our universe acts only within the confines of our universe. It may get stuck you know if the gravity on the planet is same as that of the gravity of the universe then both will be contending who wants the ball. It would bounce back if the gravity exerted by the planet is more than the gravity exerted by the universe on the ball. Who knows that planet may just be another universe in that case we will have to make a universal system like the planetary system. Good one though, for a thought experiment!!

Mohan C.,
thanks for reply, but to eliminate all these other forces, assume the planet is expanding at the same rate of the universe and the ball is thrown fast enough to overcome the planet's g force so that it meets the edge of the universe.
Maybe substitute shining a laser pointer at the edge, where do the photons go at the edge?

[neutralino]

Originally Posted by stevemc2 I spread out my thoughts here for clarity... I don't understand the concept that there is no "edge/boundary" of the universe. If space-time has been expanding since the big bang, then by inherent definition it has an outer edge of it's expansion. To say there is no "edge of the universe," then to me that means the universe is infinite.

Not necessarily. Imagine a sphere, and suppose we are living in a two dimensional world on the surface of this sphere. Note that we are not aware and cannot move in the radial dimension. Now, if this sphere expands, then we see, as beings living on the 2d surface, the distances between all objects increase; i.e. we see space expanding. Now, our surface is certainly not infinite, and it does not have a boundary. To move back to "reality," we could consider the three dimensional surface of a four dimensional sphere (which we could not imagine as easily!)

But, it is not infinite, but has an age of 14 - 15 billion years old.

Ok, so no, the universe is not temporally infinite, but when most cosmologists talk about the "universe being infinite" they mean to say "spatially infinite," which is something that could be true (we don't really know yet!)

For the sake of argument, let's define The Void as "the region empty of everything including matter, energy, space and time, and importantly, no quantum mechanical zero-point "jittery" vacuum energy of particle/anti-particle oduction/annihilation.

Whilst I know that some people here like talking about a "void," cosmologists do not mention such a thing (at least not in this context). Your definition of the void appears to be "non existence"-- since you admit that spacetime does not exist in the void, then there can't even be nothing there, but there is no "there." (Sorry, that sounds really philosophical, but I can't think of a way to explain it more succintly).

On a similar note, prior to the big bang, was there only "The Void"

What does "prior to the big bang" mean? How do you measure time "before the big bang" (if such a statement is even well defined!)

(I won't confuse things here, although I'm tempted to put in one of Hawking's arguments that the big bang arose from quantum tunneling, but that would negate my proposed definition of The Void, as the Void is completely empty, including of something to start quantum tunneling. Hawking's theory may be fruit for another thread topic....)

Indeed; could you also provide some links, since I've not heard much about that?

Well, I hope to convince neutralino, or anyone else, that the universe has "an outer edge or boundary". About 15 years ago I asked Dr. Robert Kirschner - of Harvard-Smithsonian Center for Astrophysics - this question when I was at one of his Smithsonian Institution lectures, but forgot his answer, thus I am asking it again!! Maybe I'll shoot him an email and ask him also.

I would like to think that Prof. Kirshner would agree with me! (It looks like he's been promoted since you heard him talk! )

[Profpat]

Hi Steve;

I sort of agree with Neurtralino, in that nothing exist " outside " of the universe, but I agree more with you and I that nothing is called the VOID. This allows for universal expansion, but simultaneously, as it's expanding it becomes the universe, consequently and theoritically the void does not and cannot exist in our reality.

The universe being the size of the Earth at 1 second doesn't fit our current model. Apparently the universe is 156 Billion light years wide even though it's onle 14 Billion years old. The latest theory is spatial inflation where within 10-35 to 10-32 seconds the universe expanded exponentially.

Inflation Theory

According to the latest cosmological model, the universe sprang into being about 14 billion years ago. At birth, the space was likely to have been curved and warped due to quantum effect within the tiny speck and time may be meaningless. After about 10-35 seconds, there began a brief period of exponentially fast expansion, known as inflation, that ironed out any curves or warps in space and made the universe flat (because it becomes so large). Inflation also predicts a much smaller initial region, which is required for smoothing out the distribution of matter and radiation, only leaving behind tiny quantum fluctuations that match the observed spatial variations in the cosmic microwave background radiation and provide the seeds for galaxy formation

Best to all,

Pat

P.S This expansion means the universe ( Space ) was expanding far faster than light speed. ( Another Mystery )

[neutralino]

Originally Posted by Profpat Hi Steve; I sort of agree with Neurtralino, in that nothing exist " outside " of the universe, but I agree more with you and I that nothing is called the VOID. This allows for universal expansion, but simultaneously, as it's expanding it becomes the universe, consequently and theoritically the void does not and cannot exist in our reality.

So long as this "void" doesn't exist (as you state here in the last line) then we agree

The universe being the size of the Earth at 1 second doesn't fit our current model. Apparently the universe is 156 Billion light years wide even though it's onle 14 Billion years old.

Actually, I think this figure is really a lower bound on the size of the universe (since it could be infinite).

Also, that's a nice description of inflation you give there, Pat!

P.S This expansion means the universe ( Space ) was expanding far faster than light speed. ( Another Mystery )

Well, it's not really all that mysterious!