Science aims to explain things, and by extension to explain everything. Is this even possible? Suppose the Universe consisted of a box, 20 million metres in each direction. Scientists inside this box could investigate this universe, but could they explain everything about this universal Box?
Suppose that the Box had impenetrable walls, so scientists could not probe outside of it. So they could say that the width, height, depth of the universe was 20 million metres and they could describe what was in it. They could also say that one side of the cube attracted everything in the Box and that side could be labelled “down” and the opposite side “up”.
There also might be statistical laws, so that the temperature, on average, might be 20 degrees Celsius, but could differ from that norm from place to place and from time to time. Box scientists might determine that everything appeared to be made up of tiny indivisible particles. Box atoms.
Some Box philosophers might ponder what was beyond the limits of the Box. They’d ponder the fact that starting from one side of the Box, one could travel 20 million metres in a perpendicular direction, but one could not travel 20 million and one metres. Why not?
I’m sure that they would have plenty of theories. For instance, one philosopher might contend that the Box was embedded in an infinite impenetrable bedrock, while another might say that it was obvious – the Box was embedded in nothing. No space, no time, no thing!
Meanwhile scientists probing the Box atoms might split them and discover a whole new world of sub-atomic particles. Others might conceive of space in the Box as being a seething mass of pairs of virtual particles, being created and moving apart for a brief instant and then merging into nothing, no thing, again.
But, says one bright spark, what about a particle pair created on the boundary of the Box? One particle would enter the Box, and the other would travel somewhere else! This would lead to other speculation – if the second particle travelled in another Box, then that other Box would presumably be a mirror image of our Box!
Such speculation would wait on experimentation by the Box scientists and I’m aware that I cannot push the Box analogy too far with out it breaking. But, just as in the case of the Box scientists, philosophers and scientists in this Universe have similar issue.
In our Universe there are no bounds (under current theories, I believe) but that doesn’t mean that we can’t speculate about what is beyond our Universe, whatever “beyond” may mean in this context.
The Box scientists could potentially explain every thing in the Box, maybe even the fact that it had existed, pretty much unchanged (on average) for all time, and that is periodically, over astronomically long time scale is doomed to repeat itself, time and time again.
When they go further than that, it is pure speculation, as all the data that they have relates to the Box. They have no data from outside of the Box. All the waves and particles that are observed originate in the Box. All the forces and fields are part of the Box. While scientists may speculate about “other Boxes”, that is all that they can do.
That’s the problem. The Box scientists, and the scientists from our Universe, can only observe events in the Universe in which they are embedded. Observations relate only to events in the local Universe.
Some conjectures suggest that our Universe is one of many universes all linked together in some way. Some conjectures suggest that the laws of our Universe apply in many other similar universes separate from ours. Some people conjecture that universes may exist where there are no laws or the laws that there are have no similarity in any way to the laws of our Universe.
In the Box universe these conjecture would amount to ideas that there may be other Box universes out there with similar laws to the Box universe, maybe linked in some way to the hypothetical Box universe. There may even be universes which have laws which are not at all similar to those of the Box universe. For instance a universe which springs from a single point in a vast explosion and expands at a vast rate either forever or to a certain point only to collapse once again. How bizarre!
The Box scientists would not have any way to decide whether or not their were any other Boxes as their observations would only observe events in their own Box. The only way that events in one Box could possibly affect the events in another Box would be if there was a link between them in some way.
This doesn’t necessarily mean that the event would be observable as the effect of one universe on the other universe. It would just appear as an event in each universe as it transpires as a result of the laws of the universe in question.
The theory may posit a link between two universes but the events in one universe can only result from events within that universe. If this were not so, the event in the universe would appear to happen without any causation in the universe. In other words it would be an anomaly or a miracle.
In other words, suppose a scientist in one universe knows of a law where he can cause an effect in another universe. If he can cause this effect in his universe then in the other universe something will also appear to cause this effect. Maybe this cause will be a scientist in the other universe trying to create an effect in the first universe!
This possible symmetry of cause and effect across more than one universe would mean that it would be difficult if not impossible to detect the presence of another universe by its effects on our universe.
The person in the Box universe would likely be in the same position. This means that he would never know if there were anything outside of his 20 million metre cube. He could postulate an infinite series of Boxes stacked like bricks in an endless array. Or he could postulate Boxes grouped into “houses”. Or he could postulate that his was the only Box and that speculations about universes started from “Big Bang” explosions are mere fiction.