Why Humans Will Never See the Edge of the Universe
You can think of the edge of the universe
Mind experiments that we can do
Suppose we want to take a look at the edge of the universe.How should we do it? Let’s try by means of mind experiment.
A mind experiment is one that cannot be done by physical experiments involving materials and methods. If you want to find the weight of a stone, you may use a weighing scale and take the weight of the stone, according the Newton’s theory of gravity. You use a material (stone) and method (weighing).
Mind experiment is different. Albert Einstein related one of his mind experiments. He wanted to know what would happen if he traveled with light that travels about 300,000 km per second. He concluded by means of reasoning that: his body would contract and his time would stand still. That is, his time clock would register zero. The mass of his body would increase.
Einstein could not do a physical experiment because there is no spacecraft yet that can travel as fast as light.
A mind experiment sometimes does not accomplish what we want. However, it explains why we cannot attain what we want.
First, assume that the universe started with the Big Bang about 5 billion years ago. (I have a Hub "Big Bang Theory, Origin of the Universe: What Is the Mystery?) We are not going to prove this assumption in this mind experiment. It is a starting point. Einstein did such a thing with his theories, special theory of relativity and generalized theory of relativity. He made as an assumption the Lorentz transformation. For one, Lorentz transformation says that the laws of nature in one coordinating system (for example, galaxy) are the same as in another coordinating system. He did not prove this by physical experiment. Besides he could not because there was no spacecraft that could take him to another galaxy. Even up to now that spacecraft is still unavailable.
We use light as the tracing element. This is the only one available in real life. Besides, as we know nothing else travels faster than light. That one indicated by c2 (speed of light squared) in the formula E = mc2 does not show one direction. It is rather the speed of light particles (photons) bumping into each other. (One result is splitting of an atom into two, a chain reaction in a nuclear bomb.)
We use a powerful telescope. Let us say we can make a telescope more powerful than the Hubble telescope. But no matter the power of the telescope its power will be limited by the speed of light in one direction.
Our method of physical experiment is peeping into our powerful telescope pointed toward the edge of the universe. Can we see the edge of the universe?
Let us add more considerations. The universe is finite. So, it must have an edge. However, the universe is expanding according to the generalized theory of relativity. Let’s say that the expansion is such that a point in the edge of the universe is moving away as fast as light.
Our mind experiment will show that we cannot see the edge of the universe. Why?
Our tracer is light. If we start the physical experiment now, our tracer is way, way behind the edge of the universe that started with the Big Bang billions of years ago. Our tracer, because it is light, cannot catch up with a point in the edge of the universe. That point had a head start of at least four billion lightyears. (A lightyear is the distance traveled by light in one year.) At one point in time, our tracer is at one point and the edge of the universe is also at another point. There is a wide gap between these two points. That gap will never be reduced.
But how did Einstein arrive at the prediction that the universe is expanding and that it is finite? It is by means of deductive logic that is part of mind experiment. His mind experiment (generalized hypothesis on relativity) was couched in mathematics. But his mind experiment could only be proven valid because the test of mathematics is validity. How could his mind experiment be proven as true? The test of a statement is truth (Russell, B. Human Knowledge: Its Scope and Limits. 1947). Russell was a mathematician and philosopher; a Nobel prize winner in literature (philosophy).
So, Einstein derived statements of facts from his mind experiment. These statements were in the form of predictions amenable to verification by physical experiment.
The prediction that the universe is expanding was proven by Edwin Hubble, an astronomer. That is one proof of the generalized hypothesis of relativity. Another prediction was that light bends in the vicinity of a great mass. This was verified by Sir Arthur Eddington in 1919. A hypothesis that is proven true graduates into a theory. Therefore, Einstein's generalized hypothesis of relativity graduated into the generalized theory of relativity.
Suppose we have a spacecraft that travels as fast as light and we would like to see the expansion of the universe. One thing to consider is that we should have a fuel that can jettison our spacecraft and leave the space-time of our galaxy.
This universe is composed of several galaxies (Levy, D. Cosmology 101.2003); some of them are even larger than our own. We may be able to leave the gravity of our galaxy but encounter another that of a larger galaxy. So, our spacecraft will free fall around that bigger galaxy. Free fall means that our spacecraft will travel in a hyperbola and will not impact on that bigger galaxy. A free falling spacecraft does not need fuel for its travel. The trajectory of our spacecraft will only match the curvature of that bigger galaxy. It is unlike an apple that when thrown upwards will impact (fall) on earth (Swinerd. How Spacecraft Fly. 2008).
Even if we prolong our travel, we will only be circling the bigger galaxy. In reality we cannot proceed to see the expansion of the universe. Then how did Hubble find out that the universe is expanding?. He derived it from the red shift.
Today, we don’t have a powerful fuel that can jettison our spacecraft beyond our galaxy. The satellites that had gone to Mars are still within our galaxy. Granting that we have that fuel, our spacecraft will circle around a bigger galaxy owing to the latter's gravity.
In the generalized theory of relativity, gravity is a curvature of space-time. It is not a force (as measured by weight) according to Newton's theory. That is why, even if the spacecraft is weightless, it still behaves following Einstein's theory of gravity.
Suppose reaching the edge of the universe is possible. We will not know that we are at the edge because our spacecraft will be circling around the universe in a free fall. This last statement belongs in probability that cannot be verified today or in the long future. It belongs in the law of excluded middle (Russell, B. Human Knowledge: Its Scope and Limits. 1947). The reason is that there is no evidence in its favor or against it. It is a statement that is neither true nor false. It is not knowledge.
Other tracer elements can be used like neutrino that is said to travel faster than light. This claim of neutrino's speed is still doubtful though. We can use the cosmic microwave background radiation (CMBR) that was discovered by Arno Penzias and Robert W. Wilson in 1965. CMBR is supposed to be the debris of the Big Bang. Therefore, it is traveling as fast as the edge of the universe. CMBR is similar to light in that there is a gap between our tracer CMBR and the edge of the universe. The tracer CMBR cannot catch up with the edge of the universe.
There are mind experiments that we can do that we cannot do by means of physical experiments.