Below post is a back-up of my posts in my Quora blog. I take a copy of them to here in case if something goes wrong in there.
(If you somehow have ended up on this page without any background knowledge, before continue, you better know that all below scenario is based on the idea that our universe is expanding in time -4th dimension- direction with speed of light. You can check the previous 2 posts of: 1. Do We Travel in Time Direction With Speed Of Light and 2. Expansion of the Universe as Travel in Time Direction if you are willing to understand this concept)
One of my biggest bemusements about “travelling in a 4D universe with the speed of light” idea for me was; what would be the consequence on the objects which travel with that speed. I guess, I finally have an opinion and it is actually pretty simple.
As we usually do, in order to figure out what will happen in any 4D case, we can have a look at its analogue in 3D and try to apply the same logic to our 4D scenario. In this case, we should first understand what is happening in 3D.
In wikipedia description of the Length contraction it says;
Length contraction is the phenomenon of a decrease in length of an object as measured by an observer who is traveling at any non-zero velocity relative to the object.
and futher it adds;
Length contraction is only in the direction parallel to the direction in which the observed body is travelling.
So, when an object is in motion, the observers outside of its reference frame sees that it gets shortened on its travel direction. But at the same time, since everything is relative to each other, the object which is in motion observes the other objects in its rest frame are being length contracted while not observing any length contraction on its own body. The travelling object also sees that the length contraction of the other objects is only happening on its travel direction.
The amount of the length contraction can be found by using the formula:
where;
L0 is the proper length (the length of the object in its rest frame),
L is the length observed by an observer in relative motion with respect to the object,
v is the relative velocity between the observer and the moving object,
c is the speed of light,
So, all you need is to know the proper length and the velocity of an object. If you insert your velocity same as the speed of light into the formula, contracted length becomes “zero”. If you want, you can interpret this as; an object which travels with the speed of light, will be infinitely length contracted and its length on its travel direction will become zero for other observers (the ones in its rest frame). Meaning, our 3D objects will turn into 2D objects for the observers in its rest frame.
More interestingly, as I wrote above, the object in motion will also see all the other objects around it (the ones in its rest frame) as infinitely length contracted. We can also interpret that as; for an object which travels in 3D space with the speed of light, the space around and everything else in its rest frame will be infinitely length contracted on its travel direction and it will become a 2D universe for that traveler.
Now we have everything that we need to know to investigate our 4D case. The most important thing to consider is:
Length contraction is only in the direction parallel to the direction in which the observed body is travelling.
With that feature, comes our conclusion;
In a 4D universe, if any object is in motion with the speed of light, all the universe will be infinitely length contracted in its travel direction and will turn into a 3D universe for that traveler.
I am not a physicist but am pretty sure that this conclusion is not against the Relativity Theory (Except the fact that any object with mass cannot travel with the speed of light. That can be true for a 3D universe but can be different for a 4D universe model, I am not so sure). And if that’s true, there is no way for us to tell if we are living in a (3D only) universe or travelling in a 4D universe with the speed of light, therefore observing it as a 3D one. I put this conclusion in my pocket in favor of this universe model for now. Just wanted to note it here for the record.
