Navid, if you only say "bridge" and "longest in the world", it is obvious to an English speaker that you are comparing the bridge to other bridges. If you comparing the bridge to something that is not a bridge, that is the only time that it is necessary to add any distinction. For example, you can say "this bridge is the longest structure in Iran". Then people know you are comparing the bridge to other structures in Iran (such as dams, pipelines, or tunnels) and not just to other bridges in Iran. If you just say "this bridge is the longest in Iran" people understand that you are comparing this bridge only to other bridges in Iran since you did not say anything about anything else in that sentence. We call this "context" (knowing what the speaker is talking about by listening to the other words in the sentence and not just the word we have a question about). You can "this bridge is the longest in the world", "this is the longest bridge in the world", "this bridge, compared to others, is the longest in the world", or "this bridge is the longest in the world, compared to others" and as long as you are only comparing to other bridges those examples I gave would be correct and all have the same meaning.

Hi, thank you Mr Paul

We use the reciprocal pronouns each other and one another when two or more people do the same thing. Traditionally, each other refers to two people and one another refers to more than two people, but this distinction is disappearing in modern English.

Hi Navid, You could say "this bridge is the longest in the world" and that would be fine Hope this helps