Both are perfectly correct but have different meaning by the writer. Everyone expressed doubt that he was hijacking -this implies that everyone was more certain that he was not hijacking. Everyone was sure he was not hijacking. The doubt is thus unreasonable as it were. Example: Person a: "I just got 100% on my English test!" Person b: "I doubt that, you never study" Here Person b, is feels certain that person a didn't get a100 , cause he *never* studies Next Everyone expressed doubt whether he was hijacking. -this implies the there is reasonable doubt here and the author is reminding us of the alternative he was not hijacking. Person b: I doubt whether person A will get a 100, he doesn't always study hard. In this example the doubt is more reasonable cause only sometimes he didn't study enough.

The meaning of both sentences is more or less the same, it just has a different emotive impact. In the sentence where you sue 'whether', a greater element of doubt is introduced. In this context, you have to say 'whether or not' he was hijacking to be grammatically correct.

If I doubt THAT you told me the truth, I am reasonably certain that you told me a lie. If I doubt WHETHER you told me the truth (or not), I am more uncertain: you are probably lying, but there is still a real possibility that you're telling the truth. Since you're also learning French, note that "se douter" would translate as "suspect," NOT doubt. Ils se doutent que je leur cache quelque chose.= They suspect that I'm hiding something from them.

Hi Victoria! Your question/sentence seems incomplete. You may want to add some details to it. Is it from a movie, are they hijacking a car or a plane? Using "whether" in a sentence, is expressing that there is an alternative to the situation, usually added by "or" then another alternative situation For this question, you may use: "We watched a movie earlier and everyone was confused whether the villains were hijacking the bus or just simply creating a chaos." "Everyone was doubtful about his actions whether he was hijacking the plane or he just simply went mad." More examples of "whether": I will attend the wedding whether I am invited or not. You should dance whether your partner is your best friend or your frenemy. The reunion will happen whether the venue is here or out of the country.

Yes, they have the same meaning. There are always several ways in English (and other languages) to express the same idea. Usually 'or not' goes hand in hand with 'whether'. ie. "Everyone expressed doubt whether he was hijacking or not" or ""Everyone expressed doubt whether or not he was hijacking". It would be more common as well to specify what was being hijacked (or not). ie "Everyone expressed doubt whether he was hijacking the plane or not".