Is it possible to travel to the past or the future, and is there any Shar'i (Islamic legal) objection to that?

Time travel is merely a mathematical function; it cannot be separated or abstracted. What occurred in the Isra’ and Mi’raj was a compression of time, not travel through it. The claim of traveling to the future constitutes disbelief because it inherently involves claiming knowledge of the unseen. As for returning to the past, it is impossible, for the legal evidences prohibit it. If it were permissible, then humans would be able to witness past nations and the creation of the heavens and the earth, which is negated by the noble verses, such as His saying, the Almighty: ﴿I did not make them witness the creation of the heavens and the earth, nor their own creation﴾ (Al-Kahf/51).