I usually tell my parents "it's impossible to go faster than light", but they keep telling me "soon" (soon it will be possible to go faster). I know this is impossible in theory, but how much can I trust theories? Are there other things that in theory thought to be impossible but are not anymore (either because the theory was wrong or any other thing)?
Are there other things that in theory thought to be impossible but are not anymore?
It is said that the field of Chemistry came about from the desire of Alchemists to turn lead (or other "base metals") into gold (achieving transmutation of elements). When the periodic table of elements was discovered and chemical reactions started to become well understood, it was believed that transmutation was impossible because it had never been observed in any chemical process, and it was thought that only chemical processes could change the properties of matter.
Eventually, however, radioactivity was discovered, and soon nuclear processes that could transmute elements became known. Eventually, transmutation of lead into gold was achieved (of course, at costs far exceeding the value of the gold, but still a significant feat of science and technology).
I know this is impossible in theory, but how much can I trust theories?
A theory is considered successful in science if it explains all relevant observations that have been made to date.
It is possible that the universe allows faster than light travel, but we have no observations of such phenomena to date[*] and no reason to believe that this will be discovered "soon".
[*] - It appears from some experiments that the universe must be bridging information across points in space faster than light in some cases, but in a way that cannot be harnessed to transfer information faster than light. https://en.wikipedia.org/wiki/Faster-than-light#Quantum_mechanics
A recent and famous example is the accelerated universe. https://en.wikipedia.org/wiki/Accelerating_universe
Along the history of mankind people didn't know how big is the universe and what governs it's evolution. Until the very beginning of the 20th century the scientific consensus was that the universe is static (even Einstein thought so, a fact which lead to the famous story about the gravitational constant). Hubble's observations changed this consensus in 2 ways - (1) the universe is large, far larger than anyone thought before. (2) it's expanding (!) , i.e all the galaxies move away from all the other galaxies.
Until the very end of the 20th century, the consensus was that the universe is expanding, but it's expansion should decelerate due to gravitational pull of each galaxy (prediction of every gravitational theory - Newtonian and Einstenian). In 1998 two independent teams wanted to measure quantitatively this deceleration, but instead the experimental data suggested that the expansion of the universe is not decelerating but accelerating! (it's very important fundemental notion in our current understanding of the universe and it's dynamics). Currently there is no accepted theory to explain this phenomena (the reccuring idea in disscutions about that is "dark energy", which no one has an idea what it is, so it has nothing but a cool name).
That's a big question that bugged mankind for thousands of years and was answered (at least experimentally) less than 20 years ago.