Consciousness is not part of the observer effect (which is itself named in the most infuriating way possible, specifically because it makes people think that the universe is somehow aware of when something sentient is looking at it). "Observing" a particle requires interacting with it in such a way that you meaningfully affect its current state of being, whether that be deflecting it in a different direction than it was going or changing its velocity, and therefore it is impossible at a quantum level to be a passive observer that does not influence the outcome.

In the case of the double slit experiment, if unobserved light will act as a wave with interference and if observed then it acts like a particle. The reason for this is both complicated and simple: light behaves as a wave due to probability. There's no way of observing a photon without influencing it, so therefore the best we can do is say it has a certain probability of being in this collection of spaces, which in the case of photons is a wave (because it can travel in any of a number of directions outwards from the photon emitter in the experiment, but all going away from the emitter and towards the wall the slits are cut into). For the purposes of this probability wave, the start position is the emitter and the end position is the wall behind the slits, so averaging out a large number of photons will recreate the interference pattern on the wall.

However, if you observe the photons at the slits to try and figure out which slits they're going through you have influenced the photons and thus collapsed that probability wave into a particle, and in the process created a new probability wave from that moment onwards which has the same end position as the original wave, but now starts at the individual slit. From its perspective, there is no second slit, so now the wave acts as if it is in the single slit setup because from its perspective it is, hence the loss of interference.

Nothing here has anything to do with consciousness. You can recreate this experiment with no one in the room and it will behave exactly the same, and has a sound (if very confusing conventionally) mathematical cause.

On a side note, string theory is effectively unfalsifiable and therefore completely useless as a scientific theory.