What do you think is truly impossible in the universe?

[Maximum7]

What science, technology, astronomical bodies, energy sources or engineering projects truly cannot exist in the universe?

[firestar464]

violation of thermodynamics

[wjfox]

Perpetual motion machines.

[Bird]

Faster-than-light travel.

[Tadasuke]

Going back in time probably.

Though it's possible to slow down time, you would still move forward in time, I don't think it's really feasible to go backwards. But there will be virtual simulations.

[Vakanai]

I hope I am wrong, I truly, truly do.

1. Warp/Alcubierre Drive
2. Traversable Worm Holes
Worried the amount of exotic energy needed will put an end to such dreams. Which means we will never colonize our galaxy, unless we spend tens of hundreds or millions of years to do so. And billions of years to colonize other galaxies to any great extent.
3. Because of the first two, Time Travel (pretty much this has to exist in some form if the other two prove possible)

[wjfox]

1. Warp/Alcubierre Drive

If we can harvest antimatter, it would greatly improve the available energy. We're talking billions of times more than chemical-based rockets. I believe we'll achieve this, and it's scientifically possible. But likely centuries away.

[firestar464]

It is possible

https://phys.org/news/2024-05-subluminal-warp.html