In his 1999 book "Entering Space: Creating a Spacefaring Civilization", Robert Zubrin mentioned checking the math for Bussard ramjets with Dana Andrews in their 1988 join paper "Magnetic Sails and Interstellar Travel" and found that they aren't capable of reaching more than a few percent of the speed of light before drag overcomes propulsion:

https://www.goodreads.com/book/show/54728.Entering_Space

https://www.researchgate.net/publication/236447908_Magnetic_...

That's not to say that they don't work. But they'll probably be used primarily for braking to enter orbit around destination stars.

Probably the only way to reach a high fraction of the speed of light is to construct a giant laser to beam energy to a spaceship (which uses a reflector to receive light pressure momentum) and leave it behind orbiting the origin star. That's the premise of the Breakthrough Starshot project, which is ambitious with today's technology. But with self-replicating makerbots, building one may not be a big deal.

https://en.wikipedia.org/wiki/Breakthrough_Starshot

Unfortunately the force of light pressure (by F=2P/c for full reflection) is only about 2/3 of a kg or 1.5 lbs per GW, so a TW or greater would be needed for practical thrust. However, light pressure becomes the most efficient form of propulsion above about 25% to 50% c, if fusion or antimatter is used to create a gamma ray rocket.

Personally, I find it unlikely that aliens use these methods. I think that they probably worked out how to build neutrino lasers, since they don't burn up objects in their wake, perhaps by scaling superradiant Bose Einstein condensates:

https://arxiv.org/abs/2412.11765

In embarrassingly oversimplified layman's terms, I think that works by recruiting the macro-scale quantum state of the condensate (increased cross-section or barn) to overcome the short interaction distance of the weak force. Or by cooling the atoms to such an extent that they don't have the energy to hold themselves apart anymore, which accelerates their decay. I'm sure my explanations are wrong somehow.

Soon we may be able to investigate stuff like gravity waves and how the fabric of spacetime may be able to rebound above flat to create tiny ripples that allow mass energy to escape black holes, for example. I know that current theories don't state it quite that way, but if we consider stuff like the no-hair theorem and black box thought experiments, it's hard to see how Hawking radiation could exist without the wavelike nature of spacetime. We can even experiment with it on a relatively large scale by measuring the Casimir force. If we can rebound space, then we can play with stuff like negative energy and Alcubierre drives.

I looked up a Dyson sphere made from Mercury and it would be 1.5 mm thick, so aliens almost certainly aren't building them. But Dyson rings and swarms are probably a thing.

Some people in the tinfoil hat crowd think that UFOs can move 4th dimensionally and just travel orthogonally to our space and appear somewhere else. Theoretically, that might only require the energy difference (delta v) between planets. That hinges on if gravity spans higher dimensions and also touches on the multiverse. I'm way outside my wheelhouse so I'll stop blabbering about that.

In all honesty though, I question whether aliens travel. I think civilizations ascend about 10 years after they implement AI, or annihilate themselves in a Great Filter, their equivalent of WWIII. We're already staring the secrets of the universe in the face with automated theorem provers. And FUD around that and other accelerating tech drives people to become Luddites and elect amoral people who would gladly see the world burn for profit. So things could go either way really.

In my heart, I feel like we have a childlike understanding of consciousness. It probably transcends 4D spacetime. It's not hard to imagine aliens scaling what was learned from the CIA Gateway Program and doing stuff like FTL message passing via remote viewing. At that point FTL teleportation comes into the realm of possibility, sort of like in Dune.

If so, then aliens are probably everywhere, know about us, and maybe had a hand in our evolution. The probably live in what we think of as a Matrix, where years could go by for every second of our time. Another interpretation might be that they're able to return to source consciousness and exist as one, rather than in separation like we do. Maybe they periodically choose to reincarnate in us to study what transitioning to a spacefaring civilization looks like.

I probably shouldn't have bothered writing all of this, but it's Sunday, and I also really don't want to do my taxes.