Hacker News Reader

Comment by gary_0 | Hacker News Reader

No, not "observing your own past". Instead, your past self could observe your future self. And then your future self could turn their FTL ship around and run you over, killing their past self and creating a paradox.

https://old.reddit.com/r/askscience/comments/l9sba/why_does_...

View on HN

> No, not "observing your own past".

Being able to observe your own past is a given after FTL travel, and the most important thing: it's a given that relies on almost no assumptions about the deeper mechanics of the universe. If you instantly teleport to Alpha Centauri, you will be able to see your old Earth photons for years.

> Instead, your past self could observe your future self.

Now that statement (like the Tachyonic antitelephone) relies on some assumptions that we can't test yet. My hunch would be that either an as-of-yet unknown non-local temporal dimension (or synchronization mechanism) exists at the foundations of the universe, or that FTL travel is indeed impossible. One excludes the other would be my intuition.

Sorry if I don't fully understand, I've read through link a bit but it just leaves me with one question in particular: Wouldn't your relativity have to be both different and the same simultaneously for this to work?

In the tachyonic antitelephone example, Bob and Alice send messages that travel at 2.4c, but neither observers are at that speed. Wouldn't the limits of their observation and time dilation cause the messages to appear in order?

If you were to theoretically travel into a black hole, and then somehow escape out of it very quickly, you would see yourself still in the black hole, but you wouldn't see yourself coming out of it, because that light was further from the black hole and thus moving at a different speed?

> turn their FTL ship around

I feel like this is the part that would probabally be hard (ie not possible) when going faster than light, assuming such a thing is possible.

Also whatever magical space warp it is that lets you go that fast could presumably also obscure your ability to see yourself in the distance.

"Turning around" is just stopping at some point (ie. some inertial reference frame) and then accelerating in the opposite direction with your FTL drive. You have all the time in the world to do this maneuver because you're already effectively in the past: https://physics.stackexchange.com/a/54242

There's no absolute coordinate system in space, so there's no way to "stop". You can only change your velocity relative to some other object, e.g. your past self in the example. What counts is acceleration / deceleration.

Slowing down to reach the speed of light might be as hard as speeding up to reach it; I remember it played a role in the theory of tachyons, hypothetical FTL elementary particles which could not be slowed down.

> acceleration / deceleration

Another question for the thread: Anyone here feel like they can feel the Coriolis effect if they focus enough? I haven't travelled across the equator enough to know if the north/south jetlag is different from east/west.

How would you feel the effect? If you do it's more likely to be placebo as you can't avoid knowing where in the world you are!

I imagine if there is any amount of angular momentum imparted to the water in our cells, the reversal of the force propelling it would create a turbulent episode of transition.

The differences in force are way too miniscule on that scale. If you're looking for a measurable force differential across the extent of your own body, look at gravity instead. When standing up, the gravity at your feet is about 0.5 ppm smaller than at your head (for a body height of 1.75 meters). You don't feel that either; there is just no evolutionary advantage to being able to measure such tiny differences in local gravity, especially since we have acceleration sensors in our ears that are way more efficient at telling up from down.

The coriolis force differential, besides being rather small to begin with, has the additional problem that the coriolis force is proportional to the speed at which you're moving over the earth. If you were to accelerate to a speed where the coriolis force differential might be appreciable (e.g. by sitting in a bullet train or in a plane), it will be completely drowned out by any other momentary forces imparted upon you by the vehicle (trains or planes can move mostly straight, but not in a mathematical-grade straight line).

Reply on news.ycombinator.com