5 ms·

Well, you can cross the universe in any amount of time. The human lifetime isn't special in that regard. A photon crosses the universe in zero time, and anythin

Well, you can cross the universe in any amount of time. The human lifetime isn't special in that regard. A photon crosses the universe in zero time, and anything with non-zero mass can get arbitrarily close to zero time.

dredmorbius 3y ago
As I've seen this point made in the past, it's that travelling at a constant 1g acceleration the subjective time for an astronaut would be about 45 years in traversing the observable Universe (~14 billion light years). That is, a comfortable and familiar acceleration gives a human-lifetime-equivalent journey (subjective time). If you're planning a round-trip, you'll find rather more time has transpired on what used to be Earth. <https://www.universetoday.com/129086/far-can-travel/ https://www.universetoday.com/129086/far-can-travel/>
- saalweachter 3y ago
  > If you're planning a round-trip, you'll find rather more time has transpired on what used to be Earth. Or less, depending on the rotation of the universe as a whole.
  - ben_w 3y ago
    Or no return may be possible at any speed/acceleration given the expansion of the universe.
    - dredmorbius 3y ago
      Given that the outbound journey is already technically challenging, that's very nearly quibbling, despite cosmological accuracy.
- heavenlyblue 3y ago
  That 1g constant acceleration requires practically infinite amounts of energy
  - dredmorbius 3y ago
    True. However it is also the specific context in which the lifetime-universe-traversal concept emerges.
denton-scratch 3y ago
> A photon crosses the universe in zero time That is not consistent with my observations.
- Scarblac 3y ago
  He obviously means from the viewpoint of the photon.
  - denton-scratch 3y ago
    Yeah, sorry: should have added some kind of irony flag. [Edit] Or maybe not; as I understand it, a photon doesn't "experience" time. It can't distinguish zero time from extended time. Everything's "zero time".

dredmorbius 3y ago

As I've seen this point made in the past, it's that travelling at a constant 1g acceleration the subjective time for an astronaut would be about 45 years in traversing the observable Universe (~14 billion light years). That is, a comfortable and familiar acceleration gives a human-lifetime-equivalent journey (subjective time). If you're planning a round-trip, you'll find rather more time has transpired on what used to be Earth. <https://www.universetoday.com/129086/far-can-travel/ https://www.universetoday.com/129086/far-can-travel/>

saalweachter 3y ago
> If you're planning a round-trip, you'll find rather more time has transpired on what used to be Earth. Or less, depending on the rotation of the universe as a whole.
- ben_w 3y ago
  Or no return may be possible at any speed/acceleration given the expansion of the universe.
  - dredmorbius 3y ago
    Given that the outbound journey is already technically challenging, that's very nearly quibbling, despite cosmological accuracy.
heavenlyblue 3y ago
That 1g constant acceleration requires practically infinite amounts of energy
- dredmorbius 3y ago
  True. However it is also the specific context in which the lifetime-universe-traversal concept emerges.

denton-scratch 3y ago

> A photon crosses the universe in zero time That is not consistent with my observations.

Scarblac 3y ago
He obviously means from the viewpoint of the photon.
- denton-scratch 3y ago
  Yeah, sorry: should have added some kind of irony flag. [Edit] Or maybe not; as I understand it, a photon doesn't "experience" time. It can't distinguish zero time from extended time. Everything's "zero time".