7 ms·

> "In fact, so long as you kept adhering to this plan, you could choose any destination at all that’s presently within 18 billion light-years of us, and reach i

by air7 5y ago

> "In fact, so long as you kept adhering to this plan, you could choose any destination at all that’s presently within 18 billion light-years of us, and reach it after merely 45 years, max, had passed. That ~18 billion light-year figure is the limit of the reachable Universe, set by the expansion of the Universe and the effects of dark energy. Everything beyond that point is currently unreachable with our present understanding of physics" What happens if I just keep going?

hoseja 5y ago
Notice the "presently".
Lariscus 5y ago
You would never reach your destination and at some point you would be stranded in dark space. This kurzgesagt video has some info on this topic.[1] [1] <https://www.youtube.com/watch?v=uzkD5SeuwzM https://www.youtube.com/watch?v=uzkD5SeuwzM>
- stouset 5y ago
  I don’t quite understand this. I think I’d maybe understand if space dilated to the point where you couldn’t quite hit that edge. Though now that I say it that “solution” seems to come with a whole bunch of problematic consequences. But “not getting to anything past the furthest thing we can see from earth today” seems fundamentally incompatible with the idea of reaching those farthest points in a reasonably bounded timeframe. There is stuff beyond the limits we can see, it’s just further than light has had a chance to travel so far (and may ever travel). My intuition is that as you approached those far objects, you’d observe them rapidly evolving forward in time until they reached their “present day” situation, able to see billions in light years in all directions with Earth right at the edge. But then what happens when you look at Earth? Surely you don’t see it’s billions of years old past. You’d have to see it at least as old as it was when you left. But something doesn’t seem quite right about that to me. Surely I’m missing something.
  - est31 5y ago
    Space is expanding. Which means stuff is getting further away from each other. https://en.wikipedia.org/wiki/Expansion_of_the_universe https://en.wikipedia.org/wiki/Expansion_of_the_universe If you go far enough, stuff moves fast enough away from us than the speed of light. Thus, some places can't be reached from someone starting at our present location, ever, even though we might still see their light. That light is closer to us than the object that sent it out.
    - edf13 5y ago
      Can you (Or stuff) go faster than the speed of light?
      - marcyb5st 5y ago
        Not with our current understand of physics. Even reaching the speed of light takes infinite amount of energy for things with mass. This is because the faster you go the more massive you become. Furthermore, even before reaching the speed of light you will become a black hole as more energy is added. Things without mass, instead, can only travel at the speed of light (photons for instance). Finally, there is a quirk in the math that would allow for the appearance of faster than light travel. If you compress the spacetime in front of you and expand it behind you you could can move faster than light without turning into a black hole or needing infinite energy (Google Alcubierre drive for more information). It is only an appearance than faster than light travel because you would still move at sub-luminal speed in your bubble of "normal" space time, but the compression/expansion effect would drag you through space-time at faster than light speed. This, however, require so called "exotic matter", ie matter with negative energy density (this is not anti-matter, but matter that has a repulsive gravitational field) and it is probably only a quirk of the math and nothing more.
      - simiones 5y ago
        No, the "speed of light", c, is actually the speed of energy and matter. Everything is always moving at this speed in 4-dimensional space-time. If you are not moving in space, you are moving through "time" at this speed. Since this is just the speed things move at, it makes no sense to discuss moving faster or slower than this speed.
gambiting 5y ago
As far as I understand - you'll come back to where you started.
- simonh 5y ago
  I'm afraid not, the accelerating expansion of the universe will stretch out space ahead of you faster than you can traverse it. From your perspective the galaxies will become sparser and sparser over time until you're in an ever colder eternal black void.
  - daxfohl 5y ago
    So you witness the heat death of the universe at ~50 years. Yet you and your spaceship are still intact then, and at times exponentially beyond it. Weird. Though perhaps the amount of energy it would take to accelerate that much exceeds the available energy in the universe? So maybe that's what balances it.
DarkWiiPlayer 5y ago
The way I understand it is that it's not impossible to cover the distance, but that by the time you do, the distance between your starting point and your destination will have (at least) doubled due to the expansion of space. So you're still as far away from your destination as in the beginning (or even further), but now you're the same distance from your starting position as well and are effectively stranded.
perihelions 5y ago
Well, an accelerating mass emits gravitational waves; and your spaceship is asymptotically infinite-mass; so at some point you destroy the universe. (Or at least the parts that are causally connected to you -- I guess cosmic expansion would save parts of it?) I don't know whether you observe this or not.
Gunax 5y ago
There is a misconception that expanding space prevents us from ever reaching a destination. It's counter-intuitive (https://en.wikipedia.org/wiki/Ant_on_a_rubber_rope https://en.wikipedia.org/wiki/Ant_on_a_rubber_rope) but falls out of the math. If space expands at a constant rate, you could eventually reach any destination at any velocity. But if the expansion of space is accelerating (which we believe currently), this is not true. There really would be destinations that are unreachable. You would continue to travel further from your origin, but also witness your destination accelerate away.
- stavros 5y ago
  But what if you accelerate faster than it does?
  - Armisael16 5y ago
    Your top speed is bounded by the speed of light. The thing you’re trying to reach is not, since that acceleration is mostly actually space expanding between you and it. It’s going faster that you can ever go, and as the distance between you and it increases it accelerates even more.
  - cygx 5y ago
    No matter how fast you accelerate, you'll chew up distances at a local(!) rate of less than c, so it's enough to consider the limiting case of light and skip the complications introduced by acceleration. The analogous classical problem is the "ant on a rubber rope". If the endpoint of the rope moves at a constant speed, the ant will be able to reach it, no matter the rope's length. If the endpoint accelerates, the ant might not, and only be able to asymptotically approach some point somewhere on the rope.
    - stavros 5y ago
      Hmm, but if we treat the end of the rope as an object which accelerates at x m/s^2, and I accelerate at 2x m/s^2, wouldn't I eventually reach that object? I don't know what I'm not getting...

by air7 5y ago

hoseja 5y ago

Notice the "presently".

Lariscus 5y ago

You would never reach your destination and at some point you would be stranded in dark space. This kurzgesagt video has some info on this topic.[1] [1] <https://www.youtube.com/watch?v=uzkD5SeuwzM https://www.youtube.com/watch?v=uzkD5SeuwzM>

stouset 5y ago
I don’t quite understand this. I think I’d maybe understand if space dilated to the point where you couldn’t quite hit that edge. Though now that I say it that “solution” seems to come with a whole bunch of problematic consequences. But “not getting to anything past the furthest thing we can see from earth today” seems fundamentally incompatible with the idea of reaching those farthest points in a reasonably bounded timeframe. There is stuff beyond the limits we can see, it’s just further than light has had a chance to travel so far (and may ever travel). My intuition is that as you approached those far objects, you’d observe them rapidly evolving forward in time until they reached their “present day” situation, able to see billions in light years in all directions with Earth right at the edge. But then what happens when you look at Earth? Surely you don’t see it’s billions of years old past. You’d have to see it at least as old as it was when you left. But something doesn’t seem quite right about that to me. Surely I’m missing something.
- est31 5y ago
  Space is expanding. Which means stuff is getting further away from each other. https://en.wikipedia.org/wiki/Expansion_of_the_universe https://en.wikipedia.org/wiki/Expansion_of_the_universe If you go far enough, stuff moves fast enough away from us than the speed of light. Thus, some places can't be reached from someone starting at our present location, ever, even though we might still see their light. That light is closer to us than the object that sent it out.
  - edf13 5y ago
    Can you (Or stuff) go faster than the speed of light?
    - marcyb5st 5y ago
      Not with our current understand of physics. Even reaching the speed of light takes infinite amount of energy for things with mass. This is because the faster you go the more massive you become. Furthermore, even before reaching the speed of light you will become a black hole as more energy is added. Things without mass, instead, can only travel at the speed of light (photons for instance). Finally, there is a quirk in the math that would allow for the appearance of faster than light travel. If you compress the spacetime in front of you and expand it behind you you could can move faster than light without turning into a black hole or needing infinite energy (Google Alcubierre drive for more information). It is only an appearance than faster than light travel because you would still move at sub-luminal speed in your bubble of "normal" space time, but the compression/expansion effect would drag you through space-time at faster than light speed. This, however, require so called "exotic matter", ie matter with negative energy density (this is not anti-matter, but matter that has a repulsive gravitational field) and it is probably only a quirk of the math and nothing more.
    - simiones 5y ago
      No, the "speed of light", c, is actually the speed of energy and matter. Everything is always moving at this speed in 4-dimensional space-time. If you are not moving in space, you are moving through "time" at this speed. Since this is just the speed things move at, it makes no sense to discuss moving faster or slower than this speed.

gambiting 5y ago

As far as I understand - you'll come back to where you started.

simonh 5y ago
I'm afraid not, the accelerating expansion of the universe will stretch out space ahead of you faster than you can traverse it. From your perspective the galaxies will become sparser and sparser over time until you're in an ever colder eternal black void.
- daxfohl 5y ago
  So you witness the heat death of the universe at ~50 years. Yet you and your spaceship are still intact then, and at times exponentially beyond it. Weird. Though perhaps the amount of energy it would take to accelerate that much exceeds the available energy in the universe? So maybe that's what balances it.

DarkWiiPlayer 5y ago

The way I understand it is that it's not impossible to cover the distance, but that by the time you do, the distance between your starting point and your destination will have (at least) doubled due to the expansion of space. So you're still as far away from your destination as in the beginning (or even further), but now you're the same distance from your starting position as well and are effectively stranded.

perihelions 5y ago

Well, an accelerating mass emits gravitational waves; and your spaceship is asymptotically infinite-mass; so at some point you destroy the universe. (Or at least the parts that are causally connected to you -- I guess cosmic expansion would save parts of it?) I don't know whether you observe this or not.

Gunax 5y ago

There is a misconception that expanding space prevents us from ever reaching a destination. It's counter-intuitive (https://en.wikipedia.org/wiki/Ant_on_a_rubber_rope https://en.wikipedia.org/wiki/Ant_on_a_rubber_rope) but falls out of the math. If space expands at a constant rate, you could eventually reach any destination at any velocity. But if the expansion of space is accelerating (which we believe currently), this is not true. There really would be destinations that are unreachable. You would continue to travel further from your origin, but also witness your destination accelerate away.

stavros 5y ago
But what if you accelerate faster than it does?
- Armisael16 5y ago
  Your top speed is bounded by the speed of light. The thing you’re trying to reach is not, since that acceleration is mostly actually space expanding between you and it. It’s going faster that you can ever go, and as the distance between you and it increases it accelerates even more.
- cygx 5y ago
  No matter how fast you accelerate, you'll chew up distances at a local(!) rate of less than c, so it's enough to consider the limiting case of light and skip the complications introduced by acceleration. The analogous classical problem is the "ant on a rubber rope". If the endpoint of the rope moves at a constant speed, the ant will be able to reach it, no matter the rope's length. If the endpoint accelerates, the ant might not, and only be able to asymptotically approach some point somewhere on the rope.
  - stavros 5y ago
    Hmm, but if we treat the end of the rope as an object which accelerates at x m/s^2, and I accelerate at 2x m/s^2, wouldn't I eventually reach that object? I don't know what I'm not getting...