6 ms·

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_rubbe

by 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...
    - cygx 5y ago
      > and I accelerate at 2x m/s^2 A constant lab frame acceleration is incompatible with special relativity.
    - JZumun 5y ago
      You can never go faster than c, so you can never reach anything that's being moved faster away.
- Thiez 5y ago
  Let's say the universe is turn-based, and consists of a single dimension. At the end of each turn the distance between two points in this universe doubles (because new empty space is created between them). Let the maximum speed in this universe be 300.000.000 distance-units per turn. When two points are, say, 5 units apart, we can easily travel between them (for a couple of turns, at least). But when two points are more than 600.000.000 distance-units apart, we can never again travel between them. Let's try. We're at turn 0, at point A. Point B is 600.000.000 distance-units apart. We have infinite! acceleration and accelerate to 300.000.000 units/turn, and start our travel to point B. We move 300.000.000 units. Distances double. We're at turn 1. Point A is now 600.000.000 units behind us, and point B is still 600.000.000 units ahead of us. We travel 300.000.000 units! Distances double. We're at turn 2. Point A is now 1.800.000.000 units behind us, and point B is still 600.000.000 ahead of us. We're starting to wonder if we should have stayed at point A... hopefully there is a point C that was between point A and point B that we can still get to, because we don't seem to be making much progress. The universe is a bit like that, except that it looks more continuous, it has some more dimensions, and it's not doubling quite so quickly.
  - stavros 5y ago
    Hmm, I see, thank you. So it's space itself that expands, not just the objects in it that accelerate apart from one another. But then doesn't that mean that some of that space is expanding inside the objects? Do we know what happens there? How do objects just get more space inside them?
    - cygx 5y ago
      Thinking of spacetime as some sort of expanding fabric permeating every-day objects is not a good analogy in my opinion. First, like any other gravitational effect, locally, spatial expansion would manifest as pseudo-forces that can be counter-acted by all the other forces that are far more relevant at human scale. Second, the local effect of spatial expansion should be an indirect one: Friedmann cosmology - which is how we describe an isotropic, expanding universe - is a large-scale approximation. More realistic would be 'swiss-cheese' models, where spacetime in our neighbourhood can look vastly different from the Friedmann one, except that the spacetime patches need to properly fit together to yield the correct large-scale behaviour.
- saiya-jin 5y ago
  You can't go faster than light. Space in universe is expanding faster than light.
secondcoming 5y ago
Can scientists count the number of far distant stars that are no longer visible due to space accelerating?

by Gunax 5y ago

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...
  - cygx 5y ago
    > and I accelerate at 2x m/s^2 A constant lab frame acceleration is incompatible with special relativity.
  - JZumun 5y ago
    You can never go faster than c, so you can never reach anything that's being moved faster away.
Thiez 5y ago
Let's say the universe is turn-based, and consists of a single dimension. At the end of each turn the distance between two points in this universe doubles (because new empty space is created between them). Let the maximum speed in this universe be 300.000.000 distance-units per turn. When two points are, say, 5 units apart, we can easily travel between them (for a couple of turns, at least). But when two points are more than 600.000.000 distance-units apart, we can never again travel between them. Let's try. We're at turn 0, at point A. Point B is 600.000.000 distance-units apart. We have infinite! acceleration and accelerate to 300.000.000 units/turn, and start our travel to point B. We move 300.000.000 units. Distances double. We're at turn 1. Point A is now 600.000.000 units behind us, and point B is still 600.000.000 units ahead of us. We travel 300.000.000 units! Distances double. We're at turn 2. Point A is now 1.800.000.000 units behind us, and point B is still 600.000.000 ahead of us. We're starting to wonder if we should have stayed at point A... hopefully there is a point C that was between point A and point B that we can still get to, because we don't seem to be making much progress. The universe is a bit like that, except that it looks more continuous, it has some more dimensions, and it's not doubling quite so quickly.
- stavros 5y ago
  Hmm, I see, thank you. So it's space itself that expands, not just the objects in it that accelerate apart from one another. But then doesn't that mean that some of that space is expanding inside the objects? Do we know what happens there? How do objects just get more space inside them?
  - cygx 5y ago
    Thinking of spacetime as some sort of expanding fabric permeating every-day objects is not a good analogy in my opinion. First, like any other gravitational effect, locally, spatial expansion would manifest as pseudo-forces that can be counter-acted by all the other forces that are far more relevant at human scale. Second, the local effect of spatial expansion should be an indirect one: Friedmann cosmology - which is how we describe an isotropic, expanding universe - is a large-scale approximation. More realistic would be 'swiss-cheese' models, where spacetime in our neighbourhood can look vastly different from the Friedmann one, except that the spacetime patches need to properly fit together to yield the correct large-scale behaviour.
saiya-jin 5y ago
You can't go faster than light. Space in universe is expanding faster than light.

secondcoming 5y ago

Can scientists count the number of far distant stars that are no longer visible due to space accelerating?