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all stars outside the Local Group of galaxies will no be accessible to us since they will be receding away faster than the speed of light. Can anyone explain w

by gaius 8y ago

all stars outside the Local Group of galaxies will no be accessible to us since they will be receding away faster than the speed of light. Can anyone explain what this means?

user5994461 8y ago
There is a theory that the universe is expanding faster and faster. At some point in the future, stars would be moving away faster than the speed of light and we couldn't ever see them again or reach them.
- montrose 8y ago
  How could they be moving faster than the speed of light?
  - fiblye 8y ago
    Imagine two RC cars driving in opposite directions on a crumpled up, (virtually) infinitely large blanket with an uncountable number of ripples. The cars' top speeds are 10 MPH. On top of each car is an ant watching the other car. Both cars start driving off at 10 MPH in opposite directions. From an ant's perspective on top of one car, the other car looks to be going 20 MPH away from it. Then someone starts pulling the blanket underneath the cars from opposite ends at 5 MPH. Now it looks like the other car is going 30 MPH. Now imagine for each ripple on that blanket, another hand appears and starts to pull, and the outer edges are pulled at 5MPH from the reference point of the nearest ripple. From the perspective of an ant on one end, it can eventually get to the point that the other car is warping off at 100MPH or more. And as this blanket spreads out more and more, things speed up more and more as well. The cars never technically exceed their speed limits of 10 MPH, but the space that they occupy is being pulled apart in a way that they seem to be exceeding 10 MPH from each other's perspectives.
  - gaius 8y ago
    And if they can, why can’t we and catch them? I mean we are in spacetime. If it stretches we stretch too!
    - ben_w 8y ago
      Right now the only serious effort to consider how to do so is the Alcubierre metric and its variations. Nobody knows how to build one, or what matrials could build one, or how to turn it on or off. Unless you have a cunning way to cheat, such as making the inside bigger than the outside, it requires in excess of minus the rest mass of the observable universe for a plausible sized object. There are further objections, but above my level.
  - skummetmaelk 8y ago
    Anything which moves through space-time is restricted to moving at the speed of light. Space-time itself can warp causing objects to "move" away from each other faster than the speed of light. Think of ants crawling on the surface and a balloon which is being blown up.
  - hsk 8y ago
    Because while matter cannot move faster than the speed of light the universe itself is allowed to expand faster than the speed of light. Think of the spacetime fabric stretching so fast light cannot cover the increased distance. The universe is essentially creating more distance between objects. Another way to think of it is velocity of an object is derivative of position in space over time. If space itself is moving your position relative to it isn't changing.
    - chii 8y ago
      I still can't intuitively understand this. If two objects ar stationary relative to each other, the expansion implies that at some point, the distance between those two objects will increase (without either of those objects having moved). So space is being somehow created?! What if i have a large object, will the length of that object increase? Or will it break up?
      - Findeton 8y ago
        I think you are using a concept of space-time that is quite limited. You are at most using concepts from Einstein's Special Relativity. Everything changed with General Relativity. Space is not "created", space-time is warped. Gravity is the deformation of space-time by mass/energy. The distance from point A to point B is measured by the time it takes for a light ray to get from A to B using the shortest path. But if you move objects with mass (or with energy) near A and B, the shortest path will change. Also, it's easy to measure distance in seconds when you know the speed of light.
        deleted 8y ago
        [deleted]
      - philipov 8y ago
        The object is bound together by electromagnetic forces, and the local group is bound together by gravity. These forces are much stronger than the expansion of the universe (cosmological constant), but the expansion of the universe manifests as a tiny tiny outward pressure term (too small for us to detect). It makes every atom/orbit a little bigger by changing the equilibrium point, but they don't to grow in size because the force isn't increasing. If the cosmological constant were much larger than it is, atoms would have never formed at all.
- criddell 8y ago
  So would the light from those stars also be moving outward faster than the speed of light?
  - db48x 8y ago
    No. In fact, if two objects are far enough apart, then the amount of new space created between them will be so great that light will never be able to travel between them. Neither object will ever be able to see the other.
    - criddell 8y ago
      It's really hard for me to imagine how a flashlight on a distance plant could be moving away from me faster than the speed of light but the emitted light is going slower.
  - maxerickson 8y ago
    No. It's color shifted though. https://en.wikipedia.org/wiki/Relativistic_Doppler_effect https://en.wikipedia.org/wiki/Relativistic_Doppler_effect
    - Findeton 8y ago
      Just like the sound of an ambulance.
mlevental 8y ago
the universe is expanding and the expansion is accelerating
ithkuil 8y ago
The first few paragraphs of https://en.m.wikipedia.org/wiki/Expansion_of_the_universe https://en.m.wikipedia.org/wiki/Expansion_of_the_universe do a reasonably good job at describing what I assume is the source of your puzzlement (namely how can something recede faster than the speed of light)
wavedingo 8y ago
Due to the big bang the universe is expanding... but you’d expect the expansion to be slowing down due to the drag of gravity. However, since the 1990s we’ve observed that the universe’s expansion is accelerating - the theory is that a force called “dark energy” is causing this acceleration. If the acceleration continues, there will be a point where distant galaxies will be accelerating away from each other at the speed of light. Since light is the universe’s “speed limit”, we would not be able to travel to these galaxies with known physics because even traveling at light-speed the galaxy we’re going to would be stretching away faster. Locally, however, due to gravity, it sounds like the paper is saying we would have regions of dense galaxies that stay closer together. Islands of matter in a sea of dark energy.
- oooooof 8y ago
  The idea that humanity will ever travel even to the nearest star is pure fiction. Anyone who thinks otherwise just fails to grasp the distances involved. They are so large as to be beyond understanding.
  - senorjazz 8y ago
    Unfortunately, I think you are right. We haven't done so well in taking care of this planet that is perfect for us, we evolved for it. People talking about living on dead, inhospitable planets / moons are just getting carried away with mental science fiction masturbation imho. Which is a fun activity in itself and with merit I agree, but realistically. Very soon, the very best minds we have are going to be on how to save humanity from destroying itself here within the century. Not looking to colonise the stars.
  - OscarCunningham 8y ago
    The distance to the moon is already beyond intuition, but you can still understand how to get there by doing calculations. If the numbers say that it's possible then it's possible.
  - SeanMacConMara 8y ago
    the _nearest_ star is actually quiet close :-]
db48x 8y ago
Space is expanding, pushing all the stuff in the universe apart. But "pushing" is not really the right word, because the objects aren't actually being accelerated. Instead, new space is being created between all objects. This can cause the distance between two objects to increase "faster" than the speed of light, without either of the objects actually moving.

by gaius 8y ago

all stars outside the Local Group of galaxies will no be accessible to us since they will be receding away faster than the speed of light. Can anyone explain what this means?

user5994461 8y ago

There is a theory that the universe is expanding faster and faster. At some point in the future, stars would be moving away faster than the speed of light and we couldn't ever see them again or reach them.

montrose 8y ago
How could they be moving faster than the speed of light?
- fiblye 8y ago
  Imagine two RC cars driving in opposite directions on a crumpled up, (virtually) infinitely large blanket with an uncountable number of ripples. The cars' top speeds are 10 MPH. On top of each car is an ant watching the other car. Both cars start driving off at 10 MPH in opposite directions. From an ant's perspective on top of one car, the other car looks to be going 20 MPH away from it. Then someone starts pulling the blanket underneath the cars from opposite ends at 5 MPH. Now it looks like the other car is going 30 MPH. Now imagine for each ripple on that blanket, another hand appears and starts to pull, and the outer edges are pulled at 5MPH from the reference point of the nearest ripple. From the perspective of an ant on one end, it can eventually get to the point that the other car is warping off at 100MPH or more. And as this blanket spreads out more and more, things speed up more and more as well. The cars never technically exceed their speed limits of 10 MPH, but the space that they occupy is being pulled apart in a way that they seem to be exceeding 10 MPH from each other's perspectives.
- gaius 8y ago
  And if they can, why can’t we and catch them? I mean we are in spacetime. If it stretches we stretch too!
  - ben_w 8y ago
    Right now the only serious effort to consider how to do so is the Alcubierre metric and its variations. Nobody knows how to build one, or what matrials could build one, or how to turn it on or off. Unless you have a cunning way to cheat, such as making the inside bigger than the outside, it requires in excess of minus the rest mass of the observable universe for a plausible sized object. There are further objections, but above my level.
- skummetmaelk 8y ago
  Anything which moves through space-time is restricted to moving at the speed of light. Space-time itself can warp causing objects to "move" away from each other faster than the speed of light. Think of ants crawling on the surface and a balloon which is being blown up.
- hsk 8y ago
  Because while matter cannot move faster than the speed of light the universe itself is allowed to expand faster than the speed of light. Think of the spacetime fabric stretching so fast light cannot cover the increased distance. The universe is essentially creating more distance between objects. Another way to think of it is velocity of an object is derivative of position in space over time. If space itself is moving your position relative to it isn't changing.
  - chii 8y ago
    I still can't intuitively understand this. If two objects ar stationary relative to each other, the expansion implies that at some point, the distance between those two objects will increase (without either of those objects having moved). So space is being somehow created?! What if i have a large object, will the length of that object increase? Or will it break up?
    - Findeton 8y ago
      I think you are using a concept of space-time that is quite limited. You are at most using concepts from Einstein's Special Relativity. Everything changed with General Relativity. Space is not "created", space-time is warped. Gravity is the deformation of space-time by mass/energy. The distance from point A to point B is measured by the time it takes for a light ray to get from A to B using the shortest path. But if you move objects with mass (or with energy) near A and B, the shortest path will change. Also, it's easy to measure distance in seconds when you know the speed of light.
      - deleted 8y ago
        [deleted]
    - philipov 8y ago
      The object is bound together by electromagnetic forces, and the local group is bound together by gravity. These forces are much stronger than the expansion of the universe (cosmological constant), but the expansion of the universe manifests as a tiny tiny outward pressure term (too small for us to detect). It makes every atom/orbit a little bigger by changing the equilibrium point, but they don't to grow in size because the force isn't increasing. If the cosmological constant were much larger than it is, atoms would have never formed at all.
criddell 8y ago
So would the light from those stars also be moving outward faster than the speed of light?
- db48x 8y ago
  No. In fact, if two objects are far enough apart, then the amount of new space created between them will be so great that light will never be able to travel between them. Neither object will ever be able to see the other.
  - criddell 8y ago
    It's really hard for me to imagine how a flashlight on a distance plant could be moving away from me faster than the speed of light but the emitted light is going slower.
- maxerickson 8y ago
  No. It's color shifted though. https://en.wikipedia.org/wiki/Relativistic_Doppler_effect https://en.wikipedia.org/wiki/Relativistic_Doppler_effect
  - Findeton 8y ago
    Just like the sound of an ambulance.

mlevental 8y ago

the universe is expanding and the expansion is accelerating

ithkuil 8y ago

The first few paragraphs of https://en.m.wikipedia.org/wiki/Expansion_of_the_universe https://en.m.wikipedia.org/wiki/Expansion_of_the_universe do a reasonably good job at describing what I assume is the source of your puzzlement (namely how can something recede faster than the speed of light)

wavedingo 8y ago

Due to the big bang the universe is expanding... but you’d expect the expansion to be slowing down due to the drag of gravity. However, since the 1990s we’ve observed that the universe’s expansion is accelerating - the theory is that a force called “dark energy” is causing this acceleration. If the acceleration continues, there will be a point where distant galaxies will be accelerating away from each other at the speed of light. Since light is the universe’s “speed limit”, we would not be able to travel to these galaxies with known physics because even traveling at light-speed the galaxy we’re going to would be stretching away faster. Locally, however, due to gravity, it sounds like the paper is saying we would have regions of dense galaxies that stay closer together. Islands of matter in a sea of dark energy.

oooooof 8y ago
The idea that humanity will ever travel even to the nearest star is pure fiction. Anyone who thinks otherwise just fails to grasp the distances involved. They are so large as to be beyond understanding.
- senorjazz 8y ago
  Unfortunately, I think you are right. We haven't done so well in taking care of this planet that is perfect for us, we evolved for it. People talking about living on dead, inhospitable planets / moons are just getting carried away with mental science fiction masturbation imho. Which is a fun activity in itself and with merit I agree, but realistically. Very soon, the very best minds we have are going to be on how to save humanity from destroying itself here within the century. Not looking to colonise the stars.
- OscarCunningham 8y ago
  The distance to the moon is already beyond intuition, but you can still understand how to get there by doing calculations. If the numbers say that it's possible then it's possible.
- SeanMacConMara 8y ago
  the _nearest_ star is actually quiet close :-]

db48x 8y ago

Space is expanding, pushing all the stuff in the universe apart. But "pushing" is not really the right word, because the objects aren't actually being accelerated. Instead, new space is being created between all objects. This can cause the distance between two objects to increase "faster" than the speed of light, without either of the objects actually moving.