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If the universe is 13.8B years old, how can we see 46B light years away? (2018)

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Re: If the universe is 13.8B years old, how can we see 46B light years away? (2018)

#11

Post by michelpp »

hunter2_ wrote: post #9
leonroy wrote: post #3

Earlier quoted context omitted.

The universe is expanding, at a rate which I believe is heading past the speed of light. This limits our ability to reach galaxies further away from us as they are speeding away faster than we could conceivably travel since our engines would of course be bound by the speed of light. Some more info: https://www.forbes.com/sites/startswithabang/2016/06/10/can-...

Why would some things be bound by the speed of light and not others? I always thought of this limitation as an all-or-nothing law.

Everything that moves through space is bound by the speed of light. But space itself is expanding at a rate such that two distant points can move away from each other faster than the speed of light. Think of it like two ants on opposite sides of a balloon, one ant starts moving toward the other as fast as it can go, but then you start to inflate the balloon. The distance between the two ants expands faster than an ant can move and they'll never meet.

Re: If the universe is 13.8B years old, how can we see 46B light years away? (2018)

#12

Post by lordnacho »

hunter2_ wrote: post #9
leonroy wrote: post #3

Earlier quoted context omitted.

The universe is expanding, at a rate which I believe is heading past the speed of light. This limits our ability to reach galaxies further away from us as they are speeding away faster than we could conceivably travel since our engines would of course be bound by the speed of light. Some more info: https://www.forbes.com/sites/startswithabang/2016/06/10/can-...

Why would some things be bound by the speed of light and not others? I always thought of this limitation as an all-or-nothing law.

Not an expert but the speed of light limitation seems to be derived from special relativity considerations the constancy of that speed and the use of light to carry information about events. So a train cannot accelerate to beyond c because of this. But not every phenomenon is limited by this.

As an example if you shine a torch at a really far away rock the dot can move faster than c transversally when you wiggle the torch. But the actual light is going at c to the rock and back to you.

Re: If the universe is 13.8B years old, how can we see 46B light years away? (2018)

#13

Post by drekembe »

hunter2_ wrote: post #9
leonroy wrote: post #3

Earlier quoted context omitted.

The universe is expanding, at a rate which I believe is heading past the speed of light. This limits our ability to reach galaxies further away from us as they are speeding away faster than we could conceivably travel since our engines would of course be bound by the speed of light. Some more info: https://www.forbes.com/sites/startswithabang/2016/06/10/can-...

Why would some things be bound by the speed of light and not others? I always thought of this limitation as an all-or-nothing law.

Things can't move faster than light through space. The space itself has no such limitation. You can imagine an expanding balloon and ants walking on it that have a limit to how fast they can move relative to the surface of the balloon.

Re: If the universe is 13.8B years old, how can we see 46B light years away? (2018)

#14

Post by krastanov »

hunter2_ wrote: post #9
leonroy wrote: post #3

Earlier quoted context omitted.

The universe is expanding, at a rate which I believe is heading past the speed of light. This limits our ability to reach galaxies further away from us as they are speeding away faster than we could conceivably travel since our engines would of course be bound by the speed of light. Some more info: https://www.forbes.com/sites/startswithabang/2016/06/10/can-...

Why would some things be bound by the speed of light and not others? I always thought of this limitation as an all-or-nothing law.

Roughly, anything that can be used as an information carrier is limited by the speed of light (a good rule of thumb is that such things have some mass or energy). Things that can not be exploited on their own for information transfer do not need to obey that limit (stretching of space, entangling of particles, maybe other things).

PS It is an open question how that does not lead to contradictions when you start considering energy density of the vacuum or the extremely small gravitational waves (which can in principle transmit information) created by the changes in the latter systems. But these effects are astronomically small and it is difficult to imagine even a thought experiment that can detect them.

Re: If the universe is 13.8B years old, how can we see 46B light years away? (2018)

#15

Post by diegoperini »

hunter2_ wrote: post #9
leonroy wrote: post #3

Earlier quoted context omitted.

The universe is expanding, at a rate which I believe is heading past the speed of light. This limits our ability to reach galaxies further away from us as they are speeding away faster than we could conceivably travel since our engines would of course be bound by the speed of light. Some more info: https://www.forbes.com/sites/startswithabang/2016/06/10/can-...

Why would some things be bound by the speed of light and not others? I always thought of this limitation as an all-or-nothing law.

Think like it is a shadow or a light trace.

If you point a laser pointer device to moon from earth (assuming it's a really strong laser), rotating it for a few degrees on earth will result a light mark kms long on moon. The build up time for the mark will even exceed the speed of light if your hand is fast enough. This doesn't mean you broke the law. It just means multiple objects (in this case, each photon hitting the moon) created the illusion of a single object, moving really fast. Background light trying to reach us from the edge of the universe mimics a similar behavior with the added extra spacial dimension. Locations in space aren't objects. They are some points objects can leave marks on. Tracking their speed can always create faster than light situations if you pick a bunch of them conveniently.

Re: If the universe is 13.8B years old, how can we see 46B light years away? (2018)

#16

Post by mannykannot »

ctdonath wrote: post #5

It’s not that we can see that far, it’s that what we see has moved away from where it was when it produced the light we see now.

Indeed; the issue is covered much more clearly in the article from which the most relevant illustration was taken: https://calgary.rasc.ca/redshift.htm

Re: If the universe is 13.8B years old, how can we see 46B light years away? (2018)

#17

Post by lioeters »

chongli wrote: post #7
leonroy wrote: post #3

Earlier quoted context omitted.

The universe is expanding, at a rate which I believe is heading past the speed of light. This limits our ability to reach galaxies further away from us as they are speeding away faster than we could conceivably travel since our engines would of course be bound by the speed of light. Some more info: https://www.forbes.com/sites/startswithabang/2016/06/10/can-...

That’s the case for the galaxies furthest away from us. Closer galaxies are receding less quickly and Andromeda is actually coming towards us.

Indeed, and will merge with ours to become..the Milkdromeda (TBD) galaxy. Looking forward to the fireworks.

https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_co...

Re: If the universe is 13.8B years old, how can we see 46B light years away? (2018)

#18

Post by BjoernKW »

ldoughty wrote: post #2

What does it mean: "if we left today we could only reach one third of it"? I couldn't find a the limiting factor... The stars would be dead before we arrived? My best guess. Of course, the qualification would be that we continue to be bound to the speed of light maximum.

We can only move as fast as the speed of light. At a truly large scale space expands faster than that.

Re: If the universe is 13.8B years old, how can we see 46B light years away? (2018)

#20

Post by nvrspyx »

If what we’re seeing was 13.8B light years away 13.8B years ago and are now 46B light years away, wouldn’t that mean that those things traveled ~32.2B light years in 13.8B years. Wouldn’t that thus mean they travelled faster than the speed of light?

Maybe I’m misunderstanding though.

EDIT: I guess you also have to take into account the expansion of the fabric of space as well and not just the movement of the galaxies. Still a bit difficult to grasp because I still intuitively want to say that those galaxies are traveling faster than the speed of light with the assistance of space expansion like the old Hollywood trick of actors running on a moving rug to fake superhuman speed. They’re still moving faster, just not on their own. I guess the key difference is that an actor on a sliding rug is moving through space faster even if their legs are moving at the same speed, but in the case of the galaxies, space itself is moving thus they’re not technically FTL because the speed of light is relative to its movement through space.

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