I understand that our local galaxy group is considered “gravitationally bound” and therefore exempt from the expansion from each other ((, but we don’t seem to have other galaxies collected into their own “local groups” of gravitationally bound clusters, so are we saying we’re somehow unique? Is there a trick of perception taking place?)) <—edit:this is wrong

I found this quote in the Wikipedia article on the Expansion of the universe.

While objects cannot move faster than light, this limitation applies only with respect to local reference frames and does not limit the recession rates of cosmologically distant objects.

It seems to me that if we can perceive at cosmological distance something that cannot exist, perhaps we are falsely observing an expanding universe. Maybe everything IS gravitationally bound and we’re just seeing expansion because… Relativity?

  • CarbonIceDragon@pawb.social
    5·
    6 months ago

    If you’re actually curious, or someone else reading this is, you never can get a rocket, or anything with mass, to the speed of light either, not just faster than it, but you can get arbitrarily close. However, you never notice anything stopping you going faster than your current speed, there’s no point where your rockets stop working or anything, rather, time and space stretch and squeeze such that neither you nor anybody else see you going faster than light. If you have a magic rocket that somehow has infinite fuel and can fire forever, you can actually get anywhere as fast as you want, from your perspective.

    Alpha centauri is famously about 4 light years away, but you can get there in 2 seconds, from your perspective, if you go fast enough. But, everyone on earth will see slightly over that roughly 4 years go by in the time that for you is just 2 seconds. (You’ll see them move slowly too at first, since they’re moving relative to you just as fast as your ship is moving to them, but when you slow down, you’ll see them seem to speed up until you’ll have seen them do 4 years worth of stuff by the time you stop). Meanwhile on your ship, you don’t see yourself crossing that 4 light year distance in less than the allowed time either, because space itself is squished kinda, so that the distance to alpha centauri is shortened to the point that if you’re getting there in 2 seconds, it’s now less than 2 light seconds away, from your perspective, and you’re not moving faster than light to cross that distance in that time. People outside will also see your ship compressed like this too.

    This isn’t just a regular optical illusion either, space and time really are different for the people on and off the ship (and indeed very slightly different for everyone anywhere). Nobody has the “correct” view of the universe, because everyone’s perspective is equally valid.

    • Victor@lemmy.world
      2·
      6 months ago

      Ah yes, I seem to remember a few things now, which helped me to understand this before:

      1. You have a fixed energy output (not sure what to call it, but let’s go with that, “energy output”). Either you output all of your energy in the physical dimensions, or in the time dimension (I can’t think of anything that does this though), or some in both. Most things output some energy in both, time and the physical, like us for example!
      2. Light is basically all of its energy in the physical dimension and none in the time dimension. Thus a photon doesn’t really experience time at all. To it, it feels like absolutely zero time has passed at all between traveling from one end of the universe until it is absorbed by an object at the other end.
      3. To attain the speed of light takes infinite energy, as it takes more and more energy to accelerate mass the faster you get to c, in a way that you never reach c.

      These concepts are really helpful in my opinion, to understanding the fact that you can’t reach the speed of light, as well as the whole time dilation concept.

      I hope it helps someone else.