4 ms·

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/

by 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.
  - ccozan 5y ago
    Correct explanation!
- eru 5y ago
  Well, if you take a really strong laser pointer and point it at the moon, than a flick of the wrist can make that point race faster along the moon's surface than the speed of light.
  - MaxikCZ 5y ago
    But its not really a point thats traveling faster than speed of light. The individual photons that makes the point travel always at the speed of light from your laser pointer to the moon. Its not point on the moon thats traveling, the travel is just ilusion because it looks like the point that was there a split second ago.
    - eru 5y ago
      Well, the 'point' is traveling faster than the speed of light. You are quite right that no matter nor energy nor information is traveling faster than light here.
      - MaxikCZ 5y ago
        I am not sure how to define the point, but my way of thinking is that its not the same point. The point never moves, instead it continuously vanishes and a new point is being continuously made next to it. That way, there really isnt a point that is traveling or moving, its just our perception mistaking it for a point because it looks and move like one.
        jasonwatkinspdx 5y ago
        Yeah, exactly this. If you think of it like waving a garden hose back and forth it's very clear why this is flawed logic. Also because of quantization the further away you put the target surface, to try to raise the "speed" of the not actually existing point, the more discontinuous it becomes.
        eru 5y ago
        Yes, I know all that. The thing is that lots of our 'concrete' real world objects have more in common with the later pointer point than with physical reality. Concreteness is a bit of an illusion, it's all wave functions at the bottom. (Of course, real world objects still can't go faster than light.)
- cygx 5y ago
  Nothing can outpace photons (in vacuum, that is - in a medium, cf Cherenkov radiation). However, velocity is a surprisingly tricky concept. While the relative velocity of two objects passing each other by is well-defined, velocity at a distance tends to require a clock synchronization convention, which is to some degree arbitrary. In consequence, in special relativity, the speed of light is only isotropic by convention (cf various discussions about the one-way speed of light, which became a topic of online conversations due to Veritasium posting a video about it), and in general relativity, recession velocities (ie change in proper distance at constant cosmological time) of far away, but still observable galaxy can in fact formally be greater than c.
ajconway 5y ago
So, when you reach the "edge" you were going for, you arrive at the point when everything has already moved past your horizon (including Earth), there isn't a single hydrogen atom around, so you're stuck in the darkness forever?
- est31 5y ago
  Yeah from current understanding that's your fate. Everyone's fate actually, if you see gravitationally bound structures as one unit. You'd be alone waiting for the heat death. Therefore, I recommend decelerating to somewhere interesting during the time window which allows intergalactic travel.
drran 5y ago
Expansion of the (visible) Universe is hypothesis, not a fact. Nobody pointed to an incredibly powerful energy source required to expand the whole visible Universe. A non-zero curvature of space is not confirmed also. We are able to see gravitational waves at this point. Where is a gravitational turbulence, created by the massive expansion? (Non-native speaker, but you got the idea).
stouset 5y ago
Ah, I think I understand! Something 18bnly away from us today has a light bubble that includes objects outside our light cone. If you can get there in 45y, that doesn’t mean 45y has passed at that location. In fact if it’s 18bnly away, that means you’d arrive in 18bn years from their perspective. And not 18bn years from what you saw when you left, but 18bn years from when they saw you leave. By that time, space will have expanded enough that it’s no longer possible to see past.

by est31 5y ago

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.
- ccozan 5y ago
  Correct explanation!
eru 5y ago
Well, if you take a really strong laser pointer and point it at the moon, than a flick of the wrist can make that point race faster along the moon's surface than the speed of light.
- MaxikCZ 5y ago
  But its not really a point thats traveling faster than speed of light. The individual photons that makes the point travel always at the speed of light from your laser pointer to the moon. Its not point on the moon thats traveling, the travel is just ilusion because it looks like the point that was there a split second ago.
  - eru 5y ago
    Well, the 'point' is traveling faster than the speed of light. You are quite right that no matter nor energy nor information is traveling faster than light here.
    - MaxikCZ 5y ago
      I am not sure how to define the point, but my way of thinking is that its not the same point. The point never moves, instead it continuously vanishes and a new point is being continuously made next to it. That way, there really isnt a point that is traveling or moving, its just our perception mistaking it for a point because it looks and move like one.
      - jasonwatkinspdx 5y ago
        Yeah, exactly this. If you think of it like waving a garden hose back and forth it's very clear why this is flawed logic. Also because of quantization the further away you put the target surface, to try to raise the "speed" of the not actually existing point, the more discontinuous it becomes.
      - eru 5y ago
        Yes, I know all that. The thing is that lots of our 'concrete' real world objects have more in common with the later pointer point than with physical reality. Concreteness is a bit of an illusion, it's all wave functions at the bottom. (Of course, real world objects still can't go faster than light.)
cygx 5y ago
Nothing can outpace photons (in vacuum, that is - in a medium, cf Cherenkov radiation). However, velocity is a surprisingly tricky concept. While the relative velocity of two objects passing each other by is well-defined, velocity at a distance tends to require a clock synchronization convention, which is to some degree arbitrary. In consequence, in special relativity, the speed of light is only isotropic by convention (cf various discussions about the one-way speed of light, which became a topic of online conversations due to Veritasium posting a video about it), and in general relativity, recession velocities (ie change in proper distance at constant cosmological time) of far away, but still observable galaxy can in fact formally be greater than c.

ajconway 5y ago

So, when you reach the "edge" you were going for, you arrive at the point when everything has already moved past your horizon (including Earth), there isn't a single hydrogen atom around, so you're stuck in the darkness forever?

est31 5y ago
Yeah from current understanding that's your fate. Everyone's fate actually, if you see gravitationally bound structures as one unit. You'd be alone waiting for the heat death. Therefore, I recommend decelerating to somewhere interesting during the time window which allows intergalactic travel.

drran 5y ago

Expansion of the (visible) Universe is hypothesis, not a fact. Nobody pointed to an incredibly powerful energy source required to expand the whole visible Universe. A non-zero curvature of space is not confirmed also. We are able to see gravitational waves at this point. Where is a gravitational turbulence, created by the massive expansion? (Non-native speaker, but you got the idea).

stouset 5y ago

Ah, I think I understand! Something 18bnly away from us today has a light bubble that includes objects outside our light cone. If you can get there in 45y, that doesn’t mean 45y has passed at that location. In fact if it’s 18bnly away, that means you’d arrive in 18bn years from their perspective. And not 18bn years from what you saw when you left, but 18bn years from when they saw you leave. By that time, space will have expanded enough that it’s no longer possible to see past.