I just don’t get it.
According to the theory of special relativity, nothing can ever move faster than light speed.
But due to the expansion of the universe, sufficiently distant stars move away from us faster than the speed of light.
And the explanation is…that this universal speed limit doesn’t apply to things that are really far away?
Please make it make sense!
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https://www.kobo.com/us/en/ebook/an-infinity-of-worlds
might be what you’re looking-for…
VoterFrog, here, & Sims, give much better views than most articles on it, tbh, & that book also helps.
& if you want to get into Time…
Carlo Rovelli has some yt vids, including a lecture ( for us humans, not for the physicists ) on Royal Institution’s channel, & his book “The Order of Time” might be of interest to you…
https://www.youtube.com/watch?v=-6rWqJhDv7M
His point that you can’t measure time, but you can measure change… seems valid.
& if “space” is actually a kind of process…
good luck, eh?
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An analogy I recall is being on the surface of an expanding balloon.
There may be a speed limit on surface travel, but a point far away may recede faster than the travel speed limit due to expansion.
Oh yeah but if I say the exact same thing i get downvoted because that’s “not how special relativity works”
That is not at all the thing you said. You said two objects moving away from each other each at the speed of light in non-expanding space would be perceived by each other as going at double the speed of light which does indeed not work like that in relativity.
nothing can ever move faster than light speed
…pr space-time ‘unit’ ? If max speed is defined inside spacetime, then what if space it self expands everywhere ? So the speed is a limitation of the rules governing a ‘medium’, but the medium it self might not have such limitations.
[contrived analogy] If you start to swim in a large and 20m tall swimming pool, you have a max speed. Imagine everything in the pool have that max speed. Now imagine that the poolwalls suddenly disappears and the water column drops. At the ground, water would make a pan-cake shape, where water moves faster on the edge than in the center. Objects would flow with the water, and still have the same max speed as before. Objects at the edges now moves at the water expansion speed + max speed. Objects relative to other objects in the pool can move away ‘globally’ from each other at many times the max swimming speed, while maintaining the ‘local’ max speed in the water.
This is horrible cosmology/physics on many levels, and I probably made someone go to, or roll over in their grave, but it was the only ‘medium expanding’ analogy I could come up with on the spot. Apologies for the inflicted cognitive dissonance…
Absolutely-excellent analogizing, Hoomin: communicating the essence when many just left the wall of incomprehension!
Thanks for helping lots of people.
& let the graverollers do their thing.
d :
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Think of the universe not as objects flying apart, but as the fabric of space itself stretching. The more distance there is between two objects, the more ‘new space’ is generated every second, across that entire distance.
Light travels 9.461 × 1015 meters per year. At a certain distance (the Hubble limit), more than 9.461 × 1015 meters of new space is created in a year. So for stars beyond the Hubble limit, the light sent from those stars actually ends up further from us after a year than when it started. That’s what “moving away from us faster than light” means.
That is not what the theory of special relativity says. It says that nothing with mass can accelerate to the speed of light. This may sound pedantic, but it is quite a different statement.
Aso, the expansion of spacetime isn’t much more than a “best guess” as to why we see the redshift of distant galaxies, but the truth of the matter is that we don’t really understand much yet about this universe. Not really.
Speed is distance/time. But time is relative, how time progresses is not some universal constant. And it gets WAY weirder than that. Here are some links, one of Richard Freymann explaining light much better than I can.
https://www.youtube.com/watch?v=kWCl7diBGos
And another about the mystery of information and time.
That YouTube video isn’t Richard Feynman by the way. It’s an AI voice reading a text loosely based on his work.
Thanks, I didn’t notice that. I’ll pay more attention in the future.
It got me too, last week.
Richard Freymann explaining light
Sounds like it’s not actually Feynman, it AI.
This isn’t his voice — it’s our tribute to his teaching style, created purely for education and inspiration. No impersonation intended, just deep respect for one of history’s greatest teachers. 🙏
All content is created to inspire, educate, and encourage reflection. This channel follows YouTube’s monetization policies, including clear labeling of synthetic media.
Thanks, I didn’t notice that. I’ll pay more attention in the future.
I actually started watching a video from this channel a while back and was disappointed to learn it wasn’t him. I suppose some people don’t mind and find the videos interesting, but I personally find it disturbing, as though they’re re-animating his corpse or something. 😆
Adding to other answers to your question, once you’ve grasped how it is, here is another mindfuck for you.
Eventually the space between stars will be too big for emerging intelligent species in the far future on other planets to ever even know a starry sky. There will simply put not be any meaningful light from other stars to reach them. Their entire universe will be their star system and then nothing.
The alternative might be worse. If light was faster than the expansion of the observable universe, the entire sky would eventually be bright as all the stars combined.
Make sure your answer comes from a cosmologist, not a cosmetologist! They sound similar, but I had to retake AST1430H Cosmology, because I consulted the wrong sources.
Don’t get me started on my mathematologist friend.
My cosmetologist told me I should get rid of my black holes before any light hits them.
My Cosmetologist told me I need to realign my chakras because Mercury is in retrograde.
Are you a Taurus by chance? My Astronomy teacher told me I’d be meeting someone like you today at our last reading.
I hope you’re looking for a new one that isn’t so racist.
There was a short and beautiful moment in time when I understood all of this. But then I forgot.
Same. I can actually still sorta imagine the analogy that I used to understand it myself, but I find it impossible to actually articulate.
i thought it was because the expansion doesnt actually make things move… all the space between all the things is getting bigger at the same time, making it look like things are moving… in aggregate that can look like more than speed of light
But things are actually moving.
The movement causes light emitted from those things to redshift, like the siren of an ambulance changing its pitch when it’s moving away from you. And stars we can currently still see will disappear in the future, never to be seen again, as they move outside of our observable universe, accelerating faster away from us than the light they emit.That’s the thing. Nothing is actually moving, it just appears to be. Space itself is increasing in volume.
The best analogy isn’t that everything’s moving, it’s that everything’s shrinking.
If you and a friend are stood 2 metres apart and you suddenly both shrink, proportionately, to half your height, the distance between you is going to appear to have doubled, when in fact it’s still 2 metres.
Universe expansion turns this on its head by the distance itself growing to 4 metres without either of you moving.
As to why this doesn’t happen on local scales: gravity has a tendency to hold nearby things together. And closer still, atomic forces.
As to why this doesn’t happen on local scales: gravity has a tendency to hold nearby things together. And closer still, atomic forces.
For now. As I understand it eventually space will be so stretched out even atoms will degenerate.
i thought distance caused red-shift… prolly semantics… but … do 2 stationary objects on an expanding plane ‘move’?
An observer perceives them as moving objects, i.e. they appear to move, but they actually don’t, as it’s the space between them that expands.
I always thought it was more like, since light can act as a wave, it is like the wave is becoming stretched out as the space expands which creates that redshift. The light isn’t moving any faster or slower, but it has a redder (lower energy) frequency. Like a plucked string that is pulled more taut as the space in between expands. It essentially loses energy, and at some point that energy loss will be significant enough for light from other galaxies no longer being detectable for us. As well as any new light emitted from them simply not being able to overcome the distance+expansion speed.
There is just more space being added in between us and them, as if we were on a plane of stretchy fabric or on the surface of a balloon being blown up. From their (the other galaxy’s) perspective, we are doing the same exact thing, as well as every other thing that is observable to them.
*words of someone who is not an astronomer, nor a scientist.
The expansion of space actually stretches light out into longer wavelengths called red shift.
So light doesn’t ignore it.
Spacetime, the medium through which light is traveling, is itself expanding. As someone said earlier, its akin to blowing up a balloon, except the balloon is inflating faster than light would be able to traverse the distance along the surface of the balloon.
Full disclosure, I’m not a scientist just a person on the internet, but here is my understanding.
The confusion starts with our use of units which are otherwise static and predictable in “close” cosmological terms, in this case:
- the speed of light
- a light year, a unit of measure of distance over which is light can travel in the time it takes Earth to orbit our sun one time.
How do we measure a distant object and determine its distance? By measuring the light that is emitted by that object and seeing how much it has red-shifted (with the wavelength of that light being the underlying thing being measured) with an Earthbound observer as the relative point of measurement. A longer wavelength (into the red end of the spectrum) denotes the object traveling farther away from us. This last point is right in light with special relativity.
However, what if there is another thing beyond special relativity’s effects also increasing the wavelength of the measured light. Our measurement becomes polluted by this other variable. That other variable is the expansion of the universe further lengthening the wavelength of light. Essentially the distance the light is traveling is being extended causing the additional special relativity effects to our sample. So since our measurement of distance is based upon the behavior of light traveling over a distance, and we derive that distance from the parameters of the measured light, we can (and must) subtract out the speed of light from a measurement and the difference we see allows us to measure the expansion of the universe by itself.
Its a spacetime effect. Take a partially inflated balloon. Draw a circle on it. Draw a line across the diameter of the circle. The diameter line represents the speed of light from one edge of the circle to the other. Starting from one side of the circle, measure 90% of the diameter across and draw a dot. Go from the other side of the diameter and again measure 90% across from to the other side of the circle across the diameter. Now blow the balloon up to twice its previous partial size. Both dots are still at their relative 90% of the diameter. So the length of the diameter line you started with is actually the combined value of both the speed of line as well as the expansion of the balloon. The place where this example falls apart is that if you were to instantly transport (impossible in our understanding of the laws of the universe) to that balloon light year section of space, you would not see the effects of the expansion of the universe. It would look like the original partially inflated balloon you stated with.
Objects beyond our cosmic event horizon are in a similar state to objects inside a black hole’s event horizon: we can describe them in hypothetical terms, but they’re effectively outside of our universe. There’s no longer any causal connection between us and them in either direction, and our relativistic frame of reference doesn’t extend to them.
Nobody does.
That’s why we have the hubble tension.
Cosmology is full of tensions to catastrophes… No one really knows, definitively.
The greatest catastrophe, I believe, is the vacuum energy catastrophe. Which theoretically ought allow zero point energy (energy extraction from empty space - literally a vacuum.) that’s according to quantum mechanics… But any time anyone’s ever tried to do this and when they have actually measured it… It’s infinitesimal. It’s one of (of not the) the biggest tensions in cosmology.
Just like if anyone tries to tell you how old the universe is they’re full of shit and it’s a guess based on hell of a lot of assumptions not actual hard science.
And the problem with modern cosmology is it’s been framed for so long in certain ways that most astrophysicist and cosmologists are tending to find what they think they should find. They’re not really making new discoveries or unveiling new truths they’re just trying the same old shit and getting equally shitty (meaningless) answers.
Iirc, the speed of light limits the speed at which action (information) can be transmitted through space, but doesn’t impose a limit on the “velocity” of the space itself, i.e. two objects can remain “at rest” at fixed coordinates in space but appear as being moved away from each other (even with a velocity larger than the speed of light) due to expansion of the space.
(I’m only a mechanical engineer with profession in mechanics, not a physicist, so please take this with a grain of salt)
