This educational paper [0] titled "Expanding Confusion" (2003) is a classic on the general topic and well worth the read.
Holding the two concepts of an accelerated (!) (in terms of objects [1]) expanding universe and the fixed finite speed of light simultaneously in one's Euclidean head can be dizzying, so be prepare to draw and enjoy the hard earned manual labor of counterintuitive conclusions.
Took me a minute to wrap my head around it, that explanation isn't worded that clearly, but then I got it.
That happens because time is a factor in how light from different parts of the object will reach the observer. Light from its far side takes longer and in that time the object continues to move. You can see behind the object, because its rear end moves out of the way of the light coming from itself during the travel time of that light.
How does the rear end move out of the way? Wouldn't it be blocking the light? It's not like the object could move out of the way faster than the speed the light it traveling at (ftl)
That's amazing. I've watched lots of videos about length contraction and I don't think any of them ever mentioned this (the shape of an object moving a near light speed won't change to an observer, it will just appear as if it had rotated instead of being "squeezed" as every video about this seems to imply)!
I actually think the opposite is true. The way I've heard it phrased and explained that makes the most sense to me is "everything moves through spacetime at the same rate" - it's basically the clock speed of the universe. It's just that if you move faster in a space dimension that your relative movement in the time dimension slows down.
It only seems weird to us because our senses and minds evolved in an environment where things we can perceive never differ by relativistic speeds.
Nah, its real simple. SR just comes about because you want to keep chemistry working the same on a rocket doing 99% of the speed of light as it is at rest.
Working out all the implications becomes very complex.
But then you probably wouldn't have life to observe it if the simple rules didn't have complex emergent behaviors.
A limit to the speed of causality makes physics so much simpler. Without it you'd need to factor in the interaction of every particle with every other particle in the universe.
Frankly, it may very well be KISS because the other options were so much more complex. Or they said that if we put speed of light to constant to make it simple, there were so many unforeseen edge cases because of it. The devil is in the details, perhaps?
Is it reasonable to view the Cosmic Microwave Background Radiation as being the limit of this? The remains of the big bang, maximally scaled up and red shifted as far as things can be today?
I think there's a coherent explanation of the cmb be had there, but it's not the conventional explanation.
Under this alternative, the universe cooled to light transparency some time before the moment depicted by the cmb, and anything "further away" than that hugely magnified scene just happened outside of our light cone. That is to say, it's "elsewhere" (a technical term (https://web.phys.ksu.edu/fascination/Interlude1.pdf)).
Seems to me that in this alternative, cosmic expansion could be explained as gravitational attraction between elsewhere-matter and matter in our light cone.
Imagine there's some argument to be made for why this is not the case, but I don't know it. It would require a bit of explaining re: why that point in history and not some other?
- Is it that the maximal distance is constant and that the cmb is subtly changing in ways we havent noticed (as the point of most-distant-past moves forward in time)
- Or maybe something caused the speed of light to change at that time, pruning the rest of the universe from our view.
Kinda but different scale, the CMB era universe was about 1100 times smaller than that now, so still huge.
There may be a neutrino background behind the CMB, where the universe was even smaller, and the gravitational wave background behind that with even more of a size difference.
Take this a further step. Assuming we had telescopes big enough and sensors sensitive enough, what does the structure of the deepest parts of space look like? Are there pre-galaxy-formation structures which are smaller than galaxies and yet take up huge swaths of sky? Are there structures from some point in the past that take up so much space on the sky that not very many of them can "fit", and, if so, do the calculations work out so that an equivalent explanation for having not very many of them is that the [region of the] universe [which is observable to us] was just that much smaller back then?
The furthest we can see is the physical limit of universe: we can literaly see thefirst photons after universe became transparent. That is the CMB (cosmic microwave background, and you can easily google a real picture). The problem is, while these are the oldest photons we will ever be able to see, they still are from when universe was cca 400 000 years old, and by that time it was 100 million lightyears wide. That picture tells us that the universe was extremely homogenous (altho not perfectly), and basically no such structures you talk about.
If we would like to see even further, we must give up on photons completely, and probably probe the ultra deep space gravitation waves. Those should give us picture even of completely opaque universe, as it was before then. So far we can only "see/hear" the brightest/loudest events in the universe with our gravitational waves observatories, but the fact we can even do that is astounding nontheless: we built a new sense for humanity, that no other known creature in the universe posses. LISA project should hear more.
Fascinating. A related question: When we look at Andromeda, which has a diameter of 220,000 light years, we are looking at it slightly edge on. Shouldn't the stars on the back edge be in a relatively different place in the sky than the stars on the front edge since the galaxy has moved relative to us over that 220K light years?
One thing that always bothers me is time dilation when it comes to observing distant objects like this. If it takes 4 light years for the light to travel one way (and the one way speed of light still hasn't been measured!), that is for the observer at the origin, the photons we observe, for them it is much less than 4 years, is it even in years? So if I look at alpha centauri with a telescope, it isn't really aloha centauri 4 years ago that I am observing right? It's much more recent than that?
Otherwise, if a 30yo person travels at the speed of light from alpha centauri to earth, when the person arrives will they be a 31yo(~) person who arrived 4 year later, effectively time traveling to the future? And if they return right away, will folks at alpha centauri meet a 32yo(~) person who came back 8 years later? If so, then maybe superman had the right idea about flying really fast around earth to travel in time, just not to the past.
Perhaps some billionaire will decide to spin around the solar system really fast for a few decades and skip a century or so? Haha!
A photon travelling between Alpha Centauri and Earth does not experience time it effectively arrives instantly. We as observers however will see that it took 4 years to make its journey. The idea of whether it's recent or not is irrelevant, it is all relative (to the photon it's recent, to us the light is 4 years old).
Readit News
Holding the two concepts of an accelerated (!) (in terms of objects [1]) expanding universe and the fixed finite speed of light simultaneously in one's Euclidean head can be dizzying, so be prepare to draw and enjoy the hard earned manual labor of counterintuitive conclusions.
[0]https://arxiv.org/pdf/astro-ph/0310808.pdf
[1]https://bigthink.com/starts-with-a-bang/universe-expansion-n...
Dead Comment
How when going at relativistic speeds, you start to appear to rotate to obervers even if you are going straight - you can even see behind the object!
That happens because time is a factor in how light from different parts of the object will reach the observer. Light from its far side takes longer and in that time the object continues to move. You can see behind the object, because its rear end moves out of the way of the light coming from itself during the travel time of that light.
https://youtu.be/watch?v=uTyAI1LbdgA
It only seems weird to us because our senses and minds evolved in an environment where things we can perceive never differ by relativistic speeds.
Working out all the implications becomes very complex.
But then you probably wouldn't have life to observe it if the simple rules didn't have complex emergent behaviors.
https://news.ycombinator.com/item?id=37197977
Under this alternative, the universe cooled to light transparency some time before the moment depicted by the cmb, and anything "further away" than that hugely magnified scene just happened outside of our light cone. That is to say, it's "elsewhere" (a technical term (https://web.phys.ksu.edu/fascination/Interlude1.pdf)).
Seems to me that in this alternative, cosmic expansion could be explained as gravitational attraction between elsewhere-matter and matter in our light cone.
Imagine there's some argument to be made for why this is not the case, but I don't know it. It would require a bit of explaining re: why that point in history and not some other?
- Is it that the maximal distance is constant and that the cmb is subtly changing in ways we havent noticed (as the point of most-distant-past moves forward in time)
- Or maybe something caused the speed of light to change at that time, pruning the rest of the universe from our view.
There may be a neutrino background behind the CMB, where the universe was even smaller, and the gravitational wave background behind that with even more of a size difference.
https://en.m.wikipedia.org/wiki/Cosmic_microwave_background
The interpretation of the Poynting vector is another.
If we would like to see even further, we must give up on photons completely, and probably probe the ultra deep space gravitation waves. Those should give us picture even of completely opaque universe, as it was before then. So far we can only "see/hear" the brightest/loudest events in the universe with our gravitational waves observatories, but the fact we can even do that is astounding nontheless: we built a new sense for humanity, that no other known creature in the universe posses. LISA project should hear more.
Otherwise, if a 30yo person travels at the speed of light from alpha centauri to earth, when the person arrives will they be a 31yo(~) person who arrived 4 year later, effectively time traveling to the future? And if they return right away, will folks at alpha centauri meet a 32yo(~) person who came back 8 years later? If so, then maybe superman had the right idea about flying really fast around earth to travel in time, just not to the past.
Perhaps some billionaire will decide to spin around the solar system really fast for a few decades and skip a century or so? Haha!
A photon travelling between Alpha Centauri and Earth does not experience time it effectively arrives instantly. We as observers however will see that it took 4 years to make its journey. The idea of whether it's recent or not is irrelevant, it is all relative (to the photon it's recent, to us the light is 4 years old).