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Laittakaa meemejä tänne.
In the same way that two cars driving away from each other at 60 mph have relative speeds of 120 mph with regard to each other, two bodies moving away from each other at less than the speed of light have relative speeds exceeding it. Everything in the universe is moving away from everything else and sometimes at relative speeds that exceed the speed of light. Nothing is individually exceeding the speed of light in absolute terms.
I fucking hate that aspect of Special Relativity when I did l A-Level Physics (wait, shouldn’t that be “Physic” in the US to go with “Math”?). Two spaceships head off in opposite directions at light speed - from the frame of reference of each spaceship, the other is moving away at C, not 2C, because the Universe would rather slow down time itself than let anything move faster than its stupid precious C!
Wait does that mean theoretically one thing can slow another down by just being an observer if it is also moving in the opposite direction?
It’s pretty complicated and it’s been a while since i read the layman’s-terms no-maths explanations, but think of it like driving
You’re in a car looking at another car which is driving at the same speed on s road which is parallel to you. You’re both going to look like you’re driving at the same speed and travelling the same distance
Now say that rather than being parallel the roads are at an angle to each other. Say 45 degrees. Now when you look at the other car, even though its Speedo will say the same as yours it’ll look to you like it’s going slower and it’ll fall behind you. After a while you won’t be able to see it out of the drivers side window and will have to look through the back seat window and then the rear window to see it
And the experience in the other car will be the same - they’ll see you as going slower and falling behind them
Nobody’s speed has actually changed, it just looks different from each car’s perspective
If you can mentally change “difference travelled” to “time passing”, then that’s how to conceptualise it
Cool, that's a good explanation, thanks
Relative speeds also cannot exceed the speed of light. Since there's no absolute reference frame, if this were possible it would be no different than exceeding the speed of light on "absolute" terms. Once you get up to speeds where this would matter, funny dilation effects that I'm too dumb to understand would prevent this.
Cars are not driving away from each other at more than the speed of light relatively. The road is stretching faster than the speed of light.
Yeah, but wouldn't this lead to the cars perceiving each other as moving faster than light?
No, because the space is expanding faster than light, the light can't bridge the gap and so you and the other car simply can't perceive each other at all.
The universe is mostly nothing. So obviously the universe, being nothing, expanding faster than the speed of light isn't surprising, as nothing is faster than light. 😌
We should probably use the word “growing” instead of “expanding”, would that be easier to follow 🤔
no
Take a balloon.
Blow it upto about 50mm
Make a couple dots around it
Blow it up a little more.
Now there's distance between the dots.
Imagine an ant walking between the dots. That ant is going at the speed of light (as fast as it can go) relative to the dots.
Now as it walks between the dots, blow the balloon up really big
The dots aren't moving, they're stuck to the surface of the balloon. The balloon itself is expanding. The ant is going at the speed of ant-light, but now the dots are all "moving away" faster than the ant can walk.
The speed of the ant hasn't changed, the space the ant is traveling has changed. And faster than the ant can move, because the balloon isn't limited by the same things the ant is.
WHOS BLOWONG UP THE BALOON AND WHEN WILL IT 💥
Thanks for that that's actually a really helpful analogy.
I mean i still dont understand. Brain hurty. But thanks anyway
Things cannot move through space at a speed faster than lightspeed.
This rule does not apply to space itself.
Also, interestingly, shadow boundaries can 'move' faster than the speed of light.
https://www.iflscience.com/shadows-can-move-across-a-surface-faster-than-the-speed-of-light-75112
Because a shadow isn't truly a 'thing'.
Its just an area where light bouncing off of something is not happening (as much).
Wrong, the expansion doesn't have a speed because it isn't motion. But you have to think about it longer than you'll probably want to before hitting the up or down arrow and/or scrolling.
Best intuition I've heard for this is that "things" can't move faster than light, but not everything is a "thing".
Imagine doing shadow puppets on the wall with a flashlight. You move the bunny left, shadow moves left. The further away the wall is, the faster the apparent speed of the shadow bunny. You might think that, far enough away and with a strong enough light, your shadow bunny would be racing across the sky faster than the speed of light -- and the crazy thing is, you'd be correct! The shadow (absence of light) can move arbitrarily fast. But the light itself is moving at its normal constant speed from the flashlight out into space, perpendicular to the travel of the bunny, like a garden hose spraying water. The time it takes for the shadow to even begin to move is governed by the speed of light. No information can be communicated faster than light because the light travels at the speed of light to illuminate the places where the shadow isn't.
Best analogy I heard for it is if you put a load of dots on a balloon, then inflate it. Are the dots getting further away? Yes. Is there just the same amount of rubber between each dot as when you put the dots on? Yes. Can you measure the relative speed of the dots? Yes! But have they actually gone anywhere? No...ish?
Understanding this is the easy part imo
I remember finding out the shape of space isn't a vast plane of emptiness, but more of an ever growing sphere and that messed me up.
You also need to consider what the things inside it look like to you or anyone else. The observable universe is the volume inside which light can reach us here on the earth... technically it's different for everyone because we're all in a slightly different position, making everyone the centre of the universe, but in terms of the scale of the universe, that's irrelevant levels of accuracy and you can safely state that everyone on earth is in the same place. Now because space is expanding and carrying stars and galaxies with it, those galaxies will eventually pass the point where their light could travel for an infinite time and never reach us, in other words, they would pass beyond the edge of our observable universe. Eventually, given enough time, all that will be left in the night sky (long after the earth is destroyed) will be our galaxy, and possibly bits of our local group, which are gravitationally bound and would hold their relative positions as space expands around them.
As far as I know, not even that is certain. I read something about other topologies, like a donout shape or even higher dimentional topologies being plausible as well. Interesting rabbit hole but really makes you question our how we view our reality.
Explanations why space expands are way more crazier than this.
Preemptive explanatory note: the speed of light, approximately 300,000 km per second, is the highest speed that something can move through space.
The expansion of space doesn't happen at a set speed. It happens at a rate of approximately 70 km per second per megaparsec. So if you're measuring two points half a megaparsec away from each other, then every second, the space between them grows by about 35 km. If you're measuring two points 2 megaparsecs away from each other, then every second, the space between them grows by about 140 km.
If you're measuring two points 4300 megaparsecs away from each other, then the spacetime between them grows by about 300,000 km every second. That's not to say that anything is moving at 300,000 km per second, there's just more space between them every second
Moving is just putting more space between you and something. If it walks like a horse, talks like a horse, bites like a horse... you get the idea. It's not clear to me why the increase of inbetween space in cosmic inflation "gets a pass" whereas the increase of inbetween of space from movement doesn't
Apologies for the wall of text. I spent an hour and a half trying to find the most concise way to explain this, and there kind of isn't one...
There are two ways for the distance between two objects to increase
There are two ways for the distance between two objects to increase over time. One of them involves displacement through a medium (movement), and the other involves that medium itself expanding. From our perspective, a galaxy 5000 megaparsecs away is "moving" away from us by about 350,000 km every second, but that's only because for every megaparsec between us, the space itself is expanding by about 70 km every second. If you ask a guy smack in between us, he'll say we're both moving away from him at 175,000 km every second. That galaxy isn't experiencing any acceleration in that direction; it isn't moving through space at that speed.
An attempt at an analogy
A submarine increases its depth by sinking lower in the water, right? Imagine a tub of water 1 foot deep. Put a toy submarine 6 inches from the bottom, and it's 6 inches underwater. Now add another six inches of water to the tub. Your sub hasn't moved through the water at all, but now it's 1 foot deep. The submarine can increase its depth without sinking at all.
In fact, if you let the submarine rise slower than you add water, then it can rise upward through the water as it continues to get deeper below the surface.
The speed of light is a constant
The speed of light (in a vacuum) is a constant. That is to say, every observer in every reference frame measures every photon (in a vacuum) as moving at 299,792,458 meters per second. This fact supersedes all others. It supersedes time itself. Imagine I'm on a train going 50 mph, and I throw a ball forward at 50 mph. In my reference frame, that ball is going 50 mph. To an outside observer, that ball is going 100 mph. How fast the ball is going depends on your frame of reference.
This is not true of light. If I'm traveling on a train going half the speed of light and I shine a flashlight forward, the train's speed doesn't add to the light's speed. You and I will both agree that those photons are moving 299,792,458 meters every second, in both our reference frames. This happens because we aren't experiencing the passage of time at the same rate.
The photons coming out of that galaxy 5000 megaparsecs away are also going 299,792,458 meters per second in our reference frame, even as the space between us grows by more than that in that amount of time. That galaxy isn't moving faster than light, space is just expanding.
Wtf is a megaparsec? It's a million parsecs. Tf is a parsec? A parallax arcsecond.
...Tf is a parallax arcsecond?
An attempt at an explanation for the layperson
Imagine you're standing outside. In front of you is a tree and behind that on the horizon is a mountain. You move 10 ft to your left, and the tree looks like it moved to the right, but the mountain looks like it hasn't moved at all. That's parallax. The closer something is, the more it appears to move when you move.
Imagine you are the pivot point on a big protractor. Your field of view can be divided into 360°. Every degree can be divided into 60 parts, called arcminutes. Every arcminute can be further divided into 60 arcseconds. Each arcsecond is 1/3600 of a degree.
How do these fit together? There's one more thing I need to explain.
The earth orbits the sun at around 149.6 million kilometers. That's called an Astronomical Unit. A parsec is the distance that an object would have to be, so that moving one Astronomical Unit would make it appear to shift sideways by 1 arcsecond.
Fraser Cain did a better job explaining, because he can use pictures
It's 3.26 lightyears.
just more space between
Space out of thin air... tell me, mate, can I sell it?
Well, nothing (with nonnegative mass) can move faster than light through space. Space itself can do whatever it wants to.
besides the expansion of spacetime which is the correct answer, there's also nothing keeping two objects from traveling in opposite vectors each at 60% c. Frame of reference matters too
No, spacetime doesn't expand faster than light at any point. Its just that as you accumulate the new growth over a long distance, the farther objects appear to move away faster than light from our position.
The very short, very bastardized version is that as objects move at speeds closer to the speed of light, the way everything else around them appears to be shaped and moving changes. A "stationary" object you pass seems less long than it should in the dimension parallel to your travel. The net result is that however two objects are moving relative to each other, their own speeds warp their experiences of the universe such that nothing else is observed to be doing something "illegal".
Shadows can move faster than light.
Only on the Discworld
No they can't, because they aren't moving at all. That vsauce video pisses me off so much and I've debunked it more than once over the years, but suffice to say, you could claim light moves faster than light using the same logic he uses for shadows/darkness.
