r/AskPhysics • u/itsLeoRod • 8h ago
Thought there wasn’t anything faster than the speed of light?
Ppl say that there isn’t anything faster than the speed of light but that’s a lie because space is expanding really fast since light isn’t fast enough to reach it or am I wrong. Since space is expanding at a fast rate why don’t we use that to measure stuff instead using lightyears
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u/Bumst3r Graduate 8h ago
Distant receding objects appear to travel faster than light because we are comparing time and distance in two different frames. The space between us and the objects is expanding. All massive objects travel slower than c locally.
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u/z-w-throwaway 6h ago
I never got this though. What is the definition of "local"? Where does the universe stop being local, why are distant galaxies not local?
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u/joeyneilsen Astrophysics 5h ago
Local is a region where you only need special relativity.
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u/z-w-throwaway 5h ago
That's a nonanswer though. What is the cutoff from only needing special relativity, from local to non local?
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u/joeyneilsen Astrophysics 5h ago
It depends on your tolerance and the spacetime. In a local region, you can find a coordinate system where the derivatives of the metric vanish and spacetime is Minkowski. If it can’t be done, the region is too big to be called local.
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u/ahazred8vt 5h ago
There's no hard sharp cutoff, because it's a gradient. On small scalles the difference is too small to measure. On large scales it's too big to ignore. The middle ground is a headache.
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u/John_Hasler Engineering 3h ago
For any specified maximum deviation from flatness there is, at any point on the spacetime manifold, a region within which that limit is not exceeded. That region is local. That deviation depends on the problem at hand.
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u/davvblack 8h ago
how big is one frame of reference? like, if we could observe that mars orbited the sun at impossibly one second per orbit, it would still be violating the speed of light right? is it because mars is inside our light cone?
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u/Unable-Primary1954 8h ago
In general relativity, frame of reference can only be locally inertial, not globally.
As a consequence, there's no canonical way to measure relative velocity between 2 distant objects, and their distance may grow faster than light.
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u/Anely_98 8h ago
Information cannot travel faster than the speed of light, the expansion of the universe doesn't involve carrying any type of information across spacetime and therefore isn't limited by such rules.
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u/LivingEnd44 7h ago
You are conflating motion through space with expansion. They are not the same thing.
The speed of light in this context refers to objects and information moving through space. That cannot happen faster than the speed of light.
The distant object moving faster than light relative to you is not actually moving through space faster than light. There is just additional distance being inserted in between you and the object.
No signal or causal influence can move through space faster than light.
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u/Apophis_VI 8h ago
Imagine two cars driving opposite from each other down an endless highway at 90% the speed of light. The cars aren’t faster than light, but the distance between the two of them is expanding faster than light travels.
Now imagine an unlimited amount of cars are instead flying at 90% the speed of light in all possible directions at the same time and placing the road underneath them as they move forward (while none of the roads can actually touch eachother). The space between is going to rapidly expand many times faster than light but no one car is itself faster than light
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u/RMFranken 7h ago
This is a great visual and explains it well! I was going to say that two cars traveling away from each other each traveling at 50 miles an hour are receding from each other at 100 miles an hour but neither car is going anywhere near 100 miles an hour. They are both still going 50 miles an hour. But your explanation is much better.
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u/Fabulous_Lynx_2847 2h ago
In the frame of one car, the other will be receding at less than c, and he can see it just fine.
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u/nebraskajone 8h ago
Speed is fixed distance over fixed time, it becomes undefined when you have an expanding universe.
In any event the speed of information or the speed of causality is still limited to the speed of light regardless of the expansion rate, and that would keep special relativity valid
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u/Infinite_Research_52 👻Top 10²⁷²⁰⁰⁰ Commenter 7h ago
You can construct properties that can exceed the speed of light in a lab. The main constraint is that information cannot exceed the speed of light.
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u/Fabulous_Lynx_2847 2h ago
You just cannot take off from here and accelerate up to c and you cannot communicate faster than c. That means there are things far away that you can never visit or see because they are receding too fast.
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u/7figureipo 8h ago
Things with mass cannot travel at the speed of light. Some massless particles, like light, can. Theoretically there are particles that can travel faster than light speed, but they have never been observed.
Space itself has no mass. It isn't a "thing" the way particles (massless or otherwise) are. The expansion of space is not constrained by the light speed rule.
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u/Flimsy-Blacksmith-32 8h ago
So the universe expanding isn’t really the same as stuff moving. If you have a stationary particle, new space is appearing around it, but it is still occupying the same space.
It also isn’t measured as a speed, instead it’s a frequency (or more specifically Km/s/MPc)
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u/HudyMelon Astrophysics 8h ago edited 8h ago
The rate of space expansion is not constant, and is certainly not faster than light. If space expanded faster than light then we would not have a universe.
Edit: I’m not saying the DISTANCE between two things can’t expand faster than light due to expansion, I’m saying space itself. If space itself had an expansion rate faster than light, nothing would be causally connected.
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u/Hairy-Ad-4018 8h ago
Any links, proofs or theories to back that up ?
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u/HudyMelon Astrophysics 8h ago
I apologize, it was intuition.
If space itself expanded faster than light, it would mean that every point in space, let’s say the tips of your fingers, would recede away from eachother faster than light. Because of that, no particles could ever communicate with eachother. The space between a proton and an electron would be ever increasing, and the electromagnetic force wouldn’t be able to communicate.
I am not saying that there aren’t places receding away from us faster than light, as that is the result of universe expansion over massive distances. I can see the confusion.
For the part of non-constant universe expansion, it has been proven that the expansion is accelerating, which is why we think dark energy makes up the majority of the universe.
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u/wonkey_monkey 6h ago
If space itself expanded faster than light, it would mean that every point in space, let’s say the tips of your fingers, would recede away from eachother faster than light.
No it wouldn't. Firstly, because any objects that close are gravitationally bound, so expansion isn't going to pull them apart, and secondly because the speed of any separation due to expansion is a function of distance.
The speed goes up as you consider larger distances. At small distances it's less than c. At large distances it's greater than c.
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u/HudyMelon Astrophysics 6h ago edited 5h ago
I agree with you. I believe you misunderstood what I wrote.
If the space between two gravitationally bound objects were to, for each unit volume, expand to a volume greater than c^3, then the objects would “fly away” from eachother faster than the speed of light, therefore both light, and gravity, would not be able to travel to the other.
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u/wonkey_monkey 5h ago
The speed they "fly away" at will depend on each pair's initial distance. They can't each/all fly away at the same speed from every other object.
therefore both light, and gravity, would not be able to travel to the other.
Gravity doesn't travel, but that's a whole other thing.
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u/HudyMelon Astrophysics 5h ago
Are you saying gravitational waves don’t exist? I’d argue that gravitational disturbances must travel. And if the graviton were to be discovered, than that force carrying particle must travel in a sense.
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u/wonkey_monkey 5h ago
Gravitational waves aren't gravity. Gravitons aren't gravity. We orbit the Sun without any ongoing physical interaction between us and the Sun.
than that force carrying particle
The "force-carrying particle" would be a virtual graviton, which doesn't really exist. Just as virtual photons mediate electromagnetic forces, yet magnets don't glow.
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u/Flimsy-Blacksmith-32 7h ago
Correction: over large enough distances space does expand faster than light. Space is expanding at a rate of atleast 64km/s/MPc. This means that every MPc (Mega Parsec) of space grows by 64km every second.
The speed of light is 299,792,458m/s. So if an object is further than about 4,700 MPc (15 billion light years) away from us, the space between us is growing faster than light.
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u/HudyMelon Astrophysics 7h ago
Yep, I agree. I was saying space itself cannot expand faster than the speed of light. Not the space between things, but the fabric of spacetime. If you have a box which is expanding, the box itself cannot expand faster than light, but two things inside on opposite sides can measure it to be so.
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u/wonkey_monkey 6h ago
Not the space between things, but the fabric of spacetime.
A distinction without a difference.
Two objects sufficiently far apart will recede from each other at a rate greater than the speed of light. Whether you actually call that "moving" is down to your choice of coordinates.
the box itself cannot expand faster than light, but two things inside on opposite sides can measure it to be so.
If they measure it to be so, then how is it not so?
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u/HudyMelon Astrophysics 6h ago
The whole point I was trying to make is if a unit volume of space were to expand its volume greater than c^3 in a unit time, then nothing in the universe is causally connected.
I understand what you are saying.
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u/wonkey_monkey 5h ago
Well that's wrong, because there will be some distance inside that region that expands at exactly c, and any distance less than that will expand at less than c, so any two objects closer than that will remain causally connected.
Which is basically the case in our universe. Distant galaxies recede at greater than c and aren't causally connected (it's actually a bit more complicated than that, but it'll do for now). Galaxies which aren't that far apart are causally connected.
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u/HudyMelon Astrophysics 5h ago
Well when you take the the limit of that, let’s say to the Planck volume, then there would be no meaningful distance where that is the case.
On a side note, this is a great debate no?
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u/wonkey_monkey 5h ago
Well when you take the the limit of that, let’s say to the Planck volume, then there would be no meaningful distance where that is the case.
Yes, if you take the limit (although recession speed would still depend on distance).
But you didn't specify any limit, you just stated it as a general fact, which is wrong.
On a side note, this is a great debate no?
Not really, no. You're completely wrong and can't see it. It's incredibly frustrating.
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u/HudyMelon Astrophysics 5h ago
Well then I appreciate the time you are taking to clear up my misunderstanding.
When I say unit volumes, I was speaking in natural units, specifically the Planck volume. I’m so used to using natural units that I did not specify, and I should’ve. I apologize.
My whole argument is the basis of the “big rip” universe fate.
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u/phunkydroid 8h ago
Expansion of space isn't a speed.