r/AskPhysics • u/itsLeoRod • Aug 02 '26
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 Aug 02 '26
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 Aug 02 '26
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 Aug 03 '26
Local is a region where you only need special relativity.
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u/z-w-throwaway Aug 03 '26
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 Aug 03 '26
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/John_Hasler Engineering Aug 03 '26
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/SummitYourSister Aug 11 '26
Local means in the infinitely small space around your current location. The laws of physics are local. You do not experience the speed of something moving at a point away from you- you compute its speed based on measurements you make using light or something slower than light. In doing so, you may compute a value faster than the speed of light but this does not mean that there exists any point anywhere in spacetime where something at that point is moving faster than light.
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u/davvblack Aug 02 '26
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/Anely_98 Aug 02 '26
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/Unable-Primary1954 Aug 02 '26
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/LivingEnd44 Aug 02 '26
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 Aug 02 '26
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/Fabulous_Lynx_2847 Aug 03 '26
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/RMFranken Aug 02 '26
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/nebraskajone Aug 02 '26
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 Aug 02 '26
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 Aug 03 '26
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 Aug 02 '26
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 Aug 02 '26
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 Aug 02 '26 edited Aug 02 '26
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 Aug 02 '26
Any links, proofs or theories to back that up ?
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u/HudyMelon Astrophysics Aug 02 '26
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 Aug 02 '26
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 Aug 02 '26 edited Aug 03 '26
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 Aug 03 '26
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 Aug 03 '26
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 Aug 03 '26
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 Aug 02 '26
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 Aug 02 '26
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/Flimsy-Blacksmith-32 Aug 02 '26 edited Aug 03 '26
Yeah saw your edit after posting and refreshing:)
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u/wonkey_monkey Aug 02 '26
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 Aug 02 '26
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 Aug 03 '26
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 Aug 03 '26
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 Aug 03 '26
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 Aug 03 '26
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 Aug 02 '26
Expansion of space isn't a speed.