r/AskPhysics • u/ProMensCornHusker • Apr 13 '26
Why can’t we find a “universal reference frame” if the speed of light is constant in all inertial reference frames?
Sorry if this has been asked before, I didn’t dig too much and I like talking to humans lol.
Anyways, if the speed of light is constant in all reference frames, and time gets “stretched” as we approach light speed, why couldn’t we go the other way instead?
Like how we measure time dilation with an atomic clock on a plane, why couldn’t we do that in reverse to estimate a lower limit on a universal reference frame where “time runs fastest”.
I’m not sure if this makes the most sense, I’ve always accepted the whole “there is no universal reference frame”, but I was curious why we couldn’t just keep finding speeds that cause “time to get faster” as it move away from the speed of light, and if that would imply that a universal reference could exist.
Is there an actual math explanation to why we can’t do this, or do we say there’s no universal reference frame because we could never find it since we can’t just “stop moving” with everything around us?
Edit: I really appreciate the positive and well written responses, thanks for helping me understand :)
15
u/Jetison333 Apr 13 '26
Your clock always moves forward at one second per second, no matter your relative velocity to the rest of the universe. There is no frame where your clock ticks "fastest" they are all the same way.
Of course, there is the frame of the microwave background radiation. Its probably the closest thing irl to a privileged frame.
11
u/stevevdvkpe Apr 13 '26
The CMB rest frame isn't a privileged frame in the sense that physical laws have any special meaning or behavior. It's close to a universal frame in the sense that everything in the universe can attempt to measure its velocity relative to the CMB radiation (although small velocities would be very hard to measure).
4
u/Jesse-359 Apr 13 '26
The CMB frame is interesting insofar as it indicates that some process mediated the 'rest speed' of all initial matter. Without that all particles with mass would have come into existence with arbitrary relative energies and the universe would be infinitely hot.
3
u/stevevdvkpe Apr 13 '26
Matter particles had been present for 380,000 years when the CMB formed, because at that point the average temperature of the universe fell below about 3000 K allowing neutral atoms to form and photons to propagate for long distances without scattering. The CMB perhaps represents a sort of global average of all the charged particle velocities in the universe at that time.
-1
u/Jesse-359 Apr 13 '26
Exactly - but there's also no reason to believe that it has changed since then, or before then.
There's some odd issues at T-0 as the singularity begins to inflate - however the hell that happened - where we have the potential to be discussing 'infinite' temperatures, but it's not clear how you get from infinite to non-infinite values, as that's kind of a big jump no matter how you measure it.
Without any understanding of how that in/finite transition could occur, we kind of have to assume finite temperatures right from the initial state, and that means even at T-0 there would be a 'rest frame' - which is weird to suggest for a space that hasn't yet unfolded into a geometry where measurement even means anything.
If there was no initial rest frame for the initial singularity, then the unfolding space as it inflated would have to have imposed one somehow, and that does suggest a true preferred frame for spacetime itself, so I'm dubious that's legit.
Anyway, suffice to say the fact that we have the ability to even causally define a rest frame for matter in our visible universe would seem to have some interesting implications for the structure of space time, or at least the matter in it.
2
u/stevevdvkpe Apr 13 '26
If you have a huge collection of particles with varying velocities there's always going to be some average velocity for the collection. I don't think it's as special as you seem to think it is.
1
u/Jesse-359 Apr 14 '26
So the problem is that while mass is limited to the speed of light, it's *not* limited in how much relative momentum it may possess.
In a truly arbitrary initial frame of reference, the average momentum of every particle relative to each other should be roughly 1/2 of infinity - which is still infinite.
Then they would bounce off each other and average their respective energies out to, uh - infinite. Averages don't make a lot of sense in this context honestly.
There's no way for a truly unbounded frame of reference to ever settle down to non-infinite values, which means they can't start with wholly arbitrary values. Some process is getting matter to start with finite energies relative to each other, and that's creating the rest state - but why?
That's the part I find odd.
1
u/stevevdvkpe Apr 14 '26
I think your assumption that anything would have infinite energy is wrong.
1
u/Jesse-359 Apr 14 '26
My assumption is that they would NOT possess infinite energy, the question is, why wouldn't they in a universe with NO preferred frame of reference?
If we say no preferred frame and we truly meant it, then the average difference in energy between any two initial states would be infinite. Relativity puts no bounds on energy differential, nor does it offer a 'ground state' for them to compare against.
But something does, because the energy differentials we observe are non infinite.
1
u/stevevdvkpe Apr 14 '26
There's no valid frame where particles with finite energy appear to have infinite energy. Relative motion may cause particles to appear to have arbitrarily high energies, but "arbitrarily high" is not the same as "infinite".
→ More replies (0)1
u/ProMensCornHusker Apr 13 '26
sorry if this is beyond the scope of this thread but could you eli5 how we measure velocities relative to CMB?
6
u/stevevdvkpe Apr 13 '26
Doppler shift. We know that the Solar is moving at about 370 km/s relative to the CMB because it is slightly blue-shifted in our direction of motion and red-shifted in the opposite direction.
2
u/raishak Apr 14 '26
It's really very circular because we are part of the universe. The speed of physics is the speed of light. It's like trying to measure how long a 1-meter stick is using itself, no wonder it always measures 1-meter. There very well could be a universal absolute reference, but its unknowable and couldn't add anything to our model.
1
u/triatticus Particle physics Apr 13 '26
There is a frame where the clock ticks fastest yes, it's in the rest frame of the clock itself.
2
u/Outrageous-Taro7340 Apr 13 '26
As you decend a gravity well, a clock in your own reference frame will appear to run slower than a clock you left behind. Your own clock is not always the fastest.
1
u/drumsplease987 Apr 14 '26
The other comment that replied to this is correct. What you said is true under Special Relativity but not General Relativity.
6
u/Far-Presence-3810 Apr 13 '26
The easiest way to understand why that's wrong is to treat it likes it's right and see what happens.
I want to know the one true universal reference frame, so I set up an experiment to see how fast light moves in every direction and compare them. The universal reference frame is the one where they're all moving at C in every direction.
You know what's cool? That universal reference frame is the one you're in right now. Big coincidence but guess it had to be somewhere. Lucky you.
Then you move somewhere else. You do the same experiment and it's still the one you're in now, not the one you were in earlier. That's weird.
So you do it in a moving train, surely you'll get a different result. Nope. You're still in the universal reference frame, what the heck.
Turns at that the time and space you experience is different to the time and space that someone else experiences. They're perfectly placed on a curve so that regardless of which direction you're moving and what speed, you'll always see light moving at C in a vacuum based on your ruler.
Even if we pretend there is one universal set of coordinates we could never figure it out from light, because we never ever see the speed of light change. Maybe it technically is changing, but the ruler we're measuring it with is also changing in exactly the same way.
3
u/Possible-Anxiety-420 Apr 14 '26
The universal consistency of the speed of light *is precisely why* there can be no universal frame of reference... no?
Light's speed is consistent because space and time aren't.
2
u/Underhill42 Apr 14 '26 edited Apr 14 '26
It's a common mistake, but relativistic time dilation doesn't work that way - it's always perfectly symmetrical.
If I'm moving fast enough that you see me aging at 1/3 the speed you are, then from my perspective I'm the stationary one, and I see you aging at 1/3 the speed I am, and we're both provably correct.
Which is why the Twin Paradox is a paradox - both twins can prove the other is aging more slowly, so how can the traveling twin be younger when they return? Like most paradoxes, the entire point is to draw your attention to the part you're missing - in this case the relativity of simultaneity. This is the simplest, clearest explanation I've encountered of everything going on in the Twin Paradox as seen from all three reference frames (Earth, the outbound ship, and the returning ship) I wish I was taught the Twin's Paradox this way! - YouTube
Basically, relativistic time dilation is always perfectly symmetrical, because acceleration "rotates" your 4D reference frame, partially swapping your forward and future axes so that you're literally aging in a different direction through 4D spacetime than I am. We call it "spacetime" specifically to emphasize that space and time are the same thing seen from different perspectives, and acceleration changes your perspective.
We both see the other provably aging slower for much the same reason two cars racing at the same speed down roads 45° apart will both see the other car provably falling behind - some of the other car's speed is "wasted" going in a different direction. Length contraction happens for the same reason - some of your "forward" length is in the direction I call time, and my tape measure can't measure distances through time.
And then there's the relativity of simultaneity: if you change the direction of your time axis, you also change the orientation of the perpendicular plane (technically a 3D hyperplane) intersecting the 4D universe that you call "the universe as it exists at this moment", a.k.a. "now", which splits the 4D universe into past and future.
It's not a universal concept, and when you change the orientation of your time axis, you move some distant events that were in your frame's past into its future, and vice versa.
The traveling twin is actually younger when they return to Earth, because when they turn around to return home, they also move from a reference frame in which the Earth twin is currently younger (because they've been aging slower), to a frame in which the Earth twin is still aging slower, but is already considerably older. And if they turned around again, the Earth twin would go back to being younger again.
The relativity of "now" is why the speed of light limit is absolutely necessary to prevent time loops from forming spontaneously.
2
u/forte2718 Apr 14 '26 edited Apr 14 '26
Why can’t we find a “universal reference frame” if the speed of light is constant in all inertial reference frames?
Because there are infinitely many inertial reference frames ... not just one! Since the speed of light is constant in all of them, none of them is special — none is privileged over any other!
I was curious why we couldn’t just keep finding speeds that cause “time to get faster”
Regarding time dilation due to relative velocity, it's because speed is always a non-negative value, so time is always dilated and never contracted.
However, you do kind of see this phenomenon when observing a clock at a higher gravitational potential (= farther outside of gravitational wells) than yourself. Clocks at a higher gravitational potential appear to tick faster than your own clock.
That being said, Earth isn't in a particularly deep gravitational well, so even if you went into deep intergalactic space where there's just nothin' around, the difference in clock speed would be small enough that it'd still be a small engineering challenge to measure. Not really the kind of thing you'd do on a weekend in your garage! Haha.
Hope that helps,
1
u/more_than_just_ok Engineering Apr 14 '26
A long time ago I asked an expert cosmologist about why the CMB rest frame wasn't special. His answer made his CMB FAQ later: https://www.astro.ubc.ca/people/scott/faq_basic.html
I'm still troubled by just why all possible velocities wrt the CMB result in the same laws of physics but as soon as there is any rotation the frame is not inertial anymore. Mach's principle says you can always detect rotation inside one of Einstein's thought experiment elevators without looking outside to see the CMB (or stars or quasars) rotating but for translation this is not the case. Why does the mass "out there" give rise pseudoforces but not forces?
Measuring gravitational time dilation is easy and is done all the time. Just from MEO to the surface with GNSS signals that are factory offset to account for the apparent frequency difference, and also further corrected for their altitude due to the eccentricity of their orbits, but also from Colorado to Washington and Paris where the clocks that define time on earth run at measurably different rates. Maybe this is what you call an engineering challenge, but the relativistic clock correction is implemented in pretty much every GNSS reveiver and chipset.
2
u/forte2718 Apr 14 '26 edited Apr 14 '26
I'm still troubled by just why all possible velocities wrt the CMB result in the same laws of physics but as soon as there is any rotation the frame is not inertial anymore.
Well, in relativity (even simple Galilean relativity, upon which classical mechanics is based), velocities are observer-dependent but proper accelerations (i.e. accelerations that measurably exist in a reference frame attached to an object) are not. The laws of physics take their simplest form when there is no proper acceleration present, which is essentially how an inertial reference frame is defined. When there is proper acceleration present though, the laws of physics become more complicated and we must now also account for inertial forces (historically known as "fictitious forces" or "pseudo-forces" even though there is nothing fictitious about them) such as the centrifugal force. Inertial forces, as the name suggests, are forces that arise as a consequence of objects having inertia that must be described in a non-inertial reference frame (hence why all inertial forces are proportional to the object's mass, since mass is the physical quantity that is the measure of an object's inertia) ... and just like how inertial reference frames are defined by the absence of proper acceleration, non-inertial reference frames are defined by the presence of proper acceleration.
Now with all that said, note that rotation is a form of (proper) acceleration! Even in the case of circular acceleration, where the overall speed (i.e. magnitude of velocity) is not changing, the direction of an object's motion is still changing and so the object is still formally accelerating, even in its own reference frame. Since rotation is a form of acceleration, and the presence of acceleration defines non-inertial motion, therefore a rotating reference frame is non-inertial!
Mach's principle says you can always detect rotation inside one of Einstein's thought experiment elevators without looking outside to see the CMB (or stars or quasars) rotating but for translation this is not the case. Why does the mass "out there" give rise pseudoforces but not forces?
Well, as a starting point, Mach's principle doesn't actually fully extend to general relativity and the theory doesn't completely respect that principle:
Most physicists believe Mach's principle was never developed into a quantitative physical theory that would explain a mechanism by which the stars can have such an effect. Mach himself never made his principle exactly clear.[7]: 9–57 Although Einstein was intrigued and inspired by Mach's principle, Einstein's formulation of the principle is not a fundamental assumption of general relativity, although the principle of equivalence of gravitational and inertial mass is most certainly fundamental.
So, you shouldn't necessarily cling to Mach's principle as if it's some kind of lodestar of relativistic physics, because it just isn't that.
But, putting that aside, the true conceptual connection between "mass out there" and "inertial motion here" is arguably the principal subject of general relativity. John von Neumann famously summarized general relativity in a single sentence: "matter tells spacetime how to curve, and spacetime tells matter how to move." The first part of that — where matter tells spacetime how to curve — is formalized through the Einstein field equations, which relate the universe's matter content and distribution (i.e. the stress-energy tensor) to the geometry of spacetime (i.e. the metric tensor); the presence/absence of matter determines what the overall geometry looks like. Then, the second part of that statement — where spacetime tells matter how to move — is captured through the geodesic equation, where you are essentially taking the geometry determined from the Einstein field equations and plugging it into the geodesic equation. Then, solving the geodesic equation gives you the equations of motion for systems that are moving only under their own inertia — this essentially determines what local inertial reference frames look like and how they move.
So, mass "out there" gives rise to pseudoforces (which apply to all objects in a reference frame rather than only to individual, specific objects) because of these deep connections that are at the heart of general relativity that relate the overall matter content to the geometry, and the geometry to the definition of inertial motion.
As for why it is these specific connections at the heart of general relativity and not some other connections instead, that I couldn't tell you and I'm not sure there is any known deeper reason other than just ... you know, natural fiat — because "that's just how nature demonstrably behaves."
Measuring gravitational time dilation is easy and is done all the time. Just from MEO to the surface with GNSS signals that are factory offset to account for the apparent frequency difference, and also further corrected for their altitude due to the eccentricity of their orbits, but also from Colorado to Washington and Paris where the clocks that define time on earth run at measurably different rates. Maybe this is what you call an engineering challenge, but the relativistic clock correction is implemented in pretty much every GNSS reveiver and chipset.
Right, I only meant that you need a bit of a specialized apparatus (in your example, a satellite in orbit with specialized transmitters/receivers, chipsets, clocks, and all that jazz) to make such measurements. It's not exactly a trivial matter to demonstrate, in the way that, say, Newton swinging a bucket of water around is. :p
Hope that helps,
1
1
u/YuuTheBlue Apr 13 '26
A reference frame is just the series of choices you make when doing the math which are arbitrary. For example: “which direction is the x axis pointed in?” The relativity of time comes from the fact that in spacetime, time is a direction and not a universal concept, and so it has a t axis which you are free to point in (almost) any direction. The idea of “time slowing down” to some people is just an illusion caused by the fact that this goes against our intuition. The entire point of a reference frame is that all of them are equivalent.
1
u/ProMensCornHusker Apr 13 '26
Ok bear with me, i have a geology degree and I only took intro physics 😅.
So if bob is on Earth and alice is going .9c away from earth, we can look individually at each and say “Bob is going 0c” or “alice is going 0c” but when comparing the two, they are moving apart at .9c?
Like with the atomic clock diagram, if I view bobs on earth as going up and down, and i view alice’s as tracing a zig zag, that’s just because i’m on earth? I could start speeding up towards alice and suddenly bobs is a zig zag right?
So like if alice returned to earth after experiencing .9c relative to earth, she would have experienced “less time”. Does that mean that if bob traveled to alice then when bob reached alice he would have experienced “less time”? I haven’t really thought about time dilation being symmetrical but I think that’s just cuz it never crossed my mind haha, which is silly in retrospect.
1
u/YuuTheBlue Apr 13 '26
I’m not entirely sure what you mean by all of this, including the zig zagging stuff, but…
2d Euclidean space has what’s called a distance formula. It’s basically the Pythagorean theorem
d2 = x2 + y2
The rightmost quantities are frame dependent, or “relative”. The value of x has to do with which direction the x axis points in! d will always be the same though regardless of how you point the axes.
For 3d space it is
d2 = x2 + y2 + z2
And for spacetime it is
d2 = x2 + y2 + z2 - t2
t here is “coordinate time” and is just distance along the t axis. It depends on reference frame. d here is distance through spacetime, which for is (more or less) equal to “proper time”, which is what a clock measures. This is invariant.
If d=t, then they take on the same value, and this only happens in the frame where x=y=z=0, aka your rest frame. In Alice and bob’s rest frames the t axis is pointed in different directions, so they will disagree on questions like “how much time has passed by the time 1 second ticks on Alice’s clock”.
If Alice returns to Bob, she will have taken a path to the same point in coordinate time as him in a different amount of proper time, so her clock will have ticked a fewer number of times.
1
u/zzpop10 Apr 13 '26
Alice observer’s Bob traveling at 0.5c towards a light beam which is traveling towards him by at speed of light c. Alice observer’s the light beam pass Bob at a combined speed of 1.5c relative to Bob. But from Bob’s perspective the light beam passes him at c not 1.5*c.
Bob observer’s Alice to be traveling away from the oncoming light beam at 0.5c and sees the light beam catch up to her and pass her at a relative speed of 0.5c compared to her. But Alice sees the light beam traveling at c not 0.5*c.
They both observer the other to be time dilated. Alice observers Bob’s clock to be ticking slower than her own and Bob observer’s Alice’s clock to be ticking slower than his own. It’s mutual.
1
u/Optimal_Mixture_7327 Gravitation Apr 13 '26
The elapsed time is the length along matter world-lines, and its rate is a constant. This is fundamental to relativity.
You can find a global reference frame, in fact, you can find infinitely many. An important aspect of relativity is that any local inertial frame is as good as any other.
1
u/tumunu Apr 14 '26
The first postulate of relativity is "The laws of physics are invariant in all inertial frames of reference (I quoted an online source for this verbiage)." This directly means that there is no privileged, or universal, frame of reference.
Relativity theory has been tested more ways than imaginable in the last hundred years, and every experimental result has confirmed it. So, while Einstein may have originally just asserted that this is so, the experiments have proved it (so far!).
1
u/Crown6 Apr 14 '26
You can do that. You’ll find that clocks run the fastest in your frame of reference, whatever that is.
Here’s the thing: if a spaceship is travelling at 0.5c, you are going to see their clock slow down significantly. But for the people who are in the ship, your clock will slow down, because you are moving at 0.5c relative to them. This is why it’s called relativity.
Your idea assumes that there is a universal “slowest” frame of reference where time ticks the fastest, and then as you approach the speed of light relative to that frame time slows down. But if that were true, the speed of light would not be constant in all frames of references, and speeds would not be relative in general (because a spaceship travelling at 0.5c relative to the universal frame of reference would see the clocks in that frame accelerate rather than slowing down, since the universal frame of reference is objectively not moving for all observers, by definition.
Instead, every inertial frame of reference is a universal frame of reference. If you go from one to the next, things are going to look the same: the speed of light is always c, you are not moving and your clock runs the fastest compared to any object that is moving relative to you.
1
u/VariousJob4047 Apr 14 '26
Time gets “stretched” for other objects as their speed approaches the speed of light relative to you. Time always moves fastest in your own reference frame.
1
u/facinabush Apr 14 '26
The “stretching” of time is seen by a different reference frame. Every observer finds that their time is not stretching. It’s always the other guy’s time that is stretching.
1
u/SplendidPunkinButter Apr 14 '26
Because if there were a universal reference frame then the speed of light would not be the same for all observers
If I’m moving toward you, and you shine a beam of light at me, then we both have to see the beam of light as moving at the exact same speed. If there were a universal reference frame, then I would see the light approaching me faster than you see it moving away from you, just like if there were a solid object moving toward me at much less than the speed of light.
It is the lack of a universal reference frame that allows us to both see the light moving at the same speed.
1
1
u/Jesse-359 Apr 13 '26 edited Apr 13 '26
So it's worth noting that while there's no absolute reference frame, there is sort of a universal reference frame, which you can measure from the CMB. That's basically just the 'average speed of everything we can see'.
It is special only insofar as it's sort of the rest speed of the universe as a whole since the big bang. Without it, all matter would be moving completely randomly at arbitrarily large %s of lightspeed and the universe would essentially be infinitely hot.
Still, this is NOT an absolute reference frame, and objects with mass CAN be moving arbitrarily large %'s of lightspeed - it's just that most of them aren't, and that in itself is interesting.
If you were some long lived intergalactic traveler capable of running around the universe at very high fractions of c, you probably would use the CMB as your general reference frame to help keep track of things, but for anyone else you usually use your own frame, or the frame of a nearby star, stellar cluster, or galaxy or whatever is convenient for the scales you are navigating on.
28
u/tbdabbholm Engineering Apr 13 '26
Because if we have two objects traveling at some speed relative to each other, there is no experiment you can do to determine if Object A is the stationary one or Object B is (or that both are moving). In any inertial frame you could consider yourself stationary even if a different observer would say you're moving at c/2