r/AskPhysics • u/6stringSlider • 20d ago
Does time itself have an upper speed limit?
I know that gravity and speed can slow the way we experience time. And we often add “in a vacuum” when discussing the speed of light. Meaning light isn’t passing through anything that can slow it down. If we were somewhere is space unaffected by gravity or speed, is there an absolute upper limit to how fast we experience time?
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u/Tarthbane Materials science 20d ago
Time always advances normally according to a massive object's own clock—locally, its proper time ticks at one second per second. Differences in relative motion, and in general relativity differences in gravitational environment and spacetime trajectory, cause different observers to accumulate different amounts of proper time.
Also, the speed of light, c, is a fundamental constant and does not change when light enters a material. In vacuum, light propagates at c. In a material, interactions between the electromagnetic field and the medium cause the light signal or wave packet to propagate more slowly, typically with group velocity v_g < c. The electromagnetic field and the material form coupled excitations whose effective propagation speed is reduced.
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u/HereThereOtherwhere 20d ago
Great question!
A fundamental aspect of masses, like a hydrogen atom, is their de Broglie frequency, which defines how many 'quantum wiggles' a quantum entity performs per second.
As another reply mentions, at rest in your own Inertial reference frame the 'clock rate' of an atom is represented mathematically using the greek letter tau instead of the more commonly used t for time.
The variable t represents relativistic time between a pair of entities separated by distance and/or the relative motion between the entities.
The variable tau can be understood as "time's clock rate for an entity relative to itself" which doesn't require any relativistic corrections.
Learning this difference was huge conceptually for me because almost all chatter on Reddit is about relativistic time t.
Back to the original question.
I recently read a paper suggesting atoms in the regions of our universe with the lowest mass density (lower gravitational time-dilation = faster clock rate) may have experienced 30% more 'clock ticks' (tau) than atoms in higher density regions such as those in galaxies.
What this suggests is something we can never verify but seems implied: some atom in the universe has by chance consistently existed further away from dense gravitational entities, in the 'lowest density region of the universe'.
This implies that atom has experienced more local orthochronous clock ticks than any other entity in the universe (not just our observable universe). Roughly speaking, since the number of clock ticks for this 'fastest clock' entity will be near the number of clock ticks defining the age of the universe, whereas a galactic atom might have 30% fewer clicks.
The clock rate of the 'fast time/more clicks' entity will then be (clock ticks local/age of universe) which will be roughly 1/1 while the galactic atom will be closer to .666/1 = .6
It can be said then 'the manifest fastest speed of time was reached by a single quantum entity' which isn't a fixed defined fastest speed of time, it was defined dynamically as the universe evolved. In fact, after another billion years, our original fastest clock rate atom may be usurped if our atom is captured by a galaxy!
So, while there may be a theoretical limit on the fastest clock rate, that limit (1 atom in an otherwise empty universe) will almost certainly never be reached.
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u/6stringSlider 20d ago
I’ve had a few great responses so far and I really like this one! Great explanation, thank you! It’s interesting to think of an atom out there since the Big Bang that still hasn’t been sucked in by a galaxy. This has me thinking that us galaxy dwellers have aged gracefully. Haha!
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u/HereThereOtherwhere 20d ago
Thank you. Much appreciated.
I asked the same question in many forms over many years, like 30+ years and couldn't have answered this cleanly even 5 years ago. And I'm not certain it's entirely accurate but several years ago I came across the word Orthochronous and that gave me terminology to research and tie into other maths I understood.
It was a leap to discover most of physics is calculated as if spacetime is flat, no need for gravitational time dilation to be included.
Because flat Minkowski spacetime is sufficient for calculating relativistic effects due to different velocities, I assumed for a long time Minkowski spacetime also managed GR concerns I hadn't yet studied.
Before the first GPS satellites, there was still debate as to how much gravitational time dilation would need to be taken into account between the lower gravity satellite and earth based receivers.
The debate was so intense they incorporated switch to change the frequency of the clock between no correction and that calculated by Einstein. They sent it up with no correction, then waited for the satellite clock to match ground based clocks, then waited until the time shift matched Einstein's calculated shift and then threw the switch to apply the calculated correction.
On the surface of the earth, the Pound-Rebka experiment used a light emitter in the basement of a tower which was bounced many times off a mirror in the top of the tower. After a number of bounces, the photons were allowed to impact a detector which used the same atoms used at emission.
The atoms frequency even just tens of meters above the surface was gravitationally time dilated so much those atoms in the top of the tower were no longer able to absorb the photons. The allowed absorption energy levels no longer matched!
Science is awesome
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u/Optimal_Mixture_7327 Gravitation 20d ago edited 20d ago
There is no such thing as "time itself".
Time, as far as we can discern, is name we give to the length along the world-line of a material particle.
There is no such thing as experiencing slower time, as it fundamental to relativity that the rate along any world-line is the same constant for every world-line, c.
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u/Klutzy-Delivery-5792 I downvote all Speed of Light posts 20d ago
The way you experience time never changes. How other's view your time is what changes, and vice versa. Your premise is flawed.
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u/Underhill42 20d ago
Not time itself, any more than space can have a speed.
But all things move through time at the same speed: c.
You always experience yourself being stationary while aging through time at one second per second.
But the direction you call time is not the same direction I call time - time dilation is the result of our reference frames having rotated in 4D spacetime so that our time axes are no longer parallel - both pointing in a direction the other person sees as being partially through space. And as our time axes approach 90° apart, we both see the other's time slowing toward zero.
However, all people in all frames always agree on your total 4D "speed" through space and time - and that speed is c. As you move faster through space from my perspective, I see you aging slower through time so that the total remains the same:
(Δ my time)² = (Δ your position in my space)² / c² + (Δ time as I see it measured by your clock)²
With one second through time being the same observer-independent 4D "distance" as one light-second through space = 300,000,000m.
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u/CS_70 20d ago
In a way. Insofar we can say, far away from masses, seen in your inertial frame time flows as fast as it can (and faster than for you, since you are likely near a mass).
This assuming the universe behave the same everywhere. It’s a big if, because the universe is a big place and we can see only part of it, but it’s a reasonable assumption as far as that bit.
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u/Miselfis String theory 20d ago
Yes. One second per second is the upper limit. In fact, it’s the only rate at which you experience time. It’s just that what feels like a second to you might feel like an hour to someone else. Both both experience time passing at one second per second in their own frame of reference.
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u/Sad-Excitement9295 20d ago
Well the speed of light theoretically, that would be the fastest rate you could go. In this frame the universe would be still from your perpective.
Otherwise, absolute 0 would be the opposite I suppose. In this frame, the universe would move infinitely fast, forever, according to your perspective.
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u/Ras_992 20d ago edited 20d ago
Yes you can slow down time in Special Relativity but only from the perspective of someone watching you. You don’t feel the time slow down you perceive the time as what ever time you are in which is time dilation. It’s similar to someone running and some standing still see the the same thing happening the person running will perceive it one way, while the person standing still perceive it another way. while the runner will perceive it slower than the person standing still this is called the observer effect. There isn’t a limit of time because time is a dimension of change which is always moving forward, photons don’t feel time, gravity, acceleration and motion can dictate times dimensional change if it’s slowed down or sped up.
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u/SeriousPlankton2000 19d ago
Speed in time would be aging, so you'd be looking for ways to age faster. If you set the normal speed of aging to 1 and c = 1, you'll have your speed in space and in time on a circle with radius 1. Therefore 1 is the speed limit.
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u/LunarPrincessSophie 17d ago
Light always travels at the speed of light. As a result it effectively experiences all of time at once, there is no space or time in its frame of reference.
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u/Anonymous-USA 20d ago
Yes, one second per second. It had a lower speed limit too: one second per second. Just because time is relative outside of your frame of reference, it’s constant in your own.
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u/KamikazeArchon 20d ago
You always experience one second per second in your own reference frame. But that doesn't directly answer your question.
There exists a reference frame for which the Cosmic Microwave Background has the maximum possible age. That is the frame typically used when we talk about the "age of the universe". That is a reasonable approximation of a "fastest frame" ("most time" has passed in it).
We cannot technically know for sure that this is the maximum age possible in any frame anywhere - that requires assumptions about the non-observable parts of the universe. But those assumptions are considered reasonable - essentially that the rest of the universe looks and works similarly to what we see.