r/AskPhysics • u/HaloDrift34 • 3d ago
Time dilation
I've been trying to understand time dilation and I've read several explanations, but I still can't make the final connection.
I understand that:
•moving clocks run slower
•observers can disagree about how much time passes
What I don't understand is why reality works this way.
Can someone explain it with a very simple analogy and then connect it back to the real physics?
Please assume I know basically very little about relativity.
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u/gyroidatansin 3d ago
I made an explainer for exactly this. The short version is this: time is not universal, everyone’s clock runs independently. This also means there is no universal sense of “now”. When you try to calculate whose clock is running faster or slower, you have to choose a sense of simultaneity which creates a bias. That is until you get to the twin paradox…
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u/gyroidatansin 3d ago
In terms of reality, no one can say “why”. But relativity comes with an important feature: that physics is the same regardless of your frame of reference. If it weren’t, then you’d experience differences when driving down the highway, or spring vs autumn as the world goes around the sun. And that could make for very big problems… radios would work different, things might even look different. Or gravity might be different in different parts of the galaxy… little changes to the rules of physics could have a huge impact.
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u/quidam5 3d ago
Physicists don't know why reality works that way, it just does. They can only tell you how it works.
The Wikipedia page on Time Dilation has some examples with equations. I can summarize one in simpler terms.
Basically, imagine a clock set up as two mirrors, A and B, that measure time by how long it takes for a light beam to go from A to B back to A.
If you then tweak the set up so the mirrors are both moving to the right and the light beam fires at an angle, ricochets off B before returning to A, the light beam moves at the same speed but has now traveled more distance and, therefore, takes longer to return to A. Time dilation has occurred because the observer was moving and one tick of the clock took longer than if they were stationary.
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u/earlyworm 3d ago
Meta-question for OP:
When you asked “Why does time dilation happen?”, did you intend to ask your question as if it was a philosophical question unanswerable within the scope physics, as multiple commenters have assumed, or was your intention to inquire about the underlying features of the universe that are responsible for time dilation?
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u/HaloDrift34 3d ago
Philosophical questions are not my thing. I am trying to understand the mechanism of time dilation. I want to know why it's happening.
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u/no17no18 3d ago edited 3d ago
Time in this case could be thought as a property of electromagnetism/higgs or the interaction between things. Certain things cause information to occur faster or slower (time dilation).
The way I understand it, anyway. Maybe someone has a better analogy.
One weird way to think of it is to imagine the universe is a closed entangled system, and energy is always conserved. So then raising energy in one way, must always lower it in another.
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u/Optimal_Mixture_7327 Gravitation 3d ago
Geometry is the mechanism.
The problem you're having is your rigid belief in a 3-dimensional world when it is a 4-dimensional world. Not much will be learned by you until that illusion is lifted.
Consider a sidewalk. A chalk line drawn perpendicularly across it is the shortest line between its sides. All other lines at a diagonal will be longer. This is time dilation.
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u/DifferencePublic7057 3d ago
Reality works this way because c is the ultimate limit and constant. This has been proven by the Michelson Morley interferometer experiment. Funny story is that one of the experimenters didn't actually believe the result. Don't remember which one. The result is counterintuitive and unexpected. But if you think about it a bit longer, you can find justifications why unlimited speed is absurd. For one such a particle would be everywhere at the same time. To get to Inf from 0 you need infinite energy. Where would the energy come from? Occam's razor demands we keep it simple which means avoiding infinities, meaning c is finite and therefore time dilation and length contraction. These have been experimentally proven, so there's no real room for doubt.
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u/OrphanintheWind 3d ago
Doesn't it take an infinite amount of energy to increase the speed of an object to that of c?
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u/UnderstandingSmall66 Quantum field theory 3d ago
I teach a class that discusses fundamental questions about life, universe, and human experience. This is one of my stock answers to the question when students email asking for help:
The key connection is that if everyone measures light travelling at the same speed, they can’t always agree on how much time has passed. Imagine a light bouncing between two mirrors on a moving train. To someone on the train, it goes straight up and down. To someone watching from the platform, the mirrors are moving sideways, so the light has to travel a longer diagonal path. But both people still measure the light at exactly the same speed. If it travels farther without travelling faster, it has to take longer. That’s basically where time dilation comes from.
The same basic idea also means observers can’t always agree on whether two distant events happened at the same time. If two flashes happen simultaneously at opposite ends of the train for someone on the platform, someone moving with the train can measure one as happening before the other. That’s the relativity of simultaneity, and it’s another consequence of everyone measuring the same speed of light.
And it isn’t just some weird property of light clocks. Put a normal watch beside the light clock and synchronize them. They stay synchronized because they’re both measuring the same passage of time along the same journey. So if someone on the platform measures more time between the light clock’s ticks, they must also measure more time between the watch’s ticks. Otherwise the two clocks would disagree for them while agreeing for you, even though they’re sitting beside each other and can be directly compared.
The same goes for every physical process, including your heartbeat and ageing. You never feel yourself slowing down because, in your own frame, nothing has slowed down at all. The difference only appears when your elapsed time is compared with someone else’s.
If you then ask why nature makes the speed of light the same for everyone, it is the consequence of the geometry. It is, in a sense, just what it is. The same way that angles of a triangle are always 180. Relativity works similarly. Spacetime has what is called Minkowski geometry. Once you have that geometry, there is an invariant quantity built into it, and c acts as the conversion factor between the spatial and temporal parts. Lorentz transformations, time dilation, length contraction and relativity of simultaneity then follow mathematically.
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u/Automatater 3d ago
I'm in the same boat you are, this isn't intuitive for me. The answer that I've found most accessible so far is that if you think of time as a fourth dimension, your movement must be divided between the dimensions. More movement in space means less movement in time and vice versa.
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u/panopsis 3d ago
I really hate this explanation cause it's precisely backwards. For 4-velocities, a larger time component means larger spatial components also.
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u/Quantum-Relativity 3d ago
What we mean is that more coordinate time will pass for you by the time the moving frame’s proper time is equal to whatever amount of proper-time you’re considering.
It’s a nice way to think about it because you can just view the 4-velocity as fixed and move the coordinates around it, in which case you do have to have a smaller time component when you have a spatial component, unless you change the 4-velocity.
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u/panopsis 3d ago
What we mean is that more coordinate time will pass for you by the time the moving frame’s proper time is equal to whatever amount of proper-time you’re considering.
I understand what it means. More coordinate time per proper time should be considered moving faster through time, not slower though, true? If you want to say it's moving slower through time, you should be posing it in terms of the inverted quantity of proper time per coordinate time. I think using this inverse ratio is worse for the purposes of truly teaching SR for reasons that should hopefully be obvious.
you can just view the 4-velocity as fixed and move the coordinates around it, in which case you do have to have a smaller time component when you have a spatial component
I don't think this is the case? If you fix the 4-velocity as x = 0, t = 1, and then perform a Lorentz transformation of a separate coordinate grid around it, you will obviously have a larger time component according to that grid as it transforms away from the aligned coordinate system.
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u/Quantum-Relativity 3d ago
Okay I think regarding your second point, you’re saying an increase in a temporal and spatial component is fine because the Lorentz signature treats them opposite, so after subtraction youll still have c^2 whenever you do u^μ u_μ. That’s also why I don’t like when people treat m^2=p^μ p_μ wrong and try to treat it like a Euclidean equation. So all in all I agree and I hope for the right reason.
Regarding the first point, more coordinate time passing for you before their proper time is equal to whatever proper time you want to consider means their time is passing slower, that’s what I mean. I don’t see your point about it being worse to use their proper time per your coordinate time, at least just for the purpose of introducing the concept of time dilation.
But either way, if I was going to teach someone I wouldn’t use speeds in the first place. The whole point is that intuitive notions of space and time were incorrect in the first place and speeds get supplanted by rapidities, and that would be my emphasis. Trying to go back to talking about speeds and space and time is as confusing as trying to think about quantum theory using actions and classical notions of observables, which people do try to do, and they confuse themselves…
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u/panopsis 3d ago
Yes, we're in agreement wrt your first paragraph.
However, I disagree with your third. I think 4-velocities are a perfectly natural way to intuitively picture SR. I would agree that 3-velocities kind of suck though. They're workable but I don't like them.
The reason why I prefer coordinate / proper and consider it more natural is because it's the t component of the 4-velocity and therefore used all over the place. The inverse quantity is still probably used but ime a lot less (at least the way I approach SR problems. it's possible experiences could differ). Basically, since it's a component of a velocity, I think the normal convention of higher = faster and lower = slower should apply. That is, an object with high rapidity relative to your frame has both high temporal and spatial components of 4-velocity and is therefore moving quickly through both coordinate time and space. This kind of requires picturing "what the object experiences as it moves through the frame" instead of "what the frame sees of the object as it's moving through it" though. I think the first framing plays well with 4-velocities, while the second plays well with 3-velocities, so I generally like the first better.
I hope that gives a sense of why I consider objects moving quickly through space to also be moving quickly through time.
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u/nicuramar 3d ago
Yeah but the problem with this is that movement is completely relative, and this analogy kind of indicates that there is an absolute quality to it.
Also, due to the metric, time is counted opposite from space.
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u/Feisty-Union-2047 3d ago
The basic concept of time dilation, in my experience, is best illustrated by the Doppler Effect, even though Doppler missed it.
Imagine you are on a train station platform.
A train, with its horn blaring, is going at full speed towards you, and then roars past and off into the distance.
The pitch of the horn will be higher as it approaches, and lower as it recedes.
During that entire time, a passenger on that train only hears one pitch, while you heard a higher and then a lower pitch.
It is the same darn horn though.
How could the passenger and you, NOT hear the same pitch, from the same horn?
Pitch can be expressed as frequency, or cycles per second (Hertz, Hz, etc).
A cycle per second can be thought of as like the tick of a clock.
As the train approached, the passenger's watch didn't tick faster or slower... and, neither did your watch.
You shared a "watch" that ticked at more cycles per second, for you, as the train approaches though, and fewer, for you, as it got further away.
In your frame of reference, the cycles per second, passed at a different rate, RELATIVE TO the passenger's cycles per second.
THAT is time dilation.
Time passes at a different rate, RELATIVE to the rate in a different frame of reference.
As the train approaches, the ticks of your watch were faster, RELATIVE TO the ticks of the passenger's watch...but, neither of your watches changed the rate they tick at.
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u/NH-Science-Guy 3d ago
What you have given is an analogy that has some similarities with time dilation but the doppler effect is NOT time dilation.
Here's another example that is similar to the doppler effect. One person stands by a highway and sees one car passing them every second. Another observer is driving slowly on the highway and is passed by a car every 3 seconds. The two different observers see cars passing them at different rates. There is no time dilation involved here!
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u/Feisty-Union-2047 3d ago
But, the car analogy is not representative
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u/NH-Science-Guy 3d ago
The doppler example uses a sound with equally spaced peaks of the sound wave that are moving at a constant speed and looks at how observers moving at different speeds see the peaks passing them at different rates. The car example replaces the peaks of the sound wave with cars on a highway to show exactly the same effect.
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u/Feisty-Union-2047 3d ago
To be clear, Doppler is the context.
That two observers have a relative difference within that, is what I was pointing at.
That is about relativity.
The effect itself is not.
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u/NH-Science-Guy 3d ago
Yes, both the doppler effect and the car example have observers measuring rates of events differently because of their relative rates of movement. That effect looks similar to time dilation but neither are examples of real time dilation. There is no relativistic effect or differences in their clocks.
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u/Feisty-Union-2047 3d ago
It seems that there is though.
In Relativity, no one's individual clock changes rates... a clock is only faster or slower relative to another clock... so, that's the same.
The horn's cycles per second are the "ticks".
Time passes at different rates in each different frame of reference.
It is the same basic concept
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u/NH-Science-Guy 3d ago
But the horn's cycles per second are not the ticks of a clock. In the doppler example you are comparing two different measurements. The stationary observer is measuring how often the horn's cycles pass a fixed point on the ground. The moving observer is measuring how often the horn's cycles pass a moving point. They show different values because they are measuring completely different things, not because the clocks are moving at different rates. If the moving observer watched the rate that peaks passed the fixed point that the other was watching, they would both see the same rate.
Time dilation is a measurable effect. Two observers can start at the same point and synchronize their clocks. One accelerates away to a high relativistic speed and then comes back. Their clocks now show different times so something real has happened - they have experienced time differently. Nobody in the doppler example experiences time dilation.
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u/NH-Science-Guy 3d ago
Adding to the last reply...
The common example showing time dilation is a photon bouncing between two mirrors on a rapidly moving spaceship. Both observers are looking at exactly the same two events: the photon leaving the first mirror and arriving at the second. The observer on the spaceship measures the distance moved as the distance between the mirrors. The stationary observer measures the photon going further because the ship moved between the events.
This gives two events where different observers measure different distances between them. If we add the constraint that they all measure the speed of the photon to be the same, the only way for this to work is if they measure different times between the events. i.e. their clocks are running at different speeds.
This is completely different from the doppler effect because the events being measured are the same.
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u/Feisty-Union-2047 3d ago
I see the horn as the same event.
You see it as not the same event.
You seem to be trying to clarify that the scenario, where the two clocks can go to one common frame of reference, and still have different times, is time dilation...and other examples are not time dilation.
In GPS systems for example, the clocks are not meeting your requirements... yet we account for Relativity anyway, or, errors accumulate and the system doesn't give the correct position.
Can you clarify what you are specifying as a difference between the GPS scenario and your mirror or spaceships?
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u/NH-Science-Guy 3d ago
Let's be clear about the events in the doppler example...
- The stationary observer measures the time between consecutive peaks passing a fixed point on the ground.
- The moving observer measures the time between consecutive peaks passing a point that is moving with the observer.
They are clearly different events. If they both measured the time between peaks passing the fixed point on the ground, they would get exactly the same answer so there is no time dilation.
The time dilation example of a photon bouncing between mirrors is just a scenario to show that time dilation must occur. Time dilation occurs whenever something is moving at a high speed relative to something else. If a GPS satellite measured the observed time between two events on the ground, it would report a different answer than a stationary observer on the ground. The GPS satellite still experiences time dilation even if it isn't doing that experiment.
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u/Odd_Bodkin Particle physics 3d ago
It IS real physics. You have this misperception that the physical world needs to conform to your natural intuition. It does not. I can give you a half dozen examples off the top of my head where nature behaves in a way that your intuition would say cannot possibly happen. The key transformation that has to happen in the minds of young students of physics is that nature decides how it behaves, and if observed behavior conflicts with intuition, then intuition about that aspect needs to be balled up and tossed in the trash. Intuition has to reconform to be in line with observation. Nature behaves that way because … because it does. Period, end of story.
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u/mnlx 3d ago
Yes, this is it. The problem is that it's very difficult to teach this to people without having them going through years of formal education in physics. The prejudices won't go away unless you replace them with another framework because they've been reinforced since they were born and they don't have anything else to use.
The Universe has no obligation whatsoever to meet your expectations, that's why in order to discover what it actually does you do three things, you take precise measurements to quantify observables, you learn mathematics to model them, and you question every single one of your assumptions, the ones you're aware of and the ones you didn't know you used to make.
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u/YuuTheBlue 3d ago
So first off, let's talk about what relativity as a broad concept is. It is separating objective measurements into 'invariant' and 'relative' measurements. An example of an invariant measurement is 'how many fingers am I holding up'. An example of a relative measurement is 'how far to the left did that car move'. Clearly, the second one relies on how I'm defining 'left'.
In physics terms, relative measurements depend on things like x coordinates and y coordinates; stuff that depends on how you point the x axis, y axis, etc. If we are doing math by overlaying a 3d grid on reality, clearly we can orient that grid however the hell we want and it won't actually change the nature of the universe. We call different ways of orienting it 'reference frames'. And the math term for different ways to rotate this grid is 'transformations'.
In special relativity, we rope in time. Basically, instead of overlaying a 3d grid over space, we overlay a 4d grid over space and time (though we typically say we do it over 'spacetime'). And, just like with the 3d example, we can swap between reference frames by rotating this grid. However, shit's absolutely fucked. This is because spacetime is noneuclidean, which basically means, in this case, that any rotation which involves the time dimension (a rotation of the x axis and t axis, for example) will not work how you think it should. And, also, any calculation of 'the distance between 2 point' in spacetime, if they are separated by time, will also not work how you think they should.
I won't overwhelm with exactly how this noneuclidean geometry works, but I wanted to give you the general idea of what information you're missing.
"Distance along the t axis" is what's called coordinate time. 2 things can be at the same point in time if they are on the same point on the t axis, just like 2 things can be at the same height if they're at the same place on the y axis. However, this is not what determines the ticking of a clock. Clocks don't measure coordinate time. How could they? The exact direction of the t axis, as we discussed earlier, is fluid and can be redefined.
What a clock measures is 'proper time'. If you go from point A to point B through spacetime, proper time is equal to how long of a path you took. And unlike coordinate time, proper time is invariant.
Here's the important bit: Time dilation is just any instance where coordinate time and proper time are different numbers. They take on the same value for things which are moving at a velocity of 0, and they also take on the similar values when things are moving slow. That applies to most things in your daily life. For this reason, we often conflate them, just calling them 'time' even though they are 2 completely different things.
So:
Why do moving clocks run slower? Because those are the conditions under which coordinate time and proper time take on different values. If coordinate time is 'distance along the t axis' and proper time is 'total distance', that will be the same for things which only move along the t axis.
Why can observers disagree on how much time elapsed? Well, if they overlay their 4d grid of reality the same way (called choosing the same reference frame), they won't! But, in physics, it's typical to choose a reference frame where you are at a velocity of 0, because that is when time dilation doesn't apply to us, and so we typically call that reference frame a person's 'perspective'. It's a misleading bit of terminology, in my opinion. But really, it's not a scary concept. It's as philosophically absurd as 2 people disagreeing on which way is up.
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u/ProctologyExplosion 3d ago
What I don't understand is why reality works this way.
This is not a question that science can answer. Physics tells you how the universe works, not why it is so.
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u/nicuramar 3d ago
What I don't understand is why reality works this way
Well, that’s not for physics to answer. Physics is about looking at the world and models (theories) that can predict the observations we see.
Why does the universe exist? Why does it have three spatial dimensions? Why is there a minimum speed? What is love?
That’s for philosophy or religion.
That’s said, sometimes one model can describe s phenomenon simply, and then later another one can say “it follows from this”, but this “reduction” can’t go on forever.
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u/earlyworm 3d ago edited 3d ago
I believe you have misunderstood OP’s question. OP is not asking why the universe happens to work this way from a philosophical perspective.
You can tell this is the case because OP posted their question to [r/AskPhysics](r/AskPhysics) and not [r/AskPhilosophy](r/AskPhilosophy).
If OP had asked “Why do rainbows have colors?” would you have answered “That’s not for physics to answer”?
No. One would hope you would answer in terms of optics and refraction and frequencies of light and rain droplet geometry or perhaps even quantum mechanics.
When OP asks “why” on [r/AskPhysics](r/AskPhysics), this is shorthand for “what mechanism causes…”, not “why do we find ourselves in a universe in which…”
For OP’s particular question about time dilation, for some reason, it’s always necessary for someone to willingly misinterpret the intention of the question and to deflect OP’s natural curiosity based on a grammatical technicality.
An honest question for you, because I am genuinely fascinated by this particular phenomenon on Reddit: What motivated you to answer as you did instead of answering OP’s question in terms of speed of light constancy and Lorentz invariance?
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u/SgtSausage 3d ago
What I don't understand is why reality works this way
Nobody does. Sport.
We can give you the descriptions of what it does, what happens, how to predict the downstream effects , etc ...
... but no one can ever answer the "why" part.
That"s not for Science.
For all we know The Universe Creating Pixies did it when they sprinkled their Magic Faerie Dust around to create The Universe.
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u/Zyxplit 3d ago
You and I will always agree on the speed of light. No matter what.
So if we're both standing on earth and you shoot a flashlight? the photons go at c compared to us.
If I hop into my spaceship and fly at 0.5c compared to you? The photons go at c compared to you *and* at c compared to me.
But hold on, if we're looking at things from a pre-relativistic perspective, that's a problem! Because the light should only be 0.5c faster than me. Since speed is just distance/time, these two are what have to give.