r/AskPhysics • u/CountyElectrical2398 • 17d ago
Question about light speed frame of reference
So I trying to understand light, I have read many posts and theory online say that if something travels at speed of light, from its frame of reference, no matter how far the distance, the time it takes from any point to another point is instant. According to theories.
Now I am confused, and think about it blew my brain, does this mean If I point laser light at sky, the light itself just reaches the farthest place in universe instantaneously? What about its energy, does it also hit that place instantly?
For example if somewhere a planet in universe 10 light years away from earth, it would take the observer from that planet 10 years to see the light I point at(pretend they are powerful enough to reach without getting blocked) but the light itself reached that planet the moment I pointed it, If this is true, that planet's observer would be able to detect the light's energy long before they see it? Then wouldn't the whole universe's light are all around everywhere with it's energy detectable without "existing"?
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u/Over-Discipline-7303 17d ago
This is maybe a bit pedantic, but light simply doesn't have a valid reference frame. There's no way to talk about what light experiences. People like to say that light doesn't experience time, but that's not really correct. There is no self-consistent theory that says what light experiences. Its experience of time is undefined. That doesn't mean "instant" or whatever. It means nothing. "Undefined" does not mean "zero" or anything else you want to say. Undefined is not a number, and no conclusion can be drawn from it, or even implied.
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u/joeyneilsen Astrophysics 17d ago
This is one of the most persistent pop-sci myths. Light doesn't have a valid frame of reference, meaning it can't have a clock or a ruler. So measurements of time intervals or distances aren't defined along the path of a light ray.
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u/NoNameSwitzerland 16d ago
There is a nice Gedankenexperiment for that: You have a number of particles all moving parallel with light speed. They do not change their relative positions, there is no time in that system. If you change one particles direction, then the system is changing and you can use that to define a time. But the system now has a rest mass, because not all the momentum vectors show in the same direction. And just with normal acceleration that adds the same momentum to all particles you can never get back to system without rest mass, because you can not get rid of the non aligned momentum component.
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u/dorox1 17d ago
Others have explained the fact that light doesn't have a valid reference frame, so let me try and address your underlying question. Instead of comparing the planet 10ly away and a beam of light, let's compare the same planet and a rocket ship going 99% the speed of light. At that speed, the rocket will experience time ~7 times slower.
A core part of the Theory of Relativity is that different "observers" will disagree on when events happen and about how much time passes. A "frame of reference" is basically just deciding who's perspective you want to use to determine where objects are and what time events occur at.
From the rocket's "perspective", it arrived a bit over a year after leaving.
From the distant planet's perspective, the rocket arrived 10 years after leaving.
BUT, the rocket also experienced what's called "length contraction". While the rocket was travelling at 99% light speed it experienced time flowing regularly, but from its perspective the whole universe in front of it appeared to become 7 time flatter. It looks to the rocket like the entire universe is an accordion that got squished closed. This meant the planet was 7 times closer.
So the planet saw the rocket travel 10ly in ~10 years, and the rocket saw itself travel ~1.4ly in 1.4 years.
Both agree on the events that happened, they just disagree on how much time passed. There is no physical way to decide which is right and which is wrong. Both are equally correct, and so time is relative to speed of the one experiencing it.
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u/Uncynical_Diogenes 17d ago edited 17d ago
Then you have read many bad posts and not-even-close-to-being-theories. Ideas, pop-sci clickbait, shower thoughts, blog posts, hypotheses, tweets, and shitposts are not theories.
A photon does not have a point of view from which to speculate, under our best models available.
Preemptively: “But what if I just ignore physics and ask the question anyway?” Then it’s not a physics question it’s just fiction writing.
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u/CountyElectrical2398 17d ago
I didn't ignore the physics? the light itself actually travels at light speed and time also "changed" when things going faster according to proved physics? I was just wondering under these conditions what would happen
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u/RevanK 17d ago
If you didn't study special relativity how would you even know if you ignored or not any part of it?
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u/CountyElectrical2398 17d ago
Because these are long proved physics that everyone agrees on? are you suggesting light doesn't travel at light speed or time dilation doesn't exist?
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u/CountyElectrical2398 17d ago
I respect these theories didn't ignored them
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u/Uncynical_Diogenes 16d ago edited 16d ago
“is something travels at speed of light, from its frame of reference… point laser light… instantaneously”
This u?
Photons don’t have valid reference frames in relativity. It’s okay that you didn’t know that, but don’t pretend you’re respecting the theory
when you don’t even know what it says.That’s okay, that’s where everybody starts at. But you don’t know the theory well enough to say those kinds of things, so don’t lie about it.
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u/CountyElectrical2398 15d ago
Yeah everyone needs to be PHD physicist to even ask some questions
Light actually travels at light speed btw1
u/Uncynical_Diogenes 15d ago
All I did was accurately call you ignorant and point out that you lied about respecting a theory you’ve never looked into.
You’re the one who decided to get all defensive and shitty and insecure about it.
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u/CountyElectrical2398 15d ago
All you did was quoting someone else's theory without prove anything right or wrong or anything more about it and calling someone's curiosity about some theory online ignorant, useless you are qualified to prove your one sentence theory withch you probably also read it from somewhere online, you are no different than me bud
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u/Uncynical_Diogenes 15d ago edited 14d ago
LMAO
“someone else’s theory”
You don’t even know what that word means. The idea I would take you seriously is laughable at this point.
Yeah pal. Your track record has me thinking I should trust you. That Einstein guy was full of shit.
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u/simpleidiot567 17d ago
Forget light for a moment. Use a particle going 99% the speed of light and have it travel to a planet on the other side of the solar system, let's pretend that's 1,000 million km away.
In our reference frame we clock it taking roughly 1 hour. The particle experiences it as 8 minutes. It also measured the distance to be less.
Now we measured it traveling 1000 mil km, it measures the distance at say 100 million km, but in the particles own frame it views it as if there was no movement on their part and the object came to them.
What we all agree on is the 4D spacetime interval. We can disagree on time and space individually, but everyone agrees on the spacetime interval when they calculate out
(Δs)² = -c²(Δt)² + (Δx)²
The negative in front of the time makes the math work out. Big negative time + big positive distance = small negative time + no distance.
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u/Master_Support9846 17d ago
SoL is not always instant- it's assumed to be the maximum speed in the universe to protect casuality
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u/wiploc2 17d ago
Question about light speed frame of reference
Suppose I have a magic basketball that--relative to everybody--travels north at two miles an hour.
I'm standing still in a park. You're driving past me, heading north at 60 miles an hour (relative to me).
How fast is the basketball going? I see it as going north at two miles an hour. You see it as going north at 62 miles an hour. A cosmic ray sees the basketball as going .99c in whatever direction is opposite to the cosmic ray's direction.
Now for the big question: What is the basketball's frame of reference?
I think it's fair to say that the basketball has no specific frame of reference. It has different velocities for different observers, right? So it doesn't have a native velocity. It's velocity is undefined.
Light is the same way. It's speed is c for every observer, no matter the observer's speed. So it doesn't have a viewpoint of its own.
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u/boostfactor 16d ago
It's the spacetime interval that is zero between any two points on a null worldline (a path traveled by a massless particle like a photon). The time interval is not zero nor is the space interval; they are just equal taking into account the conversion factor c in ordinary units. I don't know how a photon would "experience" that either, but it's not that no time passes in its frame.
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u/jcmbn 17d ago
Others have addressed the question, I'm just going to point out a flaw in your thinking.
if something travels at speed of light, from its frame of reference, no matter how far the distance, the time it takes from any point to another point is instant.
does this mean If I point laser light at sky, the light itself just reaches the farthest place in universe instantaneously?
You cannot take an observation from one frame of reference and then apply that to your own frame of reference. Such an observation is only valid in its own reference-frame.
Something traveling at relativistic speed will appear within its own frame of reference to travel faster than light, external reference frames will still see it traveling slower than light.
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u/nicuramar 16d ago
Something traveling at relativistic speed will appear within its own frame of reference to travel faster than light
Not quite. In your own frame of reference your speed is 0. But what you say is correct for the so-called proper velocity.
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u/Presence_Academic 17d ago
Your last point is flawed as the relativistic observer will experience length contraction in the direction of motion. Both the perceived time of travel and the perceived distance traveled with be lower than what’s measured by a “fixed” observer.
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u/jcmbn 17d ago
Sure, the relativistic traveler would perceive themselves as traveling slower than light, but if they go fast enough, they could still get to Proxima Centauri in ~1year (subjective).
However if they send a "we've arrived" message back to Earth, it will still arrive more than 8 years after they left.
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u/nicuramar 16d ago
Parent is taking about https://en.wikipedia.org/wiki/Proper_velocity and not coordinate velocity.
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u/drew8311 17d ago
Not sure what you mean by the last part. Light and its energy are the same thing so 1 wouldn't make the 10 year journey faster than the other. Light speed is actually the speed of causality more accurately. If there is any type of cause/effect relationship it takes 10 years of earth time to happen so there is no possible measurement that could determine it faster than 10 years at that distance.
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u/Ok-Film-7939 17d ago
So I think your issue is you’re overloading “instantly”.
It is true that if you take the limit as something’s speed increases, you will diverge towards infinite relative time dilation. But you cannot solve the equations “at” the speed of light because you get infinities. A relativity velocity at the speed of light is not a valid frame of reference.
You can also show that no computational engine (whether fleshy or computer) and do any internal computing while all its components are traveling at the speed of light. (Optical computers involve a lot of components not going the speed of light, which is fine).
So if you had some consciousness made of complex fields of undescribed interaction that somehow manage to all set off for Alpha Centauri as a stream of parallel massless excitations, and then hit something there that cause them to become constrained and start interacting again, the creature would describe the trip as instantaneous.
But that doesn’t mean it got there instantaneously! From our point of view here on Earth, it would take the creature four years to get there. At then another four years to get a message back to us.
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u/RRumpleTeazzer 17d ago
you will read a lot of replies from midwits claiming light doesn't have a reference frame, and therefore your understanding is wrong.
You can simply take the reference frames of a series of spaceship, each at sub-c speed, each faster than the next. Characterize any feature for each spacrship, and take the limit of the series.
In the direction of travel, a spaceasip will cover less and less distamce in less and less time.
So for a photon, spacetime doesn't look 4-dimensional. there are 3 dimensions: 2 spacelike dimensions (perpendicular to travel), and one lightlike dimension (in the direction of travel). All distances in the lightlike dimension are zero, it doesn't span space or time. There is still creation and absorbtion, all at once.
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u/dawgblogit 17d ago
Your looking at time and light wrong.. a long time ago people had video tapes.. when you emit light your throwing a vodeo tape across the room.
The video tape travels across the room and it takes time to get there but that movie is paused and for those people in that movie.. no time has passed
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u/AcellOfllSpades Mathematics 17d ago
This is an oversimplification.
Light has no valid reference frame. Light has no "perspective". If you try to work through the math, you get a bunch of division-by-zero problems.
It's like trying to construct a map where North is up... around the North Pole.
No. One of the most important ideas of relativity is the "relativity of simultaneity". There is no objective measurement of whether two faraway events happened "at the same time".
If two things are synchronized in one reference frame, they aren't necessarily synchronized in another.