6 ms·

"In the particle's reference frame, the photon would move away from it at the speed of light." For some reason I've never been able to wrap my head around this

by neor 9y ago

"In the particle's reference frame, the photon would move away from it at the speed of light." For some reason I've never been able to wrap my head around this. Anyone know a good explanation for this stuff?

maze-le 9y ago
Me neither, but I always had the idea, that it is impossible to think about events, at a velocity of c, since it is impossible to properly think about 2 events, that are infinitely separated in time. The Lorenz-Factor(γ) for a velovity approaching c approaches infinity. Thus, from the reference frame of a particle at c, the time dialation: ΔT = γ*Δt, also becomes infinite. 2 ticks on a clock (Δt) of an ordinary observer, take an infinite amout of time in the reference frame (ΔT). This doesn't prove or explain anything, but the fact that we can't handle infinities very well, in our casual thinking.
- posterboy 9y ago
  No no no, what we can't handle is objectivity! Everything is relative so it is striking me as absolutely paradox to pretend we could leave our frame of reference anyway and "ride a photon" measuring it's speed with a stop watch. Instead you simply have to deal with towers of abstractions, understand the experiments. It is counter-intuitive to claim we also couldn't see photons, but really, the disconnect between theory and phenomenology is rather large. I guess we are only really interested in the weak-electromagnetic force. And that's a probabilistic theory thanks to quantum mechanics. Discrete systems are macroscopic simplifications for didactic and deductive reasons. Gravity is next to the other three basic forces. That's a simple dualism. So time stands still for the photon, but space bends. And we can't fully explain how. And don't get me started on infinity. It's just a number big enough for all intents and purposes. Conversely, it's pretty simple to think about the opposite: Nothing. So if you you take an infinite amount of time to observe a clock at light speed from your inertial frame of reference, it will just not move at all -- there are no "2 ticks". Corollary: A photon doesn't experience time at all. And this holds in quantum mechanics, because a photon doesn't exist until observed. A photon is thus the measure of interaction, a delta on our clock. Colloquially, time is the order of events -- if nothing happens, time stands still. For sake of the argument: If you just switch the photon and observer in your example, the photon in infinite time wouldn't "see" any time pass on the outside observers clock. He wouldn't move. But this is just too simplistic, you have two points in space that practically can't move, because they don't From the frame of reference of a photon, the environment would > 2 ticks on a clock (ΔT) at c, take an infinite amout of time in the reference frame of an ordinary observer (Δt). Extending the thought, you are trying to multiply infinity by infinity. As you said, we cannot handle infinities. I guess that is because there can be only one, one singularity, one universe. That's why the speed of light is set at unit-interval.
  - erikpukinskis 9y ago
    > Instead you simply have to deal with towers of abstractions, understand the experiments Sounds like the Matrix.
  - erikpukinskis 9y ago
    > time stands still for the photon, but space bends Sounds like a processor tick.
  - erikpukinskis 9y ago
    > It's just a number big enough for all intents and purposes Sounds like a register.
TremendousJudge 9y ago
someone correct me if I'm mistaken, but for the particle the time goes so fast that it sees the 215,000 years that we see the photon take to gain a 1 centimeter lead go by in a flash
- cdelsolar 9y ago
  Yeah, basically. If you were to hitch a ride on this particle, those 215,000 years would go in a flash (basically, how much time it takes light to travel a centimeter). Wherever your final destination was, you would have aged a few fractions of a second but you just traveled nearly 215,000 light years.
- pixl97 9y ago
  https://www.youtube.com/watch?v=GguAN1_JouQ https://www.youtube.com/watch?v=GguAN1_JouQ When Time Breaks Down | Space Time | PBS Digital Studios At about 4:30 is when they talk about light speed, but you should watch before then to understand what is going on.
contravariant 9y ago
The speed of light is a result of the laws of electromagnetism. No matter how fast you move the laws of electromagnetism and therefore the speed of light wont change. I know of no way to make it intuitive though. Things improve slightly if you replace speed with 'rapidity' which places the speed of light at infinity, but then you get that objects with infinite rapidity don't travel instantly from one place to another (although this is true in some frames of reference).
- luk32 9y ago
  The speed of light is, but it does not have anything to do with relativity. Those concepts are connected by the speed of light in vacuum, yes, but there is no logical reason for that. This was an educated guess, and was confirmed by observation it's not a conclusion. Einstein started to formulate special relativity theory starting from an assumption that a "ball of light" won't change its shape for non-inertial (no accelerarion) observers regardless of their respective speed. I.e. constant speed of light was the reason not a result of this theory. (Duh. This got lenghty.)
  - contravariant 9y ago
    I'm not entirely sure what you're getting at. Actually I can't even parse the first two sentences, and the third seems to refute something I never claimed.
dragontamer 9y ago
> For some reason I've never been able to wrap my head around this. Anyone know a good explanation for this stuff? There's no explanation "needed". Light photons move at a constant speed from any viewpoint. Its just a fact that is experimentally confirmed. Perhaps what you need is an experiment that proves this to be true. But those experiments don't offer you any "explanation", they just will prove what you already have been told. https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity ------------ One thing that helped me understand it all is that "Reality" is a wave that propagates out. We call it "the speed of light", but its far more accurate to call it "The speed of reality" or perhaps "The speed of causality". Basically, when A causes B, that "event" takes time to propagate to other locations. This means that different observers sees the event happen at different times. For example: the galaxies 3-billion light years away from us appear to be forming today. We know the information is 3-billion years old, but reality itself is warped. Because the reality from 3-billion years ago was 3-billion light years away, its basically happening right now from our point of view (The "wave of reality" is finally reaching us). Calling it the "speed of light" is confusing for multiple reasons. Just call it the "speed of reality" (and light in a vacuum happens to travel at this speed) and in my eyes, it becomes way easier to understand.
- freehunter 9y ago
  I don't think they're doubting it's a fact, but rather just saying it's a fact they don't understand. Saying "no one needs to explain the theory of relativity" implies it's intuitive to everyone, which is very obviously not true. Hand-waving isn't an explanation, it's a distraction.
  - twtw 9y ago
    I think 'dragontamer read the question as "why does the photon move away from it at the speed of light?" (as I did). There isn't really an answer to that question, since it's just the way the universe works. Saying that the laws of the universe don't (or can't) have explanations is not the same as saying they are intuitive to everyone. Ultimately, some of those facts you just have to accept. It is extremely difficult to have any useful analogy or explanation that attempts to explain why light always travels at the speed of light in any reference frame. The consequences can definitely be explained well, but the fundamental laws really can't. EDIT: John von Neumann's quote about math reflects my view about this kind of fundamental physics: > in mathematics you don't understand things. You just get used to them.
dlwdlw 9y ago
I think it's like this: Time stands still for the photon and is super slow for the particle. Super slow is still infinitely away from 0. In the article example from the particles view, 0.000..1 seconds pass for the photon to be 1cm ahead being the photon is so fast. From our view however that same small time is dilated to the 200k years.
- ptrincr 9y ago
  This is all true.... However it should be said that time is not super slow for the particle, from the particles point of view (frame of reference) time passes as per normal for itself. This is what allows the photon to appear to continue to travel at the speed of light from the particles frame of reference. If the particle was carrying a clock, and we could watch it tick as it flew away from us, it would appear to be ticking super slowly, the second hand would take forever to move. Conversely if we had a giant clock and the particle could watch it as it flew away from us, it would appear to be ticking super fast, it would look as though we were travelling far into the future. The same is also true if an object is close to a very large gravity well. If someone could stand next to the event horizon of a black hole and hold a giant clock, from our point of view, their clock would appear to tick very slowly. From their point of view, it would tick normally, but our clock would rapidly tick far into the future. The universe around them would quickly travel into the future. Time and space are elastic to allow the speed of light to remain constant.
- Nition 9y ago
  There's something I don't understand about that type of explanation. Say a particle and I are moving along like so: -----[photon @ speed of light]--> -----[me @ 50% speed of light]--> Or more simplified, say two cars are moving like so: -----[other car]--> -----[my car]--> When I'm stationary, I see him go past at 100km/h. When I move at 50km/h, in the real world he would now appear to only be going 50km/h. But if I slow down my time to half speed, now he'll look like he's going 100km/h again. Because he looks to me like he's at 50km/h at double time speed = 100km/h. That seems to make sense - time slows down and the apparent speed stays at C (100km/h in this case). That's how I've understood the explanation for relativity. But it must be a very wrong understanding - because that whole idea breaks down when the other car is going along a different vector to me! In this situation: <----[other car @ 100km/h]-- -----[my car @ 50km/h]--> His speed relative to me would come out as 150km/h in the real world, and no amount of making him look like he's going faster would reduce his speed to 100km/h.
deleted 9y ago
[deleted]
lisper 9y ago
Imagine you were driving a car with a stuck accelerator an no brakes. The car always moves at 100 MPH. So you can't change the car's speed, but you can change its direction. So you can drive north, at which point your east-west velocity is zero, or you can drive east, at which point your north-south velocity is zero, or you can drive in some other direction, which will give you a mix of north-south and east-west velocities. The faster you move east-west, the slower you move north-south and vice versa. You are kind of like that car. You (and everything else) are always moving at the speed of light, but through spaceTIME, not through space. When you move through space you are not changing your speed through spacetime, you are only changing your direction. The faster you move through space, the slower you move through time, and vice versa.
- wakkaflokka 9y ago
  This is fascinating to think about, and easy to understand. Thanks. I'd like to read a book on this subject written a similar manner. I remember trying to read The Fabric of the Cosmos as a leisure reading book (I like nonfiction) but it got super tedious halfway through and I had to put it down.
  - pixl97 9y ago
    For those into videos, check out PBS Spacetime on Youtube, they have a few videos on the subject.
- whatshisface 9y ago
  The bug in your explanation is that turning the steering wheel performs a rotation (which moves vectors along the edges of circles) on your velocity vector, but in spacetime that would be a hyperbolic rotation (which moves vectors along hyperbolas). The result of this is: If I am an outside observer measuring your position with a very long ruler and your time with my wristwatch, all while watching your dashboard clock, I have access to three numbers. (Your clock, my watch, your position on my long ruler.) There are three ways to compare these numbers. If I compare my watch to your position down the ruler, I will obtain "classical speed." The behavior of this number is very confusing when it is high. I may also compare your dashboard clock with my ruler position, or your dashboard clock with my watch. Both of these can lead to a "speed," in the sense of one number changing at a certian rate with respect to another. If Vx is the rate of change of the ruler position with respect to your dashboard clock, and Vt is the rate of change of my wristwatch with respect to your dashboard clock, and c is the speed of light, then it turns out to be always true that c^2*Vt^2 - Vx^2 = c^2. If you graph this you will see that it is a hyperbola. In the parent's example, the car situation would obey V(north)^2 + V(south)^2 = 100mph^2. If you graph that it is a circle, as it differs from the above by the minus sign.
robotresearcher 9y ago
There is no explanation. It's just a fact we observe. Like magnets attracting: it's just what happens. Feynman talks interestingly about the nature of these facts: https://www.youtube.com/watch?v=MO0r930Sn_8 https://www.youtube.com/watch?v=MO0r930Sn_8
sunstone 9y ago
And from the photon's frame of reference the photo itself doesn't move at all.
- baddox 9y ago
  That part is actually harder for me to understand. The photon experiences no passage of time, right? And yet it does move through space. Or would you say that, from the reference point of a photo, there is no movement in time or space? Or perhaps it’s better to just say that the concept of reference points hits an asymptote at velocity c, so it simply doesn’t make sense to talk about the reference frame of a photon.
  - jcims 9y ago
    Can photons interact? If they can, what happens when you have two photons cancelling each other out at different distances from their origin? From the photon reference frame, both were emitted and absorbed at T=0, but in our reference frames they were emitted at different times. Does that somehow link the emission time of the two given that there is a reference frame in which they are equivalent?
- luk32 9y ago
  This is trivial. From one's frames of reference this very one is always stationary =]. What is interesting from photon's frame of reference is the image of the rest of the world.

by neor 9y ago

maze-le 9y ago

Me neither, but I always had the idea, that it is impossible to think about events, at a velocity of c, since it is impossible to properly think about 2 events, that are infinitely separated in time. The Lorenz-Factor(γ) for a velovity approaching c approaches infinity. Thus, from the reference frame of a particle at c, the time dialation: ΔT = γ*Δt, also becomes infinite. 2 ticks on a clock (Δt) of an ordinary observer, take an infinite amout of time in the reference frame (ΔT). This doesn't prove or explain anything, but the fact that we can't handle infinities very well, in our casual thinking.

posterboy 9y ago
No no no, what we can't handle is objectivity! Everything is relative so it is striking me as absolutely paradox to pretend we could leave our frame of reference anyway and "ride a photon" measuring it's speed with a stop watch. Instead you simply have to deal with towers of abstractions, understand the experiments. It is counter-intuitive to claim we also couldn't see photons, but really, the disconnect between theory and phenomenology is rather large. I guess we are only really interested in the weak-electromagnetic force. And that's a probabilistic theory thanks to quantum mechanics. Discrete systems are macroscopic simplifications for didactic and deductive reasons. Gravity is next to the other three basic forces. That's a simple dualism. So time stands still for the photon, but space bends. And we can't fully explain how. And don't get me started on infinity. It's just a number big enough for all intents and purposes. Conversely, it's pretty simple to think about the opposite: Nothing. So if you you take an infinite amount of time to observe a clock at light speed from your inertial frame of reference, it will just not move at all -- there are no "2 ticks". Corollary: A photon doesn't experience time at all. And this holds in quantum mechanics, because a photon doesn't exist until observed. A photon is thus the measure of interaction, a delta on our clock. Colloquially, time is the order of events -- if nothing happens, time stands still. For sake of the argument: If you just switch the photon and observer in your example, the photon in infinite time wouldn't "see" any time pass on the outside observers clock. He wouldn't move. But this is just too simplistic, you have two points in space that practically can't move, because they don't From the frame of reference of a photon, the environment would > 2 ticks on a clock (ΔT) at c, take an infinite amout of time in the reference frame of an ordinary observer (Δt). Extending the thought, you are trying to multiply infinity by infinity. As you said, we cannot handle infinities. I guess that is because there can be only one, one singularity, one universe. That's why the speed of light is set at unit-interval.
- erikpukinskis 9y ago
  > Instead you simply have to deal with towers of abstractions, understand the experiments Sounds like the Matrix.
- erikpukinskis 9y ago
  > time stands still for the photon, but space bends Sounds like a processor tick.
- erikpukinskis 9y ago
  > It's just a number big enough for all intents and purposes Sounds like a register.

TremendousJudge 9y ago

someone correct me if I'm mistaken, but for the particle the time goes so fast that it sees the 215,000 years that we see the photon take to gain a 1 centimeter lead go by in a flash

cdelsolar 9y ago
Yeah, basically. If you were to hitch a ride on this particle, those 215,000 years would go in a flash (basically, how much time it takes light to travel a centimeter). Wherever your final destination was, you would have aged a few fractions of a second but you just traveled nearly 215,000 light years.
pixl97 9y ago
https://www.youtube.com/watch?v=GguAN1_JouQ https://www.youtube.com/watch?v=GguAN1_JouQ When Time Breaks Down | Space Time | PBS Digital Studios At about 4:30 is when they talk about light speed, but you should watch before then to understand what is going on.

contravariant 9y ago

The speed of light is a result of the laws of electromagnetism. No matter how fast you move the laws of electromagnetism and therefore the speed of light wont change. I know of no way to make it intuitive though. Things improve slightly if you replace speed with 'rapidity' which places the speed of light at infinity, but then you get that objects with infinite rapidity don't travel instantly from one place to another (although this is true in some frames of reference).

luk32 9y ago
The speed of light is, but it does not have anything to do with relativity. Those concepts are connected by the speed of light in vacuum, yes, but there is no logical reason for that. This was an educated guess, and was confirmed by observation it's not a conclusion. Einstein started to formulate special relativity theory starting from an assumption that a "ball of light" won't change its shape for non-inertial (no accelerarion) observers regardless of their respective speed. I.e. constant speed of light was the reason not a result of this theory. (Duh. This got lenghty.)
- contravariant 9y ago
  I'm not entirely sure what you're getting at. Actually I can't even parse the first two sentences, and the third seems to refute something I never claimed.

dragontamer 9y ago

> For some reason I've never been able to wrap my head around this. Anyone know a good explanation for this stuff? There's no explanation "needed". Light photons move at a constant speed from any viewpoint. Its just a fact that is experimentally confirmed. Perhaps what you need is an experiment that proves this to be true. But those experiments don't offer you any "explanation", they just will prove what you already have been told. https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity ------------ One thing that helped me understand it all is that "Reality" is a wave that propagates out. We call it "the speed of light", but its far more accurate to call it "The speed of reality" or perhaps "The speed of causality". Basically, when A causes B, that "event" takes time to propagate to other locations. This means that different observers sees the event happen at different times. For example: the galaxies 3-billion light years away from us appear to be forming today. We know the information is 3-billion years old, but reality itself is warped. Because the reality from 3-billion years ago was 3-billion light years away, its basically happening right now from our point of view (The "wave of reality" is finally reaching us). Calling it the "speed of light" is confusing for multiple reasons. Just call it the "speed of reality" (and light in a vacuum happens to travel at this speed) and in my eyes, it becomes way easier to understand.

freehunter 9y ago
I don't think they're doubting it's a fact, but rather just saying it's a fact they don't understand. Saying "no one needs to explain the theory of relativity" implies it's intuitive to everyone, which is very obviously not true. Hand-waving isn't an explanation, it's a distraction.
- twtw 9y ago
  I think 'dragontamer read the question as "why does the photon move away from it at the speed of light?" (as I did). There isn't really an answer to that question, since it's just the way the universe works. Saying that the laws of the universe don't (or can't) have explanations is not the same as saying they are intuitive to everyone. Ultimately, some of those facts you just have to accept. It is extremely difficult to have any useful analogy or explanation that attempts to explain why light always travels at the speed of light in any reference frame. The consequences can definitely be explained well, but the fundamental laws really can't. EDIT: John von Neumann's quote about math reflects my view about this kind of fundamental physics: > in mathematics you don't understand things. You just get used to them.

dlwdlw 9y ago

I think it's like this: Time stands still for the photon and is super slow for the particle. Super slow is still infinitely away from 0. In the article example from the particles view, 0.000..1 seconds pass for the photon to be 1cm ahead being the photon is so fast. From our view however that same small time is dilated to the 200k years.

ptrincr 9y ago
This is all true.... However it should be said that time is not super slow for the particle, from the particles point of view (frame of reference) time passes as per normal for itself. This is what allows the photon to appear to continue to travel at the speed of light from the particles frame of reference. If the particle was carrying a clock, and we could watch it tick as it flew away from us, it would appear to be ticking super slowly, the second hand would take forever to move. Conversely if we had a giant clock and the particle could watch it as it flew away from us, it would appear to be ticking super fast, it would look as though we were travelling far into the future. The same is also true if an object is close to a very large gravity well. If someone could stand next to the event horizon of a black hole and hold a giant clock, from our point of view, their clock would appear to tick very slowly. From their point of view, it would tick normally, but our clock would rapidly tick far into the future. The universe around them would quickly travel into the future. Time and space are elastic to allow the speed of light to remain constant.
Nition 9y ago
There's something I don't understand about that type of explanation. Say a particle and I are moving along like so: -----[photon @ speed of light]--> -----[me @ 50% speed of light]--> Or more simplified, say two cars are moving like so: -----[other car]--> -----[my car]--> When I'm stationary, I see him go past at 100km/h. When I move at 50km/h, in the real world he would now appear to only be going 50km/h. But if I slow down my time to half speed, now he'll look like he's going 100km/h again. Because he looks to me like he's at 50km/h at double time speed = 100km/h. That seems to make sense - time slows down and the apparent speed stays at C (100km/h in this case). That's how I've understood the explanation for relativity. But it must be a very wrong understanding - because that whole idea breaks down when the other car is going along a different vector to me! In this situation: <----[other car @ 100km/h]-- -----[my car @ 50km/h]--> His speed relative to me would come out as 150km/h in the real world, and no amount of making him look like he's going faster would reduce his speed to 100km/h.

deleted 9y ago

[deleted]

lisper 9y ago

Imagine you were driving a car with a stuck accelerator an no brakes. The car always moves at 100 MPH. So you can't change the car's speed, but you can change its direction. So you can drive north, at which point your east-west velocity is zero, or you can drive east, at which point your north-south velocity is zero, or you can drive in some other direction, which will give you a mix of north-south and east-west velocities. The faster you move east-west, the slower you move north-south and vice versa. You are kind of like that car. You (and everything else) are always moving at the speed of light, but through spaceTIME, not through space. When you move through space you are not changing your speed through spacetime, you are only changing your direction. The faster you move through space, the slower you move through time, and vice versa.

wakkaflokka 9y ago
This is fascinating to think about, and easy to understand. Thanks. I'd like to read a book on this subject written a similar manner. I remember trying to read The Fabric of the Cosmos as a leisure reading book (I like nonfiction) but it got super tedious halfway through and I had to put it down.
- pixl97 9y ago
  For those into videos, check out PBS Spacetime on Youtube, they have a few videos on the subject.
whatshisface 9y ago
The bug in your explanation is that turning the steering wheel performs a rotation (which moves vectors along the edges of circles) on your velocity vector, but in spacetime that would be a hyperbolic rotation (which moves vectors along hyperbolas). The result of this is: If I am an outside observer measuring your position with a very long ruler and your time with my wristwatch, all while watching your dashboard clock, I have access to three numbers. (Your clock, my watch, your position on my long ruler.) There are three ways to compare these numbers. If I compare my watch to your position down the ruler, I will obtain "classical speed." The behavior of this number is very confusing when it is high. I may also compare your dashboard clock with my ruler position, or your dashboard clock with my watch. Both of these can lead to a "speed," in the sense of one number changing at a certian rate with respect to another. If Vx is the rate of change of the ruler position with respect to your dashboard clock, and Vt is the rate of change of my wristwatch with respect to your dashboard clock, and c is the speed of light, then it turns out to be always true that c^2*Vt^2 - Vx^2 = c^2. If you graph this you will see that it is a hyperbola. In the parent's example, the car situation would obey V(north)^2 + V(south)^2 = 100mph^2. If you graph that it is a circle, as it differs from the above by the minus sign.

robotresearcher 9y ago

There is no explanation. It's just a fact we observe. Like magnets attracting: it's just what happens. Feynman talks interestingly about the nature of these facts: https://www.youtube.com/watch?v=MO0r930Sn_8 https://www.youtube.com/watch?v=MO0r930Sn_8

sunstone 9y ago

And from the photon's frame of reference the photo itself doesn't move at all.

baddox 9y ago
That part is actually harder for me to understand. The photon experiences no passage of time, right? And yet it does move through space. Or would you say that, from the reference point of a photo, there is no movement in time or space? Or perhaps it’s better to just say that the concept of reference points hits an asymptote at velocity c, so it simply doesn’t make sense to talk about the reference frame of a photon.
- jcims 9y ago
  Can photons interact? If they can, what happens when you have two photons cancelling each other out at different distances from their origin? From the photon reference frame, both were emitted and absorbed at T=0, but in our reference frames they were emitted at different times. Does that somehow link the emission time of the two given that there is a reference frame in which they are equivalent?
luk32 9y ago
This is trivial. From one's frames of reference this very one is always stationary =]. What is interesting from photon's frame of reference is the image of the rest of the world.