<p>When people talk about the time taken for light to go from the Sun to the Earth, they're generally just considering classical Newtonian mechanics, not relativity. So we simply divide the distance by the speed of light to get the time taken.</p>
<p>There are several other variables that affect this time, since the distance between the Earth and Sun is not constant (the orbit is an ellipse, and it's perturbed by other planets' (mostly Jupiter) gravity), and Earth is moving in its orbit. Unless you describe the time taken with extreme precision, all these effects, including time dilation, are negligible.</p>

CC BY-SA 4.0
2022-03-27T13:00:41.000Z
Roger that. So, regardless of what the proton might or might not "experience", what we're talking about is how long is the journey observed by a stationary observer.
Rob Dirnens
CC BY-SA 4.0
2022-03-27T13:43:13.937Z
There’s no absolute frame of reference and therefore no such thing as a “stationary observer”. I guess you mean “observed in the Earth’s reference frame” (which is usually assumed to be inertial, even though it is not really). Also, I assume you are talking about p**h**otons, not p**r**otons. :)
Brian Drake
CC BY-SA 4.0
2022-03-27T15:07:18.713Z
Even in a relativistic sense, the speed of light is the same in every frame, so the answer is "in every frame where the distance between Sun and Earth is about 93 million miles." Due to relativistic length contraction, that excludes frames in which the Earth / Sun pair is moving sufficiently fast.
John Bollinger
CC BY-SA 4.0
2022-03-27T15:47:18.213Z
@BrianDrake Yep, both of those assumptions are correct :)
Rob Dirnens
CC BY-SA 4.0
2022-03-27T08:40:18.943Z
2022-03-27T15:10:57.183Z
2022-03-27T15:10:57.183Z
Barmar
<p>To measure the time light takes to travel, we run into an obstacle: if the observer is on Earth, they can not actually measure the point of time at which the photon is emitted from the Sun. Conversely, if the observer is the Sun (let's assume they have a magical spacesuit that protects them), they can measure the point of time at which the photon is emitted, but not the point of time at which it arrives on Earth.</p>
<p>To solve this, we might send one observer to the Sun, and one to Earth, give them synchronous clocks and have them both record the time at which the photon was sent and received. The problem with that is that 'synchronous clocks' is itself not well-defined if the clocks are at different locations. The best thing we can do, is give the two observers synchronous clocks while they are still on Earth, and then send one of them to the Sun, at a non-relativistic speed. After the measurement, this observer can return to Earth, and the observers can compare their observations. The recorded times between the emission and receiving of the photons will then be roughly 8 minutes (the relativistic corrections are small at this scale).</p>
<p>An easier way is to put a mirror on the sun and send a photon to it. The photon will return after 16 minutes.</p>

CC BY-SA 4.0
2022-03-29T04:02:50.383Z
"An easier way is to put a mirror on the sun". *John Cena "Are you sure about that?" meme*
Rob Dirnens
CC BY-SA 4.0
2022-03-28T16:30:28.337Z
2022-04-02T08:38:34.753Z
2022-04-02T08:38:34.753Z
Squala
<p>If I interpreted <a href="https://www.forbes.com/sites/startswithabang/2016/09/30/how-do-photons-experience-time/" rel="noreferrer">this article</a> correctly, then the answer to my question should be: 8 minutes is what we perceive, whereas for the photon the journey is instantaneous, due to the fact that it travels AT the speed of light, not close to it, at which point &quot;the photon itself experiences none of what we know as time: it simply is emitted and then instantaneously is absorbed, experiencing the entirety of its travels through space in literally no time. Given everything that we know, a photon never ages in any way at all.&quot;</p>

CC BY-SA 4.0
2022-03-26T18:39:26.320Z
That is a good self answer. One thing to be careful of: don't try to imagine yourself moving "at the same speed as a photon". That's not just impossible, its non-sensical. Such a frame of reference is singular and you can't use it to make any predictions.  One further point is that not only is there no time passing but space is infinitely contracted so "from the point of view of a a photon" the Sun and Earth are in the same place.  This is why we can't really talk about what it experiences...
James K
CC BY-SA 4.0
2022-03-27T02:11:47.330Z
@JamesK
 >don't try to imagine yourself moving "at the same speed as a photon". That's not just impossible, its non-sensical.
Einstein claimed that he tried to do that when he was a teenager, and that is what give him the idea of relativity. But maybe it's not a good idea for someone who isn't as smart as Einstein...
Simon Crase
CC BY-SA 4.0
2022-03-27T03:30:11.683Z
I disagree with this answer in the sense that there is no physical reference frame comoving with a photon. "What a photon sees" is not merely practically unobservable, but undefined even in principle.
d_b
CC BY-SA 4.0
2022-03-27T09:30:41.517Z
@d_b You’re probably right.   However, the „photon perspective“ makes the answer so interesting that I would rather not remove that part.
not2savvy
CC BY-SA 4.0
2022-03-28T02:16:17.917Z
@d_b I don't know enough of the mathematics to bring any rigour to this, but I believe you can sort of take the limit as speed approaches the speed of light, and see what that does to reference frames. My understanding is that would make the travel time and travel distance measured by someone moving in that frame get arbitrarily close to zero. So if you want to be mathematically rigorous yes, there is no reference frame co-moving with a photon. But there are ones that are arbitrarily close, and considering them might be a useful intuition-building exercise.
Ben
CC BY-SA 4.0
2022-03-28T13:45:07.280Z
@Ben: There's one big problem with that limit: You're applying a mathematical law to an empirically derived formula, while that formula is not validated in the range where you are applying that limit. That's the sort of math that led to the UV catastrophe.
MSalters
CC BY-SA 4.0
2022-03-29T22:02:25.473Z
@Msalters Yeah, it's a very important point that our current theories may very well break down when taken to extremes beyond anything we've ever observed. But just talking about what our theories *say*, isn't it valid to tell interested laypersons that as you travel closer and closer to the speed of light it will seem to you that your journey takes less and less time (and crosses less and less distance) in your own frame of reference? All I'm suggesting is that "in the reference frame of a photon distance and time are zero" is informal shorthand for the above.
Ben
CC BY-SA 4.0
2022-03-29T22:09:05.583Z
@Msalters Also my understanding is that the UV catastrophe idea was incredibly useful because it was *obviously* bogus, and this pointed to where the theory was wrong. Extrapolating theories from the situations that were used to derive them into new regimes is how we validate them, isn't it?
Ben
CC BY-SA 4.0
2022-03-29T23:01:17.483Z
_"... don't try to imagine yourself moving "at the same speed as a photon" ..."_ - Why not?  Sure, it's impossible, but imagining things like this often helps our understanding of physics to move forward.  Like imagining what we'd see if we were sucked into a black hole, instead of falling back on "you can't see anything because you've already been killed by the intense gravity".  Or imagining what an electron would do if we squeezed it between our fingers.
Dawood ibn Kareem
CC BY-SA 4.0
2022-03-30T00:58:19.597Z
@Ben: IMHO it's *more* interesting and useful to observe, for example, the fact that the spacelike and timelike components of the photon's worldline [cancel out](https://en.wikipedia.org/wiki/Spacetime#Spacetime_interval). It is, to some extent, meaningful to say that the photon is moving through space and not time, and this has a rigorous formal meaning, without taking any limit.
Kevin
CC BY-SA 4.0
2022-03-26T17:52:27.463Z
2022-03-26T20:03:44.783Z
2022-03-26T20:03:44.783Z
Rob Dirnens
<p>The passage of time is relative depending on whether one is the stationary observer or the object/particle traveling at the speed of light (or close to it). I get this, kind of. But, when we talk about &quot;it takes light X [units of time] to travel Y [units of distance]&quot;, where does time dilation come in? In the case of light's journey from the Sun to Earth, if we imagine the photons doing the actual speed-of-light travelling, is it 8 minutes for &quot;them&quot;? Or is it 8 minutes for us to observe their journey?
Apologies in advance if it's a stupid question.</p>

CC BY-SA 4.0
2022-03-27T16:17:44.657Z
There is no reference frame in which the photon would be at rest. The speed of the photon is c in *every* reference frame. So a photon perspective does mot exist
Thomas
<speed><time-dilation><special-relativity><frame-of-reference>
It takes light roughly 8 minutes to travel to Earth from the Sun, but based on which perspective?
Rob Dirnens
# It takes light roughly 8 minutes to travel to Earth from the Sun, but based on which perspective?

(Asked by: username_0 on 2022-03-26)

The passage of time is relative depending on whether one is the stationary observer or the object/particle traveling at the speed of light (or close to it). I get this, kind of. But, when we talk about "it takes light X [units of time] to travel Y [units of distance]", where does time dilation come in? In the case of light's journey from the Sun to Earth, if we imagine the photons doing the actual speed-of-light travelling, is it 8 minutes for "them"? Or is it 8 minutes for us to observe their journey?
Apologies in advance if it's a stupid question.

**Tags:** speed, time-dilation, special-relativity, frame-of-reference

> **Comments:**
> **username_1**: There is no reference frame in which the photon would be at rest. The speed of the photon is c in *every* reference frame. So a photon perspective does mot exist  

---

## 3 answers

### Answer by username_0 on 2022-03-26. Score: 38 | Accepted answer

If I interpreted [this article](https://www.forbes.com/sites/startswithabang/2016/09/30/how-do-photons-experience-time/) correctly, then the answer to my question should be: 8 minutes is what we perceive, whereas for the photon the journey is instantaneous, due to the fact that it travels AT the speed of light, not close to it, at which point "the photon itself experiences none of what we know as time: it simply is emitted and then instantaneously is absorbed, experiencing the entirety of its travels through space in literally no time. Given everything that we know, a photon never ages in any way at all."

> **Comments:**
> **username_6**: That is a good self answer. One thing to be careful of: don't try to imagine yourself moving "at the same speed as a photon". That's not just impossible, its non-sensical. Such a frame of reference is singular and you can't use it to make any predictions.  One further point is that not only is there no time passing but space is infinitely contracted so "from the point of view of a a photon" the Sun and Earth are in the same place.  This is why we can't really talk about what it experiences...  
> **username_7**: @username_6  >don't try to imagine yourself moving "at the same speed as a photon". That's not just impossible, its non-sensical. Einstein claimed that he tried to do that when he was a teenager, and that is what give him the idea of relativity. But maybe it's not a good idea for someone who isn't as smart as Einstein...  
> **username_8**: I disagree with this answer in the sense that there is no physical reference frame comoving with a photon. "What a photon sees" is not merely practically unobservable, but undefined even in principle.  
> **username_9**: @username_8 You’re probably right.   However, the „photon perspective“ makes the answer so interesting that I would rather not remove that part.  
> **username_10**: @username_8 I don't know enough of the mathematics to bring any rigour to this, but I believe you can sort of take the limit as speed approaches the speed of light, and see what that does to reference frames. My understanding is that would make the travel time and travel distance measured by someone moving in that frame get arbitrarily close to zero. So if you want to be mathematically rigorous yes, there is no reference frame co-moving with a photon. But there are ones that are arbitrarily close, and considering them might be a useful intuition-building exercise.  
> **username_11**: @username_10: There's one big problem with that limit: You're applying a mathematical law to an empirically derived formula, while that formula is not validated in the range where you are applying that limit. That's the sort of math that led to the UV catastrophe.  
> **username_10**: @username_11 Yeah, it's a very important point that our current theories may very well break down when taken to extremes beyond anything we've ever observed. But just talking about what our theories *say*, isn't it valid to tell interested laypersons that as you travel closer and closer to the speed of light it will seem to you that your journey takes less and less time (and crosses less and less distance) in your own frame of reference? All I'm suggesting is that "in the reference frame of a photon distance and time are zero" is informal shorthand for the above.  
> **username_10**: @username_11 Also my understanding is that the UV catastrophe idea was incredibly useful because it was *obviously* bogus, and this pointed to where the theory was wrong. Extrapolating theories from the situations that were used to derive them into new regimes is how we validate them, isn't it?  
> **username_12**: _"... don't try to imagine yourself moving "at the same speed as a photon" ..."_ - Why not?  Sure, it's impossible, but imagining things like this often helps our understanding of physics to move forward.  Like imagining what we'd see if we were sucked into a black hole, instead of falling back on "you can't see anything because you've already been killed by the intense gravity".  Or imagining what an electron would do if we squeezed it between our fingers.  
> **username_13**: @username_10: IMHO it's *more* interesting and useful to observe, for example, the fact that the spacelike and timelike components of the photon's worldline [cancel out](https://en.wikipedia.org/wiki/Spacetime#Spacetime_interval). It is, to some extent, meaningful to say that the photon is moving through space and not time, and this has a rigorous formal meaning, without taking any limit.  

---

### Answer by username_2 on 2022-03-27. Score: 23

When people talk about the time taken for light to go from the Sun to the Earth, they're generally just considering classical Newtonian mechanics, not relativity. So we simply divide the distance by the speed of light to get the time taken.

There are several other variables that affect this time, since the distance between the Earth and Sun is not constant (the orbit is an ellipse, and it's perturbed by other planets' (mostly Jupiter) gravity), and Earth is moving in its orbit. Unless you describe the time taken with extreme precision, all these effects, including time dilation, are negligible.

> **Comments:**
> **username_0**: Roger that. So, regardless of what the proton might or might not "experience", what we're talking about is how long is the journey observed by a stationary observer.  
> **username_3**: There’s no absolute frame of reference and therefore no such thing as a “stationary observer”. I guess you mean “observed in the Earth’s reference frame” (which is usually assumed to be inertial, even though it is not really). Also, I assume you are talking about p**h**otons, not p**r**otons. :)  
> **username_4**: Even in a relativistic sense, the speed of light is the same in every frame, so the answer is "in every frame where the distance between Sun and Earth is about 93 million miles." Due to relativistic length contraction, that excludes frames in which the Earth / Sun pair is moving sufficiently fast.  
> **username_0**: @username_3 Yep, both of those assumptions are correct :)  

---

### Answer by username_5 on 2022-03-28. Score: 2

To measure the time light takes to travel, we run into an obstacle: if the observer is on Earth, they can not actually measure the point of time at which the photon is emitted from the Sun. Conversely, if the observer is the Sun (let's assume they have a magical spacesuit that protects them), they can measure the point of time at which the photon is emitted, but not the point of time at which it arrives on Earth.

To solve this, we might send one observer to the Sun, and one to Earth, give them synchronous clocks and have them both record the time at which the photon was sent and received. The problem with that is that 'synchronous clocks' is itself not well-defined if the clocks are at different locations. The best thing we can do, is give the two observers synchronous clocks while they are still on Earth, and then send one of them to the Sun, at a non-relativistic speed. After the measurement, this observer can return to Earth, and the observers can compare their observations. The recorded times between the emission and receiving of the photons will then be roughly 8 minutes (the relativistic corrections are small at this scale).

An easier way is to put a mirror on the sun and send a photon to it. The photon will return after 16 minutes.

> **Comments:**
> **username_0**: "An easier way is to put a mirror on the sun". *username_4 Cena "Are you sure about that?" meme*  

---

