I just learned that, according to Einstein's relativity theory, time reaches zero for an observer (light) when traveling at the speed of light, so everything is supposed to be at the same place in the universe for light. But why does it take 8 minutes for light to travel from the Sun to Earth? Is it because we are observing it from earth? At 300.000 km/s distances in the universe are hardly zero? I can't seem to grasp this.

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    $\begingroup$ Where did you "learn" this? $\endgroup$ – skillpatrol Jul 19 '15 at 23:21
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    $\begingroup$ Consider an observer moving, relative to the Earth, arbitrarily close to the speed of light. According to that observer, the elapsed time in travelling between the Sun and Earth is arbitrarily small. According to clocks on Earth, it is about 8 minutes. According to other relatively moving observers, the elapsed time is something else. There is no universal time in relativity. $\endgroup$ – Alfred Centauri Jul 19 '15 at 23:33
  • $\begingroup$ I'm pretty sure that to an observer time would be the same, but to an observer travelling at the speed of light time would stop. $\endgroup$ – phi2k Jul 19 '15 at 23:52
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    $\begingroup$ @PhyCSx, there are no inertial reference frames with relative speed $c$. $\endgroup$ – Alfred Centauri Jul 19 '15 at 23:57
  • $\begingroup$ As you mentioned, for an object traveling at the speed of light, it would take about 8 minutes to traverse the distance from the sun to the Earth. As we know, the sun's velocity is not relativistic (no tunneling of light). So our clocks are slightly slower than a stationary clock (if we can ever find one). We also know that photons travel at the speed of light, so they don't experience any time. I don't see what your problem is. $\endgroup$ – LDC3 Jul 20 '15 at 0:07

Light travels at the speed $c$ this speed is finite and with out using any relativity we can calculate the time it takes for something travelling at this speed to reach us: $\text{time} = \frac{\text{Distance}}{\text{speed}}$ or $ t= \frac{d}{c} = \text{8 minutes}$ in this case.

For a person travelling very close to the speed of light with velocity $v$ from the sun towards the earth time does slows down, and he goes past the earth in a matter of seconds. But for us time doesn't slow we see the person with almost the speed of light and the time it takes to reach us is again $ t= \frac{d}{v}$ which will be almost 8 minutes but slightly longer.

Now for light you say time freezes completely this is not really accurate, but for arguments sake I will accept it: Then the same logic applies as before. For light it seems that zero time has passed but for us it is still 8 minutes.

This might seem like a paradox, but time is relative in Einstein's theory of relativiy.

Note that your argumentation is backwards, "time reaches zero for light, because everything is at the same place". While the more "correct" way to say it would be that the photon does not experience time and therefore everything seems to be at the same place.

Hope this helps

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The thing is, in relativity you cannot have a reference frame "chasing" a photon. You'll get singularities if you try to view the world from a photon's perspective. A photon cannot move like you and you cannot move like a photon.

As a photon, travelling along a light-like world line, experiences no proper time it's proper velocity is simply undefined. However, we can perfectly state that the speed of light is invariant for every reference frame where $v < c$.

Special relativity breaks down when going extreme (e.g. for velocities equal to or higher than $c$), but the absence of tachyons is also an experimental fact. There are probably no such things either.

All models breaks down at some point, but as long as it produces good pragmatic experimentally results, the model is useful within the set boundaries. The proper velocity of light is simply not well-defined; it's meaningless to talk about a proper velocity for a photon nor a valid reference frame in which a photon is at rest. Such concepts have no defined meaning and lies outside what the model (in this case, special relativity) can deal with.

What a scientist does when a model gives you nonsense is to go back and ask nature for hints for a better model. We do already know that the theories of relativity is incomplete, and we'll never achieve a complete theory. We can, however, do our best to continue our investigation and come up with models as complete as we can make them.

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You are correct time does stop at $c$ speed of light, however we (planet) did not place a timer inside the light but are measuring it from outside so time for us passes normally and nothing weird happens and nothing is frozen.

As a result we simply are measuring the gap of time from the 1 point to another point whilst a object (light) goes to it so as @john said its simply $t = d/v = d/c $.

On the other hand if I put a clock on the light then yes the time would be zero, but since we are not there is not problem and time can be measured normally.

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  • $\begingroup$ But when you say that "time can be measured normally" you are assuming that "normally" is the human perspective and the human perspective is correct. We've never measured anything called "time". We measure entirely physical processes and we infer measurements of time; we measure the rotation of the planet and infer that a consistent amount of time is passing with each instance of our planet's rotation. "However, we have proven in many different experimental tests that time clearly does not run at a constant rate between different frames of reference." from the phys.org article. $\endgroup$ – Derek Roberts Jul 10 '18 at 6:22

The answers to this question seem to be implying that the laws of physics are more of a relative relationship to the observer instead of "absolute laws". Why isn't there a single answer? If I asked, does light reach it's destination instantaneously or does it take 8 minutes for light from the Sun to reach the Earth, it's illogical to answer "both"? One of those is The Truth, and one is merely our perspective of the truth as human beings. From our perspective, it merely appears as if light is "traveling", from a point of origin to a destination, at a "speed". But our "perspective" as human beings clearly isn't entirely accurate. Even Carl Sagan said, "Science is far from a perfect instrument of knowledge. It’s just the best one we have. In this respect, as in many others, it’s like democracy."

Light exists instantaneously. It's illogical to say that light "travels" because light doesn't move, because light doesn't experience time. As this article on Phys.org describes it, the "time" that it takes light to travel from a point of origin to a destination has been reduced all the way to 0. We misunderstand light as "traveling" only because our perspective does experience "time". But that is just our perspective, it isn't The Truth. For example, as humans we've divided time into a lot of different parts, seconds, hours, days, weeks, months, years, decades... but time itself wouldn't recognize any of those "portions", time itself would have no way to differentiate between the very first moment of existence and the very last moment of existence; to "time" those are both within the same "moment" because "time" can't be divided, that's only a human idea... imagination. It's only on our planet that the rotation of the Earth "separates" one "period of time" from another, one day is separated from another by a complete rotation of our planet around it's own axis. But the "dimension" of time has never actually been divided, in the same way that space has never been divided. We consider the Milky Way galaxy to be a "different place" than the Andromeda galaxy but there's no wall or border or barrier that separates any one point or area in space from another, just like there's no border or barrier that can be inserted into the "dimension of time" to separate one moment from another. Time's perspective would be that all of time is the same time, there's no Cretaceous or Jurassic, the dimension of time has never stopped and reset then started over again. It's only in our imaginations that one second or one day is a "different period of time" than any other second or any other day.

Humans think that it takes 8 minutes for light from the Sun to reach the Earth because we have a limited capacity to understand the truth and as Carl Sagan said, "science is like a democracy", so whatever the majority of scientists believe, or whichever idea the most scientists have faith in, is considered to be the truth. But science can't describe light by itself. We're not even sure if we have the capacity to observe the entirety of the phenomenon that we know as light, we may only be able to observe the after math of light having existed. Like a train that passes by you so fast, you don't see the train itself and can only tell that anything happened because you felt a sudden rush of wind after the train passed. If you want to answer this question then you have to also rely on logic. Logically, both answers can't be true, light can't be instantaneous and also have a travel time. And as far as we understand it, the concept of light traveling only appears to happen within our perspective. So, is the human perspective the truth or is light's perspective the truth? If I had to wager, I'd bet that light's perspective is the truth.

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    $\begingroup$ Plenty of things have relative answers. How far away is France? We're likely to have quite different answers. How fast is the earth going? Depends on what you're measuring its speed relative to. $\endgroup$ – Chris Jul 10 '18 at 13:03

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