• We’re currently investigating an issue related to the forum theme and styling that is impacting page layout and visual formatting. The problem has been identified, and we are actively working on a resolution. There is no impact to user data or functionality, this is strictly a front-end display issue. We’ll post an update once the fix has been deployed. Thanks for your patience while we get this sorted.

How long does it take for a light to traverse distance of one light year?

M

MyK Von DyK

Member
Hi to all Henri Poncaires of the world! Now, this is a bit strange question for some as one might reply with the obvious "one year, stupid". But since in real life speed of light does not reach its hypothetical limits, what is the typical time dilation factor for a light in vacuum of real space? Besides, with a hypothetical factor of 1, would a light emitting object that is one light year away be seen to the observer in "real time" or with one year delay?😕
 
...does not directly apply not light since light has no mass.
Light, by definition, always travels at the speed of light c

(btw, special relativity DOES apply to light but all terms which contains m disapperars, you can however still use it to calculate things like radiation pressure).


 
Originally posted by: f95toli
...does not directly apply not light since light has no mass.
Light, by definition, always travels at the speed of light c

(btw, special relativity DOES apply to light but all terms which contains m disapperars, you can however still use it to calculate things like radiation pressure).
Click to expand...

Maybe he's referring to the fact that since space isn't a perfect vacuum, light will have a very slightly different speed than "c" proper?
 
Maybe he's referring to the fact that since space isn't a perfect vacuum, light will have a very slightly different speed than "c" proper?
Click to expand...

Exactly that! Now, would you please be so kind and answer what is the speed of light in a real world situation, mostly what's it's speed in a typical space environment (not a cool but unrealistic 100% vacuum). As I get it, distances between celestial objects are measured in hypothetical numbers, now, what's that for real? Say I'm looking at our nearest solar system that's exactly 5 light years away. How long does it take light to reach Earth? And, please, don't answer "5 years, silly", I'm getting bored with such adolescent replies. THX, MyK
 
Space is not a perfect vacuum but not far from it, there is not much material that either absorb or re-emitt light (which is the mechanism which "slows down" light in a material), even in a gas cloud particles are very far from each other. Moreover, light only interact with SOME of the particles out there (frequency dependent).
The means that the average relative dielectric constant is very close to one meaning the speed of light is extremely close to c.
Hence, it will depend on the frequency and what the light travels through but my guess is that the correction ix perhaps 1e-9 or so (meaning the speed is 0.999999999 *c).
But again, I don't think there is a general anwer to that question.
It is worth remembering that the speed of light in a typical metal is not much lower than c (a typical number for a copper line is 0.8*c).

So to answer your question, perhaps 5 years and 30 ms?
That would be my guess. I have never seen anyone actually try to take this effect into account in any real world situation, the effect is definitly too small to measure.

Regardless, 5 years will definitly be very,very close to the real answer.
 
Originally posted by: MyK Von DyK
Maybe he's referring to the fact that since space isn't a perfect vacuum, light will have a very slightly different speed than "c" proper?
Click to expand...

Exactly that! Now, would you please be so kind and answer what is the speed of light in a real world situation, mostly what's it's speed in a typical space environment (not a cool but unrealistic 100% vacuum). As I get it, distances between celestial objects are measured in hypothetical numbers, now, what's that for real? Say I'm looking at our nearest solar system that's exactly 5 light years away. How long does it take light to reach Earth? And, please, don't answer "5 years, silly", I'm getting bored with such adolescent replies. THX, MyK
Click to expand...

Your internet personality is amusing.

Edit: Fixed for politeness.
 
Thanks for that answer f95toli! Any links on the web for any measurements taken, I can't find any good ones, but I believe NASA took measurements of deep space density with one of their probes?

BTW, did you mean .8c for electrons or photons in a copper line? I wonder because as I remember copper becomes translucent only at extremely high temperatures, or am I missing something?
 
Originally posted by: MyK Von DyK
Thanks for that answer f95toli! Any links on the web for any measurements taken, I can't find any good ones, but I believe NASA took measurements of deep space density with one of their probes?

BTW, did you mean .8c for electrons or photons in a copper line? I wonder because as I remember copper becomes translucent only at extremely high temperatures, or am I missing something?
Click to expand...

Well it's definitely not electrons because they move very slowly in copper wires.
 
Originally posted by: MyK Von DyK
Thanks for that answer f95toli! Any links on the web for any measurements taken, I can't find any good ones, but I believe NASA took measurements of deep space density with one of their probes?

BTW, did you mean .8c for electrons or photons in a copper line? I wonder because as I remember copper becomes translucent only at extremely high temperatures, or am I missing something?
Click to expand...

Photons. Electrons travel much slower, but the mechanism that we care about is the propagation of electromagnetic waves. I think that you can reasonably expect electrons to travel on the order of millimeters per second.
 
Sure, EM, sorry for that, it's getting late in our part of the world (3am). It would be nice though, wouldn't it? Touch live wire for a sec and you're copper plated :laugh:
 
Originally posted by: MyK Von DyK
Maybe he's referring to the fact that since space isn't a perfect vacuum, light will have a very slightly different speed than "c" proper?
Click to expand...

Exactly that! Now, would you please be so kind and answer what is the speed of light in a real world situation, mostly what's it's speed in a typical space environment (not a cool but unrealistic 100% vacuum). As I get it, distances between celestial objects are measured in hypothetical numbers, now, what's that for real? Say I'm looking at our nearest solar system that's exactly 5 light years away. How long does it take light to reach Earth? And, please, don't answer "5 years, silly", I'm getting bored with such adolescent replies. THX, MyK
Click to expand...

Space is close enough to being a perfect vacuum that you can safely assume that it will take 1 light year. Include an uncertainty of +- 0.000001 light years just to be safe?

If a galaxy is exactly 5 light years away (which is a meaningless statement, which part of the galaxy are we assuming is exactly 5 light years away? What is the uncertainty in this measurement? You can't be sure it is exactly 5 light years away)...

Then it would take light almost exactly 5 years to reach the Earth

Here, use this as an example as to why it would take 5 years to reach the Earth. If you measure the speed of light using a basic rotating mirror experiment, you'll learn that the speed of light in air is almost the same as the speed of light in a vacuum. Remember, this is in an atmosphere. The index of refraction in air is only about 1.0003. If light traveled through a light year of air, it would only fall short of a light year by an insignificant distance.

So while it is true that space is not a perfect vacuum, we can assume that light travelling for 5 years would traverse a distance of approximately 5 light years. The uncertainty in your final distance measurement would be something like 1*10^-5 light years, an insignificant distance compared to the total distance.
 
Originally posted by: MyK Von DyK
Say I'm looking at our nearest solar system that's exactly 5 light years away. How long does it take light to reach Earth? And, please, don't answer "5 years, silly"
Click to expand...

I doubt that anyone has an exact answer to this question, however I can't think of a real world situation where the slight difference would even matter.
 
Your question is as useless as the "1 year, stupid" answers. Basically you are asking, "How fast is light, if its not going c due to an imperfect vacuum?". Noone can give you an exact speed, and so noone can answer your question the way you want them to. That said my answer is:

a little over 1 year, stupid.
 
Well, to be fair it would probably be possible to answer a more specifik version of the uestion:
E.g.

"What is the speed of light if the wavelength is 630 nm and the medium is a near-perfect vacuum with an average particle density (hydrogen atoms in ther ground state) of 1 particle/cm^3?"

Unless I am missing something this is a "proper" question and it should be possible to answer. However, you need to make some assumptiones (wavelenght, particle density)

 
So the fact that we're all on a massive body (Earth) that actually bends space-time continuum does zilt/zero/null/nada for our perception of time passed compared (or relative) to the non mass particle moving through vacuum of space? Let me paraphrase my question. If you measure distance to a (hypothetical) object and the origin of measurement is on a massive object (Earth), wouldn't that account for quite a large difference as opposed to taking measurement if the massive object (Earth) wasn't there?
 
Originally posted by: MyK Von DyK
So the fact that we're all on a massive body (Earth) that actually bends space-time continuum does zilt/zero/null/nada for our perception of time passed compared (or relative) to the non mass particle moving through vacuum of space? Let me paraphrase my question. If you measure distance to a (hypothetical) object and the origin of measurement is on a massive object (Earth), wouldn't that account for quite a large difference as opposed to taking measurement if the massive object (Earth) wasn't there?
Click to expand...
The bending of the light is still going to be rather minimal. And even when it does occur, we know enough of the physics to take it into account. For example, the 1919 confirmation of Einstein's General Theory of Relativity showed that the deflection due to the massive Sun, as reported in the appendix of my copy of Einstein's Relativity papers, was on the order of at most 1 arc-second. That's 1/3600 degrees. So the elongation in the path between the Sun and Earth due to the bend is around .0000000000117522152696640383458*Au, or 1.758 meters.
 
As for the speed of light, isn't that now thought to be variable, depending on local magnetic conditions where the light is passing through? I vaguely remember news a few years ago about some lab slowing light in a magnetic bottle.

Edit: Add this link.

If a particle of matter can't attain light speed then
the dual theory (light is both a wave and a photon particle) has an obvious paradox. A photon can't travel at the spped at which it travels.

Isn't gravity thought to be faster than light?

And doesn't that theory about entanglement (I don't know much about it) say "communication" occurs faster than light between two particles that were first closely situated, then moved a wide distance apart?
 
Originally posted by: scott
As for the speed of light, isn't that now thought to be variable, depending on local magnetic conditions where the light is passing through? I vaguely remember news a few years ago about some lab slowing light in a magnetic bottle.

If a particle of matter can't attain light speed then
the dual theory (light is both a wave and a photon particle) has an obvious paradox. A photon can't travel at the spped at which it travels.

Isn't gravity thought to be faster than light?

And doesn't that theory about entanglement (I don't know much about it) say "communication" occurs faster than light between two particles that were first closely situated, then separated by a wide distance?
Click to expand...
The speed of light is variable depending upon the permittivity and permeability of the media that it is passing through. But in terms of space we can assume that we have a vacuum and still be relatively accurate.

The duality of light is still retained in relativity by assigning the photon a rest mass of zero which cancels out any of the Lorentz transformations that act upon it.

 
well that depends. light has no mass and alwasy travels at c speed unless it is slowed down by matter. but if you say through vacum and assuming the light doesn't pass through any black holes and such, one year is the answer. and not a second more.
 
I beleive I read somewhere that we have measured the speed of light from all over the sky and found variations only of about 1000 or so m/s due to non-vacuum. One area where you definitely do have to worry is astronomy, if a quasar or similar is 80 million lightyears away that non one term adds up quickly. Light extinction is the prevailing inaccuracy in distance measurements however so I don't think you have to worry about really close stuff (in astronomy 5 light years is close--the sun is 8kPc from the center of our own galaxy, 5 light years is around the interstellar distance)

Quantum entaglement is when you have 2 particles which take on a certain state, when one is measured the other will be found in an opposite state. If they are separated, this will still happen regardless of how far away they are. Quantum mechanics tells us that you can't know the state of the particles before you measure them, thus no relativity is violated--information doesn't travel faster than light.

There are many cases where "stuff" can travel faster than light, take for instance evanescent waves, the surface wave can travel faster than the speed of light. Also if you point a laser point at say the moon or something else far away and sweep it across the surface, you can theoretically make the dot move faster than light.

Another way to beat light is in a medium, in a nuclear reactor there is a blue glow in the water, this is from the electrons being emitted moving faster than the light in the same medium.

Gravity is predicted to travel at the speed of light, however there is no experimental evidence for this statement....yet.

P.S. never heard of light behaving differently in a magnetic field, not sure if a photon has a magnetic moment or not.
 
You must log in or register to reply here.

TRENDING THREADS

Back
Top