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If the light from a galaxy 250 million light years away takes 250 million years to get to Earth, does that mean if one exploded today we would only discover it in 250 million years? (self.askscience)

submitted ago by Grasshop

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[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 61 points62 points ago

Yes, you are correct.

If we observe a close star, for example, say one about 250 light years away. The light we are seeing today was produced from that star 250 years ago. In another 250 years, we will see what that star really looks like today.

In this way, astronomy is a form of seeing back in time.

[–]soster506 19 points20 points ago

Cool little tidbit I've heard before that largely comes up to be impossible, but theoretically awesome to think about.

Imagine if there were a perfect mirror some 120 lightyears away from earth. If light traveling to that mirror were not interfered with drastically and we had a telescope that could resolve the reflection from earth, we could be watching the American Revolution happen 'live', so to speak, in the form of light.

[–]melanthius 7 points8 points ago

It would be awesome to find some kind of gravitational ultralens that instead of just focusing light, caused it to make a 180 and do exactly what you are referring to (naturally).

[–]Astromike23Astronomy|Planetary Science|Giant Planet Atmospheres 10 points11 points ago

This is an awesome thought experiment. If you don't mind, I'm going to steal this little tidbit for my astronomy lectures when talking about the speed of light.

[–]soster506 3 points4 points ago

Of course! I heard it from somewhere else, glad to pass the knowledge on!

[–][deleted] 5 points6 points ago

The mirror would have to be matching the rotation and orientation of earth?
At 250 light years away or a gps type of array would be needed surely? It would be moving at ridiculous speeds.
It's should probably also be countering for relativity?

[–]soster506 6 points7 points ago

Good point, these are some other assumptions I was talking about making that I hadn't thought of! Also, the photons must have an unimpeded path to the mirror from earth and back again. This means no matter in it's way, perfect vacuum, and also no gravitational effects.

We also have to look at the way light is reflected from the earth. First of all, it would be incredibly difficult to resolve individuals from the sort of range we're talking. Think about looking at the great red spot on Jupiter. We can resolve it's movement over time, but it's a very large weather pattern we're looking at - on the size scale of several earths. Another assumption we must make is that we can resolve details on many orders of magnitudes smaller than this depending on what we want to see.

Also, most telescopes (as well as microscopes) are limited by optical constraints which make seeing at very high resolution difficult. In this case, the light scattering away from the earth, carrying the information of what happened, would not be travelling in a highly collimated (uni-directional) beam like a laser, but in a more conical beam, like a flashlight. The reflection would be scattered with beams going off in different directions, so the mirror out in space would need to be absolutely MASSIVE in order to capture the entire 'scene'. At lightyear distances, light rays propagating even fractions of a degree off of the optical axis result in massive areas to which the light will travel.

Finally, the mirror would also have to be in place 120 lightyears away in time to reflect the light traveling from Earth. This brings up another cool thought: if the mirror only took five years to build, and there was colony 120 lightyears away building it, they wouldn't need to start building until 115 years AFTER the events on Earth took place in order to reflect them back!

[–]trekkie80 0 points1 point ago

Suppose an alien civilisation were to get our Arecibo message and would try to test our advances, especially to see if managed to not self-destroy.

Suppose they did perform this moving mirror trick and reflected our own Arecibo message back to us, would we find out?

Or would we think it a natural coincidence?

Any way of telling it apart?

Or what if some terrestrial 70s signal was picked up ? would we jump to think it was aliens?

[–]soster506 0 points1 point ago

I'm not sure how far it's traveled at this point, but if it were returned to us clearly, it would be very hard to think it was natural coincidence, because we would essentially be validating all of our assumptions above, which would be unlikely to naturally occur.

[–][deleted] -1 points0 points ago

Finally, the mirror would also have to be in place 120 lightyears away in time to reflect the light traveling from Earth. This brings up another cool thought: if the mirror only took five years to build, and there was colony 120 lightyears away building it, they wouldn't need to start building until 115 years AFTER the events on Earth took place in order to reflect them back!

So? They would care as much as we do about ISDN nowadays.... I don't see the mindblowingness in this unless you have a communication system above the speed of light in which case you could phone a distant planet to "TIVO that shit"...

[–]soster506 2 points3 points ago

Mainly just another interesting fact in time perception, mostly in our reference frame for time events like reflections need to occur simultaneously, but on the larger scale their's a lot of room for variation. In this case, I thought it was interesting that the mirror only needs to exist for the time period that the light reflects off.

[–]TheBBMathematics|Numerical Methods for PDEs 1 point2 points ago

Hey, it's like that Denzel Washington movie, where you can see the past at a time some fixed distance from the present.

It would suck if, when the Kennedy assassination rolled around, say, the Earth turned out to be rotated the wrong way....

[–]Cozmo23 4 points5 points ago

Wouldn't it take the light 120 years to get there and 120 years to get back? But I see what your saying. I think Google is working on adding this feature to its maps.

[–]WindsweptHydra 19 points20 points ago

soster506 did the math correctly. The American Revolution was about 240 years ago. (1770's)

[–]johnprime 1 point2 points ago

This is the greatest thing I've read it a while. Blows my mind thinking about it.

[–]Comika 3 points4 points ago

This is also why when you look at the moon, you are seeing the moon as it was 1.3 seconds ago. The light reflecting off the surface takes that long to reach the earth, letting you see the moon of the past.

[–]skyshadow42 11 points12 points ago

Also, that monitor that's a foot in front of your face is also about 1.0167 nanoseconds in the past.

Light is funny that way.

[–]ineffable_internut 12 points13 points ago

It's actually a bit longer than that, because your brain also has a slight lag in interpreting and putting together the picture your eyes are seeing.

[–]BullshitUsername 1 point2 points ago

"a bit longer" being the operative phrase

[–]rooktakesqueen 7 points8 points ago

Just seven or eight orders of magnitude, not too big a difference.

[–]nikesoccer01 1 point2 points ago

And we also live in the past because all of the decisions we make occur in our minds but take nanoseconds to have them executed. Pretty thought provoking.

[–]Clawdius 1 point2 points ago

So in a sense, everything we see is what occurred in the past? We never actually see anything as it is in the immediate present?

[–]taggedjc 3 points4 points ago

This is partly true; however, the brain apparently is able to predict the future. This is why optical illusions work the way they do. We compensate by having our brains extrapolate from the information the eyes give us to actually represent what the immediate future could be like

Note that this might actually mean everything we see is actually what will occur a very short time in the future (or, the best guess our brains can manage of this).

[–]thebpfeif 3 points4 points ago

The simple Physics equation Velocity = Distance/Time would answer that for you. Because light has a constant velocity (somewhere around 300,000,000 m/s) and light always has some sort of distance to travel to your eyes, there will always be some sort of delay, no matter how minute.

[–]laetus 1 point2 points ago

The 'immediate present' might be tricky to define.

http://www.youtube.com/watch?v=wteiuxyqtoM

[–]ayb -1 points0 points ago

This is starting to sound like a post-modern philosophy class.

Soon you'll be reading Rilke.

[–]wtallis 4 points5 points ago

Not post-modern philosophy, pre-Einsteinian philosophy. Nowadays we have hard science that settles the issue, albeit in a counter-intuitive way: Relativity of simultaneity

[–]bobsomeguy -2 points-1 points ago

Then there is that theory that says that reality only exists when we look at it.

[–]EastCoastLA 1 point2 points ago

So, if we put a mirror on the moon looking back at the earth, and could focus it on any point on the earth with amazing resolution. How many seconds can we look back in time? Wasn't their a move based on a similar concept?

[–]firstLOL 8 points9 points ago

You'd be looking about 2.6 seconds back in time. To be honest you'd be better off just using Skype.

[–]soster506 0 points1 point ago

Good point to bring up, it's the same reason there's a lag in satellite relays! The further the information travels distance wise, the longer it takes. In essence, it makes my original idea not as cool because anything that we would be able to look 'back' on now would be in the forward direction, so we may as well look at something directly.

[–]MissingHeadache 2 points3 points ago

Would the expansion of space have an affect upon this? If so, to what degree would this be?

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 0 points1 point ago

The expansion of space means that more space is being added between any two distant points - so if more space is added it indeed takes photons longer to travel. So yes, expansion of space changes the distance between two distant observers.

[–]fingersquid 0 points1 point ago

Just a side-question: How does that theory (that more space is being added between any two distant points, and the theory of the universe expanding constantly) take into account the laws of conservation of matter and energy? The matter and the energy contained in it is not being created, so where is it coming from?

[–]issiebear 0 points1 point ago

Volume is independent of matter and energy, so the law of conservation holds. In addition, the second law of thermodynamics says that the entropy of the universe is always increasing (going to more disorder), which makes sense if the universe is expanding and the matter contained is constant.

[–]fingersquid 0 points1 point ago

tagged you as smartperson#2.. I appreciate the responses though, as much as I don't understand them...haha.

[–]issiebear 0 points1 point ago

Think of a contained vacuum such as those commonly found in thermoses (the space between the outside of the container and the inside of the container). It is essentially a quantified volume of space that has no material within it. This is one example that shows volume is independent of matter and energy.

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 0 points1 point ago

What is expanding is the geometry of space-time. That is neither matter nor energy.

[–]fingersquid 0 points1 point ago

I'm going to go ahead and trust you on this, and tag you as "smart person" while I'm at it... I am very interested in space, but myself do not possess the mental capabilities to understand the physics of it.

[–]ggrievesPhysical Chemistry|Radiation Processes on Surfaces 0 points1 point ago

the galaxy was closer to us 250 mya than it is today, so it's true the light would not have had to travel the full 250m light years. Eactly by how much I'm not sure

[–]johnbarnshack 0 points1 point ago

Because the space the light is travelling through gets stretched out, so does the light. Its wavelength becomes longer and it gets redder - the Doppler effect (redshift). The light isn't slowed down though. What this does mean is that we see light from 13.7 billion years ago, but that object isn't 13.7 billion lightyears away - at the moment it's farther away because space is expanding. The light itself has only travelled those 13.7 billion light years though.

I am terrible at explaining this stuff ._.

[–]RepostThatShit 2 points3 points ago

No, it's incorrect. If a star were to explode 250 million light years away the light from the explosion would take considerably longer than 250 million years to come here because the distance would grow significantly during that time.

Expansion of the universe

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 0 points1 point ago

I was ignoring expansion - as it confuses the basic point of light travel in a conversation like this.

Otherwise I have to get into accelerating expansion, massive objects at massive distances, etc.

The basic gist is that the light is old, very old, and yes, from the source.

Plus, at short distances, such as 250 light years, expansion isn't an issue (local galactic gravity is stronger than expansion). But yes, at the massive distances, expansion has moved it even further.

However, when we make discoveries... such as the galaxy we discovered that is 12.9 billion light years away (GN-108036) - when we give that age we do not take expansion into account.

[–]Grasshop[S] 1 point2 points ago

relevant

[–][deleted] ago

[deleted]

[–]pjhorrex 1 point2 points ago

Not a super-scientific source but here's what Wikipedia says about it.

[–]chimpanzee 1 point2 points ago

You're right that we can't observe their current state by any means, since they're so far away and nothing can travel faster than light. But it may be possible to make a decently accurate guess at their current state given enough information about their past states - kind of like how we can infer that our own sun will turn into a red giant in about 5 billion years.

[–]fingersquid -1 points0 points ago

nothing can travel faster than light

Well.. they thought that neutrinos could. The jury is apparently still out though, because of satellite lag and off-timing (relevant). But then I found another article saying they duplicated the experiment and found faster than light particles again, so who knows. I think it would be fantastic if the results were true.

[–]AWaffleTooFar 0 points1 point ago

So do we actually have any idea what space looks like right now?

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 0 points1 point ago

Your question needs refinement. I don't know what you mean by space.

If something is 250 light years away, there is no way to know what it looks like right now, at that spot. The best we can know is what it looked like 250 years ago. Based on what we know about a star we can speculate as to its position and state of well being today, but we can't know for certain.

[–]Not_A_Throwawy 1 point2 points ago

So are we influenced by the gravity of things in their current positions, or the positions we see them in? In other words, does gravity "travel" faster than light, or does the same limit apply?

I realize that the Earth itself is not strongly influenced by the gravity of bodies hundreds of light years away, but I'm still curious...

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 4 points5 points ago

While not yet experimentally confirmed - gravity is also thought to travel at c, so it would be treated just like light.

[–]AWaffleTooFar 0 points1 point ago

So does everything more than a few thousand lightyears away look completely different than any images we have of it?

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 1 point2 points ago

Photons take time to travel - so yes. However, thousands of light years in cosmological time isn't very long. A star, for example, wouldn't change much over thousands of years. However, position of planets around stars... stars in their galaxies, etc... are all moving and would not be in the same position we see them at today.

[–]emikochan 1 point2 points ago

A few thousand years is tiny in cosmic terms.

[–]benYosef 0 points1 point ago

This is actually a good thing as well when it comes to studying our universe. The further we look the further back in time we are looking, this allows us to make more concrete statements about the nature/growth of our universe.

[–]joshocar -1 points0 points ago

I read somewhere that the Pillars of Creation, a famous picture from the Hubble telescope, has already been destroyed by a supernova, but the light hasn't reached us yet. Apparently, you can just see the beginnings of the blast in the photo.

[–]mdeg 0 points1 point ago

<layman speculation>

Aren't you suppose to factor the expansion of space into this equation? 250 lightyears maybe not, but OP is asking about 250 MILLION lightyears.

"Cosmological redshift is seen due to the expansion of the universe, and sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase of their distance from Earth." source

</layman speculation>

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 0 points1 point ago

I left out expansion to keep the concepts simple.

Plus, when you hear announcements of galaxies being discovered - the distances they cite do not include expansion.

[–]MjrJWPowell 0 points1 point ago

So that means that with a super accurate telescope it would be possible to see back in time to the point of the big bang?

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 1 point2 points ago

No. The earliest light we can see is the CMB which was 379,000 years after the Big Bang. Prior to that, photons didn't stick around long enough (they were absorbed pretty much as soon as they were created).

[–]Scarlet- 0 points1 point ago

On a similar note, if we were to use an "super" telescope, would we be reducing the amount of light years we are viewing?

For example, with our normal vision it takes us 250 million light years to see an explosion, what if we were to use a telescope, does that reduce it down to 249 million light years?

[–]jswhitten 10 points11 points ago

No, light years is a measurement of distance. The distance to something doesn't change when you're looking at it through a telescope. No matter how good your telescope is, you won't see the explosion until 250 million years after it happened.

[–]bobsomeguy 0 points1 point ago

When I was a kid and first learned about this stuff, I thought that if you put a camera a few hundred light years out, you could see what was happening at that spot without the delay. Then I learned about EM transmissions......

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 5 points6 points ago

No. A telescope doesn't see 'further away light'. A telescope gathers photons, just like your eye.

However, the larger the mirror, the more photons we can gather. And the longer we gaze at a single spot, the more photons we accumulate. This allows us to see things that are very faint and very far away.

But the light is still hitting the photoreceptors in the telescope instruments AT the telescope instruments - so that light took just as long to hit the telescope as it did to hit your eyes.

[–]_BEAR_ 0 points1 point ago

So is the light from the stars I see when I look at them with my naked eyes (say on a clear night), really just photons emitted from that star that have traveled x amount of miles, and are hitting my retina?

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 1 point2 points ago

Exactly!

And if I am standing right next to you, and looking at that same star, the photons hitting my eyes are also from that star but are completely different from the photons that hit your eyes.

And... we just killed those photons.

[–]jarsky 4 points5 points ago

No it would still take the same amount of time, as the light still has the same distance to travel to the telescope.

It would probably take longer than 250 million years to see an explosion 250 million light years away as you would also have to take into account expansion at those distances.

[–]banditski 1 point2 points ago

I'm the fourth person to answer, but this analogy helped me understand it when i was first introduced to the idea.

Imagine you live in a time before phones, internet, radio, etc. Your grandma is sick in another city 160 km away and you want to know how she is doing. The fastest form of communication is a carrier pigeon that flies at a top speed of 80 km/h.

Your uncle sends a carrier pigeon to you once an hour with an update, but it takes the pigeon two hours to fly to you. So the news you get is always two hours old, even though it's 'new' to you every hour when you receive an update. No matter how badly you want to get updates sooner, it will always take two hours for the pigeon to fly to you. (Carrier pigeon is the fastest form of communication, remember?)

Now replace the carrier pigeon with a photon of light. Instead of travelling at 80 km/h, a photon of light travels at ~300,000 km/second. But the concept is the same. You won't get any 'new' information until a new photon reaches you, and that photon only can tell you what was happening at the source when it left (i.e. your grandma's health 2 hours ago in the case of the carrier pigeon or what the star looked like 250 years ago for the photon).

[–]chimpanzee 4 points5 points ago

To expand on your analogy, using a 'super telescope' would be like sending an eagle that can carry a 200-page book at 80 km/h rather than a pigeon that can carry a 2-page letter at that speed. There's more information, but it doesn't get there any faster.

[–]horse-pheathers 1 point2 points ago

The primary (astronomical) function of an optical telescope is to collect more light than your eye does, allowing you to see dimmer objects; if your pupil is, say, .5 cm wide, a tiny telescope with a 5cm wide lens will collect 100 times as much light as your eye. This is why the primary trait you should consider when, say, buying a telescope is aperture; it's a direct correlation with how much light the telescope can collect.

The secondary function of an optical telescope is magnification. Most things you're going to point a telescope at in the sky, believe it or not, actually appear pretty big -- they're just too faint to see clearly -- so magnification is not a huge issue for most stargazers (and it can be easily adjusted to whatever you are looking at just by swapping in an appropriate eyepiece). This is a matter of focal length, how severely the optics bend the light to bring it into focus. Shorter focal lengths will give you a brighter image and a wider field of view in a given eyepiece, where longer focal lengths will give you more magnification. Take two telescopes, one with a focal length of 500cm and one with a focal length of 250cm, and swap the same 10cm focal length eyepiece between them, you'll get 50x magnification from the first set up and 25x from the second (it's simply a matter of dividing the telescope focal length by the eyepiece's). At the higher magnification, you're throwing a considerable amount of the light you collected away, so the lesser magnified image is going to appear considerably brighter.

Sorry for the digression into telescopy 101; the central point here is that magnification has nothing to do with moving you effectively closer to the thing you're looking at -- it's all about how you collect and bend the light that has already reached you.

(And if you ever buy a telescope, avoid anything that lists the damned magnification as a selling point on the outside of the box like the plague; it's a sure sign of a crap instrument aimed at people who don't know a thing about optics, and the small apertures that sort of scope tends to have render the high magnification eyepieces they advertize on the box effectively useless thanks to the dimming effect magnification has on the image. And don't get me started on the horribly wobbly mounts these scopes inevitably have....)

[–]Blue_Paul 0 points1 point ago

does that mean if we could get to the edge of the universe somehow we could see how it was created?

[–]emikochan 0 points1 point ago

If we look far enough out we still can't see the beginning since the early universe was opaque. Too much in too little space :P

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 0 points1 point ago

There is no edge (or center) to the universe. There is an observational sphere for every point in the universe (using that point as a center). The observational sphere is that beyond which light has not yet traveled.

We have seen objects as old (far away) as 12.7 billion light years - which is pretty early in galaxy formation.

[–]ZSinemus 0 points1 point ago

Not using a telescope that views electromagnetic radiation - we can only see back to roughly 300,000 years after the big bang, when the universe finally cooled down enough for recombination, which unleashed the first "light" (electromagnetic radiation). So we can keep looking further back in time, but once we hit the first recombination we can't look any further back, as there's nothing to "see".

[–]ordig -2 points-1 points ago

Assuming time is linear.

[–]seqqer 8 points9 points ago

Yeah, a good and really interesting example are The Pillars of Creation.

The Pillars of Creation no longer exist. In 2007, astronomers announced that they were destroyed about 6,000 years ago by the shock wave from a supernova. Because of the limited speed of light, the shock wave's approach to the pillars can currently be seen from Earth, but their actual destruction will not be visible for another millennium.

[–]SquashyO 7 points8 points ago

Actually we know something is going on a few hours before the light from the supernova gets here because we see a spike in neutrino detectors.

Most of the energy of a supernova is released as neutrinos and as they are weakly interacting they can start their journey to Earth straight away. As the gas of the star is a luminous plasma all the photons have to bounce around a bit before finding their way out.

Neutrino spikes have been detected 3 or 4 hours ahead of the light from a supernova, but it's believed that they are still traveling at light speed (so far) so it still takes 250 Million years in your example before we know something is going on.

[–]VELL1 3 points4 points ago

It is also interesting to not that some galaxies are moving away from as at a speed exceeding he speed of light. Meaning that light from those galaxies will never reach us and we'll never find out what is gonna happen to them.

In fact we have galaxies going away from us at sub-light speeds but we can see them speeding up, meaning that at some point they gonna reach the light speed and we gonna see them dissapear.

[–]lostPixels 0 points1 point ago

Nothing moves greater than the speed of light.

[–]VELL1 4 points5 points ago

That is true. However we live in a constantly expanding universe. Meaning that even if the galaxies are standing still (so to say), they will be still moving away from us. So technically you are right, but at the same time you are not.

http://en.wikipedia.org/wiki/Faster-than-light

Universal expansion section.

[–]Denode 3 points4 points ago

Picture a balloon. It is the Universe. Draw a few dots on it, those are the galaxies. The speed of light is the limit on what speed things move across the surface of the balloon, but that's not what's happening. The balloon itself is expanding. The galaxies are travelling below the speed of light, but with the very space between them and us expanding like the surface of a balloon, the distance between us and them is effectively growing faster than the speed of light.

Comprende?

[–]Gunski 0 points1 point ago

That actually made more sense than anything I've ever read on this subreddit.

[–]panzerkampfwagen -1 points0 points ago

Space can expand faster than light. Physical objects, like an atom, can't.

Shortly after the Big Bang, and I mean SHORTLY, the universe expanded faster than the speed of light.

It's hypothesises that one day, due to the expansion of the universe speeding up, all other galaxies will be moving away from ours, and each other, faster than the speed of light, but we won't be able to see them.

[–]emuApocalypse197 2 points3 points ago

Might be a stupid question but does the aid of a telescope allow us to see light that hasn't quite reached earth? Or is there no change?( or at least one of any significance)

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 8 points9 points ago

No. A telescope doesn't see 'further away light'. A telescope gathers photons, just like your eye.

However, the larger the mirror, the more photons we can gather. And the longer we gaze at a single spot, the more photons we accumulate. This allows us to see things that are very faint and very far away.

But the light is still hitting the photoreceptors in the telescope instruments AT the telescope instruments - so that light took just as long to hit the telescope as it did to hit your eyes.

[–]jswhitten 5 points6 points ago

A telescope will let you see things in more detail, and you can see things that are too small or too dim to see without one, but information is never transmitted faster than the speed of light. If the light hasn't had time to reach Earth, there's no way to know it's there.

[–]jamesfilm 1 point2 points ago

Remember when you are looking through a telescope you are seeing the same light your eye would see, only the light is hitting a mirror / lens that distorts the light , that then lets you see it in more detail In all cases the light has to travel from its source to the telescope and then to your eye for you to see it.

A telescope actually makes it take a tiny tiny tiny tiny bit longer for you to see the light as the light has slightly more distance to travel for it to get to your eye as it bounces around the telescope.

[–]Guck_Mal 1 point2 points ago

no

[–]AngryWeasels 0 points1 point ago

Unlikely as it is, the universe could be begining to end right now, or the galaxys we know may have just disappeared, and we,d be none the wiser.

[–]banned_andeh 1 point2 points ago

Shouldn't it take a little longer with the universe expanding during the time the light is travelling?

[–][deleted] -1 points0 points ago

It would probably take longer to notice it, since the Universe is expanding. If the point where the star is and the Earth are moving away from each other, then light has to cover a farther distance. This is the basis of Red Shifting.

[–]DJaySID -1 points0 points ago

so if i were to take all this and relate it to our sun, technically the light we're seeing from the sun is "old?" but exactly how old is it? the calculation would be as simple as the distance divided by the speed of light? so say speed of light was 2 m/s and the sun was 6 m away it would be 3 seconds "old?" i don't know why i'm so confused by this.

[–]lostquito 0 points1 point ago

Correct. The sun is at an average distance of about 93,000,000 miles away from Earth. Light from the sun, traveling at c takes about 8 minutes to reach us.

[–]Icedipping -1 points0 points ago

Yes, simply put, it would take 250 million years for you to notice the change.

However, if you factor in the expansion of the universe, the light would take longer than 250 million years to reach you as the expansion creates more space between you and the light.

[–][deleted] -2 points-1 points ago

If the Sun exploded you would only find out 8 minutes later.
AFAIU gravity also travels at the speed of light.
So IF the sun would suddenly just disappear it would take the 8 minutes before the planets would we slinged out of orbit. Yes/No?

[–]emperor000 0 points1 point ago

Yes.

[–]sheliakObservational Astrophysics|Spectroscopy|Interstellar Medium -4 points-3 points ago

Yes, but this is not something we are concerned about. Astronomers never say we discovered a star that exploded 250 years ago. When we say today a star exploded, we mean the day we saw it. And if we say that a supernova exploded 1000 years ago and the chines saw it, it means that they saw it 1000 years ago. Than we say it was 3000 light years away and this tells us all about the "real" time of explosion.

[–][deleted] ago

[deleted]

[–]adamsolomonTheoretical Cosmology|General Relativity 4 points5 points ago

it would take slightly longer since the universe is expanding so by the time it gets here the distance to the original location would be different.

Good point. This is not a negligible effect over 250 million light years.

though the 'effects' of gravity would be felt instantly, even though they would be infinitesimaly small

What? Gravity most certainly does not propagate instantaneously, and there'd be mighty big problems if it did!

[–]AnotherUser256 2 points3 points ago

Are you sure about that? You might want to look up the speed at which gravitational waves travel.

[–]Arcanorum -1 points0 points ago

There is no scientific study that we've done that can prove the speed of gravitational waves. It is generally assumed they travel at the speed of light, but we have not tested or proven this theory.

[–]zgott300 1 point2 points ago

One thing to remember, The total gravity from that tiny spec of sky where the star exploded doesn't really change after the explosion. All the mass is still there. It's not like the star's gravity suddenly disappears.

[–]AbrahamVanHelsing 0 points1 point ago

Also, gravity propagates at c, not instantaneously.

[–]KaneHauComputing|Astronomy|Cosmology|Volcanoes 0 points1 point ago

'effects' of gravity would be felt instantly

It is theorized (though not conclusively proven) that gravity moves at c (speed of light).

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