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A lot of the answers here are for the question below but the previous question has not been received well. So with the same question in mind, wouldn't the universe look different from different parts of the universe because from earth the furthest star we can see is Cassiopeia and it is 16,308 years ahead of what we see and then Alpha Centari is only 8 minutes and 19s ahead of what we see. But if I changed my location in the universe, would all the stars be different distances and thus be in different time frames, thus changing the picture as you fly through space?

As you approached lights, you would go forward in that lights time, as you got further from light, you would go forward in that lights time slower.

So when we take a picture of the universe, isn't it really just from our perspective?

OLD QUESTION >>>>>

Are you sure light isn't instant? The more I think about how we see things the more I realize that light lag would really change how we see things, so are we sure light isn't instant.

Light never has lag time, neither does electricity technically either. Are there properties they have in common when it comes to transfer?

Light disperses as it travels, that is why things are harder to see as they get further. Since light behaves that way, I will bring up the double slit test that demonstrates light behaves differently when observed then when not observed.

Light that was not observed exhibited a pattern that resembled a wave where as the light when observed was a beam as you would expect.

Light has at least two interesting properties in its nature, the third seems to be that it is instant.

So if light has a speed, why does everything appear without lag?

When I play with my laser and the clouds at night, the entire beam moves as one unit from the base to the tip on the clouds and never reveals any misalignment, no matter how fast I swing it back and forth.

d w
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    See e.g. http://web.media.mit.edu/~raskar/trillionfps/ for a physical setup that uses ps laser pulses to show how light "really" illuminates objects. – CuriousOne Oct 27 '15 at 08:41
  • You might want to read over this Wikipedia article on the speed of light. – Kyle Kanos Oct 27 '15 at 13:03
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    "Why does everything appear without lag"? Human vision is not a simple sensing of what is out there, and the eye doesn't work like a camera, no matter what the simplified text books for early school grades tell you. Most of what we see is constructed in the brain, quite independently from the speed of light. In reality, there are lots of lags in our seeing, due to saccades, the speed of nerve impulses, etc. They are just edited out before the result of vision becomes conscious. Summary: Don't assume that human perception is an accurate instrument for observing physical phenomena. – rumtscho Oct 27 '15 at 16:48
  • So my brain automatically corrects for light's delay by showing things in perfect concert in real time. Know how you know it is real time? When a bunch of people see something, no matter their distance, they all see it at the same time. If you can prove that isn't true, please do. – d w Oct 27 '15 at 17:00
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    You can measure the speed of light in your kitchen. Also see How has the speed of light been measured? – RedGrittyBrick Oct 27 '15 at 17:05
  • @RedGrittyBrick Wow, if that isn't like counting rings on a tree to find its age, then I don't know what is. How it has been measured is odd since we can detect units of time as small as 1 Planck. They say light travels 1.616199(97)×10−35 meters in that amount of time. So that means we can measure it physically. – d w Oct 27 '15 at 17:10
  • @rumtscho If the human eye isn't an accurate perception, then can we ever have an accurate perception? How do we know that anyway? – d w Oct 27 '15 at 17:13
  • @dw many instruments that we have are more "accurate" for certain definitions of being accurate, for example a camera can record light rays more accurately than the eye. But ultimately, everything an instrument senses has to be translated to some output consistent with human perception/knowledge structures, and there is indeed no guarantee that they can represent reality in an exact way - indeed, there are many examples where such a representation is impossible. This is not physics though, it's philosophy. For the purposes of physics, instruments are more accurate than eyes. – rumtscho Oct 27 '15 at 17:18
  • @rumtscho "a camera can record light rays more accurately than the eye" Please source this, thank you. – d w Oct 27 '15 at 17:27
  • @dw I don't have a source for such a basic claim. It is just a side effect of how cameras work and how human vision works. If you don't know these basics, take any introductory text on the two topics and compare them. http://www.amazon.de/Camera-Technology-Dark-Side-Lens/dp/0122875702/ seems to be a decent one for cameras, http://www.amazon.de/Wahrnehmungspsychologie-Grundkurs-Karl-Gegenfurtner/dp/3642550738/ is standard for perception if you read German, but I'm sure there are equivalent textbooks in English. – rumtscho Oct 27 '15 at 19:35
  • @dw I get a feeling that you are questioning whether we can know anything, if we cannot perceive it in the most direct way with our senses. If this is your basic concern (which is millenia old), this place is not for you. In that case, you need to study the philosophy of science. Once you share the basic theory of science, we will be very happy to help you understand scientific results. If you don't share the basics, there is nothing we can say to make you see differently and this place is not for you. – Martin Oct 27 '15 at 20:16
  • Is this philosophy? Gravity is energy. Force is the result of energy acting on mass. Mass is a weight defined by gravity.

    F=ma has an energy input which is "a" and an energy output "F"

    Mass is the medium used to calculate the energy based on the force of gravity.

    So energy in from gravity is expressed as constant acceleration. The product of energy and mass combined is what gives mass weight. The energy stored as weight can be transferred to another form of energy by necessary means. But gravity appears to be able to put energy into mass.

     – d w Oct 27 '15 at 20:38
  • @Martin I would go so far as to say that all molecules get its mass from energy, on earth primarily from within the field of gravity. Mass is the result of the energy interacting with the molecules. – d w Oct 27 '15 at 22:25
  • @Martin Gravity acts equally on all of some part of the molecule, which creates the effect of density. – d w Oct 27 '15 at 22:59
  • @Martin Do you think mass at rest has a multiplier of 143 N (essentially a multiplier of 1 "G") when defining its momentum upon collision from another body? Would it in a zero gravity environment? In a zero gravity environment, I think energy from impact would determine the acceleration and velocity of the mass impacted with zero resistance, like things couldn't really "crash" because the energy transfer would have no resistance. – d w Oct 28 '15 at 01:09
  • @Martin Is mass at rest a multiplier of 143 N (essentially a multiplier of 1G)when defining its momentum upon collision from another body? Would it have inertia in a zero gravity environment? In a zero gravity environment, I think energy from impact would determine the acceleration and velocity of the mass impacted with zero resistance, like things couldn't really "crash" because the object would have no momentum/inertia. Inertia is massxvelocity, assuming that things in gravity have a constant 143 N pull downward, the mass can be pushed in any direction and have the same inertial strength. – d w Oct 28 '15 at 01:17
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    @dw: Sorry, but I have no idea what you are talking about. I'm not trying to be mean, seriously, your statement are just too vague to make sense. What I do seem to grasp sharply contradicts either the general theory of relativity or the standard model - and those are the best theories we have. I understand that you try to make sense of some things you see around you, but so have thousands of people for centuries and you can really benefit from reading what they have to say! – Martin Oct 28 '15 at 11:26
  • For instance: "Mass is weight defined by gravity". No. Mass is an property that you could (in Newtonian terms) define as "resistance against acceleration" and it is this mass that appears in Newton's law. Even without gravity, you'd have mass. Then there is a second mass that seems to be affected by gravity. In the general theory of relativity, the basic postulate is that those two are the same. – Martin Oct 28 '15 at 11:29
  • I'm not going to explain this any further. The main problem here is that you are handing out your own theories - and stackexchange is not the place for this. It is just not a place to have a discussion, but it is a question/answer site based on the mainstream physical theory. We have told you how, within this mainstream theory, your question can be answered. We can also clarify unclear points, but not more. – Martin Oct 28 '15 at 11:32
  • why are shadows from space objects crossing stars instant? citations needed. I seem to recall the Transit of Venus taking several hours. – Kyle Kanos Oct 28 '15 at 19:22
  • @KyleKanos When you say transit of Venus, they are referring to the length of time it took for Venus to cross the entire sun from our point of view. I am referring to the fact that we can predict when it will start based on physics, and the shadow doesn't have a time delay when the planet gets in the way. On another note on that link you posted. How can we see the shadow when light disperses, covering up small breaks in light over distance? – d w Oct 28 '15 at 19:32
  • And that's why a citation is needed: Venus didn't "cross the star" in an instant, it took a few hours. – Kyle Kanos Oct 28 '15 at 19:34
  • @KyleKanos I didn't meant the moment went by in an instant, i meant the moment it started, we could see a shadow, no delay. Are we seeing planets in the past and shadows from the past? How can that be verified? – d w Oct 28 '15 at 19:36
  • we could see a shadow, no delay again, citation needed. We are indeed seeing things in the past when we look out in space. – Kyle Kanos Oct 28 '15 at 19:49
  • Related: http://physics.stackexchange.com/q/48328/2451 and links therein. – Qmechanic Oct 28 '15 at 19:56

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Light DOES have lag time, and it affects how we see things. It's just so fast that humans don't notice without careful experiment.

One of the most interesting results of this is that when we look into the night sky, we are seeing stars and galaxies as they were years ago. The farther back we look, the farther back in time we are seeing. The farthest objects we could possibly see are being seen as they were 13.8 billion years ago, at the beginning of the universe. We use things from this long ago to learn about the birth of our universe. One of the most important things we've seen from the early universe is called the Cosmic Microwave Background; it's really interesting and I encourage you to look more into it!

Electricity also has a speed. That speed is the speed of a wave of electrons in a metal. It depends on the material that the electricity travels through, and it is certainly slower than the speed of light. But it is typically faster than humans can see, which is why you haven't noticed it yet.

doublefelix
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  • The question is more like, why is everything always clear and accurate. You say the stars are from a distant past, but that is still assuming light has speed. I am suggesting it is instant therefore assuming I am wrong is just assuming. And electricity as far as reaction time, it is instant. – d w Oct 27 '15 at 07:03
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    @dw nope, electricity ain't instant. Lay down a copper wire across the Atlantic with a on-off switch in London and a light bulb in New York and close the switch. The light bulb will light up only some milliseconds later, not instantly. – LLlAMnYP Oct 27 '15 at 08:47
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    @dw Actually, it is not just assumed that light has a specific speed, it is experimentally proven. It is not just a thought someone had, it is actually something that can be measured. – Steeven Oct 27 '15 at 09:50
  • That is the claim, but I would like to see the math for light delay and how it effects how we see things mathematically. If a plane was flying at night in perfect dark and then a spot light was shone on it, at that exact second, when the light hits the plane, we see the plane which indicates there is zero delay, not even a small delay. I would be interested in the math on how this is accounted for. – d w Oct 27 '15 at 14:48
  • @LLIAMnYP Your claim about the delay of electricity, please provide the math for determining the delay. Thanks in advance. – d w Oct 27 '15 at 14:49
  • @dw Math is not needed to prove the assumption wrong, if experiments can show it wrong. If you set up two very precise optical measuring devices a distance apart and shine lighte past them both, then they will show different times of receiving the light input. One such result is then a counter-example against the assumption of light being instant. – Steeven Oct 30 '15 at 10:57
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Light lag also affects the way objects appear to a recording camera if their velocity relative to the camera is a significant fraction of the speed of light.

This is known as the Terrell-Penrose-Lampa effect (an Austrian by the name of A.Lampa discovered it in the 1910's, then R.Penrose and J.Terrell rediscovered it independently in 1958-59).

It basically means that, due to the finite speed of light, light rays emitted by a moving object at the same time in the object's frame reach the camera at different moments in the camera frame. And conversely: light rays recorded on camera at the same moment left the surface of the object at different times.

A major consequence of this effect is that the length contraction of a sphere moving at relativistic velocities is optically invisible to a distant camera, although it can be attested by other measurements. In other words, we see distant spherical stars that move at relativistic velocities relative to us still as spherical stars, not oblate as we would expect from length contraction alone. The fun part is, if there weren't any length contraction, the stars would appear elongated/distorted in the direction of motion.

Here are some useful starting references on this effect:

  1. Wikipedia page on the "Terrell Rotation"
  2. U. Frankfurt page on "Images of objects moving at relativistic speed and the Lampa-Terrell-Penrose effect"
  3. U.British Columbia site on the "Terrell Rotation"
udrv
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  • You said, "It basically means that, due to the finite speed of light, light rays emitted by a moving object at the same time in the object's frame reach the camera at different moments in the camera frame. And conversely: light rays recorded on camera at the same moment left the surface of the object at different times." Are you telling me that you can tell if light is instant or not by that? So if light has speed, it is still a constant flow, the way instant light would be. – d w Oct 27 '15 at 07:17
  • Judging from what you just asked here and after the other answer, I think you use a different notion of "instant light" and "light lag" then the people on this site. Can you clarify what exactly you understand by "instant light" and "lag"? For example, is it "lag" as in "lag due to finite speed", "lag due to finite time of interaction on the retina", or something else? – udrv Oct 27 '15 at 07:37
  • I'll put it this way, the sound coming from everywhere is actually effected by speed, sound is not in real time. Light is in real time, how come? – d w Oct 27 '15 at 07:41
  • Light is in real time only if emitted right next to your eye or your recording sensor, etc. Otherwise it propagates at finite speed, just like sound, only the speed is about a million times faster compared to the speed of sound in air. You see a light flash from lightning before you hear thunder, but that doesn't mean the light flash was instantly in your eyes, no? – udrv Oct 27 '15 at 07:50
  • When a sniper takes a shot, light lag is not considered on a moving target. I know that you say it wouldn't be noticed, but something like that should be noticed by someone eventually. No one has ever noticed it, but that is what it is huh? I'll pass on that explanation. – d w Oct 27 '15 at 07:52
  • If your sniper needs to shoot an object in the vicinity of the Sun's corona, he WOULD have to take into account light lag, oh yeah! About 8 minutes of it. This is how long it takes light to propagate from the Sun to Earth. So light lag was observed and accounted for alright, just not used the sniper way so far. – udrv Oct 27 '15 at 08:03
  • How far would the sniper have to be to need to take into account 1/10th of a second? – d w Oct 27 '15 at 15:32
  • Light rays would have to travel the distance from target to sniper and they do so at a speed about $c=186,000$mi/s or $c=300,000$ km/s, so the distance would be $d = c\Delta t = 18,600 \text{mi} = 30,000 $km. This is about 2.5 Earth diameters or 1/10th the distance to the Moon! Plus the sniper needs to account for projectile travel time. But there are other instances where light lag is routinely taken into account: communications with space & Moon probes, or the Mars rovers. For Mars there is a 20 minute lag that scientists have to work with constantly when they steer the Curiosity rover. – udrv Oct 27 '15 at 19:22
  • So the answer you are giving me is that as things get further away, they get more distorted, but at such a small amount nobody can tell. Well, light can measured in Planck time in theory so we should be able to do a field test. Why isn't the light just tested by aiming a beam at a mirror some distance away and using a sensitive method to determine exact travel distance in real time? Since light behaves differently when observed and not observed, you have to test it both ways too. observed and not observed! – d w Oct 27 '15 at 20:25
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    " as things get further away, they get more distorted": 1) Things get distorted only if they move relative to the camera. The faster they move, the larger the distortion. The sphere is just the special case that does not get distorted. Anything else does. 2) The propagation lag does get longer the farther things are, no matter if they move relative to the camera or stay still far away. As for "Why isn't the light just tested...": It has been, numerous times. It is called the time-of-flight measurement of light speed. See //en.wikipedia.org/wiki/Speed_of_light. – udrv Oct 28 '15 at 02:20
  • Perhaps I should have been more clear. I mean testing regular light, here on earth, observed and not observed. – d w Oct 28 '15 at 02:31
  • See https://en.wikipedia.org/wiki/Speed_of_light#Time_of_flight_techniques. Done in the 1850-60s by Foucault and Fizeau (France), then 1870s-1930s refined many times over by Michelson in the California Valley. The latter used mile-long vacuumed tube setup with mirrors. A variety of other experiments and methods since. – udrv Oct 28 '15 at 03:22
  • I just spent my time reading through them to double check, but as I said, it appears nobody has tested the speed of light the way I suggested on that site. I said observed and unobserved, on earth only, with light we can measure start and stop time from; the sun doesn't do that. – d w Oct 28 '15 at 13:53
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    None of the setups I referred to use sunlight, they use their own controllable light sources that are started and stopped at will, and their light beams travel in closed vacuum tubes here on Earth. If you want to insist that this doesn't fit your requirements, you can do so of course, but won't find much of an audience for your claims. For good reason. – udrv Oct 28 '15 at 18:18
  • I'll check the setup again. – d w Oct 28 '15 at 18:23
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You state in a comment:

If a plane was flying at night in perfect dark and then a spot light was shone on it, at that exact second, when the light hits the plane, we see the plane which indicates there is zero delay, not even a small delay.

No, it doesn't. It only indicates that the delay is less than what we can perceive. Let's do the math:

Assume that the plane is 50km away (this would be very much indeed). If I had the light bulb and shone it at the plane, the light has to travel 100km to the plane and back to my eye. Now, lights velocity is known to be 299792.458 km/s, which means that it takes the light exactly 1/2997.92458 s to do the distance. That's about 1/3000 or 1/3 of a millisecond. The typical response time for your retina (i.e. the time between when the eye starts seeing something and when your brain says "it's there") is >40 ms which is two orders of magnitude larger than the delay of the light. There is no way you could observe the effect on such a small distance.

In fact, when you say that you see the light "instantly" once you shine you light bulb at the plane, this is false information by your brain: Even if light were instant, your brain would still take >40 ms to process the information - it just deceives you by correcting for this error...

Now, if you really want to see the speed of light, shine a laser at the mirror left on the moon by one of the Appollo missions. The delay time will be two seconds, which is easily perceptible. Unfortunately, you won't see the response with your eye, though, because the focus of the laser will be too small however high the power of your laser is.

Martin
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  • "It only indicates that the delay is less than what we can perceive" Before you start, let me say this, we can measure time so small we can supposedly measure light moving 1.616199(97)×10−35 meters in 1 Planck time. So because we can measure, we should perceive. – d w Oct 27 '15 at 17:05
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    Sorry, but this is wrong. We don't measure exclusively with our eyes or ears, we measure with technical devices and then read off the results. How this is possible and why we can trust the answer is the content of philosophy of science. But as I say, point a laser to the moon, watch it return (sadly not with your eye since the relative intensity is too small) two seconds later. It's doable, you just need a clear sky and a good laser. – Martin Oct 27 '15 at 20:10
  • I like how light, something extremely visible, is so hard to study visually. lol – d w Oct 27 '15 at 21:24