# Speed of light and vision lag [closed]

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.

## closed as off-topic by Carl Witthoft, Kyle Kanos, ACuriousMind♦, Bill N, user36790 Oct 28 '15 at 2:38

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• See e.g. 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 8:41
• "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
• – RedGrittyBrick Oct 27 '15 at 17:05
• @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
• I'm certain I understand your question(s), but you've such a blind eye to evidence & lack a willingness to learn that no one here will be of any help to you. – Kyle Kanos Oct 28 '15 at 20:27

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.

• 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 7:03
• @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 8:47
• @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 9:50
• 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

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:

• 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 7:17
• 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 7:37
• 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 7:41
• " 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 2:20
• 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

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.

• "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
• 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