PDA

View Full Version : "Seeing" the speed of light

Fendercaster
2009-May-06, 04:36 AM
Let's suppose that Mars is 200,000,000 miles from earth. It would take light approximately 22.4 minutes for a beam of light on earth to reach Mars. A person standing on the surface of Mars has a camera that could film at an extreme number of frames per second. If a powerful beam of light aimed at Mars was switched on on Earth, would the camera capture the light approaching Mars 22.4 minutes later, appearing similar to an approaching car?

01101001
2009-May-06, 04:48 AM
would the camera capture the light approaching Mars 22.4 minutes later, appearing similar to an approaching car?

You see a car by the light emanating and particularly reflecting from it.

You can't do that with a light beam. The light arrives when it arrives.

robross
2009-May-06, 04:50 AM
No, because the car analogy is not really a good description of this situation. Cars travel much slower than the speed of light, whereas the light from the headlights travel to you at the speed of light. That's why you see the light from the headlights before you see the car, since to see the car it has to be close enough for photons near you to bounce off them and back to your eye.

In the case of the beam of light from earth to mars, the first time you would see anything is when the leading edge of the beam arrived 22 minutes after it was switched on. One moment you would see nothing, the next moment you would see the beam.

Rob

TRUTHisnotfacts
2009-May-06, 05:04 AM
having a bright light would help get it there more fast . Just like when its dark outside the brighter light goes out more .

Brighter is also stronger light . so having a Brightlight would help in my personal view .

robross
2009-May-06, 05:17 AM
having a bright light would help get it there more fast . Just like when its dark outside the brighter light goes out more .

Brighter is also stronger light . so having a Brightlight would help in my personal view .

The speed of light is *constant* through the same medium. There's no way a brighter light would travel faster than a dimmer light. All that a brighter light would do is look brighter when it arrived. The speed of light has known to be constant since at least 1849, so this is really something a modern person living in the 21st century should understand.

especially:
"Light is a type of electromagnetic radiation. According to electromagnetic theory, all electromagnetic radiation travels in free space at the speed of light in free space: 299,792,458 m/s. This applies regardless of the light's color, brightness, or direction. Experimentally, no variations have been found in the speed of light in any realizable approximation to free space. Laboratory measurements show that light of different colors travels at the same speed to within one part in 10^14."

Fendercaster
2009-May-06, 05:40 AM
Posted by robross:

No, because the car analogy is not really a good description of this situation. Cars travel much slower than the speed of light, whereas the light from the headlights travel to you at the speed of light. That's why you see the light from the headlights before you see the car, since to see the car it has to be close enough for photons near you to bounce off them and back to your eye.

Yes, I knew this wasn't a good analogy. Maybe comparing the approaching light to an illuminated bullet would be more accurate. Either way, I was wondering if you would see the light coming toward you if you slowed down the film enough. You have answered my question in the last part of your post.
Thank you.

Jens
2009-May-06, 07:07 AM
I think a more interesting question along the same lines would be, suppose that person A turned on a flashlight in a cloud, which disperses some of the light. Now suppose person B, who is standing some distance away from A, but not in the line of the light, could, with a very fast camera, see the beam of light gradually reflecting deeper and deeper into the cloud. I don't know if my explanation is very good. I think the answer is yes, but the camera would have to be insanely fast.

astromark
2009-May-06, 07:33 AM
In my minds eye I can 'see' what Jens is talking of. Building a camera that is that fast...?
My analogy is no better if not simple.. When from that observatory in California they fire that pulsed laser at the Apollo 17 site on Earth's Moon. They know and expect that in 2.4 seconds a readable reflection will, and does arrive. Standing on the moon with equipment able to see such would not measure that light pulse until 1.2 seconds after it was sent. Playing with the words we use does not distract from the fact that we do see the light arrive. Trouble being until that moment we do not know of it.

slang
2009-May-06, 07:41 AM
Yes, I knew this wasn't a good analogy. Maybe comparing the approaching light to an illuminated bullet would be more accurate. Either way, I was wondering if you would see the light coming toward you if you slowed down the film enough. You have answered my question in the last part of your post.

How do you understand the answer then? What do you think will be on the frames of the film? The comparison with a bullet has me a bit worried :)

astromark
2009-May-06, 09:05 AM
Utter darkness until the cones and rods of your eye are excited by arriving photons...
those frames are black until that moment when all is visible. You can not see the light till it reaches you...

cjameshuff
2009-May-06, 06:00 PM
Yes, I knew this wasn't a good analogy. Maybe comparing the approaching light to an illuminated bullet would be more accurate. Either way, I was wondering if you would see the light coming toward you if you slowed down the film enough. You have answered my question in the last part of your post.

Light travels at the speed...of light. What you see when you see a light source is the light from that source that has already reached your eye. There is no way for any light scattered from or somehow affected by that approaching light to reach your camera ahead of it. What the camera pointed at the light source sees is always the light source turning on, with no sign of anything until that light reaches the camera, no matter how long it takes it to do so.

I think a more interesting question along the same lines would be, suppose that person A turned on a flashlight in a cloud, which disperses some of the light. Now suppose person B, who is standing some distance away from A, but not in the line of the light, could, with a very fast camera, see the beam of light gradually reflecting deeper and deeper into the cloud. I don't know if my explanation is very good. I think the answer is yes, but the camera would have to be insanely fast.

If distances are large enough or the camera fast enough, the echoes of individual pulses of light can be seen. http://en.wikipedia.org/wiki/Light_echo

This is also how lidar works. And it does require a very fast camera...and a very sensitive one, since even with an intense pulse, the return received by a given pixel may be measured in individual photons. Somewhere I've seen pictures of a light pulse propagating through a room... (or more likely, pictures of several pulses propagating through the room, taken at different delays)

a1call
2009-May-06, 07:34 PM
Perhaps some of the posts are trying to say the following and perhaps not. It's not clear to me.
If you shine a laser at an angle of 89°59'59.9993", after 1 second the perpendicular to the line of sight component of the beam will only move 1 meters. In other words you should see a low speed progression of 1 meter per second to the right, of the leading point of the beam given there are reflective materials such as clouds or smoke all the way to the leading point and the beam does not attenuate below perceivable levels.
ETA: Of course one meter at a distance of 300000 km is just a dot but the point of visibly slow progression is valid as a concept I think at some smaller angles.

rommel543
2009-May-06, 08:25 PM
With the light beam coming from Mars directly at you, you're like a Helen Keller standing on a railway track, not knowing the train is coming until it hits you. Regardless of light echoes, or light spread from material reflection, the beam is not going to be seen by you or the extremely fast camera until 22.4 minutes after it leaves. One frame you are looking at the planet the next frame has the planet with a little dot where the light emitted from.

I think a better analogy would be Astromark's shining a light from California to the moon. If I was either in Florida, or in a geosynchronous orbit above Florida with the same high speed camera and the beam happened to shine through some ice crystals in the upper atmosphere, would I see the beam making its way through the ice crystals.
I know the light that the camera would be capturing is actually beyond the point that it would be seeing (kind of like the sun isn't actually where we see it in the sky, we're seeing it's location 8 minutes ago). I am wondering if it would look light the light echoes we can see from novas, only occurring on a smaller scale.

a1call
2009-May-06, 10:21 PM
Ok, Here is another try:

*- Mr. X shines a powerful laser from a point on Earth's equator parallel to the North-South axis and northwardly.
*- We assume (for the sake of argument) that the space around earth is filled with smoke for a radius of 300000 km+
*- Mr. Y is located on the same meridian as Mr. X and at a latitude of 60 degrees and on the opposite side of the Earth.
*- Then Mr. Y will see the beam from the laser take just under 1 second to trace a 58 degrees arc in the sky, which is a visibly/noticeably slow progression

ETA: Realized after posting I had not counted the delay in arrival of the reflected light so the 58 degrees trace should be more like in 2 seconds. :doh:

2009-May-07, 01:27 AM
If someone types in all caps, does someone else on the other side of the internet get the message sooner?

raptorthang
2009-May-07, 01:55 AM
One can't take a photo or see 'approaching' light. The light is 'here' when you see it. It isn't 'over there'. Any light you see has already reached your eye. It doesn't matter what route it takes. The brain then interprets the pattern of photons into images that exist within our mind and not 'out there'. When you sit before your computer screen you are seeing light that has reached your eye from the screen and translated that info into 'thoughts'...you are processing light 'from' the screen and not the screen itself.

Middenrat
2009-May-07, 02:15 AM
a1call's most recent post above astounds and excites me!
How can his experiment be realised? Such would be an amazing demonstration worthy of gratitude from all scientific educationalists.

Steve Ballard
2009-May-07, 09:20 AM
I see the original question analogous to Luke Skywalkers light saber.

When he turns on the light saber, you see the light slowly cascade
out from the handle to the end of the saber. Whooooshhhhh.

If it was pure light, you would not, nor could not visualize NOR photograph
the light cascading out from the handle to the end of the saber. It would
appear instantaneously. All at once, from the handle to end of the saber.
(Supposedly nothing travels faster than the speed of light. Even a camera
shutter.)

Obviously the light saber uses a medium much like a neon sign uses a gas
to ionize the light into a (medium) of different substance than pure light.

When the light saber is switched on, the light, or glowing energy follows the
path of this medium out from the handle to the end of the saber. Traveling
slower than the actual speed of light. You are not seeing the actual light,
you are seeing the medium reacting to the light.

You can then photograph or see that if the camera is fast enough.

Now. My question is this.

A car is traveling at the speed of light. And the driver switches on his headlights.
Will the light from the headlights go faster than the speed of light?

robross
2009-May-07, 10:23 AM
I see the original question analogous to Luke Skywalkers light saber.

When he turns on the light saber, you see the light slowly cascade
out from the handle to the end of the saber. Whooooshhhhh.

If it was pure light, you would not, nor could not visualize NOR photograph
the light cascading out from the handle to the end of the saber. It would
appear instantaneously. All at once, from the handle to end of the saber.
(Supposedly nothing travels faster than the speed of light. Even a camera
shutter.)

Well, light is fast, but not instantaneous. Let's assume for sake of argument your light-saber blade was 1 meter in length. Then the light would take 3335.6 picoseconds to move from the base of the blade to the tip.

The human eye can't perceive motion that fast, so you would indeed see it as instantaneously appearing. I don't know what kind of a camera you could use to take pictures of the light as it traveled up the blade, but that would have to be a pretty darn fast camera!!

Now. My question is this.

A car is traveling at the speed of light. And the driver switches on his headlights.
Will the light from the headlights go faster than the speed of light?

Of course not! Light always travels at the speed of light. You would see the light from your headlights travel away from you at the speed of light. Some other cool weird things is that to an outside observer, you would not be experiencing time (infinite time dilation), i.e., your watch would not be ticking, and also, the length of your car would be 0 feet in the direction of motion (length contraction) AND you would have infinite mass. Luckily, since you can't travel at the speed of light, you could never actually experience any of these strange phenomena.

Rob

Steve Ballard
2009-May-07, 11:25 AM
I feel like I have infinite mass all ready. (Where's my jogging shoes?)

So we are concluding that if you had a camera that could take photographs
fast enough you could indeed image the speed of light. In theory of course.

Argos
2009-May-07, 12:16 PM
Seeing the speed of light... Itīs interesting that the whole Relativity thing started when Einstein [according to himself] began one of his 'gedanken experiments' on what would happen [what would he see] if he tried to catch up with a ray of light.