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Light travel.

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#1 bgrass2

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Posted 25 February 2021 - 10:46 AM

Light travels 5,878.5 billion miles per year, roughly so anyone beyond that will see us in the past. At some point the view would show dinosaurs if possible. Same as us looking for little green men! Moderators, move is necessary. 



#2 Kid_A

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Posted 25 February 2021 - 11:32 AM

Yeah! You're looking at everything in the past, even things right in front of you, since light needs some time to travel from them to you. The moon is 1.3 light seconds away, so when you observe it, you're seeing it as it was 1.3 seconds ago. Pretty trippy, even when it's just seconds. 


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#3 TOMDEY

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Posted 25 February 2021 - 11:36 AM

Professional athletes also take this into account.    Tom

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#4 rocketsteve

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Posted 25 February 2021 - 04:12 PM

In our solar system, light travel times are "fairly" short...

 

Light Travel Times to Earth.jpg

 

 

 

Simply put, the farther the object is from the viewer, the farther into the past the viewer is looking.


Edited by rocketsteve, 25 February 2021 - 04:14 PM.

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#5 ButterFly

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Posted 02 March 2021 - 07:36 AM

One foot per nanosecond is a surprisingly close approximation.  This is the absolute limit for all electronic communication.  It sounds tiny, but those feet add up quickly, especially for high frequency traders.  Wall street moved uptown simply because it's closer to the main internet cable into Manhattan.



#6 therealdmt

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Posted 02 March 2021 - 07:40 AM

In our solar system, light travel times are "fairly" short...

 

attachicon.gifLight Travel Times to Earth.jpg

 

 

 

Simply put, the farther the object is from the viewer, the farther into the past the viewer is looking.

Without checking the rest, off the top of my head that figure looks low for Mars. And checking up on the numbers for Mars now, I see NASA says average communication delay to Perseverance is 5 to 20 minutes: https://mars.nasa.go...communications/

 

I post the following at risk of embarrassing myself, but here goes:

 

Looking up average distance to Mars, I see it listed as 225 million kilometers ( https://www.space.co...earth-mars.html ). Meanwhile, the speed of light in a vacuum is 299,792 km/s.
So, 225,000,000 km/299,792 km/s = 750.52 seconds

750.52/60= 12.51, or about 12 and a half minutes

 

A quick common sense check puts that in roughly between NASA’s communications time to Perserverance, slightly towards the longer side. In contrast, your chart shows a time quicker than any time NASA says it achieves.

 

I think what that chart is doing is taking the average distance from the sun of each planet and then taking the difference between those numbers and the Eath’s average distance and then using those differences as the average distances between Earth and those planets. However, that;s the average distance between the planets’ orbits, not the distance between their positions along those orbits.

 

Ex., Mars is an average of about 1.5 AU from the Sun, while the Earth is by definition 1 AU from the sun on average. So, it would seem that the average distance between Earth and Mars is approximately 0.5 AU. Since Earth is about 8 light minutes from the Sun, that would put average communications time between Earth and Mars (supposedly at 0.5 AU) at about 4 minutes - the same result as presented in your table. However, in reality, 4 minutes is an almost best case scenario, not the average time.

 

 

For reference, Universe Today has the communications time as between 3 and 21 minutes (with a pretty good explanation): https://www.universe...-earth-to-mars/

For a more detailed explanation of the average distances to the planets, there’s this: https://physicstoday...20190312a/full/



#7 godelescher

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Posted 02 March 2021 - 07:51 AM

Without checking the rest, off the top of my head that figure looks low for Mars. And checking up on the numbers for Mars now, I see NASA says average communication delay to Perseverance is 5 to 20 minutes: https://mars.nasa.go...communications/

<...snip...>

This is correct. "Average" distance would be roughly 12 minutes. The same discrepancy is probably true for the other planets, though most drastically different with Mars


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#8 therealdmt

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Posted 02 March 2021 - 08:16 AM

Anyway, I fully agree that it is mind blowing to think that distant mountain top you might look at is actually how it appeared some time before. Even looking at your loved ones next to you, or the words on the computer screen in front of you, leaves you looking ever so slightly into the past, as things were, not as they are.

 

Someone else here on the forum recently told a story related to looking at the Andromeda Galaxy — how with the enormous disc, over 200,000 light years in diameter, being tilted towards us, we see both a near and a far side, with the farther side being much farther away in time than the nearer. 
 

It’s quite a concept 


Edited by therealdmt, 02 March 2021 - 08:37 AM.

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#9 RalphMeisterTigerMan

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Posted 03 March 2021 - 12:56 AM

I watched a documentary of exactly that, light. Since the speed of light is not constant, really cool things can be done. This physicist designed an experiment where she was able to slow light down to a point where she actually captured a photon "completely stopped". She was actually able to take a picture of a "photon", a single photon! 

 

But being the kind, ethical scientist that she is, freed the photon and allowed it to go on it's merry way. LOL!

 

Can you imagine, an actual picture of a single photon! Wow!

 

Clear skies!

RalphMeisterTigerMan



#10 gwd

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Posted 03 March 2021 - 06:04 AM

Your nerve impulses travel much more slowly than the speed of light.   So when you watch a heavy object hit your toe, the pain signal reaches your brain after the image of the hammer hit.  The time difference should be perceptible, but isn't because your mind constructs simultaneous events from the different neural pathways.    It is funny that when I see my neighbor hammering something I do hear the sound after I see the strike.  

 

To get back on topic I've read that one way to estimate the speed of light is to accurately time the movements of Jupiter's moons.   How difficult would that be for an amateur astronomer?  If an amateur could do it, it seems like there should be an article describing it in an old issue of Sky & Telescope.  


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#11 therealdmt

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Posted 03 March 2021 - 01:44 PM

To get back on topic I've read that one way to estimate the speed of light is to accurately time the movements of Jupiter's moons.   How difficult would that be for an amateur astronomer?  If an amateur could do it, it seems like there should be an article describing it in an old issue of Sky & Telescope.  

The timing of a moon of Jupiter’s transits (occultations?) in relation to Jupiter itself was one of the two first ways the speed of light was measured. The calculated result wasn’t very accurate, but confirmation of the concept that light had a speed at all was as big a breakthrough as the reasonable stab at measuring it.

The basic idea is that as Earth and Jupiter orbit the Sun, the distance between Earth and Jupiter is changing over the months, from nearer to farther and eventually back again, and this was causing differences (from expected) in when the transits were observed. The guy recording all this (Roemer - had to look it up!) made the mental leap that the inconsistent times for the moon in question were due to the changing distances involved between Earth and what was being observed and that light was taking time to cover those distances.

The basic idea that if one knows the change in distance involved and the timing difference, you can calculate the speed of light seems simple enough, but when I start thinking about the practicalities involved.... One issue would be how far back to "scratch" to start from



#12 Keith Rivich

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Posted 03 March 2021 - 07:25 PM

Light travels 5,878.5 billion miles per year, roughly so anyone beyond that will see us in the past. At some point the view would show dinosaurs if possible. Same as us looking for little green men! Moderators, move is necessary. 

Since we cannot catch up to those dino light waves we have no hope of seeing them moving around. Now, Imagine an alien astronomer 65,000,000 lights years away with a ginormous scope that could resolve those dino's eating each other, what a sight!

 

What would be weird is to set up a bunch of mirrors that could reflect your image enough times to create a light second delay...you could talk to yourself in near real time!


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#13 chrysalis

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Posted 03 March 2021 - 07:43 PM

Professional athletes also take this into account.    Tom

Mitch Hedberg: "My friend showed me a photo and said 'Here's a picture of me when I was younger'. I said, "Every picture is of you when you were younger."


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#14 Jon Isaacs

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Posted 03 March 2021 - 07:49 PM

Since we cannot catch up to those dino light waves we have no hope of seeing them moving around. Now, Imagine an alien astronomer 65,000,000 lights years away with a ginormous scope that could resolve those dino's eating each other, what a sight!

 

What would be weird is to set up a bunch of mirrors that could reflect your image enough times to create a light second delay...you could talk to yourself in near real time!

 

That would take some high reflectivity mirrors.  In our lab, we had a camera that was capable of taking 5 nanosecond exposures at the rate of 1 every 5 nanoseconds.  It could take 16 images.. 

 

It is interesting to ponder the possibilities of somehow imaging a finite length beam of light.  

 

The camera is red and is mounted on a pedestal.  The two telescopes are Meade 310s, this setup was to image shockwaves  using schlieren photography.

 

4579282-UCSD Schlieren cn .jpg
 
Jon

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#15 chrysalis

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Posted 03 March 2021 - 07:57 PM

Filming the Speed of Light at 10 Trillion FPS

 

https://www.youtube....h?v=7Ys_yKGNFRQ


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#16 TOMDEY

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Posted 03 March 2021 - 08:33 PM

Reminds me of those 1970s TV "Anticipation" commercials where the Heinz Catsup flows more slowly than the inferior brands.    Tom

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#17 Jon Isaacs

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Posted 04 March 2021 - 07:17 AM

Filming the Speed of Light at 10 Trillion FPS

 

https://www.youtube....h?v=7Ys_yKGNFRQ

Interesting. Shooting the laser through the water is nice.

 

The difference between their technique and the camera we used is that they photograph many, many laser pulses, each shifted slightly in time and then put the video together, that's why it took 8 hours to process the image.  The camera we had is a "single shot" so it takes an image of a single event in real time.  

 

Jon


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#18 bumm

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Posted 04 March 2021 - 08:33 AM

I can remember how listening to live communications during the Apollo missions live showed the time delay in the speed of light.

 

Astronauts say something.

Houston replies.

 

A few seconds

 

Astronauts say something.

Houston replies.

 

A few seconds

 

Astronauts say something.

Houston replies.

 

A few seconds...  And so it went...

                                           Marty


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#19 godelescher

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Posted 04 March 2021 - 07:31 PM

Filming the Speed of Light at 10 Trillion FPS

 

https://www.youtube....h?v=7Ys_yKGNFRQ

This is amazing



#20 JMW

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Posted 04 March 2021 - 07:39 PM

Light travel is how I like to backpack. 

 

It is cool that our scope's are time machines.




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