Speed of light if observer is VERY Big

  • #1
4
0
How fast would the light travel for a SuperBig observer... immagine his head is the size of the Sun, and he is sitting in a room, then he decides to turn on the light in the room... if he is so big, would he have to wait some seconds until the light reaches his eyes? Would it mean that for this massive observer the light travels much slower than for us micro observers?
 

Answers and Replies

  • #2
berkeman
Mentor
59,059
9,157
Welcome to the PF. :smile:
How fast would the light travel for a SuperBig observer... immagine his head is the size of the Sun, and he is sitting in a room, then he decides to turn on the light in the room... if he is so big, would he have to wait some seconds until the light reaches his eyes? Would it mean that for this massive observer the light travels much slower than for us micro observers?
It would still travel at c.
 
  • Like
Likes PeroK
  • #3
I assume your giant sits in breathable air. Which means the speed of light is constant everywhere in the room. Let's say his first reaction to the light being switched on is blinking his eyes. Since your giant is so big, it will take much more time for the light to each his eyes, but it will travel at the same speed for you (I assume you sit at his feet, at the size of a pico-ant). The only "visible" effect his size would have, is that he will appear sluggish to blink his eyes. My answer does not take into effect the time between the giant reaching the conclusion that he must switch on the light, and the message from his brain reaching his hand...
 
  • #4
PeterDonis
Mentor
Insights Author
2020 Award
32,961
11,444
Would it mean that for this massive observer the light travels much slower than for us micro observers?
No, it would mean that the light has to travel a longer distance.
 
  • Like
Likes Vanadium 50
  • #5
Ibix
Science Advisor
Insights Author
2020 Award
7,399
6,487
How fast would the light travel for a SuperBig observer... immagine his head is the size of the Sun, and he is sitting in a room, then he decides to turn on the light in the room... if he is so big, would he have to wait some seconds until the light reaches his eyes? Would it mean that for this massive observer the light travels much slower than for us micro observers?
Very Long Baseline Interferometry is a way to combine multiple telescopes across the world to make a single telescope, a single "eye", the size of the Earth. We've been doing it for decades. The speed of light is still c.

You haven't really described your creature in detail. If it's just a scaled up human then its brain is huge. One of the reasons we build computer chips small is to mimimise time wasted waiting around for electrical signals crossing the chip. Something similar happens in reverse happens if you scale a brain up - its processing speed must decrease. So your creature's notion of "a really short time" would be what we would call years or centuries. So even if light takes a few minutes to cross its body, it is still immeasurably fast for the creature.
 
Last edited:
  • Like
Likes Grinkle and Romanko
  • #6
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
16,231
8,239
I must admit, I just assumed that the OP's question was intended as a joke.
 
  • #7
jbriggs444
Science Advisor
Homework Helper
9,603
4,257
You haven't really described your creature in detail. If it's just a scaled up human then its brain is huge.
One difficulty with the OP's notion of a huge person is, of course, the cube-square problem. https://en.wikipedia.org/wiki/Square–cube_law. If he's running on protoplasm, this means that he has a heat problem. Not only will his head be the size of the sun, it will radiate approximately as much energy as the sun. He does not need to turn on the light switch.
 
  • Like
Likes Imager, Ibix and PeroK
  • #8
Ibix
Science Advisor
Insights Author
2020 Award
7,399
6,487
One difficulty with the OP's notion of a huge person is, of course, the cube-square problem. https://en.wikipedia.org/wiki/Square–cube_law. If he's running on protoplasm, this means that he has a heat problem. Not only will his head be the size of the sun, it will radiate approximately as much energy as the sun. He does not need to turn on the light switch.
Also it'd have to have incredible material properties to be anything other than a sphere due to its own gravity.

I was interpreting the question as being a variant on "why is the speed of light so large". It's large in SI units because our convenient timescale, a second, is so much larger than our convenient distance unit, a meter. If you could retain our typical timescale while increasing the typical length scale then the value of c would come down (just rescaling the units). I was just pointing out that typical length scale and typical time scale are related quantities because of the physics. As has been discussed many times here, you need to tweak the fine structure constant to achieve what people probably mean by "a lower value of c".
 
  • Like
Likes jbriggs444
  • #9
Mister T
Science Advisor
Gold Member
2,608
856
... if he is so big, would he have to wait some seconds until the light reaches his eyes?
The math is pretty simple. Light travels a distance of about 300 000 km every second, so in three seconds it would travel a distance of about 900 000 km.

Would it mean that for this massive observer the light travels much slower than for us micro observers?
No. It makes no difference what lies at the two points separated by a distance of 900 000 km. Planets in a solar system or opposite ends of your observer.
 
  • #10
3
1
As has been discussed many times here, you need to tweak the fine structure constant to achieve what people probably mean by "a lower value of c".
That does not make any sense IMHO. c is the universal speed limit, i.e. 1. Changing it only changes the definition of the unit of length (or time, depending what you consider more fundamental), it does not change any physics. It is just a unit conversion factor. As such, it is not directly related(*) to the fine structure constant, which is related to electromagnetism only. Hence what you'll change when tweaking the fine structure constant is not c, but the elementary (electrical) charge.

There is only one way to make light travel with a speed that is less than the fundamental speed limit: You need to give it mass. Unfortunately that'll change the physics of light such that it'll be hardly recognisable anymore.

If people ask "What if light would travel more slowly?" they are trying to imagine seeing the finiteness of the speed of light with their own eyes. Basically the latter would require their eyes and brains to function at superluminal speed.

(*) formulas for the fine structure constant contain c only because we like to measure space and time in different units!
 
  • Like
Likes nikkkom

Related Threads on Speed of light if observer is VERY Big

  • Last Post
Replies
18
Views
2K
  • Last Post
Replies
5
Views
2K
  • Last Post
6
Replies
139
Views
13K
Replies
60
Views
10K
Replies
34
Views
3K
Replies
57
Views
2K
Replies
25
Views
2K
Replies
7
Views
2K
Replies
5
Views
854
Replies
42
Views
6K
Top