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Q & A: Where is the light that left Earth 70 years ago?

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Most recent answer: 11/19/2014
Q:
hi,van light travel to all direction and never stop travel. if that is right, where is 70 year ago light from my mother that is still traveling at this very moment? can i see it? if not why????? thanks for your time john um
- john um (age 70)
atlanta, georgia
A:

I think it's safe to say that some light that reflected off your mother 70 years ago is still out there, traveling through space. Space is mostly empty, and light can travel for billions of years without hitting anything. 

All the radio signals and television broadcasts we've sent into space are also still out there, traveling away from the Earth at the speed of light. Each year of traveling carries them about 5.9 trillion miles (one light-year) farther away from Earth. The first radio broadcasts left the Earth about 113 years ago, so they are now about 113 light-years away, farther than many of the nearest stars. (In comparison, the Voyager 1 space probe, the human-made object that is most distant from Earth, is only about 18 light-hours away, near the edge of our solar system.) To see light that left Earth 70 years ago, you would have to catch up to it by traveling faster than light, which is forbidden by the laws of physics as we know them.

What about an alien astronomer living on a planet 70 light-years away? She probably wouldn't be able to see it either. The amount of light reflected by a human being into space is very small, much less than a radio broadcast. More importantly, light spreads out as it travels, like ripples from a stone dropped in a lake. The intensity of light from a point source decreases with the square of the distance, so if you move twice as far from the source, you will see only one quarter of the intensity. The result is that after a few light-years, signals of this kind become extremely diffuse and would be difficult to detect against background noise. But the light is still out there.

Rebecca Holmes


(published on 11/19/2014)

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