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Author Topic: What would happen to light without gravity  (Read 3867 times)

Offline jeffreyH

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What would happen to light without gravity
« on: 24/12/2014 04:21:54 »
Would light be deadly without gravity to change its wavelength?


 

Online evan_au

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Re: What would happen to light without gravity
« Reply #1 on: 24/12/2014 09:23:33 »
  • If someone shines a 1 Watt torch horizontally at you, gravity does not change the average wavelength, and you don't die.
  • If someone shines a 1 Megawatt continuous laser horizontally at you, gravity does not change the average wavelength, and you could well die.
  • Repeating the experiment in intergalactic space would have similar results.
  • So the presence or absence of gravity doesn't matter as much as the power being absorbed by your body
  • ...and some parts of your body are more vulnerable than other parts. 

Perhaps you are asking about the effect of the Sun's gravity on the Sun's radiation?
  • The Einstein shift from the Sun's gravity is very slight, and does not change the Sun's spectrum in any way that a human could detect.
  • Even if the Sun's radiation was red-shifted or blue-shifted by 10%, the atmosphere would still filter out the X-Rays and most of the Ultraviolet; our eyes would quickly adapt to the change in colour temperature, leaving no visible difference in the scene around us.

What would matter is if the power of solar radiation increased or decreased by 10%, the Earth's black-body temperature would also change by about 10%, ie by about 27C, which would make some places much less habitable, and some other places much more habitable. It would certainly be lethal to a large fraction of the world's population.
 

Offline lightarrow

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Re: What would happen to light without gravity
« Reply #2 on: 24/12/2014 11:27:49 »
I think the OP question is not so silly, evan_au, because the average number of light sources in the universe increases as r3 (r is the distance from us) while their intensities decrease as r2 only. There is also the fact that light's very far sources has not yet arrived to us, so there is a limit to the increase in radiation power arriving to us in absence of redshift, however the computation is not very trivial.

--
lightarrow
 

Offline jeffreyH

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Re: What would happen to light without gravity
« Reply #3 on: 24/12/2014 13:29:45 »
The question was overstated. However, this scenario, while being unrealistic is also critical in understanding the forces involved. If we model the universe as if certain forces were absent we may gain insights that otherwise would be hidden from view.
 

Online evan_au

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Re: What would happen to light without gravity
« Reply #4 on: 24/12/2014 20:44:25 »
Ah... so the question may have been asking about Olber's Paradox?

In this case, it is the expansion of the universe which red-shifts the light from distant galaxies, and makes the Earth habitable.

The Cosmological red-shift has a larger impact on light from distant galaxies than does the Einstein (gravitational) red-shift of those same galaxies (or their stars).
 

Online evan_au

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Re: What would happen to light without gravity
« Reply #5 on: 24/12/2014 20:54:44 »
Quote
number of light sources in the universe increases as r3 (r is the distance from us) while their intensities decrease as r2 only.

The r3 number assumes a uniform distribution of matter, and ignores the "clumpiness" of matter in space.

However, when viewed as a fractal dust, the Sloan Digital Sky Survey observed that the number of galactic clusters only increases as ≈r2.

Olber's paradox is still important, because the total effect of a large number of shells of stars, each with the same brightness still adds up to an uncomfortably bright night-time sky.
 

Offline PmbPhy

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Re: What would happen to light without gravity
« Reply #6 on: 24/12/2014 21:25:15 »
Quote from: jeffreyH
Would light be deadly without gravity to change its wavelength?
No. The change in wavelength is imperceptible. I.e. the change in energy is so small as to not be noticeable or to make a difference.
 

Offline jeffreyH

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Re: What would happen to light without gravity
« Reply #7 on: 24/12/2014 22:46:28 »
Ah... so the question may have been asking about Olber's Paradox?

In this case, it is the expansion of the universe which red-shifts the light from distant galaxies, and makes the Earth habitable.

The Cosmological red-shift has a larger impact on light from distant galaxies than does the Einstein (gravitational) red-shift of those same galaxies (or their stars).

Olber's paradox is resolved if the observable universe contained within the Hubble sphere acts like a white hole and precludes the light from outside that sphere from entering our region of spacetime. There could still be an infinite number of stars out there but we will never see them unless we cross the horizon of the Hubble sphere.
 

Offline jeffreyH

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Re: What would happen to light without gravity
« Reply #8 on: 24/12/2014 22:55:47 »
Quote from: jeffreyH
Would light be deadly without gravity to change its wavelength?
No. The change in wavelength is imperceptible. I.e. the change in energy is so small as to not be noticeable or to make a difference.

Thanks Pete.
 

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Re: What would happen to light without gravity
« Reply #8 on: 24/12/2014 22:55:47 »

 

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