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Photons do not disintegrate when there is no interaction with other particles, and will continue to carry their initial momentum and information to travel through space forever. Telescopes captures ancient light emitted from far-away galaxies, and the human eye (retina) captures lesser-ancient light reflected from solar planets/moons.

By extension, can we also say that all photons reflected off of us (from the time we are born to the time we are laid to rest) that finds its way to pass through the Earth's dense atmosphere and enter the space will be packets of information that travel forever? Could this mean, hypothetically, that a super-advanced civilization could have their telescopes directed towards different localized points on Earth (angles differing by a billionth of a degree from their reference point) and that different members of their species could be witnessing different human lives at the same time?

This sounds spooky, but by this logic, can we also conclude that we are immortals in some sense, for as long as these photons continue to travel through space, they will carry raw data (wavelength, direction, intensity etc.) of us, for as long as the universe ceases to exist or the decay happens (whichever is earlier)?

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    $\begingroup$ Have you estimated the resolution of those images? $\endgroup$
    – WillO
    Commented Feb 19, 2023 at 17:57
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    $\begingroup$ Information does not convey immortality. The pictures of our great grandfathers and great grandmothers confirm my statement. $\endgroup$
    – anna v
    Commented Feb 19, 2023 at 18:36
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    $\begingroup$ Wasn't this a Star Trek episode, some 900 ly from Earth, so the other-life-form saw Renaissance Earth? $\endgroup$
    – JEB
    Commented Feb 19, 2023 at 19:59
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    $\begingroup$ "can we also conclude that we are immortals in some sense" - this is probably a subjective philosophic/metaphoric/semantic question more than a physics one. $\endgroup$
    – NotThatGuy
    Commented Feb 20, 2023 at 11:13
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    $\begingroup$ a single photon or even as many as might likely be received on said distanct telescope won't produce any kind of meaningful picture. $\endgroup$
    – Michael
    Commented Feb 20, 2023 at 17:04

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In principle, yes. The light reflected from us travels out into the universe and most of it is not intercepted on a timescale of many billions of years.

About 20-30% of visible light is probably absorbed or scattered in the atmosphere, depending on your altitude and aerosol contamination. As it gets much further from the Earth, some of the light will interact with the interstellar medium (ISM) and therefore some of the energy emitted will be used to excite atoms and molecules or even ionise some atoms. This will be the fate of almost all the light which is emitted in the direction of the plane of our Galaxy, which contains sufficient molecular gas and dust to block starlight travelling through it for any distance. We know this happens because we can "see" dark clouds in the Milky Way, that can be penetrated by longer wavelength radiation to reveal all the billions of Sun-like stars that lie behind them. Roughly speaking, about half the visible light from the Earth will be absorbed every 1000 light years when travelling in directions within $\pm 5$ degrees of the Galactic plane, so it is essentially all absorbed within a few thousand light years.

But most of the light is not travelling in the direction of the Galactic plane, and interstellar and intergalactic space has a very low density of gas and dust. The equivalent extinction number for the intergalactic medium is that light travels many billions of light years with almost no chance of being absorbed (see Zu et al. 2010).

This means that most of the light reflected from you will travel to cosmological distances (billions of light years) over the course of the next billions of years.

That this will happen is demonstrated by the fact that we can observe galaxies that are billions of light years away.

As the light travels towards cosmological distances, its wavelength is "stretched" by the expansion of the universe, becoming redder and redder. We know this happens because distant galaxies have redshifted spectra. If the universe keeps expanding, then its matter density will continue to decrease and there is little to stop your reflected radiation travelling on forever, with a wavelength that scales as the scale factor, $a$, of the universe.

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    $\begingroup$ @Bhanu Goyal If I look back at you from Alpha Centauri, I'll get zillions of photons from the Sun, plus a lot from the city that you live in. So picking out your photons from the others would be impracticable. If I'm looking from the Andromeda Galaxy the problem is worse. $\endgroup$ Commented Feb 20, 2023 at 7:50
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    $\begingroup$ On the other hand, a photon is just the interaction between two other particles, and from the photon's own point of view, it takes no time at all. The notion of "photons in transit" or "travelling" is a bit of a relativistic illusion. $\endgroup$
    – Deipatrous
    Commented Feb 20, 2023 at 8:21
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Just to help give you the sense of an answer,

  • sure, hypothetically, if we had a "sci-fi" telescope, we could clearly observe a human on the moon. Hypothetically, if we had a "sci-fi" telescope, we could clearly observe a human on a planet 100,000 ly away. And hypothetically, if we had a "sci-fi" telescope, we could clearly observe a human on planets in distant galaxies.

  • to do so would require a telescope comparable in size to the universe (!)

  • but yes, you're 100% correct that our "images" go on forever. Yes.

Hence,

Are we thus immortals in some sense?

Simply, imagine I was 10 feet away from you, and I had a "sci-fi" device to absolutely record you.

Then, I have a sci-fi device to make a perfect "print out" of you.

Is it you?

Whatever, nobody cares, it's a philosophical question.

If you feel that a perfect copy of you, recreated from your information, perhaps in the far future, means you are immortal, then - sure, the answer is yes.

If you feel that someone "seeing you, in detail", billions of years from now, means you are immortal, then - sure, the answer is yes.

The answer is simple. Yes, hypothetically it could be that in the far future, aliens re-create ("duplicate") you, billions of years after your death, from some sort of record they gathered in some way or another (using a telescope the size of the universe). All this happens in the plot of hundreds of sci-fi movies. So, that's that.

"Immortality" is just word, you can define it however you want.

If you feel that that (incredibly hypothetical) situation means "you are immortal" - that's your feeling on the word "immortal".

Another thought is: you will in fact die. [Assuming life extension medicine is not invented incredibly quickly the next few years.] And be aware that the aliens you envisage perfectly recording you in a billion years, will in fact say: "that person was mortal, and died. That person is dead.". You literally are not immortal as a factual matter, i.e. you are, factually, mortal. And depressingly, the all-seeing aliens would very much know this.

Again, the key point is that philosophical questions are meaningless and stupid, more specifically, they're just word games.

You probably know the quote from American comedian Woody Allen, "Some people want to achieve immortality by having their films and books remembered forever ... I want to achieve immortality by not dying."

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  • $\begingroup$ I think Frank J Tipler had a take on this recreation business. $\endgroup$ Commented Feb 21, 2023 at 20:29

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