Recent comments in /f/space

Pegajace t1_jcnwgbu wrote

Reply to Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

>would these photons traveling ad infinitum define the edge of the universe

They would if the Big Bang had been an explosion at a specific point in space, but it wasn’t. There isn’t an expanding sphere of photons defining the outer edge of the universe because the universe did not start at a central point. The Big Bang was a rapid growth of spacetime that happened to space, not in space, and it happened everywhere simultaneously.

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mysteryofthefieryeye OP t1_jcnucsf wrote

Reply to comment by Civil_Willingness298 in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

I know light doesn't experience time. From the photon's vantage, you are right. But in physical space, it still takes time for the photon to reach a destination. Downvote me all you want but your original comment is plainly wrong.

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jthtiger t1_jcnttul wrote

Reply to comment by ZylonBane in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

Position isn't the right word probably. Velocity is more accurate yes, but it's the velocity of the object that emits that cause the wavelength to be stretched.

My point was that the wavelength does not continue to stretch over time. So a photon won't redshift into nothing-ness.

The velocity of the photons does not change over time and therefore will not drift apart.

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ZylonBane t1_jcnt1c7 wrote

Reply to comment by jthtiger in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

Redshift has nothing to do with position. Redshift is the photon equivalent of the Doppler effect. Just as sound sources that are rapidly receding sound lower-pitched due to their waveforms being stretched out, light from sources that are rapidly receding appear shifted toward red in the electromagnetic spectrum. So velocity is what matters.

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jthtiger t1_jcnq4bb wrote

Reply to comment by FMLAdad in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

Redshift is (from my understanding) a single moment, not continuous. Light travels at a constant rate, so the wavelength is not CONTINUALLY expanding. If it did, then one of the wave fronts would have to be travelling at a different speed. The redshift is only caused by the difference in position of the object that emitted them from when two waves were emitted.

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mysteryofthefieryeye OP t1_jcnpe2o wrote

Reply to comment by Civil_Willingness298 in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

I'm sorry, that's incorrect, and no noodle has been cooked. From the point of view of the photon, sure, you are right. But the physical packet of information still travels at the speed of light, and even then that speed can be altered by an intervening atmosphere or interstellar medium. By your logic, the starlight I see outside is both here and just leaving the star simultaneously, which is 100% inaccurate.

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Civil_Willingness298 t1_jcnoq2r wrote

Reply to Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

Photons do not experience space or time. It reaches its destination 13 billion light years away or two inches away at the same exact moment it departs. Let that cook your noodle for a minute or two.

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OffusMax t1_jcnle8d wrote

Reply to comment by FMLAdad in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

Redshift is caused by the motion of the object emitting the light and the fax that light behaves like a wave.

Consider the following example. Imagine a train sitting still on the tracks. The sound waves emitted by the engine propagate away from it as expanding, concentric spheres. There is no motion so an observer hears the sound at their natural frequency.

Then the train starts to move. At each interval, it emits a new spherical wave that has moved from the position where the last sphere was emitted. That means that the distance between the sphere just emitted and the previously emitted sphere in front of the train is closer than the distance between them in the back of the train. When the observer hears the sound, they hear a higher pitched sound as the train approaches them (blue shifted) and a lower pitched sound as it recedes from them (red shifted).

The same thing happens with the light emitted from stars in a galaxy. The color of the light changes because it’s light and not sound. But it’s all caused by the way we perceive light (or sound for the train) and not anything intrinsic about the universe.

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DudeWithAnAxeToGrind t1_jcnjzmm wrote

Reply to Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

Let assume that photon is emitted from Earth, just so we can have convenient frame of reference. The same would be true if it was emitted from anywhere else.

Assuming nothing ever absorbs it, the photon would just keep going forever. However, even if the Universe itself is not infinite, and has an edge, it will never get to that edge. Not even close. I.e. as far as your question goes, the difference between infinite and finite Universe is irrelevant. The final fate of that photon and how far it can get is the same.

As it travels, its frequency will get lower and lower because the space it is traveling through is expanding; it will redshift more and more until its energy becomes so low to be undetectable.

It will only be able to reach a region of space that is finite distance from where it started its journey. This furthest point in space that it can reach is within our currently observable Universe. However, by the time that photon reaches it (after it was traveling for infinite amount of time), that point in space will be far outside of our future observable Universe. Again because space is expanding, and the space beyond observable Universe is moving away from us much faster than the speed of light, it can never get to those regions of space (it actually can't even reach the edge of currently observable Universe, because that region of space is also already moving away from us faster than the speed of light).

This also means that there is a sphere around the Earth from which the photons emitted right now towards us, Earth's current location in space is the furthest they will ever be able to get to. By the time they get here after travelling for trillions of years, they'll be so much red shifted as to be undetectable.

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triffid_hunter t1_jcnj657 wrote

Reply to comment by jmarkmorris in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

> what is the lowest frequency longest wavelength photon that is observable by state of the art equipment?

We're still receiving photons from the first moment that the universe was transparent, they're called the CMB

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triffid_hunter t1_jcnj4f4 wrote

Reply to Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

> Where do photons go if they've been emitted but are destined to never be absorbed

They just keep going forever.

We're still receiving photons from the first moment that the universe was transparent, they're called the CMB

> would these photons traveling ad infinitum define the edge of the universe

There's no edge.

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