# Starlight: are the rays that we see here on Earth parallel?

Are the rays of light emitted from stars that are perceived by an eye here on Earth parallel?

Do they travel in a straight line from the star to the eye, never crossing, and equidistant?

Do they wiggle around and cross as they move through space?

Do the rays meet at a point on the retina of the eye?

Assuming that a star is pretty far away, it seems reasonable to think that they've traveled zillions of miles without being parallel.

• I'll mention quickly that while ray optics is an extremely useful approximation because it is simple and handy, when pushed to its limit it breaks down. It might be better to think of light from a star as a series of photons and each of those as a wave that completely fills the aperture of a telescope or the pupil of an eye. There are indeed things that can slightly distort the wavefront in space but once it reaches our atmosphere it gets distorted much more – uhoh Jan 14 at 19:26
• That distortion is perceived as astronomical seeing or "twinkling" because it is constantly changing. – uhoh Jan 14 at 19:27
• Whether or not rays from a distant point meet at the same point on your retina depends on how well your eyes are able to focus. I can tell you for a fact, that they do not meet at a single point on my retinas unless I view the star through some corrective optics. – Solomon Slow Jan 15 at 18:29
• Mathematically parallel? No. Do the trigonometry. Assume that spacetime is flat (but, see answers below for more on that subject) and assume an isocoles triangle whose base is the distance between your eyes, (or the distance between your eye and my eye—depends on what, exactly, you are asking) and whose height is the distance to the star. The angle between the two sides will be really, freakingly, incredibly close to zero; but it will not actually be zero. – Solomon Slow Jan 15 at 18:35

Light rays travel along things called geodesics in spacetime (specifically they travel along a certain class of geodesics called 'null geodesics').

If spacetime is flat, which will be the case in the absence of gravity, then those geodesics are just straight lines as we normally think of them, and so yes, light rays travel along straight lines in that case.

But there is gravity and spacetime is therefore not flat. This means there there is really no definition of 'straight line' any more: geodesics are the best thing we have (that's why they're called geodesics rather than straight lines).

But in very many cases the deviation caused by spacetime not being flat is pretty small: for instance it's possible to see the difference in apparent position of stars if the light from them passes close to the Sun, but it's a small deviation.

In some cases – if the light passes close to very massive objects, the deviation can be very large (and in fact if the object is massive enough, it can be as large as you like).

The general effect of light travelling along geodesics which are not well-approximated by straight lines is called 'gravitational lensing' and it's now a well-confirmed thing which is used in practice to detect things like exoplanets (amazingly, in this case it's the lensing of the light from stars by the planet which is observed, which is just astonishing.)

So a brief answer is yes, almost always, but occasionally not at all!

Light will travel in a straight line through the 4D Space-time continuum. It does not wiggle around and is only deflected by gravity. Light from distant stars reaches us more or less parallel due to the great distance involved.

But depending on how accurately you wish to make the measurement, there is scope for some light from that star to reach us very slightly off parallel. However the difference would be extremely small indeed and might not be measurable so this is more of a theoretical prediction. The angle would be depend on the diameter of the star and its distance from us.

So to be clear photons of light from a distant star will all arrive arrive more or less parallel. But some of them may be off parallel by a miniscule amount.

• So you don't think that the rays are distorted as they move through space and time? I think they get bent all over the place. A one-year ray might be many miles off-target for Earth, until it gets bent by some force. Kinda like a sniper bullet gets moved around by the wind and heat rising, until it hits its target. – kmiklas Jan 14 at 18:41
• Light travels in a straight line through space-time, but space-time is itself distorted by gravity so light appears to bend around massive objects. So it depends what you mean by a straight line. – Slarty Jan 14 at 19:09
• "It depends what you mean by a straight line" XD Love that quote!!! – kmiklas Jan 14 at 19:11
• Note even if there were no gravitational effects light from a distant star might still not be parallel due to the fact that light may originate from either side of the star on converge on the observer. – Slarty Jan 14 at 19:11
• What do you mean by a straight line? Are you talking about a 3D or 4D straight line? – Slarty Jan 14 at 19:12

The light rays will certainly be very close to being parallel because the star is so far away and because gravitational distortions are so small in our neighborhood.
First, rays from opposite edges of the star will arrive at angles that differ by the diameter of the star divided by the distance to the star (~10^-8 radians). Second, the effect of gravitational lensing can cause the wave function corresponding even to a single photon to be split. For example, the Einstein Cross has an apparent size of 10^-5 radians. Gravitational lensing (exactly as for conventional lenses) can even cause light-rays to cross. This is true even for the wave-function for a single photon originating on distant star or galaxy billions of light-years away.

• I've been pondering this question since it was first posted in Space SE, and my feeling has been that rays initially parallel would never cross, at least parallel rays that begin 6 mm apart would never cross before reaching a somebody's pupil. But now I'm not so sure about even that. – uhoh Jan 16 at 11:26