# Detect Passage of Massive Relativistic Object

I came across an interesting article that speculates that we could detect an object of significant mass ("space-ship size") moving through our galactic neighborhood at a substantial fraction of the speed of light with existing technology.

That got me wondering (for a sci-fi novel I'm writing) whether there would be evidence of such passage near the point in space where it occurred years or decades after such passage far from any star.

The CMB interactions mentioned in the article would spread from the point of passage at the speed of light so would have long departed the area. Is it likely that some artifact of passage would still be present, such as a change to the interstellar medium (change to ionization rates, fusion of lighter elements, etc)? How quickly would any such changes disperse? For purposes of the question, assume that scientific observations are possible at the point of suspected passage.

Probably not.

I believe that your question can be stated "Can we detect a lingering effect on the local environment in interstellar space a decade or two after the passage of a relativistic starship?"

The first step is to ask just how relativistic? If it's just "really fast" (say, 0.99c) there would be one set if effects, but if it was much fast -- 0.99999c, for instance -- some new effects would come into play.

The second is to ask about the density of gas in the vicinity. If it's like normal interstellar space (typically a million atoms per cubic meter) its passage will be much harder to detect than if we're in something like a Giant Molecular Cloud where densities can be 100 to a million times greater.

First off, you won't see any lingering electromagnetic radiation: That will necessarily be light years off by a decade later and you specified local effects. That leaves two things that I can see: (a) Effects on the pre-existing gas, and (b) bits of ship that remain behind.

The interstellar medium is nearly entirely atomic hydrogen, either neutral or ionized. The temperature is typically 100-1000 K. The mean free path of these atoms is very large: 105 miles up to a million times that. The mean speed of the atoms is about 1 kps.

That means that any effect the passage of the ship has on the local interstellar medium will be entirely erased by the diffusion of the interstellar medium (IM) in a short time. Imagine that the ship blows a tube of vacuum in the IM a million miles across. Neutral hydrogen atoms from outside of this region (just a few mean free paths away) will move in at 1 kps. In a few million seconds, the void created by the ship's passage will be refilled. (A million seconds is about 10 days, so a single year is plenty of time for the IM to diffuse back to equilibrium.)

If the IM is much denser, the effect may linger as long as a decade, but we're not talking about the IM anywhere near us.

The second potential source is bits of the ship left behind. Hitting the IM at .99c is precisely the same as the IM hitting the ship at .99c, and that's quite hard radiation -- the IM's protons are hitting at 5 GeV. Unless the ship has force shields of some sort, this is likely to cause some spallation -- atoms of the ship's front end eroded into space. This might be detectable, but the problem I see is that those bits are unlikely to be dumped into space with zero transverse velocity and will thus quickly leave the local area and be undetectable a decade later.

The ship it may dump enough energy into the IM to heat it -- creating a long tube of hotter than normal IM along its path. The problem -- again -- is that the mean free path is so long that the hot atoms would immediately decamp for distant places and the colder atoms outside the ships's range would diffuse in, erasing the tracks.

If the speed were insanely high (as if .99c wasn't insane enough), possibly you might see some sort of effect where a bow wave of plasma builds up big enough to clear a really wide tube along the path, long enough that it can't fill in in a decade, or enough energy is dumped into the IM that it's still excited a decade later. The problem is that that energy -- which is large -- is coming from the ship's kinetic energy through friction with the IM. (a) This slows the ship and (b) it probably does very bad things to the ship's structure.

The last effect is that we might see some side effect of the ship's drive or shields. But since they're basically magic (Clarke's Third Law) I don't see how anyone can give you a scientific answer.

• Thank you for the wonderfully detailed breakdown. I'm trying to avoid invoking magic. I have a background in physics (but not astronomy or astrophysics), so am driven toward scientific accuracy. Your comments made me consider the Bussard ramjet concept which can potentially affect the interstellar medium in a radius of thousands of kilometers and also would eject (hot, fast-moving) helium. I suppose your calculation that even a hole a million miles across would be filled means the scoop won't matter. What about the helium, ejected opposite the motion, with little transverse velocity? Jul 27, 2018 at 19:16
• It's very hard to eject the He with small transverse velocity. The transverse velocity will be slow compared with the exhaust velocity, but that is so high that the He atoms are hot and move away at many kps. The exhaust would zip quickly away from the ship's path. (The kicker is the mean free path -- the vacuum of space is so high,and the mean free path so long, and so out of our experience that our intuition of how a gas ought to behave just gets it wrong.) Jul 27, 2018 at 19:46
• One more thought... what if the space ship passed through the heliosphere of a star system? From my understanding, there is a turbulent zone with significantly increased pressure as well as large magnetic bubbles. Might this environment delay the diffusion of fusion biproducts long enough for them to stay at detectable levels longer than in the original scenario? Thank you again for sharing your thoughts. Jul 28, 2018 at 3:32

Detect Passage of Massive Relativistic Object

... we could detect a massive object moving through our galactic neighborhood at a substantial fraction of the speed of light with existing technology.

... wondering ... whether there would be evidence of such passage years or decades after such passage far from any star.

Yes.

1. A large object passes through a galaxy over 10 light years away.

2. We begin observation of that portion of the sky, 9 years 11 months later (or sooner).

3. The following month we start to notice a change in the orbits of objects in that area.

If the massive object was a star we would see it as though it's travel through that system was occuring in real time (our time, of course all times and their frames are real; this is a simplified explanation) even though what we're seeing occurred over a decade ago.

If the massive object was a black hole we would not see it but we would see the movement of other objects in an unexpected manner, and the lensing of light.

The effects of foreground galaxy cluster mass on background galaxy shapes. The upper left panel shows (projected onto the plane of the sky) the shapes of cluster members (in yellow) and background galaxies (in white), ignoring the effects of weak lensing. The lower right panel shows this same scenario, but includes the effects of lensing. The middle panel shows a 3-d representation of the positions of cluster and source galaxies, relative to the observer. Note that the background galaxies appear stretched tangentially around the cluster.

In the case of very large masses well positioned to deflect the background the effect is more pronounced.

Click to animate

A remote light source passing behind a gravitational lens. There is a large point mass in the center acting as a lens. The aqua circle is how we would see the light source if there was no lens, while the white spots/circle is the light source as seen through the lens. If the light source is collinear with the earth and lens, the image is an "Einstein ring". When the source is off this line we see a double image. As it moves far away, one of the images gets fainter while the other one is almost not affected by the lens any more (thus coinciding with cyan circle).

• Thank you for your thoughts. However, the question is focused on whether it's possible to detect that passage close to where it occurred, not at the distance that light has traveled from the disturbance by the time we begin observation. I'll update the question to make that clearer. Jul 21, 2018 at 0:30
• @EricJ. - Such a tiny object would have immeasurably small effect on other objects in space that secondary effects would be minimal; direct observation would be better. Decades later you wouldn't be able to go to space and expect to follow an exhaust trail or a trail of 'disturbed space' and see where they went. It's your fiction novel, invent a device that does what you want. Thanks for visiting. I hope part of the answer helps with part of your question.
– Rob
Jul 21, 2018 at 0:47
• I'm trying to avoid devices (technological or plot) that break the laws of physics. I do have a background in physics (probably what drives me to be accurate) but little experience in astronomy or astrophysics. The discussion thus far is indeed helpful, even if I haven't yet solved that part of the novel in a satisfying way :-) Jul 27, 2018 at 18:59