### What's the fastest moving object in the universe?

• We know that nothing can have proper velocities larger than the speed of light in vacuum. But are there any objects in space that get close to it? Any comets, or other objects thrown by gravity or supernova explosions that were hurled to incredible speeds?

"We know nothing can travel faster than light." Do we? I've read in more than one place that Einstein's relativity prohibits accelerating to the speed of light and there are theories which permit faster than light travel provided the material never slows below the speed of light. Of course such material would be rather exotic...

@GreenMatt Interesting. Though it would seem discussing it would be off topic for this site

@Haaakon Exotic matter is theoretical, not hypothetical, and thus on topic.

@Haakon, could you please make your question specific: do you mean large enough macroscopic object, astrophysical bodies, or any types of objects in your question?

Small point to add, but the fastest observed particle with mass might be this one: https://en.wikipedia.org/wiki/Oh-My-God_particle 0.999 999 999 999 999 999 999 9951c - much faster than anything accelerated at CERN. Neutrinos also travel close to the speed of light and they have tiny mass. Not sure how close to c they get.

Mundane, Milky-way-ocentric answer: Fastest Star in the Galaxy Has a Strange Origin : "*The runaway star, US 708, is traveling at 745 miles per second (1200 km/s) — that's 26 million miles per hour (43 million km/h) —making it the fastest star in the Milky Way ever clocked by astronomers, according to the new research. Its speed will allow it to escape the gravitational pull of the galaxy, and eventually make its way into intergalactic space.*" http://www.space.com/28737-fastest-star-galaxy-strange-origin.html

9 years ago

The answer to this is surprising:

We are.

And many (if not all) other galaxies.

And they move faster than light.

See, the universe is expanding, at an accelerating rate. The fabric of spacetime itself stretches out, so that galaxies seem to move away from each other. The interesting thing is that relativity does not forbid these from moving away faster than light. While local space is flat and the local speed of light must be upheld, this need not hold at a global scale, so it is possible to have frames which move away from each other faster than $c$. Indeed, there are some galaxies that are moving away from us faster than light (the only reason we see them is that they used to be closer and moving at a slower speed). Any pair of galaxies that are 4200 Mpc away from each other (that is, with a redshift of 1.4), are moving away from each other faster than light in each other's frames (numbers stolen from the linked page).

Since the only consistent way to talk about motion is relative, one can say that we are moving away from other galaxies faster than light, since the reverse is also true. This can put galaxies in the bucket of the fastest moving objects in the universe. As for which is the fastest, I don't know, we would have to find a pair of galaxies which are the farthest apart (distance measured in the frame of the galaxy, of course), but since the universe is probably more than what we observe1, we can't pinpoint the pair of galaxies for which this is true.

For those who think that it is cheating2 to short-circuit the question with space expansion, there are other objects that go faster than light (they are not the fastest objects in the universe though), and these can be found on good 'ol Earth.

Electrons:

In nuclear reactor cooling pools3, we have a phenomenon known as Cerenkov radiation. Basically, emitted beta particles move faster than the speed of light in water. This creates an effect of similar origin as the sonic boom, where strong light emanates from the medium.

Saywhat? You think I'm cheating again2 by putting everything relative to the speed of light in a medium?

Alright, fine. Here are some fast objects that don't require space expansion to be fast, nor do they involve any trickery of semantics where the medium in which they are being measured is not mentioned. Many have already been mentioned by astromax.

• Tachyons:These are particles which go faster than $c$ — this does not violate relativity as long as they never decelerate to subluminal speeds. However, there isn't much (any?) experimental evidence for these. A lot of BSM models do predict their existence, though. So there's still some cheating here, on to bradyonic matter:

• Gluons: These are massless, and though they don't occur freely (except possibly in glueballs, though these most probably have mass) they do travel at $c$. But these can't move at any other speed, so again, this is slightly cheating. On to fermionic matter:

• Neutrinos: Now these are viable candidates. The electron neutrino is known to have very, very little mass (we have an upper bound for it, which gives ), and as a result it can easily attain very high speeds. Put it in a gravitational field, and it goes even faster. If you want macroscopic objects, however:

• Stuff spiraling around spinning black holes: Black holes have a strong gravitational field, and when rotating, they can impart angular momentum (lots of it) to nearby objects like accretion disks. Objects close to a black hole are accelerated to pretty high speeds. In fact, if an object is within the ergosphere, it moves faster than light from the point of view of certain frames of reference.

• Stuff falling into black holes: From the faraway frame, an object speeds up and approaches the speed of light as it approaches the horizon of a black hole. Arbitrarily large speeds bounded by $c$ can be attained here.

• Black hole plasma jets: Jets being flung out of black holes can get pretty fast relative to each other.

1. Due to cosmic expansion, there can be galaxies that are no longer visible to us. Some galaxies may never have been visible to us, if we start watching from when galaxies started forming.

2. I, for one, agree with you.

3. And other places where you have massive particles being emitted really fast into a medium

Nice post. A couple of things, though: 1) While galaxies may travel faster than the speed of light with respect to one another, you may want to make the distinction that this is a global (not a local) statement, 2) The actual electron neutrino (and all neutrino flavors for that matter) is not known. We may have limits on these masses, and structure in the universe would certainly look much different if they were much heavier than what people believe them to be, but that disclaimer should be present, 3) lastly, what do you mean when you say that neutrinos travel faster in a gravitational field?

Here's more of a discussion as to why no laws were harmed in the making of your post: http://curious.astro.cornell.edu/question.php?number=56. In summary, the reason why we can see things that are globally moving away from us faster than the speed of light is because they weren't always. While it's true that the rate of expansion between two distant points in the universe are moving faster than c (hence making them causally disconnected from the point at which they did so), that wasn't always the case in the past. We're simply seeing the lag - our cosmic monitors haven't refreshed yet.

@astromax Thanks for the input and the edit! (1) I _think_ I know what you mean, but could you elaborate? (2) Yes, but the upper bound is pretty tiny for $\nu_e$, so we get a rather high lower bound for avg speed. I'll edit that in though. (3) Oh, I just meant that neutrinos can be accelerated further in a gravitational field, making them even faster. (4) I mentioned that in the post ("the only reason..."), but the link is appreciated!

Well - as the previous link points out, inertial reference frames go out the window when you talk about acceleration. Locally, everybody is an inertial reference frame (and locally space is always flat - this is one of the important things about GR), so the local speed of light is always obeyed. Globally, all bets are off.

@astromax Ah, alright, I sort of assumed that wouldn't cause any confusion, but it's a good thing to mention. Thanks!

Well, sorry to say, but your post, being fun in itself, doesn't provide an answer to the question (yet?). The question is about astrophysical objects moving relativistically wrt other local object. It is not about particles, light propagation in medium or cosmological expansion. Just to avoid misleading the reader.

@AlexeyBobrick I personally count particles as objects, but I see what you're getting at. The "in a medium" bit was just a sideshow, really. In that case plasma jets are probably likely answer as mentioned in your answer (I'll add it in). Not sure if we can consider jets to be an "object" though

@Manishearth: Particles are microscopic objects, that's the difference. Jets are macroscopic, the question is also about macroscopic things.