# 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?

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"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 Nov 7 '13 at 19:54
@GreenMatt Interesting. Though it would seem discussing it would be off topic for this site –  Haaakon Nov 7 '13 at 20:12
@Haaakon Exotic matter is theoretical, not hypothetical, and thus on topic. –  called2voyage Nov 7 '13 at 20:27
@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? –  Alexey Bobrick Nov 9 '13 at 20:15

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 back 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

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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? –  astromax Nov 8 '13 at 4:04
Here's more of a discussion as to why no laws were harmed in the making of your post: 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 Nov 8 '13 at 4:24
@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! –  Manishearth Nov 8 '13 at 8:54
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 Nov 8 '13 at 13:37
@astromax Ah, alright, I sort of assumed that wouldn't cause any confusion, but it's a good thing to mention. Thanks! –  Manishearth Nov 8 '13 at 14:58

There is also another mediator particle which moves at the speed of light other than the photon. This is the gluon, which is the exchange particle for the strong force. The odd thing about the gluon is that it's never seen by itself (that is, outside of collections of other gluons).

Also, though neutrinos do in fact have mass, they are neutral particles. Why I'm bringing this up is because in supernovae explosions neutrinos actually outpace the photons - they do not interact with charged particles because they are weak, and they pass through considerable amounts of mass (namely dust and gas) before an interaction might occur. What this means is if you could detect the neutrinos coming from a supernova it would give you early warning that the photons would soon follow. This would give you time to measure its light curve (see: SNEWS: The SuperNova Early Warning System).

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Good point about neutrinos! Don't forget also about gravitons, which also propagate at the speed of light with little interaction, not being particles in a common sense, though. –  Alexey Bobrick Nov 7 '13 at 20:54
For that matter, beta particles outpace photons in the cooling chambers of nuclear reactors, but of course their $\gamma$s are orders of magnitude less than that of the caffeinated neutrinos :) Also, interesting link, thanks! –  Manishearth Nov 7 '13 at 23:09
@Alexey Bobrick I didn't include gravitons, but you're correct, they would absolutely travel at the speed of light. –  astromax Nov 8 '13 at 3:50
Sorry for a little anti-advertisement here, but I would like to underline that, being nice and interesting as a comment, this reply is not the answer to the question about the fastest moving objects in the Universe => I encourage the readers to read the other posts and the writers to think and add more on the matter: about mascrosopic, preferably astrophysical, objects, which move at relativistic velocities with respect to other macroscopic objects. –  Alexey Bobrick Nov 8 '13 at 12:11
Well it depends on what he really meant by the word particle. If he meant to say macroscopic astrophysical objects then that's what he should have stated. I know he mentions comets, but he also mentions "things hurled by supernovae", which my answer is a direct response to. I agree that off topic answers must be avoided, but you also need to be careful you're not putting words in @Haaakon's mouth. –  astromax Nov 9 '13 at 14:22

There are plenty of rapidly moving objects in astrophysics.

A good place where one can get moving relativistically is near an event horizon of a black hole. A simple Newtonian estimate illustrates the point. Black hole has all its mass $M$ hidden under an event horizon of the radius of order $r_{g}=\dfrac{2GM}{c^2}$. An object moving circularly in the gravitational field of a black hole at the radius $\alpha r_{g}$, where $\alpha>1$, would have Newtonian orbital velocity $v$ equal to $v=\sqrt{\dfrac{GM}{\alpha r_g}}=\dfrac{c}{\sqrt{2\alpha}}$.

This is a quilitative estimate of the velocity scale. From general relativity there are no stable circular orbits at $\alpha<3$, but any body will have additional acceleration when inspiralling into the black hole. To add a bit of complexity, when one starts thinking in terms of general relativity, one has to wonder, what do we really mean by objects' velocity and about such kinds of questions.

Nevertheless, the above conclusion is correct: In the field of black holes intact objects can obtain relativistic velocities, which are comparable to the speed of light.

There are many physical examples of such systems: binary megring black holes, black holes merging with neutron stars, supermassive black holes and white dwarfs, etc. While all these systems are driven to eventual merger at relativistic velocities, it is hard for any of their components to get ejected and become free floating. To my knowledge, there are no known free floating relativistic astrophysical bodies, but some of them are indeed probably produced from the pieces of material ejected away at mildly relativistic speeds during mergers involving black holes.

One other rare possibility is to have a compact binary system in the field of a supermassive black hole, which is being disrupted due to interaction with it. However, the probability for such a disruption happening when the compact binary is just about to merge is vanishingly low.

Another ubiquitous class of objects are relativistic jets, which are ultrarelativistic streams of plasma, produced mainly when some accretion onto a black hole is taking place. Particles in such jets move at highly relativictic velocities, though the exact nature of jet formation is yet not completely understood. Finally, there is plenty of relativistic particles present in the background, such as cosmic ray particles and neutrinos.

One last thing to mention would be plasmas which are at relativistic temperatures (of order $10^9 K$), and which hence contain particles (mainly, electrons) moving relativistically. It is rare that plasmas get temperatures that high, but it is definitely possible during core-collapse supernova.

Finally, at sufficiently early stages of the Big Bang, absolutely everything in the Universe was moving relativistically!

Edit: A few more things which came to my mind afterwards: 1) Man-made particle beams in particle accelerators are relativistic, macroscopic, but not astrophysical objects. 2) If there exists intelligent life in the Universe, it might have also produced relativistic objects of macroscopic, but again probably not astrophysical, scale (like spacecrafts).

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