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Comment by twic | Hacker News Reader

> The research in Japan used a beam of Helium-8, Helium with 4 extra neutrons, colliding with a regular Helium-4 atom. The collision breaks up the Helium-8 into another Helium-4 and a tetraneutron in its brief resonance state, before it, too, breaks apart, forming four lone neutrons.

I don't know how fast these particles were going in the experiment, but if they were going at the speed of light, then the tetraneutron would make it 150 femtometres from the site of the collision before decaying. By way of comparison, neutrons and protons are 1 - 2 fm across, and a uranium nucleus is 15 fm across [i]. So if the tetraneutron was actually going at a tenth of the speed of light, it would barely make it a nucleus's diameter away before exploding.

[i] https://en.wikipedia.org/wiki/Atomic_nucleus

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That's not how relativistic speeds work. If this was going near enough to the speed of light, it could travel almost any distance before decaying. Time dilation would be observed of the particle, where it would appear that time is ticking slower for the particle compared to our own frame of reference.

Actually what the parent comment said would be correct. Time dilation and length contraction do not arbitrarily change the proper distances that particles travel at near light speeds.

As observers, the detectors would see precisely what we expect if we calculate the distance using the detector's proper time when compared with the distance we experimentally measure.

Please explain why you think this is so.

This is super basic stuff https://en.wikipedia.org/wiki/Time_dilation_of_moving_partic...

The linked wiki article says that rapidly moving particles take longer to decay.

Mmm... I have the strange feeling that you both agree, let's write this with more details to avoid confusion.

The half life of a static tetraneutron is (theoreticaly) 5E-22s.

If you are at a laboratory and the tetraneutron is moving at relativistic speed, the apparent half life in the laboratory frame will increase substantially.

To calculate the distance that the tetraneutron will travel you must multiply the velocity x the apparent half life. I.E

d ~= v * 1/(1-v^2/c^2) * t_hl

where t_hl = 5E-22s = the half life of the tetraneuton

not the classic version that is

d_clasic_wrong = v * t_hl

For a more detailed explanation, with numbers and graphic you all can see the analogue experiment with muons: http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/muon.html

Anyway, IIRC a half life of 5E-22s is still very small. IIRC you can use a bubble chamber or something similar to see the trajectory of the particles. You can only see a collision and examine the debris and you will note that when the collision has some specific energy you will get an unexpected excess of collisions. You only can examine the direction and energy of the debris to make a good guess of the intermediate steps of the collision and discover that for a very short time you had a tetraneutron.

I do agree that time dilation occurs, what I disagreed with/corrected was with this: it could travel almost any distance before decaying, which I took to imply that the lengthening of particle lifetimes was not measurable in the way that you just pointed out.

In retrospect perhaps the parent comment was saying theoretically it could travel any distance which is true, but doesn't really help in estimating whether it could clear nuclear radii or not.

That's a good point, i didn't think of that! Do we know how fast the tetraneutrons were moving in this experiment? If so, we can work out the degree of time dilation, and how far it would have got.

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