At essentially the speed of light. Neutrinos travel slightly below c because they have a tiny non-zero rest mass, but the difference is so small that no experiment has ever measured it directly. The best upper bound comes from SN 1987A:
$$\frac{|v - c|}{c} < 2 \times 10^{-9}$$
In other words, the speed of neutrinos and the speed of light agree to at least 9 significant figures.
Why not exactly the speed of light?
Only massless particles travel at exactly c. Photons are massless. Gluons are massless. Hypothetical gravitons are massless. They all travel at the speed of light.
Neutrinos have non-zero rest mass. Super-Kamiokande and SNO proved this in 1998 and 2001. The heaviest neutrino has at least:
$$m_\nu \gtrsim \sqrt{\Delta m_{31}^2} \approx 0.05 \text{ eV}/c^2$$
A particle with mass m and total energy E travels at velocity:
$$v = c \sqrt{1 - \frac{m^2 c^4}{E^2}}$$
For a 1 GeV neutrino with $m \approx 0.05$ eV:
$$\frac{v}{c} = 1 - 1.25 \times 10^{-21}$$
That’s slightly less than light speed by an amount no current technology can measure.
How the speed was actually measured
SN 1987A (the strongest constraint)
The supernova in the Large Magellanic Cloud on 23 February 1987 emitted a burst of about $10^{58}$ neutrinos. 24 of them were detected at three observatories on Earth (Kamiokande-II, IMB, Baksan).
The neutrinos arrived within a 13-second window, roughly 3 hours before the visible light. The early arrival is expected: the neutrinos escape the supernova’s outer envelope freely, while the photons have to diffuse out through the still-collapsing envelope.
Over 168,000 light-years, even a tiny speed difference would have spread the neutrinos in time or shifted them relative to the photons. The observed agreement constrains:
$$\frac{|v - c|}{c} < 2 \times 10^{-9}$$
That’s the tightest direct measurement of neutrino speed ever made.
Accelerator measurements
Particle accelerators produce neutrino beams and aim them at distant detectors. The time-of-flight can be measured precisely if the beam has a sharp timing structure.
- MINOS (Fermilab → Soudan, 735 km): consistent with c to within ~0.01 %.
- T2K (J-PARC → Super-K, 295 km): same.
- ICARUS (CERN → Gran Sasso, 732 km): same.
- OPERA (CERN → Gran Sasso, 732 km): initially reported 60 ns faster than light in 2011, then traced to a loose fibre cable. The corrected measurement matched c.
Cross-correlation with gravitational waves
GW170817 — a binary neutron star merger in 2017 — was observed in gravitational waves and gamma rays within 1.7 seconds. If neutrinos had been detected (none reached the threshold), the cross-correlation would have tested whether they travel at the same speed as light to similar precision.
The 2011 OPERA episode
Briefly, in September 2011, OPERA announced neutrinos arriving 60 ns faster than light over 732 km. The story dominated physics news for six months.
In February 2012 the collaboration retracted the result: a loose fibre-optic cable between the GPS receiver and the master clock had introduced a 74 ns delay in the timing reference, and a clock oscillator had drifted by another 14 ns. After correction, the measured speed matched c.
Special relativity was not in danger. The episode is now used as a teaching example of how the scientific process catches and corrects high-profile errors fast.
What if neutrinos really were faster than light?
It would overturn special relativity and most of modern physics. There are also independent theoretical arguments against the possibility:
- The Cohen-Glashow argument: superluminal neutrinos would emit electron-positron pairs as a kind of analog Cherenkov radiation, rapidly losing energy. We don’t observe this.
- SN 1987A timing: any superluminal speed at MeV energies would have produced visible time-of-flight effects over 168,000 light-years. None were seen.
Modern measurements and theory both strongly say: no.
The short answer
Neutrinos travel at very nearly the speed of light. They’re not exactly at c because they have rest mass, but the deviation is below $10^{-9}$ — far smaller than any experiment can directly measure. For all practical purposes, a neutrino and a photon emitted at the same time will arrive at the same time, even across cosmological distances.
For more on the OPERA episode, see Anomalies and Mysteries — Part 5. For why neutrinos have mass in the first place, see Do neutrinos have mass?.
Frequently asked
How fast do neutrinos travel?
At essentially the speed of light. Their tiny non-zero rest mass means they travel slightly below c, but the difference is so small that no experiment has ever measured it directly. The most stringent bound comes from SN 1987A: |v−c|/c < 2 × 10⁻⁹.
Are neutrinos faster than light?
No. The famous 2011 OPERA result that briefly suggested superluminal speeds was traced to a loose fibre-optic cable in the GPS timing system. Every reliable measurement is consistent with light-speed travel.
Why aren't neutrinos exactly at light speed?
Special relativity caps any particle with rest mass strictly below c, no matter how small the mass. Neutrinos have at least 0.05 eV of mass (the heaviest), so they cannot reach exactly the speed of light. For a 1 GeV neutrino, the velocity is roughly 1 − 10⁻²¹ × c — indistinguishable from light speed for any practical experiment.
How was the speed of neutrinos measured?
Three ways. First, SN 1987A: the supernova neutrinos arrived within hours of the optical signal after travelling 168,000 light-years, giving the tightest constraint. Second, accelerator time-of-flight experiments at MINOS, T2K, ICARUS, and OPERA (corrected). Third, cosmological bounds from gravitational-wave and neutrino observatory cross-correlations.
Quick check
A quick check that the key points landed. Click an option to see if you got it.
Why can\'t neutrinos travel at exactly the speed of light?
Special relativity requires any particle with rest mass — no matter how tiny — to travel strictly below c. Neutrinos have at least ~0.05 eV of mass, which puts them imperceptibly below light speed.
What was the famous 2011 OPERA superluminal-neutrino result eventually traced to?
OPERA\'s 2011 announcement of muon neutrinos arriving 60 ns earlier than light was real measurement-versus-prediction tension, but the cause was a loose fibre-optic timing cable plus a faulty reference clock. Once fixed, the apparent superluminal travel disappeared.
How did SN 1987A constrain the neutrino speed?
The supernova neutrinos arrived within roughly 3 hours of the optical brightening — extraordinarily tight agreement for a 168,000-year journey. This gives the bound |v−c|/c < 2 × 10⁻⁹.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, July 19). How fast do neutrinos travel?. Neutrino Times. https://neutrino-times.com/articles/how-fast-do-neutrinos-travel/
Chicago
Neutrino Times Editorial Team. "How fast do neutrinos travel?." Neutrino Times, July 19, 2025. https://neutrino-times.com/articles/how-fast-do-neutrinos-travel/.
MLA
Neutrino Times Editorial Team. "How fast do neutrinos travel?." Neutrino Times, 19 Jul. 2025, https://neutrino-times.com/articles/how-fast-do-neutrinos-travel/.
BibTeX
@misc{neutrino-times-how-fast-do-neutrinos-travel,
author = {Neutrino Times Editorial Team},
title = {How fast do neutrinos travel?},
howpublished = {Neutrino Times},
year = {2025},
month = {jul},
url = {https://neutrino-times.com/articles/how-fast-do-neutrinos-travel/},
note = {Accessed: 2025-07-19}
} RIS
TY - GEN TI - How fast do neutrinos travel? AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-07-19 PB - Neutrino Times UR - https://neutrino-times.com/articles/how-fast-do-neutrinos-travel/ ER -