So, lets say that for one reason or another, I have a Petawatt laser pointed at me, and nothing but a small, high quality, hand mirror to deflect it.

Ignoring the obvious problems of firing a Petawatt laser in the open air, could I deflect this laser with my hand mirrior? If not, what could I expect to experience from said laser hitting me, and what would a mirror need to be constructed out of to deflect said laser.

  • $\begingroup$ If you could clarify what worldbuilding objective you are trying to achieve here, you may avoid closure for not being about worldbuilding. As is, this appears to be a physics question, and not one trying to build something. $\endgroup$
    – Frostfyre
    Commented Mar 9, 2017 at 20:44
  • $\begingroup$ Yes, but not for very long. You could probably last from a microsecond up to maybe even a millisecond, depending on how focused the beam is and how much of its power is lost in the air on the way to your mirror. $\endgroup$ Commented Mar 9, 2017 at 21:07
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    $\begingroup$ Try asking on the Physics SE for real-world answers. $\endgroup$
    – JDługosz
    Commented Mar 9, 2017 at 22:59
  • $\begingroup$ Would you specify pulse duration? A petawatt is 10^15 watts. If it only shines for an femttosecond (10^-15 seconds), total power is only 1 joule. If it is a CW (continuous wave) laser, well, that's another matter. $\endgroup$ Commented Mar 9, 2017 at 23:00
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    $\begingroup$ consider to choose between two tags science-based or hard-science, and consider not to use hard-science in the case. Consider to add information as wave length and duration of pulse and size of the beam. $\endgroup$
    – MolbOrg
    Commented Mar 10, 2017 at 2:05

2 Answers 2


Even assuming your mirror is made from something absolutely perfectly suited to reflecting the laser beam, defect-free and absorbing only 1/1012th of the 1015 watts in your laser beam, you will feel like you were hit by a truck — light has momentum.

Specifically, 1 Petawatt / (speed of light) = 3,335,640.95 newtons… actually, that's not a truck. 340 tons is what you get if you stack three tanks on a blue whale.

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    $\begingroup$ The comment on duration makes me wonder how 340 tons would feel if you only had it on you for for a femtosecond. $\endgroup$
    – JDługosz
    Commented Mar 9, 2017 at 23:04
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    $\begingroup$ I doubt your nerve impulses would work fast enough to "feel" anything before you are a fine mist on the wall........ $\endgroup$
    – Thucydides
    Commented Mar 10, 2017 at 0:11
  • $\begingroup$ @JDługosz it would feel like a broken glass in your hand, at least in this scenario... $\endgroup$
    – Mołot
    Commented Mar 11, 2017 at 23:08
  • $\begingroup$ @Thucydides one joule won't turn a human body into a mist. The totql energy limits the amount of damage that could be done. $\endgroup$
    – JDługosz
    Commented Mar 11, 2017 at 23:24

Even at the most reflective materials we know, dielectric mirrors, can only reflect about 99.999% of the light that hits them. That means 10 GW of the Petawatt laser is absorbed as heat. According to the order of magnitude table that is more than the largest nuclear power plant in the world Released as heat right in front of you. Dielectric mirrors can only have such a high performance at very specific wavelengths, but that should be possible with a laser. By comparison, an aluminium mirror would absorb an amount of power comparable to the US power consumption at most wavelengths.


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