If a photon has no mass how can it deliver energy?

A photon has no mass when it is not moving. But it has mass when it is moving at the speed of light.

Wouldn't it then have infinite mass and energy? I thought it was only able to travel at the speed of light because it was massless?  How would one stop?

This is getting to the edge of my understanding but here goes: The same mechanism that would give ordinary matter infinite mass gives a massless particle apparent mass. And a photon doesn't stop as such. It becomes energy without mass.

You're going to give a false impression to the guy if we don't expand on that a bit.

There's rest mass and relativistic mass. Rest mass is a non-varying quantity which is the same for all observers in all frames of reference (so regardless of relative motion), while relativistic mass is dependent on the velocity of the observer. A free photon is never at rest relative to anything (the wavelength changes, not the speed) so it can have a rest mass of zero while having a non-zero relativistic mass. Plank worked out through his work on black body radiation (IIRC) that E=hf (E is energy, h is plank's constant, f is frequency). Einstein worked out that E=mc^2.

So if we go from Einstein's mass-energy equivalence equation E=mc^2 and Plank's equation E=hf, since E=hf and E=mc^2 then hf=mc^2 and m=hf/c^2. Thus hf/c^2 is the mass of the photon and the mass of a photon is proportional only to its frequency since all the other numbers are constant. FWIW, Plank's constant is a mind shatteringly small number: 6.626176 x 10^-34 joule-seconds. To put that in what I think are more human readable numbers it's .0000000000000000000000000000000006626176 joule-seconds (pretty sure I got all the zeros in there).

The coolest thing about light is how it has repeatedly taught us that no matter how strange the universe might superficially appear, it's much stranger in detail.

Also, not a dumbshit question. About as far from a dumbshit question as I've ever seen on Arfcom.

Thanks for that explanation. I'll take your word for it because you lost me in the math. I read an article on Universe Today about length and time from different frames of reference. One scenario was a hypothetical spacecraft traveling to the Andromeda Galaxy at 99.999999 the speed of light. To a stationary observer on earth the distance would be two million light years but to those on board it would only be something like 655 light years. From a photons frame of reference traveling at c it would not experience time or distance at all. Essentially when it left a star in Andromeda and struck my retina the photon would be in the same place, again from it's frame of reference.

The night after I read that I was spending the night in my sleeping bag at over nine thousand feet on a remote mountain hoping to enjoy the surroundings, I say spending the night in my new sleeping bag instead of sleeping in it because I could not stop thinking about that damned photon.

It also had me wondering how light could exert pressure on nebular clouds of gas or propel Breakthrough Star Shot.

I knew there would be someone here smart enough to try to get it through my head, and again thanks.

A photon has no REST mass.  It does have finite energy and momentum, however, both of which are related to its frequency.  So in this case, you multiply zero by infinity and get an actual number...

Mike

“Now my own suspicion is that the universe is not only queerer than we suppose,
but queerer than we can suppose.”
 J. B. S. Haldane quotes (British geneticist 1892-1964)

The spooky thing is that from the photon's point of view, the trip is instant no matter how long or short it is in distance.