How can something with no mass be affected by gravity?

I was reading the thread about black holes and noticed the statement that light cannot escape from black holes. I had always known this, but I guess I never thought about it. Light is simply electromagnetic waves. Electromagnetic waves are a mode of transportation for energy. While energy moves in the form of electromagnetic waves, it has no mass. So how can light, which has no mass, be affected by the gravity of a black hole? I thought gravity only acted upon objects with mass?

Light has mass under certain circumstances, never could get my physics teacher to fully explain this to me though. But photons do have mass.

Light has relativistic mass, in that it has mass because it’s moving so fast. If a photon were to stand still (which is not really possible) it would be massless.

I remember that things get more massive as they move faster, but something with no mass becomes massive? :confused:

In a word, yes. :smiley:

Slight hijack but kinda relevant:

What is the current thinking on gravitons? (the particles that are supposed to be the quanta of gravity, and propagate it, in the same way photons are for light.) Do they tie in with Quantum Gravity, or do they contradict it? Also, if the escape velocity of a black hole is > c, how can these gravitons escape from it, i.e. how can the black hole influence its surroundings graviationally? I tried a few searches on Google but couldn’t find anything that explains this in high-school physics terms.

A photon has neither rest mass nor relativistic mass. It is sometimes considered to have gravitational mass, but only a very few physicist subscribe to this term. A photon on the other hand very definitely has energy ( E = Planck’s constant * frequency) and energy can both create and be affected by a gravitational field.

Real gravitons constitute gravitational waves and are predicted by string theory. On the other hand the gravitational force (loosely speaking) is mediated by virtual gravitons which are not constrained by the speed of light and therefore have no trouble escaping the hole. However string theory or quantum gravity is not yet a developed theory and for several reasons cannot be tested experimentally.

Also, relativistic mass is no longer used by most scientists - it is confusing and would mean that an objects longitudinal mass would be different from its transverse mass.

Doesn’t mass distort space? Even a massless particle has to move thru space, therefore it will be affected by mass. IIRC.

Somewhat related topic

Are neutrinos unaffected by black holes?

There you go! Gravity = curved space. A photon will travel in a straight line through curved space. So, it appears to curve (be affected by gravity).

Which is really related to the fact that energy and mass are different aspects of the same thing.


Is my answer right then? Do I win? Do I win?

The photon “appears” to travel in a curve because
space is curved? Or does the photon take a curve because space it travels in is curved? Didn’t those photons from a distant star that travelled in a curve because of the sun go in a curve around the sun in a famous experiment in 1900 or so. The sun was in eclipse at the time and they knew these stars were behind the sun, but still their light came to us because the rays curved around because of the huge sun distorting the space around it because it was so big.
And now I’m wondering just how big does a body have to be before it curves the space around itself? Moon size?
Half moon size?

Anything with mass would curve space, but not necessarily enough to be noticable.

My questions is: is space really “curved” or is this just a convenient way for us to picture gravity? What is it that is being “curved”?

That’s an interesting question because in order for 3 dimensional space to be curved there would have to a fourth spatial dimension for it to curve into. Time is not a fourth spatial dimension and therefore it will not serve the purpose.

What this really means is that the intrinsic geometry of space is curved. And all this means is that an object will follow a certain path through space while traversing it. Many scientists don’t like the term curvature but accept that it is so ingrained in the language that there is no doing away with it.