# I Clarification virtual photons, real photons

1. Sep 24, 2016

### Albertgauss

I need clarification on the difference between a virtual photon of a transmitted force and “radiated photons” of accelerated charges.

Imagine two electric charges separated by a distance “d” but not moving. They exchange virtual photons which tell the electric charges to attract/repel. How are these virtual photons the same/different to the real photons emitted from accelerated charge like in an antenna?

Why can’t virtual photons be detected from non-accelerating electric charges? If they are truly present, and cause the real observable attraction and repulsion of electric charges, shouldn’t they be able to be detected going back and forth between the electric charges? It seems that photons are photons.

This now applies to gravity waves. Similarly, I know that masses exchange gravity waves from which they know to attract. If such gravity waves really are present between the masses, shouldn’t I be able to detect them? Yet I know that accelerated masses are required for the detection of gravity waves also (LIGO). This seems like a contradiction. Aren’t the gravity waves always there, always moving between the masses, whether the masses accelerate or not? Or like, with electromagnetism, is there some fundamental difference I am missing between the virtual photons? of gravity and the gravitational radiation we observe from binary black holes?

Is the graviton the "virtual photon" responsible for the attraction of masses in gravrity?

2. Sep 24, 2016

### Staff: Mentor

You can't observe them. Ever.

Real photons can be counted with a detector (like a camera sensor) and propagate at the speed of light. Virtual photons cannot be counted. A camera sensor will not detect them. They cannot even be said to propagate between two points.

This is incorrect. Gravitational waves are not the source of the attraction between two masses. That belongs to the curvature of spacetime, not to the waves within it.

Gravitational waves are similar to EM waves in that they are the result of accelerated charges (positive and negative charges in EM, acceleration of masses in gravity). They both travel at the speed of light and carry energy away from their source. Neither the photons composing the EM waves nor the gravitons composing the gravitational waves (if they exist) are virtual. They are real.