Here are a few commonly heard sentences that will make my question clear:
Statements #1
“The Michelson Morley experiment once and for all did away with the ether or the idea that light needs a medium in which to travel”
“Light can travel through empty space”
Statements #2
“Light travels though the electromagnetic field”
“The photon field is only one of the fields for each particle in the standard model.”
“There must be dark matter in the universe for our current model of gravity to make sense.”
“Space is not empty. A quantum foam fills it”
You can see the confusion that arises from the conflicting statements which are commonly heard in physics.
The fact that light travels via an electromagnetic field is in clear contradiction to the statement that light travels through empty space. I suppose we could try to join empty space and the EM field into one thing by saying light is the field. Frankly, that sounds like doublespeak.
What am I missing?
Light is a disturbance in the EM field. There is no empty space. Light travels through a field.
Why do some physicists say light can travel through empty space?
Best Answer
I guess what a lot of people mean when they say that light travels through empty space is that it doesn't require a physical medium (matter) to travel through - like sound waves do.
The fact of the matter is that light does travel in the electromagnetic field, but the electromagnetic field is just a mathematical tool - as are all the other fields in QFT. You can't touch the EM field, just like you can't touch the fields that give rise to physical matter. You might say that we can touch matter and so those fields are more real. However, the sensation of touching those fields is just a result of electromagnetic interactions between the particles in ourselves and those in the object we are touching, and the reason for that being true is no more real than the reason that light can move from A to B.
To answer your specific points, the Michelson-Morley experiment proved that light travels at a constant speed for all observers. It proved that there was no physically-measurable medium in which light travels through (like sound). The electromagnetic field is how we describe light moving, however, it is not a physical object - it is just a mathematical tool and so only exists on paper.
I think the confusion in terminology comes from people trying to attach physical meaning to fields. Maybe you can attach meaning to them, and one day we will be able to look at them in a different way to how we can now. However, it must be noted that the fields introduced in QFT are mathematical tools, and that is all they are. We should not imagine them as filling space like a fluid, they simply make useful and accurate predictions.
In summary, they tell us how everything works - they will never tell us why.