Cheap Astronomy - Dear CA #142: Knowns & Unknowns

Space science, like any science, is full of knowns and unknowns, and that is unlikely to ever change.

First up, we're going to consider a fundamental particle that really might exist, but at this point in history, that is still unknown.

Dear Cheap Astronomy, do gravitons exist? Once again, we're dealing with something hypothetical here.

So, much like particle pairs that can pop out of nowhere and evaporate black holes, it's entirely possible that gravitons exist, but at this point in history, it's probably appropriate to entertain the notion that they might not exist.

Ten years ago we would have said the same thing about the Higgs boson, but that was positively identified by the Large Hadron Collider in 2012, which hence moved it from being a hypothetical particle to a real particle.

The case for the existence of gravitons is strong, where three of the four known forces, electromagnetism and the strong and weak nuclear forces, are all mediated by particles, being the photon for the electromagnetic force, the gluon for the strong nuclear force, and the W and Z bosons for the weak nuclear force.

So it makes sense that the fourth force, gravity, should also be mediated by a particle, that is, the graviton.

The problem is that gravity is the weakest force.

Indeed at the subatomic level, it's hundreds of thousands of times weaker than the other forces.

So presuming gravitons do exist, they operate at very, very low energy levels, which hence makes them extremely difficult to detect.

The standard line is that a detector the size of Jupiter would be unable to detect a graviton even if it was placed in the proximity of a neutron star, which generates tremendous amounts of gravity and hence should also generate lots of gravitons.

Freeman Dyson said that if we built a machine big enough to detect gravitons, it would collapse under its own mass and form a black hole.

So, it's unlikely we are going to positively identify a graviton any time soon.

Of course, the absence of evidence is not evidence of absence.

But until you do have such evidence, it's best to keep running with the hypothetical label.

And there is something of a case for the non-existence of gravitons as well.

According to general relativity, gravity isn't a force at all.

So rather than the Earth holding the Moon in orbit via an invisible force, the Earth's mass changes the geometry of space-time, which results in the Moon's trajectory following a roughly circular path around the Earth.

it's still feasible to envision gravitons mediating gravity by acting directly on space-time, rather than between the two massive objects, but that means that gravity works in a completely different way to any of the other forces, since those forces involve interacting particles exchanging force-mediating particles.

And that's a problem for gravity, because space-time is not composed of particles.

The mathematics of quantum-level interactions is also difficult to apply to a gravitational context since that mathematics assumes that such interactions happen against a fixed background of space and time.

However, general relativity tells us that there is no such fixed background.

Instead, space and time will be measured differently between different frames of reference.

So an external observer will observe clocks running slower and lengths contracting deep within a gravity well, just as they would on a fast-moving spaceship.

This is what people refer to as the great schism of modern physics.

Space science, like any science, is full of knowns and unknowns, and that is unlikely to ever change.

First up, we're going to consider a fundamental particle that really might exist, but at this point in history, that is still unknown.

Dear Cheap Astronomy, do gravitons exist? Once again, we're dealing with something hypothetical here.

So, much like particle pairs that can pop out of nowhere and evaporate black holes, it's entirely possible that gravitons exist, but at this point in history, it's probably appropriate to entertain the notion that they might not exist.

Ten years ago we would have said the same thing about the Higgs boson, but that was positively identified by the Large Hadron Collider in 2012, which hence moved it from being a hypothetical particle to a real particle.

The case for the existence of gravitons is strong, where three of the four known forces, electromagnetism and the strong and weak nuclear forces, are all mediated by particles, being the photon for the electromagnetic force, the gluon for the strong nuclear force, and the W and Z bosons for the weak nuclear force.

So it makes sense that the fourth force, gravity, should also be mediated by a particle, that is, the graviton.

The problem is that gravity is the weakest force.

Indeed at the subatomic level, it's hundreds of thousands of times weaker than the other forces.

So presuming gravitons do exist, they operate at very, very low energy levels, which hence makes them extremely difficult to detect.

The standard line is that a detector the size of Jupiter would be unable to detect a graviton even if it was placed in the proximity of a neutron star, which generates tremendous amounts of gravity and hence should also generate lots of gravitons.

Freeman Dyson said that if we built a machine big enough to detect gravitons, it would collapse under its own mass and form a black hole.

So, it's unlikely we are going to positively identify a graviton any time soon.

Of course, the absence of evidence is not evidence of absence.

But until you do have such evidence, it's best to keep running with the hypothetical label.

And there is something of a case for the non-existence of gravitons as well.

According to general relativity, gravity isn't a force at all.

So rather than the Earth holding the Moon in orbit via an invisible force, the Earth's mass changes the geometry of space-time, which results in the Moon's trajectory following a roughly circular path around the Earth.

it's still feasible to envision gravitons mediating gravity by acting directly on space-time, rather than between the two massive objects, but that means that gravity works in a completely different way to any of the other forces, since those forces involve interacting particles exchanging force-mediating particles.

And that's a problem for gravity, because space-time is not composed of particles.

The mathematics of quantum-level interactions is also difficult to apply to a gravitational context since that mathematics assumes that such interactions happen against a fixed background of space and time.

However, general relativity tells us that there is no such fixed background.

Instead, space and time will be measured differently between different frames of reference.

So an external observer will observe clocks running slower and lengths contracting deep within a gravity well, just as they would on a fast-moving spaceship.

This is what people refer to as the great schism of modern physics.