# Observation and Gravity

## Main Question or Discussion Point

hello,

I don't really fully understand quantum mechanics, nor could I even hope to dip my toe into even the most basic of formal mathematics it uses without at least a year of study :(

however, I have one quick question if I may

The Double Slit Experiment with electrons

As I understand it, any attempt at an observation of the electrons to detrmine which slit they go through during the experiment collapses the electrons wavefunction, making them behave as a particle (there is no interference pattern).

A non-observation produces an interference pattern suggesting that each electron goes through both slits as a wave, and interferes with 'itself'

My question is this. Since the electron has 'rest mass', what if we could use gravity (i.e. a gargantuan electron gun and slits, or some sort of high-precision 'graviton camera') to indirectly detect the which-path information, rather than using photons or detectors

would there be an interference pattern or not?

Related Quantum Physics News on Phys.org
This is actually kind of fascinating, but I don't know what you mean by a "gargantuan electron gun". Experimentally, there is no such thing as a "graviton" camera... or any way to observe gravitons, at least at this point (and in my mind ever), but as a thought experiment it's very interesting.

I do not believe there is any apparatus, even close, today that would be able to distinguish, due to mass, which slit (which have to be awfully close together) than an electron (which has a tiny mass) were to travel though. Still... it's interesting... I'm actually going to think about that a little more.

Matterwave
Gold Member
I have always wondered how gravity, or indeed other forces, plays into all this. For example, could we not detect an electron by measuring the coulomb repulsion/attraction between it and another "test" electron, without "looking" at the electron?

I have no good answer for this; however, it would seem that in QM all notion of "forces" are not well defined, only "potentials". My guess would be that the gravity would appear in QM as a "potential" to be included in the Hamiltonian of the system just as the coulomb attraction between the proton and electron of Hydrogen is included. This would lead to a wavefunction which slowly evolved according to Schroedinger's equation, and not abrupt "collapse". But take my word with a grain of salt as I haven't studied QM decoherence.

I have always wondered how gravity, or indeed other forces, plays into all this. For example, could we not detect an electron by measuring the coulomb repulsion/attraction between it and another "test" electron, without "looking" at the electron?
That's an easier one to answer, as that is 'observation' in the more classical sense. Coulomb interaction is an exchange of virtual photons, so it really would be "looking" at the electron still.

DrChinese
Gold Member
My question is this. Since the electron has 'rest mass', what if we could use gravity (i.e. a gargantuan electron gun and slits, or some sort of high-precision 'graviton camera') to indirectly detect the which-path information, rather than using photons or detectors

would there be an interference pattern or not?
Assuming gravity is a quantum force: IF you could observe an electron which-path using the gravitational force, THEN the result would be no different than if you learned this conventionally. So *no* interference pattern.

Now, the practical details of performing this experiment are complex. Like, you would need a suitable candidate quantum theory of gravity to start with... one which made predictions which could be falsified and tested. And which had a mechanism for providing the kind of detection you envision. We clearly aren't there yet.

This is actually kind of fascinating, but I don't know what you mean by a "gargantuan electron gun".
I don't want to really get into a technical apparatus discussion, I can think of at least three different set ups, but for now - I'll summarise the simplest.

Let's just say an electron gun connected to a particle accelerator of sufficient size and energy to accelerate the electrons so that their relativistic mass increase (unless I'm mistaken about relativistic mass and gravitational interaction ) enables them to gravitationally interact with your instruments

DrChinese said:
Assuming gravity is a quantum force: IF you could observe an electron which-path using the gravitational force, THEN the result would be no different than if you learned this conventionally. So *no* interference pattern.
this makes no sense to me since the electrons never stop interacting gravitationally

you seem to be saying that interference pattern disappears when somebody looks at a results screen?, or when the experiment is recorded in some way?

this makes no sense to me since the electrons never stop interacting gravitationally
Why do you choose to single out gravitational interaction ? Electrons never stop interacting electromagnetically, and more intensely so than gravitationally.

Why do you choose to single out gravitational interaction ? Electrons never stop interacting electromagnetically, and more intensely so than gravitationally.
many reasons, but I don't want to get side-tracked away from my original question

many reasons, but I don't want to get side-tracked away from my original question
Don't you think before attacking a real-life reasearch problem, it's best suited to solve the baby problem ?

DrChinese
Gold Member
this makes no sense to me since the electrons never stop interacting gravitationally

you seem to be saying that interference pattern disappears when somebody looks at a results screen?, or when the experiment is recorded in some way?
Interference always disappears whenever you can learn, in principle, the which-way information. Doesn't really matter how it is done. However, if gravity is NOT a quantum force then there won't be such a mechanism possible as you envision. (And there might not be even if gravity IS a quantum force.)

Don't you think before attacking a real-life research problem, it's best suited to solve the baby problem ?
last post as regards this

no, because in this thread I want to focus on the Graviton (if it exists) and Gravitational interaction as a method of observation

Interference always disappears whenever you can learn, in principle, the which-way information. Doesn't really matter how it is done.
if gravitational interaction can be used, then there should never be any interference pattern - ever.

Because in principle, you can always learn the which-path information and it is always being 'observed' by the rest of the universe since the gravity from the electrons have infinite range

However, if gravity is NOT a quantum force then there won't be such a mechanism possible as you envision. (And there might not be even if gravity IS a quantum force.)
this, I don't understand

but please answer as clearly as you can. Assuming you can detect the gravity on an electron to determine which-path information, In which scenario in this experiment is there an interference pattern and in which is there not?

Hootenanny
Staff Emeritus
Gold Member
Because in principle, you can always learn the which-path information and it is always being 'observed' by the rest of the universe since the gravity from the electrons have infinite range
Err... the EM field generated by the electron also has an infinite range. So what's so special about gravity?

Err... the EM field generated by the electron also has an infinite range. So what's so special about gravity?
nothing, however the virtual photons which (I think) make up the EM field can be blocked or perturbed, thus the EM field itself. The graviton (if it exists) cannot, so I have chosen gravity as the observational tool for the experiment

do you want to hazard an answer to my original question?

Hootenanny
Staff Emeritus
Gold Member
nothing, however the virtual photons which (I think) make up the EM field can be blocked or perturbed, thus the EM field itself.
Okay fair enough.
The graviton (if it exists) cannot, so I have chosen gravity as the observational tool for the experiment
The hypothetical graviton does indeed interact very weakly. However, this creates a problem for detecting gravitons: if gravitons interact very weakly, then it is going to be extremely difficult to detect them, which makes them a poor candidate for measuring other quantities - say the position of an electron.
do you want to hazard an answer to my original question?
As Dr. Chinese has said, if a quantum theory of gravity exists that could be tested and if a mechanism existed such that it allowed you to measure the position of an electron via the gravitational interaction, then no interference pattern would be observed.

The hypothetical graviton does indeed interact very weakly. However, this creates a problem for detecting gravitons: if gravitons interact very weakly, then it is going to be extremely difficult to detect them, which makes them a poor candidate for measuring other quantities - say the position of an electron.
I'm assuming then that an electron cannot be accelerated/it's energy increased to such an extent that it's gravity becomes easily detectable? (posts #1 and #6 - my gargantuan Electron Gun™)

As Dr. Chinese has said, if a quantum theory of gravity exists that could be tested and if a mechanism existed such that it allowed you to measure the position of an electron via the gravitational interaction, then no interference pattern would be observed.
and this is what I don't seem to understand, at what point will there be no interference pattern?

It is not true that you can somehow "switch off" electromagnetic interaction. What you can screen is electromagnetic charges.
no, because in this thread I want to focus on the Graviton (if it exists) and Gravitational interaction as a method of observation
So you hope to find answers to a highly speculative and difficult question, and refuse to acknowledge how much hindsight on the problem you could gain by first concentrating on a real-life well-posed problem, the answer to which is already known, and most probably captures the essence of your question. It is indeed quite interesting as an approach to science, and suffice to have me out of this discussion.

DrChinese
Gold Member
if gravitational interaction can be used, then there should never be any interference pattern - ever.
This is false. Quantum erasers are devices which allow interaction via the electromagnetic force, and yet there is no possibility of obtaining which-path information. Check out the following link which runs through some examples. As mentioned, using hypothetical gravitons to detect which-way will cause interference to collapse. But if you can't detect the which-way, in principle, then the interference pattern is not eliminated.

I might point out that although an electron may be subject to the earth's gravitional force: even if there are gravitons, you really couldn't be sure that an electron absorbed or emitted one unless it moved up or down in the earth's field without other influences. That is why you need a quantum theory of gravity to put together such an experiment. A single electron traveling 5 or 10 feet might not absorb a free graviton. Who knows? (We do know that virtual photons don't cause collapse, so we presume that virtual gravitons do not cause collapse either.)

Last edited by a moderator:
DrChinese
Gold Member
I'm assuming then that an electron cannot be accelerated/it's energy increased to such an extent that it's gravity becomes easily detectable?
Are you talking about its rest mass, which is of course fixed? Or its inertial mass, which is routinely boosted to large values in particle accelerators? What makes you think that such an electron emits more gravitons than one with a lower inertial mass?

(Besides, it isn't the electron's gravitional emissions that would be used to detect which-path. It is the earth's. But of course, none of this makes any sense to talk about without a specific candidate theory of quantum gravity so that you have something specific to test. There are many theoretical researchers actively studying this area, and there are all kinds of complex issues involved. Obviously, this is not something you come into and say, "what if X" without considering the many constraints that must be met to match already known phenomena.)

If you want to learn a bit about what is already going on out there, I might recommend flipping through some of the 500+ articles written on quantum gravity/cosmology in the past year:

http://arxiv.org/find/gr-qc/1/abs:+quantum/0/1/0/past/0/1?per_page=100

I think I'll bow out of this thread since it hasn't been helpful; with the exceptions of the second half of DrChinese's last post and whybother's earlier speculations.

My direct questions haven't been answered directly, nor even pontificated upon in any depth (for the most part) which was my original intention. And the experiment has a very simple (theoretical) set up

I realise this is a difficult question, which was why I asked it. But I don't think snide remarks about refusal of some unmentioned insights I might get from EM field interaction will offer me any of the clarity I'm looking for. I think I'll send one of my old University lecturers an email

won't post in this thread again, but will read if it continues

good day

Last edited:
Hootenanny
Staff Emeritus
Gold Member
My direct questions haven't been answered directly, nor even pontificated upon in any depth (for the most part) which was my original intention. And the experiment has a very simple (theoretical) set up
I believe both I and Dr. Chinese have given you as direct answers as possible to you 'direct' questions. In which respects do you feel that your questions have not been directly answered?

Matterwave