Understanding VLBI: The Importance of Time Communication in Interferometry

[Larry Pendarvis]

The descriptions I have seen of VLBI seem to imply that a photon is detected at more than one detector (impossible), or that two photons interfere with one another (possible but extremely unlikely).
Of course, an interference pattern can be built up from many individual photons over time, but then why is it important for the detectors to communicate time information to get the result?

 

[Jilang]

I should imagine that the time delay between the arrival at different detectors would tell you something about the direction of the source.

 

[Nugatory]

The descriptions I have seen of VLBI...

Which ones?

... seem to imply that a photon is detected at more than one detector (impossible), or that two photons interfere with one another (possible but extremely unlikely).

How about "neither of the above"? The object being studied isn't a single-photon source, and the detectors aren't photon detectors - this is ordinary radio-frequency electromagnetic radiation that we're looking at.

Of course, an interference pattern can be built up from many individual photons over time, but then why is it important for the detectors to communicate time information to get the result?

We're comparing the phase differences between the signal received at various locations. That comes down to knowing the amplitude as a function of time at each location, and the observations from each location can only be compared if all the locations are using a common time standard.

 

[Larry Pendarvis]

... the detectors aren't photon detectors - this is ordinary radio-frequency electromagnetic radiation that we're looking at.

EM radiation that is not photons does not seem ordinary to me.

 

[Larry Pendarvis]

We're comparing the phase differences between the signal received at various locations.

Do you mean that this only works with coherent radiation?

 

[davenn]

EM radiation that is not photons does not seem ordinary to me.

you misunderstand what Nurgatory is telling you

The VLBI isn't detecting photons, its detecting radio waves, trying to think of radio waves as individual photons is not correct and will lead you to much confusion

 

[Larry Pendarvis]

you misunderstand what Nurgatory is telling you

The VLBI isn't detecting photons, its detecting radio waves, trying to think of radio waves as individual photons is not correct and will lead you to much confusion

https://en.wikipedia.org/wiki/Near_and_far_field
"In the quantum view of electromagnetic interactions, far-field effects are manifestations of real photons"
Perhaps this thread belongs in the Quantum Physics forum.

 

[davenn]

Perhaps this thread belongs in the Quantum Physics forum.

maybe, maybe not

When I do radio astronomy, I am receiving radio signal from distant objects
individual photons are irrelevant ...
The principle idea behind any large baseline interferometer is to synthesise a radio telescope of the same size
to improve the resolution of the object being studied

 

[Larry Pendarvis]

maybe, maybe not

When I do radio astronomy, I am receiving radio signal from distant objects
individual photons are irrelevant ...
The principle idea behind any large baseline interferometer is to synthesise a radio telescope of the same size
to improve the resolution of the object being studied

Obviously it works, and works well. I just have trouble seeing how it works, in QM terms.

 

[Nugatory]

Perhaps this thread belongs in the Quantum Physics forum.

It does not, which is why it was moved here after you started it there. VLBI radio telescopes are not quantum mechanical devices and they don't depend on quantum mechanical effects. We're using antennas to detect oscillations in the electromagnetic field, just as with any other classical radio receiver; the only remarkable thing here is the weakness of the signal and the technical challenges that creates.

 

[Larry Pendarvis]

VLBI radio telescopes are not quantum mechanical devices and they don't depend on quantum mechanical effects.

True, but I don't think they can contradict QM either.

 

[ZapperZ]

True, but I don't think they can contradict QM ether.

They don't, but you are trying to kill a mosquito with a bazooka, which doesn't make any sense.

QM is useful when the quantum mechanical effect is obvious, or when classical picture is insufficient. Trying to solve what can clearly and EASILY be described by classical wave by using a more complicated QM picture is irrational. That is like asking someone who builds houses to use Special Relativity.

Zz.

 

[Larry Pendarvis]

They don't, but you are trying to kill a mosquito with a bazooka, which doesn't make any sense.

QM is useful when the quantum mechanical effect is obvious, or when classical picture is insufficient. Trying to solve what can clearly and EASILY be described by classical wave by using a more complicated QM picture is irrational. That is like asking someone who builds houses to use Special Relativity.

Zz.

Do you think that VLBI would, in principle, work the same, if individual photons arrived at one-second intervals, or say one-year intervals? Or one-femtosecond intervals?

 

[ZapperZ]

Do you think that VLBI would, in principle, work the same, if individual photons arrived at one-second intervals, or say one-year intervals? Or one-femtosecond intervals?

It wasn't designed to detect "single photons" of that wavelength.

This is where your understanding of detectors are lacking. We design detectors based on what we intend to detect. No detectors can operate over all range possible, and this one works as a "pick-up antenna" so to speak.

You've been told repeatedly why the picture you want to use isn't feasible here. I don't know what else anyone can say to you to get through.

Zz.

 

[Larry Pendarvis]

It wasn't designed to detect "single photons" of that wavelength.

This is where your understanding of detectors are lacking. We design detectors based on what we intend to detect. No detectors can operate over all range possible, and this one works as a "pick-up antenna" so to speak.

You've been told repeatedly why the picture you want to use isn't feasible here. I don't know what else anyone can say to you to get through.

Zz.

No one has ever seen an individual photon.
No one has ever seen a carbon footprint either, but that doesn't mean they don't exist.

 

[ZapperZ]

No one has ever seen an individual photon.
No one has ever seen a carbon footprint either, but that doesn't mean they don't exist.

This is getting silly. I never said that we don't detect individual photons. I said THIS particular detector was never designed for such a purpose! READ CAREFULLY!

Zz.

 

[Larry Pendarvis]

This is getting silly. I never said that we don't detect individual photons. I said THIS particular detector was never designed for such a purpose! READ CAREFULLY!

Zz.

http://astrogeo.org/vlbi_development/gravity/gravity.html
"Measurements of differential retardation of photons' travel time caused by the gravitational field of the Sun [1], [2] and the Jupiter [3] allowed to get precise estimates of the Post-Newtonian parameter gamma. Currently, VLBI provides the most accurate estimate of this parameter: 1.00058 -+ 0.00014 (un-scaled formal uncertainty). This is an important contribution of VLBI to fundamental physics."

http://arxiv.org/pdf/1012.2267
"... it would be interesting to estimate gravitational bounds for the
photon mass by considering the most recent measurements of the solar gravitational
deflection of radio waves obtained by means of the VLBI."

 

[Drakkith]

http://astrogeo.org/vlbi_development/gravity/gravity.html
"Measurements of differential retardation of photons' travel time caused by the gravitational field of the Sun [1], [2] and the Jupiter [3] allowed to get precise estimates of the Post-Newtonian parameter gamma. Currently, VLBI provides the most accurate estimate of this parameter: 1.00058 -+ 0.00014 (un-scaled formal uncertainty). This is an important contribution of VLBI to fundamental physics."

That is irrelevant, as it has nothing to do with how the detector operates. It doesn't change the fact that the detector is not detecting individual photons at a time. The energy of a single photon in the radio or microwave frequencies is far too low for this detector to detect. In fact there are no detectors capable of detecting single photons at these frequencies. Instead they are detecting the contribution from a great many photons that are absorbed by the detector over a short period of time.

The fact that the detector isn't detecting single photons doesn't mean that the EM radiation isn't made up of photons. It just means that the EM radiation can be accurately described by classical physics for this particular purpose.

 

[Larry Pendarvis]

This is getting silly. I never said that we don't detect individual photons. I said THIS particular detector was never designed for such a purpose! READ CAREFULLY!

Zz.

I am not sure which particular detector you are talking about. My question was about VBLI instrumentation generally.

In Principle, you could get better and better clocks so that you could match near-simultaneous detection events at the detectors. But I cannot see how one photon can be detected by two different antennas, and two photons at two antennas would be two events at slightly different times.

 

[Drakkith]

Ah. But then how can you derive phase information by comparing information from an antenna gathering photons over time, with information from another antenna gathering photons over a similar time? Unless you are dealing with coherent radiation.

The antennas directly detect the phase of the incoming EM wave. With accurate timing at each location, the phase difference between each site can be determined, which can then be used for interferometry.

 

[Nugatory]

Mentor's note: a number of posts that have nothing to do with VLBI but merely represent confusion about how antennas in general detect electromagnetic radiation have been removed.

 

[Nugatory]

... two photons at two antennas would be two events at slightly different times.

VLBI does indeed work with multiple events at multiple locations at slightly different times. However, the events in question are not observations of individual photons but rather observations of the phase and amplitude of the oscillating electromagnetic field at the various antennae at various times.

That two antennae in two different locations both detect the signal is no more mysterious than that my local broadcast FM radio station can be heard by every receiver in range.

The methods of quantum field theory can be used to explain how an antenna detects an oscillating electronic field in its vicinity, but as ZapperZ pointed out above, this is completely unnecessary. The classical wave model yields the same results and is far easier to work with.

 

[Nugatory]

This thread is closed, as the original question has been answered.