[mxplxxx (OP)]

It's interesting that the questioner persists in telling us that something can't be done

Not at all. Where have I said this? I am interested in measuring the energy of a single photon - that is the question. Have/can you answer it? According to Chiralpso, "That is correct. There is no good way to directly measure the energy of a single photon." I wonder, is there any way to directly measure the energy of anything?

The answers I have been getting refer to measurig the energy of particular types of photons in particular sets of circumstances. Nothing like a generalised  PhotonMeter that I would have thought would have been dead easy to construct.

ps I would prefer to be referred to as mxplxxx rather than the very unfriendly "the questioner"


 

[chiralSPO]

mxplxxx, I think you are misinterpreting my response. I said it is not easy to directly measure the energy of a photon, and that we make these measurements indirectly. But that doesn't mean that the measurement is any less valid (because we have rigorously established the validity of these indirect measurements).

When dealing with single particles, it is very rare that a direct measurement will be useful (think about what indirect and direct mean--how likely is it that machine the size of my head will be able to interact directly with a single elementary particle, and extract this information without having any working intermediate bits between the photon and the little LED readout that tells me what the energy is.)

[chiralSPO]

To clarify my point about direct vs indirect:

I think we can agree that when I talk to someone on the phone, that is a form of indirect communication: the sound waves of my voice are captured by a microphone, converted into electronic signals, which are then converted into electromagnetic radiation, before being received, turned back into electronic signals, which the speaker reconstitutes into my voice (this is a very simple version).

But what I urge people to consider is that talking face-to-face is also indirect. My vocal chords vibrate, bumping into air molecules, which bump into air molecules, which bump into air molecules, which bump into air molecules, which bump into air molecules, etc. etc. until those air molecules reach the ear of my listener. But wait, we're not done yet! The ear is really complex. Those moving molecules bump into their ear drum, causing vibrations that then emanate from the eardrum, and goes into a funny spiral-shaped organ, that is able to physically do a Fourier transform of the vibrational frequencies (it sorts them out by frequency due to how easily the sound waves are able to go around a bend), ultimately detecting those vibrations with little hair-like structures, that then encode the vibrations into neural signals, to be processed by the listeners ear.

My point is: when considering information transfer on a molecular scale or smaller, one must accept that there is not really any such thing as a direct measurement, other than the actual interaction between two particles (and there is no way for that kind of interaction to directly transfer the information from a single particle to any sort of macroscopic device.)

[Colin2B]

My point is: when considering information transfer on a molecular scale or smaller, one must accept that there is not really any such thing as a direct measurement, other than the actual interaction between two particles (and there is no way for that kind of interaction to directly transfer the information from a single particle to any sort of macroscopic device.)

This is an important point because many of the measurements we make are indirect.
Eg voltage is often measured by the movement of a coil inside a permanent magnet, moving a pointer. Even our weight is often the deflection of a spring.

[alancalverd]

The answers I have been getting refer to measurig the energy of particular types of photons in particular sets of circumstances. Nothing like a generalised  PhotonMeter that I would have thought would have been dead easy to construct.
ps I would prefer to be referred to as mxplxxx rather than the very unfriendly "the questioner"

A bolometer will in principle directly measure the energy of any and all photons. As with rockets, the science is indeed dead easy but the engineering is extremely difficult. It took my team 15 man-years to measure the absolute energy of a radiotherapy photon beam to a reproducibility and traceability of 0.1%.

And it all depends on what you mean by "directly". Time is what comes out of the NIST primary standard atomic clock, but you wouldn't use it to decide when to stop boiling an egg. Length is defined in terms of time and the speed of light, but you wouldn't use radar to buy curtains. So we use different gadgets to measure the energy of different photons, and most easily, in a tiny weeny bit of the electromagnetic spectrum, we just use our eyes.

Interesting to note that an unpronounceable assembly of consonants can be considered a friendly form of address. But  Llanfair­pwllgwyngyll­gogery­chwyrn­drobwll­llan­tysilio­gogo­goch is probably the only word in any human language that  has three consecutive l's, so I'll assume our friendly questioner is Welsh. Iechyd da.

[mxplxxx (OP)]

Llanfair­pwllgwyngyll­gogery­chwyrn­drobwll­llan­tysilio­gogo­goch

Full meaning: “Saint Mary’s Church in the hollow of the white hazel near the rapid whirlpool and the Church of Saint Tysilio of the red cave”.

[mxplxxx (OP)]

Llanfair­pwllgwyngyll­gogery­chwyrn­drobwll­llan­tysilio­gogo­goch

Full meaning: “Saint Mary’s Church in the hollow of the white hazel near the rapid whirlpool and the Church of Saint Tysilio of the red cave”. Actually, I am 4th generation Australian.

[mxplxxx (OP)]

According to relativity, time does not exist for such an object

Actually (I think:)), the photon itself will experience the universe as if it were stationary. An observing object will experience the photon as travelling at the speed of light but also as ageless. Which is just great for (guess) the photon's purpose in life which is to transfer state information between fermions.

[alancalverd]

Iechyd da = g'day, sport, or cheers, mate. Much as I respect a culture whose origins predate mine by about 40,000 years, I can't accept that photons have a purpose in life. That's taking anthropomorphism beyond the ridiculous. When a photon goes walkabout, it has no idea of where it's going or why - it's just a teenage drongo rushing out of the house to get rid of some energy.

And an observer doesn't see a photon as ageless! Photons of considerable age will be redshifted according to Hubble's Law, so the energy of a photon compared with the expected energy of a freshly minted one from the same quantum process gives us an idea of how long it has been in the intergalactic outback.

Irrelevant perhaps, but the Celtic branch of my family held a wedding in LlanfairPG, presumably in the hope that the English would be too embarrassed to buy a train ticket, and thus save a bit on the bar bill. But we did.

[mxplxxx (OP)]

Much as I respect a culture whose origins predate mine by about 40,000 years,

My original ancestry is Irish. We have been in Australia since about 1850. Unfortunately, my adopted country was invaded by the British in 1788. The original inhabitants had been there for about 40000 years. They had a very advanced culture that embraced living in harmony with their environment. Each aboriginal child could speak 5 languages. Unfortunately the non-technological nature of the culture made it no match for the British army. An apology for past wrongs has recently been made in the Australian parliament. 

[mxplxxx (OP)]

And an observer doesn't see a photon as ageless! Photons of considerable age will be redshifted according to Hubble's Law, so the energy of a photon compared with the expected energy of a freshly minted one from the same quantum process gives us an idea of how long it has been in the intergalactic outback.

I am aware of this. It seems to contradict relativity. Actually it may not. An EM wave is really three interacting waves/particles) ... but I will discuss this more in  New Theories, as soon as I have worked out how they interact:).

[mxplxxx (OP)]

I can't accept that photons have a purpose in life.

Bosons, of which photons are a particular type, are to the universe what events are to a finite state machine. From that perspective, they have the purpose of changing the state of the current particle based on a change of state of another (usually) related particle. Actually, it is likely that the particle reacts to the photon rather than the photon changing the particle. Or it may be an interaction of the two.

[alancalverd]

This could go two ways, equally entertaining.

If the British (which at the time included the Irish) hadn't invaded Australia in 1788, I doubt that your family would have emigrated there in 1850, so whether you are grateful, apologetic, or both, is a matter way beyond the scope of a science chatroom. Anyway it's a fascinating place and I'm looking forward to visiting my grandchildren just as soon as  it cools down.

Relativity gives us all sorts of reasons for redshift, starting from the simplest Doppler effect.

As far as bosons are concerned, or anything else for that matter, it's important to note the ambiguity of "function". Colloquially, it certainly involves intended purpose, but scientifically it merely states the difference between before and after.

[tanujagdale]

Great information..

[mxplxxx (OP)]

Anyway it's a fascinating place and I'm looking forward to visiting my grandchildren just as soon as  it cools down

Your kids have good sense! Melbourne (for ages voted the world's most livable city) , where I live, enjoys warm summers, glorious springs, mild autumns and crisp winters. Today was just glorious at 30 degrees:). Some of the best research in the world happens in Melbourne. Esp. medical - e.g. the bionic ear that allows me to hear was invented here.

I will research red shifts and relativity. Thanks for the info.

Actually, it seems to me the red shift would be different for different observations of the same photon (if that were even possible). Which makes the properties of a photon something that is shared between an observer and the photon. But that is ridiculous isn't it!

[yor_on]

no, that's perfectly correct. the energy you measure a photon is dependent on what frame of reference you have, being 'at rest' with it or 'moving' relative it. To define a intrinsic energy you have to be 'at rest' with your experimental setup. Light can both be said to 'red shift' and 'blue shift' depending on ones frame of reference, measuring it. It's in principle similar to two cars colliding, depending on relative speed(s), but in the photons frame we better avoid talking about 'mass'.

[mxplxxx (OP)]

no, that's perfectly correct. the energy you measure a photon is dependent on what frame of reference you have, being 'at rest' with it or 'moving' relative it. To define a intrinsic energy you have to be 'at rest' with your experimental setup. Light can both be said to 'red shift' and 'blue shift' depending on ones frame of reference, measuring it. It's in principle similar to two cars colliding, depending on relative speed(s), but in the photons frame we better avoid talking about 'mass'.

Whoahhh!:)

[yor_on]

One thing though. The speed of light in a vacuum as defined in physics is always 'c', so the 'red' respectively 'blue shift' then can be seen as a equivalence to the 'speed' you would use when it comes to two cars colliding, although the 'red shift' is different in that you now may measure the photon to have a lower 'energy' than it will have being 'at rest' with it. It's somewhat like two cars moving in the same direction colliding but not really as it there will be mass and speeds that define the 'energy' and their respective mass won't go 'negative' if you see what I mean. With a photons red shift it can though if we use the energy' you would measure being at rest (experimental setup) as a intrinsic energy of it. That won't happen with those cars :)

[Colin2B]

Great information..

But not such a great attempt to spam us.
Your website and signature have been changed to ours and - at the speed of light - you are banned.

[mxplxxx (OP)]

no, that's perfectly correct. the energy you measure a photon is dependent on what frame of reference you have, being 'at rest' with it or 'moving' relative it. To define a intrinsic energy you have to be 'at rest' with your experimental setup. Light can both be said to 'red shift' and 'blue shift' depending on ones frame of reference, measuring it. It's in principle similar to two cars colliding, depending on relative speed(s), but in the photons frame we better avoid talking about 'mass'.

Whoahhh!:)

Actually, it seems that energy is relative, so measuring the energy of a photon can only be done relative to the energy of another particle. Like an electron! ... but what is the energy of electrons measured in relation to? Relativity a can of worms:) . See also https://www.thenakedscientists.com/forum/index.php?topic=75704.msg566749#msg566749