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Physics, Astronomy & Cosmology / Re: How long does a gravitational wave last?

« on: 24/11/2021 03:25:56 »

I am still  wondering if we can still say  that these very asymmetric  "rings" carry away their  energy without  any loss of power at all  as they encounter obstacles in their path.

A spiral is still symmetric, just not spherically symmetric.
By time-symmetry, if matter (say a binary star) can emit gravitational waves and lose energy in doing so, then such a system can hypothetically absorb them, gaining energy in the process, but the passing wave is not going to be focused in such a way that any measurable effect will occur. It would have to look like the time-reversed wave spiraling in, which isn't how it looks when it was emitted from afar.

So would a neutron star or another black hole absorb their energy?

A lone mass like a  neutron star which seems no more capable of absorbing them as emitting them. Any mass will however deflect the waves, breaking the symmetry like water waves crossing a shallow spot. A black hole must absorb the energy as there is no worldline through them. They're also the only things that absorb dark matter.

Or does the gravitational wave go through these objects as if they were not there?

Like dark matter, it goes through them like they were transparent, but still deflected by the gravity.

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Physics, Astronomy & Cosmology / Re: A particle in 2 places at once?

« on: 05/11/2021 22:45:25 »

You seem to me to be be saying or implying that ,if measurements were finer and more efficient that the position and the momentum of a particle could be determined separately and not as a pair that are joined at the  hip

No, I’m not saying that, as Alan has explained very clearly.
What I’m saying is that the analogy @Harri was giving is inappropriate. There are two factors at work here, one is our state of knowledge of a particle because we can’t see it in the same way as looking at a player on a field. We see the player on the field because photons bounce off him and hit our eyes, those photons don’t move him or alter his momentum significantly, but they do if the player is an electron. One way we can detect an electron is to have it hit a detector eg phosphor screen, but then it’s stopped moving. Even if we use a speed gun on the player s/he has to move in order to get a doppler reading. These are the physical and practical problems.
The second problem is answered by Alan.
Reread Alan’s reply https://www.thenakedscientists.com/forum/index.php?topic=83459.msg659621#msg659621
There is an intrinsic limit to how accurately we can know these 2 properties at the same time and is quite different from the measurement problems already described. This is a limit set by the way the universe works. Reread both posts by Alan as he has put down very clearly what is an endless source of confusion to most non physicists, and unfortunately a few physicists!
For a large object like a ball kicked by a player the difference is so small that it is irrelevant around 10^-30m.

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Physics, Astronomy & Cosmology / Re: A particle in 2 places at once?

« on: 05/11/2021 22:14:24 »

You seem to me to be be saying or implying that ,if measurements were finer and more efficient that the position and the momentum of a particle could be determined separately and not as a pair that are joined at the  hip

Quite the opposite. If you know the position of a particle to infinitesimal precision, you have no information as to its momentum. People look different when they are running compared with standing still, but if you photograph a car with a very short flash, you can't tell whether it is moving forwards, backwards, or stationary Cameras have advanced to the point that you can now get "propellor disc blur" software so that photos of classic aircraft in flight look different from stationary models, but a true "snapshot" gives you no clue as to its speed.

So far, so intuitive. But intuition breaks down if you know that an electron is absolutely stationary, Heisenberg says in that case, you can have no idea where it is!

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Physics, Astronomy & Cosmology / Re: Could mass be considered as an excitation of the gravitational field?

« on: 09/10/2021 16:47:44 »

way of looking at the relationship between mass and the gravitational field

You'll have to describe what gravitational field you're speaking of. There is a gravitational potential field, which is a relative (not absolute) scalar at any point, and there is the gravitational strength field, which is the derrivative of the potential field. This is not relative, and is a vector at any given point.

Anyway ,does my question have any merit?

I think so. Mass certainly effects both, but expressing it that way differs in some ways from the typical 'excitation of field' description. Most importantly, excitations imply positive energy relative to the non-excited state, but it is negative energy with mass present. Empty space with no mass in it has more (zero) energy than does the negative energy of the same space with an object in it.

The 'excitation' doesn't move, but changes to the distribution of it is something that travels at light speed, hence gravitons and gravitational waves, which very much are excitations and fields. So there is a field, but excitations of it move at lightspeed, and gravity doesn't do that any more than gravitons are responsible for the attractions between planets.

Also,however defined it, can we imagine the gravitational field to have been established in its initial form  after the BB and to be metamorphosing (changing shape) ever since

Sounds good. The potential has been going up ever since the initial maximum negative value of the big bang. The strength on the other hand has been increasing as the energy distribution changed from completely uniform to today's very localized concentrations of mass/energy.

I think Einstein  may have used the "mollusk" description about it.

Einstein seemed to use the word to describe arbitrary (abstract) coordinate systems (ones that are not inertial or not in Minkowskian spacetime) and not about physical fields, gravitational or otherwise. That's how I read it at least.

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Physics, Astronomy & Cosmology / Re: Are possibly all massless particles just travelling waves in particle fields?

« on: 06/08/2021 09:50:48 »

Who ,I wonder was or were  the  inventors of QFT? Some unholy collaboration,I'd guess.

If science were a religion then this would be a holy collaboration.
In fact as ES says it has been a progressive development over many years, starting as far back as the 1920s. What is now QFT is the modelling that underlies what we call the standard model so you will find that it is built on the work of some very famous names, each contributing a part, and some of those parts are famous in their own right eg QED. That’s how science often works.

I didn't realise it could be used to predict scenarios in the real world
I thought  this was ivory tower territory.

Although often called theoretical physics, much of the theory is in response to the results of experiments. If the theory didn’t fit the experimental results and predict accurately, it would be discarded or changed; and that really is how science works.
It’s worth repeating what has been said, that the current theory is very successful at predictions.

I think by particle fields you may have meant multiple fields, which is correct and as ES says, each particle has its own field. The reason for this is that each particle (the interaction we detect) has its own set of properties so if you are going to describe them and their interactions using a field model then each model will have its own set of properties and the particle will be a quantum of that field.

Waves? The field model supersedes the old idea of wave particle duality (which is still used for elementary teaching) and gets over questions like which slit did the photon go through. However, particles have wave properties and this has been demonstrated for those with mass as well as massless eg electrons. The quantum objects we describe as particles have wave properties (eg they interfere) and particle properties (eg mass or momentum) but they are neither. It’s important to remember that these are models and it is important not to assign a classical reality to them. However, if it helps you to imagine them as waves there is little harm as long as you bear in mind what Stephen Hawking said about virtual particles “just don’t take them too literally”.

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Physics, Astronomy & Cosmology / Re: What happens to the spacetime interval between two events as they approach a BH?

« on: 20/02/2021 16:19:26 »

Is it fair or noteworthy  to say all these events in GR are treated as point objects even though they are actually spatio-temporally extensive?

Events are by definition mathematical points in spacetime, say the event of a photon being emitted and such.  In practice, yes, they're spread out, so one can speak of the event of the sinking of the Titanic even though the ship is fairly large and moved around quite a distance during the hours-long process. It just means we lose precision when we talk about that event.
Billiard balls are modeled as point events (the event of the contact between two balls) when in fact the contact takes finite time, involves finite (but very large) acceleration, and finite surface area of contact.

Is that where a theory of  quantum gravity might be useful?

It's just geometry. No, quantum gravity solves other issues.

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Physics, Astronomy & Cosmology / Re: How do electromagnetic waves propagate through a vacuum?

« on: 30/09/2017 19:11:07 »

I thought fields extended indefinitely in space.That image seems to show the two fields  extending a finite distance .Do they actually extend indefinitely in the direction at right angles to the direction of propagation? (the image being for illustrative purposes)

Electromagnetic fields do extend indefinitely into space. What you are seeing in the image is the amplitude of the photon, which is different from the range of the field that makes up the photon. Amplitudes are finite. Even for a hypothetical, infinite ocean, the waves in the ocean will have a limited amplitude.

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Physics, Astronomy & Cosmology / Re: Is spacetime real?

« on: 26/09/2016 22:07:20 »

geordief, your skepticism of Atkhenaken's claims are well-warranted. I would recommend being highly skeptical of those who make such extreme claims (look up some of their other posts, and you might see a pattern develop) Atkhenaken does not believe that viruses exist, or that medicine is anything other than a scam...

To those who are interested in this thread, I would also recommend paying attention to PmbPhy. He is very knowledgeable regarding physics, especially relativity (we have our disagreements too, but on this particular subject, he is an expert!) evan_au is also a knowledgeable and well-intentioned member of this forum.

Thanks ,yes of course . I look forward to PmbPhy's contributions down the line hopefully.

The phrase "logical certainly " certainly rings alarm bells  as the stand up performer  Tim Vine  might put it

https    ://www.youtube.com/watch?v=HcFd5j1cios

Perhaps I don't have enough posts to post a link on this site yet?

Yeah, I don't remember what the required number of posts is before you can link urls directly, but I think you are close. Maybe 20?

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Physics, Astronomy & Cosmology / Re: Is spacetime real?

« on: 26/09/2016 20:16:36 »

Having drifted into talking about particle spin, this might be worth looking at.  I’m not always happy with the answers here, but this seems not bad.

http://www.askamathematician.com/2011/10/q-what-is-spin-in-particle-physics-why-is-it-different-from-just-ordinary-rotation/