XPost: sci.electronics.design
From: PointedEars@web.de
Bill Sloman wrote:
> On 13/02/2026 3:44 am, john larkin wrote:
>> On 12 Feb 2026 16:23:57 GMT, ram@zedat.fu-berlin.de (Stefan Ram)
>> wrote:
>>> john larkin wrote or quoted:
>>>> Do photons have the same gravitational effects as their mass
>>>> equivalents?
>>>
>>> The electromagnetic stress-energy tensor bends spacetime. To see the
>>> Hilbert stress-energy tensor T of a source-free electromagnetic field
>>> F, please see the section "Electromagnetic stress-energy tensor" on
>>> the Wikipedia page "Stress-energy tensor". (I think this also is
>>> equation 4.2.27 in "General Relativity" (1984) - Robert M. Wald.)
>>>
>>> A single photon does not have mass, while a pair or gas of photons can
>>> have mass.
>
> You are confusing rest mass (which a photon lacks) with mass-energy.
There is no such thing as "rest mass" (in modern physics). Rest energy (the
total energy that an object has at relative rest) is sometimes called "mass
energy" because it only comes from mass:
E_0 = m c^2,
where m is mass and c is the speed of light in vacuum. The rest energy of
objects with mass zero, like photons, is zero: m = 0 ==> E_0 = 0. IOW, if
they are free particles (no potential energy), their total energy is (equal
to) their kinetic energy.
> The energy tied up in photon has mass, and that will have a
> gravitational effect.
Nonsense. Energy does not "have mass", and mass does not "have energy".
Those are physical quantities, properties of objects or physical systems,
not objects themselves. One cannot "have" the other.
It is the energy (density) of photons itself that corresponds to the
curvature of spacetime, and *that* explains a (miniscule) gravitational
effect that light has on other objects.
It does NOT explain the deflection of light: light mostly follows the
spacetime curvature that corresponds to the existence of *other* objects
(*their* energy--momentum density), *particularly* those with non-zero mass
(because their rest energy is already large thanks to the factor c^2, and
if the mass density is large, so is the energy density).
>> That's what most sources say: m means mass as used by Newton and
>> Einstein, and is what's in gravitational equations, and photons don't
>> have any.
>
> Einstein talks about mass-energy,
He does not.
> not mass,
He does.
> and clearly didn't have the same idea in mind as Newton had.
That much is true. Based on special relativity he discovered that, contrary
to Newtonian mechanics, objects can have energy even when they are not in
relative motion, due to and proportional to their mass. He wrote in 1905 in
"Does the Inertia of a Body Depend Upon Its Energy Content?" (translated):
"we are led to the conclusion: The mass of a body is a measure of its energy
content" (it is that paper where E_0 = m c^2 comes from, even though he did
not write it this way).
You should read his papers once; they are quite instructive and at least the
first ones are written so that even laypeople can understand them (with some
intellectual effort):
(Unfortunately, the Digital Einstein Archives are being commercialized now,
so free versions of the official translations and the original papers are
currently only available via the Internet Archive and some private
repositories, respectively.)
>> So when two gammas collide and create a particle pair, mass appears
>> where there was none.
>>
>> Right so far?
>
> Wrong.
No, the above is correct. (Contrary to what had been though previously, for
example by Lavoisier, mass is NOT conserved; the _total energy_ is.)
> The first LIGO observation was of two black holes merging
... and has absolutely nothing to do with two *photons* interacting with
each other. Apparently you have no clue what a photon is or what a black
hole is, or you simply cannot read comprehensibly. Which one is it?
> https://en.wikipedia.org/wiki/GW190521
>
> "At 85 and 66 solar masses (M☉) respectively, the two black holes
> comprising this merger are the largest progenitor masses observed to
> date. The resulting black hole had a mass equivalent to 142 times that
> of the Sun, making this the first clear detection of an
> intermediate-mass black hole. The remaining 8 solar masses were radiated
> away as energy in the form of gravitational waves."
Correct.
> Not only do photons have mass, but gravity waves do too.
Wrong conclusion. Also, for the nth time: gravity waves and gravitational
waves are two different phenomena.
And which part of "Followup-To" do you people not understand? *facepalm*
--
PointedEars
Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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