High School Is E=mc^2 a Bidirectional Equation in Physics?

[stevendaryl]

One should also mention that the supposed "conversion" of mass to energy is not a mysterious feature limited to quarks or nuclear reactions. It is true (or not) for all processes: the nuclear ones are where the energies get large enough to beg the question "where the heck did all that energy come from?"

Right. I always found it strange that people look at two hydrogen atoms fusing into a helium atom and saying: “Look, mass is being converted into energy!” and then proving it by adding up the masses of the hydrogen atoms and subtracting the mass of the helium atom.

But you can do exactly the same thing with chemical reactions. If two hydrogen atoms and one oxygen atom combine to form a water molecule, the resulting molecule will have a mass that is less than the sum of the parts. So that’s just as much converting mass into energy as the nuclear fusion case.

 

[vanhees71]

It doesn't matter as long as everybody knows what it means. If you don't like 'rest energy', how about internal energy?

That's usually used in the thermodynamic context. Why not simply call it "mass" (ok, here in PF we have to say "invariant mass", because there are still people liking to confuse themselves with "relativistic mass", which I'd abandon once and for all from the vocabular :-)).

 

[DrStupid]

When we say for example a charge q has electric potential energy or a mass m has gravitational potential energy we don't mean that if the charge q is reduced and converted to energy it will gives us that much electric potential energy.

I'm not sure what you mean because these are bad examples. A charge has no electric potential energy. You need at least one other charge and than the potential energy belongs to all charges and not just to a single charge. The same goes for mass and gravitational potential energy.

But when we say a rest mass m has rest energy mc^2 we mean this thing that if the mass m is reduced it will give us that much energy (of one of the known forms).

No, we mean that a system with the mass m also has the rest energy Eo=m·c². If the mass is reduced by dm than the energy is reduced by dE=dm·c² as well. Both properties are two sides of the same coin.

 

[saddlestone-man]

Here’s an illustration of “converting mass into energy” that maybe explains why I consider that phrase misleading:

Suppose you have an object that has a certain mass, $M$, and it is at rest. What you don’t know is that it is actually two objects, each of mass $m$ connected by a rope, and they are orbiting around each other. The mass of the composite object counts both the masses of the parts, plus the kinetic energy of the parts (plus whatever energy is associated with the rope).

If somebody cuts the rope, the two smaller objects will fly apart. You will find that the two masses don’t add up to the mass of the original object. Ignoring the energy of the rope, for simplicity, you would find that $M c^2 \approx 2 mc^2 + KE$, where $KE$ is the sums of the kinetic energies of the two smaller objects.

So you might say: “Some of the mass of the original object was converted into kinetic energy.” That’s sort of right, and sort of wrong. The original mass already counted the kinetic energy of the parts. No conversion happened.

Does this imply that if this experiment is going on inside a sealed box and the box is being weighed on some sensitive scales, then we will detect the rope being cut by a reduction in weight of the box and its contents?

 

[DrStupid]

Why not simply call it "mass"

I would support that. To ban the other property that must not be named due to it's equivalence with energy but keeping both, mass and rest energy is inconsistent.

 

[vanhees71]

Also this strange idea of "converting matter to energy" is nonsense. It comes from the popular-science literature and usually describes something like the annihilation process $\text{e}^++\text{e}^- = 2 \gamma$. Of course there's also the inverse pair-production process $2 \gamma \rightarrow \text{e}^++\text{e}^-$ (at least in principle, I'm not sure, whether this has been really seen in the lab using real photons, i.e., literally this process of pair production with 2 photons in the initial state).

 

[Herman Trivilino]

Does this imply that if this experiment is going on inside a sealed box and the box is being weighed on some sensitive scales, then we will detect the rope being cut by a reduction in weight of the box and its contents?

The mass of the box is the sum of the masses of the constituents, plus the kinetic energy of the constituents.
So, no, there would be no change in the reading on the scales.

 

[Herman Trivilino]

Summary:: We know that matter can be converted into energy, but can energy be converted into matter?

Not matter, mass. Mass is NOT a measure of the quantity of matter. That is one of the lessons of the Einstein mass-energy equivalence. Another is the existence of rest energy.

Routinely, subatomic particles collide with each other, producing more particles. You can add up the masses of the original particles and find that in many cases it's less than the sum of the masses of the produced particles. The kinetic energy of the original particles is converted to the rest energy (mass) of the produced particles.

 

[SiennaTheGr8]

We've had this mass vs rest-energy discussion before.

If you know what you're talking about then it's just semantics and it doesn't matter. And obviously "mass" isn't going anywhere; it's been around forever, it's shorter, and it doesn't come with a subscript.

But I do think "rest energy" (or "invariant energy" or "proper energy") is the more useful term for beginners. The whole wonderful point of $E_0 = mc^2$ is the unification of two seemingly different concepts—mass and (rest) energy. Continuing right along using "mass" on the one hand and "kinetic energy"/"total energy" on the other obscures that revolutionary and central insight. It needs to sink in that "mass" and "rest energy" are truly redundant concepts, and that you can rewrite even your Newtonian-limit equations with $E_0/c^2$ instead of $m$ if you'd like. The mysterious "measure of inertia" and "gravitational charge" that puzzled physicists for centuries turn out to be nothing but a thing's energy-content (approximately, in the Newtonian limit).

Then, "mass" carries some misleading baggage with it that takes a while to push out of your mind: an independent conservation law, an association with matter ("how can a box of photons have mass?"), and an expectation that it should be additive ("isn't mass just the amount of matter?"). "Rest energy," by contrast, carries none of that baggage and actually suggests the right stuff—that it, like any other kind of energy, isn't independently additive-and-conserved but rather must be summed with all the other kinds of energy to arrive at the conserved total energy (every high-school physics student does this with kinetic and potential energy). It's also perfectly intuitive that a system's total energy in its rest frame should give you its "rest energy." Oh, and you completely sidestep these misconceptions about "converting" between mass and energy, because it's already clear we're always really talking about converting energy from one form to another.

A strong counterargument I've seen here is the conceptual benefit of using an altogether different term for a four-vector's magnitude than for a four-vector's component. But beginners aren't typically learning (or ready for) four-vectors out of the gate.

 

[votingmachine]

Summary:: We know that matter can be converted into energy, but can energy be converted into matter?

Hello All

Does the most famous and elegant equation in physics imply bidirectionality in the way it describes the relationship between energy and mass?

There are many examples of matter being converted into energy (stars, bombs, reactors, etc), but can energy be converted into matter? At the point of the Big Bang, I presume this is what happened, but has it happened since, and has it been reproduced in the laboratory?

best regards ... Stef

Supernova's create heavy elements. Fusing heavy nuclei together decreases energy and increases mass with the e=mc^2 conversion. Human efforts to produce high atomic number elements in particle accelerators are another example of an proces for conversion of energy to mass. The really heavy elements don't last, and rapidly fission reversing that process.

 

[Telemachus]

Not exactly. Photon-photon interactions are incredibly hard to do. But you can certainly do stuff like take a couple of low mass particles and slam them together at high speed, and you may get high mass low speed particles out - energy to mass (at least in some senses). CERN does that all the time.

Hi. Doesn't interference effects account as photon-photon interaction? I think you mean something like pair creation, but wouldn't a simple double slit interference phenomenon account also as a photon-photon interaction? thanks.

 

[Nugatory]

Hi. Doesn't interference effects account as photon-photon interaction? I think you mean something like pair creation, but wouldn't a simple double slit interference phenomenon account also as a photon-photon interaction? thanks.

It does not. The quantum mechanical interference that appears in the double slit experiment is a photon interfering with itself: we're sending photons one at a time towards the barrier with the slits, each photon makes a single dot on the screen, and the interference pattern builds up slowly as more dots in one area than another.

The classical interference pattern that can be seen on the screen as a series of dark and light bands, as in Young's experiment very early in the 19th century, is observing intereference of classical electromagnetic waves, no photons involved.
(Further discussion of how photons and light aren't the same thing should probably happen in a new thread in the Quantum Physics subforum).

 

[PeroK]

Hi. Doesn't interference effects account as photon-photon interaction? I think you mean something like pair creation, but wouldn't a simple double slit interference phenomenon account also as a photon-photon interaction? thanks.

In the double slit experiment each photon interferes with itself, rather than with other photons. That's why you get an interference pattern even with one photon at a time.

 

[Ibix]

Hi. Doesn't interference effects account as photon-photon interaction? I think you mean something like pair creation, but wouldn't a simple double slit interference phenomenon account also as a photon-photon interaction? thanks.

Interference isn't photon-photon interaction in this sense. Neither photon changes the other - it's just their probability amplitudes adding where they overlap.

The point is that if you shine two laser beams at an angle so they cross, there is no effect on the beams downstream of the crossing. If you used electron beams instead they'd disrupt each other where they crossed. You could tell by looking at one beam downstream of the crossing point whether the other beam was turned on or not because of the electrons interacting with one another at the crossing. Not so with lasers.

Seeing @Nugatory's response, I won't post more here.

 

[Buzz Bloom]

There is no such thing as converting matter into energy.

EDIT completed

I found the concept in the above quote a bit confusing.

1. When an electron hits a positron, is it not a possibility that the result is photons? If so, then are the resulting photons matter rather than energy? My intgerpretaion of the quote is that the answer is YES.

2. Since a photon has no rest mass, I had previously to the quote assumed it was not matter, but just energy with some properties, like wavelength for example. If this is incorrect, and photons are matter with mass, then does a photon's energy's mass equivalent alter space so as to effect nearby matter particles gravitationally - that is effect the motion of an electrically neutral particle (e.g. a neutron)?

 

[weirdoguy]

I assumed it was not matter, but just energy

Energy is a property of things, not the things itself. Photons have many other properties besides energy.

If this is incorrect, then does a photon's energy's mass equivalent alter space so as to effect matter particles gravitationally

I don't see any reason to invoke "energy's mass equivalent". What curves spacetime is full stress-energy tensor, so energy alone is sufficient, no need of mass.

It a photon matter?

Depends on whom you ask. For me all gauge fields represent matter, but I've seen that some people don't call them matter.

 

[Orodruin]

but just energy with some properties

This is the fundamental misunderstanding. Energy is a property of things, not something that has other properties.

 

[PeterDonis]

Since a photon has no rest mass, I had previously to the quote assumed it was not matter, but just energy

Objects with no rest mass, like photons, are usually called radiation in contexts like this. That is a different category from "matter", but it is also not "just energy with some properties", any more than matter (objects with nonzero rest mass) is.

 

[Buzz Bloom]

I don't see any reason to invoke to "energy's mass equivalent". What curves spacetime is full stress-energy tensor, so energy alone is sufficient, no need of mass.

I have only very little experience with tensors. What would the tensor equation look like describing the result of the motion of a neutron when a photon with a very large amount of energy passes near by the neutron? How would this motion of the neutron compare with the motion if there was no photon passing by?

Perhaps I can simplify this. I apologize for using Wikipedia as a source of information.

Energy is [a] quantitative property. A photon is an elementary particle. Photons are massless. (They have energy.) Matter is any substance which has mass. (@PeterDonis makes clear that photons are radiation.) Consider a flat space which is empty of all matter and radiation except for a single photon. As the photon travels, what does the corresponding tensor tell us about the time changing shape of space as the photon travels along a straight line? I am guessing that when the photon is at the origin x=y=z=0 the space shape at that time moves with the photon, both forward and backward in time.

 

[Ibix]

What would the tensor equation look like describing the result of the motion of a neutron when a photon with a very large amount of energy passes near by the neutron?

No one knows. To know that would require a theory of quantum gravity, which is why we exhort people not to talk about photons in the relativity forum but talk about a light pulse. For classical light beams, you write down the EM stress-energy tensor, $$T^{\mu\nu}=\frac 1{\mu_0}\left(F^{\mu\alpha}F^\nu{}_\alpha-\frac 14 g^{\mu\nu}F_{\alpha\beta}F^{\alpha\beta}\right)$$where $F$ is the Faraday tensor. Then you slot it into the Einstein Field Equation and solve. Which is messy. Some solutions for radiation are known - I believe you need to search for pp-wave spacetimes.

How would this motion of the neutron compare with the motion if there was no photon passing by?

You'd have to solve the equations (probably numerically). I don't know the answer for any particular radiation-only spacetime off the top of my head.

 

[Ibix]

For example: the awesomely named Aichelburg-Sexl Ultraboost.

 

[votingmachine]

EDIT completed

I found the concept in the above quote a bit confusing.

1. When an electron hits a positron, is it not a possibility that the result is photons? If so, then are the resulting photons matter rather than energy? My intgerpretaion of the quote is that the answer is YES.

2. Since a photon has no rest mass, I had previously to the quote assumed it was not matter, but just energy with some properties, like wavelength for example. If this is incorrect, and photons are matter with mass, then does a photon's energy's mass equivalent alter space so as to effect nearby matter particles gravitationally - that is effect the motion of an electrically neutral particle (e.g. a neutron)?

I read his comment differently. Rather like a discussion of ice and water, where someone says there is one thing with two forms. Not a denial of either form, but that there is one thing, spoken of in two ways.

 

[Buzz Bloom]

That would require a theory of quantum gravity, which is why we exhort people not to talk about photons in the relativity forum but talk about a light pulse

Hi Ibix:

I have two questions I would like to ask.

1. Could the Electro-Magnetic (EM) field approximating a short pulse of radiation moving along the x-axis be used to simulate a photon? By "short" I mean a single sinusoid of length along the x-axis.

2. Given an appropriate 3D EM field corresponding to the (1) pulse shape, what is your guess about there being (or not) any distortion to the flat space geometry?

Regards,
Buzz

 

[Ibix]

I read his comment differently. Rather like a discussion of ice and water, where someone says there is one thing with two forms. Not a denial of either form, but that there is one thing, spoken of in two ways.

No - matter and energy are not two forms of the same thing. Matter and radiation are more or less equivalent concepts and you can turn one into the other and vice versa. Mass and energy are properties of matter and radiation, but you can't turn matter into energy any more than you can turn matter into purple.

 

[Ibix]

1. Could the Electro-Magnetic (EM) field approximating a short pulse of radiation moving along the x-axis be used to simulate a photon? By "short" I mean a single sinusoid of length along the x-axis.

You can certainly describe a short burst of radiation - the Gaussian pulse described in the wiki article I linked does that, just set $a$ large. Whether this simulates a photon or not, I repeat, we do not know. It would require a theory of quantum gravity to tell us what a photon's gravitational field looks like and we don't have one. So whether a very large $a$ actually describes anything realistic or not, who knows?

Given an appropriate 3D EM field corresponding to the (1) pulse shape, what is your guess about there being (or not) any distortion to the flat space geometry?

Of course spacetime isn't flat in the presence of any radiation. Flat spacetime means $G^{\mu\nu}=0$ which means $T^{\mu\nu}=0$ which means $F^{\mu\nu}=0$ which means no EM field. I just don't know exactly what look like in that spacetime without running the maths.

 

[Buzz Bloom]

Of course spacetime isn't flat in the presence of any radiation.

I was thinking only of flat Euclidean 3-space, not space-time. I was thinking that if the photon has a gravitational effect, it would be similar to a matter particle's gravity effect distorting space.

 

[Ibix]

That would depend on how you're defining "space". The spacetime doesn't manifestly have a timelike Killing field (it might have one, but it's not obvious from the metric) so it may not have a definition of "space" unless you want to impose one. So my answer is somewhere between "I don't know" and "the question doesn't have a meaningful answer for this spacetime".

 

[Buzz Bloom]

Hi Ibix:

I very much appreciate your resposnses to my questions. I believe I learned something of value, although not what I was hoping for. I have learned that the subject is too over my head for me to have any intuitive feel for the complexity. I had the same problem when I was trying grasp QM interpretations. So far, even at my advanced years, I still feel comfortable trying to improve my understanding about cosmology.Buzz

 

[PeterDonis]

What would the tensor equation look like describing the result of the motion of a neutron when a photon with a very large amount of energy passes near by the neutron? How would this motion of the neutron compare with the motion if there was no photon passing by?

Perhaps I can simplify this. I apologize for using Wikipedia as a source of information.

Energy is [a] quantitative property. A photon is an elementary particle. Photons are massless. (They have energy.) Matter is any substance which has mass. (@PeterDonis makes clear that photons are radiation.) Consider a flat space which is empty of all matter and radiation except for a single photon. As the photon travels, what does the corresponding tensor tell us about the time changing shape of space as the photon travels along a straight line? I am guessing that when the photon is at the origin x=y=z=0 the space shape at that time moves with the photon, both forward and backward in time.

All of this looks to me like a confused way of asking the question: how does light gravitate?

Since you have very little experience with tensors, you should probably not even try to formulate the question more specifically. You should certainly not be trying to guess what the answer is, or in what terms the answer is going to be usefully phrased. Terms like "time changing shape of space" are not useful terms.

Also, you should beware of using the concept of "photon" in a classical context, which is the relevant context for this forum. (If you want to ask questions about the quantum nature of light, those questions belong in the quantum physics forum.) In a classical context, the best concept for what you appear to be interested in is "light pulse" or "null worldline". Such a thing is not a "photon" except in the most informal, colloquial sense.

With those caveats, the general answer to the question "how does light gravitate" is to look at electrovacuum solutions to the Einstein Field Equation, i.e., solutions in which the stress-energy tensor is purely that of a vacuum electromagnetic field. If you want to take "light" to mean specifically electromagnetic radiation, instead of allowing any electromagnetic field, then you would look at the subset of electrovacuum solutions in which the electromagnetic field is a radiation field (as opposed to, say, a static Coulomb field, as in the Reissner-Nordstrom charged black hole solution), or what is called a "null electrovacuum" in this Wikipedia article:.

https://en.wikipedia.org/wiki/Electrovacuum_solution

To see how individual test objects (like your "neutron") are affected by the gravitation of light, you would look at timelike test particle geodesics in whatever null electrovacuum solution you are using as your model.

One further note: all of this is way beyond a "B" level thread, so if you are really interested in it, you should start a separate thread.

 

[PeterDonis]

No one knows.

That's a bit strong; there are classical solutions describing how light gravitates and how that affects the motion of test particles. I described them in general terms in post #60 just now. (The pp-wave spacetimes you mention are, IIRC, one of the general classes of null electrovacuum solutions.)

It is true that we don't have a theory of quantum gravity so we don't know how quantum effects might (or might not) change the classical behavior. But, as I noted in post #60, discussion of the quantum case belongs in the quantum physics forum, not here.