Also, is light a form of radiation?
Black Holes bend light
Light is a form of radiation - specifically, light is electromagnetic radiation.
Light has zero rest mass, but it does have momentum. Light can travel at the speed of light because it has zero rest mass - nothing with nonzero rest mass can travel at the speed of light.
But it has momentum, which would imply that it has mass…that’s part of the whore “is it a particle/is it a wave” dilemma of light, I think. It shares certain characteristics with classical particles, but also others with classical waves.
Also, light is EM radiation - its just part of the whole spectrum that includes radio waves and microwaves.
I’ve tried several times to get into this thread and the hamsters keep eating my stuff. So, I’ll do this without cites for now.
That’s a tricky question at best. The simple answer is NO light does not have mass. The problem with this is E=MC2 of course which necessitates mass being present in order to have energy, which light has.
To get around this, the common answer is to say that a photon has no mass because it is never at rest. Since photons travel at light speed, they must have NO mass. But, because photons never rest…you see the tautology.
I’ll probably get in deep shit w/ this next step, but here goes. The mass of a photon is supposedly .0000000000000006 of a volt. That is “essentially” ZERO according to popular science. I don’t think that is absolutely ZERO though.
and, While one photon or a trillion photons would still have essentially NO MASS, that is still not ZERO.
Considering the nearly infinite #of photons present in the universe, I think the total mass of all photons past and present would be quite large in fact.
A physics prof. at Harvard actually stopped a beam of light in an experiment, held it captive and then released it. Granted the conditions were drastically altered to do so, but that still shows evidence that the properties we consider constant (ie.light speed)
are in fact variable in different settings.
If there were absolutely NO MASS this constant could not be manipulated to this extent.
But, if you think mainstream science is ready to accept this idea, FORGETABOUTIT
Like I said before, the simple answer to your question is NO light does not have mass. (at least not in the universe as we know it)
This question has been asked and answered before many times before. Here are a few:
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=139654
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=133099
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=120495
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=99682
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=76221
Light has no mass. (Stars will bend light - you don’t need black holes. Finding the proper amount of bending around the sun was one of the ways Einstein’s equations were proven.) Light has momentum. There is no wave/particle “dilemma”. It is both. E=MC[sup]2[/sup] is the wrong equation to use. The right one for momentum is E[sup]2[/sup] = p[sup]2[/sup]c[sup]2[/sup] + m[sup]2[/sup]c[sup]4[/sup]. Check these other threads for the proper equations and detailed answers.
Granted, Black Holes do appear to bend light. I believe science explains it this way.
The light passing near a blackhole appears to be bent, but it is actually SPACE that is warped around the black hole.
How many pages of the book have you made it through, BZ?
You aren’t skipping ahead are you?
I only read the introduction, but I was just thinking about light and what it is.
You’ll have a pretty good handle on this by the time you finish the book. Actually, a good while before you finish.
After you finish that, if you want to know even more about the way light behaves, I have another book for you. Light has alot of interesting properties and behaviours that you may not be familiar with and aren’t completely obvious if you don’t look for it. The one theory that (so far) explains everything we know about the measured behaviour of light is pretty interesting as well.
Of course light has mass. That’s what the Equivalence Principle is all about. Anything which contains energy must also have non-zero mass. E=mc^2 is universal. We don’t have the option of applying it sometimes while saying that it’s violated at other times.
What’s probably confusing people is the “Rest Mass” concept. Light has mass, yet photons have zero Rest Mass. In other words, if we could move along with photons and examine them, we’d discover that they were massless particles. But from all reference frames light is always moving at c, so it always has kinetic energy, and because kinetic energy has mass, all light has mass. The brighter the light, the more it weighs.
However, everyday light has VERY LITTLE mass. Set off a ten-joule camera flash, and assume that all the energy comes off as light. How much does a 10J pulse of light weigh? It’s E=mc^2; using joules of energy, kilograms mass, and meters per second velocity:
10 = m (3x10^8)^2
m = 1.1 x 10^-16 kilograms
Hey, how about H-bombs? Supposing that a few percent of the output of a 10-megaton bomb is made of light, how much does it weigh? 10megaton= 4.2^12 joules weighs 45 milligrams, so the mass of the light emitted by the bomb might be a few milligrams worth.
Hate to disagree with you this time.
Light is consider to have no mass. The amount of matter it would take to convert to a give amount of light is generally quite small and would mass very little.
But, the light itself has no mass. E=mc^2 is the conversion factor for massless particles (that have only energy) to particles with mass. A photon is massless and forever moves at the speed of light. Matter is massive and forever moves at any speed less that the speed of light (ignoring the possiblility, at least by the numbers, that a massive particle could travel faster than the speed of light, but still never cross that speed of light barrier to travel at less.)
There are two concepts in play. Rest mass is the amount of mass something has in its own frame of reference (ie, when it’s still). For a photon, there is no such frame, and the rest mass is zero. This is what is meant when it is said that photons are massless. Relativistic mass is the apparent amount of inertial mass an object has. That is, it’s the value for m that satisfies the classical equation p = mv. This is the mass that’s in the equation E = mc[sup]2[/sup].
Really, mass in not as useful a concept in Relativity as it is in Classical Mechanics. It’s much better, in general, to talk about momentum.
No. Photons are considered to have zero rest mass.
“Very little mass” is totally different than “zero mass.” Can’t have it both ways. If light has any mass at all, then an incredibly bright pulse of light would have significant mass (and everyday light beams seem to weigh nothing only because they’re so wimpy and dim.)
I was under the impression that mass/energy equivalence says that a charged capacitor weighs slightly more than an uncharged one. And a stretched spring weighs slightly more than a relaxed one. The difference is way too small to measure, but since energy and mass are related, a change in energy always means a change in mass.
OK, thought experiment: capture a pulse of really bright light in a closed bottle which has perfectly reflective walls. Which statement is true?
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Since light has no mass at all, the light-filled bottle doesn’t weigh any more than an empty bottle. The bottle won’t weigh differently no matter how much light you put inside.
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Since E=mc^2 applies equally well to optical energy as to any other sort, the bottle will weigh more depending how much light it contains. A pulse of light measuring 10^14 joules will weigh about 1 gram.
Or my favorite: drop a very large antimatter asteroid onto a normal asteroid of equal mass, then gather the resulting gamma ray pulse and form it into a collimated beam (maybe using thought-experiment perfect mirrors?) If this optical pulse passes near you, will you be measurably accelerated by its gravity? Will it act like a travelling black hole, where you feel a “thump” as it races by?
Or to put it backwards, can we make the gravity field of an asteroid go away by converting the entire asteroid into photons? Or does its total mass stay the same before and after total conversion?
Try this, sideways: you don’t have to have mass to have a gravity field.
As mentioned already by Exapno Mapcase,
E^2 = p^2c^2 + m^2c^4 is a much more useful formula than E= mc^2 for understanding this one.
In the case where you are dealing with light, the formula simplies to E^2 = p^2c^2 as light has no mass so the last term is zero.
Considering your thought experiment bbeaty, I will consider your matter/anti matter objects to be stationary and therefore no momentum and the formula reduces to E^2 = m^2c^4.
For coverting the mass of a stationary object to light (photons, light is always photons) you could rearrange things as:
p^2c^2 = E^2 = m^2c^4
This would only be valid if you acheived a perfect total coversion of the matter to photons.
And back to your strobe example… The mass you calculated for the light coming from the strobe is actually a good number. But not for the mass of the light. That is how much you would expect the flash capacitor to weigh in excess of its normal weight before the flash was discharged and that mass was coverted into massless photons (that travel at the speed of light).