# Q & A: Mass/Light relationship

Q:
I might be a little to old to ask... But here we go if light has any mass at all. Can you condense that mass or increase the mass? If you can increase the mass wouldn't you be able to use that condensed light to push an object? Say that you have huge light bulbs and have them shine in a space where light has only a small exit? since light is always moving? I guess the only problem you would have is intense heat... Lol alright I was just courious about my therory...
- Joshua Spudeno (age 27)
Shillington, Pa, USA
A:
Many very accurate experiments have been performed trying to detect the so-called 'rest mass' of light but there is no evidence that there is any.   Light, however, does carry momentum and thus can push objects around.     There is even an idea of using the light from the sun to propel an interplanetary space craft.  I don't think it will be implemented soon.
See
LeeH

(published on 06/06/2008)

## Follow-Up #1: solar sails

Q:
- Drew (age 23)
blacksburg va
A:
thanks, Mike W.

(published on 07/01/2008)

## Follow-Up #2: Moving things with light pressure

Q:
Since light can supposedly move objects, does it affect us and does it affect large objects such as planets? For example, light from the sun and numerous other stars is constantly hitting quite a large area of Jupiter, would this affect Jupiter's orbit?
- Ryan (age 13)
California
A:
Yes, light does carry momentum and when it is absorbed or reflected from an object a force is exerted on that object.  This force is quite small and in the case of moving Jupiter around, it is negligible compared to the force of gravity from the sun.  For a very light object, however, the force can become significant.  Some people have considered using 'Solar Sails' to navigate the solar system.
See:

LeeH

(published on 03/19/2009)

## Follow-Up #3: weighing light

Q:
Q1. I know in beginning all experiments sounds to be weird but... anyways my question is if we put a source of light which is emitting enormous amount of light in a enclosed box and then if we weight the box without light and then with light emission would we will be able to prove light have mass/weight? Q2. Does heat have mass/weight?
NOIDA, U.P., India
A:
Q1.  Weird or not, that's a good question.

The answer is yes, that when the light escapes from the box the weight of the box goes down a little.

I think, from a rough calculation, that about one part in 108 of the Sun's gravitational mass comes from the light in it. That calculation could easily be off a factor of ten either way, since I used very approximate numbers.

Q2. Also yes, if you heat something up, leaving everything else the same, it has more energy, i.e. more mass and more weight. At ordinary temperatures the effect is minuscule. In think the thermal contribution to the weight of ordinary matter at room temperature is less than one part in 1013 or so.

Mike W,

(published on 04/24/2009)

## Follow-Up #4: Losing mass through light emission

Q:
when a source of light emit light is there any decrease in the mass of source
- lovepreet (age 23)
bhagta ,punjab ,india
A:
Yes, in principle.   But it's a teeny tiny amount, too small to measure.   The reason is that the 'effective mass' of a photon is its energy divided by the velocity of light squared.  The energy of a photon of yellowish light is about 2 eV  (electron Volts).  One ev corresponds to 1.6 10-19ergs.  That number divided by c2 is 1.8 10-36 kilograms.   Even though there are lots and lots of photons in a laser beam it still adds up to an unmeasurable amount.  Give up.

LeeH

Even if you radiate a kilowatt for a day, you lose about 108 Joules of energy. That's equivalent to 10-9 kg of mass. Not a lot. Mike W.

(published on 06/17/2009)

## Follow-Up #5: absorbed light

Q:
I remember hearing that when light strikes a wall the particles are in a sense "destroyed". now if you where to put a light bulb in a box and then turn it on, wouldnt all the light particles be "destroyed" when colliding with the box? And heres one more question. what if you get a massive light bulb and sourround all the sides of it with mirrors forcing the light to come out in a single concentrated beam at a black 1/2 inch peice of wall of something. would that have any impact on the object at all? ( i say black because the color black absorbs all rays of the spectrum.) although i could be wrong
- Nathan Garcia (age 17)
San Antonio, Texas
A:
1) "if you where to put a light bulb in a box and then turn it on, wouldnt all the light particles be "destroyed" when colliding with the box"
Yes, they'd just heat the box up.

2)"would that have any impact on the object at all?" Yes, it would heat up noticably. There would also be a very slight force pushing on it.

Mike W.

(published on 06/18/2009)

## Follow-Up #6: relativistic infinities?

Q:
When reading about relativity and special relativity you always run into the question about spaceships moving at the speed of light. The answer I always see is that as the ships speed approaches the speed of light it's mass goes towards infinity. Perhaps my understanding of Physics and Calculus are too limited but this answer bothers me. According to E=mc^2 the mass of the ship would be m=E/c^2. Since it is held that the speed of light is a constant I do not see how the mass could be said to go to infinity. Of course I also understand the the concept of applying that much energy to any mass is absurd, but we are talking about theory here. So, where does this assertion that mass goes to infinity come from? After all - light has mass and it moves at the speed of light. Granted, a photon has negligible mass compared to a space ship - but the theory must work for both.
- Matthew (age 37)
New York, NY
A:
You're right that if m=E/c2, then you can't have m-> infinity unless E does too. So, neither one does.

What people mean when they say 'm-> ∞' is that as v->c m keeps growing without any limit. So v can't reach c, because that would require an infinite amount of energy.

The specific form is that

E/c2=m=m0/sqrt(1-(v/c)2),
where m0 is the mass as seen by someone moving along with the object, called the rest mass.

Now for your more interesting question. If light has E and hence m, how can it then travel at c? If light had m>0 when it was  traveling at any v<c, we'd have a big problem. However, light can't travel at less than c. (But see comment below- here we're only talking about in a vacuum.). It only has E or m when traveling at c.     For light, m0 = 0.

A general form which covers the cases both of m0>0 and m0=0 is:

E2(1-(v/c)2)=(
m0c2)2

When v=c, this requires m0=0. Likewise when m0=0 it requires that E=0 or v=c. When m0 >0, it gives the formula above.

Mike W.

(published on 06/25/2009)

## Follow-Up #7: Velocity of light

Q:
In the answer to the last question you said "...light can't travel at less than c." But doesn't light travel slower in water. I thought light only travels at c in a vacuum.
- Joe
Edison, NJ, USA
A:
You are correct.  In a vacuum, light travels at velocity c = 299,792,458 meters per second.  In media with an index of refraction n, such as glass or water, light travels at a velocity of c/n, i.e. slower than that of in a vacuum.

LeeH

(published on 08/24/2009)

## Follow-Up #8: Light

Q:
If light has mass, as you've stated in the questions above, then I have magical lightbulbs... When I turn on a light switch, I accelerate the light to 'c'. The energy required to do this is apparently infinite. Ergo, my light bulbs are magical. If that is explained away by stating that light has a zero resting mass, then how is mass created by getting it to it's constant vacuum velocity? We've just broken the cardinal rule that matter (and therefore mass) cannot be created or destroyed. The third part (using Einstien's theory) is the fact that if I stand outside I'm getting bombarded with mass moving at the speed of light. I should be ripped to shreds. The inertia required to stop that much momentum would be phenomonal. I'm good, but not THAT good...
- Luke (age 32)
Brisbane, Australia
A:
We've been through these points before, but they are unfamiliar enough to bear repetition.

You aren't accelerating anything when you turn on a light. The light is born traveling at c and stays that way.

There is no rule that "matter" is conserved because "matter" is not a defined quantity.
The pre-relativistic rules about conservation of mass and energy are replaced in Special Relativity by a unified conservation of energy. The energy that makes light comes from other sources, e.g. the electrical power supplied to your house.

When you are hit by light you do indeed pick up some momentum (p) as well as the obvious energy (which you can feel). The momentum is very small, however, since p=E/c for anything traveling at c.  For example, 1 Watt of light energy hitting a surface exerts a force of 3.3 nano-Newtons on it. (6.6 nN if it's reflected rather than absorbed.) That can be measured with instruments, but not directly felt.

Mike W.

(published on 09/01/2009)

## Follow-Up #9: light in universe

Q:
Does light have mass or energy relative to the claim that it is cannot escape the gravitational pull of a black hole? Question of missing mass of the Universe in addition to dark matter? Could it simply be invisible light mass/energy generated and moving since the creation of the Universe. The light went somewhere is their a quantity of light existing at all times from all light sources. There must be or you could not see the furthest galaxy or star. Just a thought for the missing stuff. The amount of light moving through the Universe is huge! Thanks Wade
Phoenix AZ USA
A:
See the chain of answers above. These are popular questions!

Mike W.

(published on 09/15/2009)

## Follow-Up #10: light and gravity

Q:
Keeping in mind that light has mas and is acted upon by gravitational pull, shouldn't the path of light be a projectile instead of a straight line?
- Aryaki (age 17)
India
A:
Yes, you're right. In the presence of gravity, light doesn't follow straight lines. Different paths, for example, can intersect twice.
It's a little more complicated than just thinking of something following a projectile path, however, because it turns out that the curvature is twice as much as you would guess in that picture. Spacetime itself isn't flat when there's gravity, and the geometry of it is described by General Relativity.

Mike W.

(published on 09/15/2009)

## Follow-Up #11: Effect of light on universe

Q:
If light has mass, and light is spread everywhere in the universe, what gravitational effect does all that light have on the universe if any?
- Milo (age 32)
Seattle
A:
Not much at the present epoch.  The effective mass density of light is one ten-thousandth that of ordinary luminous matter, even less if you consider dark matter and other exotica like dark energy.   According to the current lore, light had a brief  ~3 seconds worth of glory after the big bang but then faded into dimness as the universe expanded.   We still see its remnants in the 3 degree cosmic microwave background.

Very localized light, for example from a supernova, can affect nearby regions but the overall effect is small.  Consider the total effect of lightning bugs on a summer's eve; it's pretty but it doesn't give you enough light to read by.

LeeH

(published on 09/16/2009)

## Follow-Up #12: light bouncing off water

Q:
So... if light does have mass AND matter, then when it is reflected on water, why doesn't it leave ripples? I've spent a lot of time wondering!
- Savannah (age 13)
Jacksonville, FL
A:
Great question. Light has only a very small amount of equivalent mass and momentum, so when it bounces off the surface of water it makes only a very small push. The effect you're thinking about is real, but just really tiny.

Mike W.

(published on 09/30/2009)

## Follow-Up #13: Light speed, teleportation, and exact timing

Q:
I'm a 5 yrs behind these postes, but hope to get your answer: Q1:If we think of light made up of differnt particles-is it possible that some travel faster than the speed of light,C? in other words, is teleportation of light particles possible and how does it work? Q2: Is it difficult to predict EXACT time of a mooving object (in the Earth's atmosphere) if the light travel is affected by the gravity and the motion? Thanks!!!
- Danijel (age 27)
Houston, TX
A:
Q1. Nope.  As usual there are two equally valid interpretations of light:  the classical fields variety ala Maxwell and Young,  and the quantum variety ala Einstein (actually Newton thought that light was made of corpuscles).  Both predict no faster than light travel, i.e. no teleportation (at least faster than light).  It's not clear whether or not the teleportation in Star Trek ("Beam me up, Scotty") implied faster than light or not.  The presence of gravitational fields does not affect this conclusion.

Q2.  Yes it's difficult to predict the exact time but it can be done.  The problem is that general relativity screws things up when you want to have a precise time in the presence of gravitational field and relative velocities.   In fact the famous Global Positioning System (GPS) has to use the theory of general relativity  in order to make corrections for the earth's gravitational potential as well as relative velocities of the various satellite components.  These involve microsecond time discrepancies, which imply 1,000 foot discrepancies in global positions.
See:
and then click on the Special and General Relativity topic.

LeeH

(published on 10/06/2009)

## Follow-Up #14: light, mass, matter

Q:
We have established that light has mass. I was wondering in what form the mass is in, and if it would be possible to slow down the particles. I'm assuming light isn't atoms, because we would have figured that out by now and I never would have needed to look up the answer to the question, "Does light have mass?" However, this mass must be somewhere, and must be some form of matter. What form of matter is light?
- Eli B (age 15)
CT USA
A:
Let me answer first in the context of simple familiar physics. Electromagnetic fields (including light) are not made up of other things. Likewise many other things (e.g. neutrinos) are not in any sense made up of electromagnetic fields, although they are also fields. These are all just independent basic constituents of the universe.  They all have some mass-energy and can have momentum.

These days, we're partway in to the process of describing fields like electromagnetism in terms of some deeper more basic fields. However, these deeper ingredients will have no more resemblance to intuitive ideas of 'matter' than do our familiar electromagnetic fields. So far as we can tell, there are no little building blocks.

At some point the word 'matter' becomes essentially a meaningless label. There's some sort of underlying mathematical description (like the equations governing electromagnetism) and that's it.

Mike W.

(published on 10/20/2009)

## Follow-Up #15: Mass in modern physics.

Q:
If light has no rest mass (weight). Does this imply that Einstein was wrong? As an object (Mass) nears the speed of light it will become infinite in (Mass). Yet Light (photons, electrons). Do just that, and weight nothing.
- Brent (age 68)
Houston, tx USA
A:
Hi Brent,

I think you might (understandably) be confused about a couple terms physicists like to use, most notably, "mass". I'd also like to point out that light is composed of photons, not electrons.

Ever since Einstein proposed special relativity, the word "mass" has come to mean very different things depending on the context. I personally think the misuse of the term "mass" is responsible for a lot of the confusion people have about modern physics.

First I'd like to distinguish mass and weight, we tend to naturally assume they are the same thing, but they are really not. Weight is essentially the gravitational force exerted by a body on an object. In our everyday life this is the force of gravity exerted by the earth on us and the objects around us. While it is true that something with more mass weighs more, they are not the same thing.

So what is mass? Before Einstein the word mass really only meant one thing: a measure of "inertia". What's inertia? It's a fancy term for "how hard something is to push". So a boulder is harder to push around than a pebble because it has more mass, which means more inertia.

Why did stuff get so confusing after Einstein? It has a lot to do with his most famous equation: Eo=mc2. What many people don't know is that this equation isn't the full story. The more useful, but less ubiquitous equation is actually E=[ (pc)2+(moc2)2] The simpler form comes out when p, which is momentum, is zero; when what we are describing isn't moving.

Why did I write the mass term as mo in the last equation? It's to clearly show that we mean it's the "rest mass" of the object we are describing.

You referred to rest mass in your question. This is generally what physicists talk about when they say "mass" in the context of modern physics. Rest mass is quite simply the mass something has in its own reference frame; its mass when it is not moving. Photons have zero rest mass. Anything with zero rest mass always moves at a speed c, which we call the speed of light.

Electrons, protons, neutrons, and pretty much everything else that we deal with in day to day life have rest mass. Anything with rest mass naturally gains more and more energy as we speed it up. This  energy  is equivalent to "inertial mass".  Inertial mass is what you multiply the velocity by to get the momentum. One could think of this as the object gaining more "mass" as it speeds up, but physicists usually don't use the term that way.

The idea of "inertial mass" has an intuitive appeal in special relativity though: as you speed something up more and more (giving it more energy) you also increase its inertial mass. Earlier I said that inertia is "how hard something is to push". If the object is gaining inertia as it speeds up, then it would imply that it also gets harder to push. Well that is exactly what happens. When you take something and try to accelerate it to a significant fraction of the speed of light, it gets increasingly difficult to make it go any faster. This is why particle accelerators like the Large Hadron Collider are so complicated. They accelerate particles to 99.9999991% the speed of light, which requires monumental amounts of energy.

Another potentially appealing property of relativistic mass is that it is additive. This makes the quantities easy to deal with conceptually, since they add just like a classical mass would. The inertial mass also is the quantity that enters into gravity, so light does indeed have weight, but not much.

In conclusion, Einstein wasn't wrong, but the terminology needed to understand special relativity can get confusing and is often misinterpreted.

Matt J.

(published on 02/28/2012)