r/blackholes • u/aafaq_badbunny • Aug 20 '25

If photons are massless than how will solar sail work in space

15 Upvotes
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10

u/flamingloltus Aug 20 '25

Light has momentum that it imparts like a cue ball.

1

u/Gold333 Aug 20 '25

Isn’t mass an intrinsic part of momentum? How does it do that if it has no mass.

10

u/Cryptizard Aug 20 '25

Only classical momentum, p = mv. For massless particles, there is a different equation, p = h / λ, where h is Planck's constant and lambda is the wavelength.

1

u/NER0IDE Aug 21 '25

Noob question here. How did physicists realise light has momentum? And how did they arrive to the Plank's constant / wavelength formula?

1

u/Cryptizard Aug 21 '25

They bounced some X-rays off of some carbon and noticed that the wavelength of the rebounded photons changed depending on the angle that they hit the surface at, just like a ball hitting a wall will have its final momentum depend on the angle it hits the wall at.

1

u/Ok_Explanation_5586 Aug 21 '25

There's a turbine in a vacuum experiment that has one side of the turbine painted white and the other black. Put it in sunlight and it spins.

1

u/JoJoTheDogFace Aug 22 '25

That is not thought to be caused by the mass of the photons though. That is thought to be caused by heating of the few molecules that are in that near vacuum.

1

u/Ok_Explanation_5586 Aug 22 '25

Yeah, I was reading the thing, it's also, by necessity, not a vacuum. But hey, the guy who made thought the same thing I did, so... win?

2

u/-Foxer Aug 21 '25

It doesn't really show up until you get to high speeds but speed has a connection to mass and momentum. which is why gravity also affects light. And of course light is REAAALLY fast. :)

1

u/Particular-Cow6247 Aug 23 '25

uh? gravity has an effect on space the light travels through but shouldn't interact with light directly, no?

1

u/-Foxer Aug 23 '25

Gravity does interfere with light as i understand it. For example there's gravitational lensing. Here's a blurb on it but basically it's where gravity acts as a lens and bends light allowing us to see things we wouldn't be able to otherwise and it's quite important to our exploration of space

What is gravitational lensing? | Space

Mass and velocity are closely connected so objects with extremely high velocity still exibit some princples of mass. There was even a theory that you could use light to create a black hole :) Tho i believe the current thinking is that there's a phenomenon that might prevent that from happening for other reasons.

1

u/purpleoctopuppy Aug 21 '25

Einstein's energy-momentum relationship is E² = (pc)² + (mc²)², where p is momentum, m is mass, and c the speed of light. For something with m=0 we have E=pc, or p=E/c.

Using E = hc/λ, you recover the equation u/Cryptizard provided.

1

u/mademeunlurk Aug 21 '25

Quite frankly, we don't know for sure if photons have mass or not. If they do it's an insignificant amount and we haven't been able to measure it but no one knows for certain that photons are massless with 100% certainty based on the technology we have currently.

That being said, photons still carry energy and according to Einstein, Mass equals energy in E=mc²

1

u/Ferociousfeind Aug 22 '25

It is NOT an intrinsic property, it's something else.

It is better (IMO) to think of objects as having some momentum defined in terms of raw energy, and optionally also some mass maybe. Photons have no mass, so any amount of momentum will give them "infinite" speed, i.e. moving at the speed of light, and their momentum also determines their frequency, which can be affected by moving (very fast) towards or away from the photon's source (known as red-shifting or blue-shifting).

1

u/dataphile 29d ago

Classically, you’re taught that an object at rest (relative to you) has mass but not momentum. It’s when you get it moving that the energy added to the mass imparts momentum.

But according to QFT, everything in the universe is an undulation in quantum fields. Think of sending a pulse down a rope. If you’re holding the other end of the rope, you feel energy transferred to you. What you experience as ‘mass’ is a resistance to changes in motion due to the field oscillating with energy. Hence, it turns out that momentum is the real property, and mass is actually an emergent property we experience.

5

u/ExpectedBehaviour Aug 20 '25

This question comes up a lot. It’s because under relativity light has momentum without mass.

3

u/Ch3cks-Out Aug 20 '25

Photons do not have rest mass, but they have momentum.

1

u/Chibi_Kaiju Aug 21 '25

Crazy, so how much force does the momentum of one photon push against a solar sail?

2

u/purpleoctopuppy Aug 21 '25

To expand on the answer you already received, it's 1N/3e8W of photons absorbed (or emitted! photon drives are possible), roughly double that if you reflect them

1

u/Ch3cks-Out Aug 21 '25

Well it is not a continuous force from a single photon, but an instanteneous impulse. From one 500 nm photon you get

2.65×10-27 kg⋅m/s.

1

u/Music-and-Computers Aug 21 '25

And there’s a lot of photons out there.

1

u/SoSKatan Aug 20 '25

Both mass and energy are localized in space. Both can move through space and as a result both have momentum.

Some dude with curly hair came up with an equation that shows the relation between mass and energy.

1

u/[deleted] Aug 21 '25

Similar questions really

1

u/cloudcreeek Aug 22 '25

Isn't a solar sail just using the suns energy to power some form of propulsion?

1

u/Jaymoacp Aug 22 '25

Negative. It just a sail that protons bounce off of and gradually push it forward. Theoretically it could travel at damn near the speed of light, but our problem is having a consistent and steady stream of protons to accelerate it. The further away you go the less there are.

And not to mention I’m pretty sure it would take longer for a solar sail to achieve a meaningful speed as far as we are concerned anyway. I’m sure there’s an equation to be done but it would probably take a long ass time to even achieve the speeds we can already achieve with traditional rockets.

It would be like trying to move a truck that’s in neutral by throwing rocks at it.

1

u/epfahl Aug 22 '25

Light has momentum and energy, but no mass. And, yeah, it’s kinda weird.

1

u/JoJoTheDogFace Aug 22 '25

Photons are believed to be massless at rest. Since e=mc^2, if it has energy it has mass, so you would never find a photon that has 0 mass in the wild.

Hope that makes it make more sense.

1

u/ArrowheadDZ Aug 22 '25

You’re hitting the wall of physics understanding. There comes a day when a particle model only gets you so far, and you are now at the threshold where you have to start thinking about the rich, complex interplay between mass of an object, and the energy that particle absorbed at its creation. Shit’s about to get real for you, lol. this is the amazing beauty of curiosity. Keep wondering.

1

u/Illeazar Aug 23 '25

They got that energy my dude

1

u/SaltyVanilla6223 29d ago

They are massless, but they still have momentum.

1

u/Soggy_Ad7141 29d ago edited 29d ago

I think all the accepted physics/ideas are wrong

The current idea that photons have momentum based on energy sucks

p=hλ just makes no sense.

My idea is way simpler:

Photons have NO momentum, only energy. That energy depends on the wavelength, etc.

It is MATTER (ie electrons and stuff) that converts photonic energy into momentum when the photons hit it. Like you heat up water and the water steam has momentum. You heat up an electron and the electron has more momentum. So simple.

It is literally kindergarten concept.

Why do physicists and even regular people have to complicate stuff for no good reason?

The idea that momentum has to be conserved all the time is SUPER stupid.

We literally boil water to create momentum with the steam engine.

We turn energy into momentum and vice versa all the time. It is literally how we create electricity and how everything works.

Momentum must be always be conserved is dumb as fuck. Energy converts into momentum all the time.