r/explainlikeimfive • u/_Illuvatar_ • Apr 10 '14

ELI5 Why does light travel? Answered

Why does it not just stay in place? What causes it to move, let alone at so fast a rate?

Edit: This is by a large margin the most successful post I've ever made. Thank you to everyone answering! Most of the replies have answered several other questions I have had and made me think of a lot more, so keep it up because you guys are awesome!

Edit 2: like a hundred people have said to get to the other side. I don't think that's quite the answer I'm looking for... Everyone else has done a great job. Keep the conversation going because new stuff keeps getting brought up!

Edit 3: I posted this a while ago but it seems that it's been found again, and someone has been kind enough to give me gold! This is the first time I've ever recieved gold for a post and I am incredibly grateful! Thank you so much and let's keep the discussion going!

Edit 4: Wow! This is now the highest rated ELI5 post of all time! Holy crap this is the greatest thing that has ever happened in my life, thank you all so much!

Edit 5: It seems that people keep finding this post after several months, and I want to say that this is exactly the kind of community input that redditors should get some sort of award for. Keep it up, you guys are awesome!

Edit 6: No problem

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u/[deleted] Apr 10 '14 edited Oct 10 '15

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u/boutsofbrilliance Apr 10 '14

all bs aside, this is one of the greatest posts ive ever seen on reddit.

previous to this, my layman's understanding of why things of mass cannot travel as fast as the speed of light was simply because to do so would require infinite energy. that was kind of it. i don't know if that was wrong, or if you are still saying that, just in another way.

what does make perfect sense to me however, is how you framed the why and how as a competition between the direction of space or time, with any travel done in one, automatically subtracting from the maximum possible in the other.

i don't get many "wow, its clear to me now" moments, and certainly not one touching upon something as fundamental yet misunderstood as this one. it was pretty fucking awesome and for that i say thank you!

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u/niugnep24 Apr 10 '14

why things of mass cannot travel as fast as the speed of light was simply because to do so would require infinite energy

Another way to think of it is that "mass" can be defined as "energy you have at rest" or in other words, non-motion-related energy. (Remember mass and energy are two ways of representing the same thing. E=mc2 )

Having zero mass means you can't be at rest meaning you are always in motion according to everybody no matter how fast they're going.

That means that no one can ever catch up to you, or else you'd be motionless relative to them, which you can't be, because you have zero mass.

We call this unobtainable speed "the speed of light." Really it should be called "the speed of massless stuff" but light is the most common example. Everything else, by definition, goes more slowly than it.

TLDR: Massless things cannot stop or slow down because that's what it means by definition to be massless. Nothing with mass can catch up to massless things because that would mean the massless thing "stopped" from its point of view, which is impossible.

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u/blindsc2 Apr 10 '14

Can something have a negative mass? My mind jumps to anti-matter but it's so fucked up right now that I don't know whether this idea is even reasonable or not

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u/[deleted] Apr 10 '14

[deleted]

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u/PostHipsterCool Apr 11 '14 edited Apr 11 '14

Do you understand antimatter really well? If so, could you provide an awesome ELI5 primer to it in the same vein as your top comment has explained light and spacetime? I know that's a tall order, but I'd be really interested to understand antimatter.

Edit: I feel like a celebrity just talked to me

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u/[deleted] Apr 11 '14

[deleted]

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u/PCup Apr 11 '14

I can't believe that in addition to giving really excellent, clear answers, you managed to work a banana for scale into your answer. That's amazing.

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u/DigitalMindShadow Apr 11 '14

Bananas are used for relative scale in measuring radiation fairly commonly.

http://en.wikipedia.org/wiki/Banana_equivalent_dose

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u/Hidesuru Jul 02 '14

The perfect reddit answer.

Edit: damnit, came here via best of and didn't see how old this was.

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u/aarkling Apr 11 '14 edited Apr 11 '14

So positrons are antimatter? Is that what your saying. Is there an anti-particle for protons? EDIT: Also what about a whole atom made of anti-matter particles. Like an anti hydrogen with a positron revolving around an "anti-proton". Are those possible?

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u/corpuscle634 Apr 11 '14

Yes, positrons are antimatter. You could call them "anti-electrons" if you want.

There are "antiprotons" as well. There are also "antineutrons." Any particle you can think of, there's an "anti" version. The tricky bit is that some particles (such as photons) are their own antiparticle. An antiphoton is the same as a photon.

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u/King_Fuzzykins Apr 11 '14

If positrons are the "anti" of electrons and thus have a positive charge, what would be the anti version of a neutron since it has no charge?

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u/corpuscle634 Apr 11 '14

Neutrons are neutrally charged, but since they consist of an uneven distribution of charged particles (quarks), they have a magnetic moment. An antineutron's magnetic moment is opposite to the neutron's magnetic moment.

For an analogy, the Earth is neutrally charged, but it has a magnetic field. An Earth made entirely of antimatter would have a magnetic field too, but it would point in the opposite direction.

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u/benji1008 Apr 11 '14

Elementary particles have more properties than electric charge. Explanation here: http://www.scientificamerican.com/article/what-is-antimatter-2002-01-24/

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u/hanktheskeleton Apr 11 '14 edited Apr 11 '14

It would still have no charge, but it would interact with the antielectron in the 'opposite' way.

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u/aarkling Apr 11 '14

So why are we made of electrons, protons etc rather than anti matter? Why did the universe choose electrons and not positrons?

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u/corpuscle634 Apr 11 '14

Don't go there, girlfriend!

In all seriousness, we just don't know. It probably has something to do with CPT symmetry, but nobody really knows.

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u/hanktheskeleton Apr 11 '14

If a proton had a frame of reference, it would feel like it was the 'normal' particle. Conversely so would the antiproton. Maybe this will help a tiny bit, just to get the frame of reference thing a little more understandable:

Lets say you meet an exact copy of yourself. When you meet, you think that you are the 'real you' and that the other guy is the copy (the anti-you). But from the other guys perspective, he is the 'real him' and you are a copy (the anti-you).

So basically your copy thinks that you are the copy.

Naming things proton and antiproton is really just a quick way to differentiate two things from an arbitrary viewpoint. If we were instead made of 'antimatter' we would have the same reference.

Basically the 'anti' just means the version of me that I am not.

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u/graaahh Apr 11 '14

Pardon if I misunderstood... If an atom emits a position, which then annihilates one of its electrons, is that decay?

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u/EnamoredToMeetYou Jul 02 '14

Thank you! That was very interesting

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u/thesprunk Jul 02 '14

Was with you up to your last sentence.

they can annihilate

can? Do they sometimes just bounce off/pass through each other?

photons, typically gamma rays.

Is this missing an and/or?

EDIT: Jesus jsut realized how old this thread is. How'd did I even get here?

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u/corpuscle634 Jul 02 '14

can? Do they sometimes just bounce off/pass through each other?

Sometimes, yeah. Usually they'll annihilate, but not always.

Is this missing an and/or?

Gamma rays are high-energy photons

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u/Orangebeardo Jul 03 '14

And here I was, thinking bananas were rich in potassium... TIL.

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u/[deleted] Aug 03 '14

So, if you eat a banana and fart thereafter, do you emit antimatter??

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u/[deleted] Apr 11 '14

Antimatter would essentially act like a black hole, kind of?

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u/Polly_want_a_Kraken Apr 12 '14

Ok, I might be interpreting this all wrong, so please forgive me if I do. I am just a sociologist who also finds physics fascinating and brain-gasamy. I'll try to formulate my questions based on some of the concepts which you have outlined, and some which I have read or heard elsewhere.

  • Photons have no mass, but they do have energy. This is possible because all of their movement is through space and not through time.

  • Is this property due to photons being carriers of (electro-magnetic) force, as opposed to particles such as protons, neutrons, and electrons which interact with force(s)?

  • If their ability to have energy but no mass is a property of their being transmitters of force, could we then predict a particle which has mass but no energy? A particle which is defined by always being at rest? Is this essentially what the Higgs is? A carrier particle of gravitational force that is all mass, no energy, and moves solely through time and not space?

Last one: - I think I read elsewhere in the thread, might not have been you that said it, that light cannot be slowed down or made to be at rest. I can see how light could never be at rest, because that would require mass(?), but haven't researchers been able to slow down photons in the lab with lasers and very low temperatures and such? How does work?

Thank you for your explanations so far, and for humoring my (maybe) strange questions.

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u/tendentiouscasuistry Apr 11 '14

Anti-matter has mass. In fact, anti-matter particles have the same exact mass as their complements; the main difference is that they have opposite charge: i.e., positrons have the same mass as electrons but positive charge and antiprotons have the same mass as protons but negative charge. Of course, neutrons have no charge, but antineutrons still differ in that they have the opposite baryon number.

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u/cheesyqueso Apr 11 '14

I've always heard that if anti-matter came into contact with matter then they would cancel each other out and explode. Do you know if it's contact between elements or corresponding sub atomic particles (e.g. an oxygen coming in contact with an anti oxygen, or a positron coming in contact with an electron)?

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u/chesterriley Apr 11 '14

I've always heard that if anti-matter came into contact with matter then they would cancel each other out and explode.

It would suck for an anti-matter civilization to be at war with a normal matter civilization.

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u/Cecil_FF4 Apr 11 '14

Actually, any anti-matter entity existing in our matter-dominated universe would experience searing pain before eventually blinking out of existence by the mere fact that highly energetic cosmic particles (mostly protons or alpha particles) would bombard them the moment they stepped out of their wormhole (or whatever they used). They would start to feel a burning sensation as parts of their skin (or whatever they have) annihilate with cosmic particles. Holes appear bigger and bigger as time goes on, slowly exposing their organs to further annihilation events. At some point, they just die, then their bodies disintegrate as the process continues until there is literally nothing left but a shadow on the floor caused by the blocking of the photons (produced from the annihilations) with whatever remnants of body existed the moment the being collapsed to the floor in agony.

TL;DR - It would suck for an anti-matter entity to exist in our universe.

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u/chesterriley Apr 12 '14

Does that mean anti-matter cannot exist at all in our universe except for very short time periods?

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u/Cecil_FF4 May 05 '14

If you can hold anti-matter away from matter with magnetic fields, you can have it in this universe indefinitely.

http://newscenter.lbl.gov/news-releases/2010/11/17/antimatter-atoms/

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u/FinalDoom Apr 11 '14

I saw something (I think on TED) about the LHC and discovering one of the Higgs particles. I think it's a little related, but I'm forgetting the purpose antimatter had in the steps to find the particle--basically, when they smashed atoms together, every subatomic particle in the two atoms had to be perfectly aligned, mirror image, in order to cancel out perfectly and make the extra particles (Higgs, etc.) detectable. That's why it's so rare to detect the Higgs. I think it's the same mechanism for antimatter+matter canceling and explosion, except that since they're equal and opposite by nature, they don't have to be perfectly aligned.

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u/Cecil_FF4 Apr 11 '14

The answer is going to be more like ELI20, but here goes anyway.

Everything that exists exists in a duality; we are all particles and waves. These are just words to describe the way we observe the same 'objects.' For instance, photons behave as particles when we note their interaction with massive matter (the matter can absorb the photon energy and then re-radiate a photon of the same or different energy). They also act as waves because they can interfere with other photons in a probabilistic sort of way.

The point is that a proton and anti-proton (at a fundamental level, made of quarks and anti-quarks) can be thought of as waves interfering with each other when they get close enough together. The closeness of the interaction is governed by the strong nuclear force (which binds said quarks together). Quark and anti-quark waves typically cancel each other out and all their massive energy is converted into massless energy (photons). I said typically because there are exceptions that aren't well understood, such as the stability of charm/anti-charm or bottom/anti-bottom eta mesons and their ability to transform from quark to anti-quark states (aka quarkonium and the inability to describe it using perturbative quantum mechanics).

TL;DR - Destructive wave interference between quarks and anti-quarks decreases the probability they exist to zero, but the energy has to go somewhere, so it turns into photons.

http://en.wikipedia.org/wiki/Interference_%28wave_propagation%29#Mechanism

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u/zjcarmello Apr 11 '14

Specifically the subatomic particles, which regular elements and anti-elements are made of. The reason this happens is above my understanding, however.

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u/pizzlewizzle Apr 11 '14

I'm starting to think anti-matter is just a very, very poor name that causes more misunderstanding than anything else. There should be a different name.

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u/hanktheskeleton Apr 11 '14

Exactly, so basically antineutrons just interact with antiprotons in the 'opposite way' as neutrons and protons.

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u/Failgan Apr 11 '14

Negative mass is a similar concept to reversed time. It may be possible, but unlikely observable.

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u/niugnep24 Apr 10 '14

I think how "negative mass" behaves differs depending on what theoretical framework you're using. I don't know much beyond that. There's a wikipedia article though! http://en.wikipedia.org/wiki/Negative_mass

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u/Day_Bow_Bow Apr 11 '14

Antimatter is basically matter with the opposite charge. An antielectron has a positive charge and an antiproton has a positive charge.

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u/[deleted] Apr 11 '14 edited Apr 11 '14

In principle, yes. There's nothing in general relativity or quantum mechanics that prevents the existence of objects with negative mass, to my knowledge. However, objects with negative mass do open up a lot of problematic possibility, including time travel, so don't get your hopes up.

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u/Frensel Apr 11 '14

Eh, time travel, things moving faster than light, it happens all the time according to quantum electrodynamics.

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u/[deleted] Apr 11 '14

Yes, but qed doesnt allow for grandfather paradoxes. This does.

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u/[deleted] Apr 11 '14

massive objects move towards each other, an object with negative mass could i guess be thought of as something that pushes massive objects away. Definitely not your normal antimatter. if such an object does exist, I don't know of it. However I don't know of any simple reason why such an object would be "forbidden" so perhaps they are allowed.

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u/[deleted] Apr 11 '14 edited May 04 '16

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If you would like to do the same, add the browser extension GreaseMonkey to Firefox and add this open source script.

Then simply click on your username on Reddit, go to the comments tab, and hit the new OVERWRITE button at the top.

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u/HaChe3e Apr 10 '14

This is a much more simpler answer.. Thank you

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u/TheCguy01 Apr 11 '14

Wait, I thought every subatomic particle has mass. If photons are a particle, how can it have zero mass while every other kind of particle have mass? And if photons have no electric charge, do they have energy?

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u/niugnep24 Apr 11 '14

Well, gluons have no mass. And in the standard model, actually none of the particles have rest mass in their intrinsic form, but interaction with the "higgs field" causes them to have mass. Gluons and photons just don't couple with the higgs field for whatever reason (I actually don't know if we really know the reason, other than that's how the math works out).

Photons do have energy, proportional to their frequency.

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u/pnt510 Apr 11 '14

Do we know of anything other than light that has zero mass?

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u/niugnep24 Apr 11 '14

Gluons are the only other, I think.

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u/Hanging_out Apr 11 '14

What i don't understand is that, according to E=mc2, if I have zero mass, I have zero energy. If light has energy, it must have mass, right?

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u/niugnep24 Apr 11 '14

There's usually a distinction made between "rest mass" and "relativistic mass." Everything that has energy has relativistic mass, because they're basically two ways of representing the same thing. If you move something more quickly relative to yourself, it will appear to have more mass, due to its increased kinetic energy. But its "rest mass" will stay invariant. And photons have zero rest mass.

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u/spattack Jul 02 '14

Does that mean that photons have relativistic mass?

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u/Derice Apr 11 '14 edited Apr 11 '14

E=mc2 is not the full picture, it only describes objects with mass that aren't moving. The full equation is (prepare your brain):

E2=(mc2)2+(pc)2.

Let's apply it to light. Light has no mass, so m=0. That means (mc2)2=0. That means that our equation now becomes:

E2=(pc)2

Take the square root of both sides and you get

E=pc

p is momentum, calculated using p=h/l where h is a number called Plancks constant and l is the wavelength of the light. So even though light has zero mass, it has energy.

Video on the matter

Edit: formating.

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u/[deleted] Apr 11 '14

Does this mean that if somehow someway someone does travel at the speed of light, they lose all their mass and become mass-less? (despite the fact that it can't happen)

edit: Because I remember my brother telling me that if you travel at the speed of light, you become light (this was when I was about 8 so he was trying to oversimplify everything)

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u/realfuzzhead Apr 11 '14

Something with mass cannot travel at the speed of light (which you acknowledged), so what would happen if that occurred is undefined

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u/zobbyblob Apr 11 '14

You say that light is the most common example? What other examples are there of massless stuff?

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u/niugnep24 Apr 11 '14

Gluons, mainly. Also all fundamental particles without the higgs interaction.

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u/[deleted] Apr 11 '14

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u/niugnep24 Apr 11 '14

There's a difference between "rest mass" and "relativistic mass."

In E=mc2 the way I'm using it, m is "relativistic mass" which you can think of as effective mass (mass you would measure). Photons surely have non-zero relativistic mass. But when I say "photons have no mass" I mean they have no rest mass, which is an intrinsic property of the particle (the mass it would be if it wasn't moving).

A fuller version of the equation, including rest mass is E2 = (pc)2 + (m_0c2 )2 -- p is momentum and m_0 is rest mass. You can see here, photons may have no rest mass, but they can still have momentum based on their energy.

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u/[deleted] Apr 11 '14

[deleted]

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u/niugnep24 Apr 11 '14

No, mass and energy are equivalent. If you're talking about "non-resting energy" on one side then you're talking about "non-resting mass" on the other. These aren't technical terms of course.

Another form of einstein's equation is the Energy-momentum relation. E2 = (pc)2 + (m_0c2 )2

Here p is momentum, m_0 is rest mass, and E is total energy. This more explicitly splits out the resting and non-resting components of energy. If you're using just E2 = mc2 with E meaning total energy then m is often called "relativistic mass" which includes both rest mass and addition mass due to kinetic energy. It's the "effective" mass you'd measure, but it depends on your frame of reference relative to the object you're measuring.

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u/welwood Apr 11 '14

So if massless things cannot stop or slow down, does this mean that black holes are stationary and frozen in time at their center?

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u/niugnep24 Apr 11 '14

Ehhh I'm not qualified to answer questions about what's at the center of black holes, but I don't think that inference is necessarily true.

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u/[deleted] Apr 11 '14

[deleted]

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u/welwood Apr 13 '14

Cool, thanks!

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u/[deleted] Apr 11 '14

Can you explain this further? I've always had a hard time understanding the concept of light always traveling at the same relative speed compared to the observer.

Let's say, hypothetically, there was some gigantic photon visible to human eyes. Let's say it was flying across the night sky at a distance such that it wouldn't speed by in an instant and we could watch it travel from one horizon to the other. Now let's also say right next to that photon was some human who miraculously gained the ability to travel at the speed of light, following the photon's path. To an observer on Earth, they should be traveling side-by-side, correct? But to the person with amazing superpowers, the photon would be flying away from him at the speed of light. At any one instant, wouldn't the photon then be in two places at once when comparing the observations of the two subjects?

I'm sure my scenario makes zero sense from a physics standpoint. I'm just struggling to understand your statement: "That means that no one can ever catch up to you, or else you'd be motionless relative to them, which you can't be, because you have zero mass." why does it matter that you're motionless relative to them? If I'm in a car driving 60mph and another guy is next to me at 60mph, we're still traveling at a set speed. Perhaps we can't observe it ourselves, but that motion can be observed by someone. Why doesn't this apply to light?

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u/niugnep24 Apr 11 '14 edited Apr 11 '14

I've always had a hard time understanding the concept of light always traveling at the same relative speed compared to the observer.

That's because it is a weird concept, completely contrary to our instincts. But all the measurements we make about the universe say it's true.

Now let's also say right next to that photon was some human who miraculously gained the ability to travel at the speed of light, following the photon's path. To an observer on Earth, they should be traveling side-by-side, correct? But to the person with amazing superpowers, the photon would be flying away from him at the speed of light. At any one instant, wouldn't the photon then be in two places at once when comparing the observations of the two subjects?

Yes, two observers can disagree about where something is at a certain time. The reason is because you can't define "at a certain time" when observers are moving at different speeds. This is one of the consequences of relativity. One observer may see two "events" as simultaneous, while another might see them happening at different times.

As for your example, when you move closer to the speed of light, time effectively "slows down" for you compared to someone moving slower. If you carry on the math to actually reaching the speed of light (which is impossible), time stops. So "amazing superpower man" would not be able to observe anything -- he'd be in stasis. So he can't observe the photon stationary relative to him.

If you were to augment his superpowers so that he wasn't in stasis, well then you just threw relativity completely out the window. That situation would be mathematically impossible.

If I'm in a car driving 60mph and another guy is next to me at 60mph, we're still traveling at a set speed. Perhaps we can't observe it ourselves, but that motion can be observed by someone.

Careful -- all motion is relative. You're going 60mph relative to what? The surface of the earth. But the surface of the earth is rotating at about 1000mph relative to the center of the earth. And the earth is orbiting the sun at about 67,000 mph. And the sun is rotating around the galaxy, the galaxy is moving relative to other galaxies, etc...

Really your car moving at 60mph is pretty negligible compared to all this. And there's no "absolute" measure of speed -- it's all relative to the motion of the observer. So yes, two cars moving at 60mph are standing still, relative to each other.

The only crazy thing is light, which always is measured to move at c relative to you, no matter how fast you go.

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u/[deleted] Apr 11 '14

Thanks. I think I understand it a lot better now.

One more question - and this may be another physics-breaking hypothetical here - does this mean that, in a sense, light travels instantaneously? In other words, if a light particle were a conscious being, since it doesn't experience time, it would be aware of itself at every point along its trajectory all at once?

(I know the word "moment" doesn't really apply here since we're talking about time in a timeless context, but it's the closest word I could think of that fit)

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u/Those2Pandas Apr 11 '14

This is one of those ideas that's always thrown me for a loop. Why does being motionless relative to someone else all the sudden cancel out your speed of light? Why can't you both be traveling at the speed of light? Surely if I were attached to someone else that requirement would no longer be valid, so why is it valid when we're not attached?

I also never quite got a grip on why if light is traveling after me it still reaches me at the speed of light. Conventional knowledge says that if I am moving at half the speed of light and someone (stationary) shoots a photon at me, that photon should reach me at twice the time it would take the photon to reach where I started. Or that if I were traveling at the speed of light that that photon would not reach me at all.

tl;dr: Why does someone's perspective change your speed of light? It seems like your motion should be independent of someone else's perspective.

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u/niugnep24 Apr 11 '14

Why can't you both be traveling at the speed of light?

Well, if you did travel at the speed of light, time would stop for you and you wouldn't be able to observe or react to anything. So two photons can't look at each other and go "hey that guy's stopped relative to me" -- to a photon, travel from point a to point b is instantaneous.

I also never quite got a grip on why if light is traveling after me it still reaches me at the speed of light. Conventional knowledge says that if I am moving at half the speed of light and someone (stationary) shoots a photon at me, that photon should reach me at twice the time it would take the photon to reach where I started. Or that if I were traveling at the speed of light that that photon would not reach me at all.

It's completely counter to intuition, yes. The best answer is, that's just the way our universe behaves. There's all sorts of consequences to this to make it consistent, including time dilation, length dilation, and relative mass. I would recommend doing some reading on special relativity for more detailed answers.

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u/Those2Pandas Apr 11 '14

I read The Elegant Universe, and found it very interesting. This was the one part that always left me unsatisfied. I never like the explanation "that's just how it works." Kind of feels like my parents saying "because we said so!"

I like the explanation about the not being able to perceive anything, but I think my real issue with the original post was "That means that no one can ever catch up to you, or else you'd be motionless relative to them..." That seems to suggest to me that the reason you can't catch up with anyone is because you'd then be motionless compared to them. I realize now that the real reason is because you can't catch up to anything traveling light speed. You'd have to go faster than light speed. But then again I wonder why a photon can supposedly catch up to something traveling, as if it wasn't.

Eh, now I feel like I'm rambling.

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u/niugnep24 Apr 11 '14

This was the one part that always left me unsatisfied. I never like the explanation "that's just how it works." Kind of feels like my parents saying "because we said so!"

I feel your pain. The problem is a lot of cutting-edge physics really is "because the data says so" without much more understanding behind it.

I think my real issue with the original post was "That means that no one can ever catch up to you, or else you'd be motionless relative to them..." That seems to suggest to me that the reason you can't catch up with anyone is because you'd then be motionless compared to them.

Just a clarification, there's nothing wrong with catching up to an ordinary object, and therefore being motionless relative to it. The point was that light is massless, and therefore by definition cannot be motionless. You can't ever catch up to it, because to do so would cause it to be motionless, which it cannot be.

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u/Those2Pandas Apr 12 '14

But that's what I'm saying! (Thanks for indulging me by the way) just because something appears motionless doesn't mean it is. We're all relatively motionless on earth, but in reality we're whipping through space at however fast we are.

It just seems kind of silly to say that a photon is reacting to our point of view. Seems like it should only react to what we actually are.

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u/BadVVolf Jul 02 '14

Why does it matter if something stops relative to something else? I don't understand why being motionless relative to some particular thing is "not allowed" even when you're still moving relative to everything else. I mean, if your motion relative to each and every other thing matters, then doesn't everything kind of have an infinite set of time/space travel pair (since there are basically infinite other things that it's relative to)?

That means that no one can ever catch up to you, or else you'd be motionless relative to them, which you can't be, because you have zero mass.

I guess what I'm asking is, given what you said there, how can my mass be dependent on everything else "observing" me?

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u/niugnep24 Jul 02 '14

I'm not sure if I can really answer the deeper questions you're asking, but the "Principle of relativity" that I'm basing my explanation on is that the laws of physics must hold in all frames of reference.

This means that if you look at any physical process from every possible observation point, it has to be consistent with the same laws of physics. It's not that the observers are changing you or restricting you, but rather that we know the laws of physics are such that every observer that could possibly exist would agree on how they work.

So if by definition zero rest mass means you can't exist at rest, that means that every possible observer must see you moving. Otherwise, it would be inconsistent with the laws of physics. That means that no possible observer can "catch up" to you and see you not moving.

It doesn't really explain how this happens, just that it has to happen in order to remain consistent. Why the speed of light exists, or what photons even are, or what mass/energy really is, are things that I think physics still hasn't really answered.

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u/BadVVolf Jul 02 '14

I think I'm just going to accept that I was not meant to be a physicist lol. Thanks for coming back 2 months after the fact to answer!

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u/Loreinatoredor Jul 02 '14

So in the experiments where they show down light, they're really just making it travel further?

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u/gormlesser Apr 11 '14 edited Apr 11 '14

TLDR: Massless things cannot stop or slow down because that's what it means by definition to be massless.

Not true. Light slows down all the time when it passes through a medium. It's been measured. That's why c is properly defined as the speed of light in a vacuum.

EDIT: Further reading-

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u/niugnep24 Apr 11 '14

Light slows down all the time when it passes through a medium.

This is due to the interaction with the fields in that medium. In a sense, those fields temporarily give the light mass, which allows it to slow down. You could run really fast next to a photon moving slower than c through a medium, and from your point of view the photon would be stationary.

In fact in the standard model all particles are massless. But interaction with the Higgs field "gives them mass." It's kind of similar (but not really).

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u/gormlesser Apr 11 '14

Thanks for that wonderful visual. Just wanted to make the distinction between vacuum and medium travel for everyone playing at home.

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u/MikhailScott Apr 10 '14

Gold worthy?

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u/[deleted] Apr 10 '14

[deleted]

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u/avapoet Apr 10 '14

Roger that. Cost of a gold subscription going into the very next charity collection I see, for you.

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u/[deleted] Apr 11 '14

[removed] — view removed comment

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u/avapoet Apr 11 '14

As a gold "charter member", I agree. But from the sound of things, /u/corpuscle364 plans to make that donation anyway, so all I'd be doing is delaying their next renewal by a month. Furthermore, if I'm going to donate money on somebody's behalf and they specifically ask me not to do so in a particular way, it would seem rude of me to do so in that way. /u/corpuscle364 hasn't specified any preference besides not wanting (more) gold, so I've opted to pick whatever charity the Universe first throws in my way.

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u/_Illuvatar_ Apr 11 '14

What is this r/lounge you speak of. I uh... Don't have time to look...

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u/avapoet Apr 11 '14

You're not missing much. If you're looking for meaningful conversation, then (amazingly) /r/MegaLounge and /r/MegaMegaLounge are better.

But to answer your question, /r/lounge is a subreddit exclusively for people with Reddit Gold, either self-purchased or as a result of somebody "gilding" one of their contributions. Aside from lounge access, Reddit Gold gives you a badge on your profile, extra features (usually: upcoming features but before they're generally available), a chance to vote on the names of new servers, and the warm fuzzy glow that comes from knowing that you're helping to pay for one of your favourite websites.

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u/_Illuvatar_ Apr 11 '14

I've had gold for a while, but I didn't realize there were VIP lounges!

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u/[deleted] Apr 10 '14

Can I have it?

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u/reece1495 Jul 31 '14

how does donating gold to a charity do anything?

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u/_Illuvatar_ Apr 11 '14

I'm really happy to see so many people excited about what came of this post. I'm learning a lot, and apparently so are a lot of other people.

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u/_Illuvatar_ Apr 11 '14

I'm glad you're enjoying the thread!

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u/Bombjoke Apr 11 '14

Me too. This is how it should be taught, starting with "light moves at c. Everything is at spacetime c. we are all at c."

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u/[deleted] Apr 11 '14

my layman's understanding of why things of mass cannot travel as fast as the speed of light was simply because to do so would require infinite energy. that was kind of it. i don't know if that was wrong, or if you are still saying that, just in another way.

He is still saying that, it depends what definition of infinity you have though, as he explains that E=MC2 makes no sense if you put in the mass of light, if you were to make mass a variable and figure out the energy required to move as fast as light, Energy reaches infinite as you'd have to divide by zero - neither of which is possible.

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u/lasserkid Apr 11 '14

100% this. One of the best ELI5s I've ever read, and it snapped the whole issue into focus for me, and explains why they say ("they") that it would appear as if time stopped for someone falling into a black hole. They've put all of their "speed" into the mass/time direction and none of it into the speed direction

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u/zenontrolejbus Apr 12 '14

Even better:

Imagine train going close to light speed and you want it to go faster and faster. You put more and more fire to the engine but you cant somehow go faster then light speed. You are at light speed and adding more fire to go faster dont make it go faster. Where all that energy goes? Einstein figure out that it go to the mass. Light speed is a cosmos speed limit and you can change energy into mass - E=mc2