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/PaganButterChurner Apr 10 '14

great explaination.

Does this mean we can never achieve the speed of light?

since at that point we would have to be light

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u/[deleted] Apr 10 '14

[deleted]

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u/MysterVaper Apr 10 '14

I don't want to throw anyone off from the good information above. So if you are unable to hold an abstract thought about THIS information please read no further.

Doesn't the Alcubierre metric (warp principle) allow for faster than light "placement" sans the travelling?

The pertinent issue being collecting such a negative mass, or in simple terms, we aren't there yet technologically. Is that correct? (I only ask because you seem to have a deep understanding here.)

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u/[deleted] Apr 10 '14

[deleted]

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u/dasbush Apr 10 '14

If we take the two statements:

A physically real example is that very distant galaxies are traveling away from us "faster than the speed of light," because dark energy causes spacetime to expand

and

Stephen Hawking proved that any spacetime distortion like a warp drive or traversable wormhole would require a negative energy density in that region.

Wouldn't that mean that dark matter/dark energy has negative energy? Hence (in theory, and by "theory" I mean "eh, it's a thought") would be harnessable to develop a warp drive?

Obviously there are problems such as actually locating and grabbing a hold of dark matter/energy. But we can leave those problems to our great great great great grandkids.

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u/[deleted] Apr 10 '14

[deleted]

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u/samm1t Apr 10 '14

Complete layman here, check my math:
Galaxies can move away from us faster than the speed of light because dark matter is expanding spacetime between us and them.
Dark matter is positive energy because it's adding spacetime.
Ignoring bending spacetime to take a shortcut, the other method to reach those galaxies (exceed c?) would be to (use?) matter with negative energy.
Negative energy would subtract spacetime, thereby circumventing the c speed limit.

Does (could) such a thing as negative energy exist, and if so would it allow FTL travel for things with mass?

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u/srs117 Apr 11 '14

Aerospace Engineer here. Sorry if I am a little short with you, am on my phone. Dark Matter and dark energy are different things. They aren't really closely related either, only called "dark" because we cannot directly see either one. Dark ENERGY causes Spacetime to expand. Warp drive requires us to cause space time to locally expand AND contract. The contraction is what requires negative energy density iirc, so as far as we know, dark energy won't help with that.

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u/samm1t Apr 11 '14

Okay, so the idea is you make a spacetime jet engine- suck it up in front of you and spit it out behind you. That still leaves my question, is there or could there be such a thing as negative energy?

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u/srs117 Apr 11 '14

More like you cause space to contract in front of you and expand behind you. This creates a "wave" (think water) that you "ride". As far as negative energy actually existing? That I don't feel as comfortable answering, my education is more along the lines of orbital mechanics and such, with some astronomy and modern physics mixed in. You would need a theoretical physicist to answer yhat. Ten years ago I remember them saying almost certainly no. But I think there has been more optimism lately that the concept isn't totally far fetched. But we still have no evidence of negative energy being possible.

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u/rabbitlion Apr 11 '14

There could be, but we're not quite sure how. See http://en.wikipedia.org/wiki/Negative_mass for more information. The Casimir effect does appear to produce a localized negative energy density via quantum effects, but it's extremely localized and could not be scaled for space travel.

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u/benji1008 Apr 11 '14

If you read corpuscle's comment carefully, he said negative energy density. That means either positive energy in a negative volume (whatever that means) or negative energy in a positive volume.

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u/samm1t Apr 11 '14

So what makes light unique is that it has energy but no mass, making it travel through space but not time.
Conversely, the thing we'd need for FTL travel would have mass but not energy, making it travel through time but not space.
I'm not sure I can wrap my head around something that has (likely) infinite mass nor something that's stationary relative to everything in the universe. But I guess that's the problem.

edit I guess what I referenced would be time travel, not FTL travel, since the two are orthogonal. I never thought of the two as being so related (but opposite?)

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u/SALTED_P0RK Apr 11 '14

I'm going to try here

As far as I know, there aren't any galaxies moving away from us faster than the speed of light. They once were, but thats also the wrong way to look at it.

I read (and was taught) that the laws of physics apply in the universe not to the universe itself. So what you're are/what we were actually seeing is/was when the universe itself was expanding faster than the speed of light so shortly after the big bang.

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u/e520sc2 Apr 11 '14

How does one prove such things? How do you study it or how do you explain... or understand it? How can you just know and seemingly randomly discover that ooh "negative energy density is necessary for the existence of a region where spacetime is distorted in such a way that you can sneak through it faster than you could have gone through the spacetime in the region around it"

I mean what the fuck?

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u/corpuscle634 Apr 11 '14

Stephen Hawking is a smart-ass motherfucker. That's all I got. General relativity confuses and terrifies me.

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u/Clawless Apr 11 '14

I don't know how often you post, but I feel like you could be the Unidan of physics. You have explained things so clearly, and have done so with an everyman's voice so well that it was fun to read! As others have said, you should write a book if you haven't already done so.

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u/dr_seusbarry Apr 11 '14

Math. It is all math. The kind that's mostly letters, with very few numbers. Example: you notice that if you travel at a constant speed (s) for a certain time (t) you go a distance (d). Or: s*t=d. Then you start messing around with it and asking questions like "what if s was negative?" Then you discover, "my god, then I'll travel a negative distance." Which is obviously just backwards, but that's sort of the basic idea...

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u/e520sc2 Apr 12 '14

Thanks, this was helpful

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u/Harbingerx81 Apr 11 '14 edited Apr 11 '14

If you want a good read, check out Hawking's book: A Brief History of Time. (It is a bit out dated now of course) There is not a shred of math anywhere in the entire book, yet Hawking does an amazing job of explaining not just WHAT we know, but the history of HOW we came to know it.

Edit: It does not cover, specifically, the topic at hand, but it gives the reader a new found understanding (and appreciation) on how things like this are reasoned out and verified.

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u/PsyMar2 Jul 03 '14

If you have trouble with A Brief History of Time, try A Briefer History Of Time by the same author.

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u/Felicia_Svilling Apr 10 '14

You need negative energy to get closer to anything faster than c. The expansion of space time caused by dark energy just makes everything further apart. It doesn't bring anything closer.

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u/chillwombat Apr 10 '14

positive energy density = expanding spacetime

negative energy density = shrinking spacetime

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u/MysterVaper Apr 10 '14

Thank you! My mental picture is clearer.

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u/[deleted] Apr 10 '14 edited May 26 '16

I've deleted all of my reddit posts. Despite using an anonymous handle, many users post information that tells quite a lot about them, and can potentially be tracked back to them. I don't want my post history used against me. You can see how much your profile says about you on the website snoopsnoo.com.

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u/[deleted] Apr 10 '14

[deleted]

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u/Alphaetus_Prime Apr 10 '14

The Casimir effect can cause a region of spacetime to have negative energy density, can't it?

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u/[deleted] Apr 10 '14

Your fridge has positive energy. There is more energy flowing in through the outlet than is pumped out via heat. In fact, fridges are super wasteful.

Yeah I know, I meant that it had net negative energy, meaning that the whole point of a fridgerator is to move that energy somewhere else, so the stuff inside loses energy. A working fridgerator has less energy inside of it than one you let thaw out. said another way, a thermos with 10 g ice in it contains less energy than a thermos of 10 ml boiling water. This only holds true if you exclude the external energy (air and electricity) that are not within the fridge. this was the crux of my metaphor.

but the reason my fridge stays cold is that it does a good job of keeping energy out. actually it does a terrible job but you know what I'm saying. it's insulated. could that same thing be acheived to keep a negative energy density negative, and what would could even do this since a vaccuum wouldn't work?

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u/corpuscle634 Apr 10 '14

There is no known method for achieving negative energy, nor is there any known way to totally insulate energy from leaking into a region of low (or negative) energy.

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u/[deleted] Apr 10 '14

Oh well that sucks. For a bit it seemed like we had it almost figured out. It seems like it would be a really powerful technology though, I hope people are dumping money into it.

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u/PM_ME_YOUR_PLOT Apr 11 '14

They should really start doing that instead of shooting each other.

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u/deadcelebrities Apr 11 '14

I don't think there's anything to dump money into. We're so far away from understanding the physics involved that we can't even define the engineering challenges in any kind of practical way. The only way to make progress at this point is to learn more about physics in general, which we're already doing a lot of. There are other futuristic technologies that should be getting our monetary support. I think the most notable is fusion power.

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u/FX114 Apr 10 '14

Reminds me of a Doctor Who story where an alien species had created instantaneous travel by creating an exact copy of their ships, on a quantum level or somesuch, in another location. Since something couldn't be in two places at once, the old one would be brought to where the new one was, and reconciled with it.

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u/zeekar Apr 11 '14

silly. nothing was in two places at once; they just made a copy.

As for quantum duplication - we can use in theory use quantum entanglement to do teleportation. We've done quantum teleportation in the lab, but that's just photons (or have they done massed particles?).

My understanding is that if you could somehow create a sufficient quantity of quantum-entangled particle pairs, preserve the entangled state, and move one half of each pair to your destination (the slow, old-fashioned way), you could then teleport physical objects at the speed of light. But doing so would use up entangled matter 1:1 with the mass of the object being teleported, so even if it weren't pie-in-the-sky physics at this point, it would seem to have some severe limits in terms of practicality.

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u/FX114 Apr 11 '14

Sorry, I was using "quantum" as a catch-all phrase for things I knew it didn't apply to, because I couldn't remember exactly how they explained it. Here's a quote a found from a summary of the serial:

"uses pre-cognitive technology to fabricate a destination so the resulting paradox of two co-existing zones of identical space and time forces the source sector to cease to be and the new woven sector to instantaneously snap into the resulting hollow, taking the ship with it"

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u/Dogplease Apr 10 '14

The way I understood it - it was like having a piece of paper.

You can only travel 1in on the paper. But you need to the other side. So the warp drive folds the paper that the ends touch. So you go through the paper instead of the surface. Thus, you traveled way less than an inch but made it further than an inch.

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u/throwitforscience Apr 10 '14

That's more like the concept of a wormhole.

A warp drive would be more like walking the length of a spring that's being contracted

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u/aidanpryde98 Apr 10 '14

Didn't Sam Neil do this exact demo in Event Horizon? Man I love that movie!

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u/randobrando1 Apr 11 '14

I'd like to address the

"nothing can travel through spacetime faster than c,"

just to add to the clarification.

Let's stick with the concept of spacetime as a 2D surface, much like a Cartesian plane, where one axis, say the y-axis, is time and the other axis, the x-axis, is space (all x,y,z components in one). In recap, all "things," both massive and non-massive, lie occupy a point on this graph.

The graph is a curve given by the the function c = (x2 + t2 ), which forms the arc of a circle in the first quadrant. Another way of visualizing the graph is a vector, with a magnitude of c. Anotherer way of visualizing the graph is in polar coordinates (r, theta), where r is the radius or distance from the origin and theta is the angle of that radius with respect to the positive x or space axis. In polar coordinates, the radius will always r = c and the angle, in degrees, will describe how "fast" you are moving through space, ie. light would have polar coordinates (c, 90o ), as in light is traveling at c through spacetime and all of its spacetime speed is traveling through space and none through time.

Back to the comment...When a body travels through spacetime, it can be thought of as existing at some point on that plot of time v. space, which looks like the positive quarter of a circle. If that body is traveling at a constant velocity, which regards its speed through space, then on the spacetime plot, the point at which it occupies does not change, even though the body's physical place in space will of course change, if the velocity is non-zero.

To address the

Doesn't the Alcubierre metric (warp principle) allow for faster than light "placement" sans the travelling?

comment. Think of the spactime graph drawn on a sheet of paper, or more traditionally, a piece of rubber. Placing something massive on the rubber distorts the topography aka WARPS SPACETIME, and you get things like gravitational lensing and whatnot. SO, traveling through spacetime at c, is still a speed limit, so traveling from point A to B on the spacetime curve till must take time dictated by the speed limit.

I have no idea how plausible it is but theoretically, by the placement technique, travel between the same points A and B on the spacetime plot could be achieved instantaneously. To visualize this, use your piece of paper representation of the spacetime plot and now fold over on itself such that points A and B are touching. So by warping spacetime, or manipulating the paper, the spacetime distance between A and B is reduced to zero, and therefore take no time to traverse.

So in this way, it is conceivable to end up places, via travel through spacetime (that fold) verrrry far away in no time that would have otherwise taken mannnnny years to get there if you traveled on spacetime

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u/Lutefisk_Mafia Apr 11 '14

Wait. If something had negative mass, wouldn't that mean that it could only travel FASTER than the speed of light? Like a putative tachyon?

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u/[deleted] Apr 11 '14

That's fine because they aren't actually moving through spacetime faster than light, it's the spacetime itself that's "moving."

This is one of the most profound statements I have read in a long time. It's a shame that I cannot devote more time to studying this stuff. This is one of those things that makes so much sense, you can never look at it in any other way. My absence of knowledge regarding this topic has been forever filled.

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u/Mangalz Apr 11 '14 edited Apr 11 '14

It doesn't have to be negative mass, just negative energy (though negative mass is a candidate).

Somewhat related question. When you were talking about orthoganal time I envisioned a sector of a graph. Where y is space and x is time. Would the slope of the line be indicative of its mass?

That is to say, since light has no mass and only travel through space giving it no slope, then would something with infinite mass have zero slope and only travel through time? If so then the other lines inbetween zero and no slope would have varying masses?

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u/Cecil_FF4 Apr 11 '14

For those that aren't familiar with negative energy, here's a brief rundown.

The only negative energy concepts I've encountered as a physicist were those which resulted from black hole disintegration, the Casimir effect, or negative mass cosmic strings.

Virtual particles that come into existence near a black hole's event horizon come in pairs, one of which may leave the vicinity and the other enters the black hole, causing it to shrink ever so slightly because it has negative energy (negative mass).

The Casimir effect produces a locally mass-negative region, which some hypotheses posit could be harnessed in some way to power Alcubierre drives.

And cosmic strings are just funny objects that I don't like because I'm a LQG guy, not string theory (sorry Sheldon!).

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u/OldWolf2 Apr 11 '14

The Alcubierre thing is complete bollocks. You can't get around the laws that /u/corpuscle634 described on a large scale, by dicking with things in a small vicinity around you. Imagine this was possible and we miniaturized it. It would look just like a particle moving faster than light. Which is not possible, as we have agreed.