A question posed on another forum asked, “If two spaceships were travelling side by side, 100 yards apart going 99% of the speed of light, would they be able to look through the portholes and see each other?”
It was answered by science writer, Wayne Boyd thusly, “If you were traveling on a spaceship at 99% the speed of light relative to Earth, then from your perspective, you and your neighboring spaceship would both appear to be stationary relative to each other. You would be able to see the other ship and vice versa quite normally. To you, it would be Earth that is traveling at 90% the speed of light, not you”.
But from what (I think) I’ve learned this isn’t true. For us to ‘see’ something, light has to go from the object being seen to our eyes. In the above question, even though the frame of refence of the ships is relative to each other, light has to travel from one ship to the other to be seen.
Take Einstein’s elevator in space thought experiment, light shown through a hole in one side would not shine straight through to the other side, if the elevator was traveling at relativistic speeds. So, for the purposes of the question, couldn’t the two spaceships represent each side of the elevator?
Factual question: “If two spaceships were travelling side by side, 100 yards apart going 99% of the speed of light, would they be able to look through the portholes and see each other?”
Everything we know says yes they can see each other.
Relative to each other the ships are NOT traveling at relativistic speeds. They are going at 0% of the speed of light as they measure themselves. Presuming otherwise assumes a ‘preferred’ frame of reference, which violates the premise (relativistic universe).
How can you claim it is the ships moving at high speeds rather than the ships being stationary and the rest of the universe moving at high speed?
Also, even if they were traveling at 99% of the speed of light relative to each other, they’d still see each other but not as stationary. They’d also see the light moving between them as going at a constant speed (the speed of light), albeit red or blue shifted, depending on whether they were moving towards or away from each other.
ETA: elevator example - well, no. The person outside the elevator is not in lockstep with the elevator. The elevator is moving relative to that person. You would need a pair of elevators moving in precise sync to make it analogous
Right. It may have been the case that the earth was moving at 99% of the speed of light, and then the ships both departed earth and decelerated themselves to zero speed. This highlights the unspecified detail in the OP’s factual question:
Going at 99% of the speed of light…relative to what? What is the point of reference relative to which they are moving at 99% of the speed of light, and what’s so special about that point of reference?
Or God reached out and accelerated everything on Earth except for these two ships standing side by side on launchpads.
Right - this gets to the heart of the question and the heart of most questions I’ve seen about relativity. There’s an underlying, hidden assumption of a preferred frame of reference. An ‘absolute’ reference against which things like speed are measured.
That’s light that’s emitted from a source that the elevator is moving relative to.
That’s also not Einstein’s actual elevator thought experiment, because Einstein’s version had the elevator accelerating, not merely moving, and the light source was on board the elevator. But that’s not actually relevant; this is obviously some other person’s elevator thought experiment.
This has always slightly bothered me. Special relativity assumes that there is no absolute frame of reference for either position or velocity.
But (and I know this is apples vs oranges, or math vs physics) the cosmic background radiation does ‘seem’ to provide a velocity frame of reference for the universe. Assuming we could measure it with sufficient accuracy.
Of course that gets a bit trickier now that it seems that ‘normal’ matter is only about 4% of the whole shebang…?
Well, sure, the CMB can be set as a frame of reference but there’s nothing more extra-special about it, either. The laws of physics work the same if you use that as a frame and that’s the heart of the matter.
A lot of people like to use the Earth (or sometimes the Sun) as their preferred frame. Which makes sense - we live here. Where they go wrong is trying to claim there’s something special enough about it that it dictates what happens in other frames as in the OP.
The CMB just applies that sort of anthropocentrism in a different way. If you have a velocity of 0.99c relative to the CMB, fine. But if the two rockets in the OP were relative to the CMB rather than earth, nothing changes. They see the light from each other just fine. A ‘preferred’ frame of reference means the laws of physics ‘prefer’ or are different for that frame than for others. But that’s just not true.
If you want to set everything relative to the CMB, that’s your prerogative. That may mean some complicated algebra at times, but there’s no inherent issue with that.
But claiming there’s something special about the CMB that necessitates that there is something different about it in terms of the actual physical effects (as described by the math), that doesn’t happen. And that’s been verified by several experiments now.
it seems to me the problem is not understanding “frames of reference”. The question implies the pre-relativity concept that light travels in a fixed medium of aether, like sound in air. You are imagining that photons leaving the porthole of one ship are like confettii tossed out a car window, to be swept backward in the stationary air (or aether).
A better analogy would be to consider two cars (electric, of course) in a vaccum, travelling at a constant speed. Bob throws the confetti out his window. with no wind to hinder it, it travels direct sideways at the same forward speed to the car until it enters the window of the adjacent car. from the point of view of the two cars, everything is standing still and you just tossed the confetti from one car to the other.
You can do the same thing with spherical chickens, since it’s a vaccum - and we’re ignoring gravity.
Obviously, if there’s acceleration involved, the chickens and confetti get left behind and it’s a more complex situation.
My analogy is to just remove everything else in the universe. It is nothing but empty space for those two spaceships. As far as they are concerned, they are not moving at all. There is no experiment that will tell them they are moving.
Same idea as if they were approaching each other. Each will think they are stationary and it is the other moving towards them. No way to tell (or if it is some combination of each moving).
Putting earth and the rest of the universe back in only shows how you move relative to them.
Understood. Einstein spent years trying to find the math for a theory that would be independent of any specific co-ordinate system before he arrived at General Relativity.
It’s just a niggle… probably a lingering vestige of schooldays when the ‘physics’ they taught us was a simplified ‘lies to Children’ as Terry Pratchett once put it…
Yeah, this. The very first rule in anything dealing with relativity is, “What inertial reference frame are you using?” Until you define that, it’s pointless to discuss anything else.
Once you’ve defined your reference frame, then the answer is: things happen just like they do in any other inertial reference frame.
In the OP, the reference frame to use is the one that contains both ships, and they are not moving relative to each other, regardless of what everything else in the Universe appears to be doing. So, to the ships, it’s no different than if they were sitting on the surface of the Earth looking at each other. So they’d be able to see each other fine.
It’s counter-intuitive, but then, all of relativity is counter-intuitive. If you can’t figure out how to deal with that, then maybe physics isn’t for you.
The point of relativity (as I understand it IANAP) is that it doesn’t matter if you are in the ship going the same speed, or a stationary observer. The light will appear to be travelling at the constant speed of light relative to both the ship and the observer. So both the stationary observer and the other ship will see the ship, but the stationary observer will observe all the the weird relativistic effect like the ship appearing to be much longer (or shorter? I forget how this works ) which the other ship will not.
Shorter, from a stationary observer on Earth, I think.
If one ship shined a flashlight at the other, it would appear to go straight across to the other, from the point of view of the other ship, and at an angle from the point of view of someone on Earth, right?
Right. And since the two observers disagree about the distance the flashlight’s light traveled, but (of course) agree about the speed of light, they disagree about length of time that it took. That’s where time dilation comes into play.
I think one simple way to think of this is: what’s special about the fact that they are two spaceships? What if they were two seats inside the same spaceship? What if they were two passenger compartment pods, but joined together as part of the same spaceship? What if the link between the two pods was just a thin beam? What if it was a piece of twine?
All of those scenarios are the same with respect to relativity - the two people in the two spaceships are in the same reference frame just as they would be if they were in adjacent seats in one spaceship.
I don’t think there’s any mockery intended. It’s simply a fact of relativity that many things are counter-intuitive. It’s not that you don’t understand, everyone is fine with asking questions if the askee is willing to listen to answers.
The point (I think Horatius was trying to make) is that inevitably you’re going to run into situations that simple defy human intuition. It’s really difficult to grasp those things, and it never really makes sense to our lizard brains. This was to warn you of that in case you get frustrated.
Actually, the CMB doesn’t define a reference frame. It defines a whole bunch of reference frames, a different one for every point in the Universe. If I, here, look at the CMB, and define a reference frame, and someone else in a far-distant galaxy looks up, they’re seeing a different CMB, and that CMB will give them a reference frame that’s different from mine. It’s really just a variation of the old method of “Pick a convenient large object, and call that object your reference frame”, except that instead of picking the Earth or the Sun, we’re picking the biggest thing we can see.
I think the thing that tripped me up, was a memory from a book on relativity that I read many, many years ago. An illustration in that book of light ‘curving’ after entering a slit in the side of the elevator stood out in my memory. I think it was mentioned upthread that that only applied to an accelerating elevator. Again, it’s been many years since I read that book and I’m sure I’m conflating the two (accelerating and going fast).
Then he could have stopped after the first sentence and not added the second, snarkier one.
Whatever, I know I don’t know this stuff… that’s why I asked.
) which the other ship will not.