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TomDRV t1_j26gfc8 wrote

Reply to comment by Aseyhe in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey

Hang on, if all motion is relative, why can't I pick an object moving in the direction I would like to travel at 99% of the speed of light, the travel at the speed of light relative to that, thereby traveling at 199% speed of light relative to my starting location.

Is it because the speed of light limit is relative to the 'fabric' of space? But in that case, would it not be possible (at least theoretically) to measure speed as an absolute based off whether it is stationary on space's 'fabic' or not?

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Narwhal_Assassin t1_j26kz5b wrote

The speed of light is the same for every reference frame, and no object with mass can ever go at or above that speed. If you stood on an rocket going at 99% the speed of light relative to the earth and threw a rock at 10% the speed of light relative to you, that rock wouldn’t be moving at 109% the speed of light relative to earth. Instead, the Lorentz equations tell us the rock would move at about 99.2% of the speed of light relative to earth.

Talking about the “fabric of space” isn’t really an accurate way to describe the universe because it implies that there is some sort of universal background that everything takes place against. In reality, it’s more like every single object in the universe has its own “fabric” of space that it sees, and two different objects might completely disagree about what the fabric looks like, and both could be correct. It’s very confusing and not helpful for these scenarios (relativistic speeds).

Tl;dr: very high speeds are not intuitive and don’t work the way you might think. Just remember that nothing can ever go faster than the speed of light in any reference frame, and there’s no known way to “cheat” this. There also is no “absolute” speed: everything is relative to something else, whether it’s the earth or the sun or the CMB or whatever

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John_Fx t1_j273y12 wrote

if you want to break your mind, consider that if earth were the only object in the universe it would be impossible to move because there would be no reference frame.

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randomnickname99 t1_j27r5e7 wrote

Let's say I have two guns that shoot bullets at the speed of light. I simultaneously shoot one to my right and one to my left. If I follow correctly, I can look left or right and see a bullet moving away at the speed of light. But if the bullets looked at each other they would only see themselves moving apart at the speed of light.

Here's the part I never understood though. Let's say I was standing directly between two walls that were 600,000 km apart. When I shoot the guns I should be able to see each bullet travel for one second before hitting the wall. But from the bullet's perspective that's impossible, because they would have had to travel apart at 2c to do so. How is that reconciled?

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echohack t1_j282aid wrote

In special relativity, simultaneity depends on your reference frame. In one reference frame, event A can occur before event B, but in another, event A can occur after event B. There is no absolute ordering of events that are separated in space time, unless the events are causally connected.

>But if the bullets looked at each other they would only see themselves moving apart at the speed of light.

Additionally, the reference frame of the bullet (photon) is not defined. There is no reference frame where a photon is at rest, so you cannot use special relativity to consider the perspective of the "bullet."

If the bullets were traveling at almost c, each bullet would regard the other bullet as traveling at almost c. You may observe the bullets moving away from each other at almost 2c from your reference frame and hitting the walls simultaneously, but the bullets would not. See closing speeds and other examples of apparent superluminal speeds.

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randomnickname99 t1_j29cbvv wrote

Whoa, that's exactly what I was looking for. Thanks! I didn't realize the order of events could change like that.

The space time diagrams in the link also make it a lot easier to follow.

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Yaver_Mbizi t1_j285j4z wrote

I'm pretty sure "relativity of simultaneity" describes what you're talking about.

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rckrusekontrol t1_j287j5h wrote

There’s more thorough explanations here already, but quite simply- one bullet would not perceive the other hitting its wall at the same time as itself. Remember that to see the bullet hit the wall, the light from the event has to travel to your eyes. You are equidistant. If the bullet had eyes, that light has to travel that extra distance- wall to wall. It would hit the wall, and slightly later would see it’s companion hit it’s wall.

A more mind bending “paradox” is the ladder paradox in which a ladder contracts to fit in a barn too small for it. I can’t explain it better than wiki here.

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desepticon t1_j272gd7 wrote

If you shot a neutrino, or whatever, in one direction and then another neutrino in the opposite direction, wouldn't they be going faster than light with respect to each other?

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Narwhal_Assassin t1_j27jbp5 wrote

Nope! Let’s say each neutrino is going 51% the speed of light, in opposite directions. If neutrino A were to look at neutrino B, it would only see B traveling at about 81% the speed of light. B would see A going the same speed, but in the other direction.

Now, if you’re on the ground watching these particles fly, you would see them move apart with the gap between them growing at 102% the speed of light. However, the individual objects would only move at 51% C, so nothing is violating physics

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Raflesia t1_j27izsh wrote

They would be moving away at the speed of light from any reference point. One neutrino would "see" the other moving away at the speed of light.

If you stood between them they would both be moving away at the speed of light from you. If you shot one away and then accelerated to the speed of light in the other direction then that neutrino would still be moving away at the speed of light.

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acrabb3 t1_j28jcvn wrote

How would the neutrinos perceive each other's (and their own) velocity relative to the start point?
The most cohesive answerer I can think of is that they would see the other neutrino as still at the start point, and therefore everything at the start would appear to be frozen in time.
Which makes sense, since no new information could catch up with them without going faster than light

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Raflesia t1_j29lvb8 wrote

Yup, you got it.

My previous answer is technically incorrect in the sense that "objects" at light speed don't really experience time anymore, but people have a tendency to ignore that bit when trying to explain relativity in hypothetical examples.

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jbp216 t1_j27jy00 wrote

Yeah, with those speeds, your pattern recognition of what you would expect to happen breaks down, there’s a reason relativity is so confusing to most people

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Benjaphar t1_j26zlcr wrote

> There also is no “absolute” speed: everything is relative to something else, whether it’s the earth or the sun or the CMB or whatever

Couldn't we say that the speed of light is the absolute standard?

If it were possible to measure it accurately somehow, you could send photons away from you in opposite directions and determine your true motion through space based on how quickly the photons moved away from you. Let's say you got lucky and happened to pick the direction of your true motion as one of your two directions. The photons moving the same direction you are moving would be blue-shifted as they are receding from you at c - (your true speed) and the ones going the opposite direction would be red-shifted at c + (your true speed).

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MattieShoes t1_j27bwfe wrote

You can't see the photons going away from you. The only way to measure the speed of light is by round-trip -- ie. stick a mirror out there and let the photons come back, and measure round-trip time.

... but I'm sure you see where this is going -- if the speed outward was faster, the return trip will be slower, canceling out the difference.

Now, you could stick an observer out there to try and measure the transit time in one direction... but how do you synchronize your watches without relying on the speed of light? Well, you could sync them before moving apart, but the act of traveling apart will make time pass at different rates for you, so your watches instantly become un-synchronized. The equations you might use to cancel out this desyncing all rely on... the speed of light :-D

There's a fun video on it, lemme find it.

EDIT: found it! Veritasium

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Benjaphar t1_j27cn2k wrote

Yes, Veritasium covered those issues in a really interesting video earlier this year. My question wasn't really about the logistics of the measurement for the experiment, but rather if you could hypothetically get the measurements, could you calculate your own speed relative to light emanating from your position.

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Aseyhe t1_j26nbd0 wrote

> Hang on, if all motion is relative, why can't I pick an object moving in the direction I would like to travel at 99% of the speed of light, the travel at the speed of light relative to that, thereby traveling at 199% speed of light relative to my starting location.

See relativistic velocity addition

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