Anything with mass can’t travel at the speed of light, but a massless particle, such as a photon, completes its trip instantly from its perspective. A photon is created, departs, and arrives at its destination simultaneously.
It’s also worth noting that it also experiences zero distance. If you’re willing to tie your brain in knots, a photon doesn’t exist. Instead, space-time flexes so that 1 point touches another, momentarily. Energy is transferred, and space-time recoils back. That flex would be mathematically identical to a photon traversing the intervening space-time.
There’s a reason we use photons however. Such twisted space is effectively impossible for our brains to usefully comprehend.
I don’t know if that analogy works, because from the perspective of an observer, a photon doesn’t travel instantaneously. It travels at the speed of light.
That’s why I said space-time, not just space. Generally worked with in the form of [X,Y,Z,iT] to make them all behave space like. Basically 2 4D positions become the same position. The fact that the 2 positions are displaced in time is almost incidental. The rules for the transformation however still have to collapse down to the same underlying measurements, so it’s a lot more complex than 2 arbitrarily points.
What I remember from watching discovery years ago is simply that “time” will slow down so that it takes longer and longer for that object to reach the speed of light. This is the only kind of " time travel" that is theoretically possible.
Not really.
Take for example a star 1 light year from sun. If we start this theoretical machine, it WILL take 1 year for that machine to reach there. Same for 100 light years distance. That’s the amount of time it takes for light to travel that distance.
We’d see that in the redshift: one direction would be more redshifted than another. Instead, we see all points in space moving away from all other points (except points mutually within gravitationally bound systems), and the rate of expansion between two points (recessional velocity) is directly proportional to the distance between them: the more distance, the faster they expand.
Edit: To answer the question in the title: Strictly, we don’t. We know, as you pointed out, that our measurements don’t agree. We also have good evidence that the rate of expansion was different in the past (much, much faster) in the early universe.
That makes sense, but how would we then be able to distinguish how much of the redshift is due to the metric expansion of space and how much is due to their velocity vector component in that direction?
Inflation is supposed to explain this: It could provide the initial impulse to kickstart the velocities. I think the general idea is that the fact that everything is moving away from each other to begin with is explained by inflation, and the fact that this expansion is accelerating is explained by dark energy. Take all this with a grain of salt, here we approach the limits of my tenuous understanding, but what I do understand is that none of this is experimentally verified: No “inflaton” has been found, or any other mechanism to otherwise explain inflation theory has ever been produced such that we could test it, and no working model of dark energy has ever been produced (to my limited knowledge) that we could test or detect.
Tl;dr: I’m pretty sure it’s untestable anyway, we basically will never know during our lifetime short of some breakthrough in physics.
Right, but the velocity component would still be present in some form due to the gravitational attraction between bodies. I don’t know how significant this would be compared to the redshift value from the initial kick from inflation, or if it is possible to separate the two components somehow.
Gravitational attraction is not a relevant factor on the largest scales where dark energy takes over. To be more precise, it’s possible to measure the effects, and to describe a specific distance limit between two bodies where they can no longer become gravitationally bound and are doomed to eventually expand out of each others’ event horizons. That limit is the precise boundary between gravitational dominance and DE dominance.
To be specific, literally everything outside of the Virgo Supercluster (home to Andromeda and Milky Way among others) is outside of this limit, and will eventually become impossible to detect because the light between us and them isn’t moving as fast as the rate of expansion between us and them. Everything within the supercluster is gravitationally bound, and will eventually (iirc, grain of salt on this one) form a supergalaxy.
The Hubble constant is an interesting one – it isn’t actually a constant, but if you reframe it as a partial differential equation, the law is very predictable. en.wikipedia.org/wiki/Hubble's_law#Time-dependenc… – so in many ways, it’s just a misnamed phenomenon, and shouldn’t be called a constant at all.
Any place in the universe beyond which we’ve directly sent probes – is assumed to be like those parts we already know. Part of the reason we make this assumption is that: main sequence stars appear to behave identically across vast reaches of space and time. Thus we assume that physics hasn’t changed significantly (at least within the period of time where main sequence stars exist). Because if the physics was different, the stars would be different (spectra, lifecycles, etc.).
I’ll present a tickler I learned in cosmology decades ago, for hand-waving.
Run the big bang backwards – imagine all of the matter and energy of the universe collapsing to a single point. Which point is at the centre? They all are. Run time forwards again and all the points expand outwards from each other, but which point was at the centre that you can use to reference the centre of the universe against? They all were. Thus, I am the centre of the universe. And so are you ;)
This made my brain melt until I learned to visualize this using lower dimensional surfaces (like Riemann spheres). Imagine a beach ball being inflated. It is a two dimensional surface. You’re an ant on the beach ball and all the other points are getting further away, but it’s happening in a uniform way. (The Hubble parameter is something like the rate at which air is added to the beach ball.) Now, run this beach ball backwards through time – it shrinks and shrinks until it becomes a single point, where all points overlap – every point is the centre of the beach ball universe. Run this forward in time again and ask: which point on the surface of the beach ball is the centre of this two-dimensional universe? And the answer is “all of them” and the universe should be uniform in its expansion properties.
But it is a changing 1.23% on the same plane. Both respective planets have no significant satellites. Venus spins wonky. I’m not saying any of it is related, but it is curious.
Venus is loosely around solar lap 20M, Earth 12M, Mars 6.5M in the last 4.5 billion years. How many 1% differences stack in patterns before there is a problem?
Two more variables that are going to affect the number of encounters are when the "final" orbits of the inner planets were established (the Nice model suggests there was much disruption early on) and that Mars' orbit is very elliptic so it's rarely lining up at its closest approach, which is still pretty far. If anything we'd more likely see some correlation between Earth and Venus if there is any.
This is the first I’ve heard of the effect Mars has on Earth’s Milankovitch cycles (unsurprising, given that the paper is recent and the effect is quite small with a very long period). Earth presumably has a similar effect on Mars, but measuring this would be quite difficult. Keep in mind that we’re able to do this for Earth by analyzing drill cores (that paper uses data from 293 scientific deep-sea drill holes), which we can’t really do for Mars currently. Using other methods, we’ve been able to measure the effects of axial tilt and precession for Mars, but the effect from orbital interactions with Earth would be much more subtle. I’d be surprised if you could find anything on it in the literature.
I also would not expect the Moon to make much of a difference. The Earth-Moon distance is <1% of the Mars-Earth distance even at closest approach, so the Earth-Moon system is essentially a point mass to first order. Additionally, the mass of the moon is ~1% that of the Earth, so the effect there is quite small as well. As I mentioned, measuring Earth-Mars Milankovitch cycles is already difficult for Earth (we apparently only recently did so) while likely infeasible for Mars (currently), and detecting the effects from the Moon would be harder still.
I’m hanging cannibalism on the end of extreme, but intending on the broader scope of most extreme behaviors. It is easier to approach than the sexual and predatory counterparts.
Like if there is no potential “greater” social authority likely to interfere, is there a population density that determines overall accountability? Is it the randomness of personalities and spectrums? Is there any evidence of a change over time and social evolution?
They are hard questions. I wonder if any observational evidence exists around the various dwindling native groups that exist(ed) in various degrees of isolation. It is also a question of how fixation, paranoia, and anxiety may have evolved in the human species over time. It would be really interesting to be able to contrast this kind of behavior potential now versus the deep past.
No. Unless that hot water is very, very hot vapor, you’re just adding more mass that’s going to be cooled by the original cold water. And even with vapor, the heat transfer between a hot gas and cool liquid just doesn’t happen fast enough, the vapor will be in the atmosphere before the water heats up very much.
Go out there with a blow dryer or heat gun I guess lol. Adding more water isn’t gonna help you get rid of all the water. But I was curious if it was just a homework problem or what lol
It would mostly depend on surface area available then. If you can add warmer water and also increase the surface area available to evaporate, then you could evaporate it faster
But if you can use the hot water to heat up the cold water a little bit, that can help. That would mean bringing them into thermal contact but not allowing them to mix.
Bonus question. What will freeze faster: a cup of 100 ml hot water or 100 ml cold water? Both are uncovered.
Oh no, not the mbappe Mpemba effect effect. I refuse to accept that as a real thing, there is just no way the warm water freezes faster. I’ve read dozens of articles about it, eventually finding some that confirmed for me it’s probably just measuring error or subtle differences that aren’t being noticed. But that left me thinking if I had to search so hard for the one article that confirms my gut instinct I shouldn’t lean into it too hard
Like you have two cups of identical water, eventually the warm water becomes the cold water. If I then use that previously warm water as my cold water and start the experiment over with another glass of warm water, what now? And don’t tell me water has memory.
My favorite explanation is imagine two cars on a track 100 meters long. The far end is the track is hard asphalt and cars can drive fast. The track gets rougher and muddier the closer you are to the finish line, so the first 50 meters are covered in seconds, the next 25 meters are slower, and the final 5 meters the cars are crawling. You start one car at the 100 meter line and one starts at 10 meters. If you’re observing this race from the top of a 50 storey building above the track, you’d understandably think “wow, that car that started far away was so much faster! For sure it won” even though in the last few feet it was neck-and-neck.
The reason the hot one freezes first is because the hot one evaporates more, thereby lowering it’s mass. The amount of energy that must be removed from water to cool it is small compared to the amount of energy to freeze the water. Therefore, the mass of the water that freezes determines the total energy much more strongly than the starting temperature.
Actually, it doesnt exist.
Previous experiments accounted for evaporation by using sealed containers, and still observed mpemba phenominon.
A recent-ish study managed to control factors for all the proposed reasons of the mpemba phenominon, and found no difference between freezing cold and hot water.
They found the location of the temperature probe to be more of a factor than anything else. youtu.be/SkH2iX0rx8U
Essentially, any observations of this can be accounted for by margins of error.
So in isolation, hot water does not freeze faster than cold water.
Any observation of this are from environmental effects (extra nucleation sites in the water, different freezing conditions etc).
So the laws of thermodynamics still hold.
However, what these environmental factors are and how they contribute arent yet understood.
Thanks for the video. As it notes, the observations are real, but the explanation may not be known.
However, preventing the evaporation and then finding that the process does not occur kind of proves the evaporation theory, so I’m not sure that point works the way that you or Derek claim it does, unless I’m misunderstanding.
Lastly, I’m not claiming nor do I believe that there is some mystical way of violating the laws of thermo. I’m claiming that when the mass of water is reduced that the total latent heat is also reduced; that is completely consistent with thermo.
I mean, if you can create a vacuum, water at any temperature will boil-freeze. And the ice will sublimate afterwards above cryogenic temperatures, but I’m not sure how fast.
Even if you don’t mix the steam with the water, heat will seep in through the surface. At thousands of degrees you bet that water is gone fast - explosively - as long it’s not super deep. If this is for drying something, you can add a bunch of other hot inert gasses to dilute or push it out after, so when you cool everything back down it doesn’t re-condense.
If you have to add liquid water, it might be impossible, although I can’t say for sure there isn’t some weird non-linear evaporation effect that allows it to technically work on very cold water. Intuitively, you are always adding more additional water than additional heat, but water is crazy and breaks usual rules for matter fairly often. I’ll do a bit of digging and edit.
Edit: Research turned up nothing. As far as I can tell, water evaporation is calculated as being a linear rate. Like the light thing someone else posted, that doesn’t necessarily mean there isn’t a counterexample, just that it hasn’t been found and publicised well enough for a quick search around. So yeah, no wetting away a puddle.
askscience
Oldest
This magazine is from a federated server and may be incomplete. Browse more on the original instance.