# Atomic unit of time, does it exist? [duplicate]

I am having difficulty understanding time at the most fundamental level, especially I am wondering whether there exists an indivisible unit of time (i.e. whether time at some fundamental level is discrete). If the question is non-nonsensical, please help me with how to think about time.

Research for and against this notion are very welcome.

• Time does not flow for photons. Their unit of time is $\epsilon$? or perhaps zero? Mar 19, 2017 at 9:42
• Keep in mind that 'atomic unit of time' normally refers to the time unit in the atomic system of units, i.e. the time it takes to advance by one Bohr radius if you're going at velocity $\alpha c = \frac{1}{137}c$. That's independent of your question but it will derail your googling attempts with those keywords. Mar 19, 2017 at 11:05
• Possible duplicates: physics.stackexchange.com/q/35674/2451 , physics.stackexchange.com/q/9720/2451 , physics.stackexchange.com/q/89975/2451 and links therein. Mar 19, 2017 at 16:07

At this point, nobody knows.Certainly there is no experimental evidence in favor of such an unit. On the other hand, there is no evidence against it, except that we have been unable to find it, yet.

However, by putting together $G$ (Newton's constant of gravity), $h$ (Planck's constant) and $c$ (the velocity of light), we can compute the smallest meaningful time coming at about $10^{-44}$ second. At this scale, quantum effects should be dominating gravity and hence, because Einstein's theory links gravity and time, dominating the ordinary notion of time. Simply put, any 'time' smaller than this would hold no meaning according to our notion of time.

• Augmentation: You're referring to the Planck time . If we assume time to be quantized, too ( a fair assumption) then there could be a "minimal unit of time" - whether it is the Planck time is, afaik, unknown.
– Nox
Mar 19, 2017 at 10:49
• @Nox No, that is not a "fair" assumption. The currently accepted quantum theory, i.e. quantum field theory, does not quantize time and space - both are continuous. Mar 19, 2017 at 13:09
• @ACuriousMind Is it possible to make time or space discrete and be Lorentz-invariant? I've always assumed not.
– user107153
Mar 19, 2017 at 15:26
• @ACuriousMind I am well aware that time as well as space are assumed to be continuous in QFT.What I meant by "fair" is that having arrived at a quantized description of matter it is fair to try ang quantize time and space themselves. If memory serves me right there's a theory which actually assumes that at least space is quantized and derives from that assumption that distant $\gamma$-ray bursts should have a delay between "blue" and "red" when arriving at earth.
– Nox
Mar 20, 2017 at 13:04

The question can be answered in two ways -

1. Is time discrete for us? Yes, it is. How discrete? will depend upon the smallest event we can use to measure the time. If we can not measure time with higher precision than that, then it does not make sense to talk about smaller units of time than that as there is no way we can measure it. Just like how high the temperature can be - as high as we can measure.

2. Is time discrete in nature? No! that can not be true. Simply because - suppose there is a smallest unit of time in nature. That smallest unit has to belong to the smallest possible event in nature. That means all such events in the whole universe have to be synchronized, otherwise, you can come up with a smaller event by intersecting boundaries of two such events.

Also, suppose there was a smallest unit of time in nature, then what would wake up the nature when that time is elapsed? That means, there has to be a more precise nature underlying the nature, eventually making it a continuous nature.

• I don't think the whole universe would have to be synchronized. Einstein's theory links gravity and time, and allows for time to be stretched and compressed. Mar 20, 2017 at 4:43
• I don't understand your 'wake up' metaphor. Mar 20, 2017 at 4:43
• @TokeFaurby: Yes, and time stretching/compression is another reasoning that time can not be discrete,
– kpv
Mar 20, 2017 at 16:02
• @TokeFaurby: For the smallest unit of time, we need a wake up mechanism. For example, if our smallest time is a heart beat, then we can not differentiate within a heart beat and can only observe one beat to another. That is the wake up metaphor. Why we set the alarm? To wake us up as we can not tell when that particular time will be. Same way, if nature has a minimum time interval, then nature needs to know when that interval is over and then something more precise needs to monitor when that interval is over.
– kpv
Mar 20, 2017 at 16:05